Textbook-of-Zoology-1947 | Hypothesis | Observation
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INSTRUCTOR'S COPY
103

Compliments
of

THE
ST.

C. V.

MDSBY COMPANY
SAN FRANCISCO, CAL.

LOUIS. MO. and

SENT AT THE REQUEST OF

MR. FRANK

A.

VOLK

will Your opinion of this book appreciated when your review of

be
tt

has been completed.

.B

TEXTBOOK
OF

ZOOLOGY
BY

GEORGE EDWIN POTTER,

Ph.D.

Professor of Zoology, Agricultural and Mechanical College of Texas, Formerly Professor of Zoology, Baylor University

SECOND EDITION

With 445 Text

Illustrations

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ST.

LOUIS
1947

THE

C. V.

MOSBY COMPANY

Copyright, 1938, 1947,

By The

C. V.

Mosby Company

(All rights reserved)

Printed in U.

S.

A.

Press of

The

C. V.

Mosby Company

St. Loviis

DEDICATED TO

PROFESSOR FRANK A friend and an

A.

STROMSTEN

inspiration

to the student

/
f-i

PREFACE TO SECOND EDITION


The present edition represents a revision
of certain parts of several chapters, such as those dealing with Annelida, Genetics, Eugenics, Internal Regulation and Endocrines, Physiology, and Phylogenetic A brief section on Mammalian Development Relations of Animals. has been added. Numerous minor corrections or improvements have

been

made throughout.

Several illustrations have been added and

others improved.

In addition to the acknowledgements included in the preface to the first edition, the author wishes to acknowledge the help of Dr. Kelshaw Bonahm of the Fish and Game Department, Agricultural and Mechanical College of Texas, on the chapter dealing with Pisces. At this point recognition is made of assistance given by Mr. Gordon Gunter in revising the list of animals of the Texas Gulf Coast in the chapter on ]\Iarine Zoology. The author is also indebted to Dr. Fred L. Kohlruss, Biology Department, University of Portland (Oregon), Many valued suggestions have for numerous useful suggestions. been received from individuals in a number of other institutions where
the first edition has been in use.

The author's indebtedness and

appreciation

expressed to Mr. Phil T. Williams who has furnished a number of the new illustrations. Finally, appreciation is expressed to Agricultural and Mechanical College of Texas for cois

also

operation in numerous ways to assist in making this revision.

George E. Potter.
College Station, Texas

PKEFACE TO FIRST EDITION


The important problems of life are common to all animals (including man) as well as to plants. It should be the purpose of a textbook in general zoology to present the animal kingdom in a logical and natural way and at the same time carry the interpretation of the facts
in terms of the principles involved. strike the ideal balance
factual,
It is exceedingly difficult to
sufficient,

between the necessity of presenting

"type" material
knowledge of

in order that the student will have the

requisite

classification, structure, function,

development,

and organography
rules,

to appreciate the discussion of principles,

and the

opposite temptation to go into endless discussions of theories and

comprehension of which is unquestionably beyond the who has not become grounded in fundamentals of animal make-up. Of course it is usually hoped that the laboratory division of the course wiU supply this needed foundation. It seems reasonable that the ultimate aim of the teacher of introductory zoology should be to bring the student to a fundamental and well-grounded understanding of the principles involved in all of the living processes. It is extremely difficult to skim this information from the top of the entire body of zoological knoAvledge, as one can skim cream from a crock of milk, and hand-feed it to the waiting student mind. Apparently there must be a certain amount of personal acquisition of the principles of the subject through attaining a clear-cut knowledge of tlie complete biology of a series of representative animals. Each of these representatives, since it is a living organism, demonstrates certain of these principles. In order to bring this out there must be a rather close coordination between the studies of the laboratory and
the

capacity of the student

the presentation of principles

by the textbook. Based on a recognition of the above-mentioned situation and

also

on the realization that the majority of students taking elementary zoology plan to go no further in the field, the author has attempted to strike a workable combination of the two schools of teaching and still cover the fundamental knowledge of the subject. There has been
a definite effort to lead the student to think of biology as related to humankind and to himself. It is hoped that the book will overlap
the laboratory studies just far enough to
lift

the student out of the

laboratory into his o^vn correct interpretation of the facts discovered

PREFACE
there.
It is of course

assumed that the teacher

will naturally elabo-

rate

in the course. The upon particular phases of the topics anticipation of this and limitations of space have reduced the volume

taken up

of detailed information included.

Many
this

animals from west of the Mississippi River are featured in book. There has been no attempt to limit the scope of the work

to this region,

but since many southwestern and western forms are available and serve as very good illustrative material, they have been It is hoped this wiU make the book more useful and meanutilized.
ingful to students in these regions, as well as

more

teachable.

The introduction
generation,

of chapters on

Biological

Effects
is

of Radiation,

Wildlife Conservation
outline,

a slight

Animal Anomalies, Animal ReMarine Zoologij, and departure from the usual textbook

but each of these seems to the author to have enough of special value and current interest to warrant presentation. The chapIts ters on Regulatory Glands, Animal Distribution, The Animal and Environment, Animal Parasitism, Comparative Emhrijology, Animal
Behavior, and Paleontology are also presented with the feeling that they are of exceptional general interest to all students, as well as

being thoroughly zoological.

The arrangement of the chapters on animal groups has been somewhat in the order of complexity and systematic relationships. The
chapters are written in such a way, however, that this order may be modified in any manner to suit the teacher. The chapters dealing with typical Protozoa, Hydra, Planaria, Annelida, Arthropoda, and Amphibia are somewhat amplified and include more detail because

they are so often chosen as typical groups for study. Throughout many the book the genus and species names have been italicized, and

names

of structures

and functions have

also

been italicized the

first

time they occur.

The author

is

of several teachers

indebted and extremely grateful for the cooperation and specialists who have contributed manuscript

For this service acknowledgment is made Teague Self, University of Oklahoma, Annelida; Elmer P. to: J. Cheatum, Southern Methodist University, Mollusca, and assisted with Marine Zoology; Vasco M. Tanner, Brigham Young University, Arthropoda; Mary Fielding, Public Schools, Waco, Texas, collaborafor chapters in their fields.

Newman, Baylor University, collaboraOttys Sanders, Southwestern Biological Supply Co., tion on Pisces;
tion on Elasmohranchii; Rose

PREFACE

Amphibia, and assisted with Marine Zoology; Leo T. Murray, Baylor University, assisted by James E. Blaylock, Ranger Junior College, Reptilia; Helen Joe Talley, University of Oklahoma, collaboration on Regulatory Glands; T. C. Byerly, United States Bureau of Animal Industries, Ayiimal Regeneration; Titus C. Evans, University of Iowa, Medical College, Biological Effects of Radiation; Willis Hewatt, Texas Christian University, A^iimal Distrilution, and assisted with

Marine Zoology; A. 0. Weese, University of Oklahoma, The Animal and Its Environment ; Sewell H. Hopkins, Texas Agricultural and Mechanical College, Animal Parasitism ; J. G. Burr, Texas Game, Fish, and Oyster Commission, marine data; Walter P. Taylor, Texas Cooperative Wildlife Service and United States Bureau of Biological
Survey,
Wildlife

Conservation; A. Richards, University of Okla-

homa, Comparative Embryology ; Frank G. Brooks, Cornell College, Genetics and Eugenics; Iva Cox Gardner, Baylor University, Animal Behavior; W. M. Winton, Texas Christian University, Paleontology

To Mr. Ivan Summers goes immeasurable


art

credit for the excellent

work he has put

into this edition.

Dr. Titus Evans, of the

University of Iowa, Medical College, has also been of great service Mrs. Ruth M. with his excellent talent in creating illustrations.
Sanders, Miss Joanne Moore, and Mr.
assisted

Edward O'Malley have

each

The drawings used in XLII on Genetics and Eugenics were made by Miss Betty Chapter R. Smith of Cornell College. The author is grateful to all of these
by contributing certain illustrations.
individuals for their valuable services.

The author wishes

to

acknowledge also the friendly and helpful

advice which has been offered by Professors D. B. Casteel, T. S. Painter, and E. J. Lund of the University of Texas, and Professor

Asa Chandler of Rice Institute. Finally, appreciation is expressed to Baylor University for the cooperation which has made the writing of this book possible. George E. Potter.
Waco, Texas.

CONTENTS
Introduction The Biological Point of View, 17; Science and the Scientific Method, 18; Zoologj-, a Biological Science, 19; The Subdivisions of Zoologj-, 19; Classification of the Animal Kingdom, 25; Vital Eelations of Animals and Plants, 27; Attributes of Life, 30; Balance in Nature, 31; Zoology as Belated to Man, 33; Agriculture and Zoology, 34; Fisheries and the Application of Zoologj-, 34.

CHAPTEE ___________________
I

PAGE
17

History op Zoology

_________________
_______________
CHAPTER
III

CHAPTER

II

36

Protoplasm and the Cell

49

Living Matter, or Protoplasm, 49; The Cell Principle, 49; General Characteristics of Protoplasm and the Material of the Cell, 53;

Fundamental Properties or Activities of Protoplasm, 54; Physical Nature of Protoplasm, 55; Chemical Nature of Protoplasm, 56; Structure of a Typical Animal Cell, 58; Cell Division, 61.

CHAPTER IV
Phylum Protozoa
in

General

_____________
77.

65

Characteristics, 65; Classification, 65; Colonial Protozoa, 75; Tropisms

and Animal Reaction, 77; Economic Relations of Protozoa,

CHAPTER V
Euglena op Class Mastigophora

____________

81

Habitat and Characteristics, 81; Structure, 81* Food and Assimilation, 81; Respiration and Excretion, 83; Reproduction and Life Cycle, 83; Behavior, 84; Locomotion and Flagellar Movement, 84.

Amoeba op

Ck<vss Sarcodina

______________
CHAPTER
VII

CHAPTER VI

85

and Habitat, 85; Structure, 86; Metabolism, 86; Reproduction and Life Cycle, 89; Behavior, 91; Amoeboid Movement and Locomotion, 91.
Characteristics

Paramecium of Class Inpusoria


Characteristics

_____________
;

93

and Habitat, 93 Structure, 93 Metabolism, 95 Reproduction and Life History, 96; Behavior, 100; Locomotion, 102.
;

6164.3

10

CONTENTS

Metazoan Organization

__________----CHAPTER IX

CHAPTER

VIII

PAGE
103

General Characteristics, 103; Cellular Differentiation, 104; Cellular Organization, 105; Development of Sexual Reproduction, 111; Meta-

zoan and Ontogeny, 113.

Phylum

Porifera

_____-___-------Protozoa, 129.

119

Sponges, 119; Classification, 120; Fresh-Water Sponges, 121; The Simple Sponge, 122; Habitat and Behavior, 122; External Anatomy, 123; Internal Anatomy, 124; Metabolism, 127; Reproduction and Life History, 127; Economic Relations, 129; Phylogenetic Advances
of Sponges

When Compared With

Phylum Coelenterata
Classification of the

______---------;

CHAPTER X

130

Phylum, 131 Hydra, 144 Habitat and Behavior, 144; External Anatomy, 146; Internal Anatomy, 148; Metabolism, 151; The Nervous System and Nervous Conduction, 153; Reproduction and Life Cycle, 153; Regeneration, 156; Economic Relations of the Phylum, 156 Phylogenetic Advances of Coelenterates, 156.
;
;

Phylum Ctenophora

______---------____---------CHAPTER XII

CHAPTER XI

157

Habitat and Behavior, 157; Anatomy, 157.

Phylum Platyhelminthes
Classification,

160

160; Planaria, 163; Habitat and Behavior, 163; External Anatomy, 165; Internal Anatomy, 165; Metabolism, 170; Reproduction and Life History, 170; Regeneration, 173; Economic Relations of the Phylum, 174; Phylogenetic Advances of Platyhelminthes,

174.

Phylum
tat

Nemathelminthes

_____---------175
CHAPTER XIV

CHAPTER

XIII

Classification, 175; Asearis,

A Representative Roundworm, 179; Habiand Behavior, 179; External Anatomy, 179; Internal Anatomy, 181; Reproduction and the Life Cycle, 183; Relations to Man, 183.

Molluscoida, Trochelminthes, and Chaetoonatha

___---_

184

Molluscoida, 184; Bugula, 184; Trochelminthes, 188.

CHAPTER XV
Phylum Annelida (By

J. Teague Self) Earthworm, 199; Internal Anatomy, 201; Reproductive Organs, 202;

___-__-----

194

Digestive System, 203; Circulatory System, 205; Respiratory System,

CONTENTS
206; Excretory System, 206; The Nervous System, 208; Eeproduction, 209; Eegeneration, 212; Importance of Annelids to Man and Other Animals, 215; Phylogenetic Advances of Annelida, 216.

11
PAGE

Phylum Echinodermata

_______________
CHAPTER XVII

CHAPTER XVI

21

Habitat and Behavior, 226; External Anatomy, 226; Internal Anatomy, 227; Reproduction and Life Cycle, 234; Regeneration and Autotomy, 234;
Classification, 217; Starfish of Class Asteroidea, 226;

Economic Relations, 235.

Phylum Mollusca (By Elmer

P. Cheatum)

_________
;

236

General Characters, 236; The Snail, 237; Habitat and Behavior, 237; External Anatomy, 239; Internal Morphology, 243; Respiration, 244; Circulation, 244 Nervous System, 245 ; Excretory, 245 Reproduction and Life Cycle, 245; Fresh-Water Clams, 248; Habitat and Behavior, 248; External Features, 249; Internal Anatomy, 250;
;

Digestion, 250;

Respiration, 251; Circulation, 252; Nervous System and Sense Organs, 252 Excretion, 253 Reproduction and Life Cycle, 253; Economic Relations of the Phylum, 255; Classification, 256.
;
;

Phylum Arthropoda
Classification,
;

________________
; ;

CHAPTER XVIII

263

Crayfish of Class Crustacea, 266 Habitat and 263 Behavior, 267; External Structure, 268; Internal Structure, 271; Metabolism, 278; Reproduction, 278; Regeneration and Autotomy,

281

Economic Relations, 281

Characterization of Other Crustacea,

282;

Recapitulation Theory, 284;

Phylogenetic Advances of Arthro-

poda, 286.

CHAPTER XIX
Phylum Arthropoda
288.

(Cont'd)

(By Vasco M. Tanner)


287;

______
287;

287

Onychophora and Myriapoda,

Onychophora,

Myriapoda,

CHAPTER XX
Phylum Arthropoda
Arachnida, 292;
(Cont'd)

(By Vasco M. Tanner)


Classification

______

292

Spiders, 292;

of the Arachnida, 295.

CHAPTER XXI
Phylum Arthropoda
(Cont'd)

(By Vasco M. Taimer)

______
;

300

Head, 301; Thorax, 305; Abdomen, 307; Body Wall, 308; Metamorphosis, 308; Classification, 309; Hemimetabolous Insects With Incomplete Metamorphosis, 320; Holometabolous Insects With Complete Metamorphosis, 321; Other Orders, 333 Social Life Among the Insects, 334 Guests, 339 Economic Relations, 340; Useful Insects, 341.
Class Insecta, 300;
Insect Characteristics, 301;
;
I

12

contp:nts

CHAPTEE XXII
Representative Insects (By Vasco M. Tanner) The Locust, 343; The June Bug, 354; The Honey Bee, 357.

________

p^^j^

343

Phylum Chordata
Characteristics,

_______-____-_--CHAPTER XXIV

CHAPTER, XXIII

360

Phylogenetic Advances Classification, 361; 360; of Chordata, 362; Protochordata (Lower Chordates), 362; Subphylum Hemichordata, 362; Subphylum Urochorda, Molgula, 365; Subphylum
Cephalochorda, Amphioxus, 368.

The Vertebrate Animal:


Classification, 410.

Subphylum Vertebrata

_______

375

Cyclostomata

__________________
412
; ;

CHAPTER XXV

412

Economic Relations of the Class, 413 The Lamprey, 413; Habitat, 413; Habits and Behavior, 415; External Structure,
Classification,

415; Internal Structure, 415.

Elasmobranchii

__________________

CHAPTER XXVI

422

Economic Relations of the Class, 425; The Spiny Dogfish, 426; External Features, 426; Muscular System, 427; Skeletal
Classification, 422;

tory System, 435;


Squalus, 439.

System, 427; Digestive System, 430; Circulatory System, 431; RespiraNervous System, 435; Urinogenital System, 437; The Eonnethead Shark, Reniceps (Sphyrna) Tiburo Compared to

Pisces,

True Fish

_________________

CHAPTER XXVII

442

Classification,

Bony Fish

445; Economic Relations of the Class, 455; Typical Yellow Bullhead and Some Comparisons With Yellow

Perch, 457; External Features, 457; Digestive System and Digestion, 458; Circulatory System and Circulation, 459; Respiratory System,

463; Excretory Organs, 464; Skeletal System, 464; Muscular System and Locomotion, 467 Nervous System, 469 Reproduction and the
;

Life History, 470.

CHAPTER XXVIII
Class Amphibia

(By Ottys Sanders)

___________
Amphibia
in the

472

United (Tailed Amphibians), 484; States, 484; Order Caudata (Urodela) Order Salientia (Anura) (Tailless Amphibians), 485; Economic Importance, 486; Necturus Maculosus, the Mud Puppy, 487; Food and Digestive System, 489; Circulatory System, 490; Respiratory System and Breathing, 491; Urinogenital System, 492; Skeletal System,
Classification, 482;

List of Families of the


CONTENTS
494; Muscular System, 496; The Nervous System and Sense Organs, 497; The Bullfrog, 497; Habitat, 497; External Structure, 497; Digestive System and Digestion, 499; Circulatory System, 502; Respiratory Organs and Eespiration, 514; Excretory System and Excretion,

13
PAGE

515;
525;

Skeletal

System, 517;

Muscular System, 523; Nervous

The Sense Organs, 528; Reproductive Organs, 531; Embryology, 532; The Toad, 538; Habitat, 538; External Features,
System,
539;
541
;

and

Internal Structure, 541; Respiratory and Digestive Organs, Urinogenital Organs, 541 ; Blood Vascular System, 542 ; Skeleton EmNervous System and Sense Organs, 543 Muscles, 543
;

bryology, 544.

CHAPTER XXIX
Reptilia (By Leo T. Murray and James E. Blaylock) Fossil Reptiles, 546; Classification of Living Reptiles, 547; Class Reptilia, 547; Order Testudinata (Chelonia), 548; Order Squamata, 551; Order Rhincocephalia, 560; Order Crocodilia, 560; The Horned Lizard, 561; Habits and Behavior, 561; External Structure, 562;
Digestive System, 563; Respiratory System, 566; The Circulatory Sys-

______

545

tem, 566; The Urinogenital System, 571; The Nervous System, 573; The Skeletal System, 574; Muscular System, 576; The Turtle, 577; Habits and Behavior, 577; External Structure, 578; Digestive System,

578; Respiratory System, 579; Circulatory System, 580; Urinogenital System, 581; The Nervous System, 581; The Skeleton, 581; The

Muscular System, 581.

AVES

_____________________
Classification, 584;

CHAPTER XXX

582

Economic Relations, 596; Domestic Chicken, 598;

Habits and Behavior, 598; External Structure, 599; Digestive System, 601; Respiratory System, 603; Circulatory System, 604; Excretory System, 606; Nervous System, 607; Skeletal System, 609; Muscular System, 612; Reproduction and Life History, 613.

Mammalia
tive
lar

Classification, 616;

_________-__-__----A
Economic Relations, 637; The Cat,
Structure, 639;

CHAPTER XXXI

616

Representa-

Mammal, 639; External


System,
644;

Skeleton, 641;

The Digestive System, 646;

Circulatory

MuscuSystem

648;

Respiratory System, 649; Nervous System, System, 650; Reproduction and Life History, 652.

650;

Excretory

Animal Anomalies

_________________

CHAPTER XXXII

654

Harelip and Cleft Palate, 656; Diaphragmatic Hernia (Open Diaphragm), 657; Polydactylism (Extra Digits), 659; Conjoined Twins, 659; Hermaphroditism, 663; Cardiac Anomalies, 664; Abnormalities of Brain and Sense Organs, 664.

14

CONTENTS

CHAPTER XXXIII
The Endocrine Glands and Their Functions

_________

. PAGE
666

The Thyroid Gland, 667; The Parathyroid Glands, 670; The Suprarenal Bodies, 671; The Pituitary Gland, 672; The Thymus Gland, 675; The Gonads and Sex Hormones, 675; The Pancreas, 677.

CHAPTEE XXXIV
Regeneration (By
era,

T. C. Byerly)

_____________
Platyhelminthes,

681

Introduction, 681; Regenerative Capacity, 681; Protozoa, 681; Porif-

682;

Coelenterata,

682;

683;

Annelida,

684;

Mollusca,

686

Arthropoda,
688; 692;

686

Echinodermata,
Aves,

686

Chordata,

687;

Amphibia,
for
696,

Reptilia,

690;

690;

Mammalia, 690;
695;

Basis

Regeneration,

Adaptability and

Regeneration,

Summary,

CHAPTER XXXV
Biological Effects of Radiations

(By Titus C. Evans) The Structure of the Atom, 697; Biological Effects of Sunlight, 700; Infrared Radiation, 700 High Frequency Oscillations, 701 Effects of Roentgen Radiation, 702 Ultraviolet Radiation, 701 The Funda;
;

_____
Action
710.

697

mental

Action

of

Roentgen
of

Radiation,

707;

Biological

of

Radium, 708;

Effects

Other Radiations, 709;

Summary,

CHAPTER XXXVI
Animal Distribution (By
Willis

Hewatt)

__-_______711

Life Regions and Zones of the Earth, 711; Migration of Animals, 716; Means of Dispersal and Barriers, 717; Effects of Man Upon
Distribution, 718.

CHAPTER XXXVII
The Animal and
Its Environment (By A. 0. Weese) The Principal Biotic Formations, 724; Adaptation, 727; Succession, 727; Animal Populations, 730; Seasonal Changes, 733; Summary, 733.

______

719

CHAPTER XXXVIII
Animal Parasitism (By
of Parasitism, 736;
tion

Sewell H. Hopkins)

_________

735

Social Relations of Animals, 735; Origin of Parasitism, 736; Degrees

The Successful Parasite, 737; Means of Infecand Transmission, 740; Parasitism and Host Specificity, 741; Parasites and the Groups in the Animal Kingdom, 742; Some Repre-

sentative Parasites, 747.

Marine Zoology

-.._______-________

CHAPTER XXXIX

766

CONTENTS

15

CHAPTER XL
Wildlife Conservation The Abundance of Animals, 789; The fensibles, 792; The

(By Walter

P. Taylor)

____-----

PAGE
78-1

Wild Animals, 784; The Natural Range of Wild Coming of Civilization and a Declaration of IndeProblem of Restoration, 794.

CHAPTER XLI
Comparative Embryology (By A. Richards)

_____-__-

798

MAMMALLA.N DEVELOPMENT Organs and Systems, 817.

__________--_-CHAPTER XLIII

CHAPTER XLII

812

Genetics and Eugenics (By Frank G. Brooks)

______--

821

a Great Discovery, 821; Mendel's Law, 821; Derivatives of Mendel's Law, 823; The Physical Basis, 824; Plotting Crosses, 825; Complications of Mendelian Inheritance, 827; Inheritance of Sex, 831; Linkage, 832; Sex Linkage, 832; Crossing Over, 834;

The

History

of

Mutations, 836;

Human Heredity, 836; Matings Among Defectives, 839; The Differential Birth Rate, 839; Family Size in Eugenic Groups, 841; Family Size in Dysgenic Groups, 842; What Can Be
844.

Done? 844; Some Eugenic Measures,

CHAPTER XLIV
Animal Behavior (By Ina Cox Gardner)
Introduction,
;

____-_---;

846

Reflex Tropistic Behavior, 849 846 Chain Reflex Behavior, 851; Habitual Behavior, 852.

Behavior,

850

CHAPTER XLV
Paleontology (By W. M. Winton)

_______-----

854

CHAPTER XLVI
Phylogenetic Relations op Animal Groups and the Theory of Evolution Colony Formation in Certain Protozoa, 864; Development of the Gastrula, 865; Trochophore Larva, 865; Peripatus and the Wormlike Ancestry of Arthropoda, 865; Eehinoderms and Their Larval Relations, 866; Ancestry of the Vertebrates, 866; Basis for the Theory of Evolution, 870; Darwin and Studies of Evolution, 885,

_____-___--------

863

TEXTBOOK OF ZOOLOGY
CHAPTER
I

INTRODUCTION
the air, on land, or in the sea,

In whichever direction we turn or wherever we go, whether in we are surrounded by living creatures. Their very presence presents problems and fills us with curiosity.
ask questions.

We

From whence do

source of their energy?

Why

are there so

What

is our relation to other living questions and endless numbers of similar ones kindle the interest The constant endeavor on the part of of every thinking person.

What is the many different kinds? things? What is life? Such


they come?

man

to

answer these questions and solve the problems of the origin


of life has given us the field of study
is

and nature
Biology

known

as biology.

word derived from two Greek words, hios, life, and and is the name universally applied to the study Since living things fall of living organisms and life processes. largelj^ into tAvo general categories, plants and animals, such a study deals with the forms and phenomena exhibited by both.
a
logos, discourse,

The Biological Point


Nature
is

of
;

View

ever inviting investigation

operation about us, but she hides the truth.

her forces are in constant The biologist looks


in

upon himself
into

as a seeker after truth, as one striving to get a glimpse

the

mysteries

of

life.

As he succeeds
existence
of

obtaining

these

glimpses, he soon realizes the

certain fundamental

features

He

common to the structure and function of all living forms. soon recognizes the oneness of all life, and himself as a part of one great organic system, each unit of which has some relation to the whole. A biological concept may rest upon observations, which may be changed from day to day by the discovery of new facts, but the biologist, like the chemist or physicist, is justified in holding to
a theory or hypothesis as long as
for further investigation.
17
it

provides a true working basis

18

TEXTBOOK OF ZOOLOGY
Science and the Scientific Method

"Trained and organized common sense" was the definition of by Thomas H. Huxley, an eminent English biologist who lived from 1825 to 1895. That was his way of saying that scientific knowledge is simply an extension and organization of the knowledge based upon common observation and experiment conscience given

cerning the facts of nature. Facts are indispensable building stones


of science.

Facts must be gleaned from careful observations and experiments which have been rigidly checked and will yield identi-

cal results

Science lays

with frequent repetition and by numerous observers. its foundation on accurate observations and depends
Established facts

on the ability of the senses to reveal the truth.


rei^resent truth,

and the

scientist respects truth while to


little

him

tradi-

tion or

mere opinion counts for


is

as such.

There

nothing mysterious in the

scientific

method, although the

and much involved. The method is simplicity itself; to observe, to identify by comparison, to experiment, The scientist is not a mato coordinate, to deduce, to conclude. from thin air as Thurston gician Avho can draw his conclusions seemed to produce almost anything imaginable. Complete and accurate observations of the objects or phenomena under investigation followed by honest interpretation, are the aims of the scientist. Our powers of observation have been increased by the development of the microscope and many other instruments. Experiments are devised to bring to light the features not readily revealed by
steps are often tedious
direct observation.

After the facts are thus established, the qualities of the thing observed are compared and the essential or fundamental ones are separated from the nonessential. This requires accurate logic and

keen judgment.
These fundamental facts are then classified with respect to previously established facts, and, on the basis of the relationships of
qualities,
It
is

deduction

may

be

made

of the principles involved.

by

this systematic

method of investigation that science has

been established. An idea, as it first develops from preliminary observation and experiment, is known as a hypothesis. When the conclusions have been further verified by repeated examination, observation, and experimentation, the hypothesis becomes a theory.

INTRODUCTION
There
is

19

always a considerable volume of evidence which supports the Finally, all indication that it is a true statement. the theory advances to a principle or law after it has been so thoroughly and critically tried as to be generally accepted and assumed This process requires the accumulation of the combined as a truth.
theory and gives
restilts of

many
drawn

people truth
is

numerous investigators over a long period of time. To is absolute, not relative, and a conclusion once In fixed and may not be withdrawn for any reason.
always subject to modification or even

science

conclusions are

abandonment as investigation continues. Scientific hypotheses are frequently shown to be untenable, theories are occasionally found fallacious and discarded, but up to the present time our scientific However, at any time sufficient principles have remained valid. evidence is produced to show the absolute fallacy of a so-called principle, the scientist will put aside sentiment and prejudice and
accept the results of repeated investigation. Science is, therefore, a chajiging, increasing body of knowledge which is ever becoming

more thoroughly

established.

Zoology, a Biological Science

The name, zoology, which is derived from the Greek words zoos, animal, and logos, discourse on, refers to the study or science of The natural sciences, as distinguished from the social animals. sciences, are conveniently divided into two groups: the physical sciences, such as chemistry, physics, and astronomy, which deal with nonliving bodies; and the biological sciences, such as botany and Zoology and zoology, which are concerned with living organisms. The expression constitute the science of biology. botany together animal hiologij is often used as a synonym for zoology. A person

who

study of zoology is known as a zoologist. There erroneous popular impression that zoologists were was at one time an simply "bug-hunters." This conception of the field has been greatly expanded until now it is considered one of the valuable and serious
specializes in the
fields of science.

The Subdivisions
Although zoology
of biological science,
is
it is

of Zoology

only one of the divisions of the general field such a broad field in itself that it is neces-

sary to subdivide
It

it

into several divisions for convenience in study.

has been a relatively short time since all of the known biology, geology, and related subjects were studied under the head of natu-

20
ral history.
to such

TEXTBOOK OF ZOOLOGY

But now the subject matter of zoology alone has grown magnitude that it has become necessary to divide it into numerous special fields. These subdivisions may be summarized as
follows
1.
:

Morphology

is

the study of the form

and structure of the


study oc-

bodies of animals.

Its organization as a special field of

Fig.

1.

Divisions of study

in the field of biology.

(Modified from several authors.)

curred at about the beginning of the nineteenth century. further divided into several branches.
that

It is

now

A. Gross anatomy, which literally means cutting up, includes all may be studied of form and structure of bodies by dissecting them. Human anatomy, which is one of the fundamental subjects of

study in the preparation of the medical student, is usually separated from comparative anatomy. The latter study comprises a compara-

INTRODUCTION
tive

21

in turn are

study of the form and structure of the other animals and these compared, finally, with the human anatomy. The dissection, observation, and study of the parts, form, and relationship
of parts of the digestive system of the cat

would be

a good example

of anatomical

study.

Galen, A.D.

131-201; Vesalius,

1514-1564;

Cuvier, 1769-1832.

B. Histology or Microscopic Anatomy,

is

a study of the microscopic

structure of the various parts of the animal body.


ate to comprise the substance of the organism.
C. Cytology is the

The

histologist

studies the relationship and arrangement of the cells as they cooper-

study of the minute structure of the

cells

which,

we will learn, are the units of structure of all living matter. Cytology, as usually studied, includes not only the morphology of the cell
but a great deal of the physiology in addition. This field of study has yielded many fundamental concepts of the factors involved in
the living process.
2.

Taxonomy

is

the subdivision which deals with the classifica-

arrangement of organisms according to their natuThis field is often spoken of as systematic zoolral relationships. ogy. The number of described species of animals as given by diftion or orderly

ferent authorities ranges from 840,000 to well over a million. One well-known writer says there are probably no less than 2,000,000
species of living animals.

Besides these, there are large numbers of It can readily be seen that a system for putting extinct forms. these large numbers of different kinds of animals into a known
order
is

one of the

first

prerequisites for dealing with them.

On

much

is systematized for some One can see that it would be next to impossible to do business if a company were to provide a large floor space, go out and buy the thousands of different kinds of articles that are handled by a department store, and just throw all of them

smaller scale, the department store

of the

same reasons.

on

random. Few customers would return a second time if they had to wait hours while the clerk hunted among ladies' shoes, children's toys, and men's underwear for the toothbrush the
its floors at

customer desired. Instead of this, the store is divided into general departments, and the goods are completely classified within these departments. To get the toothbrush, the customer can be directed to the proper department and counter, where kind, color, size, and

22

TEXTBOOK OF ZOOLOGY
In this

price are all orderly arranged.

way

the large unwieldy

number

of different kinds of articles

become simply managed.

The relationships of animals are discovered from similarity of structure, from facts of distribution, from embryological similarities, and many other comparisons. A group in which the members
are very closely related
is

likely to be comparatively small.

These

groups are ranked together according to evident relationships.


Zoologists recognize a

number

of large divisions of the animal kingdivi-

dom based on certain general characteristics. Each of these sions is known as a phylum and is divided into classes, each
is

class

divided into orders, each order into families, each family into

genera, and each genus into species.

Taking the

classification of

man

as an

example we have:

Phylum rChordata Subphylum :Vertebrata


Class

rMammalia Order :Primates Family :Hominidae Genus :Homo


Species :sapiens

The scientific name of man is written, Homo sapiens Linnaeus. Such a name is composed of the genus name and species name, and followed by the name of the person who wrote the first authoritative

description of the particular species.

This always gives a


it

double

name

to a

kind of animal, and for that reason


in Latin instead of

is

the

hinomial system of nomenclature.

This system was originated by

Linnaeus.

The names are


is

common

vernacular

and almost universal language. The common names would be almost certain to vary with each different language, but the Latinized form Homo sapiens Linn, is the same in
because Latin
a constant

Russian as
3.

it is

in English.
is

Physiology

the study of the functions of the various parts


It involves

of the organism as well as its living process as a whole.

a consideration of metabolism, growth, reproduction, sensitivity,

and adaptation.
functions,

In this field
as

is

included the study of


circulation,

many

special

such

digestion,

respiration,

excretion,

glandular secretion, nervous activity, muscular contraction, and

INTRODUCTION
others.

23
in the developing

Many

of the processes

which occur

eman

bryo are also included here.


to as

Much

of the present study referred


is

cytology

is

physiological.

Physiology, like morphology,

old branch of zoology; physiology, however, remained in a crude state long after morphology was fairly well developed. Physiology

depends upon an understanding of physics and chemistry on one hand, and anatomv on the other. This field of study could not develop until the sciences of physics and chemistry came forward
during the nineteenth century.
the study of the abnormal structures and abnormal functioning of life processes. It is really the science of disease There is a fundamental similarity in in all of its manifestations.
4.

Pathology

is

diseases in the different groups of animals,

and

a study of pathol-

ogy is likely to involve certain forms from all groups of animals. This field has advanced rapidly during the last seventy years.
is a study of the origin and development of the usually involves the changes occurring in the organism from the time of fertilization by the union of two cells, one derived from each parent, through the numerous cell divisions,
5.

Embryology
It

mdividual.

growth, organization, and differentiation leading to the adult conThis process includes both morphological and physiological dition. changes. The beginning of this study dates back to the work of K. E. von Baer before the middle of the nineteenth century. In recent years the field of experimental embryology has developed
rapidly.

The development of the individual may be referred

to as

ontogeny.
is the division which deals with the study of variaresemblances, and their inheritance from one generation to tions, the next or from parent to offspring. The characteristic features of an animal or plant may be transmitted to the offspring somewhat
6.

Genetics

independently of one another, bringing about a variety of combinations in the progeny. Fairly definite laws governing this inheritance of qualities have been established by the geneticists. Some of
the factors which control this distribution of characteristics are morphological in their nature, others are physiological.
7.

Phylogeny

is

a study of the origin


of organisms.

and relationships
It is

of the

based on the results of studies of morphology, embryology, genetics, zoogeography, and


different groups

and races

paleontology.

24
8.

TEXTBOOK OF ZOOLOGY
Ecology
is

a study of the relation of the organism to its ento bring

vironment.

Many

made by animals
ism to the

adjustments in structure and function have been them into harmony with the conditions
in

Such conditions as the relation of the organwhich it lives to temperature, to light, to food, to competition, to enemies, to mating, and many other facThis study usually tors, all become a part of an ecological study. draws somewhat upon a knowledge of all branches of biology. This branch of the field has become prominent in comparatively recent
of the environment.

medium

3"ears.
9.

Zoogeography or geographical distribution

of animals

is

condis-

cerned with the extent of the regions over Avhich species are
tributed and the association of species in individual regions.

In
con-

some respects

this field is closely related to

ecology.

It is

affecting their distribution.

cerned with the regions in which species exist and with the factors The regional distribution of an animal group is limited in part by the extent and relations of favorable environmental conditions, but no species occupies all of the regions

where environment would permit. The point of origin of the group may be cut off from other favorable regions by unsurmountable obstacles. Conditions which prevent dispersal of animals from one
area to another are
deserts

known
all

as barriers.

Oceans, mountains, forests,

and land are

barriers to different types of animals.

Even

a slight difference in the salinity or acidity of the water becomes a

barrier to

many

aquatic animals.

The

failure of a species to
it

occupy
its

a suitable region usually means that

has been unable to reach

that region, perhaps because of the topography of the region,

geological history, or the remoteness of the place of origin of the


species.

found

The English sparrow, which originated in Europe, was not America until after it was introduced by man, and in relatively few years it became a dominant bird.
in
10. Paleozoology is a study of the animals of the past as they are presented by their fossil remains. Parts of many of the ancient animals are embedded and preserved in the sedimentary rocks. The relative age of the fossils is determined from the depth of Many of the probable the rock strata in which they are found.

lines of descent of animals


fossils.

have been discovered by studies of the concerning the facts and the fate of extinct species has been learned through this field of study. Paleozoology is ordi-

Much

INTRODUCTION

25

narily studied under the head of geology, and the geologist uses it in determining the relative ages of the rock strata which compose
the crust of the earth.
Classification of the

Animal Kingdom

Very few people realize how many different kinds of animals there are and how greatly they vary in size, structure, ajid habits of life. The estimated number of kinds is all the way from 1,000,000 To date, approximately 840,000 species have been to 10,000,000.
Cbordata
(Frog)

/^rthropoda

(Crayfbh)

MoUusca
(Snail)

Annelida {Lumbricus)

Echinodermata
{Starfish)

Bryo^oa
(Bugulaj
Rotifera

NemathMnthes
{Ascahs)

Platyhelminthes

(Tapeworm)
Ctenophcra (Be roe)
sea walnut

Coelenterafca
Fori f era

(Hydra)

(5cypha)

Protozoa
(Paramecium)

ANIMAL KINGDOM
Fig.
2.

Phylum

relations in the animal kingdom.

named and described. In order that be known and definitely recognized,


kinds has been devised.

different forms of animals

may

scheme of grouping related

The entire kingdom


or
all

is

single-celled animals,

divided into two subkingdoms: Protozoa, and Metazoa, the many-celled animals.

The secondary groups are phyla, and they in turn are divided into classes. The principal groups subordinate to the class are order, The principal phyla are listed and defamily, genus, and species.
scribed briefly:

Phylum

Protozoa.
cells,

Individuals consist either of a single


of

cell

or of

aggregates of

by each

which are performed

all

the essential

26
functions of
life.

TEXTBOOK OF ZOOLOGY

aquatic in habit.

They are mostly microscopic in size and largely Some live in the ocean, some in fresh water, others
as parasites in

in soil water, and still others About 15,000 are known.

man and

other animals.

Phylum Porifera (Sponges).


tached.

Aquatic

Most

of

them are marine.


pores.

fibrous, calcareous, or siliceous

metazoans which live atThe body is supported by spicules, and the body wall is per-

forated by
species.

many

There are approximately 3,000 known

(Jellyfish). All are aquatic and most of They possess radial symmetry, a single gastrovascular cavit.y, and tentacles provided with stinging bodies, nemato cysts. The described species number at least 4,500.

Phylum Coelenterata

them are marine.

or Comb Jellies). Free swimmarine animals that possess biradial symmetry. They are triploblastic and hermaphroditic. Less than one hundred species are known, and twenty-one of these are American.

Phylum Ctenophora (Sea "Walnuts


delicate,

ming,

Phylum Platyhelminthes (Flatworms).


mented,
bilaterally

These

are

flat,

unseg-

"Flame worms. cells" are characteristic excretory structures. These animals may be free-living or parasitic. Tapeworms, liver flukes, and the freeliving, aquatic Planaria are commonly known. Approximately 6,500 species have been described.
symmetrical,
triploblastic

Phylum Nemathelminthes (Threadworms

or

Roundworms).

Un-

segmented, bilaterally symmetrical, elongated worms which possess both a mouth and an anus. Some are free-living, others are parasitic. The hookworm, ascaris, and the "horsehair worm" are com-

mon

representatives. About 3,500 species are known. Phylum Echinodermata. Marine animals which have

spiny

skin and the body wall usually supported with calcareous plates.

and have tube feet as organs of locoThe common representatives are starfishes, sea urchins, sea cucumbers, and sea lilies. There are about 4,500 known living
radially symmetrical

They are

motion.
species.

Phylum Annelida (Jointed worms). This group is characterized by segmented body, well-developed body cavity, and nephridia as tubular excretory structures. They live in marine waters, fresh water, and in the soil. The earthworm and leech are well-known examples of the phylum. There are at least 4,500 known species.

INTRODUCTION

27

Phylum Arthropoda.
aquatic or terrestrial.

The

representatives of this group

may

be

Their bodies are segmented, and they have

segmented appendages.

The group includes crayfishes, lobsters, crabs, centipedes, scorpions, and all insects, such as bugs, beetles, butterflies, flies, etc. This is by far the largest single phylum. Some
authors believe as

many

as 675,000 species belong to

it.

Phylum

Mollusca.

Unsegmeuted animals that are usually en-

The single muscular "foot" is a charCommon forms include clams, snails, slugs, acteristic structure. and octopuses. About 78,000 species have been recognized.
closed in a calcareous shell.

Phylum Chordata.
eral

Segmentally

constructed animals with bilat-

symmetry and

an endoskeletal axis or notochord at


;

some

stage.

Many of our best known animals belong here the phylum includes lampreys, sharks, bony fish, frogs, salamanders, alligators, snakes, Apturtles, rats, birds, horses, sheep, cows, monkeys, and men.
proximately 40,000 species have been described in the group.
In addition to the above generally recognized phyla, there are several other more or less independent smaller but distinct groups.

Most

of these groups

have certain of the wormlike characteristics.


contractile,

Many

authors have dignified each of these as a phylum.


frequently
similar

Nemertinea,

nearly unsegmented, Trochelminthes unsegmented and

They are wormlike forms;


to

certain

larval stages of annelids and molluscs, rotifers being typical; Bryozoa colonial, marine, or fresh-water forms, of which there are about 1,750 known species Brachiopoda marine animals enclosed in a bivalve shell, the majority of which are fossil; Phoronidea

marine worms living in chitinous tubes in shallow water; Chaetognatha marine, transparent, carnivorous worms of which Sagitta is an example; Sipunculoidea unsegmented, elongated marine worms, living either free, in tubes, or in snail shells. A number of these are sometimes described under the phylum name
sessile

MoUuscoida.
Vital Relations of Animals and Plants

There are certain single-celled organisms that are claimed as animals by zoologists and as plants by botanists. As a matter of fact, it is not easy to draw an absolutely clear-cut line of distinction. Of course, it is easy to recognize the extremes. Anyone

28

TEXTBOOK OF ZOOLOGY
Chiorophyll

Sunlight

\nt(zrmedigte

decomposition

products

Respiration

formoldehyde

^
fats

\ Bacteriai
decomposition

Living ^/Carbohydrates , animals \ (and proteins)

/
Dead
organisms
cycle

Pig.

3._The carbon

and the fundamental

living process.

Inyalra -^rnn

fO-)

Hrnnnir- food

a AnimQ/

Sl l /~^trf

i-.t

of Oa

well as the food Fig. 4. The metabolic processes of plants and animals ^s (Redrawn by permission f om Wolcott, manufacturing process (photosynthesis). Company, Inc.) Animal Biology, published by McGraw-Hill Book
i

INTRODUCTION

29

holding a sunflower in one hand and a frog in the other has no The difficulty in determining which is animal and which is plant. animal forms depend on green plants for their distinctly typical

a d

a
S o

O
>, Si

m o

ttl

fe

o >> o C
<D

ho

C
0)

bo

8100H ia KOIidHOSaV

existence.

All organic animal food

is

derived either directly or

indirectly

from the process

of photosynthesis carried on
is

plants to manufacture starch which

stored in

by green plant tissue. Green


in sunlight

plants by effect of their chlorophyll

(green pigment)

30

TEXTBOOK OF ZOOLOGY
air,

cause carbon dioxide, a gas of the

and water
is

to unite in the

formation of a simple carbohydrate.


of plants as well as of animals.

This

the basic food material

During daylight hours while photo-

oxygen is discharged into the air as a product of the process. This oxygen adds to the atmospheric supply and is used by animals in respiration. The carbon dioxide discharged by respiration of plants and animals is made use of by plants in this synthesis of material. The excretory products of animals contain nitrogen which is easily transformed into a soluble form and absorbed by plants to be combined with the simple carbohydrate, already described, to produce protein.
synthesis
is

in progress,

In general, plants, by utilizing the radiant energy of the sun, and


chlorophyll, as a catalyst in causing the combination of water and

carbon dioxide, form the potential material for the animals, because
they extract simple substances from the earth and unite them into

complex foodstufl^s, such as the starches and proteins. Plants are devoured by animals, and some animals are in turn devoured by others. The complex substances are then broken down with liberation of energy, and the by-products excreted are again incorporated in the earth to be available to other plants.
Attributes of Life

Most

of us think

we know what
is

life is,

but

if

asked to define
task.

it,

we

find ourselves confronted

by an almost hopeless

The ques-

tion.

What

is

Life?,

the greatest riddle in the biological world.

The term life is an abstraction with no objective reality except as is a phenomenon related to the activities of living units. The following statement has been given and is probably as nearly a definition as can be found Life is a continuous series of reactions in a complexly organized substance, by means of which the organiit
:

zation tends to adjust itself to a constantly varying environment.

Numerous
rial

attributes of living material

may

be given.

Living mate-

has the ability to carry on active chemical reactions without losing its body form. It is responsive to changes in the environmental conditions therefore, it is said to be adaptive. Living material is able to sustain and reproduce itself under favorable conditions. The size of living organisms varies within definite limits.
;

Much more

will be said of living material in the following chapter.

INTRODUCTION
Balance in Nature

31

The influence exerted by one animal or one group of animals on


another can hardly be estimated until one of them leaves the picture.

In an established animal community which might be said to

all groups are held in boimds by their enemies. Balanced animal communities can be found the world over, and we are only beginning to get a notion of the extensive ramifications of the forces concerned in maintaining that balance. Quite clearly most animals live in a state of repression because relatively few of them become pests and overrun the country. About eighty-five years ago someone who had admired the remarkable spirit of the English sparrow in its native European home thought this hardy little bird would be a cheerful addition on this side of the Atlantic, Consequently, a few pairs were landed in Brooklyn. In the short years that have elapsed, this sparrow has proved so hardy and free of enemies here that it is now our dominant bird. Every city in the United States, as well as many in Canada and Mexico, has a large permanent population. Its nesting and perching habits in the heart of great cities are a source of great annoyance and expense to building owners. Also, they consume enormous quantities of the farmers' grain.

be balanced,

The story of the rabbit in Australia is likewise an interesting example of the effect of balance or lack of it. Not many j-ears ago Australia did not have a rabbit within its boundaries. It was hoped and intended by English immigrants there, that a few imported pairs of rabbits would increase sufficiently so that the old English sport of riding to the hounds might be developed in Australia. To the surprise and dismay of these people the rabbits flourished until now they are jeopardizing the enterprises of man. Many men are kept employed full time doing nothing but hunting rabbits.
Again,
of

we have an example

of the effect of the natural agents

The Japanese beetle which was recently introduced in the United States hy accident has ravaged the vegetation in several eastern states and threatens other areas. When our investigators went to Japan to study the enemies of the beetle in an effort to find a means of control, they had to search for weeks to find a seriously infested area. So impressed are some biologists berepression.

coming with the potential danger of interfering with the natural balance, that even when some irritating pest is under discussion,

32

TEXTBOOK OF ZOOLOGY

a 0] C
bo

u o

<u

bo

a o S d
m S

o
0)

o o

0)

Pi

s
0)

d
ft >>

o C

bo
i-t

INTRODUCTION
whose extermination
is

33
it

easily possible, they will advise against

until all phases of the animal's existence are thoroughly investigated.

To wipe out this form might remove the cheek on others that are still more obnoxious. Because of the danger of interfering with the normal balance or equilibrium in nature, our government and many others have placed a restriction on importation of plants or animals. One must have permission to bring either into this country.
Zoology as Related
to

Man

The values of the study of zoology may be placed in two classes: and practical. There is hardly a field of endeavor in the realm of human activities which is not greatly influenced by zoology and biology generally. The study of philosophy, the formulation of our conception of religion, the comprehension of social welfare problems, and many other similar intellectual and social accomplishments are greatly facilitated by a knowledge and recognition of biological principles. From the purely practical or economic side, of course, agriculture, medicine, and their related sciences have
cultural profited enormously.
biology.

In fact, these fields are in themselves applied


of inheritance of

Most

of the great discoveries as to the nature

of disease, the

manner
of

human

characteristics,

and control and

the

knowledge

fundamental physiological processes occurring in

our

own

bodies have been attained by studies on other animals.

What is found to be true in a dog, frog, rabbit, rat, monkey, or guinea pig, usually has its application to man. The lives of these laboratory animals have made untold and inestimable contribution
to the welfare

and comfort of man.

The

loss of their lives is con-

stantly saving millions of


uses of other animals
is

human

lives.

One

of the

most obvious

as a source of food supply.

All of the phyla

and

classes of larger animals furnish at least a

places on our

menu

cards, particularly

frogs, fish, crabs, lobsters, clams, oysters,

few species that find mammals, birds, turtles, and even snails.

Many
of
to

animals are important because of their destructive tend-

encies in regard to articles valued

man.

by man, or to the health and life Most of the predaceous animals today are not a menace
but they do destroy
It is likely

man

directly,

many

domesticated as well as

useful wild animals.

that the parasites which live on

and

in the bodies of men, and on domesticated plants and animals have been much more costly than the depredations of the more conspicuous predators.

34

TEXTBOOK OF ZOOLOGY
Agriculture and Zoology

It may frequently bring a smile to the lips of an onlooker to see a full-grown and perhaps intelligent zoologist enthusiastically attempting to learn what, when, and how much a little boll weevil

eats or when, where, of such information

and how

it

lays its eggs

and

yet, the discovery

may

influence the activities of our entire cotton

A recent instance of the economic importance of zooknowledge is found in the saving of the entire citrus industry in Florida from the Mediterranean fruit fly. Injurious insects alone cause an annual loss in the United States of more than one and one-half billion dollars worth of products if they could be sold at the price the remaining portion brings. With proper knowledge of animal life and application of this knowledge it is likely that at
industry.
logical
'

least half of this loss could be prevented.

Losses almost as impor-

tant are caused each year by the parasitism of our domestic animals

by

bacteria, protozoans,

worms, and
is

insects.

application of parasitology, which


this loss.

a field of zoology,

The knowledge and would avoid

Agriculture has benefited greatly from the application of the and animal breeding. Much fundamental knowledge has come from the extensive studies on the
principles of heredity to plant

genetics and breeding of the


easily kept in the laboratory

common

fruit

fly,

Drosophila.

It is

and mated. It produces a new generation about once every nine days. More improvement of strains of animals and plants too, can be made in one man's lifetime than was previously possible through ages. The United States Department of Agriculture and the United States Department of Interior have
taken the lead in

much

of this type of zoology.

Fisheries

and the Application of Zoology

A very practical and profitable application of zoology has been made


The annual salmon catch alone on the Pacific coast has been known to be worth $25,000,000. The fishing industry cultures, collects, and markets not only fish of many kinds but also oysters, clams, lobsters, crabs, shrimp, and even sponges. The United States Fish and Life Service does an extensive and remarkable work in the study, propagation, and care of this natural zoological reEven with this work and that of all the State Fisheries source. Departments, the natural fish life does not flourish as it might, had
in the fishing industry.

INTRODUCTION

35

live.

our public more appreciation of conditions necessary for a fish to A fish needs suitable water conditions including proper gas content, salt balance, nesting places, vegetation, and freedom from
chemical or
oil

pollution.

The

strictly intellectual

has given

man

are no less valuable than the tangible gifts.

understand something of that her operations are in accord with definite principles, gives one insight to the solution of many of the problems of life. Many of the superstitious dreads of unseen monsters have been eliminated by the knowledge of the fundamental principles of life processes. In recent times, it is probably true that nothing has influenced the thinking of the world more than the ideas, principles, and knowledge growing out of biological study.

and cultural endowments which zoology To the orderly conduct of Nature and to see

CHAPTEE

II

HISTORY OF ZOOLOGY
This brief chapter
is

works and
This
is

lives of a selected

organized to afford a slight preview of the few of the historic pioneers of zoology.

not an attempt to give a complete history of the subject. of numerous pioneers in special fields are being conthroughout the text rather than in a given chapter. sidered There were individual persons interested in and studying natural

The works

history long before there

was any

orgajiized field of study recog-

nized under the name of natural history or the more limited diviSome of the translations from the sions of it, including zoology. later from the Greeks indicate that there had early Egyptians and been some concern for the problems of life as well as medicine a

number

of

centuries

before
living

Christ.

Some
all

of

the

early

Greek
life.

scholars believed that the ocean supported

of the original

Hippocrates, a Greek

from 460

to 370 B.C.,

was the

first to

think of medicine on a scientific basis. Aristotle (384-322 B.C.) was an outstanding Greek philosopher and scholar. To him goes the
of study which is based and drawing conclusions on gathering facts from from a study of these facts. His observations on the structure and development of embryo sharks, chicks, and many other animals, as

credit for establishing the scientific

method

direct observations

well as his introduction of animal classification, are contributions which caused him to be called a biologist. He had the assist-

ance of the armies of Alexander the Great in collecting materials. Alexander had been one of Aristotle's pupils and had become interested in the development of scientific endeavor. He made a grant
of 800 talents ($200,000 or more) for use
gations.

Thus even

in those times

by Aristotle in his investiendowments were being set up

for the support of research.


totle

The other Greeks who followed Arisadded very little of importance. Early Roman Scholars. From shortly before the time of Christ and extending for about sixteen centuries was a period of "dark ages" in scholarly endeavor. However, a few contributions of note were made. Pliny (a.d. 23-79), a Roman general, compiled a 37volume work in which much of the scientific knowledge of the time

36

HISTORY OF ZOOLOGY

37

and traditional superstitions are woven together. His work was limited to compilations, and because of the indiscriminate mixing of fact and fancy it is not scientifically valuable. It does reflect the tendency of the time in that scientific observation had given

way

to speculation.
(a.d.

Galen

131-201), coming in the midst of the

"dark ages"

as

he does, should be particularly credited for the contributions he made. He was of Greek ancestry but moved to Rome early and

became a successful physician. His anatomical studies were made principally from direct observations on elephants, Barbary apes, and

Fig-.

7.

found

In

Aristotle (384-322 B.C.), Ursinus. father collection of Fulvius


tlie

of naturalists. From a (Visconti, Iconographic

bas-relief

grecque.)
Inc.)

(From Locy, Growth


swine.

of Biology, published

by Henry Holt and Company,

During

his time

it

was

strictly against the

law to make

was not allowed this privilege. Unfortunately, Galeai did not take advantage of the work of certain of his predecessors who had been privileged to study human bodies.
dissections of the
so he

human body

His conviction in the matter of direct observation as a basis of study handicapped him in this respect. His textbook on anatomy became the authority for the next eleven or twelve centuries.

Andreas Vesalius (1514-1564). The return of interest in zoology came about through the medical schools. Vesalius was aji active

38

TEXTBOOK OF ZOOLOGY

Fig. 1719. Inc.)


5.

8.

(A.D. 131-200), anatomist. Acta GalenLocy, Groivth Biology, publishedMedicorum (From by Henry
of

Berolinensium, Vol. Holt and Company,

HISTORY OF ZOOLOGY

39

young student and was not


Galen's textbook.

satisfied to

accept the authority of

Therefore, after beginning his medical education at Brussels, he transferred to Padua where human dissection was

Iislied

Fig. 9. Vesahus (1514-1564), anatomist. Frontispiere. facsimile edition of 1728. Nortliwestern University Library. (From Locy, Grotvth of Biology, pubby Henry Holt and Company, Inc.)

then allowed.
the
first,

He

later

since the time of Aristotle

became professor of surgery there. He was and Galen, to prove that direct

40

TEXTBOOK OF ZOOLOGY

observation is the only true criterion of knowledge. Vesalius is thought of as the "father of modern anatomy," and his teaching is really responsible for the rapid development of biology and medicine following his time.

William Harvey (1578-1657). Following closely upon the epochmaking work of Vesalius and inspired by several of his pertinent observatiojis on the anatomy of the circulatory system, "William Harvey, an Englishman, began experiments on the movement of blood in the vessels. Galen, Vesalius, and three or four others had

Fig.

10.

History William

Harvey

(1578-1657),

father

of

physiology.

(From

Garrison,

of Medicine, published

by W. B. Saunders Company.)

suspected a circuit of the blood from the heart to the lungs and
return, but
first to

Harvey was the

first to

demonstrate circulation, and the


This

arrive at an idea of a complete circulation of all of the blood


vessels.

through a closed system of


in 1628.

new

idea

was presented

He

also did notable

work in embryology.

histologist,

Marcello Malpighi (1628-1694) was a famous Spanish anatomist, and embryologist. His observation of blood corpuscles in capillaries, studies on glands, and his work on the structure and

metamorphosis of the silkworm take rank with outstanding con-

HISTORY OF ZOOLOGY
tributions to zoological knowledge.

41
of the

Numerous organs

human

body are named for

early microscopists, he

renowned had to build his own microscope. Antonj van Leeuwenhoek (1632-1723) lived almost contemporaneously with Malpighi and like him made many contributions to the
this

scientist of his time.

Like other

Leeuwenhoek (1632-1723), pioneer micromotist. (From a painting by Fig. 11. Veekolje, 1685. Reprinted by permission from Locy, Growth of Biology, published
by Henry Holt and Company,
Inc.)

development of the microscope. He is said to have possessed a total Further study on of 419 lenses, most of which he had ground.
capillary blood circulation, first descriptions of spermatozoa, extended observations on bacteria and microscopic animals, and his

42

TEXTBOOK OF ZOOLOGY

valuable contributions to the development of the microscope are


the enviable accomplishments of this man.

gist,

Carolus Linnaeus (1707-1778) was a very eminent Swedish biolowho, like many early students of this subject, was educated

as a physician.

(1628-1705),

He followed somewhat in the footsteps of Ray who had paved the way by fixing a definite conception

Fig. 12. Linnaeus (1707-1778), an outstanding Swedish biologist of his time. (Reprinted by permission from Locy, Growth of Biology, published by Henry Holt and Company, Inc.)

of a species and introduced the use of anatomical features in distinguishing the larger groups. Linnaeus believed in a rigidly fixed

species

and had divided the animals into six classes, 32 sub-classes, and numerous genera and species. In spite of his idea of the invariability of species his classification system was so simple, clear, and flexible that it has persisted to the present time. His was the

HISTORY OF ZOOLOGY
first

43

natural system of classification, and it is known as the Binomial System of Nomenclature. Each individual not only fits into larger general groups by this sj'stem, but it is specifically known by the genus and species names used together, hence the two names. Linnaeus is said to have classified and listed 4,378 species of plants and animals.

Almost immediately following Linnaeus came the Frenchman,

Lamarck

(1744-1829),

who among

other important things

is

credited

with being

first to realize

that there are different lines of descent


in 1866, as used

and that no living species is absolutely fixed. Much later, Ernst Haeckel organized the modification of this system
in

modern

times.
is

Georges Cuvier (1769-1832)


of comparative anatomj^

credited with establishing the field


of

He was
tlie

French ancestry and largely

self-educated by his studies at


cal structures bear his

seashore.

A number

of anatomi-

name.

really established

Karl Ernst von Baer (1792-1876), a Russian biologist, is one who embryology as a field of study. His notable paper on the development of the chick was published in 1832. He estab-

lished the

"germ

layer theory,*' thus explaining the unfolding and

differentiation of the various organs of the developing animal.

The

recapitulation theory, Avhich


of his

is

explained elsewhere, came as a result

work and thought.

Johannes Miiller (1801-1858), a German scientist, is referred to as the founder of comparative physiology and the first to apply the facts of physics and chemistry to living protoplasm. His work was
a great impetus to

modern physiology.

Matthias Schleiden (1804-1881) and Theodor Schwann (1810-1882) are the two Germans who in 1838-1839 arrived at one of the most important generalizations of biology, the cell theory (principle).
This
is to

be discussed further in the following chapter.


is

Louis Agassiz (1807-1873)

commonly regarded

as the father of

American zoology and a renowned student of comparative anatomy. His great inspiration has permeated through his students to nearly every institution in the land. He was a recognized paleontologist
as well as zoologist.

He

is

responsible for one of our

first

and oldest

Marine Biological Laboratories.

44
Charles

TEXTBOOK OF ZOOLOGY

Darwin

(1809-1882),

studies on the problem of the


species of organisms arise.

an Englishman, made extensive manner and means by which new


effectively developed the thesis

He very

that they originate by a process of natural selection.

This was based

on the idea that no two individuals are exactly


tions are constantly appearing,

alike, that

new

varia-

or groups best suited to their


persist

and produce progeny.

and finally that those individuals environment would be the ones to His conception of the factors and

P^ig.

13.

(From Locy, Agassiz (1807-1873), great American pioneer zoologist. Biology and Its Makers, published by Henry Holt and Company, Inc.)

new species, pictures a constant struggle for existence among organisms, with those whose natural variations happen to fit them best to the changing features
limitations determining the development of

of the environment persisting as

crowded out. Those become extinct.

least fitted to the

dominant species and others being environment would naturally

Darwin did not claim originality in his idea. Lamarck, Buffon, and Erasmus Darwin, grandfather of Charles, had presented similar

HISTORY OF ZOOLOGY
ideas before him.

45

It was the vast accumulation of facts covering a period of twenty years which commanded the attention of scientists as well as the public generally. In 1858 he read a joint paper with Alfred Russel AVallace, a contemporary who had reached the

same conclusion, on the theory of natural selection. That same year Darwin published his book Origin of Species which is a classic in its field and familiar to all scholars.

'

>T.:"

Fig.

(From 14. Charles Darwin (1809-1882), the author of Origin of Species. Garrison, History of Medicine, published by W. B. Saunders Company.)

Gregor Mendel (1822-1884) was an Austrian monk who carried on experiments with the breeding of garden peas in the cloister garden. From his work there, he derived the original laws of heredity. His results were first published in an obscure Swiss paper in 1866 and were not really discovered and appreciated until 1900. He was the founder of genetics. He crossed different kinds of peas and found that the offspring in the first generation all resembled one parent. When these oft'spring were interbred he found that three-fourths of their progeny resembled one grandparent, and the remainder resembled the other. From these facts he referred to characteristics of the former group as dominant and those of the
latter as recessive.

The facts which he established are now known


of Heredity.

as Mendel's

Laws

46

TEXTBOOK OF ZOOLOGY

,(S,

C^^f^e^a^-^

Fig. 15. Greg-or studies of heredity.

made between 1861 and

Mendel (1822-1S84), the Austrian monk who pioneered In Plaque by Theodor Charlemont based on "Fuschia picture" 1864. The signature is his, taken from an autobiography.

(Courtesy of the Journal of Heredity.)

HISTORY OF ZOOLOGY
Louis Pasteur (1822-1895) was a French scientist

47

who had been

trained in chemistry but became one of the outstanding pioneers in applied biology and medicine. In 1861 he put an end to the

controversy regarding spontaneous generation of living organisms

and established the idea that all life in present times comes from life. He showed that living organisms cause fermentation and demonstrated that these organisms and others could be killed by heating them to a certain temperature. He showed that materials thus heated and then sealed would not ferment until after they were exposed to the organisms in the air. The pasteurization
preexisting

Fig.

16. Louis Pasteur (1822-1895), one of the benefactors of mankind. (From Garrison, History of Medicine, published by W. B. Saunders Company.)

process grew out of these experiments. He rescued the silk industry of southern Europe by discovering the organism which killed the insects, and he also discovered an immunization process and treatment for hydrophobia.

Thomas Henry Huxley (1825-1895) Avas one of the most popular English scientists of his day. He was one of the principal champions of Darwin's ideas and theories. Comparative anatomy and paleontology were greatly advanced through his influence.

August Weismann (1834-1914) was a German

zoologist

who

started

out as a physician after having been trained in that

field.

He was

48

TEXTBOOK OP ZOOLOGY
fields of

an outstanding scholar in the


is

for his theory, that there He plasm from generation to generation.

best

known

heredity and embryology. is continuity of germ

(1848-1935), a Dutch botanist, brought out the mutor tion theory, which is important to all modern biological conceptions. His idea was that species have not arisen through gradual selection requiring thousands of years for each but by jumps through sud-

Hugo DeVries

den, though small, transformations.

He

is

widely

known

for his

experimental studies in plant breeding and with evening primrose.


omists of America.

genetics, particularly

E. D. Cope (1840-1897) was one of the greatest comparative anatHe dealt not only with living forms but with

fossil materials as well.

The work of all those mentioned and hundreds of others has given us the background for our present knowledge and grasp of zoology and medicine. History is being made so rapidly in these fields durdifficult even to catalogue the imextremely active field, particularly portant contributions. It is an The printed program in the realm of the experimental endeavors. American Zoological Society, which for the annual meeting of the is made up largely of titles and abstracts of new papers to be pre-

ing the current years that

it is

sented,

is

a small book in

itself.

The works and lives of such prominent pioneer zoologists of the Southwest as Jacob Boll, Gustaf W. Belfrage, Lincecum, Vliet, Walker, "Webb, and others have been described in the recent book by Dr. S. W. Geiser of Southern Methodist University, entitled
Naturalists of the Frontier.
read.

This book

is

extremely interesting to

CHAPTER

III

PROTOPLASM AND THE CELL


Living- Matter, or
Little is

Protoplasm

known concerning
called, but

the origin of living matter, or protois

plasm, as

it is

more and more

being learned about

its
is

nature, characteristics, structure, and activities.

Living matter

ahvays active in some degree, and this activity attracted the attention of scholars at a rather early date, but serious study of the
material was not begun until approximately one hundred years ago.

some of the simple microscopic animals he was studying were composed of a soft, gummy substance and called it sarcode, which means "flesh." He was able to test its solubility and its behavior with alcohol and acids sufficiently to satisfy himself that it differed from ordinary gelatin or albumin, with Avhich it might be confused. In 1840, Purkinje, a Bohemian biologist, gave living matter the name protoplasm, which comes from the Greek protos, first, and plasma, anything formed or molded. In 1846 von Mohl, a German botanist, saw in plants a granular, viscous substance similar to that already seen in animals', and called it protoplasm. He was instrumental in bringing this name into common use. During these years it had gradually dawned on biologists that this matter is found in all livthe
of Dujardin, in 1835, realized that

Frenchman by

name

ing things.

The
Cells

Cell Principle
superficially described during the

had been seen and even

latter part of the seventeenth century

the eighteenth century, but their significance

and numerous times during was not realized.

Hooke, an Englishman, in 1665 in observing cork with the microscope he had made, saw the spaces in it and called them cells because they reminded him of prison cells. This name later came to be applied to the real cells. It was an unfortunate term, for cells do not have a hollow structure but are typically semisolid bodies. Leeuwejihoek saw spermatozoa and bacteria and included them with single-celled animals as "little beasties"; Malpighi had described
49

50

TEXTBOOK OF ZOOLOGY

the nature and appearance microscopically of several organs of the body; Grew had made rather extensive microscopic studies of plants,

and in 1831 Kobert Brown had discovered the nucleus of the cell, but not until the work of Schleiden in 1838 and Schwann in 1839 was the cell theory formally enunciated. The former a botanist and

Fig. 17. Matthias J. Schleiden (1804-1881), noted German botanist who helped establish the cell theory. (From Locy, Growth of Biology, published by Henry

Holt and Company, Inc.)

the latter a zoologist, each

working independently, came

to the

same

conclusion and in 1839 collaborated their ideas. This theory, as they gave it, was in substance, All living things (plants and animals)
are composed of
cells.

PROTOPLASM AND THE CELL


It is

51
that they and

no discredit

to this

theory or these

men

many
dis-

other biologists of the time had erroneous ideas concerning the


essential features of the cell.

Although Brown had recently

covered the nucleus, the


part,
cells since practically

cell

wall was thought to be the essential

though now we know it is not a universal structure of all no animal cells have a cell wall. The notions of the origin of cells and the functional significance were almost wholly fantastic, yet the cell theory proved to be such a unifying generalization and inspiring stimulus to investigation that it became the turning point in the development of biological study.

Fig. 18. Theodor Schwann (1810-1881^), the German zoologist who, in 1838 and 1839, collaborated with Schleiden in formulating the cell theory. (From Garrison, History of Medicine, published by \Y. B. Saunders Company.)

The bare statement that living beings are composed of cells soon became inadequate as studies of cells progressed. It was soon found that some tissues are made up not only of cellular structures but included also certain noncellular materials produced by the cells. The matrix, so abundant between the cells of cartilage, was soon found to be noncellular and to be produced by the cartilage cells which became embedded in it. This matrix is not strictly living matter since it is inactive and passive as far as life processes are concerned. Connective tissue fibers fall in the same category. Since living bodies are composed of such an abundance of this noncellular

52

TEXTBOOK OF ZOOLOGY
produced by the
:

m aterial

cells,

the cell prmciple soon

came

to be

things are composed of cells and cell products. stated thus conceptions of the nature of the nucleus, the With the years, the
all living

membranes, and the composition of protoplasm itself have all added their contributions to the present understanding of the meaning and application of the cell principle. The cell is now regarded
cell

as a physiological unit as well as a structural one, and as almost a corollary to the original statement of the principle, namely, that

the activities of the organism equal the


cells.

sum

of the activities of its

the embracing of the functional activity of the cell as a part of the principle underlying living processes comes also the inclusion Cell division, growth, tissue formaof heredity and development.

With

tion,

migration of

cells,

formation of

cell

products, chromosome rela-

and modifications have come to be recognized as being brought about in or by the cells. Through the rather rigid and constant set of developmental changes for which the cells are responsible, there is developed a new individual which usually resembles
tionships
its

parents quite closely.

cell theory on biological thinking and progress on fundamental thinking generally, can hardly as well as its effect be over-estimated. The conception of this idea was one of the great landmarks in development of biological ajid scientific thinking. It was the first great generalization in biology. It is comparable in

The influence of the

the field of biology to Newton's law of gravitation in the field of physics. Up until this time there had been no single fundamental
idea applied to living material that

was recognized

as being univer-

This conception focused the thinking of all biologists sally true. in the same direction and therefore it had a great unifying influence. Deliberation and meditation on this fundamental idea seemed
to

prepare biologists for other great generalizations which followed quite rapidly. Many new problems arose with this new knowledge
of plants

and animals. Comparative morphology was extensively investigated, and physiology now has become physiology of cells
as a result of this impetus. of cellular

An understanding of the permeability membranes, the transformation of energy by chemical


cells,

reaction within

the roles of electrolytes in living substance,

and the principles of heredity are some of the results of this


conception of
life

new

embraced

in the cell theorj^

PROTOPLASM AND THE CELL

53

General Characteristics of Protoplasm and the Material of the Cell

To begin
variation

with,

it

may

be said that this substance has a variable

degree of fluidity under different conditions.

The range
It
is

of this

may

be from semisolid to semiliquid.

viscid

and

less,

It is more or less granular, nearly colorand more or less translucent; however, it is never perfectly transparent. The trauslucency causes a mass of it to have a lustrous gray appearance. As a constituent of protoplasm there is always a considerable percentage of water, which conditions the degree of

gelatinous in consistency.

viscosity.
WT.
:

i->.

-v

"<.*.

'-^J'.'

.'...

Km

"

/
*'
'''.

'"

..'

J'y-

'

...

i-

--.-.,.

--

f
*
'

>v^^._.^.J^"^-?'^%^,^

Fig.

19.

structure of The
is

living protoplasm as seen in tlie starfish.

(From Wilson,

Cell,

published by The Macmillan Company.)

Protoplasm
is

in a colloidal state of the

emulsoid type.

colloid

a substance of gelatinous nature, permeable by crystalloid solutions,

and diffusing not at all or very slowly through animal or vegetable membranes. In the emulsoid, or colloidal emulsion, the substances are distributed through the more watery or dispersion medium. A colloid is identified by the presence of particles which are groups of molecules dispersed through a more fluid or watery phase. These particles, of course, are larger than molecules, but they are too small to be seen with the ordinary microscope. It is possible for water and substances in solution to enter protoplasm from without, and this is With loss of water from the dispersion medium the disreversible. persed particles of the colloid become congested by loss of general
fluidity.

This condition

is

increased water in the dispersion

known as the gel state. When there is medium and the particles move with

greater ease in the more fluid medium, the colloidal state tends to be-

54

TEXTBOOK OF ZOOLOGY
sol.

be due to chemical changes in dispersion medium of the colloid. the dispersed particles or in the The ability of protoplasm, because of its colloidal nature, to change

come

This transfer of water

may

from

sol to gel state

and back

to sol repeatedly is the basis of

many

of the vital activities, such as utilization of food, disposal of waste,

and movement.

Fundamental Properties or

Activities of Protoplasm

In addition to the general characteristics, there

may

be mentioned

and
all
1.

described briefly a

number

of important activities

common

to

protoplasm.

These properties are:

Irritability, which refers to the capacity present in all protoplasm for responding to changes in environmental conditions, or

external stimuli.
2.

Conductivity refers to the fact that the impulses produced by


cell

stimuli or irritations at one point in protoplasm are conducted to

other parts of not only a single


3.

but also to adjoining

cells.

Contractility,
is

which

is

the power of contraction and relaxation

that
4.

common

to the substance of every cell.

Metaholism, the process of continual exchange of food and fuel

materials being built into the protoplasm, while, at the same time, materials there are being oxidized to liberate kinetic energy, such as
heat and movement, and produce waste by-products.

recognized as any increase in volume. When the rate of the building side of metabolism exceeds the oxidation rate in the protoplasm, there is storage of materials in the mass of the protoplasm
5.

Growth

is

and hence growth. All protoplasm has this capacity. 6. Reproduction is the capacity for producing new individuals of the same kind. All living organisms are capable of this by some means. Simple cell division is the most primitive process of reproduction

among

animals.

Consciousness, which refers to the awareness of one's


ence, is frequently given as a property of protoplasm.

own

exist-

It is certain

that some protoplasm possesses consciousness, but evidence of this


quality
is

rather intangible.
all

Spontaneity

is

also considered a prop-

erty of protoplasm

source of

by some. To be certain that the activity and reaction comes from within is likewise rather difficult
is

of definite proof, so this

simply mentioned here as another prop-

erty which

is

often listed.

PROTOPLASM AND THE CELL


Physical Nature of Protoplasm

55

Protoplasm is a semifluid material which is heavier than water and somewhat more refractive to light. Its physical constitution
is

similar to glue or gelatin, rather than to crystalloids, such as

sugar or ordinary table salt (sodium chloride).


in the

Instead of being

form of a true solution

like salt in water, it consists of sus-

pensions of relatively large molecular aggregations varying roughly


particles keep

between 0.0001 and 0.000001 of a millimeter in diameter. These up an expression of energy in that they move against each other as though they were dancing in a limited space. This activity can be seen only with a special optical arrangement known

as the ultramicroscope and the

phenomenon

is

known

as

Brownian

movement
from a

(characteristic of colloidal substances).

fuses slowly or not at all

Protoplasm difthrough animal membranes. It changes

fluid or sol state to a

more

solid or gel state

and may return

in the other direction.

Ordinarily the viscosity of the continuous


is

phase or supporting liquid


times that of water.
that of water.

only three or four times that of water,


it is

while with the dispersed particles included

only eight or ten

The

viscosity of the nuclear fluid is only twice

Since glycerin has a viscosity about a thousand times

as great as water, it will be realized that


fluid in its active state.

essential to

most protoplasm is quite Changes in viscosity accompany and are the activity and functioning of it.
is

Protoplasm
are

not a single compound;

it is

a colloidal system of
Colloidal systems

a number of chemical compounds existing together.

known

as

disperse systems of the

emulsoid type.

The more

watery or continuous part of the system is loiown as the dispersion medium, while the particles or molecular aggregations constitute the dispersed phase. An important consequence of the colloidal systems in protoplasm is the enormous surface of particles exposed
to the continuous phase.
If a sphere of material

has a radius of
centimeters.

one centimeter

its

total surface will be


is

12.6 square

Now,

if

the same volume of material

in colloidal particles of the

average size given above, the total surface of these will be approximately 7,000 square meters. This increase in surface is one of the

many important

significant effects of colloidal organization of substances, because reactions occur at these surfaces. By the presence

of salt ions in the continuous phase

and these becoming adsorbed

56

TEXTBOOK OF ZOOLOGY
of the colloidal particles, they acquire an electric Protoplasm exhibits these several phenomena because of

upon the surfaces


charge.
its colloidal

nature.

Chemical Nature of Protoplasm

Up

to

exact chemical analysis.

the present time, protoplasm has eluded complete and Nevertheless the compounds of living

matter are composed of several elements, many of them the most ordinary and abundant in the world. The list of elements necessary to make human protoplasm could be gathered in almost any locality on the face of the earth. As a rule the elements found in protoplasm are oxygen, carbon, hydrogen, nitrogen, sulphur, phosphorus, calcium, sodium, chlorine, magnesium, iron, potassium, iodine, and frequently others like silicon, aluminum, copper, manganese, bromine,

and
first

fluorine.

part of the

The most abundant of these are found named in the list. A few of them are usually given as constitut:

ing approximately the following percentages of protoplasm

oxycarbon 18 per cent, hydrogen 10 per cent, nitrogen 3 per cent, calcium 2 per cent, phosphorus 1 per cent, and all others makijig up the remaining 1 per cent. These elements are found combined to form compounds. The organic compounds include carhohydratcs, fats, proteins, and also enzymes. The inorganic com-

gen 65 per

cent,

povmds consist of several inorganic salts and water. The carbohydrates, which include starches and sugars, are compounds of carbon, oxygen, and hydrogen. The proportion of the hydrogen to oxygen in the molecule is the same as found in water, two to one. The principal carbohydrate found in protoplasm is the monosaccharid, or simple sugar, glucose, whose formula is CeHioOeThis is actually built into some parts of the cell, but its chief function is to furnish the most available source of energy by its ready
oxidation.

When

a molecule of glucose

is

burned, the potential

energy

is

released as kinetic or mechanical energy, and there are

formed

(H2O) and six molecules of carbon converted to a starchlike substance, glycogen, for storage in the various animal tissues. This substance must be reconverted to glucose before it is available for production
six molecules of v.'ater

dioxide

(CO2).

Glucose

is

of energy. Fats, like carbohydrates, are composed of carbon, hydrogen, and oxygen but in more complex molecular arrangement. There is much more carbon and hydrogen with less oxygen, which allows the fats

PROTOPLASM AND THE CELL


to

57

combine with more oxygen in oxidation and therefore release Fat is extremely well adapted as a form of material for storage, since Aveight by weight it contains more potential energy than any of the organic group. Such common substances as lard, butter, tallow, whale blubber, and cottonseed oil are good examples. Fats serve a double function in protoplasm constitution of a part

more energy.

of the structure of the cell and, secondly, the storage of food.

Proteins constitute the bulk of the foundation or framework of the

and are the most abundant organic constituents. and nitrogen, with the frequent addition of traces of sulphur, phosphorus, magnesium, and iron. All of the proteins have large molecules, each being comcellular structure

They

are composed of carbon, hydrogen, oxygen,

posed of thousands of atoms


teins
rent,

as

an

illustration, take

hemoglobin of

the red blood corpuscles with its formula C7i2Hii3oN2i40245FeS2. Pro-

have a slow rate of diffusion, high resistance to electric curand usually coagulate upon heating or upon addition of acids, alcohol, or salts to form a clot. Egg albumen, gelatin, and lean meat are common examples of proteins. They are split into numerous amino acids which serve as the building stones of the stable

portions of protoplasm.

Enzymes are substances whose exact chemical nature is not yet known, but whose importance to protoplasm is probably unequaled. Chemically and physically they seem to be more like proteins than anything else. These substances are found not only in the cells, but they are also secreted by cells into the digestive tract and into the blood stream, where they act as organic catalysts. The general
function of the catalyzer or catalytic agent is that of facilitating and speeding up certain chemical exchanges without the agent itself

The well-known example of catalysis amount of platinum in increasing the rate at which hydrogen and oxygen combine to form water. A particular enzyme is usually specific for one kind of reaction, but not for the species of animal in which it will function. Enzymes taken from one species will usually facilitate the same kind of specific reaction in other species. The digestive enzymes may be thought of as an example. Of these, pepsin will bring about the same general reaction, whether it is in the stomach of a frog or of a man, under favorable conditions. Since many enzymes influence only one specific type of chemical reaction and since there are numerous
entering into the reaction.
the effect of a small
is

58

TEXTBOOK OF ZOOLOGY
it

types of reactions going forward in active protoplasm,


that there must be

is

seen

numerous enzymes present

in the cells of every

organism.

Water

constitutes 60 to 90 per cent of protoplasm

and maintains
efficient

many

substances in solution.

Water

is

not only a very

solvent; but it is important to protoplasm because of its comparatively high surface tension, because its presence gives the protoplasm a consistency compatible with the range of variation necesThis sary for metabolism, and because of its high specific heat. latter point is important in maintaining protection against sudden and extreme temperature changes in the living organism. Young cells contain more water than old ones, young organisms likewise contain more than old ones. The relative amounts of water in relation to other materials of the protoplasm vary in different cells and
in different species.

present in considerable numbers but in They are electrolytes, and therefore split up in aqueous solution into ions, which are able to combine with all the other substances in protoplasm. The chlorides, phosphates, iodides, carbonates, and sulphates of sodium, potassium, calcium, magnesium, and iron are important salts of living cells. The relative proportion of these salts is kept at a fairly constant level, and slight
salts are

The inorganic

relatively small amount.

changes in this balance have regulatory effects on metabolism.

From

the chemical standpoint, living protoplasm

is

considered

the most complex of all systems of compounds.

Even

the proteins,

as a part of protoplasm, are


stances.
its

more complex than any other sub-

is quite unstable in that it changes composition in response to every change in the environment, and when active it is not the same for any two consecutive moments. The exceeding variability of protoplasm chemically, makes possible

In a sense, protoplasm

all

of the necessary adjustments of living matter to

its

environment.
it is

On account

of the extreme complexity of protoplasm


all

not sur-

prising that the chemistry of


pletely understood.

of its activities is not yet

com-

Structure of a Typical Animal Cell

The quantity of protoplasm comprising a single cell varies within wide limits therefore cells vary greatly in size. The majority of cells,
;

but not

all

of them, require considerable magnification to be seen.

Cer-

PROTOPLASM AND THE CELL


tain of the single-celled blood parasites are about as small as
cells

59

known.

any They are barely seen with our highest magnifications.

At

the other extreme of size

we may

refer to another parasitic

single-celled animal, Porospora gigantea,

which

lives in the intestine

of the lobster,

and may reach from one-half to two-thirds of an

inch in length.

Egg

cells,

including the yolk,

Some

of the nerve cells,

though of less mass,


long

may exceed this may be several

size.

feet

in length.

Muscle

cells are relatively

also.

Plasma /Atmirane

Eciop/asm

Chondriosoma
En</op/asm

Lin in

Chromaf/n
Mucf^us

Nucteof Sap
Vcict/o/e

A/</c/zar

Membrane

Plastid

Fig. 20.

Diagram

showing a typical animal


is

cell.

The shape
almost

of the typical cell

spherical; but due to the effects

and unequal growth They vary greatly in shape and include platelike, cubical, columnar, polygonal, and spindleshaped forms. The particular form of any cell is not a haphazard matter but strictly controlled by morphological and functional
of mechanical pressure, specialized functions,
all cells

are far from this shape.

necessities.

60

TEXTBOOK OF ZOOLOGY

cell consists of a

mass

of jellylike cytoplasm surrounding a

The outer surface of the cytoplasm is modified, the protoplasm having more density here to form the plasma membrane, or This cell membrane, which is the outer covering of the animal cell. membrane is living and semipermeable. In some types of cells two separate membranes may be distinguished. In plant cells the plasma membrane is covered by a cellulose cell wall. The cytoplasm usually includes the larger part of the substance of the cell. It may be subdivided into the more nearly clear, structureless fluid, hyaloplasm, and the interspersed fibrillar substance known Within the cytoplasm, lying near the nucleus, in as spongioplasm.
nucleus.

most animal cells is the centrosome. Its substance is known as kinoplasm and is made up of two parts, the larger ce7itrosphere, enclosing a (two if divided) centriole. Vacuoles are often present as small

lis

'

Fig. 21. A camera lucida drawing, showing- the details which appear on the upper 5?urface of a fully developed salivary gland chromosome (large vesicle type)

from Simulium fly larvae. The longitudinal threadlike bands are called chromonemata, and these consist of a linear series of granules, the chromomeres, which have a specific arrangement of grouping. lA is a semidiagrammatic representation of the types of chromomeres and the ways in which they are connected. At a in the main figure there are two rows of dotlike chromomeres which tend to associate in pairs. The band labeled h is composed of 15 or 16 vesiculated chromomeres closely pressed together, c to li are other groupings of chromomeres along the chromonemata of the chromosome. (From Painter and Griffen Chromosomes of Simulium, Genetics 22: 616, 1937.)
:

Scattered through the cytoplasm also are numerous rod-shaped bodies known as mitochondria. Threadlike Golgi elements or apparatus may be observed in the cytoplasm, particularly near the nucleus. Secretions produced in the cell
cavities filled with water, gases, or oils.

may may

be stored as gi-anules in the cytoplasm, also certain inclusions

be seen here.

The nucleus, which is usually round and centrally located, is surrounded by the cytoplasm and separated from it by the nuclear membrane. This membrane, like the plasma membrane, consists of a part of the protoplasm whose density is somewhat greater than the adjacent portions. The protoplasm which constitutes the nucleus is
usually

known

as karyoplasm.

The more nearly

fluid,

transparent

PROTOPLASM AND THE CELL


portion of this
is

61

haryolymph, or nuclear sap, while the meshwork it is called linin net. Supported on this net is a dark-staining granular or fibrillar substance known as chromatin, which is thought to be the center of functional activities of the nucleus. The threads of chromatin are called cliromonemata. During division of the cell this granular material becomes arranged into definite bodies, the chromosomes. It is generally thought that in these bodies are located the units of material (genes) which function in the transmission of hereditary characteristics from one generation to the next. There are usually one or two knots of more dense chromatin in the nucleus which are called karyosomes. Then besides these, most nuclei have anotlier body composed of material thought to be temporary storage products of nuclear metabolism, the nucleolus, or plasmosome. Mitochondria, similar to those of the cytoplasm, are also found in the nucleus. The cell is often spoken of as the unit
of fine fibers extending through

and function in living material. Both nucleus and cytoplasm are necessary for its normal activities. It is not entirely possible to define the part each plays in the metabolism of the whole. Since the development of the microdissector by Dr. Chambers, it is possible to dissect the nucleus of a cell. Cells that are deprived of their nuclei are unable to carry on assimilation, although catabolism goes on until the cytoplasm is depleted.
of structure
Cell Division

The
limit

cell is limited in its size, as is

the complete organism.

This

of

size

is

fixed

primarily by the physiological necessities

which are transmitted through the surface of the cell. There is a definite relation between volume and surface in any mass of material, and this may be expressed in a ratio. With variation of the size of the mass, the volume varies according to the cube of the
diameter while the surface area varies according to the square of the diameter. When the limit of growth is reached the cell divides, and this restores the proportion of the surface area to volume that will again permit growth. Remak, in 1855, was the first to describe cell division. His idea was that the nucleolus split first, then the
entire nucleus,

and

finally the

cytoplasm divided, placing each porThis direct method of division

tion with its share of the nucleus.

was called method of

amitosis.

Its actual occurrence is quite rare.

The usual
There
mitotic

cell division is

far

more complex and

less direct.

are several preliminary changes or phases which must occur before


the actual cleavage of the cell into two

new

ones.

This

is

62
cell

TEXTBOOK OF ZOOLOGY
division,

more

briefly

mitosis,

or indirect

cell

division.

This

method

of division

was

first

described by Fleming in 1878, though

Schneider in 1873 described

much

of the complicated process.


is

Although the process of mitosis


for convenience in stud3% these
CEL-l-

a continuous series of changes, changes will be set out as six phases.

MEfvBRANE

CENTROSPHERE
tvlUCUEAR

CENTBIOLE

MEMBRANE
CHROMATIN
NUC!_EOI_US

CYTOPLASM
EARL-Y
SPINDL.E

prophase:
EQUATOR
P1_ATE
r

Al_

ASTRAU
RAYS
C H ROMOSO M E

ASTER

LATE PROPHASE
ADJACENT
CEI_i_

MEXAPHASE

Fl

INTER ZONAU BE RS

EARLY ANAPHASE
CL-EAVAQE

LATE ANAPHASE

FURROW

TELOPHASE
Figr.

DAUGHTER
mitosis
:

CELLS

22.

The

fertilized Ascaris eggs. phase, late prophase,

daughter

cells.

(indirect cell division) as .shown in in order resting cell, early proanaphase, late anaphase, telophase, an'^ (Drawn by Titus Evans.)
in

stages

typical

They follow each other


early

metaphase,

Following the resting cell condition come the first changes, and the early prophase condition is seen. In this stage the centriole has
divided,

and the two pieces have moved considerably apart.

The

PROTOPLASM AND THE CELL

63

surrounding protoplasm has produced some rays radiating from each centriole. These two bodies are now known as asters because of their
starlike appearance.

The two

asters taken together are called the

aniphiaster.

The nucleolus disappears and the chromatin which ap-

parently up until this time has been somewhat dispersed through the nuclear substance in the form of granules, becomes organized into
long, possibly tangled,

chromatin
its

fibers or threads.

the chromatin retains


tion to the next.)

linear arrangement

(Some hold that from one cell genera-

Each

consists of a double linear series of chromatin

bound closely together to form the thread. two series in the thread is known as a chromonema (pi., chromonemata) The chromatin bodies comprising the chromonemata are often called chromomeres (Fig. 21).
bodies like two chains

single one of these

In the middle prophase the centrioles have migrated still farther from each other and the splindle fibers between the centrioles as well as the astral rays around them have become well established. According to modern explanation, each of the chromatin threads now shortens and condenses to become a chromosome. There is a constant

number

of these in the cells of each species.

During the above

In the late prophase, the centrioles have reached the polar positions on opposite sides of the nucleus. The spindle extends between the two asters and
to degenerate.

changes, the nuclear

membrane begins

the chromosomes become arranged on the spindle fibers in an orderly fashion midway between the centrioles to form the equatorial plate.
It

has been reported that, in certain

cells at least,

the prophase stage


al-

requires about eight minutes.

The nuclear membrane now has

most completely disappeared.

The chromosomes, arranged in the equatorial plate, now each split longitudinally, placing one chromonema in each part. The characteristic feature of the metaphase is this splitting. Following this stage each "of the new chromosomes, resulting from this splitting or division, migrates along the spindle fibers toward its respective centriole, or pole. This period is the anaphase. These " half -chromosomes " each soon come to have two chromonemata, and they carry the chromatin material of the new cells which result from the ensuing division. The explanation of the movement of these chromosomes from the equatorial plate out to the poles is not entirely forthcoming, although there is general belief that the spindle fibers, with which they are associated, are involved in the phenomenon.
still

The metaphase follows with no interruption.

64

TEXTBOOK OP ZOOLOGY

the chromosomes approach the poles of the spindle they crowd very close to each other. At this time a constriction of the cytoplasm begins in the plane of the equatorial plate. This is the beginning of

As

the

The cytoplasm perfects its constriction and divides into two parts. A nuclear membrane forms to enclose each chromosome gi'oup, and immediately the chromosomes begin to separate from the group, although certain ones still clump together. The chromosomes progressively lose their identity and their staining qualities. The nucleus resumes its granular appearance of the resting cell. One or more nucleoli soon become evident. The fortelophase stage.

mation of the new nuclear membrane excludes the centrosome, so it takes its normal position just outside the nuclear membrane in the cytoplasm. At about this time the centriole divides into two. These two new cells resulting from the division are spoken of as daughter cells. These cells have each received the same quantity and quality of chromatin material. Following the organization of these daughter cells, which are in the resting stage as far as division is concerned, growth is rapid until they reach their typical limit of size. For most average cells under optimum conditions, it is stated that this requires less than two hours. Then after a further period of from one to twelve
hours, another mitotic division

may

take place.

of this process in all types of organisms, both plant

The universality and animal,

and the regularity of the occurrence of the phases of the process The great precision with it is of vital significajice. which the chromatin is divided between the two cells seems to Chromatin is recogindicate that this is a most significant step. nized as the material which makes possible the inheritance of qualities from cell to cell and, in case of sex cells, from generation to generation. The purpose of the splitting of the chromosomes in the metaphase stage seems to be to provide each daughter cell with
suggest that
This equal division of chromatin, both qualitatively and quantitatively, has given rise to the thought expressed in the phrase, "continuity of protoplasm," and that present chromatin comes from pre-existing chromatin. In 1855
identical hereditary qualities.

Virchow, a German pathologist, declared the doctrine that all cells must be derived from previously existing cells, in his statement, "omnis cellula e cellula." This supposes that in the first living material created were inherent all of the possibilities which have been realized in all living things that have existed since.

CHAPTER IV

PHYLUM PEOTOZOA
first to

IN

GENERAL

The animals included in this group are usually said to be the have existed on earth and, therefore, they are considered the oldest. Being single-celled, they are usually referred to as the simplest known animals, although many of them are perhaps more complicated than numerous many-celled or metazoan forms because
of the extensive modifications of the one
sally placed first
cell.

Protozoa are univer-

when animal groups

are placed in the order of

complexity, beginning with the simplest. It has been supposed and with reasons to support the supposition, that modern Protozoa have

descended,

without changing their single-celled condition, from

primitive organisms that were also the ancestors of Metazoa.


Characteristics

The great majority of Protozoa are microscopic creatures. Most them live in water while a few live in the body fluids of other animals. Certain types are found living rather abundantly in the soil water. They are found in almost all conceivable shapes. Some have irregular, changing shapes others are nearly spherical, oval, spindle-shaped, cylindrical, and vase-shaped. Most Protozoa exist singly as an independent cell, but some are organized into groups called colonies. A few are encased in hard coverings or shells which are made up of a secretion from the cell alone, or of a combination of
of
;

such a secretion with a foreign material like sand.

"With the excep-

tion of one class the Protozoa have characteristic locomotor organs. ^to'

Classification

This group

is

often spoken of as a subkingdom as well as the

first

phylum number
genera,

In spite of the exceedingly large of species and microscopic size, the phylum has been quite
is

of the animal kingdom.

systematically classified and

divided into classes, orders, families,

The phylum is usually divided into four classes, each characterized by a distinctive locomotor structure or by the
species.

and

total lack of such features, as in

one of the
65

classes.

66
1.

TEXTBOOK OF ZOOLOGY
Class Mastigophora (mas
ti

gof '6 ra) which

means whip bear-

ers,

includes forms that possess one or more whiplike extensions of

The number of flagella is limited, and means of locomotion. In some species they serve the organism in feeding. The flagellum is a contractile There are some species in which exist both flagellate structure. and amoeboid stages. This seems to show a rather close relation
the cytoplasm, or flagella.

they serve the animal as

its

of this class to the next.

This class also has a close relationship


of its representatives possess chlorophyll.

with plants in that

many

111

\>asis

Monosiga

Cercomonas

Cht lononas

Codonosiga.
Phaous
Trachelmonas

Peranema
Fig.

Maatigamoeba

from and Sons,

Group of representative Curtis and Guthrie, Textbook


23.

Mastigophora. (Reprinted by permission of General Zoology, published by John Wiley

Inc.)

(Figure of Chilomonas modified.)

These forms are frequently classified as plants


class

b}^ botanists.

The

Mastigophora is divided into two groups: (a) the animal-like forms which may be holozoic, saprophytic, or entozoic, and (b) those more plantlike forms which may be holophytic, saprophytic,
or entozoic.
material.

Holozoic refers to forms which ingest and digest food

Saprophytic refers to the habit of absorbing nonliving organic matter in solution directly through the surface of the body. Entozoic is a name applied to forms which live within the bodies of
other animals, as in the intestine or the blood stream.

PHYLUM PROTOZOA

IN GENERAL

67

A larg-e
form.

number

of Mastigophora live in quiet streams, ponds, lakes,

Euglena is a very common!}^ studied fresh-water an interesting marine form which is pelagic (lives at the surface) in its habits and appears as a thick, creamy scum. This soupy mass of organisms may cover an area of hundreds of
Noctiluca
is

and in the ocean,

square rods.

When

stimulated, these animals are luminescent

and

at

night frequently give up an attractive greenish or bluish white light

Uroglena
water.

is

a fresh-water form which

is

often found in water supply

basins and causes a pungent, oily odor and unpleasant taste in the

Giardia (Fig. 386), Trichomonas, Chilomastix, Retortamonas


in the

and Enteronomas are all genera with representatives occurring digestive tract of man.

Arcella

Actinophrya

Fig.

24.

Group of typical Sarcodina.

(From

Curtis

and Guthrie, Textbook

of

General Zoology, published by John Wiley and Sons, Inc.)

2.

Class Sarcodina (sarkodi'na, fleshy) or Rhizopoda (rizop'Oda,

root foot).

^A distinctive

feature of nearly

all

species of this class

is

the capacity to form protoplasmic processes called pseudopodia (false

which are temporary structures and can be withdrawn. The aniable to accomplish locomotion by extending the protoplasm into these pseudopodia. The representatives of this class include many freeliving forms as well as numerous parasitic ones. A number of the repfeet)

mal

is

resentatives of class Sarcodina secrete an external shell of lime, silicon,

some bind in sand or other solid substances with one The class is commonly divided into five orders, (a) Amoebina are irregularly-shaped forms with lobelike pseudopodia. Some of the species are naked, and others are covered by a shell. Amoeba proteus is the free-living naked form which is commonly
chitin, cellulose, or

of the secretions.

studied.

Endamoeba

histolytica is the

Arcella, which secretes its shell,

and

Difflugia,

most common parasitic form, which constructs its

68
Shell of

TEXTBOOK OF ZOOLOGY
secretion, are two of the most sand cemented together by a (b) Foramimera is an forms,

commonly observed
reticnlar.

shell-bearing

pseudopodia are very slender a^d order of shelled forms whose the through small pores The pseudopodia are extended

sheU

Only a very few of this group

live

fresh water.

The vast

^^iijA^l^

"s

:.M

iU-

e
crw^^A^
Parker and

Foraminifera PoZysfomeZZa Fig. 25.-Life cycle of one of the from it ^ youn^^^ spheric individual; B amoeboid 9^11^ .f.^^P^^^ cell ;D,nmcrosphericin^^^^ dividual developing from amoeboid Kearawn gametes escaping from it F, union of gametes. Zoology, published by The Macmillan Co.)
;

Haswell,

PHYLUM PROTOZOA
majority are marine, and Glohigerina

IN GENERAIv
is

69

a typical example.

The

dis-

integrating calcareous shells of this organism constitute a great mass


of material on the bottom of the ocean which
is

known

as globigerina

ooze and from which chalk

is

formed,

(c)

Mycetozoa, are character-

ized as being able to produce

enormous plasmodia containing hun-

dreds of nuclei and contractile vacuoles, as well as having ability to

Shells of different Foraminifera. A, Rhahdamina abyssorum (X4.5) Fig. 26. B, Nodosaria hispida (X18); C, Globigerina buUoides (X55). (From Borradaile and Potts, The Invertebrata, published by The Macmillan Company, after various authors.
;

reproduce by spore formation.


of
is

They live quite commonly in masses decaying vegetable material upon which they feed, (d) Heliozoa
a group

with thin, radially arranged, threadlike, unbranched

pseudopodia.

Actinophrys
(e)

sol is

common one found


is

in fresh-water
fine,

streams and ponds,

Badiolaria

a marine group with

ray-

70
like

TEXTBOOK OF ZOOLOGY
pseudopodia and a
shell

composed largely of

silica.

The pseudo-

podia extend through the relatively large apertures in the shell. group includes 3. Class Infusoria (infuso'ria, crowded). This
those single-celled animals covered with small hairlike, cytoplasmic They occur in both fresh and marine processes known as cilia.

waters as free-swimming organisms. There are a few parasitic forms, notably Balantidium coli. Paramecium, Stentor, and Vorticella are

-ZT^ffflara 0i^

PHYLUM PROTOZOA
sitic

IN GENERAL

71
(b) Heteroin the cyto-

genus which inhabits the large intestine of the frog,

trichida possess a well-developed undulating

membrane

pharynx.

The body

cilia

are small or partially absent, but the cilia

of the oral region are well developed.


possesses membranelles.
fresh- water genera while

In some forms this oral region

Stentor, Halteria,

and Bursaria are common


is

Balantidnim (Fig. 389)


other mammals,

a parasite in the

intestine of

man and some

(c) Tlypotrichida possess


;

cirri or structures

cipally on the ventral side.

formed by fusion of cilia these are found prinThe cell is flattened dorsoventrally and

Prorodon

From ton

iupiom

StyhnyoMa

Laorymaria

i^m
lionotw

Fig. 28.

Curtis and Gutlirie,

Representatives from Textbook

class Infusoria.

of

(Reprinted by permission from General Zoology, John Wiley and Sons, Inc.)

(Figure of Frontonia modified.)

most of the genera use creeping as their means of locomotion. Stylony cilia, Oxytricha and Euplotes are common fresh-water genera.

Kerona

is

a parasitic form and

is

often found creeping over the ex-

ternal surface of fresh-water Hydra,

(d) Peritrichida is an order composed of sedentary ciliates with a whorl of oral cilia continued into a depression in which are located the oral spot and aperture of the

contractile vacuole.

At

the base of this depression

is

located the

mouth. There are no body cilia in certain phases of the life history. These forms are typically attached by stalks. Vor'ticeJla is probably

72
the most

TEXTBOOK OF ZOOLOGY

common

living genus.

Epistylis

and Carchesium are

well-

known

colonial genera.

stalks while Epistylis is

Vorticella and Carchesium have contractile attached by noncontractile branching stalks.


it

The second

subclass, Suctoria or TentacuUfera, as

is

sometimes

called, includes

animals that are not ciliated, except during a free-

swimming

stage which

may

occur following division or encystment.

These are attached forms with protoplasmic projections which are used in the capture of food. Most of them are marine, but Podophrya
is

an example of a fresh-water genus.


4.

Class Sporozoa (sporozo'a, seed animal).

These

protozoans
there

in their early stages are often amoeboid, but in the completed life

history locomotor structures are wanting.


is

During the

life cycle

a spore stage.

The animals

of this class are entirely parasitic

^^(^^''f^^T^ Ep
;

th e

Hum

i<m^l

:,;

hfH-Early stae

.Intermediate sta^e

-natarc 5ta^e

Fig.

stages of

29. its

Gregarina attached an epithelial development are shown within adjacent


to

cell of host's cells.

Intestine.

Other

and they are usually transmitted to other animals in the spore They often pass from one host in its feces and enter another in contaminated food or drink or they are drawn from one host by a blood-sucking animal and transmitted to the blood of ajiother. All Sporozoa reproduce by sporulation in which asexual, multiple fission is followed by gamete formation, and the gametes fuse to form a zygote. The spores are produced by the parent animal
stage.
;

it is encysted. These little cysts, which are secreted by the protoplasm of the animal, are protective and enable them to withstand adverse conditions. The cyst is dissolved upon entrance to a host and liberates the organisms.

dividing into fragments while

This class of Protozoa is among the most widely distributed of the animal parasites, and their life cycles are often quite compli-

PHYLUM PROTOZOA
cated.

IN GENERAL

73

There are three subclasses of the class, and each of these is divided into some orders. The first subclass is Telosporidia in which
the spores produced have neither a polar capsule nor polar filament.

In

this

group are three orders,


other insects,

(a)

called gregarines, inhabit the cells (cystozoic)

Gregarinida (Fig. 29), commonly of earthworms, cockin

roaches,

and occasionally vertebrates


(b)

their early

stages, but later they

may become

free in the cavities of the host.

They

may

attain considerable size,

Coccidia are minute monocysted

forms which are permanent intracellular parasites of molluscs, arthropods, and vertebrates, including man. The life history involves a
period of asexual reproduction (schizogony) and a period of so-called sexual reproduction which ends in spore formation (sporogony). (c)

The representatives of this order live chiefly in the Again the life cycle involves both schizogony and sporogony. The former occurs in the blood of the vertebrate and the latter takes place in such hosts as insects, leeches, and ticks. The malaria parasite and the causal agent of
Haemosporidia.
red blood corpuscles of vertebrates.

Texas fever in cattle are the most important forms,


Cnidosporidia is the name of a second subclass, the spores of which contain from one to four polar capsules each with a coiled polar filament. There are two orders (a) Myxosporidia are found chiefly as
:

fish parasites,

The and urinary bladder are usual seats of infection for the free forms, while the gills and muscles of the fishes are choice tissues for the cysts. Myxidium and myxoholus are
gall-bladder, uriniferous tubes,
characteristic genera,
(b) Microsporidia
is

but occur occasionally in reptiles and amphibia.

have in each spore a single

polar capsule.

This group

parasitic chiefly in arthropods,

and

occasionally in other invertebrates, fish

and amphibia.

simple spores.
the

The third subclass Acnidosporidia includes forms which produce Again there are two orders: (a) Sarcosporidia. As

name

infers these occur in muscles of several

mammals.

The

encysted forms attain a length of several millimeters, and ultimately each becomes a mass of sickle-shaped spores. The complete life cycle
is
is

known

not known, but the saclike, encysted form in muscles of mammals as Miescher's tube or sac. (b) Haplosporidia are single cells,

each with a single nucleus, and they have a relatively simple structure. This order parasitizes fishes and certain insects, notably the
cockroach.

74

TEXTBOOK OF ZOOLOGY

Plasmodium, the malaria parasite, is one of the Haemosporidia, and its life cycle will be given to illustrate the intricate life history Life cycles in which there are primary of certain of these forms.

and intermediate hosts are quite common among parasites. This example will illustrate also the relationship of insects to diseaseproducing organisms. There are three species of human malaria(a) Plasmodium vivax, which causes tertian causing organisms: (b) Plasfever, is characterized by an attack each forty-eight hours, modium malariae, which causes quartan fever, is characterized by an
attack every seventy-two hours,
(c)

Plasmodium falciparum,

caus-

ing estivo-autumnal or subtertian fever, the attacks of which recur each day, or there may be a somewhat constant fever. The parasite
(see Fig. 393)

generations

may live in the blood of man by which may continue throughout the

a series of asexual
life of

the person.

The

parasite, while in the spore stage, invades the red corpuscles,

where it reproduces by a sort of multiple division called sporulation, in which there are numerous nuclear divisions before the mass of cytoplasm divides. The new individuals (merozoites) are freed by destruction of the corpuscle and almost immediately enter new corpuscles where repetition of events occurs.

Some

of these merozoites

become sexual

cells

(gametocytes).

Part of the gametocytes develop

into macrogametes, spoken of as female,

and others become micro-

If a female gametes which develop from the male gametocyte. this person, the mosAnopheles mosquito bites and sucks blood from quito becomes infected with gametocytes of Plasmodium. A union of the flagellate microgametes with the egglike macrogametes takes place in the stomach of the mosquito. The union is commonly called fertilization, and a fused cell or zygote thus formed soon becomes a

wormlike form, known as an ookinete. This ookinete enters the wall of the mosquito's stomach where it encysts in the form of a ball with a shell, and is now called an oocyst which grows at the expense of the adjacent tissue. This cyst protrudes like a little wart on the outside of the wall of the stomach. Inside of the oocyst the nucleus divides repeatedly, forming sporohlasts. These enlarge and
motile,
coalesce,

while slender, spindle-shaped sporozoites develop within,

each with a chromatin dot as a nucleus.


is

The capsule of this oocyst crowded full of these sporozoites which may number 10,000 or more, and there may be 500 capsules in one mosquito. Depending somewhat on the temperature, it requires twelve days or more for

PHYLUM PROTOZOA
this

IN GENERAL

75

development to go on in the mosquito.

sporozoites

make

their

way

to the salivary

where they may remain for weeks. When tliis some of the saliva with sporozoites flows into the wound, and the
' ' '

These little parasitic gland of the mosquito mosquito bites a man,


'

process of asexual multiplication begins over again in the red corpuscles of this person as a

new

host.

Colonial Protozoa

There are some species of Protozoa in which the individual cells groups called colonies. This formation frequently results from incomplete separation of the cells following division. In some
exist in

Codonosi^a

Carchejium

Joanne Moore.)

Fig. 30. Different types of colonial Protozoa. Eudorina, a simple colony; Pandorina, within gelatinous envelope Ceratium, a linear colony Carchesium, stalked infusorian colony; Codonosiga, a stalked flagellate colony. (Drawn by
;

of these forms only two cells adhere, but in others the cells may remain attached after many divisions, with the result that thousands of cells are built into the group. In. some species there is a jellylike, spherical envelope inside of which the colony of cells remains. In certain species the cells are stalked, and the new cells remain attached to the stalk, giving a branching colony. Pandorina and Eudorina are typical examples of the former, while Epistylis and Carchesium are typical examples of the latter. These types of colonies are known as spheroid and arhoroid or dendritic respectively. Colonies like that of Ceratium with individuals ar-


76

TEXTBOOK OF ZOOLOGY

ranged in a line form a linear one, and colonies of irregular arrangement are spoken of as gregaloid. Tlie difference between these colonial Protozoa and simple Metazoa is a difference in the relationship of single cells to the group as a whole and not a simple difference

B
Fig. 31. Volvox. A, Mature colony containing- several daughter colonies. B, Formation of daughter colony by development of a parthenogonidium. (From White, General Biology j The C. V. Mosby Company.)

numbers of cells. In the colony each cell is an independent or almost independent individual so far as the functions of living are concerned. In the structure of Metazoa the cells are specialized and
in

PHYLUM PROTOZOA

IN GENERAL

77

distributed, so that certain groups carry out a definite portion of the entire metabolism.
cells

They are and reproductive (germ)

classified into general


cells.

body (somatic)

Certain of the spheroid proto-

zoan colonies, such as Volvox, have a rather striking resemblance to the blastula stage in the early development of metazoans. Both are
spherical organizations of
cells.

Tropisms and Animal Reaction


Organisms, whether plant or animal, of all degrees of complexity respond to various kinds of stimuli. The important stimuli which call out immediate or direct response by the animal are light, bodily contact, chemical change, temperature, gravity, mechanical cur-

The response to a stimulus may be Tropism, which means turning, refers to the reaction of an organism to a stimulus. Taxis may also be used here if the response involves the movement of the organism as a
rents,

and

electric currents.

either positive or negative.

whole.

Tropisms are named with respect

to the stimulating agent,

and the common ones usually recognized are


a. b.
c.

Phototropism, response to light

d.
e.
f.

Thigmotropism, response to contact Chemotropism, response to chemical changes Thermotropism, response to temperature
Geotropism, response to gravity Rheotropism, response to mechanical currents Electrotropism or galvanotropism, response to electric currents
is

g.

If the animal

attracted to the source of the stimulation and turns


is

toward
repelled

it,

the response

said to be positive.
is

If the
It

organism

is

by the stimulus, the response

negative.

has not been

thoroughly determined in a certain way. The


sary to get a response

why an animal responds to a specific stimulus minimum strength of stimulus which is necesis known as the threshold. The simpler ani-

mals under a given set of conditions respond to these stimuli in a certain way not because of power of choice, but because they cannot behave in any other way. The Protozoa are controlled in their behavior largely by tropisms.

Economic Relations

of Protozoa

Man has not yet found a way or need to eat Protozoa directly as food material, although he does draw on it indirectly by a food

78

TEXTBOOK OF ZOOLOGY

chain including water fleas, larger crustaceans, and fish. Too, the protozoans are not classed as predators on man as would be the

but many of them are parasites. Many diseases of man and animals are caused by Protozoa. Most of the diseases of this origin
lion,

are more prevalent in the tropical and subtropical regions of the


earth.

Such diseases may attain

sufiScient

importance to render
;

by man for example, much of northern South America and Central America was, at one time, ruled by yellow fever and malaria, and the same applies for
large portions of continents uninhabitable
sleeping sickness in Africa.

There are other Protozoa that render


fertilize the soil.

water unfit for drinking or help

Amoebic Dysentery.
intestine of
this,

man

on the inside of the walls of the There results from are caused by this disease.

Ulcers

severe diarrhea and dysentery.


if

From

the intestine the infec-

allowed to continue, will be carried to the liver where The infection is usually obtained serious abscesses are formed. directly through drinking water or eating food which has been
tion,

contaminated with the encysted organisms from fecal matter. About 10 per cent of our population are said to be carriers of these organisms. The causal agent is one of the Amoebae, Endamoeba histolytica (see Fig. 391), and it can be rather successfully eliminated from human
beings by use of such drugs as emetine, carbarsone, and chiniofon,

administered by a physician.
in

human

beings, but, so far as

amoeba

coli,

other Amoebae have been found known, they are not pathogenic. EndEndolimax nana, and Endamoeba gingivalis are such

Some

examples.

Foraminifera which is an order in class Sarcodina has some economic importance because of the limestone which is formed by the concentration of the material of the dead tests or shells. A genus by the name of Globigerina is one of the best known members of the group. It is about the size of a pinhead, and as it dies, it sinks to the bottom of the ocean where the mass forms the globigerina ooze which
hardens into solid chalk.
Badiolaria
is

another order in the same

class.

Each

of its repretheir skeletal

sentatives has a complicated skeleton of silica.

From

remains comes an ooze on the sea floor sometimes hundreds of feet deep. From this is formed quartz or flint.
disease of

African Sleeping Sickness. This malady is the most important man caused by flagellate Protozoa. Technically the disease

PHYLUM PROTOZOA
is

IN GENERAL

79

called trypanosomiasis for the


it,

genus name of the animal that

Trypanosoma gamhiense or Trypanosoma rhodesiense. These organisms are transmitted by the tsetse fly, Glossina palpalis (Fig-. 388), and the disease is limited to that area in Africa where this fly is found. The organisms (Fig. 387) live free in the blood and collect in the lymph glands, spleen, liver, and other organs. In final phases it collects and attacks the brain. The infection will bring about loss of appetite, severe emaciation, extended coma, which ends in
causes
into years.

death usually within three or four months, or it may be extended Such animals as antelope, cattle, and some wild game

are susceptible to the disease


plicates the control of
it.

lutely fatal,

and may serve as carriers. This comThe disease has been considered absobut recently a drug, arsphenamine, an arsenic com-

pound, has been tried with partial success.


Chag'as' Disease. A closely related flagellate, Trypanosoma cruzi, causes this disease in Central and South America. It is transmitted

through the bite of Triatoma, one of the true bugs which is closely to our common blood-sucking form, the "kissing bug." Chagas' disease affects dogs, monkeys, guinea pigs, armadillos, as well as man. The sjonptoms are continued fever; swollen lymph
related

glands, liver,

and spleen; anemia; and disturbance

of the nervous

system.

Malaria. The life history of Plasmodium, the sporozoan which causes this disease, has already been discussed under the general topic of Sporozoa. The disease is one of the oldest and most widely dis-

among men. It was the first disease proved to be directly caused by a protozoan parasite. As early as 1718 a worker by the name of Lancisi ventured the statement that mosquitoes or gnats might transmit malaria however, it was not until about the opening of the present centuiy that this relationship was understood. In 1881,
tributed
;

tients.

Dr. Laveran found a curious parasite in the blood of malaria paSeveral years later Laveran and IManson independently sug-

gested that the organism might be transmitted by some bloodsucking insect. After several years more of investigation. Major

Ronald Ross, an Englishman, was able to prove that the female


Anopheles mosquito
is

responsible for the transmission of malaria.

If houses are screened to keep out mosquitoes at all times, or if

aU malaria patients or carriers are thoroughly screened in, or if all mosquitoes and mosquito breeding places are destroyed, the chain

80

TEXTBOOK OF ZOOLOGY
Mos-

of necessary relations for production of the disease is broken.

by draining swamps which by placing mosquito fish (top minnows) in the pools to eat the larvae, or by covering the water with a film of oil which keeps out air and smothers the larvae as well as discourages females from laying eggs in such water. Another means by which the chain may be broken is to cure the carriers by killing all of the Plasmodia in their blood by use of quinine, properly administered under a physician 's direction. Quinine is a specific drug for this disease.
quitoes are destroyed

serve as breeding

places,

Texas Fever.

The small sporozoan, Babesia higemina, causes this

by destroying red blood corpuscles. The red cormay be reduced from an average of 7,000,000 per cu. ml. to less than 1,000,000 per eu. ml. The disease is transmitted from cow to cow by the cattle tick and its young.
disease in cattle

puscle count of the host

Nagana, similar

to African sleeping sickness in


all

man, dourine, a

sexual disease of the horse, and surra, are

diseases of domesticated

animals and are caused by trypanosomes.

In some parts of the world

they have considerable economic importance.

There are many other diseases that are rather similar to the above which may be caused by Protozoa, although the organisms have not
been specifically isolated.
fatal to

Such

diseases as

Rocky Mountain spotted

fever, transmitted by the Rocky Mountain spotted fever tick and

and unmosquito Aedes comfortable disease, transmitted by the yellow fever (Stegomyia) ; as well as perhaps rabies, scarlet fever, typhus fever,
;

man dengue

or breakbone fever, a very unpleasant

smallpox, and trachoma should be considered with this possibility.

The cost of the above-mentioned and other Protozoa to man throughout the world in money, loss of time, and suffering is almost inestimable. A good protozoologist is one of our most valuable economic assets.

CHAPTER V

EUGLENA OF CLASS MASTIGOPHORA


Habitat and Characteristics

The most common species are Euglena viridis and Euglena gracilis which are found abundantly in fresh water. This genus is also quite well represented among marine animals many Euglenae possess chloroplastids which give them the possibility of photosynthesis. They are usually found living in the surface waters of ponds, sluggish creeks, and lakes. Euglenae are sometimes classified as plants by botanists because of the presence of chlorophyll. It is a form which illustrates certain plant characteristics and animal characteristics in the same organism.
;

Structure

The microscopic,

single-celled

body has about the shape and proit

portions of a cigar with a blunt anterior and a sharp posterior end.

At
is

the anterior end, attached near the mouth,

bears a very

slender, almost transparent, whiplike filament, the flagellum.

This

an extension of the cytoplasm.


(ectoplasm)
is

ectosarc
cuticle.

The superficial layer of the cell or covered by an extremely thin portion, the

Most of the euglenoid forms have spiral markings (striations) on the surface of the body. The mouth of the cell is near the anterior end, and extending inward from it is the gullet or cytopharynx. Beside the cytopharynx is the reservoir or large vesicle. Just anterior to this is the stigma, which is red in many individuals
of E. viridis.

Bodies of collected protein material

may

be seen in

eoiuiection with

most of the chloroplasts which are distributed

through the cytoplasm. These bodies are called pyrenoid dodies. Within the inner portion of the cell or endosarc (endoplasm) is located the nucleus. It is usually obliterated from view by the abundant chloroplasts. Small contractile vacuoles empty from the endoplasm into the reservoir.

Food and Assimilation


The food problem among Euglenae
the biological standpoint.
to It

as a group is interesting from seems that some Euglenae are able ingest other small organisms through the mouth and cytopharynx
81


82
to

TEXTBOOK OF ZOOLOGY

be digested in a vacuole within the endoplasm; this has been called holozoic nutrition as typical of animals. E. viridis probably
does not possess this possibility.
assimilate dissolved nutriment

Others, like E. gracilis, are able to

surface (saprophytic nutrition).

by absorption through the general cell In fact, this species has been maintained for more than two years in a nutrient solution in darkness.

-Fhgellum
'Mouth

-Stigma
Contractile
:0'.

Vacuole
^Reservoir
)<.

'

.
^^

Ctirom atopt) ore

A\^
1^5

-Nucleus

Fig.

32.

Euglena
like

viridis

Ehrenberg.

chlorophyll-bearing flagellate.

Those forms
plant.
salts,

E. viridis that are abundantly endowed with chloro-

phyll obtain their food largely by photosynthesis as does the green

This process utilizes water, carbon dioxide, dissolved mineral

and with the aid of light and chlorophyll builds up organic food substances. The final stage of the carbohydrates formed by this

EUGLENA OP CLASS MASTIGOPHORA


process
is

83

paramylum, a granular substance much

like starch.

Grains

of this substance

may

be observed throughout the endoplasm of these

Euglenae when living in favorable conditions. It is not likely that all three of these fundamental types of nutrition are found in any one
species of Euglena, but all are represented in closely related species

of these flagellates. *o^

Respiration and Excretion


Respiration
is

carried through the general surface of the cell

membrane. There may be some utilization of the carbon dioxide produced in the metabolic activity by the process of photosynthesis in forms where it exists. Likewise, some of the excess oxygen produced by photosynthesis may be used in metabolism. "Water and
waste products collect in the several small contractile vacuoles

which empty into the reservoir, a permanent


with the exterior.

vesicle

communicating

Reproduction and Life Cycle

Binary longitudinal

fission is the

common means

of reproduction.

This division occurs only in the motile state (or active phase) in

some species, in a quiet but not encysted condition in other species, and in a few others, fission occurs only while encysted (encysted phase). E. viridis may divide by longitudinal binary fission in either the motile or encysted condition. According to some authors the original flagellum is retained by one-half, while a new flagellum is developed by the other, but there is also some rather authentic work which shows that the old flagellum entirely disappears during division, and a new one is developed in each daughter cell. During adverse conditions, such as drought or increased chemical concentration,

Euglena becomes encysted. In this condition it becomes spherical in shape, nonmotile, and secretes a thick gelatinous envelope about itself. During the encysted phase, division takes place. There may
be a single division or there may be several. Upon the return of normal, favorable conditions these cells emerge from the cyst and assume the active phase. Some observers have reported as many as
thirty-two

young

flagellated individuals

coming from a single


cell.

cyst.

On
side

rare occasions two individual Euglenae come together side by

and fuse permanently

into a single

This

is

somewhat

similar to the zygote formation in sexual reproduction.

84

TEXTBOOK OF ZOOLOGY
Behavior

near the surface of the water if the light there is not too intense, and when in the active phase swims about. This animal displays positive phototropism and is easily stimulated by changes in intensity of light. If the light is too intense, there A medium light is optimum for it. will be a negative response. There is naturally an attraction to light in those forms which utilize

Euglena usually

lives

it in

the manufacture of food

by photosynthesis.

Direct, intense

When Euglena swims however, is injurious to them. through the water, its anterior end with the flagellum goes foremost and is first to reach any injurious or distasteful environment. When it encounters such a condition in the medium, it stops and turns sharply in another direction and attempts to move out of danger. This is known as the avoiding reaction. In these and other
sunlight,

reactions this cell exhibits the irritability that

is

characteristic of all

protoplasm.

Locomotion and

Flag'ellar

Movement

are not actively

Contractions and expansions take place in Euglenae when they moving about. These movements resemble waves of contraction (peristaltic contraction) passing over the cell. Some

move about in a crawling fashion by taking advantage of this movement. This activity is known as euglenoid movement. The chief method of locomotion is swimming by means A of the whiplike movements of the flagellum through the water. spiral path is followed due to the continuous turning of the body.
of the larger species
is made up of an elastic outer sheath which encloses an axial filament composed of one or more contractile fibrils.

The flagellum

CHAPTER VI

AMOEBA OF CLASS SABCODINA


It is likely that

no microscopic organism has attracted so much


as

attention and popular interest as

by the public generally


animal
life
is

Amoeba. Amoeba is recognized a simple and low form of life. Even the

writers of fiction speak of the range of the span of complexity of

Amoeba
the

as extending "from Amoeba to Man." The pedigree of probably as long as that of any of the animals we know and involves hundreds of times as many generations as many of

common animals

yet

Amoeba remains

in a relatively primitive

and simple state. Little or nothing is known about the real ancestry of Amoeba. There are many kinds or species of Amoeba, some simpler and some more complex than Amoeha proteus. Chaos diffluens is a very desirable species for study. Recently Chaos chaos Schaeffer has been rediscovered. It is enormous in size and can be
seen with the unaided eye.

Characteristics

and Habitat
marine water,

The many kinds


soil,

of

Amoeba

live in fresh water,

or as parasites in the fluids of the visceral organs of higher

types of animals. Amoeda proteus may be collected in a variety of places where conditions of water, temperature, and organic food are favorable, such as debris from watering troughs, bottoms of

ponds,
there

spring pools, drain ditches, abandoned tajining pits, in streams where the water runs over rocky ledges, and wherever
is

face of submerged lily pads.

abundant aquatic vegetation. It is often found on the surA mass of pond weed may be brought into the laboratory in some of the pond water and allowed to stand in the container a few days. If amoebae are present, they will likely be in the brown scum which forms, or in the sediment at the

bottom.

lustrous, irregular

The general appearance of this animal is that of a slate-colored, mass of gelatinlike substance with slowly-mov-

ing, fine particles within.

When
S5

it

is

active, the outline is con-

stantly changing.

86

TEXTBOOK OF ZOOLOGY
Structure

Amoeba
Its

proteus

is

one of the largest of the fresh-water forms.


is

about M.00 iJ^ch (0.25 mm.), while its extreme diameter is Y^o inch or barely visible as little specks to the unaided human eye. The animal owes its irregular shape to the fact that protrusions of its own substance are formed at its surface.
average diameter

These are

known

as pseudopodia, and they are constantly changing

in shape in the active

animal by the flowing of the protoplasm.

Under favorable conditions the protoplasm can be differentiated two portions. The firmer, somewhat tougher outer portion, the ectosarc (ectoplasm), is nearly homogeneous and includes the plasma membrane (or plasmalemma) the more fluid inner portion, endosarc (endoplasm), is much more granular and contains the cytosome, cell
into
:

'fj

VACU01_E

FOOD

^^
1

"

'^T^'-

Z-^Z-Z^^^>

^
'^

VACUOLE
NUCl_EUS

>-* f '.'** V-''^^S? -^ ^U",

PSEUDOPODIUM
ECTOPL.ASM

^i^

Fig.

33.

Drawing

to

show the appearance and structure


proteus.

of

a living

Amoeba

inclusions as well as the nucleus.

The larger bodies


contractile

in the cytosome

are

food vacuoles,
fluid

single,

shiny,

vacuoles

containing

watery

and varying

in size; water vacuoles; various granules;

mitochondria; fat globules; and crystals.

Some authors

distinguish

two types of protoplasm in the endosarc; the inner more fluid, plasmasol in which the streaming movements take place and, surrounding this a more viscous, passive portion, the plasmagel. The nucleus usually appears somewhat dense and granular, and is located in the portion away from the end which is advancing in a moving specimen.
Metabolism
This refers to the constant building up (anabolism) of living protoplasm and its concurrent oxidation (eatabolism). It includes
all activities

necessary for maintenance of

itself

and

its race.

These

AMOEBA OF CLASS SARCODINA

87
of life

phenomena are the same as those found in the highest forms but reduced to very simple terms. Here we may study the
metabolic cycle in progress within the confines of a single

entire
Its

cell.

phases are as follows:

Food.

Its

prey consists chiefly of smaller Protozoa, small single-

celled plants, such as diatoms

and desmids, and portions of filamen-

be used to some extent and rotifers (small Metazoa) are sometimes devoured.
tous algae. Bacteria

may

Amoeba has no definite mouth but the food is taken body by engulfing it at any point that comes in contact with it. A pseudopodium is formed at this point, and the end of it flows around the food particle until the particle is entirely enclosed. A droplet of water is included with the food to form what is called a food vacuole. These vacuoles move about in the endoplasm.
Ing-estion.

into the

E^ejfion

xcretion
Fig. 34. Diagram to show the phases of the metabolic process as it occurs in amoeba. (Redrawn by permission from Wolcott, Anional Biology, published by McGraw-Hill Book Company.)

Digestion.

The food gradually disintegrates and much of

it

goes

into solution in the fluid of the vacuole.


is

The function of digestion

complex materials into a soluble, absorbable form. It assumed that the surrounding cytoplasm secretes enzymes into the food vacuoles of Amoeba to perform this function, since enzymes serve this purpose in larger animals where exact study can be made
to convert
is

on the process.

circulatory system

is

not necessary since the

88

TEXTBOOK OF ZOOLOGY

vacuoles with the food in the process of digestion circulate so widely in the endoplasm that all parts of the cell may receive

nourishment by direct absorption.


Egestion.

Indigestible material or
is

debris that has been ingested

with the food

carried to the surface of the cell

egested by simply being left behind as

and cast out or the animal moves away.

This is the process of transforming the digested Assimilation. food material into protoplasm. In Amoeba the digested food material is absorbed directly from the food vacuoles by the surrounding cytoplasm. Since the vacuoles move rather generally through the endosarc, most of the protoplasm of the cell is in rather close contact with the dissolved food.

This is a process whereby the gas, carbon dioxide Respiration. (CO2), leaving the protoplasm, is exchanged for oxygen (O2) entering it. Such a process is essential to all living protoplasm. In Amoeba this exchange is carried on primarily through the general

body

surface.

The water

in

which the animal

lives

must contain

go on. Amoebae, however, are able to and do live in rather foul water where the oxygen content is rather low and the carbon dioxide high because Amoebae may live several of the decaying vegetation present. hours in water from which the oxygen is removed before asphyxiadissolved oxygen in order that this diffusion
tion occurs.

may

The

contractile vacuole likely assists in discharging CO2.

The chemical union of the oxygen with the organic substance of the protoplasm liberates kinetic energy and heat. This is known as oxidation and is a burning process which goes on within the protoplasm. Water, some mineral matter, urea,
Catabolism or Dissimilation.

and carbon dioxide are residual products of


Excretion.
liquids

this process.

These by-products of metabolism in the form of waste


of.

must be disposed

They cannot be allowed

to

accumulate

beyond certain limits in the living organism if life is to continue. Urea and uric acid, which are protein by-products, excess water, and salts, are discharged from the body of Amoeba by way of the conThe contractile tractile vacuole along with some carbon dioxide. vacuole is formed by the union of small droplets of liquid under the plasma membrane. It fills out with liquid which is forced out through the membrane as the vacuole disappears. Its location apparently is not fixed in the cell but is often near the nucleus. The contractile vacuole is absent in some forms, and in such cases, ex-

AMOEBA OF CLASS SARCODINA


cretion occurs only
likely

89

by diffusion through the cell surface. some excretion by this means in all Amoebae.

There

is

Growth.
plished

If there is increase in the

volume
If
is

of a body, this is

spoken of as growth.
material

In

all

living organisms

growth
is

is

accom-

by addition to the protoplasm. is added to the protoplasm than


In

food

plentiful,

more

used up in the oxidation

which produces active energy.

other words,

growth occurs

when

the rate of anabolism exceeds the rate of catabolism in the

organism.

Reproduction and Life Cycle

The
adult.

life

history of the niajiy-celled animals to be studied later

includes a series of changes from egg, through

In

Amoeba

the cycle

is

likely only partly

embryo state, to known, because it

Nucleus
Contractile iracaole

Fig. 35.

-Diagram
fission

to

show

fission

in

amoeba.

A, Beginning of the process; B,

nearing completion.

(Drawn by Joanne Moore.)

is difficult

to

maintain cultures in perfectly normal conditions for


Ordinarily, the
it

sufficiently long periods to get this

complete story. animal grows when conditions are favorable until


tain size;

attains a cer-

when

this limit of size has

been reached growth ceases.

Why

does the cell cease to grow?

Why

should

it

not attain the

size of a

man?

Or why should a

tree not continue to

grow

until

it

reaches the sky, or a

man

take on the proportions of an elephant?

We have not been able to put our fingers on any one factor that completely controls growth. We do know of certain relationships
it. It will be recalled that all materials used by a must pass through the cell membrane, and likewise all waste substances must be discharged in a similar manner. Mathematics states that the volume of a cell increases according to the cube of

that influence
cell

90
its

TEXTBOOK OF ZOOLOGY
diameter; while
its

its

surface increases only according to the

square of
a growing
point

diameter.

In other words, the amount of material in

cell increases

surface needed to surround

approximately twice as fast as the plane it. It is logical, then, to assume that a
the surface area will not be sufficient

may

be reached

when

for the passage of necessary materials into


is,

and out

of a cell.

There
it

however, considerable variation in the size of cells; hence

seems there must be other factors besides volume and surface relaModified surface and difference in the rate of tion in operation. metabolism certainly would be factors affecting the
organism.
curs.
size

of the

When Amoeba
fission,

reaches the limit of

size,

a division oc-

Binary

by which two new individuals are produced,


Calkins, an authority on Protozoa, states

has been definitely established, and some other methods of reproduction have been presented.

New
Pig. 36.

cells

/Nuclear

fragment
of sporu-

Diagram

to

show amoeba encysted and undergoing the process lation. (Drawn by Joanne Moore.)

that

Amoeba

starts out as a tiny pseudopodiospore


It

which has only


It

one pseudopodium.

then passes through a growth period and


it

increases in complexity until

reaches the full-grown condition.

fission into two daughters. When grown to nearly twice its original size, fission is repeated. Environmental conditions and the variety of Amoebae determine the number of times this phase is repeated. Occasionally the fission seems to be an amitotic one. At the close of the fission phase, there is a period of encystment and subsequent sporulation. During the en-

then divides by binary


ter has

each daugh-

cystment the protoplasm undergoes several divisions to produce the


several pseudopodiospores

which

later break

from the cyst

as infant

Amoebae.

It is felt that the

complete details of the

life cycle of

many

common Sarcodina

are not yet available.

AMOEBA OF CLASS SARCODINA


Behavior

91

All of the activities of an animal which come in response to in-

make up the "behavior." The activities under discussion include the formation of pseudopodia, ingestion of food, locomotion, and others. Amoeba proteus
ternal or external stimuli
of the animal

exhibits either positive or negative reactions to various stimuli.

An

environmental change to which an animal reacts is known as a stimulus, while the reaction of the animal is called the response. The movements made by an animal in response to stimuli are called
tropisms.

Amoeba

exhibits all of the tropisms discussed in Chapter


contact,
it

IV.
gentle

To physical
;

responds positively
is

if

the impact

is

otherwise the response


its

strong light and finds

responds negatively to optimum in a moderately reduced light.


negative.
It

When some

part of the body surface of this animal comes quietly into contact with food, there is a characteristic response. This part
of the protoplasm stops flowing while other parts continue, thus form-

ing a pocket around the particle of food. The edges of the pocket fold in, meet, and join so as to enclose the object. This attraction to

food is likely a positive chemotropism. Amoeba reacts negatively to concentrated salt, cane sugar, acetic acid, and many other chemicals which have been tried. Amoebae have an optimum temperature range between 15 and 25 C. Temperatures approaching the
freezing point inactivate the animal, while temperatures above 30 C. (86 F.) also retard their activities and may soon become fatal.

weak electric current has aai effect on the physical condition of the protoplasm on the side nearest the cathode. The tendency is toward the sol state here, hence the animal turns toward the cathode. According to Jennings, who has done extensive research on behavior of Protozoa, these activities are "comparable to the habits,
reflexes,

and automatic

activities of higher

animals."

He

also feels

that

Amoeba probably

experiences pain, pleasure, hunger, desire, and

the other simple sensations.

Amoeboid Movement and Locomotion


The flowing or streaming of the protoplasm and extending the cell some direction by the formation of pseudopodia is usually called amoehoid movement. It is so named from the perfect exemplification of such activity by Amoeba. Locomotion is accomplished by the pseudopodia, and the process of their formation in most Amoebae.
in

92

TEXTBOOK OF ZOOLOGY

moving Amoeba proteus as The pseudopodia are temporary locomotor structures. Most zoologists explain this movement as being due to the contraction of the more viscous ectoplasm, particularly in This brings about a forward movement in the "posterior" region. the more fluid eudoplasm (plasmasol) which causes an outflow at points where the ectoplasm is thinnest, or where surface tension is lessened. As this plasmasol approaches the advancing tip of the pseudopodium, it turns to the sides and changes to more solid endoplasm (plasmagel). This process continues, pushing the advancing At the opposite side, the plasmatip farther and farther forward. gel continues to become plasmasol to provide for fluent material. At the side of the animal away from the advancing pseudopodium, the cell membrane (plasmalemma) moves upward and over the upper side of the body it continues to move forward to the tip of the
Successive pseudopodia are formed in the
it

goes in a given direction.

- - Particle -"" Pseudopodium


i

Fig. 37. Successive positions in the movements of an amoeba viewed from the side. Notice the formation of new pseudopodia and tlae engulfing of the particle on the surface. (Modified from photographs by Bellinger, 1906, Journal of Experimental Zoology.)

pseudopodium where it dips down and is laid on the substratum over which the animal is moving and becomes a part of the stationary portion. If the specimen has several pseudopodia, one or more

may

be developing while others are receding.


is

In the

latter, the

back through the centers of the pseudopodia toward the main mass. Temperature and other environmental facflow of plasmasol
tors affect the rate of locomotion.

mounted some of the animals on the edge of a slide in by the projecting edges of two cover glasses and observed their movement from side view by tilting the microscope to a horizontal position. He describes their movement as a sort of walking on the progressively forming pseudopodia. The new pseudopodia are formed at the advancing margin of the cell.
Dillinger

a groove formed

CHAPTER
This animal has been the subject of

Yll

PARAMECIUM OF CLASS INFUSORIA


much study and the victim Paramecium caudatum is probably the species most commonly studied. It is easily available and is large in size, ranging between 0.2 and 0.3 mm. in length.
of considerable experimentation.

Characteristics

and Habitat

Paramecium is an active, cigar-shaped animal, just about large enough to appear as small white specks in the water. It has a definite axis and permanent anterior and posterior ends, but it is asymmetrical in shape. Paramecia are easily cultured by collecting some submerged pond weeds and allowing them to stand in a jar of the pond water for several days. Or some natural creek or pond water may be placed in a jar with some old dry grass and allowed to stand about ten days. These animals occur abundantly in any water which contains considerable decaying organic matter. They thrive in all streams, creeks, or ponds polluted by sewage. They tend to congregate at the surface and particularly in contact with floating objects, where they frequently form a white scum.
This animal
is

a great favorite in zoology laboratories.

Structure

Paramecium
the heel part
;

anterior portion, which

sometimes described as being slipper-shaped. The is blunt but generally narrower, represents while the posterior portion, which is generally broader
is

but pointed, represents the sole portion.

At one side is a depression, the oral groove, which passes diagonally from the anterior end to about the middle of the body. It is broad and shallow anteriorly but it becomes narrow and deeper as it ends in a mouth, which leads to the gullet. The groove usually extends obliquely from right to left in P. caudatum as the animal is viewed from the oral side. Occasionally cultures are found in which the majority of the individuals show the groove extending from left to
93

94
right

TEXTBOOK OF ZOOLOGY
from
this view.

The body

is

covered with fine hairlike

cilia

which are of even length except in the oral groove and at the posThe cilia within terior extremity, where they are noticeably longer.
the gullet are fused together into a sheet, forming the undulating

membrane.

CON-r RACTIL.E

VACUO l_E
ORAl_

GROOVE

MACRONUCI_E.US

CONTRACTILE VACUOLE
TRICHOCYSTS
PEI_l_ICl_E

Fig. 38.

Diagram
cell
is

showing the structure of Paramecium, much enlarged. by T. C. Evans.)

(Drawn

which

is divided into the outer, tough, nongranular ectosarc composed of ectoplasm. The outer surface of it is a thin, elastic cuticle or pellicle which is marked in hexagonal areas by the distribution of the cilia. The cilia are direct outgrowths of the ectosarc. There are a great many spindle-shaped cavities located in the These ectosarc with their long axes perpendicular to the surface. structures, trichocysts, are filled with a semifluid substance and each opens to the outside through the pellicle. The endosarc, composed of

The

PARAMECIUM OF CLASS INFUSORIA


endoplasm,
is

95

within.

It

contains food vacuoles,

vacuoles, macronucleus,

and other granular masses.

food vacuoles are formed, one at a time, at by a mass of food material coming in with a droplet of water, a process similar to that described in Amoeba. The vacuoles circulate through the endoplasm in a rather definite course. This activity is

two contractile The numerous the inner end of the gullet

The contractile vacuoles are located near each end Each vacuole has several radiating canals entering it. These vacuoles expand and contract alternately. The macronucleus is located slightly posterior to the center and somewhat beside the mouth. It is relatively large and rather bean-shaped. The micronucleus is located in the curved surface of the macronucleus and is
called cydosis.

of the animal.

much

smaller.

P. aurelia, another species, ordinarily has two micro-

nuclei instead of one.

Metabolism

The same general

activities as described in

Amoeba and

others

occur, differing only in certain details.

These same vital functions

must take place

in all living things (organisms).

Oral qroove

J^l^

Nucleus

^ %

Mouth
Gullet

if

Anus
-Food vacuole

Fig. 39.

Cydosis

in

Paramecium, showing the course of the food vacuoles through the endoplasm while digestion is in progress.

Food.

stitute the principal items

Smaller protozoans, bacteria, and particles of debris conon the menu for Paramecia. Ingestion. This animal hunts food, and when locates a
its
it

re-

gion where food

is

abundant,

it

settles

down and becomes

relatively

96
quiet.

TEXTBOOK OF ZOOLOGY

swept through the oral groove by the beating and carried back through the mouth into the gullet. Finally it passes by means of the action of the undulating membrane into the endoplasm in the form of one food vacuole after another. These food vacuoles move in a definite course through the endoplasm. Since this course is in the form of a cycle, the circula-

The food

is

action of the

cilia,

tion

is

known

as cyclosis.

Dig-estion, Assimilation, Respiration,

and Catabolism or Dissimi-

lation all occur in a

manner very

similar to that described for

Amoeba.

Egestion occurs at a definite anus.

is by and expelling of fluid by the two contractile vacuoles, or it may occur to some extent by diffusion through the entire cell membrane.

Excretion of the waste products of metabolism in solution


of the alternate filling

means

Growth occurs as it does in Amoeba and in all other organisms. Under favorable conditions the storage of nutrient materials, like starch and fats, occurs in the cytosome. Nutrition in this animal is holozoic, and its living process is essentially like that of all higher
forms of animal
life.

Reproduction and Life History

itself is asexual.

The actual reproduction is by transverse binary fission which in The cell divides transversely into individuals, and
one after another.

this is repeated for long series of generations,

During this division process in P. caudatum, both the macronucleus and the micronucleus divide, the old gullet divides into two, and two new contractile vacuoles are formed by division of the old ones. The micronucleus divides by mitosis, but the division of the macronucleus is not distinctly so. The time required for the completion of a division ranges between thirty minutes and two hours, depending on environmental conditions.
Division is repeated at least once each twenty-four hours and under especially favorable conditions, twice a day. It has been estimated that if all survived and reproduced at
a normal rate, the descendants of one individual over a month's time

would number 265,000,000 individual paramecia.


P. caudatum is a conjugating form of Paramecium, while P. aurelia and others seem not to conjugate. Conjugation is a temporary union of two individuals with exchange of nuclear material. Calkins carried some cultures of P. caudatum through a long series of generations and observed that conjugation occurs at intervals of approxi-

PARAMECIUM OF CLASS INFUSORIA

97

mately every two hundred generations. When two paramecia are ready to conjugate, they come in contact, with their oral surfaces
together,

and adhere in this position. A protoplasmic bridge is formed between the two individuals. This union resembles a sexual act and has recently been described as such. The conjugants are
Shortly after
the adherence of the conjugants the nuclei of each undergo changes.

usually small, rather unhealthy appearing individuals.

The micronucleus enlarges and divides, forming two micronuclei, while the macronucleus undergoes disintegration and final disappearance. Each of these two new micronuclei again divides to form
four, three of which disintegrate, but the fourth divides again, forming one large and one small micronucleus. Sometimes the smaller of these nuclei is spoken of as the "male" nucleus and the larger, as the "female.'' In each animal the smaller nucleus moves across the protoplasmic connection to the other animal and fuses with the larger nucleus there. Each individual now has a fusion nucleus. The two conjugants now separate, and very shortly the fusion nucleus of each divides by mitotic division each of these divides, forming four nuclei in each animal, and these four divide to form eight. The descriptions of the subsequent events vary somewhat. At least it is known that four of the eight nuclei enlarge and become macronuclei; three of the others degenerate, and one remains as a micronucleus. This micronucleus divides, and almost immediately the entire animal divides by binary fission with two macronuclei and one micronucleus going to each cell. These daughter cells then divide to produce a total of four Paramecia which have the typical number of one micronucleus and one macronucleus of the active phase. Following this comes the long series of generations formed, one after the other, by transverse binary
;

fission.

The whole series of changes involved in conjugation has been compared to maturation of germ cells and fertilization in sexually reproducing metazoans. The degeneration of the three micronuclei is compared with reduction division in maturation, and the fusion
of the small "male" micronucleus with the larger "female" micronucleus of the other conjugant is compared to fertilization.

phenomenon, known as endomixis, has been found occurring in P. aurelia by Woodruff. It occurs in a single individual. This species has two micronuclei and one macronucleus. At regular intervals of about every forty or fifty generations, the macronucleus

98

TEXTBOOK OF ZOOLOGY

Fig. 40.

Conjugation

and subsequent divisions

in

Paramecium, showing

activi-

ties of the micronucleus.

The shaded ones have been resorbed.


within the cells instead
treneral Biology.)

Circles are micronuclei and crescents are macronuclei. The divisions for micronuclei actually occur (From "White, of outside, as figured for convenience.

PARAMECIUM OF CLASS INFUSORIA

99

disintegrates, and the micronuclei undergo two divisions which produce a total of eight. Six of these disappear, and then the cell divides; one of the remaining micronuclei goes to each. This nucleus then undergoes two divisions. Two of these four become macronuclei, and two remain as micronuclei. The micronuclei then divide again as the entire cell divides to form daughters, each with two micronuclei and one macronucleus, the typical condition for
this species.

Endomixis may occur

in P.

caudatum

also.

Endo-

mixis seems to have about the same effect as conjugation.

100

TEXTBOOK OP ZOOLOGY
Behavior

is an active swimmer and necessarily shows ready Its behavior consists of its environmental factors. response avoiding reactions, responses to food spiral course in locomotion,

This animal
to

material, contact

and other minor

reactions.

Its reactions to stimuli

Amoeba; however, it seems not to be affected by ordinary light. It reacts either positively or negatively to contact, change of chemical constitution,
are somewhat similar to those described for

change in temperature, to gravity, and


sponse to contact
is

to electric current.

The

re-

positive, negative to ultraviolet light, negative

:;

/^

a
'

"

JS'
rj,-?-.V."."-:-t;^-_f
,-_.' . .

19-

pSn^-";Y=iV;-".-'

Be-

ss-

Reactions of paramecia to temperature, a, Paramecia are in a Fig. 42. trough with temperature at 19 C. uniformly through the water. The animals are generally scattered. In & the temperature is held at 26" C. at the left end and 38 C. at the other. The animals are collected in the end of lower temperature. In c, the temperature is 25 C. at one end and 10 C. at the other, and the Paramecia are congregated in the region of higher temperature. (From Jennings, Hehavior of the Lower Organisms, published by The Columbia University Press.)

to

sodium chloride, positive

to

weak

acetic acid,

and positive

to the

negative pole of a weak, galvanic electric current. The optimum temperature for Paramecium ranges between 24 and 28 C. (71
P.).

placed in moving water, the animals will swim upstream.

Gravity causes the anterior end to point upward, and when If Parain contact

mecium comes
will

with a solid object when


its

it is

moving,

it

back away, swing on

posterior end to a slightly different

PARAMECIUM OF CLASS INFUSORIA


direction

101

and try again.

This

the "avoiding reaction/'

may be repeated, and is known as Such a reaction really involves simply

one or more negative responses. These animals are constantly sampling the water and avoiding the conditions which are least favorable. This may be repeated in all directions. The same type of
persistence
is

practiced in attempting to surmount a solid barrier.

Fig. 43. Diagram of the course and movement of Paramecium through the water. Notice the spiral path. (From Jennings, Behavior of the Lower Organisins, published by The Columbia University Press.)

102

TEXTBOOK OP ZOOLOGY

Such successive attempts to gain the result desired constitute what known as the "trial and error'' mode of behavior. In an effort to defend itself when severely irritated, Paramecium will discharge the contents of the trichocysts, which harden on conThese threads tact with the water and form a mass of fine threads.
is

will entangle

many

of the aquatic enemies of these animals.

Locomotion

The beating action of the cilia against the water serves as the means of locomotion. The stroke of the cilia is rather oblique and this coupled with the increased length of the cilia along the oral groove causes the body to turn on its long axis while swimming. The total effect of these activities causes the course followed through the water to be that of a spiral. Paramecium may reverse the direction of the stroke of the cilia and thus move backward just as a car can be thrown in reverse. The cilia are contractile outgrowths of the ectosarc. Each has an Contraction of the protoplasmic elastic sheath and a fibrillar core. substance on one side, bends the cilium in that direction. The reverse stroke is much more passive. The movement of one tier of
principal
cilia

seems to stimulate the adjacent ones to bring about coordi-

nated, rhythmic ciliary activity and movement.

CHAPTER

VIII

METAZOAN ORGANIZATION
All animals whose bodies consist of few or
as a unit are called metazoans.
of

many

cells

functioning

In most respects the vital activities

Metazoa are similar to those of Protozoa. Since Metazoa are more or less like compound Protozoa with some degree of intercellular differentiation,
it

is

thought by

many

authorities that they

arose through organization of single-celled organisms. of

In some forms

compound

or colonial Protozoa, only two cells adhere together after

cell division,

but in others the

cells

The size of different thousand similar cells. In the most complicated protozoan colonies
di\^sions.

may remain attached after many colonies may range from two to two
cells.

there

may

be several different types of

The representatives

of class Mastigophora are the most likely ancestral forerunners

Metazoa. The colonial forms, such as Gonium, Pandorina, Eudorina, Pleodorma, and Volvox, are rather plantlike in characterof
istics,

but a series of this type shows the possibility of the relative

complexity of different colonial forms.

There are several genera of

animals which are intermediate between Protozoa and Metazoa, but


for the most part the two groups are fairly distinct.

General Characteristics
This group includes
cells are definitely
all of

the strictly many-celled animals.

The

organized and classified morphologically as well as physiologically. There is a well-regulated division of labor.

Among

the single-celled animals each


its

cell,

like primitive

man,

is

largely independent of

fellows, doing for itself all that is neces-

sary to carry on living processes.

In the many-celled animal, as in

a highly developed society of men, certain individual cells become

more

proficient in doing certain kinds of work,


is

and as a

result, a

special group

able to care for a particular function necessary to

the life of the entire organism. for other functions.


its

In return, other special groups care


the products of

In this

way each exchanges

labor for the products of the labors of the other groups.


society this becomes

In

human

more and more complicated as


10.1

civiliza-

104
tion advances; so zoans.

TEXTBOOK OP ZOOLOGY

it is Avith development of complexity in metaAnother characteristic of Metazoa is the presence of a definite center of control localized in a particular group of cells which becomes the nervous system in higher forms.

Cellular Differentiation

In Protozoa there
ferentiation,

is
it

seen fair development of intracellular difpossible for one part of a cell to

particular function,

perform a and for other parts to perform other functions. The complexity of Metazoa is not the result of great complexity of the individual cells, but it is due to the special differences between them. The presence of a variety of cells within one body is spoken The modification of metabolic of as intercellular differentiation.

making

Fig. 44.

Typical

germ

cells.

A,

ovum

of the female; B, spermatozoa of the male.

activity is the basic factor in the

development of

all differentiation.

become specialists in a particular phase of the metabolic activity. Some become protective surface cells, others secrete special enzymes, still others specialize in excretion, and so on.
Certain groups of
cells

The

entire

metazoan body

is

usually divided into germ


cells

cells,

which
cells,

are specialized for reproduction, and somatic

or body

which compose the remainder of the body and are grouped in

layers.

The germ

cells

are set aside early in the

life

of the individual for

reproductive purposes.

They develop in the reproductive glands or The protoplasm of these cells is known as germ plasm. The female germ cells are eggs or ova, and those of the male are spermatozoa. When the germ cells reach maturity, they become separated from the body and may give rise to a new generation.
gonads of the two
sexes.

METAZOAN ORGANIZATION

105

About forty years ago Weismann presented the idea of the continuity of heredity from generation to generation by way of the germ plasm. The germ plasm, according to this idea, gives rise not only to the protoplasm of the germ cells of the new individual but to the somatic
In Protozoa the entire material of the individual is passed on to the two offspring and, for this reason, this protoplasm is spoken of as being immortal. Potentially, germ plasm is likewise
cells as well.

immortal.

The protoplasm
is

of the somatic cells

rebuilt with each generation,

known and when the


is

as somatoplasm.

This
all

individual dies,

of

the somatoplasm perishes.

somatoplasm serves as a means of conveyance for the germ plasm through the current
final analysis, the

In

generation.

Cellular Organization

The simpler Metazoa are composed of only two kinds of somatic cells. These cells are grouped according to kind in two layers. With advanced differentiation, a rather wide variety of cells has
been produced.

tissue is

an organization of similar

cells into

a group or layer

for the performance of a specific function.


intercellular substance is characteristic of

certain

amount

of

most tissues and enhances their usefulness. The entire living mass of the metazoan animal body may be classified under five fundamental (four by some authors) kinds of tissues, and when it is so distributed, there is nothing left.
These classes of tissues are:
epithelial, protective or covering; sus-

tentative, connective or supporting; muscular, contractile; nervous,


irritable or conductive; vascular, circulatory.

Epithelial Tissue.

sheet of cells that covers external or in-

is known as an epithelium. The epidermis or outer layer of the skin and the layer of column-shaped cells lining the inside of the intestine are good examples. According to function, this type of tissue can be classified as protective epithelium, glandular epithelium, and sensory epithelium. The epithelium which covers external surface of an organism usually de-

ternal surfaces of the

body

velops various protective structures in the different groups of animals the hard, homy chitin of insects scales of fish homy plates
:

and scales of reptiles feathers of birds hair ajid nails of mammals. The glands of the body are developed from epithelium. Secretions
;

106

TEXTBOOK OF ZOOLOGY

from these various glands lubricate the surfaces, contain enyzmes for digestion of food, supply regulatory substances directly to the blood, serve as poison to other animals, and some are repellent to
enemies.

Sustentative Tissue.

This type comprises


most
cases,

all tissues

whose func-

tion is to bind together or support the various parts of the body.

Connective tissue

is,

in

composed

of slender cells with


is

an abundance

of intercellular material.

This tissue

almost uni-

versally present in the various organs throughout the body. Tendons, the tough cords that connect muscles to bones, of which the "hamstring" is a good example, and much of the dermis of the skin

are composed of connective tissue.

up

the framework of the body

called supporting tissues.

Bone and cartilage, which make and support the other tissues, are In crayfishes and grasshoppers the supCartilage
is

porting tissue

is chitin

instead of bone or cartilage.

com-

posed of scattered

cells

interspersed with abundant, homogeneous,

granular, semisolid matrix or intercellular substances.

Bone

is

some-

what

similar, except that the

matrix has been replaced by a heavy

deposit of calcium phosphate

and calcium carbonate, two


cells.

solid salts.

The scattered
its ability to

cells are

present as bone

Muscular or Contractile Tissue. This is distinctive because of contract and in that way produce movements. Cells adapted to this function are more or less elongated and fiberlike. There are three types of muscular tissue smooth, involuntary, and nonstriated, as found in the wall of the intestine striated, voluntary, skeletal, as found in the muscle of the arm and striated, inSkeletal, voluntary, cardiac, as found in the wall of the heart. voluntary muscle is made up of large multinucleate (many nuclei) fibers, each composed of many fibrils (myofibrils) along which are evenly distributed dense and light areas, which give the general appearance of stripes across the cell, because the dense areas on the adjacent fibrils come at the same level. The smooth involuntary muscle is composed of individual, spindle-shaped (fusiform) cells, the cytoplasm of which is largely myofibrils but without striations and therefore smooth. There is a single oval nucleus, centrally located. The outer membrane of a muscle cell is the sarcolemma. The cardiac involuntary muscle is said to be made up of individual cells, highly modified in arrangement. The definition of cells in
:

METAZOAN ORGANIZATION
this tissue is rather difficult, but the fibers are faintly

107

segmented by

thin intercalary disks which define areas each with a single nucleus.

The

cells

branch laterally

to join each other quite frequently, pro-

ducing a condition of netlike branching known as anastomosis.

Nervous Tissue.

This

is

specialized to receive stimuli

and

trans-

mit impulses which have been set up by some stimulating agent in some part of the body. The structural features consist of nerve cell bodies and their processes. Two kinds of processes are recognizable
:

(a) the axo7ie, usually a single

unbranched

fiber except for

infrequent collateral branches; and (b) dendrites, frequently

much

QO
Fig. 45. Typical cells and tissues from epithelial cells Z, section through a portion (in center), bone cells, lacunae, canaliculi,
;

cartilage showing cartilage cells in tion of tendon composed of white fibrous connective tissue 5, longitudinal view of smooth (involuntary) muscle cells; 6, striated (voluntary) muscle; 7^ motor nerve cell, showing process; 8, human red blood (nonnucleated) corpuscles and human white (nucleated) corpuscles. (Drawn by Titus Evans.)
;

vertebrate animals. 1, Squamous of bone showing Haversian canal and matrix; 3, section of hyaline lacunae, and matrix between lacunae i, sec;

branched and arborlike. An axone may be several feet long, e.g., one extending from the spinal cord to the hand or foot. Dendrites may
be lacking.
the dendrites

The impulses are conducted toward the cell body over and away over the axone. A nerve cell body together

with

its processes is called a neuron. The neurons approach each other and pass impulses from one to the other at the synapses, where

108

TEXTBOOK OF ZOOLOGY

the brushlike ending of the axone of one comes into close proximity

with a dendrite of another.


of the nervous tissue

In this

way an impulse can


The

be trans-

mitted from one part of the body to other parts.


is to relate
is

chief function

the organism to its environment.

Vascular Tissue.

This
is

fluid tissue consisting of cells

known

as

corpuscles in a fluid
the plasma or fluid are the two

medium

called plasma.

The

cells are the

red

corpuscles (erythrocytes) and white corpuscles (leucocytes), while

the intercellular substance.

Blood and lymph

In the

common vascular tissues. Lymph has no red corpuscles. blood of mammals the red corpuscles are without nuclei
fish,

while in

frogs, turtles,

and birds these


is

cells

are nucleated.

The

chief function of this tissue

the transportation of digested food

and oxygen

to the cells of the

body and the removal

of waste by-

products of metabolism from them.

An

organ

is

an arrangement of two or more tissues as a part of

the body which performs some specific function or functions.

Some

organs are made up of


scribed.
cavity.
It is

all of

the different types of tissues just de-

For example the stomach is an organ with an internal covered and lined with epithelium; the wall contains two strong layers of muscular tissue blood vessels carrying blood, and lymph spaces bearing lymph, branch through the wall nervous tissue reaches all parts of the organ to receive stimuli and distribute impulses; and connective tissue serves to bind all the others in
;

proper relation.

an aggregation of organs properly associated and some general function of life. There are ten different systems usually recognized: system
is

related to perform

a.

The Integumentary System


Its principal

is

composed of the skin and

its out-

growths, such as hair, nails, scales, horns, hoofs, and similar structures.

purposes are protection, primarily, with some

degree of excretion and respiration, some absorption, and regulation


of

body temperature.
b.

The Skeletal System composes the supporting framework of the The bony and cartilaginous tissues make up the material of this system. The vertebral column, skull, ribs, sternum, and bones of the limbs are the general parts of the vertebrate skeletoji, and they serve for the support of the body as a whole and for the probody.
tection of the internal, vital organs.

METAZOAN ORGANIZATION
c.

109

The Muscular System consists of muscles, the voluntary, striated group moves skeletal parts and accomplishes locomotion the nonstriated, involuntary g-roup is concerned with the movements of the internal organs (viscera), and the cardiac muscle produces the
;

heart action.

{JuqularVdn

Carotid Artiry

Trachea
Sabclavian V. Precaval V.
Dorsoil Aorta Pulmonary A.
..Left Auricle

LeftVentrick

Xu.na
.Diaphracfm

Liver
,

Liuodenam

'-

-_

.Stomach
Gall Bladder

Jro-nswrseColon

D<?5c ending

Colon

.1

Ascandinc^ Colon

Fig. 46.

Ventral

view of human maniltin showing parts of the principal systems. (Drawn by Edward O'Malley.)

d. The Digestive System of the higher animals includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and ac-

cessory glands.
it is

The general form of the system

is

that of a tube, and


in-

frequently called the alimentary canal.

The functions of

110

TEXTBOOK OF ZOOLOGY
and very
it

gestion, digestion, egestion, absorption, secretion,

little ex-

cretion are performed


in solution so that
e.

by

this system.

In general,

puts the food

it

may

be absorbed by the blood.

The Respiratory System consists of structures capable of delivering oxygen to the body and eliminating carbon dioxide. In some forms the general surface of the body serves the purpose, but in all
higher forms there are special structures for this function. Tracheae are found in insects, gills of various modifications in many aquatic
the lungs are the
bronchi.
f.

Metazoa, and lungs in the terrestrial vertebrate forms accessory to nasal passages, pharynx, larynx, trachea, and
;

The Circulatory or Vascular System


and lymphoid glands.

is

a very extensive one con-

sisting of the heart, arteries, veins, capillaries,

lymph spaces, lymph The general functions are: (1) to distribute blood carrying food, oxygen, and hormones from glands of internal secretion to the tissues; (2) collect and transport to the point of exit carbon dioxide, liquid wastes, bacteria, and other foreign
nodes,
g.

matter.

The Excretory or Urinary System and kidneys.

is

made up

of tubular struc-

tures and accessory parts, such as flame


tubules, green glands,

cells,

nephridia, Malpighian

In the mammals, the ureters,

urinaiy bladder, and urethra are accessory to the kidneys. The kidneys withdraw liquid waste products of metabolism from the blood and deliver them to the outside of the body. The nitrogenous substances, urea

and uric acid dissolved

in water, are the principal prod-

ucts discharged,
h.

The Endocrine System includes a number

of different glands

located in various parts of the body.

These glands discharge chem-

The ical substances, known as hormones, directly into the blood. hormones cooperate to regulate the metabolic activity of the entire body. The thyroid gland of the neck region, adrenals located near the kidneys, and the islands of Langerhans of the pancreas are They go under the names of typical examples of these organs. ductless glands and organs of internal secretion also. i. The Nervous System is an organization of the nerve cell bodies and their processes in such a way as to receive stimuli, carry sensations, correlate them, and coordinate the activities of the parts of the
body.

By

the function of the sensory portion of the system, the anirelates itself to
it.

mal becomes aware of the environment and

In

vertebrates the principal parts of the system include the brain, spinal
cord, peripheral nerves, autonomic nerves, sense organs,

and ganglia.

METAZOAN ORGANIZATION

111

j. The Reproductive System is an organization of glands, ducts, and accessory structures which function in the reproduction of the species. More discussion of this system is found below.

The body might be thought of as being constructed by relating cells form tissues, tissues to tissues to form organs, organs to organs to form systems, and sj^stems to systems to form the metazoan organism. These will all be studied in more detail in connection with
to cells to

the study of specific animals.

Development

of Sexual Reproduction

Keproduction makes great advances among the metazoans.


simple fundamental process of reproduction by
cell

The
or

division

binary fission has been studied already.

not possible for most metazoan animals, but, in general, this type of animal begins life as a single cell resulting from the fusion of two sex cells,
This
is

one produced by each parent. In some of the colonial Protozoa and also in Sporozoa, as well as possibly in Paramecium, there seems to be the beginning of sexual reproduction. The individuals in a colony by peculiarities in cell division become differentiated into

two types:
reproduction

(a) the ordinary, nutritive individuals,


is fission
:

whose means of

and

(b)

reproductive individuals or gametes


inactive macrogametes

of two forms

the large,

egglike,

and the

smaller, motile microgametes.


cells unite to

In reproduction these two types of

from which a new colony arises by number of the Sporozoa, both sexual and In a asexual generations occur. The zygotes, which are formed in the sexual phase or generation, produce a number of spores which deform a
single zygote,

repeated divisions.

velop sporozoites (already studied under Plasmodium.). These become nutritive trophozoites and are capable of production of another generation of gametes. Conjugation of Paramecium is also looked upon as a forerunner of sexual reproduction.

In simple Metazoa there are likewise two forms of reproduction


asexual (without sex), including buddijig and fission, and sexual,

which involves the union of two germ (sex) cells, one male and one female. In simple forms like sponges and jellyfish the germ cells arise from general formative interstitial cells between the two primitive germ layers to form temporary gonads. When the germ cells are mature, they break through the wall to the outside of the body. Again, among the simpler metazoans a single individual produces both male and female germ cells. Such an organism is said to be

112

TEXTBOOK OF ZOOLOGY
Most of the types of animals in the

hermaphroditic or monoecious.
hermaphroditic.

phylogenetic scale, up to and including the worms, are normally

Infrequent examples of hermaphrodites occur either normally or


occasionally abnormally here

and there among the higher groups of

metazoans, even in man.

In higher forms the usual method of reproduction involves germ


cells

produced by two individuals.

Each

cell is either

male or female,
to be

the gonads of the other sex having degenerated in that individual.

The sexes are separate under such conditions and are said
dioecious.

There are some forms, particularly

insects, in

which

it is

possible

for the unfertilized q^ cell to develop without union with another

germ

cell.

This

is

known
lice,

as parthenogenesis.

The

case of the ordi-

is a good exIn the spring an egg which was fertilized and laid the previous fall hatches to produce an individual known as a stem-mother. This individual feeds on the sap of the particular plant on which she

nary aphids or plant


ample.

known

to every gardener,

lives

and grows

to maturity.

Instead of mating (there are no males

in her generation)

she produces a series of eggs

(macrogametes)

which continue

to develop

without union with a sperm (male germ

cell). Another generation of female aphids arises from these eggs which in turn reproduce in a similar manner. A series of female generations appears in succession during the summer. No males are produced until the last generation of the season, and this time there are both males and females. These mate, the females lay fertilized eggs which pass through the winter and hatch as the first generation next spring. These individuals are the stem-mothers for the new season. Some authors speak of this process as "virgin birth." The honey bee queen can control her offspring to some degree. If her eggs are not fertilized, the offspring are all males (drones). If the eggs are fertilized, as most of them are, only females are produced, these becoming queens if fed abundantly on proper food or workers if fed otherwise. In regard to this state of affairs Lane puts it this way, "So it comes about, that though a drone bee may become the father of thousands of daughters, he never has a son, nor did he himself have a father."

The eggs
sea urchins,

of a

number

of animals, such as frogs, molluscs,


artificially

worms,

and others have been

stimulated to continue

METAZOAN ORGANIZATION
development by application of chemical,
agents.
electrical, or

113

mechanical

This goes under the


is

name

of artificial parthenogenesis.
life

Metagenesis

a phenomenon occurring in the

history of a

number

of scattered species of Metazoa, including the coelenterate,

Ohelia; two or three marine worms; and Salpa, the tunicate (a chordate animal).

This process

is

an alternation of production of sexual

individuals in one generation and asexual in the next.


in each case differs

The

offspring

from

its

parents.

This

is

spoken of as alternation

of generation.

studied shortly), there

In Ohelia, a coelenterate related to Hydra (to be is a plantlike, asexual, colonial form, which gives rise to sexual, free-swimming medusae (Fig. 59). The medusae produce eggs and sperms which unite in the water and develop into asexual colonies. Metagenesis really involves two methods of repro-

duction in successive generations of the same species. The significance is somewhat uncertain, but possibly it insures better and more complete distribution of individuals than could be secured

by only the

budding colony.

Many

of the sexually reproducing plants have a

similar alternation of sexual

and asexual generations.

Metazoan and OntogenyOntogeny refers to the development and life history of the individual organism, produced sexually from the union of germ cells or gametes. This process is quite generally similar wherever it
occurs, differing only in detail.

Embryological development

is

an

expression referring to the processes which occur during the earlier portion of the life of the individual.

The male and female germ


mature
state

cells

or gametes are produced in their

respective gonads as previously described.

when they
cells.

are

first differentiated,

They are in a very imand are called pri-

mordial germ

of the germ cells within the gonads, except for the latter part of the process in ova which reaches completion after the cells leave the ovary. It consists of a series of mitotic cell divisions which is

The maturation (gametogenesis) or development


still

occurs while they are

modified at one point to bring about a fusion and subsequent reduction in the number of chromosomes in the cells. In brief maturation
is

the preparation of germ cells for fertilization which may follow. The development of the male germ cell is known as spermatogenesis, and the development of the female germ cell is oogenesis.

114

TEXTBOOK OF ZOOLOGY

Oogenesis begins with the primordial

germ

cell

within the ovary.

These

cells are typically spherical or

oval with a prominent nucleus,


cells of

having the normal number of chromosomes for the somatic


the species.

This number of chromosomes

is

known

as the diploid

number. For purposes of illustration the process will be described for a form whose diploid number of chromosomes is eight. The primordial
cell

divides

by
is

mitosis to

form two oogonia.


cell.

divides similarly.

As

typical of mitotic division, each

Each of these chromosome

divides with the division of the

stitutes the multiplication period of the


Oogenesis
Primordial
_

This series of divisions conmaturation process. In some


Spermatoqemsis
Primordial

qermcell

f^ WC

qerm

cell

J'permafco-

gonia

--Zbromosome
Primary oocyte
Secondary oocyte
f.
i

-(fi\\

\'^(/
v

Primary ipermatocybe
_ Secondary

"\

/
/(

spermatocyte

Nature

ovum
[it.

//T
{

jSpermatid

fj

polar body

Mature
spermatozoon

Zr>d.

polar body^--''
rertili5ed

ovum

(Zygote)

Fig. 47. Maturation of the germ cells. Oogenesis includes the maturation divisions of the female germ cells or ova, and spermatofjenesis is a similar process of division in the development of mature male germ cells or spermatozoa.

instances each of these cells divides once more.

Next, each of these oogonia passes through a growth period without division. During this time the chromosomes in each unite in pairs and fuse together. This fusion is spoken of as synapsis of chromosomes. At the close
of this growth each of these cells is called a
of these oocytes divides as

by

meiosis,

though they were single ones in

primary oocyte. Each the fused chromosomes dividing normal division. This division,

therefore, results in cells with half the somatic (diploid)

number

of

chromosomes and is spoken of as the reduction division. The cytoplasm does not divide equally; nearly all of it goes to one of the

METAZOAN ORGANIZATION
cells in

1]5

each case.
is

This large
first

cell is called

the secondary oocyte and


of these cells has four

the small one

the

polar body.

Each

chromosomes.
the mature

Following this the secondary oocyte divides to form


polar body.
Occasionally the
first

ovum and another


away

polar body divides, but none of them have any further significance
after cariying

half of the chromosomes.

They now degenerate,

by the surrounding tissue. The series of divisions and changes following the primary oocyte stage constitute the maturatio7i period of the process. The ovum containing the haploid number of chromosomes is now prepared to unite with a mature spermatozoon in fertilization.
their protoplasm is reabsorbed

and

Spermatogenesis
like oogenesis, is

germ

cells

is completed within the tubules of the testis, and, a series of mitotic cell divisions. The primordial divide by mitosis to form spermatogonia, and this process it

continues just as

does in oogenesis, until the division of the prithe growth period.


is

mary spermatocytes which have developed during

When
and

the primary spermatocytes divide, the division

an equal one

all

of the resulting cells are typical secondary spermatocytes

with the haploid number of chromosomes. These cells divide to form spermatids. Each spermatid then undergoes a change of shape or transformation to form the mature spermatozoa, each with its half
or, in this ease, four chromosomes. The change from spermatid to spermatozoa does not involve a cell division but simply rearrangement. The spermatozoon is a slender, motile cell composed of head, middle piece, and tail. It is now able to swim in fluid and prepared to unite with a mature ovum.

number

The maturation process


tant reasons.
the
cells,

First,
is

there

is very significant for at least two imporduring the fusion and subsequent divisions of given opportunity for variation of the genetic com-

position.

Secondly, the

number

of

chromosomes
it

is

reduced to half

in each
cells to

mature germ

cell,

thereby making

possible for the

germ

unite without doubling the typical

number

of chromosomes

in each

new

generation.

Each

species has a definite

and constant

number

of chromosomes.

Fertilization involves the union of a mature ovum and mature spermatozoon to produce a fertilized ovum or zygote. The spermatozoon SAvims to the q^q and enters it by penetrating the outer membrane which is called the vitelline membrane. For most animals, as soon as one sperm enters an egg, the chemical nature of the vitelline

116

TEXTBOOK OF ZOOLOGY

membrane changes and prevents entrance of others. The head of the sperm carries the nucleus and soon takes the form of a rounded male pronucleus inside the cytoplasm of the egg. The egg nucleus is known as the female pronucleus. The male and female pronuclei finally fuse The to form the fusion nucleus, and the fertilization is complete.
significance of fertilization is largely centered

around two important impetus for the development of an embryo from the egg under most normal circumstances; however, parthenofunctions.
First, it is the

genesis replaces this function in some cases.

Secondly,

it

brings about

Fig. 48. Diagrams showing cleavage in the young embryo of Asterias. 1, Fertilized ess (zygote) 2, two-celled embryo following first cleavage division; S, the four-cell stage the eight-cell stage //, 6, morula 5, the sixteen-cell stage stage (solid) 7, blastula stage (hollow) 8, early gastrula stage (infolding of cell layer at one side) 9, later stage of gastrulation. The infolded layer is the endoderm. (Drawn by T. C. Evans.)
;
;

the
of

means for inheritance of


ancestry.

characteristics

from two different


the
diploid

lines

This union

also

restores

number

of

chromosomes.
Cleavage is a series of mitotic cell divisions beginning in the zygote immediately following its formation. These divisions occur in rapid order with but very little intervening growth, and the
resulting cells adhere to each other in a body.

yolk material

is

In eggs where the scant and evenly distributed, the ensuing cleavage

METAZOAN ORGANIZATION
divisions extend completely

117

through the zygote, forming nearly

equal

cells.

If the

yolk

is

concentrated in one end of the egg, the

divisions of the developing

divisions all of the cells of the

rate that

it

appears as

if

embryo are unequal. During the early body divide at so nearly the same the zygote were being cut with a knife or
This process provides for the rapid in-

cleaver into smaller parts.


crease in the

and growth of the embryo which is necessary before any special parts can be formed. Cleavage will be described more fully in a later chapter under the discussion of the development of the frog.
of cells divisions proceed, a hlastula is formed by the development of a cavity (blastocoele) within the spherical mass of cells, the wall of which is now a single layer. The formation of the blastula, which usually comes at the sixty-four cell stage or later, marks the end

number

As

of cleavage. The blastula stage of an animal like a starfish or a frog resembles somewhat a hollow rubber ball whose wall is made up of a large number of pieces cemented together.

As

cell

divisions continue in the blastula, a gastrula

is

finally

The blastula does not simply increase in circumference, but there comes a time when the wall on one side pushes in (invaginates), finally meeting the wall of cells from the other side. This gradually crowds out the cavity and forms a wall of two layers of cells. The outer layer is known as the ectoderm (outer skin) and represents the portion of the wall of the blastula which has not folded in. The inner layer, or that resulting from the infolding of the wall of the blastula, is called endoderm (inner skin). As division of cells in this wall proceeds and the infolding continues, the two margins of the infolded part come nearer and nearer each other. This gradually encloses an outside space which is lined by the endoderm and represents the primitive digestive tract or archenteron. This is the beginning of the two primitive germ layers, ectoderm and endoderm. In sponges and coelenterates development stops here.
formed.
In higher forms, immediately following gastrulation, a third germ mesoderm (middle skin), is organized from cells usually contributed by one or the other or both of the other germ layers. In some cases it arises as two saclike outgrowths from the endoderm,
layer, the

one on each side in the gastrula. These pouches push into the remains of the blastocoele. In other cases separate cells are shed from ectoderm or endoderm or both, or from an undifferentiated

118

TEXTBOOK OF ZOOLOGY

portion to orgajiize as a distinct layer between the other two. The position of the mesoderm is external to the endoderm and internal to the ectoderm. It nearly encircles the endoderm. Sooner or later a space forms within the mesoderm, causing the outer limb of it to join the ectoderm and the inner to join the endoderm. This cavity
the coelom or future particular parts of the
is

body cavity. From each body are derived.

of the

germ

layers,

The fate of the germ layers is determined as cell division and development continue. The division proceeds at different rates in different regions and at different times resulting in various infoldings, outpushings, and extensions which finally bring about the formation of all parts of the mature individual. The ectoderm gives
rise to the external surface cells or

epidermis of the skin and to nervous system; the mesoderm furnishes the muscles, skeleton, the circulatory system, blood, excretory, and reproductive systems besides nearly all connective tissue and the endoderm produces the internal linings of the digestive tract, respiratory tract, and such
;

outgrowths as the liver and pancreas.

CHAPTEK IX PHYLUM PORIFERA


SPONGES
The name
of this
this,

phylum, Porifera (p6

rif'er a),

means

''pore-

bearers," and

these animals certainly are. This group is thought to be sort of an aberrant type with peculiar relations, but the group is often considered the simplest and lowest type of

Metazoa, notwithstanding the presence of a simple mesoderm which is lacking in Coelenterata. For a long time sponges were thought
to be plants, and it was not until 1857, only a little over ninety years ago, that they were fully acknowledged as animals.

They are
rocks,
etc.,

sessile in habit,

being fastened to piers, pilings,


is

shells,

of locomotion. Most sponges, bath sponges included, live in the sea. There are only a few small fresh-Avater forms. They have tissues but are without organs. The body is in the form of a hollow sac with many canals
life.

for

There

entire lack

making connection between the internal The pores of these canals are essentially mouths. There is only one general exit from the cavity. All sponges have some type of skeletal structure; some possess hard, calcareous, or siliceous spicules, and others have a flexible fiberlike
piercing the walls and

cavity and the outside.

material as a skeleton.

The organization of the sponges is a loose one, and the interdependence of part upon part is not great. An animal with hundreds of mouths cannot be very highly organized. Some authorities show a rather close comparison between sponges and colonial Protozoa. The sponges possess collar cells or choanocytes which are similar to
the cells of the colonial mastigophoran, Proterospongia.

There are workers who hold that sponges may have arisen from ancestor with the choanoflagellate type of colonial Protozoa. For a time sponges themselves were considered colonial Protozoa. The sponges do not have a distinct enteron or digestive cavity, but digestion is entirely intracellular (within cells). The germ layers are not well-established the layer which seems to begin
a

common

like

endoderm develops

into the external layer.


119

The

so-called ecto-

120

TEXTBOOK OF ZOOLOGY
to line the internal cavities

and its function is circuThe middle layer is very poorly differentiated, being hardly more than a matrix, and is hardly recognizable as the mesoderm of the typical triploblastic animal. If the type can be classified according to germ layers, it might be considered a modiBecause of these pecufied diploblastic (two germ layers) form. liarities, some authors have called sponges Mesozoa or Parazoa.

derm comes

lating the water.

Classification

Class Calcispongiae.
acterized

Single,

shallow-water, marine forms, char-

by calcareous

spicules.

There are two orders.

Fig. 49. Glass sponge or Venus's flower basket, Euplectella sp., is probably the most beautiful of the sponges. (Courtesy of General Biological Supply House.)

Order Homocoela.
is

Simplest type, possessing a very thin body wall


cells.

with pores as perforations in individual


lined with choanocytes.

The

internal cavity

Leucosolenia.


PHYLUM PORIFERA
Order Heterocoela.
radial canals.

121
wall.

Moderately
Sponges

complex

Choanoeytes in
siliceous spicules

ScypJia (Grantia).

Class Hyalospongiae.

which possess

with three axes and six rays or a multiple of six. Spicules are white and like spun glass. Often called glass sponges because of
this skeleton.

Venus's flower

'basket.

which have either nontriaxial siliceous spicules or spongin or no skeleton. They have complicated canal systems and are often quite large and brightly colored. A few fresh-water forms are known.
Class Demospongiae.

Forms

Order Tetraxonida.
deep water.
Thenea.

These are ordinarily attached

to the

bottom in

Order Monaxonida. Includes shallow-water, marine forms and one family of fresh-water sponges (Spongillidae). There are less than two dozen fresh-water sponges known in this country. Spongilla, Haliclona, or finger sponge, and Cliona, or boring sponge.
^faiocytes
^

.'*/.;{ 'I

'**-/*

ftatocyiea cong-etfaie to jorm gemmates

inWrgzd^zrnmuka

,'^-*i-

K"'-'^ Mi-

Fig.

50.

Spongillaj

showing reproduction.
House.)

(Courtesy General Biological

Supply

Fresh-Water Sponges
In the southwestern part of the United States, at least in central there are four species of fresh-water sponges: Spongilla
TrocJwspongilla horrida, Asteromeyenia plumosa, and Ephy-

Texas,

fragilis,

datia crater if ormis.

Of the

four,

Spongilla fragilis seems to be

the most abundant in this area.

Most

of

the

colonies of this

of an species are irregular in shape, averaging approximately as 6 inches by 2^ inch in diameter; but there are some as large inches. Usually they are not over 1/4 of an inch in height. Most of the colonies are irregular in shape, but some are cushion-shaped and a few are branched. Most of the large colonies of sponges in this region are dark grey or chocolate brown in color and are found

122

TEXTBOOK OF ZOOLOGY

on logs either floating in the water or submerged. In some parts of the country there is the idea that sponges require clear water,
but in the region referred to they
horrida and Ephydatia crater if ormis)
(particularly Trochospongilla

grow abundantly in muddy ponds and in muddy streams whose turbidity equals 110 parts of In this region again the growth of the solid matter per million. sponge and apparently gemmule formation is a perennial process. The maximum production of gemmules seems to be in the late autumn and throughout the winter, even following periods of low These gemmules are ordinarily detemperature in the spring. posited in a pavementlike layer on the object to which the sponge is attached, sometimes covering several square inches. The species
are usually identified by

means of microscopic

differences in the

gemmule

spicules as seen

when

crushed.

Order Ceratosa.

A group of important sponges of which man uses


The representatives of this order have and are found in subtropical and tropical marine

at least a dozen different ones.

skeletons of spongin

waters.

Euspongia, the bath sponge.

Order Myxospongida.
ton.

These sponges are entirely devoid of

skele-

Haliscara.

THE SIMPLE SPONGE


Scypha coronata*
is

(Ellis

and Solander) has been mistakenly

called

"grantia," the European form, by most textbooks for years.


a commonly studied representative of the phylum.
is also

This

It is available

and

comparatively simple in structure.

It is

not as simple,

however, as Leucosolenia.

Habitat and Behavior


This type lives attached to rocks in relatively shallow marine
water.

The animal
is
is

is

attached by the basal or proximal end; the

A colony may be formed by budding. drawn in through the pores or ostia on the sides of the body, then by way of the canals into the internal cavity. This water is forced up through the cavity and out at the osculum or exit opening at the top. The osculum and ostia maj^ be closed and there
opposite end
free or distal.

Water

A Case of Incorrect Identification American genus is Scypha. M. "W. de benfels. Pasadena, Calif., Science Vol. 85, No. 2199, Feb. 10, 1937, p. 199.

Lau-

PHYLUM

P0R1FEE.A

123

may be contractions of the entire body. These movements are accomplished by individual contractile cells. These reactions may involve the entire body, or they may be local. Laymen and many zoologists think of sponges as sluggish, inactive forms, because they are sessile. On the contrary, these ajiimals work day and night to keep a continuous current of water to supply their vital needs. It is reported that an average sponge will pump approximately forty-five gallons of water through his body in forty-eight hours. Activities and coordination in Scypha and sponges generally are quite limited by lack of a nervous system. Individual cells respond directly to stimuli, and impulses are conducted simply from cell to This results in very slow transmission cell in a primitive fashion. and is called neuroid transmission. of impulses
External Anatomy

The average length of Scypha


It is

is

about three-fourths of an inch.

rather goblet-shaped with the excurrent opening, osculum, at the top. A row of picketlike spicules or spines encircles the osculum,

-oiisS^Wy.i.LV;-:^;-^;.,;^..-,,

Fig. 51.

Scypha coronata (Grantia), showing habit

of

life.

124

TEXTBOOK OF ZOOLOGY

and other less conspicuous spicules are distributed over the body. The ostia are the incurrent pores through which water is taken into the body. They are quite evenly distributed over the wall. A dermal
epithelium covers the outer surface of the animal.

Internal
Internally there
is

Anatomy
is

a large central or gastral cavity which


is

simply

a water cavity and

not comparable to a stomach or enteron.

In

10%
4^ I

Dctfnal

EpiiKctium
.

InkaltAI Canal

kl>Mn

"

11... cu.-

Charr.bcr

DiAfftftm of

Akod Sponge

FlaftlUitd

Oitmbc

(Radul Canal) \

S-'

Caiiral Co.ihcl.um

Stereogram to illuatntc ample Leucen Sponge

Dermal Oatta

^Subdarmal Cavity

Ch^lcnf Cnal

FUstlUied

Cham^r

Diagram
StetcogrKiT) to illustiate

L, S, of rSagon

(Icucon)

type of canal

3truc:ure such 9 occurs

Sycon Sponge

in the

Demospongiae

Fig.

52.

Structure (Courtesy Pacific Biologicalsponges, shown of different types of Laboratories.)

diagrammaCcally.

more complex sponges there may be several or even many such cavities, each one opening distally by an osculum. Communicating with and radiating from this cavity is a set of radial canals. They join the cavity through small pores called apopyles and extend nearly to the outer surface of the wall where they end blindly. Lying between these and extending inward from the ostia are the incurrent canals. They connect with the radial canals by rather numerous
apertures called prosopyles.

This canal system not only serves to

PHYLUM
carry the water, but
animal.
it

PORIFERA

125

substantially increases the surface area of the

This seems to be a definite provision to allow increase in volume by keeping the ratio of surface to volume.
In sponges generally, there are three types of canal systems, identified as the ascon, sycon,

simplest, the second intermediate,

and rliagon types of which the first is the and the third, the most complex. The canal system of Scypha is of the sycon type. The character of the skeleton is a diagnostic feature in the classification of sponges.

Some have

a skeleton of calcareous spicules,

others of siliceous spicules, others of the fibrous spongin,


others have no skeleton.
is

and still Spongin of the ordinary bath sponge, which


silk

simply the skeleton of one of these animals, resembles


It is

chemi-

cally.

formed by some special

cells called spongioblasts.

The

honaxon

Triaxon
.raxon
Fis
53.

Hon ax on
by Joanne Moore.)

Jriradiatc
found
in different sponges.

Types

of calcareous skeletal spicules

(Drawn

spicules are of several tj^pes with a

number

of modifications of each.

The monaxon type


;

consists of simple straight spines; the triradiate

type consists of those that have three rays joining each other in one plane the tetraxon type has four rays radiating from a common point
in four different planes
;

the triaxon type possesses six rays lying in

three axes;

and the poly axon type has numerous rays. The cells which produce spicules are known as sclerohlasts. The histology of Scypha presents a peculiar arrangement of a number of different types of cells. The outer, dermal layer is composed of simple,
flat,

epithelial

cells,

contractile cells

(myocytes),

wtich secrete the substance for anchorage, as well as the sclerohlasts. A great many of the cells in this layer do not have distinct boundaries, making it a syncytium. In Order Ceratosa, the
gland
cells


126

TEXTBOOK OF ZOOLOGY

spongioblasts are located in this layer.


lined with flat

pavement

cells.

The incurrent At each prosopyle there

canals are
is

a single

large dermal cell, a porocyte, which surrounds the aperture. In the middle layer are found the reproductive cells and some amoeboid wandering cells. The radial canals and the internal wall of the central cavity have similar histological structure since the former are The cells here are primarily special outpouchings of the latter. ehoanocytes, peculiar to sponges, interspersed with scatshaped

Jpicule

Sckroblast

Dermal

cells

AirchaeccYbe.
Co/lencyte
-flaciei lum

DertnalceU

collar

Choanoo/ie -I
(Oslhrceli)

Porocyte s^.

Ovum

'4^~

Fig. 54. Histology of wall of a simple sponge in longitudinal section. (Redrawn and modified from Lankester, Treatise on Zoology, published by The Macmillan Company, after Minchin.)

Each choanocyte has at its free margin a funnel or collar opening to the central cavity and a flagellum or whip extending from the funnel. The flagella agitate the water and
tered, flat, epithelial cells.

drive the suspended food particles into the funnellike


the collar cell where a food vacuole
fashion.
is

formed in the

cell in

mouths of amoeboid

Of course the spicules appear in a histological section. The entire arrangement is quite similar to a large colony of semiindependent cells which do not function as integral parts of a tissue as do the cells of higher animals. It has been found that indi-

PHYLUM PORIFERA

127

vidual cells can be separated from each other by squeezing some

types of sponges through the meshes of a silk cloth.


living cells, if kept in favorable

conditions,

From these new sponge will

reorganize.

Metabolism
sponge obtains food from the water which is continually passway of ostia, through the canals and central cavity, and out the osculum. Microorganisms and other particles of organic matter are drawn in with the water. The current is produced by the flagella of the choanocytes and contractility of the walls. It is
ing by
controlled by the contractility of the cells surrounding the ostia. As the current sweeps the potential food particles into the collar cells they are seized and ingested by pseudopodia, according to

At any rate the food particles are taken into the cytoplasm of certain of the cells. Digestion is intracellular (within cells) in the food vacuoles and the process is much the same as has been described in Protozoa. The digested material is assimilated
some authors.

by diffusion from cell to amoeboid wandering cells.

cell.

This

may
is

be augmented by the
carried on

Respiration

by

diffusion

through the general surfaces, and the exchange of gases O2 and CO2, is made with the surrounding water. Catabolism, or the union of oxygen with the fuel substance of the cell to liberate energy, goes on in the cells in some degree as long as they are alive. Excretion is largely by general diffusion through the surfaces, perhaps assisted by the wandering cells. Egestion is probably accomplished

much

as

it is

in

Amoeba.

Reproduction and Life History

Scypha is able to reproduce both asexually and sexually. The former may be by budding or by the formation of gemmules. Budding involves the branching of new individuals from the external surface of an old one. These new individuals finally become free from the parent. Sometimes a colony is formed by the buds remaining attached to the parent. Gemmule formation or internal budding is another type of reproduction, found particularly in fresh-water sponges. Groups of cells become separated from the surrounding deep tissue by limiting membranes, which become infiltrated with siliceous materials. They are usually formed during adverse conditions and can withstand desiccation and other severe

128
circumstances.

TEXTBOOK OF ZOOLOGY
In fresh-water forms these gemmules are formed in

the middle layer of cells; the parent individual then dies; and the

following spring

new

individuals emerge from the gemmules.

Sexual Reproduction occurs here for the first time in our discussions. Sponges are usually hermaphroditic, but the germ cells

Sexual I^eprodaction
Fig. 55.

Methods

of reproduction in Scypha.

(Courtesy General Biological Supply

House.
Osculum breaks thru here
-Dermal epithelium

Qranular cells (Dermal epithelium)


Segmentation cavity
Ragellafced cells

Gasbral epithelium
Gastral cavity (future cloaca)

{Qastral epithelium)

--Dloitopore

Typical f ree-5W/mming

Typical /Amphiblaifcu/a

Awphiblastula
Fig. 56.

at time of attachment
sections of

Diagrammatic

Scypha larvae.

of the male usually mature before those of the female. The reproductive cells are produced in the jellylike middle layer. Fertilization takes place here and cleavage division progresses. At about the blastula stage the embryos are liberated through the wall of the

body as free-swimming, ciliated larvae. These later settle down, become attached, and are modified to form adult, sessile sponges.

PHYLUM PORIFERA
Economic Relations

129

Many

sponges are beneficial to man, and there are a few wliich


Oysters and some other Mollusca are injured or

are detrimental.

destroyed by certain sponges which attach themselves to the mollusc's body or by others which bore through its shell and thus
kill
it.

Of
posits

positive importance, lesser items include the large flint de-

some species and those used for The chief importance lies in the use of the spongin skeletons of certain groups for bath and surgical sponges. With rapid industrial development sponges have also become useful as a fabric material. The demand has brought about the establishment of sponge farms where they are raised from fragments about an inch square or slips, like plants. Selected sponges of good quality are cut up, and the pieces fastened to hooks or wire on a weighted frame. The "seeded" frame is sunk in the ocean and left for a year or two for the sponges to grow. If everything has gone as
siliceous spicules of

from

ornaments.

expected, the slips will then be marketable.

Sponges are usually harvested by diving or hooking between May and October. Dredges which are sometimes used for this purpose are not very satisfactory, because they drag down and kill so many young sponges. Sponges die quickly when taken. They are allowed to rot for a day or two, then beaten and squeezed under water, washed, dried, and sometimes bleached. Next they are trimmed, sorted, and placed on the market. The value of the crop each year is at least $5,000,000. Most of the commercial sponges come from the Mediterrajiean Sea, Red Sea, coasts of Florida, the Bahamas, the West Indies, and Central America.
Phylog-enetic Advances of Sponges Wlien

Compared With Protozoa

flagellata of the

This group appears to be somewhat advanced over the ChoanoGenus Proterospongia in Class Mastigophora. In
cells are partially

sponges the

entiated for separate functions.

here in a simple way.


else

organized into layers and are differSexual reproduction is developed The group has advanced so little that little

can be said.

CHAPTER X

PHYLUM COELENTERATA
The phylum name, Coelenterata (sel eu ter a'ta), means "hollow intestine," and all of the representatives bear this out by possessing a single large cavity in the body.
this cavity,

There

is

a single opening to
anus.

and

it

functions as both
;

mouth and

There are

two general types of coelenterates the polyp form and the jellyfish form. They are all modified gastrulas, have radial symmetry, and possess tentacles with "sting bodies" or nematocysts. Most of the The species are marine, but there are a few fresh-water forms. body wall is composed of two layers of cells, and for that reason they are said to be diploMastic. These two layers are the outer ectoderm and inner endoderm. Most of the representatives do not develop skeletal structures, but coral polyps produce hard, calcareous cases around themselves.
In several species there
is

the

typical alternation of generations of attached

and free-living forms.


at least a

Most coelenterates are attached or very sedentary for


part of the
life

span.

The radial symmetry is correlated with an attached habit of life. good many of the attached forms look much like plants and were

so described for a long time.

The digestive process

is

principally extracellular, being accom-

plished by enzymes which are secreted by special cells of the endoderm


into the internal or gastrov oscular cavity.
digestion, however, takes place within the
ticles of partially

limited

amount
by
these

of the

endoderm

cells after

parcells.

digested food have been engulfed

This

is

called intracellular digestion.

Excretion and respiration are

carried on
tion
is

by the general surfaces

of the body.

Asexual reproduc-

accomplished by budding and fission. Sexual reproduction, involving production of ova and spermatozoa and their union in
fertilization, occurs here too.

The group

is

considered

among

the simplest of metazoans and

shows, in a simple way, typical features of this great division of


the animal kingdom.

Hydra

will be studied in detail, because it is


130

PHYLUM COELENTERATA
readily available, easily collected and handled, and
of multicellular animals of simple formation.
is

131
representative

The study of Hydra

as a simple metazoan will go far in giving insight into the much more complex make-up of the body and life of man.
Classification of the

Phylum

The phylum
orders.

is

divided into three classes, each with three or four

Class Hydrozoa. These are typical polyp forms, many of which produce medusae forms by budding. The group includes marine,
colonial polyps, or hydroids, floating colonial hydroids, such as Portuguese man-of-war, one special gro"up of corals, some smaller
jellyfishes,

and the fresh-water polyps.

Fig. 57.

m, mouth stomach;

mn, manubrium Structure of Gonionemus.


;

n,

t,

tentacle; ve, velum.

ad. Adhesive pad li, lithocyst g, gonads nematocyst ra, radial canal re, ring canal st, (From "White, General Biology.)
; ; ; ; ;

stage.

Order Leptolina a group which has a sedentary or sessile polyp Such examples as Hydra, Ohelia, Gonionemus, Canipanularia, Tubularia, and Craspedacusta are well-known forms. The first one is a fresh-water polyp form and is the best known of the group. The last one listed is a fresh-water form with a small polyp stage lacking tentacles but with a disclike medusa possessing many tentacles. Hydra, of this order, will be discussed as a general representative of the phylum, but since Gonionemus and Ohelia are common marine forms, a brief description of them may be included

here.

132

TEXTBOOK OF ZOOLOGY

Gonionemus is a small jellyfish form, measuring about a centimeter across, and is found in the pelagic waters, along our eastern shores. Its shape reminds one somewhat of an umbrella with a fancy fringe but with practically no handle and made of clear cellophane.

The exumhrella
is

is

the convex upper, or aboral side while the

suhumhrella
its distal
is

the concave, lower, oral side.

short stalklike part,

the manuhiHiim hangs

down from

the center of the subumbrella.

end

is

the mouth, bordered

by four oral lobes.

At The mouth

the aperture leading into the internal or gastrovascular cavity

which has four radial branches or canals. These radial canals join a circumferential or marginal or ring canal. A circular ledge or fold of tissue which extends inward from the margin of the subumbrella and partially encloses this saucer-shaped cavity, is called the velum (craspedon). From a few to more than eighty almost solid tentacles hang down from the margin of the subumbrella. The cell structure of this animal is made up of an outer ectoderm and an inner endoderm, with a large amount of jellylike mcsoglea between these two genn layers. Wa\y, leaflike folds hanging in the subumbrella and radiating from manubrium to margin are the gonads. A planula-like hydroid form develops from the egg. The animal is able to swim about by drawing water into the partially enclosed cavity of the subumbrella and expelling it through the aperture formed by the velum with enough force to move the animal in the opposite direction. The pressure is developed by contraction of the body.
Obelia
colony,
is

a marine, colonial type resembling a branched plant in

appearance.

The individuals are attached


it is

to

each other in the

and

fastened to a rock or other substratum by a root-

like hydrorhiza.

They are

distributed in the Atlantic Ocean

and

Gulf of Mexico out to forty fathoms in depth. The colony begins as a single individual which buds, but they do not separate from the preceding or parent generation. This may continue for several generations. From the hydrorhiza there is an upright stem, the hydrocaidus. This stemlike part gives off lateral branches, hydranths; at the end of each is a mouth and tentacles. These are feeding
polyps.

Also as branches of the stem, there are the hlastostyles which

are modified, nonfeeding polyps capable of producing medusae.

The

medusa
colony.

is

the third type of individual connected with an Obelia


perisarc,
is

The

which

is

In some parts this

ringed,

composed of chitin, covers the colony. and it expands at the base of the

PHYLUM COELENTERATA

133

Obelia habit

Mouth
tb/drothecaCoelenteron

''

hntoderm tctoderm

Qonotheca
MedU5Q-bud
B\a5bo5t\/ie -

Radiol canal ^^

Repiroductive

Moubh
Jcatocyjfc
Tenbacles-

7^^-tirf^

Kedusa
Fig.
58.

Obelia

Obelia, hydrozoan colonial coelenterate, showing: asexual generation, sexual generation (medusa), structure, and habit of life. (Courtesy of General Biological Supply House.)

134

TEXTBOOK OF ZOOLOGY

to form a bowllike case or hydrotheca which supports it. Another modification is the taller, more enclosed case, gonotheca, which nearly encloses the Mastostyle. The blastostyle with this coverFibrous processes connect the ing is often called the gonangium.

hydranth

perisarc to the soft,

inner parts (coenosarc).

The cavity
is,

of the
there-

hydranth

is

continuous with that of the hydrocaulus, and

fore, a part of the gastrovascular cavity.

Medusa

JJ

e x Sperm -from

another

medusa

^__ ..Ferbilucd eqq


\*"
"^

Mature,
qonancjlcim.^^g

^ cell stage.

^"^
\

*;>

Cleavacfe

BJastula.

^ \arva

mature

Position of colony
Fig. 59. Life cycle of Ohelia, illustrating polymorphism and metagenesis. Adult hydroid colony with mature gonangium gives rise to sexual medusa which is produced in the gonangium and set free in the water. Germ cells produced by the medusae complete the cycle. Blastula and planula are free-swimming. (Redrawn and modified from Wolcott, Animal Biology, published by McGraw-Hill Book

Company,

Inc.)

The coenosarc

is

made up

of an outer layer of cells, the ectoderm,

and an inner endoderm layer lining the cavity. The mouth of the hydranth is located in a domelike hypostome at the free end. There are between twenty and thirty solid tentacles attached around the basal margin of the hypostome. The hydranth captures and ingests small aquatic organisms as food by the aid of stinging bodies (nematocysts) produced in certain ectoderm cells of the distal portions of the tentacles. The digestion of this food is accomplished
just beneath the perisarc,

PHYLUM COELENTERATA
in the internal cavity.
esses, a single

135

With

the exception of reproductive procwill be

hydranth of Obelia

found similar

to

an entire

hydra, to be studied soon.

The reproductive cycle is both sexual and asexual, alternating between the sexually produced polyp or hydroid generation and the asexually produced sexual generation, the medusa or jellj'fish form. The medusae arise as buds from the special individuals, blastostyles, escape through the distal pores, and develop to sexual maturity as free-swimming individuals. The sexes of these are separate; some produce eggs, and others, spermatozoa, which are discharged into the water at maturity and unite to form zygotes.

Fig. 60. Diagram of a siphonophore colony (Physophorida) A. Pneuniatophore B, C, swimming bells D, protective zooid E, sporosac F. G, dactylozooids H, feeding polyps (gastrozooids) (From Van Cleave, Invertebrate /, nettling cells. Zoology, published by McGraw-Hill Book Company, after Claus.)
.

The zygote develops into the free-swimming, ciliated planula stage which soon attaches and develops into a polyp from which a new colony arises. After producing a generation of medusae, this colony disintegrates, and after producing germ cells, the medusae die. This
process,

involving alternation of generation,

is

described as meta-

genesis in Chapter VIII.

Obelia presents a very good example of metagenesis as represented


in animals.
to distinguish

The medusae them from

of this sort are spoken of as

hydromedusae

the scyphomedusae or jellyfishes of Class

Scyphozoa.


136

TEXTBOOK OF ZOOLOGY

Order Trachylina. This order includes two suborders of hydromedusae which come from the egg- directly with no polyp stage. Campanella and Liriope are generic examples. Order Hydrocorallina. This group resembles the corals by producing strong calcareous skeletons. They have extensive, branched hydrorhiza and powerful nemato cysts (stinging 'bodies). Rudimentary

medusalike bodies develop on the coenosarcal canals. Millepora, the staghorn or stinging coral, as it is called, is a good example.

Fig.

61.

(From Hegner,

Physalia, the Portuguese man-of-war a floating colonial coelenterate. College Zoology, published by The Macmillan Company.)

Order Siphonophora.

This

is

a pelagic order of colonial coelen-

terates with extreme polymorphism.

common

tube of the coenosarc

unites the five kinds of individuals of the colony,

continuous from one individual to another.


coenosarcal tube
is

and this cavity is The blind end of the

an

air-filled,

bladderlike float (pneumatopJiore)

PHYLUM COELENTERATA
with a superior
this float.
crest.

137
into the water beneath

The polyps hang down


of polyps include
:

The types

gastrozooids (nutritive or

feeding), dactylozooids -with nests of nematocysts and having long


tentacles (tactile

ductive zooids,

and protective), gonozooids which are male, reproand others which produce ova-bearing medusae. Swimoccur just below the pneumatophore.
to

ming

bells (nectocalyces) often

Most of the individuals are specialized


care for only limited functions.

such a degree that they

This specialization and diversity of

forms
is

Physalia, the Portuguese man-of-war,


;

such that the entire colony appears as a single individual. is a typical example. Its sting quite poisonous bathers coming in contact with the trailing tenis

tacles,

which bear batteries of nematocysts, suffer severe pain.


Scyphozoa.

The coelenterates belonging here are large having an alternation of generation in which the medusa form is dominant. The scyphomedusa has an eight-notched margin, lacks the velum (therefore acraspedote), and has gonads connected with the endoderm. The polyps have four longitudinal endodermal folds, called taeniolae, which form gastral tentacles or filaments in the medusa. These jellyfish have a complex system of branched radial canals and abundant marginal tentacles as well as oral tentacles.
Class
jellyfishes

Several of the representatives of this class are thought by some


zoologists to exist generation after generation only as medusae, but

may be that the polyp form has not been discovered yet, if it exists. There are records of individuals of this group twelve feet in diameter, and possessing tentacles one hundred feet in length.
it

Order Stauromedusae. Conical or vase-shaped medusae which usumarginal sense bodies (tentaculocysts). The tentacles are distributed perradially and interradially. Lucernaria and Haliclystus
ally lack

are usually cited as examples.

Order Peromedusae. These are cup-shaped, free-swimming forms with four interradial tentaculocysts. The tentacles are adradial and
perradial.

They occur

in the open sea.

Pericolpa and Periphylla.

Order Cubomedusac. Forms which have rather cubical shape, four perradial tentaculocysts, interradial tentacles, and are chiefly tropical.

Charyhdea

is

an example.

Order Discomedusae. Scyphozoa whose medusae are dominant, saucer-shaped, and almost transparent. Some of them are more than seven feet in diameter. There are usually eight or more tentaculocysts perradially and interradially distributed on the margin

138
of the bell.

TEXTBOOK OF ZOOLOGY
Tentacles are usually present also on the margin of is the most numerous and extensively distributed

the bell.

This

group of Scyphozoa. Aurellia and Stomolophus are common examples.


the typical example, and, like most jellyfishes, is composed largely of water. When they are dried, only a thin film

Aurellia*

is

remains.

This

is

the Gulf of Mexico.

common one and ranges from New England It may reach a foot in diameter.
Cut surface
of

to

body wall

Stomach

Qonad
^

Jub-gsnital pit

Upper portion of
-^r

manubrium
Lateral mouth

Radial canal

- -Sub-umbrella space

Grtutor
muscle

Manubrium
(cut surface)

Central

mouth

Oral tentacles

Fig. 62.-

-Cabbage-head

jellyfish,

Stomolophus meleagris, a very common form


Bisected to show internal structure.

in

the Gulf of Mexico.

The animal has no velum as do the hydromedusae, but there is a square mouth on the subumbrellar side with wing-shaped, liplike
oral lohes or arms.
side of the animal.

suhgenital pit

lies in

each quadrant of this

The mouth leads through a short passageway


There
is

into the angular gastrovascular cavity which in turn has four lateral
gastric pouches containing the fringelike gonads.
also a

row of small gastric filaments here carrying nematocysts. A large number of branching radial canals extend from the gastrovascular cavity out to the margin of the bell, there joining a circumferential
This spelling is according to Mayer's monograph, proposed by Peronas Le Sueur was so spelled.

The generic name

flrst

PHYLUM COELENTERATA
-Chrysaora)

139

(in

Lonq tentacles Chryiaoro)

Larqe subqenibal pit (as

in

TentaculocyJt

Admdial canal
Perradial canal
Jhort5f'mp/e oral
-

arm

(Aurelia)

i^-:-

: lIfJ&;^-# .

-^^

>-:,

-Intenadial canal

.'iv.

*.

-iy

:T*L^5=f'^^^"""V

Admdial canal

vV-i^^:?-----^'"'?^"^'
'S^_]_; '

j
)

V^ _ _ Aon(7 ribboh-like oral arm (in Chrysaom) "^


-

/^C"""^

'Small 6iibgenibal p/fc (aj /n Aurelia)

^
'^

Qastric pouch Short tentacles


(Aurelia)

'^

Marqinal lobes (as in Chrysaora)


Fig. 63.

-Aurellia

and comparative structure


in

Pacific

of jellyflshes. (Modified from figure Biological Laboratories' catalogue.)

Stages

in

the

development
of the scyphis

Stages in the -development


^of the jbrobila

toma

Planula

larva

MM ^^
Sperm from separate adult
Zygote

Fig.

64.

Life

cycle of Aurellia

aurita, showing staj^es from germ ephyra stage which precedes the adult condition.

cells

to

the

140
canal.

TEXTBOOK OF ZOOLOGY

The eight tentaculocysts are symmetrically located at eight The tentaculocysts are sense organs of equilibrium. The pigment spot over each is likely sensitive to light. Near it is the olfactory pit. Reproduction involves both sexual and asexual generations. Germ cells are produced by the pinkish gonads in the gastric pouches, and they pass out through the mouth with the water. Fertilization takes place, and the egg develops into a free-swimming plajiula which after attachment becomes a tubelike polyp that reproduces by budding most of the season. Then the polyps form medusae by strohilization, i.e., constrictions are formed around the body making it resemble a stack of saucers the upper one periodically frees itself and swims away. The polyp with all of these constrictions is known as a strohila, and the new medusa is called an epliyra.
points ou the margin, each between marginal lappets.
;

animals in this class conform to polyp orThey have an ectodermal esophagus and longitudinal partitions called septa (mesenteries) incompletely dividing the gastrovascular cavity. Muscular
Class Anthozoa.

All

ganization and

may

be colonial or solitary.

bands are found in the septa. The mesogloea is quite abundant and contains a good many cells that resemble primitive connective tissue cells. Many of these animals produce a calcareous external skeleton called coral. Both sexual and asexual reproductissue

tion are

common.

Subclass Zoantharia.
It includes sea

This

typically occurring in multiples of six,

group has numerous paired septa, and plain tubular tentacles.

anemones and

corals.
;

Order Actinaria. These anemones are usually solitary polyps they have many complete septa and numerous tentacles but no skeleton. Sagartia, Cerianthus, and Bletridmm are common examples.

Metridium usually

lives attached to rocks or to solid bodies in

They average about three and two or two and a half inches in diameter. The free end of the jar-shaped body is covered with tentacles which are provided with nematocysts. The entire body can be expanded and contracted, and it can change its location by "seooching" on its lasal disc (attachM end). The mouth is located in the center of the crown, and food is forced into it and on through the gullet (stomodeum) bj^ action of cilia on the tentacles and part of the lining of the mouth. At each side of the gullet is usually a
the water near shore, even in tide pools.
or four inches in height

PHYLUM COELENTERATA
ciliated groove, the siphonoglyphe,

141
is

through which water

constantly

carried into the gastrovascular cavity for respiratory purposes. The

gastrovascular cavity is divided into radially arranged compartments by the primary septa or mesenteries which extend from the wall of the gullet to the inside of the body wall. The primary septa in the axis of and extending parallel with the siphonoglyphes are called directives. At the basal end these cavities are continuous

Fig. 65.

Sea

anemone, Metridium inarginatum, showing external features.

with the main central cavity. Between the primary septa are secondaries which do not quite reach the wall of the gullet, hence their medial ends are free in the cavity. Between these and the primaries are some tertiary septa which are still shorter ajid also attached to the inner surface of the body wall. A quarternary set is represented by mere ridges on the inner surface of the wall and
interspersed among the others. There is a band of muscle running vertically on the face of each septum next to the muscle on the adjacent septa of the same rank. Below the gullet the mesentery has secretory filaments which in turn bear long, threadlike acontia.
is

142

TEXTBOOK OF ZOOLOGY

These protrude through pores (cinclides) in the body wall to the outside, and they are supplied with nematocysts and secretory cells.

They

serve as defensive as well as offensive structures.

Asexual reproduction by budding from the margin of the basal Occasional longitudinal fission may disc is practiced by this animal. occur. The gonads develop on the edges of the lower part of the
septa to provide for sexual reproduction.

The sexes are

distinct.

Cinclade, with

Aconlium protruding

SlereogTam of Anthozoan Polyp

MeKtilcrie fiUment,

EnJoceet, cKamter

between
,

two metenteriei
the

of

ume

pair

"-^^

Cxaeoel. cKsmber

between pair* ^

Hollcw
Tentacle

o( meicnteriei

'

Vf)i.tfsl

^'

Directive*

Siphonogtyph
SpSincler

(Ventral bet of Primary Meccnteriea)

Diagrammatic T.

S. of

Anthozoan Polyp

fi\

level

A-A
Otagrammatic

S. of

Anthozoan Polyp

at level

B-B

Fig. 66.

Diagrams

to

show the structure

of the anthozoan, Metridium, of Pacific Biological Laboratories.)

(Courtesy

Mature ova and spermia are discharged into tlie water of tlie cavity and escape through the mouth to unite in fertilization outside. The development includes cleavage and planula stages, before the new individual attaches and changes form. Order Madreporaria. The representatives of this order secrete an The indiexternal limestone skeleton most of them are colonial. viduals of colonies communicate with each other by coenosarcal connections. Otherwise they are similar to anemones. Astrangia, Madrepora, and Oculina are examples.

PHYLUM COELENTERATA
Astrangia
secretion
is

143
it is

the

common

coral polyp,

and

quite similar to a

small sea anemone to which calcium carbonate has been added by

from the ectoderm

cells as well as

having budded to form


In time

a colony of numerous individuals.

Coral poljqDS vary in size from

one-sixteenth of an inch to several inches in diameter.

continually growing colonies of these animals can produce enormous

stony barriers (reefs) in the


in length

sea.

One such

reef

is

over 1,100 miles

and from ten

to twenty-five

fathoms deep.

Many

corals

are of beautiful colors.

Order Antipathidea. An order composed of branching colonies whose individuals are joined by a branched tubular axis which is covered by an epidermal layer. Cirripathes and Antipathes are typical examples.

srri,.^^.^
Trv^.-

^*

A
Fig.
67.

"^^^
coral,

Common

when cleaned

Astrangia danae. A, Stone produced by the animals B, polyps in natural habitat.


;

Subclass Alcyonaria.

The features of this division include


uncommon.

eight

hollow, feathered tentacles, eight mesenteries,

and one siphonoglyphe.

Colonial and pol^'morphic forms are not

Order Alcyonacea. A colonial group which has calcareous spicules but lacks an axial rod. Body walls of individuals fuse together as one. Alcyoninm is the type example. Organ pipe coral belongs in
this order.

Order Oorgonacea. This is another colonial coral which is sessile and has a calcareous axial rod. The colonies are bilaterally symmetrical. The common sea fan, Gorgonia, as well as the precious Corallium rubrum are well known examples. Order Pennatidacea.
fied so that

Another
is

colonial

one portion

submerged

in the substratum.

form whose body is modiThe colony

144
takes a bilateral form,

TEXTBOOK OF ZOOLOGY

stem which

and the individuals are born on a disc or axial supported by a hard skeleton. There may be dimorphism of zooids within the colony. Renilla and Pennatula, sea pens
is

and sea

feathers, are typical examples.

HYDRA
Habitat and Behavior

Hydra (Chlorohydra)
of the
active

viridissima
is

is

likely the

most common hydra


is

Southwest.

It

the

small green hydra which

very

and has short

tentacles.

This species has the green color

because of the presence of a unicellular alga, Chlorella vulgaris, in

endoderm cells. The plant uses some of the by-products of metabolism of the hydra, and the hydra benefits by receiving oxygen from the photosynthesis of the alga. This kind of a relationship is
the
called symbiosis.

Most of the hj^dras are found


of the water.

in cool fresh water, attached to the

surface of plant leaves, smooth sticks, debris, or even the surface film

The brown hydras, such as H. americana, H. carnea, and Pelmatohydra oligactis, are sluggish and have longer tentacles
than the green ones.*

Hydra
good.

is

a sedentary kind of animal


if

and may remain stationary

for a considerable period of time

living conditions are uniformly

the environmental conditions are changing, and the need of food, it becomes quite active, moving about from place to place. It keeps the tentacles extended, ready to grasp any food which may come into its reach. Nematocysts or sting bodies are discharged when the tentacle comes in contact with potential food, and if it chances to be a small animal, it will likely be paralyzed by the toxin which is injected by the nematocysts. The prey is then carried to the mouth and tucked into it by the tentacles. Frequently hydra is able to stretch its body over articles of food which are actually larger than the hydra usually is in normal condition. Hydra will eat only when it is hungry and will not react to food at other times. It is more sensible than many people in this respect. On the other hand, it has been authentically reported that a hungry hydra will perform the characteristic feed-

When
is

animal

in

found

American Microscopical

Recent taxonomic information concerning Hydras of the United States may be in the papers of Libbie H. Hyman, published in tlie Transactions of the
Society, Vols. 48, 49,

and

50.

PHYLUM COELENTERATA
ing movements

145

Thus
to

when only beef extract is in solution in tlie water. responds to a chemical stimulus alone, but it will not respond a mechanical stimulus only.
it

These animals show response to a number of environmental conAny sudden change is likely to bring about a negative response. If the stimulus is of a general nature and of considerable
ditions.

Fig-. 68. Locomotion in hydra. Successive positions taken when progressing by somersaults. (From Jennings, Behavior of the Loiuer Organisms, published by The Columbia University Press.)

and the body and is not too strong, the animal will contract in the affected area, by the withdrawal of one tentacle. The movements of the animal are performed by contraction and relaxation of the contractile fibers connected with certain of the cells. The activities come in response
intensity, the
also.

animal will contract

all

of the tentacles

If the stimulus is restricted to one locality

to internal as well as external stimuli.

146

TEXTBOOK OF ZOOLOGY

present in hydras.

The common tropisms, which have been described previously, are They respond to light and will find an optimum Green hydras intensity which varies with the different species. react positively to sunlight and withstand moderate temperature; hence they are adapted to the Southwest. They likewise possess an optimum for temperature and prefer relatively cool water. They seem not to become particularly uncomfortable until the temperature gets up to 31 C. then they attempt to find lower temperature. As the temperature is lowered on them, they simply become less and less active and finally cease to move as the freezing point is approached. As pointed out previously, both chemotropisms and
;

thigmotropism are concerned in food-taking. Contact stimuli are of considerable significance in a sedentary animal like this. It remains attached in contact with some solid body most of the time. Sudden mechanical stimulation like stirring the water or jarring the attachment of the animal will cause it to contract vigorously.
Locoynotion is accomplished in at least four ways. Gliding from one point to another by partially releasing the basal disc and slipping it to a new location is common. Or the animal may bend over and cling to the substratum by the tentacles, release the basal disc, then draw the body toward this point, where the basal disc is reattached. This process is consecutively repeated and is called "loopOccasionally the animal bends over, holds by the tentacles, then turns a "handspring" or "somersault" to attach the basal disc on the substratum beyond this point. The fourth means by which locomotion is effected is by dropping to the bottom, then secreting a

ing."

bubble of gas at the basal disc and floating back to the top on that.

External Anatomy

Hydra
is

is

a macroscopic animal, but

it is

relatively small.

Its

body

quite contractile, being able to extend from a contracted length

two or three millimeters to a length of eighteen or twenty millimeters. The column or body is a tubular, cylindrical trunk which ordinarily stands in a vertical position. In some forms the distal (free, oral, or anterior) end of the column is much stouter than the proximal (attached, aboral, or posterior) end, but in H. viridissima there is only a slight tapering toward the basal end. Attached around the free end of the column is a circlet of from four to seven Tentacles may fingerlike tentacles, which extends free in the water.
of

PHYLUM COELENTERATA

147

Stretch out to be slender threads five to seven centimeters in length.

group in capturing and The mouth is located in the center of the distal end of the column and is surrounded by the tentacles. This conical elevation between the bases of the tentacles in which the mouth is located is called the hypostome. The mouth when closed and viewed from the top looks something like an asterisk.
either singly or as a

They are very useful

delivering food to the mouth.

Mouth
Tentacle

Hypostome
Battery of

Nematocysts

Bud

Basal Disc

Fig.

69.

Hydra

showing external features.

appears simply as an indentation or notch in the The proximal or attached end terminates in a 'basal disc or foot, which secretes an adhesive substance which helps the animal in attaching to objects. From one to several luds are often found on the sides of the trunk, and these occasionthe side
it

From

conical end of the hypostome.

ally bear

buds before the

first is

separated from the original parent.

Buds

are lateral outgrowths of the column

and are found when the

animal has favorable living conditions. Budding usually occurs at about the middle of the body in H. viridissima. Occasionally there

may

be observed rounded projections on the side of the column which

148

TEXTBOOK OF ZOOLOGY Both ovaries (female gonads) and

are seasonal reproductive organs.


testes

be formed on a single individual, but they are usually seen on separate individuals. If these projections are conical and located nearer the tentacles, they are testes or sper-

(male gonads)

may

they are more nearly knoblike and are located nearer the base, they are ovaries. This animal possesses radial symmetry, but it is arranged with an axis of polarity from basal disc to hypostome, which is essentially equivalent to what is called the ventroAll of the metazoans have a dorsal axis of more advanced forms.
maries;
if

primary axis. Sedentary and sessile animals very commonly have radial symmetry, while the motile or free-living organisms tend toward bilateral symmetry.
Internal

Anatomy

Another feature of the organization of this animal is the diploblastic structure which consists of two layers of cells or the germ layers surrounding an internal space, the gastrov oscular cavity or enteron. These are studied on stained sections. The outer one is the ectoderm, which is thinner and is composed of four types of cells. The most numerous ones are typically cuboidal in shape and serve both as contractile units and as the general external surface of the body; they are appropriately called epitheliomuscular cells. Each of these cells consists of a polyhedral outer or epithelial portion and a basal portion which is drawn into one or two long, slender, fibrils which extend in a direction parallel to the length of the animal.
These
cells contract to

shorten the length of the animal.


or nettle

Interspersed

occasionally
tributed over

among
all

these cells are the larger cnidohlasts in which


cells
cells.

develop the neniato cysts, stinging


the

These are

dis-

body except the basal

disc,

but they are

much

more numerous near the distal part of the column and on the tenThe nematocysts are usually contained in little raised tacles. tubercles in the ectoderm. Each tubercle contains a large barbed one and several of a smaller variety. Four different kinds have been described. Since the large barbed type is the most conspicuous, it will
be described here.
sac of fluid within

which

In the cnidoblast the nematocyst appears as a is inverted a barbed stalk with a coiled

thread attached.
cnidoblast
is

Projecting out of the superficial surface of the

a triggerlike process called the cnidocil, which

when

chemically stimulated causes the cnidoblast to discharge the nemato-

PHYLUM COELENTERATA
cyst.

149

Chemicals, such as weak iodine, acetic acid, or methyl ^een,


to the water, will bring this about.

when added

Contact will not.

In this reaction the stalk and thread are everted, probably by de-

velopment of pressure.
another form the sac
elastic
;

This type of nematocyst produces a hypnoit is

toxin which anesthetizes the animals into which


is

discharged.

In
is

small, the stalk

is

barbless,

and the thread


it

becomes coiled around the object against which charged, and thus impedes locomotion of the victim.
it

is

dis-

Cnidocil

Filament
Nemaboo/st

Nucleus

a._:il1

6orb__. Stalk--

Bag-

Remains of
Cnidoblasb

Barbless nemafcocyifc

Fig. 70. Nematocysts and their function. A, Cnidoblast containing an undischarged nematocyst, after Schneider B, nematocyst everted and extended but still held in the cnidoblast, after Schneider; C. portion of tentacle, after Jennings; D, insect larva attacked by hydra, after Jennings E, leg of small aquatic insect with barbless nematocysts on its spines, after Toppe. (Redrawn and modified from
; ;

Wolcott, Animal Biology, McGraw-Hill

Book Company,

Inc.)

The cnidoblasts are produced by a third type of cell, the interstitial which is small and rounded. These are formative cells about the size of the nuclei of the epitheliomuscular cells and quite densely granular in nature. They crowd in between the other cells, especially
cell,

near their bases.


blast
is

As

nematocyst

is

discharged, the entire cnido-

replaced by an interstitial
or spent cell of the

cell

migrating into position,

A
ir-

damaged

body may be replaced from the

inter-

stitial cell.

Besides these three types, there are the scattered,

150

TEXTBOOK OF ZOOLOGY

TEMTACLE

MOUTH
TESTI S

GASXROVASCUl_AR
CAVI TY

ECTODERM
MESOSLOE
EfslDODERM

OVARY

BASAU
Fig. 71.

DISC

Diagrammatic longitudinal layers. typical


cell

section of hydra, showing

mature gonads and

(Drawn by Titus

C. Evans.)

ECTODERM
EPITHEl_IO-

MUSCUL. AR
CELL.

NJTERSTI Tl Al_ CEI_U


I

NEMATOCYST
C N DOBL. AST
I

MESOGUOEA
DIC3ESTIVE

CEUUS
G l_ A IM D CEI_U

Fig.

72.

of hydra. gastrovascular through orthe column (Drawn by Cross-section enteron. cavity

The central space Titus C. Evans.)

is

the

PHYLUM COELENTERATA
regular, slender, neuroepithelial cells

151

by

intercellular processes.

These

cells fit

which are joined into a net between the others and


cells

are either sensory or motor in function, thus receiving external


stimuli

and

also

causing contraction of the contractile

at

proper times.

Beneath the ectoderm and embedding the bases of the cells is a very thin layer of noncellular substance called mesogloea. It is produced by the cell layers and serves as attachment for them, particularly for the fibrils of the epitheliomuscular cells.

In some of the
lines
cells

other coelenterates, this layer


the

is

exceedingly thick and heavy.

The inner, thicker cell layer of the wall is the endoderm which lumen of the gastrovascular cavity. The most conspicuous
cells

here are the nutritive-muscular


tures attached to

which are long, vacuolated structhe mesogloea by fibrils which extend in it parallel

to the circumference of the animal.

By

contraction these cells in-

crease the length of the animal


cells often possess flagella at

by reducing its circumference. These the free margin and at times engulf

particles of partially digested food like an amoeba.

It is seen then, that they serve both as muscles and as digestive cells. Glandular cells are also present in this layer. Being slender, they wedge

is

themselves between the nutritive-muscular cells and secrete what probably a digestive fluid into the gastrovascular cavity. Neuroepithelial

and

interstitial

cells

are

also

interspersed

among

the

other cells of this layer.

The general morphology

of the adult

animal is very similar to the gastrula stage of the developing embryo of more complex metazoans.

Metabolism

The food
small bits

hydra consists of small insect larvae, minute worms, of organic matter in the water, water fleas, and other
of

small Crustacea.

Ingestion of the food has been described already. entering the mouth the morsel of food is moved some distance in the cavity by successive wavelike contractions of the column progressing from distal to proximal. Such serial contractions are usually called 'peristaltic contractions. Here in the upper half of the

Upon down

enteron digestion takes place.

The wall possesses many more of the and the food material disintegrates into smaller particles here in this region. The digestion which occurs here is spoken of as intercellular digestion and is brought about by enzymes produced by the secreting cells of the endoderm. The disgland
cells in

the endoderm,

152
solution of the food

TEXTBOOK OF ZOOLOGY by the enzymes


is

augmented by the churning The flagella present on the nutritive-muscular cells create currents of water which also hurry The dissolved material is presumably absorhed by the the process. cells of the endoderm, and by diffusion the nutrient solution reaches
effect of the contractions of the

body.

the ectoderm cells just outside.

Small particles of the partially

di-

gested substance are engulfed by the free ends of


particles are taken in food vacuoles, and the digestion

many
is

of the

nutritive-muscular cells by virtue of their amoeboid activity.


there just as
in

These

completed

it

is

an amoeba or Paramecium.

This illustrates

something of the primitive organization of hydra as a metazoan.

Fig. 73. Hydra with of meat. (From Curtis

body turned inside out in attempting: to Ingest a piece and Guthrie, Textbook of General Zoology, published by

John Wiley and Sons,

Inc.)

As

into

remembered, this process of converting the digested food an integral part of the protoplasm is known as assimilation. The food is distributed to all parts of the enteron, which extends into the tentacles and buds, by the action of the flagella and by bodily contractions. There is no separate system of transportation
will be

This dissolved material reaches the remote parts of the protoplasm by diffusion through the membranes and protoplasm generally. The gastrovascular cavity has the dual
or circulation of nutriment.

function of digestion and circulation.

PHYLUM COELENTERATA

153

Many

of the animals

will not digest.

used as food have hard skeletal parts that These indigestible portions are ejected from the

cavity through the

mouth by reverse

peristalsis,

and the process

is

known
branes.
is

as egestion.

Eespiration furnishes the necessary exchange of

oxygen and carbon dioxide by diffusion through the plasma memThe dissolved oxygen in the water in which the animal lives
the source of this element.

Catabolism or dissimilation takes place in the protoplasm and


involves the union of oxygen with the substance of the protoplasm

transform potential energy there to kinetic energy and heat. this oxidation there are produced some waste byproducts in solution including urea, uric acid, and water which must be expelled from the body. In hydra this excretion is accomplished by diffusion through the general surface of the body. There is some indication that there may be accumulation of waste products in endoderm cells as cytoplasmic granules, which finally escape through the gastrovascular cavity and mouth. It will be noticed that these phases of metabolism are, in general, very similar to the comparable processes in Protozoa and the same similarity will be noticed when they are compared later with the higher forms of animals, because the protoplasmic requirements are the same in all
to

Accompanying

animals.

The Nervous System and Nervous Conduction


The neuroepithelial cells are distributed among the other cells of germ layers. There is a greater abundance of them on the hypostome, basal disc, and tentacles than along the length of the column. The greatest concentration of these cells is in the hypostome around the mouth, which makes this region in a sense comparable to a primitive brain. These cells all over the body are in contact with each other by means of their processes forming what is called a nerve net. When one sensory cell is stimulated, all of the sensory cells seem to be stimulated in some degree. A sufficiently strong stimulus affecting any sensitive point will stimulate the entire body. This is a definite
the

organized type of nervous system but not a very

efficient one.

Reproduction and Life Cycle


Reproduction is both asexual and sexual. Asexual reproduction accomplished very efiiciently and quite rapidly. This process is essentially reproduction by somatic cell division. Nutrient mate-

is

154
rial

TEXTBOOK OF ZOOLOGY

accumulates at some point near the middle of the column. The first appears as a slight superficial bulge. The cell division at this point is very rapid, involving considerable activity in interstitial cells. This enlargement rapidly increases in size to form a stalk. An extension of the eiateron extends into the bud, which is essentially an outgrowth of the body wall. Tentacles appear as outpushings of ectoderm and endoderm, and in the terminal position a mouth is developed. After the bud has attained some size, a constriction occurs between it and the parent. This closes the enteron between bud and parent, and the bud finally separates to

bud

become a free individual.

Embrvo
?6md.l

tiydra.- <exua.l l^production

Fig. 74.

Methods

of reproduction in hydra.

(Courtesy General Biological Supply

House.)

Sexual Reproduction. During the summer and fall particularly, hydra reproduces sexually. This involves the production, maturation, and union of germ cells. Testes may appear first and ovaries later on the same individual or both gonads may be present at the same time in which case self-fertilization is possible. As a rule,
these animals are hermaphroditic or monoecious as suggested before,

but

it has been reported that individuals of separate sex (dioecious) have been found. The germ cells or gametes develop from interstitial cells which accumulate at a certain place between the ectoderm and endoderm, where they multiply by division to form oogonia in

PHYLUM COELENTERATA
the female gonad

155
All phases of

and spermatogonia

in

the male.

maturation (gametogenesis)

may

be obsei-^^ed in the testis and ovary.

The testis produces large numbers of motile spermatozoa, which when mature emerge periodically from an opening in the tip of the testis and are discharged free in the water. In the ovary a single egg develops at the expense of the other oogonia, which are engulfed bodily and used for food. This one cell grows rapidly, and when

Fig. 75. Development of hydra. 1, Fertilized ovum; Z, two-cell stage; S, blastula stage; i gastrula, showing ectocferm (ec) and endoderm (en) cc, cleavage cavity (blastocoele) m, cyst; p.b., polar bodies. (After Tanreuther, Biological Bulletin, Vol. 14.)
;

mature

it fills

the ovary.

Fertilization

is

accomplished by the en-

trance of spermatozoa through a rupture in the overlying ectoderm

and

cross-fertilization usually prevails.

the mature ovum,

and

this zygote

A single sperm unites with undergoes the total and equal

divisions of cleavage here in place. The process continues until a hollow llastula of many cells is formed. Then follows the formation
of the gastrula by a shedding of cells into the cavity (blastocoele) from the inside of the original layer of cells. These new cells on the inside become organized as an endoderm layer, while the original

156
outer layer
is

TEXTBOOK OF ZOOLOGY

now known

as ectoderm.

Further changes involve the

secretion of the thin mesogloea which seals the two layers together.

In the meanwhile a shell is produced about the outer surface of the embryo, and this encysted body falls from the parent to the bottom. If conditions are favorable for development, it increases in length within the cyst; when it has attained some size it breaks out, after which tentacles and a mouth appear at one end, while the enteron
develops within the endoderm.
This individual steadily grows and

soon attains adult condition. When the zygote is formed in the fall, the embryo does not emerge from the cyst until spring.

Regeneration

As is the case in many invertebrate and a few vertebrate animals, Hydra is able to replace mutilated parts or an entire animal from a
be formed from very small This process is known as regeneration, and while it is not normally a method of reproduction or multiplication, it is of great advantage to the animal. This pheportion of one.
pieces

Complete animals

may

(% mm.

in diameter) of a hydra.

nomenon was first discovered 1744 by Trembly.

in animals

from studies on Hydra

in

Economic Relations of the Phylum


is not worth much in dollars and cents to man number of different ones are used as food by some of the useful fish. The corals are of importance both positively and negatively. Many of them are valuable as ornaments, while the

The

entire group

directly.

large coral reefs are very costly to navigation of marine waters.

Many

corals are quarried for building stone, and in some instances they protect the shore from being washed by the waves.

Phylogenetic Advances of Coelenterates


(1)

Definite

organization

of

diploblastic

condition;

(2)

well(3)

defined gastrovascular cavity with one opening, the mouth;

presence of tentacles with

nematocysts or sting-bodies (5) continuance of sexual reproduction; (6) distinct radial symmetry and, (7) a nerve net.
(4)
;

CHAPTER XI

PHYLUM CTENOPHORA
This
is

a group of exclusively marine animals, most of which are

pelagic (float near the surface).


lives

There

is

a limited

number that
(te

and moves about on the bottom.


a class in this phylum.

Ctenophora

nof '6 ra

comb-bearing), because of their similarity to coelenterates, are often


classified as

There are only twenty-one Amer-

ican species representing this phylum,


sea walnuts or

comb

jellies.

and they are commonly called Most of them swim by means of eight

called swimming plates or combs. These animals and transparent, with a faint tint of pink, purple, or They are often phosphorescent. There are two classes in the blue. phylum: (1) Tentac^data, with a pair of tentacles present either in the larva stage or throughout life. Mnemiopsis leidyi is a luminescent, transparent form Pleurobrachia hachei has long tentacles on a relatively short, oval-shaped body; and Cestus veneris, Venus 's girdle, may be four feet long and only two inches in width, bandlike, transparent, with an iridescence showing violet, blue, and green colors. (2) Nuda, with no tentacles at any stage; Beroe ovata, about 10 to 12 cm. in length, conical in shape, and rather common, is an

rows of fused

cilia,

are quite clear

example.

Habitat and Behavior

These are primarily surface-living forms with rather wide distriThey move about very slowly through the water with the oral end forward and the two long tentacles trailing if tentacles are present. The tentacles have adhesive or glue cells (colloblasts) which produce a secretion, and with these they capture any small organisms making contact with them.
bution but most abundant in tropical seas.

Anatomy
The
size of different individuals of this

group ranges from

five

millimeters to four feet in length, and the shape

may

be spherical,
is

pear-shaped, ribbonlike or cylindrical.

The symmetry

said to be

biradial since there are eight rows of radially arranged paddles or

plates which are equally distributed on each side of the


157

median

158
line.

TEXTBOOK OF ZOOLOGY

These paddles are tlie locomotor organs. When seen from the side, the paddles resemble a comb. The mouth is in the oral end of the body and leads into the stomachlike stomodeum which is connected with a series of canals running through the body. This stomodeum is lined with ectoderm and leads to an infundibulum or
gastTOvascular cavity proper which joins the stomodeum at right
iflgitial

or

StomacS Plane
Right CastrovBicular Canal

51

Left CAitrovfticular Cansl

lUdial

CanaU

^^

Intcrradial Cant,

Tentaeulftt

'NviVrC^j

Sheath

""~-;,.r
, j^r) .i i^j.^ 'Yvi^^--'''
' ;

'
T.
i

Tranaverse or Infundibular Plane

A
Tentacle
-S

'-i
/.'/
;.-
"^

Aboral 5cne Orgon

...;',:;''"""

~ -

Plate

row with

Bilobed

Stomodaeum

Infundibular Canal underneath

Diagram looking down on aboral pole

Pleurobrachia bachei
Aboral End

'C^ Radial
\

Canal

Inlerradial

Canal

o..

Bilobfff

Stomo<faeuin

InfunJibuTum

Plate

Row

with

'

Slomodaeuni

branch of infundibular canal ayilem undetnealh

Mouth
Oral End

Perspective!

Drawing a}ong infundibulai plane

Pcispcctivt Drawing along sagittal or

3tomodaeum pUne

Yig, 76.

Pacific

comb

jelly,

showing external features and structure.

(Courtesy

of Pacific Biological Laboratories.)

angles.

This cavity

is

lined

egested through the mouth.


called excretory canals.

with endoderm; undigested food is The six canals mentioned above are

There are two blind canals extending from

the infundibulum beside and parallel to the stomodeum; these are called paragastric canals. The tentacular canals lead to the meridio-

nal canals lying beneath the ciliated plates.

There are two blind

PHYLUM CTENOPHORA
tentacular pouches connected with the outside
of the infundibulum.
;

159

one

lies

on each side

The

solid

prehensile tentacles emerge from

these sacs.

Around

the aboral surface of the body-

is

a collection of sense

structures or statocysts, which serve as organs of equilibrium by


stimulating- the cilia of the bands on the side against

which the inbody changes. These animals are monoecious (hermaphroditic) the ova are formed in ovaries along one side of each meridional canal and spermatozoa
ternal calcareous ball (statolith) rolls as the level of the
;

Fig. 77.

Sea walnut (CourtesyjeUy, or comb Mexico.

of General Biological

Beroe ovata, from Atlantic Ocean and Gulf of Supply House.)

along the other. The mature germ cells rupture into the infundibulum and pass through the stomodeum to the exterior. The fertilized ovum develops and finally metamorphoses to the adult stage. There is no alternation of generation. The animals are triplohlastic instead
of diploblastic as are Hydras, because, instead of a simple mesogloea,

there are, in addition, differentiated muscle fibers lying between the

ectoderm and endoderm. This is a morphological advance when compared to the coelenterates. These animals have no nematocysts and therefore do not irritate bathers as do jellyfish, but they do serve as food for a large number of valuable fish. Otherwise they have no economic importance.

CHAPTER
The representatives
of

XII

PHYLUM PLATYHELMINTHES
Phylum Platyhelminthes
(plat
i

hel min'

thez, broad worm) are usually called flatworms and in many ways show considerable advance over the coelenterates. Some of the species are parasitic, and the remainder of them are free-living. The common fresh-water planaria is an example of the free-living type while the parasitic flatworms are known as flukes or trematodes and tapeworms or cestodes. All of these worms are bilaterally symmetrical and triploblastic. The nervous system in the free-living forms is of the "ladder-type," and centralization is developed. They possess a fairly well-differentiated mesoderm, and along with it have developed some systems of organs. The alimentary cavity functions as a gastrovascular cavity and has only one opening to the exterior. The excretory system is composed of a pair of longitudinal tubules, branch tubules, and" flame cells.'' The gonads are within the body and are connected with the exterior by accessory organs. There are definite muscle cells, and excretory and reproductive systems composed of the new mesoderm layer.
;

part, quite

The representatives of the two parasitic classes have, for the most complex life histories and special adaptations. They

are very important economically because of their injury to

man

and the domesticated animals.


Classification

There are four recognized classes in the group.


Class Turbellaria (turbela'ria
sists

little stirring).

This

class con-

of a

forms.
there
is

group of soft-bodied, elongate and usually free-living The surface layer or epidermis is ciliated in patches, and
a plentiful supply of secreting cells in this layer.
is

digestive tube

single,

three-branched, or many-branched.

The The

mouth is located ventrally. There are both land and water forms. Four orders are known: Acoela, Rhabdocoelida, Tricladida, and Polycladida. Planaria and Stenostomum are examples.
160

PHYLUM PLATYHELMINTHES
Class

161

Trematoda (tremato'da having pores). These animals, commonly called flukes, have no epidermis but a thick nonciliated cuticle. The body is either leaf-shaped or elongate and has from one to many ventral suckers. This entire class is parasitic, and the immature stages frequently make use of snails and crabs as hosts for a phase of their life history. This group is divided into only two subclasses Monogenea and Digenea with orders Gasterostomata and Prosostomata. Paragonimus, Clonorchis (Fig. 397), Fasciola (Figs. 398 and 399) are genera representing the class.
:

Class Cestoda (ses to'da


acterized

girdle form). This group


cover,

is

also char-

by a heavy cuticular

and a

long, ribbonlike

body

divided into sections called proglottides.

These tapeworms each have a knoblike "head" or scolex on the This structure is supplied with suckers for attachment and sometimes has hooks. There is no alimentary tract, and the group is parasitic. A developmental stage of the life history is the bladder worm or cysticercus which lives embedded in the musanterior proglottid.

cular tissue of several different animals.

orders:

The class includes five Pseudophyllidea, Cyclophyllidea, Tetraphyllidea, Trypanorhyncha, and Heterophyllidea. Taenia (Figs. 400, 401, and 402), Diphyllohothrium, Ilymenolepis are examples.
Class Nemertina.

It seems difficult to know where to classify group since some systematists give it the rank of phylum while others give it lower ranking. The Nemertinea (nem er tin'e a unerring) as individuals, are unsegmented "band worms." Most

this

of them are free living and marine. A long proloscis, the neioly developed blood vascular system, the alimentary canal, two apertures, and cilia over the body are all characteristic of this type. There is present a mesoderm, nervous system, and excretory system, but there

seems to be no coelom. The animals feed on the bodies of other animals and on certain types of general organic matter. They usually live in burrows in sand or mud or beneath solid objects. The larger ones reach a length of ninety feet. The animals are frequently brightly colored. There are numerous mucus-secreting glands in the skin which may produce a tubelike dwelling for the worm. The two muscular layers of the body are so effective that an extended

worm

of fifteen feet
is

Locomotion
the body.

may contract to less than two feet in length. accomplished by the cilia and the contractility of The proboscis is a very characteristic organ which is

162
in the

TEXTBOOK OF ZOOLOGY

form

of a hollow tube turned

back through the body inside

of a cavity called the proboscis sheath.


sheath, the proboscis

By

contracting the saclike

may

be everted and extended from the anterior

part of the body.

The sexes are ordinarily separate and each individual possesses gonads which are located laterally and between the intestinal
_ Rhynchodeum
>

._ -Ocellus

qanqlia

.-J^loric

caecum

.^-Lateral
\

mrve

ffhynchocoe/

/nfccjfcinc

Proboscis

\\\

vSfcylefc

_..Qonad

-MetractorM.

Anus
Fig. 78. Fig. Fig. 79.
socialise

length published by The Macmillan Company, after Coe.) Fig. 79. Structure of the nemertine worm, Prostoma rubrum, as it appears when flattened. (Redrawn and modified from Hegner, College Zoology, published The Macmillan Company, after Coe.) by

A nemertine worm, Lineus about 15 cm. (Redrawn and modified


78.

with the body coiled. Natural from Hegner, College Zoology,

Both eggs and sperm are discharged from respective individuals through a dorsal pore and fertilization occurs in the surrounding water. Following cleavage there is a helmet-shaped larva
pouches.
called pilidium.
of the body
plate
is

Cilia develop

on the lappets at the lower margins


This

and on a patch

at the opposite pole or apical plate.

the principal nerve center of the animal in this stage.

The

PHYLUM PLATYHELMINTHES
adult appears after metamorphosis. ing larva

163

In some forms there is a creepThe vascular system is composed of longitudinal vessels connected by transverse loops. The vascular fluid is usually colorless. The excretory system includes the usual longitudinal tubules and flame cells characteristic of the phylum. Either one pore or several communicate with the exterior. The central nervous system consists of two ganglia and three longitudinal

known

as Desor's larva.

A pair of grooves with cilia along each side of the cephalic portion are sensory and are called cerebral organs. Other tactile organs and eyes are usually developed. Proscords passing through the body.

toma, Cerebratulus, Tetrastemma are representatives.

Georaqe cavity

L'lji^^i^ -Mesenchymal cell

Stomach

Lctodermal invagination

.fctodermaf
invagination

EsophacjUi

Ventrolateral lobe

Fig. 80.

(Redrawn and modifled from Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.)

Structure

of pilidium larva of the

nemertlne

worm

in partial section.

PLANARIA
Habitat ajid Behavior
This free-living, fresh-water, flatworm thrives beneath the rocks,
logs, leaves, algae, or debris at the

brooks and pools.

They must have pure,

bottom of shallow spring-fed clear, cool water. These

164

TEXTBOOK OF ZOOLOGY

animals are rather gregarious and

when

at rest will group together

beneath objects where the light


tively to bright light.

They respond negais not intense. They usually feed upon minute plants and animals, dead animal bodies, and living forms, such as small arthropods and molluscs. Planaria partially encompasses the food with the body, while the pharynx is protruded to eat it. If tiny scraps of meat are placed in a dish with hungry planarians, they will form a wad of living protoplasm about it. The mouth is located at the middle of the ventral side of the body, and the pharynx is everted through it as a prohoscis which is used to draw food within. It is
interesting to watch these animals passing the proboscis about over

apparently sucking up the nourishing very minute quantities of meat juice are If fluids from the meat. liberated in the water at specific points, the planarians are atthe surface of fresh meat,

tracted to those points.


Eye
Genital pore

Side of head

PharjTix sheath

Proboscis
in

Fig. 81.

Hegner, McBride.)

Entire planaria with pharynx extended position for feeding. (From College Zoology, published by The Macmillan Company, after Shipley and
is

The locomotion

accomplished in an easy gliding fashion by the


in this

action of the beating cilia and muscular contractions of the body.

enhanced by the secretion smooth track for the moving animal. It glides over a surface, even the under side of the surface film of water, and adjusts itself easily to any irregularities because of the soft, flexible nature of the body. The ciliary action and muscular contractions are both rhythmic and progress in waves from anterior to posterior.

The

ability to

of slippery

move along mucus which

way

is

essentially lays a

The behavior of

this

animal

is

of a reflex or automatic type.

receiving or receptor sensory cell transfers the impulse produced

The by

a stimulus to a ganglion cell or adjustor in the central nervous system which in turn transmits an impulse to an efferent cell carrying it to a muscle or gland. The planarians respond to several tropisms. They possess negative phototropism and thermotropism (as regards high

PHYLUM PLATYHELMINTHES

165

temperatures). They react positively to contact (thigmotropism) and water currents (rheotropism). The responses to chemicals are positive in case of food juices and the like; while they are negative to alkalies, acids, strong salts, alcohol, etc. The common species are Plaiiaria niaculata, P. agilis, and P. dorotocephala.

External

Anatomy

The body is elongated, flat, broadly wedge-shaped at the anterior and tapering to a point at the posterior end. It is triplohlastic since the ectoderm, endoderm, and mesoderm are all differentiated and present in a clear-cut fashion for the first time in our studies so far. The symmetry is distinctly hilateral. In Planaria maculata there is considerable pigment in the skin; while in Dendrocoelum
lacteum there
is

much

less.

On

the dorsal side of the anterior region

are two pigment bodies called eyespots which are sensitive to light.

At each

side of the

"head"

region

is

a pointed, sensitive, extension

of the epidermis in the

These are sensitive to


sound.

form of a lappet or "ear," called an auricle. touch and chemical stimulations but not to

The mouth is located in the midventral portion of the body. The pharynx may be protruded through the mouth in the form of a long, trunklike prohoscis which is used in feeding. Posterior to the mouth is a small, constricted, scarlike aperture, the geniial pore. Externally the epidermal cells are soft and the general surface is nearly covered with patches of cilia which are cytoplasmic extensions
of these
cells.

These

cilia

along with muscular contractions accomof fully developed active P.

plish locomotion.

The average length

maculata

is

about three-fourths of an inch.


Internal

Anatomy

The ectoderm covers the outer surfaces of the body and composes nervous system the endoderm lines the intestine and its branches; while the mesoderm constitutes the muscular, excretory, and reproductive systems. The undifferentiated mesoderm lying outside the intestine is composed of a meshwork of large cells and is called mesenchyme or parenchyma. Many of the structures of the animal, which have been observed in none of the forms previously studied, have come into existence with the development of mesoderm. The digestive system is composed of a mouth in the midventral position; a prehensile pharynx held in the pharyngeal chamber or
the
;

166

TEXTBOOK OF ZOOLOGY

CX

PHYLUM PLATYHELMINTHES
buccal cavity which
testine,
it

167

nearly

fills;

a three-branched enteron or in-

which branches immediately from the anterior end of the pharynx into an anterior trunk; and two lateral trunks that turn posteriorly, one along each side of the pharynx, and extend nearly to the posterior end. The pharynx is in the form of a cylindrical fold projecting through the full length of the pharyngeal chamber. It is attached only at its proximal or anterior end and is perfectly free otherwise. When it is extended or protruded through the mouth opening which it fills, it forms a proboscis whose length may be as
great
as,

or greater than, that of the entire body.

The trunks

of the

enteron have
ticula

many

lateral, blind extensions or pockets called diver-

which greatly increase the surface exposure of the organ and among most of the other tissues of the body. The whole arrangement represents a complicated gastrovascular cavity whose
project
wall
is

endodermal.

Cilia

Excrelioiy tubule

Fig. 83.

Flame

cell of

planaria.

The excretory system is new to our study and is composed of a set of tubules which relate themselves to all parts of the body. There are two principal, longitudinal, coiled tubules, one along each side

many small branches and open by minute pores located just posterior to the eyespots, and by several other pores along the length. Each of the numerous smaller branch-tubules has at its blind end a flame cell which is hollow and contains a mass of
of the bod}', which receive

long

cilia

that are continually beating in a direction toward the

tubule, the

movements appearing something

like

a flickering flame.

168

TEXTBOOK OF ZOOLOGY
cellular walls of the tubules as well as the flame cells arise in

The
this

the mesoderm.

Under

strict definition,

some authors object

to calling

arrangement a system.
is

Another mesoderm organization

the muscular system.

It is

com-

posed of an outer circular layer just under the epidermis; an outer longitudinal layer just medial to the circular layer; oblique bundles of fibers and at the medial margin of the mesoderm is another ir;

regular, internal, longitudinal layer just medial to a circular layer.

By

the alternate activity of these layers the animal

is

capable of

great extension and contraction.

Another advanced development is the "ladderlike" nervous system which consists of two contiguous lobes of nerve cells just ventral to the eyespots, two ventrolateral longitudinal nerve cords, transverse commissures, branch nerves, and sensory end areas of the
epidermis.

The double ganglion


It
is

at the anterior is the central portion

of the system.

known

as the

cephalic ganglion
etc.,

and gives

branches to sensory areas of the head, auricles,


the longitudinal nerve cords.

besides joining

The transverse commissures connect


transverse commissure meets a
cell

the two longitudinal cords at from 15 to 20 points like the rungs of

a ladder.
bodies.

At each point where a


is

longitudinal cord,
these points.

a small ganglion composed of a few nerve


to the

The branch nerves extend


is

surrounding tissue from

The reproductive system


reproduction
is

fairly well developed in

most species
Its

except P. dorotocephala which rarely develops sexual organs.


entirely
is

by asexual

fission.

The sexual reproduction


is

of other planarians

hermaphroditic, which

rather characteristic

of sedentary animals.

The male organs consist of numerous globular testes located in the parenchyma through most of the length of the body. Vasa efferentia are slender, thin-walled ducts leading from the testes to two larger, longitudinal ducts, the vasa deferentia. These in turn lead posteriorly, enlarge to become seminal vesicles, and converge to form the penis or cirrus, the copulatory organ. This
opens into the
cloaca,

common cavity called the genital atrium or geiiital which opens externally at the genital pore. Some authors The describe glands which pour a seminal fluid into the system. female organs in the same animal consist of two ovaries located well toward the anterior, a tubular oviduct leading posteriorly from each to join the genital atrium at a common point near its posterior end

PHYLUM PLATYHELMINTHES
by way of the vagina.
oviduct along
its

169

There are numerous yolk glands joining each length a glandular structure of questionable func;

CerebraL
(jancjlion

Auricle

LorKjitudinaL

nerve cord
Testis
/

Vasa efferenbia

Lateral

nerve

lumen of pharynx
Intestine

Vos
deferens

l_(

K-/rax^
H
:

Oviduct

Pharyngeal

Mouth
Seminal
vesicle

\_^^

chamber
Penis

5eminal receptacle

Genital pore

Fig. 84.

Reproductive

system of the planarian worm.


side only.

Male organs shown on one

tion, in the form of a blind tube with an inflated end, is connected with the genital atrium. It has been suggested that the fertilized eggs accumulate and are retained here for a time. The system is

170

TEXTBOOK OF ZOOLOGY

notably quite elaborate, and it is found generally that the flatworms have a highly specialized reproductive system.

The planarian worms and the representatives


sess

of this

phylum

pos-

no skeletal system, no respiratory system (breathe through the skin); no coelom or body cavity; and no circulatory system; this function, however, is performed by the branched enteron. It is significant that the reproductive system upon which the continuance of the race depends is highly specialized, this succeeded by the digestive system responsible for nourishment of the individual, and this followed by the nervous system which relates the organism to its
surroundings.

Metabolism

and The food may be parThe principal tially digested by a fluid produced in the pharynx. Here the process of digestion occurs in the cavity of the enteron. process is similar to that of Porifera and Coelenterata, being both intercellular and intracellular that is, part of the food in the intestinal cavity is digested by secretions from cells in their walls, while other food particles are engulfed by pseudopodia extended from cells lining the cavity and are digested in food vacuoles inside the cells. Absorption and assimilation take place through the plasma memThe food
is

principally animal tissue with some plant matter,

ingestion takes place through the proboscis.

branes of adjacent
other
cells,

cells.

Since the diverticula of this system pene-

trate all parts of the body,

and the diffusion of materials supplies all no circulatory system is necessaiy to transport nutriment. There is no anus, so all indigestible material is egested by way of the Respiration is accomplished through the general surface mouth. epithelium, and oxygen is distributed by diffusion through the protoplasm and fluid-filled spaces of the parenchyma. Catabolism or dissimilation takes place in the cells by union of the oxygen with the organic components of the protoplasm. Excretion or elimination of nitrogenous waste liquids is cared for by the flame cells and system of tubules. The flame cells absorb these wastes from the surrounding tissues and force the fluid into the tubules by the action of the cilia.
Reproduction and Life History
Sexually the individuals are hermaphroditic. The spermatozoa germ cells mature in the testes, then pass through the vasa

or male

efferentia

and vasa deferentia,

to the seminal vesicles

where they

PHYLUM PLATYHELMINTHES
are stored in advance of copulation.
into pockets
ovaries, pass

171

Here they become organized


the

known as spermatophores. The ova mature in down the oviducts where yolk cells or nurse cells

are

added by the yolk glands, through the vagina to the genital atrium, and probably from here to the uterus or seminal receptacle where

Young planana batchinq

tq(j

capsule or cocoon
cocoons and the young hatching.
Vitelline cells

Fig. 85.

Planarian

Wandering
cells
"

Ectoderm

Wandering
cells

Endodenn
Provisional

Endoderm
-Primitive gut

Wandering

pharynx

cells
ceUs

Vitelline

Mouth

Development of Planaria later blastomeres from segmented egg


Fig. 86.
;

lactea.

1,

S,

have differentiated into ectoderm, endoderm, a provisional pharynx, and wandering cells; 5, cellular differentiation more advanced; 6. embryo becomes flattened and ovoid. (From Hegner, College Zoology, published by The Macmillan Company, after Lankester after Hallez.

stage

E!gg surrounded by yolk; 2, four still later, after blastomeres if,


;

Cross-fertilization is practiced by these Planarians have been observed to copulate with an apparent exchange of spermatozoa in the form of spermatophores. In copula-

they are thought to be stored.


animals.

172

TEXTBOOK OF ZOOLOGY
is

tion the cirrus or penis

protruded from the genital pore to enter


In
transferred from each animal to the

the genital pore and extend into the uterus of the other copulant.
this

way spermatozoa may be

other.

Spermatozoa have been found along the oviduct as far as

the anterior portion, so fertilization likely occurs somewhere along


this tube.

each
to

is

At breeding time zygotes are found surrounded by a large number of yolk


cell

in the atrium,
cells

and

(nurse cells).

Each yolk
which

contributes

its store

of nourishment to the egg cell

it is

attached.

From one
cells,

many

thousands of yolk

to several zygotes, surrounded by become enclosed in a capsule-like shell

Fig. 87.

Fission

as

it

occurs in Planaria dorotocephala.

secreted by the genital atrium

and known
is

as a cocoon.

These are

expelled from the atrium and each


sides of

attached by a stalk to the under

submerged stones or vegetation in the water.

In the cocoon

the embryo passes through cleavage divisions, blastula stage, gastrulation

and even later stages before the cocoon ruptures and the small wormlike planarians escape into the water.

Asexual reproduction by transverse fission occurs quite frequently when the mature animals become slowed down. The individual constricts and then divides into anterior and posterior portions each of which forms the missing parts by rapid cell division. The axial

PHYLUM PLATYHELMINTHES
orientation of the tissue
is

173

retained

i.e.,

an anterior portion develops

in the position of the original anterior portion,

and

a posterior poris

tion at the original posterior position.

This process

not funda-

mentally different from budding in Hydra or strobilization in the

Scyphomedusae.

The retention

of the axial orientation during fission has been

explained by Dr. Child of Chicago University.


sesses a well-defined axial organization in

The animal poswhich the "head" porBe-

tion as usual has the highest metabolic activity of the body.

ginning at the anterior there a sudden increase occurs.

is

a gradient of decreasing metabolic

activity until a level just posterior to the

mouth

is

reached, and here

Posterior to this the decreasing gradient

again folloAvs to the posterior tip of the body. The level where the metabolic rate suddenly rises represents the point of fission or the
This seems to indicate a kind of zooid organization in the animal. In larger, older individuals there may be other such points of increased metabolism posterior to this first one.

anterior end of the second individual.

Such zooids are the

result of successive

functional isolations of the basal structure accompanying growth


in length.

This graduation of the rate of metabolism along the

principal axis of an axiate animal has been called an axial gradient

by Dr. Child.
of metabolism.

When

the animal

is

young,

it is

relatively short

and

the entire body, but particularly the ''head," carries on a high rate

The head at

this time holds a

length of the organism.


longer,

As

the animal

dominance over the grows older, it becomes

and the entire metabolic rate decreases. This means that the head loses its dominance over the entire length. A new center of dominance and increased metabolism is established just posterior to the point where this "head" dominajice fades out.
Regeneration
This group shows remarkable powers of replacing lost or mutilated parts of the body.
It can be cut into several pieces, and each piece will replace the missing parts about as the process is carried

out in
ate a

fission. A piece from the middle of the animal will regenerhead portion at the anterior margin and a tail portion at the

posterior margin.
will be

found

in a later chapter

more complete discussion of this phenomenon on Animal Regeneration.

174

TEXTBOOK OF ZOOLOGY

Economic Relations of the Phylum The planarians and other free-living flatworms are of practically no economic importance, but the phylum includes a large number of forms, principally Trematodes and Cestodes, which are parasitic Such groups as the in higher vertebrate animals, including man. intestinal flukes, liver flukes, lung flukes, blood flukes, pork tapeworm, beef tapeworm, margined tapeworm of dog, gid worm, hydatid worm, common tapeworm of dog, chicken tapeworm, dwarf tapeworm, sheep tapeworm, tapeworm of horse, and fish tapeworm They cost many thousands of dollars and are all serious parasites. each year. A more complete discussion of this topic much debility will be found in the chapter on Animal Parasitism.
Phylogenetic Advances of Platyhelminthes
Anteroposterior principal axis, (2) bilateral symmetry, (3) a germ layer, the mesoderm, (4) an excretory system of flame cells, (5) central nervous system extending with the axis
(1)

distinct third

of the body, (6) specialized gastrovascular cavity, and (7) permanent sexual reproductive organs.

CHAPTER

XIII

PHYLUM NEMATHELMINTHES
unsegmented roundworms or threadworms. Some of the Nemathelmiuthes (nem a thel min'thez, threadworms) are free-living in soil, fresh water, and salt water; some are found living in plant tissues and others live in animal tissues as parasites. The majority of them are microscopic, but a few are macroscopic in size. These worms are long, slender animals whose bodies are more or less cylindrical but tapering toward each end. The range of length is from i/4 mm. to four feet. They differ from
This group
is

known

as the

the flatworm not only in shape, but also in that the intestine has

two openings, there

is

a dorsal as well as a ventral nerve cord, they

are mostly dioecious, and there


also lack respiratory
definite

They is a total absence of cilia. and circulatory systems, true coelom, and locomotor organs. The group is very widely distributed
attention.

and is deserving of considerable known forms are Ascaris (pigworm

Some

of the better

or eelworm), "horsehair snake,"

hookworm, pinworm, Trichinella, Filaria, Guinea worm, whipworm, and eye worm. The former will be discussed in some detail in this chapter, and several others will be considered in the chapter on Animal Parasitism.
Classification

Three

classes are usually recognized,

although some authors prefer

to use only two.

The three

classes are

Nematoda, Gordiacea, and

Acanthocephala.
Class Nematoda (nem a to'da threadworm) is a group occupying almost every possible habitat capable of supporting life. There
free-living, fresh water, marine, and soil-inhabiting speand large numbers of parasitic forms living at the expense of other animals and plants. This is a very important class parasitically. In size they range from %o mm. to more than a meter in length. Locomotor organs are found in a few forms, no segmentation is present, and there is no true coelom. Chemical sense organs called amphids are nearly universal, while eyes and tactile organs are common in the free-living forms. The skeletonlike cuticle, common to all, is shed periodically like the molting of arthropods. The nervous

are

many

cies,

175

176

TEXTBOOK OF ZOOLOGY
is

system

extend posteriorly.

composed of a circumpharyngeal ring from which cords The It is a sensory-neuro-muscular system.


is

structure of the free-living forms


that of the parasitic forms.
habitats

generally more complex than


to

They are adapted

a wide variety of

and can withstand many rigors of natural adversity, such as freezing, high temperatures, droughts, and other unfavorable conditions. Large numbers of free-living forms have not been named and described. Representatives of this class have an intestine but no proboscis.
Order
forms.
loides

Ascaroidea.

It

includes

both

parasitic

and

free-living

Ascaris (Fig. 90), the common intestinal worm, is the most abundant. Enterohius vermicularis, the human pinworm; Strongystercoralis,

another parasite of man; Ascaridia lineata, the

chicken worm, and Toxocara canis of dogs are other familiar examples. Ascaris lunibricoides will be discussed later as a typical example of

Nemathelminthes.

Order 8trongyloidea. This is an entirely parasitic group. The males have caudal bursae with rays. The club-shaped esophagus is without a posterior bulb. The hookworms of man, the Strongylus

roundworms

of horses,

and Syngamus trachea which causes gapes


all

in

birds by obstructing the windpipe are

common

representatives.

Order Filaroidea.

This

is

a completely parasitic order, modified

for living in such tissues as lymph, blood, connective tissue,

and

muscle of chorda te animals, and transmitted by certain insects. Two distinctive characteristics are: (1) lack of bulb on esophagus; (2) lateral paired lips or entire absence of lips. Guinea worm, eye worm,

and Filaria are the common humaai

parasites.

Some

species cause
vessels.

elephantiasis (Fig. 390) through occlusion of blood

and lymph

This disease results in enormous swelling of the ai^ected parts. These organisms are transmitted by mosquitoes. Several Filaroidea are
parasites of horses

and

dogs.

Order Dioctophymoidea. This is another parasitic group living body cavity, and alimentary canal of mammals and birds. The genus Dioctophyme includes the largest roundworms, some reaching more than three feet in length.
in the kidneys,

Order Trichinelloidea.
cells. The common known examples.

This parasitic group has a peculiar


is

cuticle

lining the esophagus, outside of which

a single layer of epithelial


well-

trichina (Fig. 396)

and the whipworm are

PHYLUM NEMATHELMINTHES

177

Class Gordiacea (gor di a'she a, a knot). Superficially these animals resemble the nematodes, but the fundamental structure is quite

and therefore, it is likely proper to give them the rank of They are free-living as adults but as larvae are parasitic on May flies and other insects. They leave this host and take up abode in a terrestrial form like that of grasshoppers or beetles. After complete development the adult "hair snakes'' escape into
different,
class.

Fig. 88.

Hair

snake, Gordius, an aquatic roundworm.

the water of some stream, puddle,

or watering trough. These females again lay eggs in the water in long strings. In the adult

worm the intestine is a straight tube, often without a mouth, but opening at the posterior end by an anus. Some have no intestine at all. The outer surface of the body is covered by a cuticle. The body is cylindrical and without lateral lines, excretory organs, or circulatory system. There are four longitudinal spaces or sinuses

178
in the

TEXTBOOK OF ZOOLOGY

parenchyma;

in the adult female the

lined with peritoneum.

The nervous system

two lateral spaces are is composed of a midThere are


cells

ventral cord with a nerve ring at the anterior end.

rudimentary eyespots and scattered sensory

over the body.

Structure of Acanthocrphala. Internal structure of the genital organs a young female nematode, Neoechinorhynchus emydis. (From Van Cleave, Invertebrate Zoology^ first edition, second impression, published by McGraw-Hill
Fig. 89.

of

Book Company,

Inc.)

Sexually the group

is

dioecious with the gonads opening into the


Fertilization takes place within

posterior end of the digestive tract. the body of the female. are the

Gordius aquaticus and Paragordius varius


of the group.

common examples

PHYLUM NEMATHELMINTHES
Class Acanthocephala (a kan th6 sef 'a
la,

179

thorn head) includes a

"spmy-headed worms," which is absolutely parasitic in its habits. The adults are from a few millimeters to fifty millimeters in length and have an elongated, flattened body when found in
group,
as

known

the intestine of a vertebrate but become distended to a cylindrical

shape when removed to some solution outside the body.


sible proioscis is

The protru-

a peculiar and characteristic structure located at the


It
it is

anterior end of the body.


spines,

bears numerous recurved hooks or

and

in

many

species

capable of receding into a proboscis

no digestive tract in this parasite, and its food is absorbed through the surface of the body even though it is covered with a cuticle. A single ganglionic mass constitutes the central nervous system.
receptacle or sheath.
is

There

The male reproductive organs are the


glands joining the cirrus which
is

testes

and a

set of

cement

held in the copulatory bursa at

The bursa is capable of eversion. The sexes are separate, but the female has no permanent gonads. Egg masses develop early and completely rupture to produce a considerable number of embryos in the body cavity. Finally the embryos are discharged by way of the uterus through the genital pore which is located posteriorly and is the only external aperture of the body. Not only man, but mice, rats, pigs, fish, turtles, and in
the posterior portion of the body.
fact all classes of vertebrates serve as hosts for these animals.

ASCARIS,

A REPRESENTATIVE ROUNDWORM
Habitat and Behavior

The animal which is frequently studied as a representative of this phylum is Ascaris lumbricoides which frequents the digestive tract
ing suitable food and environment.

dependent on its host for furnishThe only time this organism is at the mercy of the elements of nature is during the egg stage when it may remain potent for months or even years if it falls in an environment unsuitable for development.
of

men and

hogs.

It is entirely

External Anatomy
This
is

a length of

one of the largest nematodes, females commonly reaching from eight to fourteen inches and males averaging six

180
to twelve inches.

TEXTBOOK OP ZOOLOGY

Males are always more slender and have a curled The mouth is guarded tail instead of the blunt tail of the female. by three lips, two in lateroventral positions and one dorsal. These lips have small papillae on their surfaces, two on the dorsal and

Fig.

90. Male and female Ascaris or eel-worm.

one on each of the ventral. The shape of the body is generally The smooth surface is marked by cylindrical with tapering ends. four longitudinal lines, two lateral, one dorsal, and one ventral. The genital pore in the female is located on the ventral midline
approximate!}^ one-third of the length of the body from the anterior

PHYLUM NEMATHELMINTHES
extremity.

181

The anus

is

located near the posterior tip of the body,

and

in the male the reproductive aperture

and two penial

setae or

spicules are located just within this opening.

Internal

Anatomy
thin, outer,

The body wall

is

composed of the

smooth

cuticle, the

epidermis, whose cells run together, and a thick layer of longitudi-

nal muscle fibers, whose medial margins are rather baggy.


/?70uth
circumesopfiagea/

There

aterc/s

ov/c^ucts I \

oyan'es pseudoooe/

excretort/ p/)ari/r}jc ^er7ti;a//x>re

/r?i>sC>/r?6

i)oc/i/

yya//

Ct/7a5

tiactas deferens

se/7?//7a/

ves/ch

e/acL'/atory

afc/ct

pseadocoeJ
psei/cfocoe/

cut/c/e

intestine

ep/c/er/T}/5\

body ivcr//
^em/'na/ ves/c/e

musc/e
cutic/e

seta/ soi

inteet/'ne recta/r? ej(cretoru cana/


pe/ifcfl_

o//dact

setae:
a/7i/:

'/Vi/sc/e

oyan/

at/c/e

ner/e oord

ep/der/nh

3
Internal anatomy of Ascaris Iwmbricoides. A, Diagram of lateral view Fig. 91. of dissection of female B, cross-section of tlie midregion of the body of female D, reproductive system of C, longitudinal section of posterior portion of male female; E, reproductive system of male. (From Curtis and Guthrie, Textbook of General Zoology, published by John Wiley and Sons, Inc., modified from Leuckart.)
;

are thickenings of the epidermis in the positions of the longitudinal


lines.

The excretory tubes follow the


is

lateral lines.
is

The body cavity


lined externally

of this animal

a primitive or false coelom which

by the mesoderm of the body wall and internally by the endoderm

182
of the intestinal wall.
is

TEXTBOOK OF ZOOLOGY
Ordinarily the coelom,

when

fully developed,

lined both laterally


is

This

the simplest type of animal in which the


is

and medially with mesodermic peritoneum. body cavity or

coelom

mesodermic.
lies in

In higher forms the outer coat of the intestine is The alimentary canal is quite straight and simple and There is no need for the dorsal part of the body cavity. found.
is

great specialization of the digestive system since the food

taken

from the digested material in the intestine of the host. A contractile pharynx, which acts as a pump, draws fluid into the long epithelial intestine from which it is absorbed by the other tissues.

The narrowed posterior portion

is

the rectum and leads to the anus

fertilization /Tjemi^ranC'

Sfye//

ze/^ade
Fig. 92. Fertilized coides. (From Curtis

ovum, 4., and amoeboid spermatozoa, B, of Ascaris lumbriand Guthrie, Textbook of General Zoology, published by-

John Wiley

&

Sons, Inc., after Leuckart.

at the posterior portion of the body.

The two

laterally located,

longitudinal duets open externally by a single pore near the anterior

end of the body.

There

is

a nerve ring

gives off a large dorsal longitudinal nerve

around the pharynx which and a large ventral


testis is

longitudinal nerve.

There are usually four other smaller longitudiIn the males the a thread-

nal nerves and some connectives.


like structure

which is much coiled in the cavity. This tube enlarges posteriorly to become the vas deferens which in turn enlarges still more before reaching the aperture to become the ejaculatory duct. In the female the threadlike ovaries join the coiled oviducts which lead forward and join the two uteri. These tubes join in the vagina, which
is

a short tube leading to the genital pore.

PHYLUM NEMATHELMINTHES
Reproduction and the Life Cycle

183

The animals copulate, and at this time the spermatozoa are introduced into the vagina of the female to fertilize the mature ova in the oviducts. A mature female may contain as many as 27,000,000 eggs. These eggs pass from the host with the feces. Some workers
have reported that each female worm in an infected host may produce a crop of eggs in excess of two thousand per gram of feces. Based on this figure, the daily production is computed to be something like 200,000 eggs. These eggs are so resistant that they can be successfully cultured in 1 to 2 per cent formalin, and they may be stored successfully for four years in a refrigerator. The life history is completed only in case the eggs are swallowed by a susceptible host.

They hatch

in the small intestine of the host

and

then go on a ten-day journey by way of the blood stream to the By burrowing liver, thence to the heart, and thence to the lungs. make their way to the throat, esophagus, out from here, these larvae

and back

stomach and intestine. After reaching the intestine, the larval worms, 2 to 3 mm. long, grow to maturity in two to two and one-half months. They likely live a little less than a year in
to the

the host.

Relations to

Man
is

Heavy

infestation in
is

man may

cause severe hemorrhages and set

up pneumonia that

often fatal.

Anemia

often the result of

such infection; in certain cases the organisms may even tangle in masses and block the intestine until surgical operation is necessary The toxic substances from these parasites may to remove them. bring on coma, convulsions, delirium, nervousness, and other similar symptoms. Drugs like chenopouium, santonin, and hexylresorcinol

have been used successfully under physicians' directions as a cure. Effective sanitary disposal of fecal material is the most successful
preventive.

CHAPTER XIV
MOLLUSCOIDA, TROCHELMINTHES, AND

CHAETOGNATHA
chapter, because they are

These groups are rather conveniently considered in the same more or less isolated, small groups of the

unsegmented worm type.

MOLLUSCOIDA
This
is

the

name
It is

of a

group composed of two

classes, as

they are

treated here.

usually considered a

authors prefer to give each of the classes


fication of the latter plan

phylum name, but many phylum rank. The justi-

may

be questionable.
animals) includes a group of co-

Class Bryozoa (bri 6 zo'a

moss

lonial animals often called Polyzoa,

which are similar

to colonial

hydroids in their manner of growth and forms. It is true that their Nearly all bryozoans structure distinguishes them very readily.
are marine, although there are a few fresh water forms.

In ex-

ternal appearance they resemble certain of the corals and hydroids.

was a long time after their existence was laiown that they were separated from that group. The subclass Ectoprocta includes forms in which the mouth is surrounded by tentacles and the anus is not
It

Bugula is an example of a treelike type of this Another type is one that grows as an incrusting organism. The second subclass Endoprocta is characterized by the circlet of oral tentacles which also encloses the anus.
enclosed in this area.
subclass.

Bugula
Bugula is a common marine genus, the individuals of which are associated in a treelike colony that lives attached to some object
in the water.

These individuals are called zooids of which the


as polypide.

soft

parts are

known
it

They

are within the primitive coelomic

muscles make

The presence of retractor withdrawn into the There are some smaller vaselike part of the chitinous skeleton. individuals whose shape is similar to that of a bird's head and whose bodies are smaller than the other zooids. These are called avicularia, and they are found on the surface of the colony. Their
cavity, the wall of

which

is

the zooecium.

possible for each zooid to be

184


MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA
185

movable jaws seem to serve as grappling hooks Avhich operate to keep the colony free from other small organisms and debris which may be present in the habitat. Yihramdaria (vibracula), which con-

Esophaqus.

-Jntestine
Avicularium

/ jaws open

Muscle to body
wa//
'

Muscle

_run/cu/uj (Mesentery)
Ooecium

Fig. 93.

Bugula, a marine bryozoan, showing the structure and habit of

life

of

two zooids from a colony.

another modified type of zooid, are filamentous, whiplike appendages. They are thought to be variations of the avicularian
stitute

modification.

186

TEXTBOOK OF ZOOLOGY

The mouth of the larger, regular zooid is located at the free end and is surrounded by a tuft of ciliated tentacles. This arrangement is known as the lophophore and has the shape of a horseshoe when expanded. Within, the digestive tube is U-shaped and termi-

3^

liaublt

K rXUAVO'ELLA,

f-

PECTIMATELLA
Fig. 94.

Kabi-1-

"

CKI^TATELLA
life

Three

forms of fresh-water Bryozoa showing the habit of (Courtesy of General Biological Supply House.)
is

for each.

nates at the anus, which


digestive tract
is

located just outside the lophophore.

The
tis-

held in place by strands of mesenchymatous tissue


Special strands of this
is

extending from the wall of the coelom.


sue are termed the funiculus.

The body

triploblastic

and there-

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA


fore composed of ectoderm, eudoderm,

187

and mesoderm. The nervous system is centered in a ganglion or mass of nerve cells located in the region of the mouth and from it, nerves extend to the tentacles.
Reproduction
is

accomplished either by budding or sexually.

Ovaries and testes


cavity.

make

their appearance either in the funiculus

and fertilization occurs in the body The early development goes on in a modified region of the zooecium, called the broad-pouch or ooecium. When the embryo escapes, it is a free-swimming, ciliated larva which is similar to the trochophore larva found as a developmental stage of certain Annelida and Mollusca. Its form resembles certain adult Rotifera. This larva becomes attached and transforms into a parent individual, the zooid of which will form a neAv colony by budding.
or in the lining of the coelom

The branching Plumatella, which


in the

is

supported by a secretion of
is

calcium carbonate, and the slimy Pectinatella, whose skeleton

form of a gelatinous mass, are the two forms most frequently found in fresh Avater. These fresh water types may be developed from winter eggs, enclosed in shells, or new individuals may be produced as internal buds. These buds are called statoMasts. They are produced in autumn and may either float on the water or sink to the bottom. They withstand the rigors of winter and are stimulated by it. So far as is known this group has little if any economic value.
Class Brachiopoda (brak i op'o da arm and foot) is a group of marine forms, the individuals of which possess bivalve shells. For this reason they are sometimes confused with the clamlike molluscs. The brachiopods, however, have dorsoventral valves, while the molluscan valves are lateral. The shell is secreted by a mantle which
lines the valves. The tip of the beaklike valves is penetrated by a foramen which serves as an opening for the peduncle. This fleshy organ makes permanent attachment to some object in the water. Internally, the lophophore is a conspicuous and characteristic structure of this type of animal. This organ is composed of two coiled appendages which bear numerous ciliated tentacles. The cilia produce water currents in the longitudinal groove and carry food particles to the mouth.

The digestive tract is U-shaped and is composed of the mouth, lophophore, gullet, stomach, and ventrally directed intestine. This

188
tract ends blindly in

TEXTBOOK OF ZOOLOGY

many

bracliiopods.

The

entire tube

is

lined
is

internally with ciliated epithelium.

segmented, true coelom

present, but the septa are a little bit difficult to distinguish. Extensions of the coelom enter the arms and mantle of this type of animal.

About two pairs

serve in excretion.

and produce the gonads also. The sexes are distinctly separate and mature germ cells are discharged into the coelom, thence to nephridia and outside. Fertilization takes place in the water, and a free-swimming, ciliated larva
of nephridia are connected with the coelom

The coelomic

cavities

hatches from the egg.


^ophophore
Digestive gland

Adductor muscle
I

Stomach
Heart

Dorsal valve Dorsal mantle


'

Intestine

Nephridium
Muscle
Pig. 95.

Mouth
to

Diagram of a sagittal section of a brachiopod show (From Hegner, College Zoology, published by The Macmillan

internal organs.

Company.)

forms.
mals.

Magellania flavescens and M. lenticularis are commonly studied They are entirely marine and represent an old line of aniis

group

There are relatively few modern forms in existence. of little economic significance.

The

TROCHELMINTHES
The
rotifers (Rotifera) are

common examples
el

of this

group known
In early

as the Trochelminthes

(trok

mm'thez

wheel

worm).

times they were called "wheel animalcules." There is very little difference between the trochophore larva of this group and the
adult animal.
this

Gastrotricha constitutes another small division of

group but will not be discussed in detail here.

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA


Rotifers are plentiful in fresh water, and a few of
the sea.

189

them inhabit

They are microscopic in size, and they are often associated They are very resistant to adverse conditions produced by drought and may be distributed in dry form. The body of a rotifer is bilaterally symmetrical and can be divided into head, trunk, and foot. It is covered externally by a cuticle. The so-called head is rather largely a troclial disc comwith Protozoa.
posed of various modifications of two bands of cilia over the anterior end and around the mouth. These cilia are in active motion, often
creating two sets of water currents so as to resemble two rotating

They are responsible for obtaining food and for locomoThe mouth is located in an anteroventral position. The trunk tapers toward the posterior and contains numerous organs. At the posterior end is the tail or foot Avhich is forked or toelike in many species. Here, too, in many forms, are located some cement or adhesive glands which assist the animal in adhering to most surfaces. The foot as a whole serves in locomotion, pushing the animal along.
wheels.
tion.

The

internal organs include several systems which

lie

in the rather

extensive body cavity or false coelom.

The

digestive system begins

anteriorly at the
food.

mouth which

receives other small organisms as

It is a cavity leading to the

pharynx.

Inside the

pharynx

is

a mill-like organ or mastax, composed of chitinous jaws, which mas-

The movements of these jaws may be observed in certain rotifers when alive. A short tubular esophagus leads to the pouchlike stomach, and extending posteriorly is the smaller cylindrical intestine which leads by way of the cloaca to
ticates the particles of food.

Nearly the entire internal surface of the alimentary is which aid the movement of the food material through it. The stomach and intestine are lined internally with endoderm.
the anus.

canal

lined with cilia

The excretory system


of flame cells,

is

well developed and consists of a

number

similar to those of flatworms;

and two winding

nephridial ducts which lead posteriorly to a contractile bladder. This bladder is pouchlike and empties into the cloaca. (The name
cloaca
is

applied to any cavity which serves as the posterior portion

of the alimentary canal


genital system.
It

and

also receives products of the urino-

opens externally by

way

of the anus.)

The flame

cells are distributed in the

body wall from the

anterior, posteriorly.

Some authors

believe that the bladder functions also to assist in

190
respiration

TEXTBOOK OF ZOOLOGY

by collecting the excess water and carbon dioxide. The oxygen is received into the body with water which diffuses through the body wall. A large ganglion, located in a dorsoanterior position and several nerves extending to sense organs and muscles con-

- Tactile organ
-r\

.Brain

Pharynx

Eye
Tooth
"Mastax Salivary gland

Salivary

gland
Salivary

gland

Stomachintestine

Vitellarium

-Egg

Flame
Blasendarm"
_

cell

Excretory
canal

Contractile bladder

-Rectum

Anus
-

Foot-gland,

Fig. 96. A common fresh-water rotifer, Philodina roseola, showing internal structure, a, dorsal view; b, ventral view. (From Hegner, College Zoology, published by The Macmillan Company, after Hickernell.)

stitute the

nervous system.

cuticle over a thin layer of ectoderm.

dermal tissue

The body wall is composed of an outer Under this layer is the mesowhich includes mesenchymatous cells and muscle fibers.

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA


This group of animals
is

191

and dimorphism (striking difThe males are usually much smaller and may even live as a parasite on the female. The males lack a well-developed digestive system and are therefore very short lived. In the female of most species there is one ovary which produces the eggs. Connected with this gonad is a yolk gland or vitellarium. In a few forms there are two ovaries with no distinct
bisexual,
is

ferences in form of the two sexes)

present.

yolk gland.
or even a

Rotifers

may

be oviparous (lay eggs), ovo viviparous,

few are viviparous. The eggs produced during the summer are thin-shelled, of two sizes, and develop parthenogenetically.
Female
Large Egg
-

Small Egg

/
Females
Males Sperm

Late SuKiner

Winter Eggs (Fertilized)

Pass Winter in thick shell

Females

Females

/ Eggs'"
/
Females
I

^ Eggs
^
k f

Parthenogenesis Parthenogenesis Many Generations Manv Generations


Females
Small Egg
i

Svunner

season

"i

Large Eggs
Fig. 97.

Life

cycle of the rotifer, Hydatina.

The smaller type produce males.


zation.

The eggs produced during the


fertili-

winter are thick-shelled, produce females only, and require

The eggs when mature, or the young if born alive, are carried by the tubular oviduct to the cloaca and are discharged to the exterior through the anus. The less highly developed males possess a single testis in which spermatozoa are produced. In some there is a peculiar type of copulation during which the special copulatory organ composed of a protrusible cirrus seems to perforate the body wall of the female. At this time the eggs of the
female are fertilized.
In oviparous forms, the fertilized eggs are

usually carried in the body for a time and then discharged by

way


TEXTBOOK OF ZOOLOGY

192
of the oviduct.

They then

lie

dormant and inactive

in the

water

for a period before hatching.

There is considerable similarity between the adult rotifer and the trochophore larva of some annelids,
Spines

-Hooks

-4

. Brain

"Mouth
Alimentary
canal

(_

Ventral ganglion

Ovary
- Oviduct

-Fin

Genital -pore

Anus
Vas deferens

Seminal vesicle

Testis

Fig. 98.

Sagitta hexaptera, an arrowworm, drawn to show internal organs. (From Hegner, College Zoology, published by The Macmillan Company.)

mollusks, Nemertinea and others.


rotifers.

This resemblance has prompted

the theory that the above groups are rather closely related to the

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA


Class Chaetognatha (ke tog'na tha

193

horse's
make

small marine

worms

are often called arrowworms,

mane, jaw). These and they are well


Horizontal
it

adapted

to

livmg at the surface of the ocean.


rapidly.

fins supit

port the animal at the surface and also

possible for

to

The prehensile mouth with its bristles have given the animal the name of "bristle jaws" in addition to other names. The body is divided into three divisions head, trunk, and tail. These are separated by septa and the coelomic cavity is separated into right and left cavities by a longitudinal mesentery. Internally is a tubelike intestine which extends from the mouth

move about

at the anterior, to the anus located near the base of the caudal fin

or

tail.

The nervous system

consists largely of a supraesophageal

ganglion or brain, ventral ganglion, branch nerves, two eyespots,

and other sensory organs.


or excretory structure.

These animals are lacking in circulatory

Each individual
tozoa, that
is,

is capable of producing both ova and spermahermaphroditic condition prevails. The ovaries the

are located in the posterior portion of the body cavity and the

mature ova are carried to the exterior by an oviduct on each side. The testes are located in the cavity of the tail portion. The spermatozoa are discharged into this cavity and delivered to the exterior by a pair of slender vasa deferentia or sperm ducts, which enlarge to become seminal vesicles near the aperture. The fertilized ova become small adults without a typical ciliated larval stage. Sagitta is the best known genus of the group.

CHAPTER XV

PHYLUM ANNELIDA
(By
J.

Teague Self, University of Oklahoma)

The Phylum Annelida (a nel'i da, form of a little ring) comprises an extremely large group of worms characterized by (1) the presence of a coelom surrounded by two layers of muscle, (2) metameres
or segments, (3) a ventrally located segmental nervous system, (4)

segmented, non jointed, chitinous appendages in most cases, (5) an excretory system composed of nephridia, and (6) a nonchitinous
cuticle covering the body.

type of free-living habitat where moisture

These worms are found in almost every There are is present.

many forms which

live in the ocean, either

swimming

freely, bur-

rowing in the sand, or living in especially prepared tubes. Freshwater streams and ponds are inhabited by numerous forms of annelids, and moist soil is usually alive with terrestrial earthworms. From this it is evident that the phylum as a group has become adapted to many varied living conditions and comprises one of the large groups of the animal kingdom from the standpoint of numbers. In the process of adaptation the annelids have become diversified in their anatomical features until only a very few characters, such as those mentioned in the beginning of this chapter, are common to the entire phylum. Even then, these distinguishing features are sometimes modified until only an expert can recognize them.

The Phylum Annelida may be divided into four


Chaetopoda I. Order 1. Polychaeta Order 2. Oligochaeta Class II. Archiannelida Class III. Hirudinea Class IV. Gephyrea Order 1. Echiuroidea Order 2. Sipunculoidea
Class
Class Chaetopoda (ke top'O da, hair and foot).
the most

classes:

This

class includes

commonly known forms

of the

phylum.

There are marine,

194

PHYLUM ANNELIDA
fresh-water,

195

and terrestrial forms; and they all possess setae (chaetae), The setae are or bristlelike appendages on the body segments. They bear in pits of the integument. chitinous and are embedded make them movable and therefore useful muscle attachments which The coelom, which surrounds the straight digestive in locomotion. tract, is divided between the segments by partitions known as septae.

99. Representative annelids. From left to right, Arenicola cristata, lug Amphitrite ornata, marine annelid with branching gills Hirudo medicinalis large medicinal leech (upper center) Aphrodita ornata, sea mouse (lower center) Nereis vii-ens, sand worm or clam worm Lumb7-icus terrestris earthworm or angleworm. (Courtesy of Denoj-er-Geppert Company.)

worm

Fig.
;

Typically, each coelomic space possesses a pair of nephridial tubules

which communicate with the coelom at one end by means of a ciliated, funnellike opening, the nephrostome. The other end opens to the outside by means of a nephridiopore. The nephridia remove nitrogenous
waste materials from the coelomic cavities and from the blood.

196

TEXTBOOK OF ZOOLOGY
of each segment
is

The inner body wall

made up

of an inner longi-

Segmental nerves which are derived from segmental nerve ganglia innervate the metameres and coordinate the movements of the body. The segmental
ganglia communicate with each other through connections extending from one segment to the other. At the anterior end is the brain, which is composed of a suprapharyngeal and a subpharyngeal ganglion joined together by a pair of commissures. The brain, however,

tudinal layer and an outer circular layer of muscle.

has

little to

do with the coordination of different parts of interseg-

mental and intrasegmental reflexes, so that the stimulation in one segment automatically stimulates the adjoining ones. Reactions which
require immediate coordination of the whole body are controlled

by three giant nerve fibers which run through the entire length of the nerve chain. The primary function of the suprapharyngeal and subpharyngeal ganglia is to relay sensory impulses. The principal
vessels of the circulatory system are a dorsal one,

through which the blood moves forward, and a ventral one through which the blood moves posteriorly. These are connected in the an-

body by a varying number of paired, segmental The dorsal vessel exhibits wavelike contractile movements (peristaltic contractions) which force the blood anteriorly. The latter passes through the hearts, which also pulsate, then backward through the ventral vessel to the skin, intestine, and other organs. Hemoglobin is suspended in the blood plasma of some Chaetopoda in others, a green pigment known as chlorocruorin is found in still others no known blood pigment occurs. The principal vessels and hearts have valves on their inner surfaces which prevent the blood from flowing in the wrong direction.
terior region of the

hearts or connectives.

The

class

Chaetopoda may be divided into two orders; namely,

(1) the Polychaeta and (2) the Oligochaeta.

Order Polychaeta. The polychaetes (majiy bristles) are typically marine Chaetopoda. One of the most widely known forms of this group is Nereis virens or the clamworm, which may be studied as a
representative form.
fleshy parapodia.
stitute the
It

possesses

many

setae

(chaetae)

located in

In this case the parapodia with their setae con-

segmental appendages.

The parapodium

is

divided into

a dorsal notopodium and a ventral neuropodium, and each surrounds a large seta, or aciculum, which serves as a point of attachment for

PHYLUM ANNELIDA
the parapodial muscles.
present.

197

A dorsal and a ventral cirrus are usuallyThe notopodium and the neuropodium each have a large group of setae. The parapodia are used principally as locomotor and respiratory organs.
The head of Nereis seems
to

have resulted from the fusion and


It is

composed of a prnstoand two pairs constitutes the first segment and bears four paii*s of cirri or tentacles. The pharynx is equipped with muscles by which it can be everted, and a pair of chitinous jaws which protrude
specialization of the anterior segments.

mium, which bears a pair of eyes. The peristomium

of tentacles, a pair of palps,

Fig. 100. External anatomy of Nereis virens and parapodium. A, anterior end and posterior end; B^ parapodium (enlarged); 1, palp; 2, terminal tentacle; 5, prostomium 4, eye 5, lateral tentacles 6, peristomium 7, segment 8, parapodium 9, anus; 10, anal cirrus; XI, dorsal cirrus; 12, gill plate; 13, setae (chaetae) Ui, notopodium; 15, neuropodium; 16, ventral cirrus; 17, aciculum.
;

(Courtesy of General Biological Supply House.)

when

the

pharynx

is

extended.

The jaws serve


is to

in capturing small

organisms and crushing anything which


ceeding segments are
all alike

be swallowed.

The

suc-

except the posterior one which bears

a pair of ventral ciVrt extending posteriorly.

The digestive tract consists of an essentially straight tube. The mouth opens directly into the muscular protrusible pharynx, which may be everted by use of protractor muscles to form a sort of prohoscis. The pharynx leads into the relatively narrow esophagus

198

TEXTBOOK OF ZOOLOGY

which extends through about six segments and which has a digestive gland opening into it from both sides. The remainder of the digestive tract is a straight intestine which continues to the last segment, where it opens to the outside.
Prostomlal

Prostomlum

tentacles

Feristomlal
tentacles

Parapodia

Pharynx

CEsophageal glands

(Esophagus

Intestine

Nephrldla
Dorsal vessel

Ventral
vessels

Nerye-cord'

Fig-.

101.

Internal anatomy of Nereis virens. (From Hegner, College Zoology, published by The Macmillan Company, after Parker and Haswell.)

The circulatory system


vessels.

is

composed principally of a dorsal and a

ventral blood vessel joined in each segment by a pair of connecting

The blood

is

forced anteriorly through the dorsal vessel and

PHYLUM ANNELIDA
passes posteriorly through the ventral one.
Its

199

movement

is effected

by wavelike contractions in the walls of the dorsal vessel. It reaches the parapodia and digestive tract through lateral branches of the ventral vessel and is then returned to the dorsal one by parietal
branches.

body except the peristomium has two nephridia from the coelom to the outside. The nephridium consists of a ciliated funnel, nephrostome, and a coiled tubule which ends in its external opening, the nephridiopore. The nephridia serve to convey the excretory and reproductive products to the outside. The

Each segment

of the

opening directly

sexes are separate

those in the anterior end of the body. walls of the coelom

and there are gonads in all the segments except The sex cells arise from the

and when ripe pass to the outside, fertilization The fertilized egg develops into a trochophore larva, which metamorphoses into the adult animal. In the central nervous system there are two suprapharyngeal ganThese are connected by means of comglia dorsal to the pharynx.
taking place in the water.
missures to the suhpharyngeal ganglion ventral to the pharynx. A nerve chain, composed of segmental ganglia joined by intersegmental
connections, extends posteriorly on the ventral side of the the anal segment.

body

to

Lateral nerves from the ventral nerve chain in-

nervate the various organs of the worm.

Two
is

eyes receive nervous

connections from the brain and the animal


detect

apparently able to

moving

objects.

Order Oligochaeta. The best known example of the order Oligochaeta is Lumhricus terrestris, the common earthworm, which is used almost universally as a laboratory specimen. Lumbricus is not as common in the Southwest as are other large forms of earthworms,
(Dipocardia) but
is

used here as an example because


its

it

is

so well
to

known and because


the entire order.

features represent so well those

common

EARTHWORM
The body of Lumbricus terrestris varies from six to fourteen inches and gives the appearance of a number of rings joined in a The rings are the body segments, or metalinear arrangement.
in length

meres, and vary in

number up

to 175.

In the adult the number of

segments from the anterior end to the posterior end of the clitellum

200

TEXTBOOK OF ZOOLOGY

is

remains constant, while the number posterior to this varies. This because growth is accomplished by the addition of segments poste-

rior to the clitellum.

The prostomium
mere.

is

a sort of knoblike lobe at the anterior end,


It is

projecting out over the mouth.

not considered a true meta-

The

first

segment

is

incomplete due to the opening of the


Pros'f'omi'u/n

Openjngf ofoviducf

^Open!n0 of
"^Mpis

deferens

^Sem/'ncfl

groove

XXVIxxxrr-i

zm

C//fe//u/7i

xxxvir-

^Sefae

A'lus
Fig.
102.

numerals.

Company,

External anatomy of earthworm, (Prom Wolcott, Animal Biology,

ventral view, segments in published by McGraw-Hill

roman Book

Inc.)

mouth through its ventral side. In studying the earthworm it is customary to number the segments with Roman numerals beginning
at the anterior end.

This simplifies the study because external as

well as internal structures are definitely related to specific seg-

PHYLUM ANNELIDA
ments.

201

The openings

of the oviducts through -which the eggs pass

XIV.

minute pores, one on each side of segment The pores of the seminal receptacles occur in pairs, one pair in the groove between segments IX and X, and one between X and XI. The openings of the vasa deferentia (sperm ducts), which
to the outside are seen as

convey sperms to the outside, are located, one on each side, in the XV. In sexually mature worms, segments XXX, XXXI, or XXXII to segment XXXVII are swollen to form
anterior part of segment

the clitellum, a sort of saddle-shaped structure, the function of

which

is

to secrete the cocoon in

which eggs are deposited during

reproduction.

Each segment except the first and last bears four pairs of chitinous setae. They are fine, stiff bristles which may be located by passing the hand lightly over the worm. They are moved by protractor and retractor muscles and serve to help the worm move
through the
soil.

pair of nepliridiopores (the external openings

of nephridia) is situated on the posterior ventral side of each seg-

ment except the first two or three. The body of the earthworm is covered by a thin, transparent cuticle which is secreted by the epidermal cells just beneath it. It contains numerous minute pores through which secretions of the unicellular glands beneath are poured and through which gaseous exchanges between the blood and moist soil can take place. It serves also as a protection against physical and chemical injury
to the animal's

body.
Internal

Anatomy
open along the mid-dorsal

The body
line,

of the earthworm, if cut

gives the general appearance of a tube within a tube, the digestive tube being the inner one and the body wall the outer one.

The space between them is the coelom. The constricted regions dividing the segments on the outside correspond to the positions of the septae which divide the coelom into separate segmental compartments.

These coelomic divisions communicate with each other


of pores in the septae so that the clear fluid

by means

which fills The septae are absent between segments I and II and incomplete between segments III and IV, and XVII and XVIII. The walls of the coelom are lined by a
the coelom can circulate freely.
thin layer of cells

known

as peritoneum (mesothelium).

202

TEXTBOOK OF ZOOLOGY
Reproductive Organs

The earthworm

is

hermaphroditic, the organs of both sexes being

present in every animal.


ovaries are female organs,

The seminal receptacles, oviducts, and and the testes and seminal vesicles are male
In sexually mature individ-

organs.
uals they

The seminal

vesicles are three pairs of light-colored bodies

located in segments IX, XI, and XII.

may

extend back through the septae as far as the fifteenth

segment.

If their contents are

examined with a microscope, they

Fig. 103. Diagram showing reproductive system and nervous system in segments VIII to XV of an eartliworm. The seminal vesicles have been cut away in somites X and XI to disclose the testes and sperm funnels, es, egg sac nc, nerve cord ov, ovary sf, seminal funnel sm, septum between two somites sp, sperm
; ;
; ;
;

duct (vas deferens) opening in the fifteenth somite; sr, seminal receptacle; sv, seminal vesicle; t, testis; vd, oviduct. (From White, General Biology, published

by The

C. V.

Mosby Company.)

are seen to contain the various stages of developing spermatozoa

coming from the sperm mother cells. The testes are the two pairs of very minute bodies projecting into the seminal vesicles in segments X and XI and cannot be seen without first removing the dorsal part of the seminal vesicles. The union of the vasa effer-

PHYLUM ANNELIDA

203

entia coming from the vesicles on each side forms a single pair of

vasa deferentia in segment XII.

The seminal receptacles are pairs IX and X. The ovaries are two minute bodies located one on each side of segment XIII.
of small white bodies located in segments

Digestive System

The mouth cavity extends through segments I to III and leads muscular pharynx which extends through segment The pharynx plays the part of a sucker in securing food for V.
into the bulbous,

Fig. 104.
I

to

XXI.
;

Diagram of dorsaleg, calciferous an earthwormcrop region dissection of buccal cavity glands


in
be,
; ;

cr,

of segments dv, dorsal blood


;

vessel

eo,

esophagus
;

stomach-intestine pe, peristomium (From White, General Biology.)

g,

gizzard

n,
;

nephridium sb, subpharyngeal ganglion st, II-XXI, somites pJi, pharynx pr, prostomium.
;

the animal.
the

The esophagus is a straight narrow tube extending from pharynx through the fourteenth segment. In segments X to XII three pairs of yellow lateral pouches open into it. These are the

204

TEXTBOOK OF ZOOLOGY

calciferous glands, the secretions of whicli help to neutralize the acid

organic matter taken as food.

The esophagus opens

into the crop,

a larger, thin-walled structure, which extends through segment

XVI.

This

is

followed by the muscular gizzard in segments

XVII and

XVIII.

thin-walled intestine extends to the anus, which opens to

the outside through the last segment.

The
sole,

intestine

is

not a simple tube but has a large fold, the typhloits

protruding into

lumen from the dorsal

side giving

it

more

absorptive surface for the assimilation of food.


<lv

The coelomic

side of

Fig. 105. Cross section of the earthworm through a posterior segment, ch, chloragogue cells cir, circular muscle fibers coe, coelom cti, cuticle dv, dorsal Id, lateral blood vessel la, lateral neural vessel ep, epidermis int, intestine branch of dorsal vessel loti, longitudinal muscle fibers n, nephridium nc, nerve cord sb, subneural blood vessel se seta, ty, typhlosole vv, ventral blood vessel. (Prom White, General Biology.)
; ; ;

the

intestine

is

covered with a layer of brown

cells,

known

as

chioragogen

cells,

whose function

is

doubtful.

They are generally

believed, however, to play a part in the excretion of nitrogenous

wastes.

The food

of the

earthworm

consists of almost
its

matter which

may
soil

pass through

digestive tract.

any kind of organic The animals

remain in the

during the daytime and work their

way through
I
I

I I I

PHYLUM ANNELIDA
it

205

by passing

it

continually through the digestive tract.

At night

they come to the surface of the ground, usually remaining partly


within or very close to the burrow, and feed on dead organic matter, such as leaves. Food is drawn into the mouth by suction pro-

duced by the muscular pharynx. In the pharynx it receives the from the pharyngeal glands and is then passed on through the esophagus, where it receives the secretions from the calciferous glands. It is then passed into the crop and is stored there long enough for the secretions of the calciferous glands to neutralize the organic acids which may be present in the food. It is then passed into the gizzard, where it is ground by contractions of the muscular walls of that organ. This process is aided by sand grains which are swallowed along with the food. From the gizzard the food is passed into the intestine where digestion is completed and the absorption of digested materials is accomplished.
secretions

Circulatory System

The blood of the earthworm consists of a clear liquid plasma in which there are numerous colorless cells. The red color of the blood,
as seen in a living specimen, is due to a pigment known as Jiemoglohin suspended in the plasma and not in the corpuscles as is the case in many animals. A complicated system of blood vessels makes up the circulatory path of the blood. The principal ones are: (1) the

dorsal blood vessel, (2) the paired hearts (usually five) in segments

VII

to XI, (3) the ventral blood vessel, (4) the subneural trunk, (5) the parietal vessels, (6) the typhlosolar vessel, and (7) the intestinovessels.
it

integumentary
teriorly

and forces
it

The dorsal vessel conveys the blood analong by waveiike contractions. The paired
which distributes
it

hearts receive the blood from the dorsal vessel and by pulsating move-

ments force

into the ventral vessel

to the
is

body
taken

wall, the uephridia,

and the

intestine.

In the intestine food

up

in the skin gaseous exchanges are


soil
;

made with

the water in the

moist

and

in the nephridia the nitrogenous wastes are removed.

The lateral neural vessels receive freshly oxygenated blood from the skin; hence the nervous system receives the most highly oxygenated blood. From the lateral vessels it passes into the subneural, where
it

flows posteriorly,

the parietal connectives.

the typhlosole into

and then returns to the dorsal vessel by way of The blood flows from the intestine through the dorsal vessel by dorso-intestinul vessels. An-

206

TEXTBOOK OF ZOOLOGY

terior to the hearts the dorsal vessel carries the blood posteriorly

and the ventral


is

vessel carries it anteriorly. The circulatory system equipped with numerous valves which keep the blood from flow-

ing in the

wrong

direction.

Dorsal vessel

- vessel ,- Ventral vessel


fieart

Intostlno-tegxuueniai;

Sub-neural vessel

Septa
Dorsal vessel

Septa

jl

IX

Septa

VIII

Intestlnotegtimentary
vessel

Dorsal vessel

CEsopbagus

Typhlosolar
vessel

Ventral vessel

Kephridium

Ventral
vessel

Sub-neural vessel

Lateral-neural vessel

Sub-neural vessel
Afferent intestinal vessel

Efferent Intestinal vessel;

Dorsal vessel

Typhlosolar vessel
Ventral vessel Sub-neural vessel

Parietal vessel

Fig. 106. Circulatory system of the earthworm. A, longitudinal view of vessels In somites VIII, IX, and B, transverse section of same region C, longitudinal view of the intestinal region D, transverse section of the same region. (From Hegner, College Zoology, published by The Macmillan Company, after Bourne, after Benham.)

Respiratory System
Respiration in the earthworm
is

carried on through the skin

which

is

well supplied with blood.

Since the animal always lives


is

in a moist environment, this type of respiration

possible.

Excretory System

The function
the last one.

of excretion is cared for principally

nephridia, which are found in each segment except the

by the paired first two and


The
cilia of

A
,

single

nephridium

consists of a ciliated funnel (the

nephrostome)

a thin coiled tube, and a nephridiopore.

the nephrostome create a current which takes the fluid containing

PHYLUM ANNELIDA

207

nitrogenous wastes from the coelom into the tubule where it can pass to the outside through the nephridiopore. Also the wastes in the blood are excreted by way of the nephridial tubules. The nephro-

stome is located in the posterior part of the segment and leads into the tubule of the segment just posterior to it. The nephridium coils

two or three times before reaching the nephridiopore.

Tl^t

a, ampulla between the 6il, ciliated canal coe coelomic epithelium; ext, external opening ^nephridiopore) Zc^ nn^Mr'^f'^^^'^ ^''T' septum between somites; nst, nephrostonie^ (internal openfneT ?;/**in^ter.^iii T^'' ^anal. (From Parker and Haswell Textbook of Zoology, mjWikherf by The MacmiUan Company, after published h^ r^^t'^^^"" Meisenheinmer, after MazE^irski.)

^...^.'S-.

ciliated

107. structure of a nephridium from earthworm, and nonciliated portions of the intracellular canal

208

TEXTBOOK OP ZOOLOGY

The Nervous System


The "brain"
ganglion.
of the

earthworm

consists of the suprapharyngeal

ganglion, two circumpharyngeal connectives, and the suhpharyngeal

The ventral nerve cord extends posteriorly the length of the body with a ganglion and three pairs of nerves in each segment.
-prosfomium

-SvLpharyn^Zo/ Gang/fo/7

Posterior <Sejfmeti^a/ A/erve

Bo</y IVa//

Sefi/um

^et-^e Core/

Fig. 108.

Anterior
is

portion of the nervous system of an earthworm from dorsal view.

Each ganglion
dition

really the fusion of two, a deviation

from the con-

found

in

many

annelids and arthropods where there are two


is

ganglia in each segment and the nerve cord

double.

The suprasegment

pharyngeal ganglion

lies

dorsal to the

pharynx

in the third

and the suhpharyngeal ganglion lies ventral to the pharynx in the fourth segment. Nerves from these two ganglia innervate the first three segments and the prostomium.

PHYLUM ANNELIDA

209

Stimuli are received by sensory cells and are passed into the ventral nerve ganglia by the afferent nerves. The stimulus is modified in the ventral

efferent neurons.

ganglia and sent to the responding organs by Nerve impulses then have the nature of a simple

reflex except that the ventral ganglia are connected

neurons which conduct stimuli from one to the other.


this

arrangement a stimulus applied to any part of cause responses to occur in a wavelike manner in both directions from the point of stimulation. Located in the dorsal part of the

by association Because of the body will

nerve cord are three giant fibers which serve as the sole means of conducting an impulse directly from one end of the body to the other. By this means the worm can contract its entire body at one
time.

Reproduction

earthworm is hermaphroditic. however, each egg being fertilized by a sperm from another individual. In reproduction two animals come together with their anterior ends pointing in opposite direc-

As has already been

described, the

Self-fertilization does not occur,

tions

and the ventral surfaces


Apertures
of

of their bodies in close contact

from

5eminal receptacles

Aperture of Vas deferens

C/it el lam

5eminal droove

Pore of oviduct

Dorsal
blood vessel

Intestine

5eminal
groove

body

wall

Band of mucus
secreted
elite II

am

Fig. 109. Reproduction in earthworm showing copulation and the cocoon. A, two worms enclosed in bands of mucus B, transverse section showing the seminal
;

grooves

O, cocoon.

210

TEXTBOOK OF ZOOLOGY

the anterior end to the clitellum.


closed passage
is

With

their bodies in close contact a

formed between the genital openings of the two inSperms pass from the testes out through the seminal dividuals. vesicles and vasa deferentia to the closed passage and move through it
blastocoef

t^eso blast cell

Endoderm

Coelomk
cavities

Hesodermic
tissue

Ectoderm

Development of the earthworm. A, blastula, surrounded by a memFig. 110. brane B, section of a blastula showing blastocoele and one of the mesoblast cells cell) of mesoderm layer; G and D, showing stages in the beginnmg of (primary gastrulation E, side view of gastrula showing invagination F, section of gastrula along a line to show polar cells, mesoderm layers on each side of them and the H, gastrxila in archenteron G, later stage showing cavities in the mesoderm cross section /, longitudinal section of a young worm after formation of the mouth and anus J, same as I but in cross section K, cross section of later stage. (After W^ilson, Embryology of the Earthworm, Journal of Morphology, 1889.)
;

PHYLUM ANNELIDA
to the seminal receptacles of the mate,

211
stored.

where they are

In

the meantime, the clitellum of each individual secretes a

band

which binds them together at these two points. After each has received sperms from the other, they separate by working themselves through the bands secreted by the clitella. This leaves each animal with a band which is gradually worked off toward the anterior end. As the band passes over the openings of the oviducts eggs are released into it, and as it passes the openings of the seminal receptacles sperms which came from the reproductive mate are released.
tion

Both ends of the band close, forming a cocoon and development take place.

in

which

fertiliza-

Regeneration and grafting in earthworm. A, anterior five segments Fig. 111. regenerated from posterior part of a worm B, tail regenerated from the posterior D, portion of a worm C, tail regenerated from an anterior piece of a worm union of three pieces to make a long worm E, grafting of two pieces to form a pieces grafted together to form a double bodied worm F. anterior and posterior new worm. The stippled areas in A, B, and C represent regenerated tissue. (From Hegner, College Zooloc/y, published by The Macmillan Company, after Morgan.)
;

Cleavage in earthworms is of the unequal holoblastic type. Soon after the segmentation cavity is formed, a certain cell, known as the
mesohlast
cell, is set
off,

move

into the cleavage cavity,


cells

and the cells resulting from its divisions and will form the mesoderm. As the

mesoblast

move

into the cleavage cavity, gastrulation occurs

by invagination to form the endoderm and ectoderm. The gastrula elongates and the archenteron opens at both ends to form the mouth and anus. The mesodermal cells which fill the space, between the ectoderm and endoderm develop segmental cavities which are the
coeloms of the metameres. At this stage the animal constitutes a tube within a tube and from it the organs of the adult develop.

212

TEXTBOOK OF ZOOLOGY
Regeneration

Earthworms have been used extensively in regeneration experiments because they possess the ability to regenerate lost parts. It has been demonstrated that when the anterior end is cut off, in front of the eighteenth segment, the segments from one to five will be regenerated. If the cut is made posterior to segment eighteen a new anterior end will not regenerate on the tail half, but instead another tail will develop from the cut surface. This produces an animal with two tails and no head, and death from starvation reWhen any part of the tail region is cut off, the lost parts sults. readily regenerate. Numerous grafting experiments have also been
performed on earthworms.
grafted to the cut surface
to
it

Almost any part of an individual


properly located) of another will fuse
of earth-

(if

and grow. In this way numerous unusual forms worms have been produced.
Class Archiannelida (ar ki a nel'i da,
first

Annelida).

This

class

includes numerous small marine forms which resemble Chaetopoda


in a

number

of ways.

It is

now

believed that they have been de-

rived from that group by changes usually involving the reduction


or loss of certain structures.
setae

They are very small and lack both and parapodia. Internally they are very similar to the earthworm. The best known example of this group is Polygordius, which has a long cylindrical segmented body with a pair of tentacles

on the prostomium.

Two

ciliated pits are present as a retenis

tion of juvenile characters.

The troehophore larva


a,

common

to

the entire group.

Class Hirudinea (hlr u din'e

leech).

These

animals are com-

monly known

They are usually flattened dorsoventrally, possess both an anterior sucker and a posterior sucker, have characteristically thirty-two segments and possess no external appendages. The anterior sucker is formed from the prostomium and first two segments, and the posterior one comes from the last seven. Each segment shows externally a variable number of annuli or rings, making the animal appear to possess more segments than are really present. Leeches are commonly parasitic and live by sucking blood from other animals. In a typical leech, of which Hirudo medicinalis is a good example,
as the leeches.

there

is

a muscular pharynx, a short esophagus, midgut or crop,

PHYLUM ANNELIDA
intestine,

213
is

and ectodermal rectum.


it

Blood which

sucked from an-

other animal receives a ferment from the salivary glarids of the

pharynx, which prevents


times
before
its
it

the diverticulae of the crop.

from coa^lating. It is then stored in The animal is capable of ing'esting three own weight in blood, and, since several months may elapse is all digested, frequent feedings are not necessary. The

Fig. 112. Development stages of Polygordius, an archiannelld. A, blastula C, early trochophore larva D, optical section of trochophore larva showing apical plate with eyespot, head kidney and preoral band of cilia E, trochophore larva with posterior growth region F, segmenting larva G, advanced larval condition in which the trochophore larva is seen with head, mouth, eyespots, and tentacles; H, adult worm, showing faint external segmentation. (From Newman, Textbook of General Zoology, published by The Macmillan Company, after Whitman.)

B, gastrula

214
coelom
is

TEXTBOOK OP ZOOLOGY
very

much reduced due

to the excessive
is

development of the

mesodermal
through
present.
it

tissue.

Each animal

hermaphroditic.

placed on the skin of another leech, and they apparently


into the ovaries, where fertilization occurs.

Sperms are work Development

takes place in a cocoon produced

by the

clitellum.

Two

nephridia are

The nervous system

is

typical of the annelids.

Glossiphonia fusca, a leech, showing annulation, segmentation, and Fig. 113. dt. ej., ductus an, anus 2-70, annuli internal organs. I to XXVII, somites ejactulatorius ga and in, intestine ifflv, crop or stomach oe, esophagus ov, ovary; po d, male aperture; po ?, female aperture; pro, proboscis; te, testes. (Prom Ward and Whipple, Fresh-water Biology, published by John Wiley & Sons, Inc., as modified from Castle, Bulletin of Museum of Comparative Zoology.)
; ; ; ; ;
;

Class Gephyrea

(je

fi

re'a,

bridge).

This

class is a

group of

marine annelids which are nonsegmented, have no appendages, and They are usually comparatively large possess a trochophore larva.

and

live in shells, crevices,

and such other places

as will afford protec-

In this class, the representatives of the order Echiuroidea have tion. a well-developed prostomium, used in capturing prey and in locomotion. In Bonellia, the female is the normal individual, while the

PHYLUM ANNELIDA
male has no proboscis,
the female.
is ciliated,

215
segmental organ of

and

lives in the

Representatives of the order Sipunculoidea have no


in the adult.

prostomium

Nephrostomo

Male genital pore


Alimentary canal Alimentary canal

Anus

male Bonellia viridis. Female (above) has bifurcated proboscis (below) is ciliated over the surface and much smaller. (From Hegner, College Zoology, published by The Macmillan Company.)
Fig.
114.
;

Importance of Annelids to

Man and

Other Animals

Even though no casual observer would consider that annelids have any important relationship to other living organisms, they have been found to be of great importance in a number of ways.
Darwin concluded from some forty years of observation that the earthworms in an acre of ground could bring to the surface in one
year as

many

as eighteen tons of feces,

known

as castings.

This

in-

dicates without doubt that these animals are of great value, because
in stirring the soil they cover

up

objects, causing
soil also

Their continuous burrowing through the


a necessary condition for plant growth.

them makes

to decay.
it

porous,

Earthworms have
sites

also less desirable

qualities.

secondary hosts for parasites of several animals.


birds, pigs,

They serve as Most of the para-

having the earthworm as a secondary host live as adults in and other animals which use the worms as food.
serious problem in

some of the irrigation districts of the Southwest by burrowing through levees until they are too porous to hold water. Before irrigation was started they

They have created a

did not appear to be at

all

numerous, but with the presence of water

they have become very abundant.

216

TEXTBOOK OF ZOOLOGY
in the bleeding of individuals

The medicinal leech was once used


as a treatment for various ailments. as parasites on turtles

Various forms of leeches live of animal life in the water. They are not at all averse to attacking human beings when the opportunity presents itself; however, they cause no great injuries and are important only as pests and as secondary hosts for some

and other forms

parasites, thus spreading certain diseases.

Marine annelids are important only as food for larger forms. In many regions the burrowing forms along the tide levels literally form a good grazing ground for fishes. The fish simply swim along, biting off that part of the worms protruding from the mud or sand. Instead of dying the injured worms regenerate one or more new heads and go about their business.
Phylogenetic Advances of Annelida
Segmentation, (2) coelom, (3) alimentary canal with defined parts, (4) closed circulatory system, (5) excretory system of nephridia, (6) muscular system, (7) concentrated mid-ventral nerve
(1)

cord connected to a dorsal pair of suprapharyngeal ganglia, (8) respiratory system (gills on parapodia of Nereis), and (9) improved
sense organs (eyes, palps, and tentacles of Nereis).

CHAPTER XVI

PHYLUM ECHINODERMATA
The Echinodermata (e ki no dur'ma ta hedgehog skin) constitute backward phylum of animals which are thought to have undergone a certain amount of retrogression in structural features. That is, they seem to have a lower level of organization than that possessed by some of their ancestors. The modern echinoderms, as the group is commonly called, possess several distinctive characa rather
teristics.

Some

of these characteristics are as follows


;

skin cov-

ered with spines

lack of segmentation

triploblastic radial

sym-

metry, subduing a primitive bilateral symmetry; water vascular or ambulacral system and tube feet circumoral nerve ring ajid radial nerves; a calcareous skeleton composed of plates; pedicellariae
;

and a coelom. The external opening into the water vascular system is called the madreporite. It is located on the dorsal or aboral side, at an interradius between arms in such a position that a line drawn through it and on through the radius of the opposite ray, will divide the body into two similar halves. There will be a half ray and two complete rays in each half of a five-rayed animal divided
in this way.
Classification

In earlier classifications as in the case of Cuvier, this entire group

was included along with coelenterates in a group called Radiata, The basis for this was the apparent similarity of radial symmetry. It was later discovered that the coelenterates have a typical primitive radial symmetry while the Echinodermata have only a secondary radial symmetry which is derived from a bilateral condition. This is indicated quite definitely by the fact that the larvae of echinoderms have a typical bilateral symmetry. The change which occurs seems to be an adaptation to a sedentary habit. This phylum is usually divided into five classes of modern forms including such

common
bers,

animals as starfishes, brittle stars, sea urchins, sea cucumlilies.

and sea

Class Asteroidea.

The

central disc usually with five

general features of the body include a arms or rays radiating from it. There

are some species which do not adhere to this pentamerous condition


217

218

TEXTBOOK OF ZOOLOGY

stricted

and have up to forty rays. The rays are not usually sharply confrom the central disc. An ambulacral groove is present in each ray. This groove is formed by a particular arrangement of
Skeletal plates also support the aboral side of the
central disc as well as the region

skeletal plates.

around the mouth.

Surrounding

the spines on the skin are distributed some pincherlike calcareous


^^SlK*n,

Fig. 115.

brittle star, oral view.

organs called pedicellariae.


species.

These vary considerably in different


aborally.

The anus and madreporic plate are located


is

The
sys-

mouth
tems

located on the oral or ventral side of the central disc, and

the radiating parts of the vascular, ambulacral,


lie

and nervous,

orally along the ambulacral groove of each arm.

Asterias,

Astropecten, Pisaster, Solaster, Oreaster and Echinaster are representative genera.

PHYLUM ECHINODERMATA
Class Ophiuroidea.

219
stars.

Brittle

stars

and serpent

There

is

small central disc with five long, slender rays which are clearly

from the disc. The rays are lacking in ambulacral The tube feet do not serve in locomotor functions but are grooves. Both the madreporite and mouth are located ventactile only.

marked

off

trally.

On

the oral side of the central disc, five interradial sets of

toward the mouth in the formation of jaws which are operated by muscles in chewing food. The anus is not The brittle present. The viscera are confined to the central disc. serpent stars are quite active and live in the shore waters. stars and At low tide they may be found under rocks and debris, but they move about and feed during high tide. The ability of autotomy
skeletal plates project

Fig. 116.

Oral view ofofa Generalstar belonging class Ophiuroidea. basket Biological Supply House.)
to

(By courtesy

(self -mutilation)

is

so well developed here that

arms

will

become de-

tached by merely grasping them.


entire animals alive.

This makes it difficult to collect Ophioderma, Ophiura, Ophiothrix and Gorgono-

cephalus are

common

Atlantic and Gulf Coast genera.

Class Echinoidea.
tives of this
still

Sea

urchins and sand dollars are representalost the

group the members of which have

rays but

retain the pentamerous (five division) condition of the body.

The sea urchins are globular or hemispherical, while the sand dollars are disc-shaped. The skeleton or test is composed of five rows of closely fitting plates which are usually arranged into five pairs of inter-ambulacral rows. The position and condition of these rows

220
of plates can be
its

TEXTBOOK OF ZOOLOGY
compared
to

body

until the tips all

a starfish with its arms turned up over touch each other. The surface of the

skeleton bears processes which support movable spines.

Tube

feet

be thrust out through perforations in the plates of the ambuThese rows correspond in position to the ambulacral lacral rows. grooves of the starfish. The plates of the inter-ambulacral rows
Madrepori'fe
W':-

may

9^
ItT{-eranjbu/acra(

p/afes

*^

Anybu/cfcraf

p/afes

Fig. 117. Dried test of the sea urchin, Arhac'xa. A, shows arrangement of the (From plates on the aboral side; B, oral view showing mouth and perioral area. Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.)

The mouth of this type of animal is located ventrally (orally), and it is guarded by five projecting skeletal processes called teeth. These converge over the aperture and are set in a skeletal case which is composed of many hard ossicles and contains the muscles for moving the teeth. The teeth are used in
are not perforated.

PHYLUM ECHINODERMATA

221

Aristotle's lantern,

removing algae from rocks for food. This arrangement constitutes and the esophagus leads internally from its
aboral part.

Aborally, each ambulacral area ends in a single ocular

plate at the periproct.

series of

genital plates interspace the

ocular plates of the periproct, and one of these genital plates has

become the madreporite.


center of the periproct.

The anus

is

a small pore located near the

Jtomac/?
flmbalacral

Ridge

Fig. 118.

portion of the test of the sea urchin has been removed to internal organs.

show the

Upon

first

view, a dissected sea urchin seems to be almost entirely

gonads, internally.

In each pentagonal section there


is

is

a mass of

the gonadal structure which

held in place by a band of tissue

known

from the aboral wall. Small from each gonad through a pore in its adjacent genital plate. The general arrangement internally includes a water vascular system composed of a stone canal leading from the madreporite to a circumoral or circumesophageal canal which encircles the mouth. A radial vessel extends from the circumoral canal through
as the genital rachis extending

genital ducts lead

the length of each ambulacral arch.

In each ambulacral plate are


is

found two pores for a tube foot which

a lateral branch of the

222
circumoral canal.

TEXTBOOK OF ZOOLOGY

Each foot is connected with a bulblike ampulla. These tube feet along- with some mobile spmes constitute the locomotor system. Five interradial pouches or branchiae or "Polian vesicles" are in communication with the circumoral canal. The esophagus leads from the aboral part of Aristotle's lantern into the
flat,

dilated stomach.

The stomach extends almost around the

inter-

nal wall of the body.


direction

From

it

the intestine leads in an opposite

and

in sea urchins finally

opens externally by the median

aboral anus.
rior

In the sand dollar the intestine passes along the posteto the anal

ambulacral plate
to

of the disc-shaped body.


allel

opening which is near the margin branch from the esophagus runs parit.

the stomach and finally joins

This tube

is

heavily

Fig.

119.

Thyone,
is

tlie

common

sea cucumber.

ciliated internally

and

known

as the siphon.

Its function is con-

jectured to be either respiratory or a means of washing refuse from

The principal organs of respiration are the interradial pouches and the tube feet. The nervous system is composed
the intestine.
of a circumoral ring with radial cords extending into the ambulacral areas.

Strongylocentrotus, Arbacia,

Tripneustes,

Clypeaster,

and Echinarachnius (sand


group.

dollar) are representative genera of the

Class Holothurioidea. The eehinoderms of this class have only an incomplete skeleton, the body is elongated, the mouth surrounded by tentacles is located at one end and the anus is at the other. These animals are called sea cucumbers because of their shape and

PHYLUM ECHINODERMATA
color.

223

some remnant of the pentamerous condition in that there are five double rows of tube feet extending lengthwise on The five sides of the body of some forms, others have less or none.
There
is

expanded body of a holothurian is soft like a bladder partly filled with liquid and the body wall is very muscular. The madreporite The row^s of tube feet serve as structures of is located internally.
Oral
R/n0
Po//<frtf

fen-f-ac/es

cai/7a/

ves/c/es
Soncrcf

Afnpa//ae
cr -\V^/r?^esf/'^e

!_'-

b/ooc/ yessef

bfood vessel
/nusc/es

/nusc/e ba/7cf
t

^^fBi
^

fiespira-^ory

Coviencrr7
/?a(;f/'a/

n7U5c/es

Fig.

120.

Diagram showing the internal anatomy of a sea cucumber. (From Wolcott, Animal Biology, published by McGraw-Hill Book Company.)

locomotion and for clinging.


assist the tentacles in

Some

of them, next to the mouth,

procuring food. Within the cloaca are the openings of two long tubular respiratory trees which receive water to assist in respiration. The tube feet, tentacles, cloaca, and other

organs serve in respiration. These respiratory trees function also as excretory organs. The madreporite draws water from the inside
of the body cavity.

224

TEXTBOOK OF ZOOLOGY
of

The digestive system


esophagus which
is

most sea cucumbers consists of a short

supported by a skeletal structure at the point

body cavity. This structure serves as attachment for the tentacles and retractor muscles. Following the esophawhere
gus
it

enters the

is

a short but rather inflated stomach which leads to the long, This tube
is

by a mesentery which is attached to the midventral line of the body wall. The intestine is thickened in its posterior portion to become the muscular cloaca which contains the openings of the two respiratory trees. In the coelom are fine longitudinal muscles that lie in the ambulacral areas. The gonad and genital duct are in ambulacral areas. They are found free at one side of the esophagus and stomach. This duct opens exteriorly by a pore beside the mouth. The food of the sea cucumbers is largely the organic material derived from mud which is ingested. This class of animals possesses a striking power of autotomy and subsequent regeneration. When they are irritated or disturbed, the muscles of the body cavity contract and produce internal pressure sufficient to cause either the body wall to split near the anus where the viscera are ejected or the viscera are forced out the mouth. Other animals, attempting to attack the sea cucumber, are rendered helpless by becoming entajigled in the visceral mass. The sea cucumbers can then regenerate the lost viscera in a short time. This power to eviscerate itself is a unique charactercoiled intestine.

partially supported

istic

of the group.

Representative genera of this class include

Thyone, Holothuria, Cucumaria, Leptosynapta, Aphelodactyla and


Caudina.
Class Crinoidea.
stalks,

Most

of the sea lilies live attached

by long

the

At the free end many-branched arms which make up the calyx. The branches In most forms some lateral proof the anus are called pi^inules.
but a few are free.
of the stalk are located
five,

jections, called cirri, are distributed at regular intervals


stalk.

along the

The mouth

is

located in the uppermost center of the calyx

and

is

surrounded by the anus.

The anus
ciliated

is

also to be

found on the

oral side of the calyx within the enclosure

made by

the arms.

On

the oral surfaces of the

arms are

amhidacral grooves which

serve to transport food to the mouth.

Modified tube feet are present,


feet.

but they serve more as tentacles than as

They lack ampullae

PHYLUM ECHINODERMATA
and are
lactile

225

and

slightly respiratory in function.

Gonads are

borne on the arms.

The skeleton

is

well developed in all crinoids

and for that reason many

of the ancient forms are preserved as fos-

^TALKZD CKIWOID
Fig.
121.

-Distal portion

of a

stalked crinoid.

(Courtesy of General Biological

Supply House.)
sils

in widely distributed limestone layers of the earth's surface.

The

nervous and circulatory structures parallel the ambulacral grooves

and

encircle

the

mouth.

Neocomatella, Pentacrimis, Rhizocrinns,

Metacrinus and Antedon are representative genera.

226

TEXTBOOK OF ZOOLOGY

STARFISH OF CLASS ASTEROmEA


Habitat and Behavior

The

starfish lives

along the shores and in the shore waters (to a

depth of over 125 feet) of our stony coasts of the Atlantic and
Pacific,

with scattered ones occurring in the Gulf of Mexico.

few

scattered individuals

may

be found on

muddy

or sandy shores, but


to pilings,

they are quite scarce.


old boats,

They are often found clinging


in the water.

and other objects


tide they

By

action of the tube

feet they are able to cling very tenaciously to almost


object.

any

solid

At low

may

be found under the rocks, out of the

sun,

where they are protected from the heat and drying. Due to a food relationship they are usually found in the same area with marine clams, oysters, and rock barnacles. During the day they are rather inactive, but at night they are much more active and respond to such stimuli as light, temperature, contact, and chemicals. It has been demonstrated experimentally that starfishes may form They ordinarily live and move about with the oral side habits. next to the substratum, and if turned over, will right themselves in the same way time after time. If the arms which are habitually
used for this are incapacitated, they will acquire the habit of using another combination of rays in this act.

External Anatomy
of a central disc and some (usually five) The mouth is located in the center of the under or oral surface while the upper or aboral surface is covered with spines of various lengths. On the arms these spines are arranged somewhat in rows. Between the spines the exposed skin
is

The body

composed

radiating arms or rays.

is

extended into projections known as papula or dermal dranchiae. There are some small pincherlike structures, called pedicellariae, arranged around the bases of the spines, which serve to keep the surface of the exposed papulae clear of debris and foreign material. The pedicellariae are composed of two jaws or Mades and a basal There are large and small plate with which the jaws articulate. pedicellariae. In an eccentric position on the aboral side of the central disc is found the calcareous, sievelike madreporite. The portion of the central disc and two rays adjacent to the madreporite

PHYLUM ECHINODERMATA
constitute the hivium.

227
their adjacent por-

The other three arms and


compose the trivium.
rows of tube

tions of the central disc

On

the oral side sur-

rounding the mouth

is

a perioral membrane or peristome.


feet, radiates

An amhuthis along

lacral groove, containing

from

the oral side of each arm.

each
the

arm is called an eye. mouth and along the margins

reddish pigment spot in the end of The spines are longer and stronger around
of the ambulacral grooves than

elsewhere.

Fig. 122. Pacific coast

The

published by The Macmillan Company.)

ochre starfish, Pisaster ocJiraceus, an abundant form along- the (XVi). (Johnson and Snook, Seashore Animals of the Pacific Coast,

Internal

Anatomy

fectly rigid.

and hard without being perdue to the presence of the calcareous skeletal plates throughout, which are bound together by connective tissue and muscular fibers. These plates are often called ossicles.
is

The body wall

relatively strong
is

This condition

They lie in a The skeleton


articulated,

flat

position in the aboral portions of the

of the ambulacral grooves consists of four

body wall. rows of

oblong ossicles in each arm.

These ossicles are ar-

228

TEXTBOOK OP ZOOLOGY

ranged with the flat sides together, like cards in a filing ease. The two middle rows of ossicles are called amhulacral plates. Amhulacral pores, through which the tube feet project, are located between these The outer rows of plates, forming the margin of the groove, plates.

Dissection of the starfish, Asterias. The aboral wall has been reFig. 123. the trivium and a portion of the central disc. One ray of the biviuni has been turned to expose the oral surface and tube feet. The organs have been removed from one ray of the trivium to expose the skeleton. Am., ambulacra! groove; C.S., cardiac stomach; D.B., dermal branchiae; E, eyespot Cr, gonads; M, madreporite Os., ossicle; P, pelicellariae P.C., ploric caeca; Py., pyloric sac; ic, T.F., tube feet T.F.2, arrangement of tube feet in skeletal ray. rectal gland (From White, General Biology, published by The C. V. Mosby Company.)

moved from

PHYLUM ECHINODERMATA
are shorter and are
ossicles

229

known

as

adamhulacral plates.

Five

flat oral

surround the mouth. Within the body wall and extending into the arms is a large eoelom which is lined by a pcrito7ieum and filled with coelomic fluid. In The this cavity are located the organs of most of the systems. vertically from the digestive system is a modified tube extending mouth on the oral side to the minute arms at the aboral surface. From the mouth a short esophagus leads to the double-pouched stomach. The larger cardiac portion (or pouch) receives the esophagus and is separated aborally from the pyloric portion by a marked constriction. A large pair of branched glandular structures, known

Fig. 124. Diagram of a cross section through a ray of a starfish, avi, ampulla hca, db, dermal branchia amb, ambulacral ossicle coe, perivisceral coolom hepatic caeca musj muscle oSj ossicle pd, pedicellaria ph, perihemal space ra, Sp, septum m sp, spine rv, radial blood vessel radial canal radial nerve rn, radial radial blood vessel (rv) tf, tube feet; v, valve between tube foot and
; ; ; ;
;

canal.

(From White, General

Biology.)

as hepatic or pyloric caeca,

joins the pyloric

is located in each arm, and each pair pouch by a duct which seems to be a continuation of this pouch. These glands and possibly the pyloric pouch produce digestive enzymes in solution. The fluid secreted by the wall

of the cardiac portion probably does not contain enzymes.

short

rectum or intestine leads aborally from the pyloric pouch to the poreTwo brown, like anus at the exterior surface of the central disc. branched pouches arise from the rectum. These are known as rectal caeca or glands and probably have excretory function. In feeding, the starfish catches its bivalve prey in the five arms and humps

230
over
shell.
it.

TEXTBOOK OF ZOOLOGY

The tube

feet are attached to the shells, and,


is

by cooperais

tive activity,

an enormous pull
is
is

exerted on the valves of the


everted

After the shell


its

open, the stomach of the starfish

through

mouth and

are digested in situ.

An

spread over the tissues of the prey which abundance of digestive fluid secreted over
its

the food causes the mollusk to be digested in

own

shell

and

this

material

then taken into the stomach of the starfish. It is reported that between four and five dozen clams may be eaten by a single starIt has also been shown that a starfish may survive fish in a week. After feeding, the stomach is withdrawn after months of fasting. into the body cavity by five pairs of retractor muscles, one pair exis

tending from the pyloric portion to the ambulacral skeleton of each arm. The branched, treelike gonads fill the remaining space in each

arm and the external tween adjacent arms.

'pores

from them are located

in the crevice be-

Fig. 125. Longitudinal section through the central disc and one ray of a stara, anus am, ampulla car, cardiac stomach coe, perivisceral coelom ey, intestine eyespot hca, hepatic caeca m, mouth tnp. madreporic plate nr. i, ra, radial nerve ring oe, esopliagus os, ambulacral ossicle py, pyloric sac canal re, ring canal rca, lectal caeca sc, stone canal sp, spine tf, tube feet. (From White, General Biology.)
fish,
; ; ;

composed of the madreporite, stone Tiedemann's bodies, Water is taken in through lateral canals, ampullae, and tube feet. the sievelike madreporite on the aboral side of the central disc and

The water-vascular system

is

canal, circumoral or ring canal, radial canals,

is

conducted by the S-shaped, calcareous stone canal (hydrophoric

canal) to the ring canal, which encircles the mouth.


of the water through the madreporite

The movement
is

and stone canal

accomplished

by the action

of

cilia,

which

line

them.

On

the medial surface of

the ring canal are nine small Tiedemann's

(racemose) bodies, the

stone canal joining the ring canal where the tenth might be expected.

The five radial canals extend cells. one in the roof of the ambulacral groove of each ray. Numerous paired lateral canals arise along the length of each radial canal. Each ends shortly by connecting with its ampulla and tube
These bodies produce amoeboid
distally,

PHYLUM ECHINODERMATA
foot.

231

The ampulla is bulblike ajid is located above the roof of the It is connected through its ambulacral groove in the coelom. ambulacral pore with the contractile tube foot which hangs down
into the ambulacral groove.

The

distal or free

has a slightly inverted, suckerlike shape.

end of the foot The proximal pair of

tiadreporite

Radial canal

J^

Stone canal

Tiedemanri's body

Tube
Fig. 126.

foot

Diagram

of water-vascular system of the starfish.

Fig. 127.

Starfish

"walking" on glass. Notice the extended tube of General Biology Supply House.)

feet.

(Courtesy

ampullae in each arm of some starfish lack the tube feet and are sometimes erroneously called Polian vesicles. Alternate tube feet are farther from the radial canal than the others on each side. The ampullae and tube feet function effectively in locomotion, the ampullae contracting to force water into their respective tube feet

232

TEXTBOOK OF ZOOLOGY
TU A.

po

Fig. 128. Development and metamorphosis of the starfish. A, Dorsal view of early ciliated larva showing ciliated bands, and left and right coelomic pouches. B, Ventral view of bipinnaria larva showing the extension of the left and right coelomic pouches. C, Dorsal view of the same larva showing the left madreponc pore and water tube, and the fusion of the left and right coelomic pouches to form an anterior coelom. D, Dorsal view of an older larva showing the budding of the five water tubes from the left coelom. E, Left side view of a still older larva showing the water vascular system developing from the water tubes, and the rays of the adult starfish developing on the dorsal side. F, Brachiolaria larva in process of metamorphosis. The larva has settled on the preoral region which is greatly shortened. Q, Aboral view of a young starfish showing the developing spines. (Legend continued on opposite page.)

PHYLUM ECHINODERMATA
to

233

ampullae and tube feet are muscular. In large starfish the tube feet may be extended an inch or two. The sucker ends of these tube feet work like a vacuum cup and will adhere effectively to surfaces over which the animal is
extend them.

The walls

of both

drawing

When the pressure is released by the ampulla, the itself. tube foot contracts and draws the animal forward. When water is again forced into the tube, it releases its grip and is again extended. By alternation of the activity of tube feet in different parts of the body the animal is able to move itself from one place
to another.

The

entire water vascular system

is

a modified part

of the coelom.

thin-walled system of vessels running parallel to the water vascular is the circulatory system. It is enclosed in a perihemal

space.

In addition to this the coelomic fluid, which occupies the all of the organs, serves as a circulatory medium This fluid in that it absorbs the digested food and distributes it. bears amoebocytes which are cells capable of picking up particles

coelom and bathes

of waste material

they pass through the

dermal branchiae, where These dermal branchiae are pouches of the coelomic wall which extend outward between the skeletal plates and have the additional function of When these pouches are completely extended, they respiration. nearly cover the exterior surface of the animal, and thus expose

and carrying them

to the

membrane

to

the exterior.

an enormous area

to the

water for respiration.

Excretion is carried out in part by the amoebocytes which have been produced by the Tiedemanu's bodies and have migrated to
the coelomic cavity.

The

rectal caeca serve in respiration to

some

There is a certain amount of diffusion of dissolved wastes through the dermal branchiae and the walls of the tube feet.
extent also.

The nervous system is radially arranged about the oral ring which From the oral encircles the mouth just orally to the ring canal. length of each arm and ends in the ring, a radial nerve extends the
pigmented eyespot.
grooves.

These nerves
is

lie

in the roof of the ambulacral

The aboral surface


;

supplied by a less conspicuous aboral


arms
f,
;

a,

anus
;

ac,

adoral ciliated band


fixation
int,

m, mouth Ic, left coelomic pouch lateral arm I, mp, madreporic pore and water tube pad, posterodorsal arm band pr, preoral ciliated band re, right coelomic pouch sp, (Modified spines; st, stomach; w. five water tubes of the water vascular system. from Wilson and McBrlde. By permission, The Macmillan Co.)
intestine
; ;

b, brachiolar ad, anterodorsal arm anterior coelom es, esophagus dr, dorsal surface developing rays
; ;
;

ci,

point of

md,
;

median dorsal arm

po, postoral ciliated

234

TEXTBOOK OF ZOOLOGY
Branches of these

nerve which extends from an anal nerve ring.

nerves extend to the numerous nerve

cells distributed in the epi-

The pigment eyespots at the tips The pediof the arms are photosensitive and sensitive to touch. There is little cellaria and tube feet are also sensitive to touch. centralization except in the oral ring and radial cords, still there
dermis above the nerve cords.
is sufficient

centralization for the necessary coordination exercised

by the animal.
Reproduction and Life Cycle

I
The repro-

The

starfish is dioecious

i.

e.,

the sexes are separate.

ductive systems of the two are similar and each consists of five paired gonads lying in the cavity of the rays beside the pyloric
caeca.

They open to the exterior by pores in the angles between Mature eggs produced in ovaries of females and mature arms. spermatozoa discharged from testes of males are freed in the ocean

water where they unite in fertilization. Total, equal cleavage is the type of division which follows fertilization, and this finally gives rise to the many-celled, free-swimming, ciliated hlastula. The wall of this infolds to form a gastrula. Following this the rounded body becomes

somewhat elongated and lobed. Ciliated bands develop over its surface and it is known as hipinnaria. This larval stage has bilateral symmetry, and the larva swims about near the surface for weeks by the aid of its ciliated bands. A later modification of the hipinnaria in which there are several extended symmetrical processes,
is

known
is

as the hrachiolarian stage (Fig. 128).

Following this condi-

and symmetry superimposes the bilateral. The presence of the bilateral symmetry in these larval stages seems to indicate that the ancestors of echinoderms were likely animals with this type of
tion

a metamorphosis during which

many

processes are formed,

the radial

symmetry.
Regeneration and Autotomy
Regeneration
is

the

name

applied to the power some animals have

to replace mutilated or lost parts.

The

starfish has this

phenomenon
or all of the

quite well developed with regard to

its

arms.

Any

arms

of the starfish

(Fig. 367.)

may be lost and the missing parts regenerated. An arm with a small portion of the central disc will

regenerate the missing parts under favorable conditions. A mutilated arm or one caught in the grip of some enemy may be cast off

PHYLUM ECHINODERMATA
by breaking
disc.

235
it

loose at the constricted point


is

where

joins the central

This ability of self -mutilation


is

known
new

as autotomy.
part.

Follow-

ing autotomy there

regeneration of a

Economic Relations

Compared with many other animals the echinoderms


unimportant economically.

are relatively

The sea cucumbers of several different as food by the Chinese and other oriental people. species are used The larger animals, some of them two feet long, are eviscerated, boiled, soaked in fresh water, dried or smoked and sold under the

name

of heche-de-mer or trepang.
It is

This dried product


is

is

semileathery

and gelatinous.

quite expensive and

usually served as a

very palatable soup. The chief fisheries are found along the shores of China, the East Indies, Australia, and the Philippines; some, however, are taken in California, Hawaii, and the "West Indies.

Sea urchins of several kinds furnish a sort of caviar known as "sea eggs." The egg masses are taken from the sexually mature females and are eaten either raw or cooked. Each specimen contains

considerable

spawning.

quantity of roe at the season just before Production of "sea eggs" has become quite an industry

in the Orient, Italy,

and the West

Indies.

ticularly noted for their production of this

The Barbados are parcommodity.

Perhaps the
relationship
is

starfish is the

most important of the group, but its almost entirely of negative importance. It is one

and snails. The starfish around the oyster beds of the Atlantic, grows in enormous numbers attacks the oysters, and feeds on them, leaving only the empty shells. A single starfish may eat as many as two dozen oysters in a day. Oyster hunters formerly attempted to protect the oysters and clams by dragging "tangles" made of frayed rope over the beds, catching large numbers of starfish, breaking them in two, and dumping the scraps back into the water. The fallacy of this was realized when their power of regeneration was learned, so at present they are usually dropped into boiling water or thrown on the bank
of the worst enemies of clams, oysters,
to dry.

delicious food

Salted or smoked starfish roe by many people.

(eggs)

are considered a

The

brittle stars

cal indices

and crinoids have little value except as geologiand biological specimens. Their skeletal parts contribute

to the formation of limestone.

CHAPTER XVII

PHYLUM MOLLUSCA
(By Elmer P. Cheatum, Southern Methodist University)

GENERAL CHARACTERS
clams, oysters,
externally,

The phylum Mollusca includes such familiar animals as the snails, and cuttlefish. Even though they appear different
all

are

soft-bodied,

unsegmented, usually bilaterally


present which
it

symmetrical, and most of them produce a shell composed principally^ of

calcium carbonate.
in the

muscular foot

is

may

be modified for different functions.

In the snail

is

used for

clam for plowing through the substrate, and, in the Covering at least a portion of the body is a mantle or dermal fold, the outer surface of which secretes the shell in most species. Between the mantle and main body is a mantle cavity which is usually either provided with gills or modified into a primitive pulmonary sac for use in respiration. Jaws are present in the snails, slugs and cephalopods. Within the mouth cavity of many species is the radula, which is an organ composed of fine chitinous teeth arranged in rows and used in
creeping
;

nautilus or squid for seizing and holding prey.

rasping food.

Approximately 78,000 species of mollusks have been described,


hence they constitute one of the largest groups of animal
life.

With very few exceptions they


ajid in the sea.

are sluggish animals and occupy a

diversity of habitats, occurring abundantly on land, in fresh water,

Although most of the species

live in

moist sur-

roundings, a few inhabit arid regions.


cuttlefish, are strictly

Some

species,

such as the

carnivorous;

many

of the snails are herbivo-

rous,

and others feed as scavengers.

The oyster and other species

that are attached during adulthood feed on the floating organisms


in the sea.

From the standpoint of their ancestry, the veliger larva of various marine forms bears close resemblances to the trochophore larva Whether or not they are direct descendants of the of the annelids.
236

PHYLUM MOLLUSCA
annelids
is

237

a matter for conjecture since some morpboJogists regard this similarity in larval forms as an example of adaptive parallelism Certainly morphological eviin a similar type of environment.

dence shows a close relationship.

THE SNAIL
(Detailed description based on Helix)

Habitat and Behavior


Snails occupy a variety of habitats. fresh water, salt water, brackish water,
live in the arid sections of

They occur abundantly in and thermal springs; they the country and occur abundantly in the

tropics

where certain arboreal forms are found. Some species belonging to the genera Caecilianella and Helix live underground, feeding
Apical or^an

Eye
[Mesodermal band

Apical or^an

Stomach
Preoral

Endoderm

Embryonic mMcle
Prototroch

EsopbaS

ciliated ring

Head

kidney.

Qtocfst

Blastocoelz

Mesoderm
Jelotroch

Mesenchyme

_,

MS

Rnal vesicle

A, Trochophore larva of Eupomatus (a polychaete annelid), side Fig. 129. (After Shearer.) B, Veliger larva of Patella (a marine snail) frontal secview. (After Patten. (Drawn by Joanne Moore.) tion.

embedded in moist and Helix aspersa, humus. Certain species, such as Helix hortensis excavate holes in rocks and live in them. Although most snails are
on roots of plants
;

many

other species live deeply

not tolerant to extremes of cold, Vitrina glacialis lives in the Alps above the timberline where the rocks are covered with snow most of the year; even some of our fresh-water snails in this country, such
as

Lymnaea
Land

palustris,

Physa gyrina and Helisoma

trivolvis,

when
ice.

frozen gradually, can live at least several weeks in solid cakes of


snails are

most

active either during a light rain or

immedi-

ately following.

In heavily shaded woodlands where surface moisture prevails, snails are active during the day as well as at night. The same species of snail that exhibits both diurnal and nocturnal activity in the

woodland may show only nocturnal

activity in an open,

238

TEXTBOOK OF ZOOLOGY

exposed habitat.

Movements

of

incident with moisture rather than darkness.

periods of cold, land snails may beneath dead logs, dense mats of humus, crannies in or under

appear to be coPreceding prolonged move to protected places, such as


snails

most land

Lymnaiza

Lymnoea
buUmoides
techelia

palusbris

Lymnaea
stagnalis
Ferrissia

(Lymnaeidae)

excenthca
(Ancylidaej

Physa bumerosa
Physa anatina
(

Fhysidoe)

HelisoiriQ

Tropicorbis liebmanni
Fig-.

Menetus
dllatatus

tnvolvis

lentum
(Planorbidac)
t

130.

Some

common

fresh water pulmonate snails.

rocks,

and there begin their period of hibernation. During this condition of torpidity the body of the snail may be well protected by one or several thin parchmentlike membranes called epiphragmas which are stretched across the shell aperture. When warm weather arrives, the membranes are broken and the snail resumes its activities.

PHYLUM MOLLUSCA
Water
snails are active all four seasons,

239

provided open water is available. Naturally their movements are slowed down in the winter due to cold, but when the pond or stream is frozen over, the movements of Lymnaea, Physa, or Relisoma may be observed through the During periods of dry weather when ponds and creeks dry up, ice.

manner are During this condition epiphragms may be formed in certain species {Lymnaea palustris), the same as in land snails; these structures probably funcsnails

embed themselves

in moss

and mud, and

in this

able sometimes to survive long periods of drouth.

tion in retarding water loss.

At least a few species of land snails possess a homing instinct. Helix aspersa, H. pomatia, and Polygyra roemeri have all been observ^ed to occupy as "home" a definite place and go out from this home on nocturnal foraging trips, then return by sunrise the next
' '
'

'

mornmg.

humboldbiono
chisosensis

Polyqyra

bulimulus dealbatus
liquabilis
terrestrial snails.

Rurnina

roemen
Fig. 131.

decoUata

Common

vary considerably in snails; some of the aquatic genera, such as Lymnaea and Helisoma may live two to four j^ears whereas some species of Helix may live to be six or eight

The

life

span seems

to

years old.

Parasitism

and commensalism are both exemplified by

certain

species of snails.

commensalistic relationship exists between the

rare mollusk Lepton

The former feeds on


of sponges,

squamosum and the crustacean Gehia stellata. by the latter. A few species echinoderms, annelids, and mollusks are parasitized by
secretions produced

various species of mollusks.

External Anatomy
Shell.

The

shell of the snail

may

be in the form of a low, broad,

or flattened spiral (Humholdtiana chisosensis and Polygyra roemeri),


or a long, tapering spire

(Lymnaea

stagnalis)

on the other hand,

240

TEXTBOOK OF ZOOLOGY
shells are

some
or

shaped

like

house roofs (Patella that lives in the sea

Ferrissia,
is

a fresh-water form).

The worm

shell

(Vermetus

spiratus)

so loosely coiled that it superficially resembles a

worm.

Some

shells,

such as those belonging to the genus Murex,


to the shell a grotesque appearance.
is

may have

long peculiarly curved spines extending out from the main shell

body that give

In the sea and

land slugs the shell

either rudimentary, internal, or absent.

Polyqyra texasiana

^ Polyqyra dorfeuiHiana

Helicina orb'iculata

f?etinella

tropica

indentata paudiirata

_ .... Zonitoides

orboreus

Euconulus chersinus trochulus

Strobilops lobyrinthica

(qostrocopta

armifera

texasiana

Succinea

Qvam

Zuqfandina
sincjleyana
Fig. 132.

Pupo'ides marq'inatus
terrestrial snails.

Phiiornycu^ carolinensis

Common
is

If the shell

is

held with the aperture toward the observer and the


left,

aperture
the shell

is is

on the

the shell

said to be sinistral; if on the right,

dextral.

Most

species are normally dextral, but occa-

sionally a reversal occurs

which has been found to be inherited.


of

i
is

The

shell

which

is

largely composed of carbonate

lime

secreted by the mantle and usually consists of three layers.

Emis

bedded within the

latter

may

be pigments that give the occasional

brilliant colors to certain species.

The thickness
salts in the

of the layers

dependent on the richness of lime


snails living in an acid

environment; thus,

bog have thin transparent shells, whereas the same species inhabiting an area rich in lime salts have thicker,

PHYLUM MOLLUSCA
perhaps opaque
P. alholahris
if

241

shells.

Certain species, such as Polygyra roemeri,

and P. texasiana are capable of repairing broken shells

damage is not too severe. The body of the snail consists of a head, neck, foot, and visceral hump. The head of a land snail (Helix) has one pair of
the

Body.

Limacina ausMb (Ptefopod)

Vemetus
{Worm

spirgtus
shelf)

Muxextenuispina
[Venus's comb)

Aeolis

Urosalpinx {Oyster drill)


, ,

(sea slug)
Fie:. 133.

Teredo navalis. (Ship worn)

Marine mollusks.
different light

true tentacles which are probably sensitive to contact and smell, and

a pair of stalked
intensities,

"eyes" which can possibly detect

but are not sight organs. Our common genera of water snails (Lymnaea, Physa, Helisoma) have their eyes situated at the

242
base of the tentacles.

TEXTBOOK OF ZOOLOGY
Just in front of and below the tentacles
is

is

mouth.

Located on the side of the head


is

the genital pore.

The

broad muscular foot


deposits a

covered with a mucus-secreting integument.


is

Just ventral to the mouth

the opening of the pedal gland which


snail usually glides;

highway

of

mucus over which the

Respiratory aperturTz

Velum

Mouth

Genital aperture

'

^V^,

Tentack

Respiratory aperture

Jtaihed eye

Edqe of wantle

Foot

~1'
Genital aperture
Fig,
134.

Fresh-water
snail,

and land

snails

Lymnaea; B, land

with bodies expanded. snail, Humboldtiana.

A, fresh-water

the gliding

movements are scarcely


The
visceral

perceptible.

In some marine

snails the surface of the foot is covered with cilia, the latter facili-

hump, which encloses the digestive, circulatory, respiratory, excretory, and reproductive systems, is protected by the shell which is lined with the mantle. A thick
tating movement.

PHYLUM MOLLtJSCA
collar is

243
foot,
;

produced where the mantle joins the

this mantle-collar is the respiratory aperture


is

and just beneath back of the latter

the anal opening.

Internal Morphology
Digestion.

Just within the mouth of a snail


The
latter is

known

as the buccal mass.

is a rounded organ composed of a ribbon of

Fig. 135.

Arrangement of
/
''

teeth in the radula of a snail.

^Hermaphroditic Duct
_,Orotesfis

,'Semma/ Receptacle
'//nfesfine

//

Albumen Gland ^,Hearf /VasOeferens ,Of/djcf

'Dart Sac
ytlcfcovs

Gland
0/ane/

/ VoQina

/ /Solvarv
/Penis

/Crop
''

^-Tentacle

enilal

Pore

Anierior
''Tentacle

-Pharynx
'Mouth
^\

^Cerebral Ganglion .bra I Canal, yry ^Sa/ifo. ry Qucr

Fig. 136.

Internal anatomy
On

of Helix.

Shell removed.

minute recurved

teeth,

the radula, supported and

nective tissues and muscles.

the roof of the

moved by conmouth is a horny

jaw which

pulls food into the

the radula into fine particles and


into the buccal cavity

mouth cavity. It is then rasped by mixed with saliva which flows


lie

from salivary glands that

on each side of

244

TEXTBOOK OF ZOOLOGY

The masticated food is then passed into the esophagus the crop. which widens, forming the crop. Here the food may be mixed with a brown liquid produced by the digestive gland which occupies most of the visceral hump. Enzymes produced by this gland convert starches into glucose, and, in the case of Helix, the ferment
is

powerful enough to dissolve the cellulose of plant


ing the protoplasm so that
it

cells,

thus releas-

be utilized. From the crop, food enters the stomach and is passed on into the intestine where absorption takes place. Feces are discharged to the outside through the

may

anus.

Respiration

Land and most fresh-water pulmonate

snails breathe

by a fold

of

the richly vascularized mantle which has been

modified into a
gills.

primitive lung, whereas the branchiate snails breathe by true

In

all

probability pulmonate snails that inhabit the deep water

of lakes use the


chiates.

pulmonary sac
the water
is

as a gill

and breathe

like the bran-

When
air

cold, it is not necessary for aquatic pul-

monate

snails to

make

periodic trips to the surface in order to re-

new

but when the water becomes sufficiently warm, cutaneous respiration alone is inadequate and the snail must come to the surface to get additional oxj^gen. The pulmonary sac
their

supply,

of aquatic pulmonates not only serves in the capacity of a gill or lung but also may serve as a hydrostatic organ, thus enabling snails Such movements are probto ascend to the surface by flotation. ably made possible through contraction of the mantle walls, thus decreasing or increasing the volume of air. Most of the marine species are gill breathers, and some, such as the sea slugs, have

external feather-like

gills.

Circulation

I
is

The
less,

blood of the snail consists of a

plasma which

usually color-

but in Helisoma, hemoglobin is dissolved in the plasma, thus giving it a red color, and in Lymnaea and some species of Helix the blood has a bluish tinge due to the presence of a copper-containing pigment, hemocyanin. In the plasma, float the colorless corpuscles. The blood serves as a transporting medium whereby digested food, excretions, secretions, and gasses may be carried from one part of the body to another. The heart, which consists of an auricle and a ventricle, lies in the pericardial cavity. Blood is pumped from the

PHYLUM MOLLUSCA
ventricle through a

245

common

aorta which divides into two branches,

one of which supplies the head and foot, and the other carries blood The terminal branches of these arteries comto the visceral hump. municate with a hemocoele or series of sinuses. Veins carry the
blood from the hemocoele to the mantle walls where it is purified and then passed through the pulmonary vein to the single auricle

and on into the

ventricle.

Nervous System
Encircling the esophagus
is

a ring of nerve tissue which includes


the cerebral ganglia, situated

three pairs of ganglionic swellings:

above the esophagus, supply nerves to the anterior regions of the body; the pleural, pedal, and visceral ganglia lie below the esophagus and are connected to the cerebral ganglia by commissures.

From

of the body.

them, nerves extend out to the visceral hump and basal parts The arrangement of ganglia and their connectives is

of taxonomic importance.

Excretory
a yellow gland situated near the heart. Its ureter, a thin-walled tube, parallels the rectum and opens near the anus.

The kidney

is

Reproduction and Life Cycle

Most fresh-water and

terrestrial

sea slugs, are hermaphroditic.

pulmonate The majority

snails, as well as the

of the

marine shelled

gastropods and our fresh-water branchiates, such as Pleurocera, The reproductive system Goniohasis, and Amnicola are unisexual. of a unisexual snail is relatively simple but is exceedingly complex
in the hermaphroditic species.

In bisexual
occurs.
testis.

(hermaphroditic)

snails

cross-fertilization

ordinarily
ovo-

The

ova, as well as spermatozoa, are produced

by the

Some

snails are protogynous, since the ovotestis functions


;

first as

an ovary and later as a testis others are protandrous, since male gametes are first formed, followed by the production of ova. Spermatozoa pass from the hermaphroditic duct into the sperm duct, and enter the vas deferens which terminates in a muscular penis. By means of the latter organ sperm are transferred into the seminal receptacle of another snail. Ova are passed from the ovotestis into the hermaphroditic duct, and from there into the oviduct

246

TEXTBOOK OF ZOOLOGY

which terminates in a thick-walled muscular vagina. During this journey the ova are fertilized by sperm from the seminal receptacle and coated with albumin from the albumin glands. Both the penis and vagina have a common genital opening to the exterior.
Album in qland

;^

Liver

^Hermaphroditic qland

Crop

WJf^

Oviduct

-
--

Oort sac

Mucous

qiands

Fig. 137.

(Modified, after Cooke, Genitalia of Helix aspersa; act of union. bridge Natural History. By permission of The Macmillan Company.)

Cam-

Most

of the fresh-water snails deposit eggs in clear gelatinous

masses on submerged objects, such as twigs and rocks. The land snails usually deposit their eggs singly or in clusters in well-protected places, such as in rotten wood or beds of humus. The eggs

PHYLUM MOLLUSCA

247

D
3. '

OO

D
"Egg masses of common snails. A, Lymnaea (fresh-water; gelatinous mass); B, Heliosoma (fresh-water; gelatinous mass); C, Physa (fresh-water; gelatinous mass) D, Pleurocera (fresh-water branchiate; tough gelatinous mass) E, Polygyra texasiana (terrestrial; eggs in cluster) F, egg capsules of Busy con.
Fig. 138.
;
; ;

Qoniobasis comolenfis (Pleuroceridae)

Coivpeloma decisum (Vwiparldoiz)

Amnicola comolensis
Fig. 139.

CochliopQ texana (Amnicolidae)


fresh-water branchiate snails.

Some common

248

TEXTBOOK OF ZOOLOGY
In

are covered with thin shells which prevent undue water loss.

some marine

snails,

such as Busycon, eggs are deposited in disc-

shaped capsules which are spaced equally apart and held together

by a tough band. Some snails, such as the fresh-water Campeloma, have a brood pouch in which eggs are deposited and the young are born alive. The latter is ovoviviparous reproduction in contrast to
oviparous reproduction, as described above.

FRESH- WATER CLAMS


(Detailed description based on Lampsilis)

Habitat and Behavior Mussels or clams are usually found partly buried in the mud, sand or gravel of ponds, lakes, or streams. By means of the muscular foot which is protruded from between the two valves at the

Proptey-Q

Anodonto
^tevjortiana

Qaadrula
forsheyi

purpurata

Amblema
costota
Fig. 140.

Leptodea
fraqilis

Carunculina
texasen5is

.Musculium
rejrnsst

Some

common

fresh-water bivalves.

anterior end of the shell they plow their

way

slowly through the


the

stream or pond bed, feeding on the microscopic organisms in the


water.

At

the posterior end of the shell are two openings

ventral siphon which pulls in food and water, and the dorsal siphon

through which wastes and deoxygenated water are eliminated.

PHYLUM MOLLUSCA
Movement
water.
sels are
is

249
Scallops

varied

among

the pelecypods.

may move

rapidly by suddenly contracting the valves, thus ejecting a jet of

Oysters are motile in their larval stages but in the adult

stage are attached to rocks and other objects.

Many marine mus-

attached to objects on the bottom or along the shore. Attachment is made possible by the dissolution of a part of the under valve and adherence of a portion of the body thus exposed.

The life span of clams may be relatively long. It has been estimated that Anodonta, one of our common genera of fresh-water
clams, attains its

maximum growth

in twelve to fourteen years.

External Features
Shell.
is

Unlike the snail whose shell is of one piece, the clam shell composed of two parts called valves (hence, bivalves) which are

attached together at the dorsal surface by a hingelike ligament.


,

Liqamentous hinqe
-^

Umbo
^

Ventral siphon Dorsal siphc

j^

^^p_^-i^^

Growth

linns

foot

&.

Anterior protractor retractor muscle

Pallial \inz

6
shell features of

Fig. 141.

External

(A) and Internal (B)

Lampsilis anodontoides.

The oldest part of the shell is the umho which protuberance near the top of the valves and

is

is

usually a rounded frequently eroded

due to carbonic acid in the water. Extending out from the umbo on each valve in a concentric manner are the growth lines of the shell, evidenced as slight, medium, or heavy ridges.

250

TEXTBOOK OF ZOOLOGY
shell is covered
is

The

by a horny, pigmented periostracum.

Under-

lying this

the prismatic layer composed of carbonate of lime.

The inner mother-of-pearl or nacreous layer consists of many thin, usually smooth plates, that in reflected light produce an iridescence
in

many

species.

Internal

Anatomy

The valves are held together by two powerful transverse muscles, the anterior and posterior adductors. Upon cutting these muscles the shells gape open, exposing the underlying organs. The valves are lined with a mantle which secretes the shell. On the inner surPerkaniial cavity

Rectum
Poit retractor
Post, adductor
I

Manble cut free

'

AuHcle

Ant retractor
Anbsyior

Ex siphon

adductor
I

Protractor
Fig.
142.

Ext

labial

palp

Left

qill

plate

(
removed and

-Lampsilis anodontoides with the left mantle partially turned back to expose the underlying organs.

i
face of each shell

may

be seen the curved pallial line which extends

between the two adductor muscles and indicates the partial attachment of the mantle. Teeth which strengthen the closure of the shell may be present where the two valves come together. Between the two walls of the mantle is the mantle cavity which contains the leaflike gills, the foot, and visceral mass.
Digestion

During the activity

of the clam a constant current of water

is

maintained in the mantle cavity. Food material is circulated forward to the mouth which lies between ciliated labial palps. Upon

PHYLUM MOLLUSCA
entering the mouth, food
the saclike stomach.
is

251

passed through a short esophagus into

comes in contact with a digestive ferproduced by the digestive gland which is discharged into ment each side of the stomach through ducts. The crystalline style, a diverticulum of the intestine, and found only in mollusks, produces an enzyme mixed with the stomach content which undoubtedly The food, having been facilitates the digestion of carbohydrates. mostly digested and partly absorbed in the stomach, is passed on
it

Here

ryjfcalline .style

1-* t^ucous qiands

%-Intest\r)e
Fig. 143.

Cross

section through the style sac and intestine of Lampsilis anodontoides. (Modified after Nelson.)

into the intestine

which makes one or more loops in the foot, passes through the pericardium and terminates in the anus near the dorsal
Respiration
Respiration is carried on through two pairs of vascularized gills which hang down into the mantle cavity on each side of the foot. Oxygenated water drawn in through the ventral siphon is passed through a rather complicated series of water tubes in the gills.

siphon.

Oxygen

is

absorbed by the capillaries and carbon dioxide passed

into the water

where

it

is

discharged to the outside through the

dorsal siphon.

252

TEXTBOOK OF ZOOLOGY
Circulation

composed of a ventricle and two auricles lies The ventricle, a muscular organ, surrounds in the pericardium. drives blood forward through the anterior aorta the rectum and and backward through the posterior aorta. Both aortae give off arteries which ramify all parts of the body. Most of the returning blood is carried to the kidneys by means of the vena caval vein. Within the latter, nitrogenous wastes are removed, and the blood then flows to the gills through afferent hranchial veins; after puri-

The heart which

is

fication in the gills

it

is

efferent hranchial veins.

returned to the auricles by way of the The blood is colorless and contains several

types of white corpuscles.

Nervous System and Sense Organs


Situated on each side of the esophagus is a cerehropleural ganglion, the

two ganglia being connected by means of a cerebral comPerlcardial wall


Post,

aorta
1

Ventricle

Reno 'pericardial pore Excretory pore


\

Vertical

^^^-

odductor M.
\

Auricle

water tubes
txhalQnt Siphon
''

kidney

^^^^

Liver Ant.aorta -Stomach Cerebral commisjure


i j

Ant .adductor
muscle

Inhalant

siphon

I
,

Qji,

Mantle

^'^e"
\ \

Labial
Visceral Q.

Qonad
Intestine

root

Mouth

P'^'P'

Pedal q.

Cerebro pleural &.

Fig. 144.

Internal

organs of Lampsilis anodontoides.

missure which passes above the esophagus. Each ganglion gives off two nerve cords, one of which passes ventrally and posteriorly
to the pedal ganglion situated at the junction of the visceral

mass

PHYLUM MOLLUSCA

253

with the foot. The other nerve cord extends backward, terminating in a visceral ganglion which is usually located just ventral to The visceral as well as the pedal the posterior adductor muscle.
ganglia are united.

The sensory organs of the clam are primitive.


visceral ganglion
is

Covering each

a patch of sensory epithelium called the osphradium, the function of which may be to test the purity of the short distance water brought in through the respiratory system.

a statocyst which functions in equilibrium. It is composed of a small calcareous concretion, the In addition to statolith, which is surrounded by sensitive cells. the sensory organs named, there are many sensory cells distributed

back of each pedal ganglion

is

along the mantle edges and elsewhere which probably react to light

and touch.
Excretion
Paired kidneys lie on each side of the body just below the periEach consists of a glandular portion which excretes cardium. waste, and a thin-walled bladder that is connected with an excretory pore through which wastes are discharged to the outside.

Reproduction and Life Cycle

The small bivalves belonging to the family Sphaeriidae (Sphaerium) are hermaphrodites, but in the larger ones the sexes are usually separate. The paired gonads are situated in the foot; the testis is usually whitish in color and the ovary reddish. A short duct leads from the gonad and opens just in front of the excretory pore. Sperm are passed to the outside through the dorsal siphon and
enter the female clam through the ventral siphon.

The ova, having

been discharged through the genital apertures, become lodged in various parts of the gills, depending upon the species. Within the Thus, the gills serve as brood pouches gills the eggs are fertilized. or marsupia and may become greatly distended due to the tremendous number (as many as three million) of developing embryos.

The small bivalve

larva,

which ranges in
is

size

from about 0.05

to

called a glochidium and has a single 0.5 millimeter in diameter, adductor muscle for closing the valves which may or may not be hooked. Extending out from the center of the larva is a long secre-

tory thread, the hyssus.


to the outside

In most clams the glochidia are discharged through the dorsal siphon. They fall to the floor of

254

TEXTBOOK OF ZOOLOGY
lie with their jaws agape, or snap jaws on any object. If the soft filament of a fish's gill or a of the fish comes in contact with the glochidium, it will close

the river, pond, or lake, and


their
fin

PHYLUM MOLLUSCA

255

group up around and eventually cover the parasites. Thus a cyst is produced about the glochidium and within this structure the
larval clam undergoes metamorphosis.
its host,
life.

It shortly

breaks loose from

drops to the stream or pond bed, and leads an independent


in a river system can be
fish-hosts.

The rapid dissemination of mussels accounted for by the movements of their

Economic Relations of the Phylum


Mollusks have been used as food by
civilization.
fish

man from

the beginning of

Oysters, clams, scallops, snails, and the

arms

of cuttleIt

are found in the

menus

of peoples all over the world.

has

been estimated that the oyster industry along the Atlantic Seaboard approximates 40,000,000 dollars annually. Along the Texas
coast alone. Federal statistics

show that 51,719 barrels

of oysters

were sold in 1932. Buttons are made from the shells of the large heavy river clams and along the Ohio, Missouri, and Mississippi rivers the button industry amounted to 5,000,000 dollars in 1931.

Within some of the clams are found pearls which are formed by some irritating particle, such as a parasite or sand grain that becomes lodged between the mantle and the shell. Iridescent protective layers of mother-of-pearl are deposited around the foreign parThe Japanese have been successticle, thus producing the pearl. ful in artificially stimulating pearl production by planting small objects, such as pieces of mother-of-pearl, between the mantle and
shell of pearl-oysters.

Pulverized clam shells are also being used as a calcium supple-

ment

to chicken feed.

Shells have also been used as a


of the eastern coast of

medium

of

exchange.

The

wampum

North America
brightly colored

consisted of strings of cylindrical beads

made from
and
still

clam

shells.

Shells have always been

are used for orna-

mentation.

Crushed

shells are

used in road construction.

Some mollusks

are injurious to

human

interests.

Among

these

might be mentioned the marine snail, Urosalpinx cinerea, which drills into and feeds on oysters and other pelecypods; the common shipworm, Teredo navalis, attacks the wood of ships and pilings, making extensive excavations. Certain species of snails serve as the intermediate host of parasitic flatworms or flukes. The liver fluke

256

TEXTBOOK OF ZOOLOGY
is

(Fasciola hepatica) whose intermediate host


snail,

the small fresh-water

Lymnaea

huUnioides, causes the disease, liver rot in livestock,

particularly in the sheep of the Southwest.

Since shells are easily fossilized they serve as excellent guides to


the geologists in determining the type of rock formation

and

relative

age of the strata,

CLASSIFICATION
Classification of this

phylum

is

based on the nature of the foot,

and respiratory organs; shape and structure of the shell; arrangement and structure of the nervous and reproductive systems.
Class
I.

Amphineura
Includes the Chitons, which are found abundantly on rocks between
tide

marks along the Atlantic and Pacific Coasts.


departed
least

This class apits

pears to be the most primitive in the phylum, and

members

have

from

the

ancestral

condition.
shell,

Bilaterally
if present,

symmetrical body; tentaculess head, eyes absent;


consists of eight overlapping plates.

foot but other species


spicuus.

Most species have a flattened are slender and wormlike Ischnochiton con-

Class

II.

Pelecypoda
Includes the bivalve moUusks, such as the oysters, clams, scallops,

and

cockles.

More than ten thousand


is

species have been described,

of which approximately four-fifths live in the ocean.


the class into orders

Division of

based on giU characters.

Order

1.

Protobranchiata

Marine species;
Order
Filibranchiata

gills consist

of short, flattened leaflets; dis-

tribution along the Atlantic and Pacific Coasts.


2.

Marine species; gills composed of long filaments which hang down into the mantle cavity. The edible scallops and the sea
mussel, Mytilus, exemplify this order.

Order

3.

Eulamellibranchiata

Fresh-water and marine species; with two platelike gills which hang down into the mantle cavity on each side of the
foot.

Family

1.

Unionidae Fresh-water clams or mussels; shell large or relatively


large; valves equal and

umbo

anterior to center.

Family

2u

Sphaeriidae

Fresh-water species.

Shell small;

umbo median

or pos-

terior to middle of shell.

PHYLUM MOLLUSCA
Order
4.

257

Pseudolamellibranchiata

Marine species;
trate this order.

gills

plaited into vertical folds;

shell

freillus-

quently inequivalve.

The oyster (Ostrea) and Pecten

ischnochiton

Dental ium

Loliqo brevipennis

Polypus bimaculatus (Octopus

Class Amphineura, Repre-sentatives of three classes of mollusks. Fig. 146. Ischnochiton; Class Scaphopoda, Dentalium ; Class Cephalopoda, Loligo brevipennis (squid) and Polypus bimaculatus (octopus).
Class III. Gastropoda

Includes the snails and


valve.

slugs.

species have been discovered

Approximately fifty-five thousand and described. Shell, if present, uni-

258
Order
1.

TEXTBOOK OP ZOOLOGY
Prosobranchiata

Mostly marine, but fresh-water and land forms are represented.

As

the

name

implies, the gills are situated in the

mantle cavity anterior to the heart.

This order embraces


all

such animals as the limpets, abalones, and periwinkles,


as Goniohasis,
lata,

of which live in the sea; also a few fresh- water genera, such

Campeloma and Pleurococera; Helicina


is

orbicu-

a terrestrial southern species which

frequently arboreal

in habit, comes under this order.

Order

2.

Opisthobrmichiata
Strictly marine.
Gills,

when

present, are situated jjosterior

to the heart; shell, if present, small.

Includes the sea slugs.

In the sea butterflies (pteropods), the foot may be modified Some of the into two fins which are used in swimming. heavier types have broad cephalic discs, adapted for burrowing in the sand. Many are found in coral beds and in seaweeds, their vivid colors harmonizing with the background.
Order
3.

Pulmonata

Mostly terrestrial and fresh-water snails. Gills are absent, the mantle cavity serves as a pulmonary sac; shell usually present, sometimes rudimentary or absent. Suborder 1. Basommatophora Fresh-water species; eyes located at base of tentacles;
external
shell

present.

Includes

the

families

Lym-

naeidae, Physidae, Planorbidae and Ancylidae.

Suborder

2.

Stylommatophora
Terrestrial
snails

and slugs; stalked


retractile tentacles;

retractile
shell in

eyes,

and one pair of


or absent. Class IV.

form of

elevated or depressed spire, rudimentary and concealed,

Marine.

Scaphopoda Mantle edges grown together along ventral side forming tube, with a shell of same shape and open at both ends. Commonly
as tooth shells.

known
Class

Approximately 300

kno-u-n living

species.

(Dentalium.)

v.

Cephalopoda

The most highly organized of the mollusks. A definitely formed head is present which bears a pair of eyes that superficially resemble the eyes of vertebrates. The foot is modified into arms or tentacles. They are carnivorous animals and many of them are used as food by man. (Nautilus, Loligo, Polypus.)
Marine.

Order

1.

Tetrabranchiata

The chambered nautilus (Nautilus) is a representative of this order. The animal inhabits the last chamber of a flattened spiral calcareous shell. As the name Tetrabranchiata
implies there are four gills; also four primitive kidneys and

PHYLUM MOLLUSCA

259

Siph uncle

1 __ - ' ^ Jepta

Fig. 147.

Sectional

view of internal structure of Nautilus.

Cyrtoceracone

Qyroceracom
Orthoceracone

Goniatlte

Ceratite
Fig. 148.

Ammonite
of the cephalopods.

Evolution

260

TEXTBOOK OF ZOOLOGY
four auricles;
ink sac
absent.

This suborder

reached

its

peak of development in the Silurian and Devonian periods and is one of the most clear-cut examples of evolutionary
development in the invertebrates.

During the Ordovician period the cephalopods constituted one of the chief groups of marine animals. Even though at that time cephalopods with coiled shells existed, the predominant ones were the orthocones (those with straight This latter group in all probability gave rise to the entire conical shells).
series of

coiled shells, culminating in Nautilus.

In

all

nautiloids a series of

partitions,

termed septa, extend the

full length of the shell.

The point of union

with the septa and sides of the shell may appear as a straight, curved, angulate This line is called the suture and in fossil shells whose or highly complex line.
outer shell coating
is

is

lost, it

stands out rather conspicuously.

The suture

line

used as a taxonomic character for the group.

Order

2.

Dibranchiata

Octopods and squids are representative types.


or absent;

Shell internal

two

ent;

mouth

and two primitive kidneys; ink sac pressurrounded by 8 to 10 tentacles which are
gills

furnished with suckers.


all

This order includes the largest of

moUusks, the giant squid (Architeuthis princepsj which


attain a total length, including arms, of over fifty feet.

may

The squids and octopods are noted for their ability to change color by the rapid contraction or expansion of chromatophores Their juovements are rapid and are produced in their skin.
by expelling water from the mantle cavity through the muscular siphon with such force that the animal
is

jerked backof
the

ward.

In the

squids,

fins

along

the

sides

body

facilitate locomotion.

Loligo hreviyennis

is

the small squid found along the Gulf coast.


it is

Wheu
The

taken out of the water

usually a mottled red or tan.

mass and mantle cavity are enclosed by a thick muscular mantle. Beneath the skin along the back is a primitive endoskeleton in the form of a feather-shaped shell. The squid is predaWithin the tory, feeding on almost any animal it can capture. The large jaws moved by powerful muscles. pharynx are two terminates in a pharynx connects with an esophagus which in turn muscular stomach. Digestive juices from the liver and pancreas
visceral

and after the food is partially dipassed into a thin-walled cecum where digestion is completed and absorption takes place. Wastes are discharged through the anus which opens near the base of the siphonal fold. The blood
are emptied into the stomach,
gested,
it is

PHYLUM MOLLUSCA

261

(.

Sucker

Hectocotylhed arm.

Cartilage

Siphon

Anus
Muscle

Esophaaus

Rectum
Inkiac
Anb. aorta

5yitemic heart

Pen

^
J
rl\

Lt.

post cava

Spermabophoric sac

stomach

:-il

Pen

Skowachpo'uch

-Cub edge of body wall


.Fir)

Fig. 149.

Dissection

of squid to sliow internal anatomy.

262
system, which
is

TEXTBOOK OF ZOOLOGY
closed,

is composed of arteries, veins, and two Blood is oxygenated in two feathery gills which project into the mantle cavity. The two light-colored triangular kidneys are situated anterior to the branchial hearts and discharge their contents through small papillae, one located on each side of the intestine. In squids the sexes are separate. The male reproductive system is composed of a testis, vas deferens, spermatophoric sac, and penis the female system consists of an ovary, oviduct, ovidueal gland, and nidamental gland.

branchial hearts.

--

Cornea
Eyelid
Iris

Lens Ciliary M.

Retina

Optic (ganglion

Fig. 150.- -Longitudinal section through eye of squid. (Redrawn and modified after Borradaile and Potts by permission of The Macmillan Co.)

The nervous system of cephalopods shows a high degree of spewhen compared with the nervous system of other mollusks. The "brain" is composed of a close association of ganglia around the esophagus and is protected by a capsule of tough tissue resembling cartilage. Nerves radiate out from the central nerve mass to the various parts of the body some of the nerves terminate in large ganglia, such as the stellate ganglia in the mantle. The eyes of the squid are supported by pieces of "cartilage" and are relatively complicated. Statocysts, which are similar but more
cialization
;

complicated than those described for the clam, are situated near
the brain mass.
in function

Ciliated pits which are supposed to be olfactory open in the form of a slit just back of each eye.

CHAPTER

XVIII

PHYLUM ARTHROPODA
Arthropoda (ar throp'O da, joint foot) is the name of the largest known group of animals. As the name implies, all representatives of the phylum have paired, jointed appendages and a definite tendency toward specialization of them. Their bodies are triploblastic, segmented, bilateral, and covered by a chitinous exoskeleThe coelom is modified by a marked reduction as a result of ton. The segmentation or metamerism of specialized vascular spaces. the body is expressed in a high degree in this phylum and there The segments is a definite relation of appendages to segments.
have undergone greater specialization and greater regional differentiation than was the case in annelids. In forms where there is little or no differentiation of segments, the condition is referred to as homonomous, while a highly differentiated condition of segments This as found in most arthropods is spoken of as heteronomous. distinct head, thorax, and abdomen. The appendgroup has fairly ages on various segments are typically homologous with each other.

Some

are modified as sense organs, others as

mouth

parts, others

for walking,

swimming, and reproduction. The skeleton is entirely exoskeletal, composed of chitin, and fits exactly the shape and contour of the body. Since it is fairly unyielding to growth, it becomes necessary for the arthropod to shed the skeleton periodically during its growing periods. This molting
or ecdysis, as
sions of this
it is

called, is quite characteristic of

many

of the divi-

phylum.

The circulatory system is of the ojyen type, since there are large sinuses or spaces surrounding most of the organs instead of a continuous circuit of blood vessels. The nervous system is of a modified
ladder type with a ventrally located cord. The digestive system shows specialization in that it is divided into distinct regions as an
adaptation to special types of food which require mastication.
Classification

This phylum is divided into two sections and at least five classes some authors recognize as many as eight. The sections are deter-

mined according

to the

means
.

of respiration.
263

264

TEXTBOOK OF ZOOLOGY
gill)

Section I. Branchiata (brankia'ta, forms for the most part.


Class
etc.
I.

gill-breathing, aquatic

Crustacea, craj^fish, crab,

pill

bug, barnacle, water

flea,

o m
oi a>

> u
"a5

S
0)

OS
>.a

^
P
CS
EI)

Q)

O O

*M

-<

P O ?>> > m
o

dB
cS

I"

PHYLUM ARTHROPODA

CLASS

CRUSTACEA

265

Order Copepoda, cyclops, fish louse (Argulus). Order Cirripedia, goose barnacle (Lepas), rock barnacle (Balanus), Sacculma (Fig. 404). (Some authors prefer to rank Branchiopoda, Ostracoda, Copepoda,

and Cirripedia
craj^fish,

as subclasses, thereby dispensing with Entomostraca.)


pill

Subclass Malacostraca,

bugs, sow bugs, sand

fleas,

lobsters,

and

crabs.

Order Isopoda, pill bugs and sow bugs. Order Amphipoda, sand fleas and beach

fleas.

Order Decapoda,

crabs, crayfish, lobsters,

and shrimps.

Section II. Tracheata (tra ke a'ta, rough) both terrestrial and aquatic arthropods which breathe by tracheae, book lungs or book gills. This section is divided into three divisions depending on the
primitiveness of the characteristics.
Division A. Prototracheata. The primitive form with some arthropod characteristics and certain annelid features, such as

nephridia.

Onychophora, Peripatus, the wormlike arthropod. Division B. Antennata. More highly specialized forms with one
Class II.

pair of antennae.
Class III. Myriapoda, centipedes and millepedes (thousand legs) having one or two pairs of appendages on each segment.

Order Chilopoda, centipedes.


Order Diplopoda, millepedes.
Class IV.
Insecta, beetles, bees, locusts,
etc.,

all

with three pairs

of thoracic appendages

and most of them with wings.

Order Thysanura,

silver moth.

Order Collemhola, springtails. Order Ephemerida, mayflies. Order Odonata, dragonflies and damsel Order Plecoptera, stone Order Order Order Order Order Order Order
flies.

flies.

Emhiidina, embicls.

(Texas, California, Florida.)

Orthoptera, crickets, grasshoppers, roaches. Isoptera, termites or "white ants."

Dermaptera, earwigs.
Coleoptera, weevils

and

beetles.

Strepsiptera, stylopids (parasites in insects),

Thysanoptera, thrips.

266

TEXTBOOK OF ZOOLOGY
lice.
lice.

Order Corrodentia, book Order Mallophaga, bird

Order Anoplura, body lice ("cooties"), crab Order Hemiptera, true bugs, as squash bug.
Order Romoptera, plant Order Neuroptcra, aphis
lice,

louse.

scale insects, cicadas.

lions,
flies.

ant lions.

Order Trichoptera, caddis


Order Lepidopiera, Order Mecoptera, scorpion

butterflies

and moths.

flies.

Order Dipt era, true flies, mosquitoes. Order Siphonaptera, fleas. Order Hymenoptera, wasps, ants, bees.
Division C.

Arachnoidea

(ar ak noi'de

a,

spiderlike).

without antennae but

Avith tracheae,

book lungs or book

gills,

group and

four pairs of thoracic appendages.


Class V. Arachnida, spider, mite, scorpion, king crab, Order Scorpionida, scorpions. Order Pedipalpi, vinegarroon and tarantula.
etc.

Order Pseudoscorpionida, book scorpion. Order Phalangida, daddy longlegs or harvestmen.


Order Palpigradi, one Texas species. Order Araneida, spiders. Order Acarina, ticks and mites.

Order Xiphosura, king crab or horseshoe crab.


This

summary
its size

of the classification of the

phylum has been placed

may realize the magand the great variety of animals included. The number of species described under the phylum is approximately one-half million, and there are large numbers still undescribed and
early in the chapter in order that the student

nitude of

unnamed.

CRAYFISH OF CLASS CRUSTACEA


Since this animal represents a relatively simple type of arthropod and is so generally well known, it serves ideally as a representative

more detailed study. The genera Canibarus and Potamdhius or Astacus are commonly found in the streams of North America. The former is distributed east of the Rocky Mountains and
species for a

the latter on the Pacific slope.

PHYLUM ARTHROPODA

CLASS

CRUSTACEA

267

Habitat and Behavior

For the most part crayfishes (crawfishes, crawclads, fresh-water lobsters) are inhabitants of fresh-water streams and ponds where
there

calcium carbonate in solution for purposes of skeleton formation. These animals may be found moving about on the bottom, or they may be in hiding under some stone or log, or they may be in the mouth of a burrow beneath the water's edge.
is

sufficient

Some

from the original horizontal burrow to the surface of the earth and deposit mud around the opening of a tunnel. They are much more active at night than during the day. It is possible for them to walk about on the bottom of the stream or pond, moving the body in almost any direction. Their
species carry air tunnels vertically

Fig.

swamps

152. Cavibarus
ol:

clarkii, the the Southern States.

swamp

crayfish, a very common species in the (Courtesy of Southern Biological Supply Co.)

swimming

habits are rather peculiar in that they dart

backward

through the water, as a result of the strong downward stroke of the tail. One stroke of the tail will carry the animal a yard and The daytime is this is commonly sufficient to avoid the enemy.
usually spent in hiding under objects or in the mouth of the burrow. Crayfishes may at times desert their aquatic habitat and go

foraging out over


build their

swampy burrows down

land.

In some localities certain species

to the subterranean

out in the fields and become important pests. chemoreception are important senses in this animal.

water table right Sight, touch, and

The crayfish captures other animals, such as tadpoles, small fish, and aquatic insects, by waiting in hiding and suddenly seizing

268
them.
color

TEXTBOOK OF ZOOLOGY

The crayfish is quite well protected, due to its protective which matches the background, its chitinous skeletal covering, and its pinchers. In spite of this, they are captured by water snakes, alligators, turtles, fish (such as bass and gars), frogs, salamanders, herons, and raccoons in particular. Many have been exterminated by the drainage of swamps, and by their use as food for man.
External Structure

The chitin-covered body is divided into cephalothorax, abdomen, and appendages. The cephalothorax is a compound division of the body including the thirteen most anterior segments and is divisible The boundary between these is marked by into head and thorax. The shellthe oblique cervical groove on each side of the region.
like

covering whose lateral edges are free,

is

known

as the carapace.

The portion anterior

to the cervical groove is the

head or cephalic

portion, while the portion posterior to the grooves is the thorax.

drawn out to almost a point, The mouth is located on the ventral side of the head portion and not at the tip of the rostrum where most people look for it. The lateral portions of the carapace are known as hranchial areas or hrancliiostegites, and they cover the
anterior end of the cephalothorax
is

The and

this portion is called the rostrum.

gills.

Their ventral edges are

free.

On

the ventral side of the thorax

between the twelfth and thirteenth segments (about the level of the fourth walking leg) of the female is a cuplike pouch called the
annulus or seminal receptacle.
receipt
It serves

in

reproduction for the

and storage of spermatozoa.


to the thorax,
is

The portion posterior


"tail" by fishermen,
the posterior end of this.

really the

which is frequently called aldomen, and the tail proper is at


is

The abdomen

divided into six typical

segments and the terminal telson, which has no appendages but is often called the seventh abdominal segment. The anus is found on
the ventral side of this part.

The

skeletal part of the

abdominal

seg-

ment

consists of:

the dorsally arched tergum; a thin, overhanging

and the slender ventral sternum in the form of a narrow bar extending from side to side. A thin arthropodial membrane extends between successive sterna and allows for movement of the segments upon one another.
lateral plate, the pleuron;

PHYLUM ARTHROPODA

CLASS
..3

CRUSTACEA

269

Protopodite

A2
Endopodite

A.l
Exopodite

Protopodite.:."

Endopodite

External ope'ning of nephtidium.

M.
^.Endopodite

Mx.l
Endopodite
Exopodite
Exopodite

Endopodite

Mx,2
Epipodite

Mp.l
Epipodite

Mv.2
Exopodite

Endopodite
Protopodite,.,.

Mp,3
Eindopodite..,

Protopodite
Epipodite-

Endopodite
Protopodite k
.ChitinouB thrrada

Epipodite

Examples of cephalic and thoracic appendages of the crayfish, ventral Fig. 153. M, mandible A. 2, antenna L. i, fourth walking leg view. A. 1, Antennule fir.st maxilliped Mp. 2, second maxilliped Mp. 3, third maxilhped Mx. 1, Mp. 1. (From Newman, Outlines of General Zoology, first maxilla; Mx. 2, second maxilla, published by The Macmillan Company, after Kerr.)
;
; ;
; :

270

TEXTBOOK OF ZOOLOGY

cal

The appendages are paired, with one pair attached to each typiThey are all desegment. There are nineteen such pairs. veloped on the same plan from the typical biramous (two branched) appendage. The five anterior pairs of abdominal appendages are

form except for the modification of the first two in connection with reproduction. This group is known as smmmerets or pleopods and all have the fundamental parts consisting of a basal protopodite composed of coxopodiie, joining the body and the hasipodite; the exopodite or lateral branch and the The first two endopod'ite or medial branch each have many joints.
quite typical of the primitive

are

much reduced

in the female, but in the

male the protopodite


for
at-

and endopodite are fused and extended to serve as an organ The posterior pair of swimmerets, transfer of spermatozoa.

tached to the sixth abdominal segment, are broadened into fanlike They are known as uropods and have structures for swimming.
oval, platelike exopodite

and endopodite.

The posterior

five thoracic

appendages are the walking legs or pereiopods.

These are uniramous

due

to the complete reduction of the exopodite.

Each
is

is

composed
Join-

of the two joints of the protopodite

and

five of

the endopodite.

ing the coxopodiie

(first

segment of protopodite)
gill

a sheetlike struc-

ture which supports a


anterior walking legs

and some chitinous threads. The three possess pinchers or chela which are formed by and defense. The

the terminal segment being set on the side of the second segment.

The walking
maxillipeds.

legs are used in locomotion, ofi'ense,

three anterior segments of the thorax bear three pairs of biramous

The parts are quite typical

in

most respects.
except the
first

Each has
bear
gills.

an epipodite joining the basipodite and

all

These appendages are used in getting food to the mouth.

head are attached five pairs of appendages. Just posterior to the mouth and immediately in front of the first maxilliped are two pairs of maxillae, the second of which overlies They are both leaflike and modified. The epipodite and the first. exopodite of the second are fused to form a bladelike hailer or scaphognathite which fits over the gills and by its movement helps circulate the water for respiration. Its endopodite is slender, but the protopodite is broad and foliate. The first maxilla is reduced The jawlike manto a leaflike protopodite and small endopodite. dible at each side of the mouth is composed of hard protopodite

To the segments

of the

PHYLUM ARTHROPOD A

CLASS

CRUSTACEA

271

with teeth ajid a fingerlike endopoclite, which is tucked under the anterior edge of the former. This appendage is used for chewing. In front of these are the antennae which are biramous and are sometimes called "feelers."
parts,

They

consist of the protopodite of

two

a long many-jointed, filamentous endopodite and a

relatively

short, fan-shaped exopodite.

which are biramous and


are similar in these.

feelerlike.

Anterior to these are the antenmiles The exopodite and endopodite

The principle of homology is excellently illustrated by the appendages of the crayfish. In general, homologous structures are those which have similar structure and similar origin but may have
similar or different functions.

By way

of contrast, analogous struc-

tures are those which,

when compared, show

different structure

and

origin but similar function.

During early development appendages of the crayfish is similar to all others. Some become modiOther illustrations of homologous structures fied with development. are the human arm and the bird 's wing. In organisms like crayfish
where the appendages of successive segments are homologous to each Homologous other, the condition is spoken of as serial homology. stiiTctures are found in many animal groups and are used in establishing relationships. It ha.s been suggested that the parapodia of Nereis
represent possible forerunners of crustacean legs. They are both typically biramous and both take about the same position on the body, as well as having a similar segmental distribution. There is also considerable similarity in their structure.

each of the

Internal Structure

Beneath the
tation

shell-like, chitinous

exoskeleton there

is

a very rep-

resentative set of systems.

is retained in a degree in the circulatory system. Earlier in the chapter it was pointed out that the coelom is modified as a provision for increased blood sinuses which have occupied much of the space.

most higher animals the segmenthe muscular system, nervous system, and to

As

in

the branchial areas of the carapace may be found the paired, feathery gills held in the gill cavity or There are three types of gills present here: branchial chamber.

Respiratory System.^

Under

pleurohranchiae, attached to the sides of the thorax; podohranchiae,


arising

from the epipodites of the thoracic appendages; and arthro-

272
branchiae, which arise

TEXTBOOK OF ZOOLOGY

ages.

from the coxopodites of the thoracic appendSeveral of the segments have lost the pleurobranchiae. The scaphognathite moves in such a way over the external surface of
the gills as to
is

move the water in an anterior direction. The water brought under the free edge of the branchiostegite or branchial area of the carapace and moved forward to be discharged by an anterior aperture. An almost constant stream of water is pumped over the gills to facilitate the exchange of oxygen and carbon dioxide between the blood in the capillaries of the gills and the surrounding water. The aerated blood is then carried to all of the
tissues of the body.
Pericardial sinus

Carapace removed

Heart Ostium

hasck
Gonad
Intestine
Digestive

$land

Efferent vessel

Hasc/e
artery

Gill

Ventral thoracic

Nerve cord

Carapace

Wntral sinus

Fig.

154.

Diagram

of cross section throug-h the posterior thoracic crayfish. Arrows indicate flow of blood.

region of a

The digestive system is in the form of a modified canal and is composed of mouth, esophagus, stomach, and intestine. The mouth
opens between the mandibles on the ventral side of the third segment. From this the short, tubular esophagus leads dorsally and joins the ventral side of the stomach almost directly above the

mouth. This larger anterior portion of the stomach is the cardiac chamber. Within its wall are a number of hard chitinous bars, known as ossicles, which bear teeth capable of mastication of food when moved over each other by the muscular activity of the wall. This grinding apparatus is known as the gastric mill. Between the cardiac chamber and the posterior or pyloric chamber is an arrange-

PHYLUM ARTHROPODA
meut

CLASS
to

CRUSTACEA

273

of bristles which serve as a strainer that allows only properly

masticated food to pass through. ably smaller and curves

The pyloric chamber

is

consider-

downward

continue posteriorly as the

tubular intestine which extends almost directly posteriorly through


the center of the abdomen to the anus in the last segment.
digestive glands

Large

(hepato-pancreas)

lead into the pyloric chamber

through hepatic ducts.


tive

The

secretion of these glands contains diges-

enzymes.
consists of a heart, the

The vascular system


the

pumping organ;

the

arteries, definite vessels; the sinuses,

series of blood spaces;

Nood which

circulates.
5
6 7 g

It consists of the fluid


<?

and plasma containing

10

Lateral view of a dissection of the crayfish to show many of the Fig. 155. S, internal organs. 2, circumesophageal connective 1, supraesopliageal ganglion ophthalmic artery J/, stomach, cardiac portion 5, lateral teeth 6, median teeth antennary artery; 8, testis; Oj, hepatic artery; 10, ostiuin 11, heart; 12, dorsal 7, abdominal ganglion; 28, ventral abdominal artery; 29, nerve cord; SO, rectum; gland; 17, esophagus; 18, mouth; 19, subesopliageal ganglion; SO, stomach, pyloric portion; 21, opening of hepatic duct; 23, digestive gland; 23, ventral thoracic artery; 2i, sternal artery; 25, opening of vas deferens; 26, thoracic ganglion; 27. abdominal arterj' IS, vas deferens; li, intestine: 15, renal opening; 16, green (Modified from Turtox Key Card of Crayfish. Courtesy General Bio31, anu9. logical Supply House.)
;

white corpuscles but without red ones.


sorbs oxygen
is

The hemocyanin which abFresh blood


is

dissolved in the plasma.


it

almost clear

and

colorless,

but

takes a blue color after standing in the air for a

The heart is somewhat flattened and angular in outline, and has a muscular wall which is perforated with three pairs of slitlike ostia. When the muscular wall of the heart is relaxed, the slits open,
short time.

274

TEXTBOOK OF ZOOLOGY

and blood is drawn in from the surrounding pericardial sinus in which the aerated blood accumulates. When the heart contracts, blood is forced into the anterior region of the body through the single anterior median artery, paired antennary, and paired hepatic arteries all of which arise from the anterior end of the heart. The large dorsal ahdominal artery extends from the posterior tip of the heart posIt supteriorly through the abdomen just dorsal to the intestine. of the body wall. The sternal artery plies the intestine and muscles is a large branch arising from the dorsal abdominal artery just after It passes ventrally through the nerve cord and it leaves the heart. divides into a posterior, ventral ahdominal artery and an anterior,
ventral thoracic artery.
portions of the body.

These branches carry blood to the ventral Besides the pericardial sinus already men-

i
i

tioned, there are others returning the blood to this one.

The

sternal
it

sinus

is

the

main
gills.

one,

and

it is

located beneath the thorax.


gills.
is

From

several branches lead into the

This provides for a course


collected

through the

From them

blood

by branchio-cardiac

canals and delivered to the pericardial sinus.

perivisceral sinus

surrounds most of the alimentary canal and collects the venous blood

from

it.

This kind of system

is

called the

open type because of the

large irregular spaces or sinuses instead of an evenly constructed set


of veins which

make

a complete circuit of the course.


consists

The excretory system

principally

of

pair

of

large

bodies located in the ventrolateral portion of the head.


richly supplied with blood

These are

and draw the nitrogenous wastes and

excess water from the blood to deliver them externally through


excretory pores located in the coxopodites of the antennae.

The nervous system is of the same structural plan as that of the earthworm, which is a modified "ladder type.'' The two longitudinal cords have come together in the ventral line and run the entire length of the body to form a ventral nerve cord with ganglia. This arrangement constitutes the central nervous system. The ganglia of the anterior three segments are fused into the "brain" or supraesophageal ganglion which is located anterior to the esophagus and is joined to the cord by two circumesophageal commissures or connecone passing on each side of the esophagus. From this dorsal ganglionic mass, nerves pass to the eyes, antennae, and antennules.
tives,

The most anterior portion of the ventral cord

receives these com-

PHYLUM ARTHROPODA
missures.

CLASS

CRUSTACEA

275

This portion, which consists of the fused ganglia from segments three to seven, is known as the suh esophageal ganglia. Ner\'es go from it to the mouth parts, first and second maxillipeds,

green glands, esophagus, and muscles of the thorax. Each segment posterior to the subesophageal ganglia possesses a segmental ganglion
with branches to its respective appendages and muscles. The sense organs include antennae, antennules, sensorj' hairs, statocysts, and

^ _ _ Supraesoplmqeal qanqlion
- -Orcumesophaqeal connective

'Subesophageal qanqlion
Thoracic ganglion

ms^

Ring for sternal artery

Jit

abdominal ganglion

Lateral nerve

JVentral nerve cord

Tegmental division

Terminal cjanqlion

Fig-.

156.

Dorsal

view of nervous system of crayfish. Notice merging of anterior thoracic ganglia with subesophageal ganglion.
tactile

organs (sensitive to touch), the endopodite of which is a relatively long jointed filament. The exopodite The basipodite and coxopodite are is much shorter and fan-shaped.
eyes.

The antennae are

closely fused to the ventral side of the cephalic region.

An

excretory

pore opens to the exterior through the coxopodite of each antenna. The hairlike processes along the edge of the carapace, on the legs,

and other parts of the body are

also sensitive to touch.

The anten-

276

TEXTBOOK OF ZOOLOGY

nules are tactile and each has two slender filamentous processes, the

exopodite and endopodite.


esses each

antennule has a saclike statocyst in


is

In addition to these slender jointed procThis its coxopodite.


is

structure
skeleton

an infolding from the outside and


hairs.

lined with exo-

and sensory

Inside of each are small particles of

solid material, such as grains of sand,

which are called

statoliths.

As the animal changes its position the statoliths move about inside of the statocyst and stimulate the sensory hairs. From these stimulations the crayfish is able to determine its orientation in space,
i.e., it

normal walking position, on its back, or standing on its head. These organs serve for equilibrium. When the crayfish molts, the statocysts are temporarily lost and new ones form as the new skeleton develops. If there are no solid objects in the water in which a crayfish lives during molting, there will be no statoliths in the statocysts and the ajiimal has an im-

knows whether

it is

in

paired sense of equilibrium.

Experimenters have placed only iron filings in the water at such a time and the animals present have used them for statoliths. By bringing a magnet near the crayfish in this condition the statoliths are moved and the animal goes
through numerous peculiar contortions in attempting to respond to these stimulations of orientation. Besides the above functions the
antennules provide the chemical senses of smell and taste.

stalks,

mounted on movable They are described as compound because each one is composed of a large number of inEach of dividual sight units, each of which is essentially an eye. these units is called an ommatidium, and the crayfish has about 2,500 in its eyes. A single one is rather spike-shaped, tapering from the
The
eyes,

which are

of the

compound

type, are

one on each side of the head region.

broader superficial end to the rather pointed internal extremity. A single ommatidium has an outer cornea which is transparent and

supported by some corneagen cells on the vitrella. Beneath this is the rather long crystalline cone beneath which is the rhabdom, an-

Surrounding the latter are sensory cells making up the retinula. The wall of the ommatidium possesses pigment cells along the sides of the crystalline cone and in the retinula. The distribution of the pigment varies with the intensity of the light. The stronger the light the more these cells are expanded and the more direct must be the ray of light to reach the retinula, because
other lenslike structure.

PHYLUM ARTHROPODA

CLASS

CRUSTACEA
is

277
reduced.

the possibility of reflection within the

ommatidium

In

light the pigment is partly toward the basal portion of the ommatidia which allows more refraction of rays by the crystalline cones and a combination of

dim

concentrated partly toward the outer and

images in several adjacent units.

In brighter light only the ray from


~
cells

Cornea - Corneagen

Crystalline

cone"

Distal retinal

pigment

cells

%:

^^^

-f1

Proximal retinal pigment cells

-Mm

Rhabdome--

Basement membrane'

Nerve fibers

a
Fig. 157.
of

Longitudinal

section of
;

pigment when
is

light is present

in the latter the distal

pigment

is in the

ment

concentrated inwardly.

ommatidia from eye of crayfish, a, position position of pigment when in the dark. Notice outward position and the proximal pig(From Hegner, College Zoology j published by The
b,

Macmillan Company, after Bernhards.

directly in front of the cornea will reach the retinula

and stimulate

the nerve cells there.

These cells are connected internally with


of vision

the optic nerve.

The type

produced

in the

compound eye

278
is

TEXTBOOK OF ZOOLOGY

"mosaic" in that there is registered only a single image by the Each ommatidium which is in focus on the object registers eye. an image of that part. As the object moves, new ommatidia are stimulated and movement is indicated by the rate of stimulation of successive ommatidia. The farther the object is from the eye, the The crayfish eye is often fewer ommatidia will be stimulated. termed a modified appendage because an antennalike structure will
regenerate in case an eye
is

mutilated.

Metabolism

The crayfish ingests principally flesh from bodies of fish, snails, tadpoles, insects, and other animals, some caught alive and others found dead. The maxillae and maxillipeds hold the morsels while they are crushed by the mandibles. Mastication continues in the cardiac chamber of the stomach and chemical digestion begins in The digestive juices possess enzymes which the pyloric portion. convert the food into soluble form, and as it passes along the intestine, it is absorbed by the blood and distributed to the tissues
over the body.
assimilation.

This conversion of food material into protoplasm

is

The
it

external phase of respiration has put oxygen in


is

the blood, and


cells.

distributed throughout the protoplasm of the

to kinetic

The energy stored in the food material is released or converted form by union Avith the oxygen (oxidation) in the proto-

plasm.

From

this

union there
is

is

excess heat produced.

Mechanical

and chemical

activity

the result of the harnessing of this energy.

As

a by-product of this cataholism, excretory materials, such as excess water, urea, uric acid, and other substances are formed in solution

and are
of these

The green glands relieve the blood collected by the blood. and deliver them to the exterior. Of course growth results

when

excess food materials are built into the cells at times

when

the rate of anabolism exceeds that of catabolism.

Reproduction

and the mating takes place either in the spring or fall or perhaps both. The spring hatch become well developed before winter. The eggs produced
These animals are dioecious
(sexes separate)
in the fall

may

not be laid before spring.

PHYLUM ARTHROPODA

CLASS

CRUSTACEA

279

Pi.

D.
158. Development of the crayfish. A, Toun^ crayfish clinging to swimmerets of mother. B, Second larval stage (2) attached by its chelipeds to hairs (Pl.H.) on a swimmeret (PI.) of the parent. The molted shell of the first larval stage {l) is clinging by chelipeds. portion of the egg-membrane (wi) and shell (Sh.) are still attached to the swimmeret by a stalk (St.). When the first larva hatches it remains attached to the shell by a filament (A/.). By means of these filaments the young remain fastened to the mother during development. C, First larva hatcliing through shell. D, Tlie second larva. (Reprinted by permission after Andrews, 1916, Smithsonian Contributions, Vol. 35.)

Fig.

280

TEXTBOOK OF ZOOLOGY

In the case of Cayiibarus clarkii the adults retire to holes or bur-

rows at the water's edge during the summer.

It is

here that the eggs

are laid and carried by the female until after hatching; then the

young cling to her swimmerets. In late summer or fall, soon after the young hatch, the adults become very migratory at night, particularly in rainy weather. In this way they help to distribute the young
to

new water

holes.

The female reproductive organs are composed of a bi-lobed ovary and beneath the pericardial sinus. During development the eggs appear in the ovary. Two oviducts lead, one from each side of the ovary, to a genital pore
located beside the pyloric chamber of the stomach
in the coxopodite of the third walking leg (pereiopod) of each respective side.

The ova develop in


(oogenesis)

follicles in the ovary.

The maturation
the eggs

divisions

take place here and,

when mature,

break into the central cavity of the ovary, from which at the time of
laying, they pass out through the oviducts.

The male reproductive

organs are composed of the bi-lobed testis located dorsal to the pyloric

stomach and ventral

to the heart.

Spermatogenesis takes place here

and mature spermatozoa are shed. The tubular vasa deferentia extend posteriorly and ventrally to open externally on the coxopodite During copulation (mating) the sperm of each fifth walking leg. cells are transferred by the two pairs of anterior swimmerets (pleopods) of the male from the apertures of the vasa deferentia to the
annulus (seminal receptacle) on the ventral side of the thorax of the female. Later, vv'hen the mature eggs are laid, they are likely
fertilized as they pass posteriorly in the groove

between the legs on

The fertilized eggs are fastened to the the two and appear much as small bunches of swimmerets by a secretion The later development continues shot-sized grapes hanging there. here, and they are aerated by movements of the swimmerets through
sides of the body.

the water.

Cleavage divisions follow over the surface of the eg^ and the em bryo develops on one side of the mass. The body form with segments and limb buds appears, and hatching occurs in from five weeks to two

The larvae grasp the swimmerets with their chela and remain with the mother for about a month. Two or three days after
months.

hatching they pass through the

first

molt or ecdysis; that

is,

they

PHYLUM ARTHROPODA
shed the outer
cuticle.

CLASS

CRUSTACEA

281

This

is

repeated seven or eight times during

the first season to allow for growth.

The average

life

span of the

crayfish that reaches maturity

is

about four years.

Regeneration and Autotomy


This power
is

limited to the appendages and eyes in this animal,

but

it is

quite well developed in these parts.

The

possibilities

and

rate of regeneration are greater in younger animals.


lost legs or

Mutilated or

mouth parts

are readily restored.


ability to allow a
if it is

The genus Cambarus has the


break
off at

walking leg to

a certain line or joint

caught or injured.
is

new

leg will develop

from

tliis

stump.

This phenomenon
to help in this

called autot-

omy.

There are special muscles

and a membranous
if

valve stops the passage of blood through the leg, thus preventing excessive bleeding.

Bleeding will stop more quickly


it

the break oc-

curs at such a point than

would otherwise.

Autotomy often
life.

makes

it

possible for the animal to sacrifice a leg to save its

Economic Relations
Crayfish and the entire class Crustacea are of considerable im-

portance to man.

The

crayfish, lobster, crab, shrimp,

and others

are used directly as food to the extent that it is an industry valued The at several million dollars annually in the United States. numerous smaller genera, like Daphnia, Cyclops, Cypris, Gammarus, Asellus, and Euhranchipus, comprise a large part of the food
of

many

of our food fish either directly or indirectly.

human consumption.

minute ones also feed many clams and oysters and The shrimp and crab fisheries

finally

The more end in are the most


is

important of the Crustacea on the Texas coast of the Gulf of Mexico.


In the Mississippi valley and on the Pacific Coast the crayfish

becomes a serious pest in the cotton and corn fields of Louisiana, East Texas, Mississippi, and Alabama, They fill the swampy land with their burrows where they come up to the surface and eat the young plants. Frequently their burrows do serious damage to irrigation ditches and earthen dams. Crayfish also capture numerous small fish which are either immature food
used extensively as a food.
It fish or potential

food of such

fish,

282

TEXTBOOK OF ZOOLOGY
Characterization of Other Crustacea

Besides crayfish the order Decapoda includes lobster, shrimp, and

They all have ten walking legs for which they are named. The crayfish and lobster are verj^ similar except in size. The shrimps and prawns are marine and resemble the crayfish except that they do not have the great pinchers (chela) and the abdomen is bent sharply downward. The crabs are quite different in shape
crab.
in that the
is

cephalothorax

is

broader than

it

is

long, the

abdomen
Crabs
to sev-

poorly developed, and folded sharply beneath the thorax.

of different kinds vary in diameter


eral inches.

from

few millimeters

There are four species of swimming crabs


the most important and best known.

in the

Gulf

of Mexico, of which the hlue or edihle crab

{Callinectes sapidus,

Fig. 408)

is

The lady crab and


is

calico crab are also interesting species.

When

the blue crab

cap-

tured at molting time


times
it is

it

is

called the soft-shelled crab.

At other

the hard-shelled crab.

They maj^ be caught

in baited nets

or on pieces of meat on a line with which they are brought to the sur-

The hermit crab (genus Pagurus, Fig. 408) is smaller and lives in empty gastropod shells by backing into the shell and carrying it around. Due to the cramping and inThe activity the abdomen has become soft and partly degenerate. fiddler crab (genus Uca, Fig. 408) is another very abundant form found on the coast of the Gulf of Mexico and elsewhere. These are small semiterrestrial crabs which burrow in tunnels, and may thus honeycomb large areas of salt marshes. They can run quite rapidly, often moving sidewise, and thej^ are peculiar in that one pincher
face
lifted out in a dip-net.

and

of the male, usually the right,

is

much

enlarged.

This gives the

appearance of a fiddle and the other, reduced pincher resembles the bow. The large pincher is used in a nuptial dance, and occasionally a large number of these little crabs will be seen raising and lowering these enormous pinchers in concert.
Asellus communis is a common fresh-water form found in streams and pools. A salt-water genus, Idotea, is found in the ocean. The pill bug (Armadillidum) and the sow bug (Oniscus asellus or Porcellio sp.) are terrestrial, living in damp places under logs, stones, or heavy vegetation, and in cellars or greenhouses. Their legs are arranged Respiration is in two groups, which point in opposite directions. carried on by gills on the ventral side of the body and for this

PHYLUM ARTHROPODA

CLASS

CRUSTACEA

283

reason they must live in moist plac6s.

a garden pest in There are a number of aequatic forms which are parasitic on fish and others, such as the gribble (Limnoria), which tunnel in submerged wood. The amphipods are sand and beach dwellers which may be found burrowing or jumping around on the seashore or walking on the bot-

They are

that they eat leaves of delicate plajits.

tom The

in fresh water.

Gammanis

is

the best

known

fresh-water form.

legs of representatives of this order are divided into

two groups,
These are

with the legs of each group pointing toward each other.


of particular value as fresh-water fish food.

Entomostraca as a group is composed of many smaller crustaceans occurring in great numbers in both marine and fresh waters. The fairy shrimps (EuhrancJiipus) are delicate, transparent and feathery
appearing.

They

are about three-fourths of an inch in length.

They

Fig. 159.

Asellus, a

common

logical

fresh-water crustacean. Supply House.)

(Courtesy of General Bio-

up and their long, leaf-like appendages the body; these appendages serve also as respiratory organs. The^^ live in cool streams during the spring ajid fall. The summer is passed in the egg, which can withstand complete dryness. Many of them are parthenogenetic, hence, males are rare. The common marine form is Artemia, often called brine shrimp. The water fleas including Daphnia of order Branchiopoda, Cyclops and Diaptomus of order Copepoda and other small Crustacea constitute an important common group. Daphnia is one that is enclosed in a delicate bivalve shell. The second pair of antennae are very large and are used in swimming. The shell is beautifully marked and terminates in a caudal spine. They are only about

swim with hang from

the ventral side

one-tenth of an inch in length. Cyclops is another common freshwater form with the antennae shorter than the cephalothorax whose

284

TEXTBOOK OF ZOOLOGY

body length is also about one-tenth inch. It has a single median eye, and the females frequently are seen with a pair of egg sacs attached at the base of the abdomen. Diaptomus, another Copepod, is a common form of about the same form and size as Cyclops, except that the antennae are nearly as long as the body. Argulws is a genus of Copepods which is parasitic on fish, and the individuals are called fish lice or carp lice. They are flat creatures and are found running around over the scales of their hosts. Some of the other forms are parasitic on the gills and fins of fish and their bodies become greatly modified. The ostracods are small, swimming, bivalve forms that are sometimes called swimming clams. This group has beautifully marked valves; in fact, these animals are the most beautiful found in the
plankton.

Adult barnacles of order Cirripedia bear so


layman.

little

resemblance to

other Crustacea that they are usually overlooked as such by the

They are completely encased in a thick shell of several sections and have the general appearance of an oyster or clam. They are sessile in habit as adults, though free-swimming in the
larval stage.

Their entire

life is

spent in marine waters.

There

are several characteristic barnacles, rock barnacles on rocks, whale


barnacles from ships and pilings, and gooseneck barnacles of the
stalked type.
like bristles

After attachment, the legs become modified into featherwhich are used in gathering food. Sacculina (Fig. 404) is a genus related to true barnacles which has gone parasitic on crabs and has lost all resemblance to animal form. It settles on the body of a crab, makes its way to the interior and there becomes a branched mass of tissue which penetrates by roots to all parts of the body of the crab. After a time a baglike portion forms and projects externally on the ventral side of the abdomen of the crab.
Recapitulation Theory

statement of this idea, which was developed by von Baer, Haeckel, and others, and is so well illustrated by the comparison
of the phylogenic

and embryonic stages of certain Crustacea, may

well

This theory maintains that certain developmental stages or structures of the individual are related to ancestral conditions. That is, the individual in its development tends to repeat in an abbreviated fashion the history of the development
at this point.

come

PHYLUM ARTHROPODA
of the race.
is still

CLASS

CRUSTACEA

285

Briefly stated ontogeny recapitulates pliylogeny.

There

some doubt as

to the validity of this generalization in direct

application.

classical

example which

is

frequently cited

is

that of the devellarva,

opment

of the shrimp, Penaeus,

which hatches out as a nauplius

Nftuplius t*c{<

Fig. 160.

Nauplius

stage of the barnacle, Balanus. logical Supply House.)

(Courtesy of General Bio-

iSchizopod
Fig. 161,
Fig.
161.
in

Fig. 162.

Zoea and Megalops stages of developing Crustacea. Supply House.) Crabs include these stages their development. (Courtesy of General Biological Fig. 162. Schizopod or mysis stage through which the shrimp and lobster pass. (Courtesy of General Biological Supply House.)
having a single median eye and only three pairs of appendages. Following the molt, this nauplius changes to become the Prozoea stage, possessing six pairs of appendages. The next molt brings on segmentation and some change in form. This stage is called the Zoea

286

TEXTBOOK OP ZOOLOGY

and resembles very closely the adult Cyclops of modern Copepoda. The Zoea transfonns during further molts and growth to a stage with thirteen segments and a distinct cephalothorax which resembles the Gammarus is adult Mysis and therefore is called the Mysis stage.
also in about this category of phylogenetical development.

Follow-

ing the next molt the mysis stage becomes a juvenile shrimp with nineteen segments. The life history of the barnacles and Sacculina

has illustrated quite forcibly the possibility of such a relationship. There are extinct forms also whose adult condition was that of one This idea generally has served as of these developmental stages.
a great stimulus to the study of embryology and the theory of evolution as well as serving to establish natural relationship of

animal groups.
Phylogenetic Advances of Arthropoda
(1)

Greater specialization of segments,

(2)

paired, jointed ap-

pendages, (3) chitinous exoskeleton, (4) gill and tracheal respiration, (5) dioecious reproduction, (6) development of eyes and other sense organs, (7) green glands and malpighian tubules (insects) as excretory organs, (8) organization of social life.

CHAPTER XIX

PHYLUM ARTHPtOPODA

(CONT'D)

ONYCHOPHORA AND MYRIAPODA


(By Vasco M. Tanner, Brigham Young University)

Onychophora

An

interesting group of arthropods,


is

now

considered as the class

Onychophora,

restricted

to

the more tropical

and semitropical
These primitive

regions of the earth south of the Tropic of Cancer.

nocturnal forms, according to Austin H. Clark, are found in areas that vary in annual temperature from 50 to 80 P.; in fact, most of
the species are confined to habitats in which the temperature does not vary beyond the limits 60 to 70 F.

The onychophores are characterized


restrial

as soft-bodied, wormlike, ter-

forms with internally segmented bodies. The body may be divided into a head and abdomen. On the head is one pair of annulate antennae and a pair of jaws. The body bears many pairs of legs which are not distinctly jointed, but are provided with transverse pads

and apical

claws.

Kespiration

is

by means

of tracheae

which communicate with spiracles that are, in some species, arranged in rows on the body. The excretory system consists of nephridia arranged in pairs in the body segments and opening to the outside by a pore at the base of the fourth and fifth legs. The genital organs
discharge at the posterior end of the body.
ventral,

The nervous system

is

consisting of separate longitudinal nerve cords connecting


of ganglia.

number

pair of eyes
consists of

is

located at the base of each

antenna.

The body wall

an unsegmented dermomuscu-

lar covering.

In commencmg on the ancestry of the Onychophora, Prof. J. W. Folsom has the following to say: "Onychophora, as represented by Peripatus, are often spoken of as bridging the gulf that separates the Insecta, Chilopoda and Diplopoda from the Annelida. Peripatus
indeed resembles the chaetopod Annelids in
its

segmentally arranged

nephridia, dermomuscular tube, coxal glands

and

soft integument,

and resembles the three other


paired
ostia,

classes in its tracheae, dorsal vessel with

lacunar circulation, mouth parts, and salivary glands.


close,

These resemblances are by no means


287

however, and Peripatus

288

TEXTBOOK OF ZOOLOGY

does not form a direct link between the other tracheate arthropods and the annelid stock, but is best regarded as an offshoot from the base of the arthropodan stem."

Very

little is

known about

the habits of the onychophores, except

that they live under stones and the bark of trees, feeding upon small insects and spiders which they capture in a slime produced and
forcefully discharged

from glands which open on the oral


in a season.

papillae.

Many

of the species are viviparous, a single female producing as

many

as thirty living

young

About seventy-three species and fifteen genera are known from the two families Peripatopsidae and Peripatidae. A number of species in the family Peripatidae are found in tropical America; Macroperipatus perrieri (Bouvier) is found at Vera Cruz, Mexico; while
Australia, Tasmania, the species of the family Peripatopsidae are confined to New Guinea, New Zealand, Cape Colony, and Chile.

The

classes

Diplopoda and Chilopoda are considered by some au-

The more recent students thors as orders of the class Myriapoda. of these groups, however, have adopted the plan of classification followed here.

Fig-.

163.

Peripatus capensis. Entomology.

Natural

size.

Redrawn by Nelson

(After Moseley A. Snow.)

from

Folsom's

Myriapoda
The Diplopoda are terrestrial arthropods commonly called millepedes. The body is composed of three regions: the head, thorax or trunk, and the ahdomen. The head bears a pair of short antennae, ocelli, and mouth parts consisting of a pair of mandiUes and a pair
of maxillae.

Just back of the head

is

a segment with a well-developed


of this group as a seg-

tergite, the collum, considered

by some students

ment which has played an in some of the other groups

important role in the formation of the

head

of arthropods. A however, to belong to this first segment, as does a single pair of legs These four segments are said to to the three following segments. Ducts from the reproductive organs open at constitute the thorax.

single pair of legs seem,

the base of the second pair of legs on

tlie third body segment. an indefinite number of segments, each The abdomen consists of consisting of a tergum and two sterna. Each sternum bears two pairs

PHYLUM ARTHROPODA

CLASS

ONYCHOPHORA

289

of legs and two pairs of spiracles. The spiracles are closed by a valve and communicate with tracheal pockets and unbranched tracheae.

Embryological evidence supports the belief that the abdominal segments have resulted from the fusion of two segments. The legs are jointed very much as in the insects. The tarsus consists of

the three segments.


is

The heart

a dorsal structure with side valves and an anterior

The diand intestines. The excretory wastes of the body are removed by two or four pairs of Malpighian tubules which discharge their excretions into the ingestive tract consists of a mouth, esophagus, stomach,
testines.

tube to the head similar to the arrangement in the insects.

th

Snow.)

Fig. 164. Julus terrestris. millepede. Side view of anterior end. a, antenna; ab. abdomen; colj collum (first thoracic segment) e, a group of ocelli go, genital opening m, mandible g, gnathochilarium h, head I, labium th, thorax. (After Borradaile and Potts, Invertebrata. Redrawn by Nelson A.
;

A common

living plant substance.

The millepedes feed upon vegetable matter, decaying as well as They are slow-moving, wormlike creatures,

living in dark, moist places.


selves

When

disturbed they usually roll them-

laid in damp earth, and young hatch they are very small, consisting of only a few segments and three pairs of legs. The diplopods are found in most parts of the world. In the United States there are six important families and about 120 species. The family Julidae is widely distributed. The species Julus hortensis Wood; J. virgatus; J. hesperus Chamberlin; and Spiroholus margin-

up

into a little

coil.

The eggs are

when

the

290
atus are fairly

TEXTBOOK OF ZOOLOGY

common

in

many

parts of the United States.

In the

family Polydesniidae, Polydesmus serratus Say is a common species in the United States west to the Mississippi River. Members of the class Chilopoda differ from the Diplopoda by their
dorsoventrally flattened bodies, consisting of fewer segments which

bear but one pair of


as a rule,

legs,

and by

their long antennae.

The Chilopoda,

move

faster than millepedes.

mature

The Chilopoda are carnivorous, preying upon adult as well as imThe mouth parts insects, also upon spiders and Mollusca. consist of a pair of mandibles and two pairs of maxillae. A pair of

Fig. 165.

Scutigera forceps.

The long-legged house centipede.

(From a paper by

C. L. Marlatt,

Farmer's Bulletin, No.

627, U. S. Dept. of Agriculture, 1914.)

poison claws are located on the

ments are

legless,

and each

of

body segment, the last two segthe other body somites bear a pair of
first

seven-jointed walldng legs.

The circulatory system is well developed it extends the full length body and gives off in each segment lateral vessels or arteries. The nervous system consists of connected paired ganglia in each leg-bearing somite and a subesophageal and supra-esophageal ganglia which supply nerves to the eyes, mouth parts, antennae, and other
;

of the

parts of the head.

PHYLUM AETHROPODA
The
digestive system
is

CLASS

ONYCHOPHORA

291

fairly simple, consisting of the esophagus,

stomach, and intestines.

pair of ]\Ialpighian tubules

the anterior part of the intestines.

connected to a pair of spiracles

empty into The tracheae are branched, being on each body segment.

Fig. 166.

Scolopendra.

The large centipede of the Southwest.

Dr. R. V. Chamberlin, one of the leading students of the millepedes

and centipedes, reports the following species of centipedes for the Western United States: Scolopendra morsitins Linn.; S. heros Girard and Arthrorhahdmus pygmaeus Pocock of the family Scolopendridae. The species S. heros is a large dark greenish colored species found in Kansas, Arizona, Southern California, and Texas.

CHAPTER XX

PHYLUM ARTHROPODA
ARACHNIDA

(CONT'D)

(By Vasco M. Tanner, Brigham Young University)


Probably the most heterogeneous class of arthropods is the class Arachnida, which is now a pigeon-hole for the spiders, scorpions,
mites,
ticks,

pseudoscorpions,

harvestmen,

whip

scorpions,

bear

animalcules, king crabs,


as the Pentastomoids

and several other

less

common

orders, such

and Pycnogonids.

In attempting to give the characteristics of this class we will list only the obvious characters and then remind the reader that the members of this group have more likenesses to the Arachnida than to any other assemblage of animals, hence, no doubt, the reason for having
this class intact.

The spiders and their relatives are characterized by having two main divisions of the body, the cephalothorax and the abdomen; no antennae or wings; mouth parts consisting of but two pairs of appendages, the chelicerae and pedipalpi; four pairs of walking legs,
each consisting of seven joints; lung books, which are remarkable

number and arrangement in the various families abdominal appendages which have been modified into spinning organs; and no metamorphosis, the development being direct.
respiratory organs; simple eyes, that are definite in
;

Spiders

The spider's body consists of two divisions: the cephalothorax and abdomen. The first division of the body, the cephalothorax, results from the fusion of the head and thorax. Six pairs of appendages The first pair, the are to be found upon this part of the body. chelicerae, are two-jointed, consisting of a basal part or mandible and a terminal claw. The mandible is stout and covered on the inner surface with small teeth and setae. The poison glands in the mandibles discharge their poison through the movable sharp-pointed claws. The poison serves as an aid to kill insects and other animals

used by the spider for food.

are located in front of the legs.

The pedipalpi, consisting of six joints, They are used in handling the food,
292

PHYLUM ARTHROPODA
as feelers,

CLASS

ARACHNIDA

293

and

in the males the terminal segments are used as copu-

latory organs.
of

These palpal organs are useful in the

classification

many

of the families of spiders.

The basal segments, or maxillae,


of appendages
of seven segments; namely, the

are used in chewing the food.

The four following pairs

are the legs.

Each

leg

is

made up

coxa, trochanter, femur, patella, tibia, metatarsus,

and tarsus. The eyes are located on the front of the head and usually are eight in number some species have fewer, but never more than this number. The cephalothorax and abdomen are connected by a narrow waist. On the under side of the abdomen, just back of the last pair of legs,
;

clearly shown. ment Station.)

Fig. 167. Ventral view of adult female, black widow spider, Latrodectus mactans, hanging from web. The hour glass on the ventral part of the abdomen is (From Knowlton, by permission of the Utah Agricultural Experi-

are the front pair of breathing openings or

slits

which communicate

with the lung books.


organs,

Near these

is

the opening of the reproductive

which in the females is protected by a plate called the epigynum. Near the end of the abdomen are three pairs of spinnerets. In front of these is an opening to the tracheae and just posterior to

them

is

the anus.

The digestive system of the spiders is well adapted for its fluid food. The mouth is located just behind the chelicerae. Through its small opening the liquid portion of the prey is sucked up by means of
muscles which are attached to the dorsal wall of the cephalothorax

294

TEXTBOOK OF ZOOLOGY

stomach.

and the anterior portion of the stomach, which is called the sucking The posterior portion of the stomach gives off five caeca which are supplied with blood vessels from the anterior aorta. The intestine passes through the small waist connection into the abdomen and becomes enlarged in two portions, one for the reception of the hepatic ducts from the liver and the other at the posterior end for
the formation of a stercoral sac or pocket.

The excretory wastes are removed by the Malpighian tubules, which discharge into the posterior portion of the intestines, and by vestigial
coxal glands.

The vascular system consists of a muscular tube and veins, located above the intestine. The heart from the body by means of three pairs of ostia.
teriorly into the

or heart, arteries,
receives its blood
It is

then forced
also pos-

anteriorly through the aorta into the cephalothorax

and

abdomen.

The nervous system is composed of a large ganglion in the cephalothorax which is connected with a dorsal brain by a nerve ring around the esophagus. Nerves pass from the ganglion to the limbs and the
abdomen.

The respiratory functions are carried on by the lung books or sacs, which contain a number of thin plates through which the blood passes on its way to the heart. In the posterior part of the abdomen
is is

a pair of branching air tubes or tracheae.

This system of breathing

found only in the arachnids.

The reproductive organs consist of the ovaries in the female and The female has two sets of openings, one to receive the sperms into the sacs, from the tips of the pedipalps, and the other is the exterior opening of the oviducts. The male and female openings are near the lung books. The eggs are fertilized within the body of the female and then laid in silken bags or cocoons. In many species these egg bags are carried on the abdomen of the female.
the testes in the male.

The rate of growth of the eggs varies according to the conditions. For example, eggs laid in the autumn develop slowly all winter, while those laid in the summer develop within a few days. The hatching takes but a day or two. The young spider is pale and soft bodied, but in a few days it molts for the first time and then begins to look like an adult spider. As the spider grows, it molts a number of times, the development being direct. If the young spiders do not get out
of the cocoon soon enough, so that they have their freedom, they

PHYLUM ARTHROPODA

CLASS

ARACHNIDA

295

begin to eat one another. Many spiders live only one year, developing from over-wintering eggs in the spring, growing to maturity during the summer, laying eggs, and then dying in the fall. Some spiders live two or three years, while other species live many years.
Classification of the

Arachnida

For purposes of

this chapter

we

shall confine our discussion to but


:

(1) Araneida, the five of the common orders of the class Arachnida true spiders; (2) Acarina, the mites and ticks; (3) Scorpionida, the scorpions; (4) Phalangida, the daddy longlegs; and (5) Xiphosura,

the king crabs.

According
families.

to

a recent study,

Natural Classification of Spiders, by


is

Dr. A. Petrunkevitch, the order Araneida


Species of
of

made up

of sixty-two

many

of these families are rarely encountered,


in America.

and some

them are not found

The following eight


:

families, however, are

common

in the United states

1.

Lycosidae, the

wolf spiders.

This family contains

many

of the largest native species.

They are found mainly upon the ground running around in search The females carry the cocoons attached to their abdomens. of food. The eyes are arranged in three rows of four, two, and two. The species, Lycosa gulosa, L. kochii, L. frondicola, and Allocosa parva
are
2.

common

species in the western states.

Theridiidae, the comb-footed spiders.

The spiders

of this fam-

ily are found on low growing vegetation, fences, buildings,

and

at

They build rather loose webs from times under boards and rocks. which they hang back downward. Members of this family have a toothed comb on the tarsi of the fourth pair of legs, three claws, and eight eyes. The black widow, Latrodectus mactans, reported as a poisonous species, is a member of this family. The hourglass on the
ventral part of the
3.

abdomen

is

scarlet in color in the live spider.

Thomisidae, crab spiders.


lie

These crablike spiders do not con-

struct webs, but

in wait in the flowers for insects that visit them.

They

They possess eight are often highly colored for protection. eyes in two rows; the two anterior pairs of legs are longer than the
other legs
;

and the body

is

rather

flat.

3Iisumena vatia and Xysticus

nervosus are
4.

common

species.

Drassidae.

or in silk tubes on shrubs

long enough to

The drassids live mainly on the ground, under stones, and grass. The spinnerets are generally extend a little behind the abdomen. The eyes are eight

296
in

TEXTBOOK OF ZOOLOGY

number and arranged in two rows, but in this family there are They differ from the wolf spiders in that the body is much longer than wide and flattened slightly on the back.
only two tarsal claws. This
is

one of the large families of spiders.

Drassus negledus

is

representative species.

jumping spiders. Members of this family attract attenThey live on fences, tion by their jumping and bright coloration. buildings, plants, and on the ground. They do not spin webs for the capture of prey but only for their protection and cocoons. The eyes of this group are most distinctive. They are arranged in three rows,
5.

Attidae,

occupying an area on the cephalothorax known as the ocular quadrangle. The eyes on the front row are the largest. The males and This family is females differ considerably in size and structure.
world-wdde in distribution. It is represented by three hundred and fifty species in America north of Mexico. Some of the common species of our fauna are: Phidippus worhmanii; P. formosus; Icius similis;
EuopJirys monadnock; and Salticus senicus.
Argiopidae, the orb-web spiders. The spiders of this family build rather large typical webs for the purpose of capturing their prey.
6.

After building the web they lie in wait for some insect which may serve for food to become entangled in it. This is a large family, the members of which have eight eyes, and three claws on the tarsus.

They are confined to suitable places on vegetation, buildings, fences, and holes, where they construct their webs and then remain near by to watch them. Some of the common species of this family are Tetragnatha lahoriosa; T. extensa; Metargiope trifasciata; Neoscona benjamina; and Aranea gemma.

Most of the species of Tetragnatlia are found in moist places they Their bodies are round and build their webs over running water. common garden spider, found in Metargiope trifasciata is the long. the fall of the year in potato, tomato, and beet fields. It has a silvery white color and is about an inch in length when full grown.
;

Representatives of this family are 7. Aviculariidae, the tarantulas. They are confined to the south and southwestern United States. large, hairy, black spiders with eight eyes and chelicerae projecting

forward.

The trap-door spiders live in tunnels dug in the ground and provided with a hinged door which closes the entrance to the
tunnel so perfectly that
it is

almost impossible to locate the tunnel.

The

tarantulas, which are large nocturnal species, live

under

rocks,

PHYLUM ARTHROPODA

CLASS

ARACHNIDA

297

in holes, and under debris in the daytime. They feed upon beetles and other ground-living insects. Eurypelma steindachneri and E.

hentzi are
8.

common

tarantulas.

Pholcidae, the pholcids, are spiders with long legs and six or

eight eyes.

They build

loose

protected corners of buildings.


States.

webs in dark places, such as cellars and There are six genera in the United
is

Pholcus phalangioides

a large species.

The abdomen

is

elongated in form, and the legs are one and one-half to two inches
species are

used to carry the eggs. Three found in the genus Physocyclus : two of them are Physocyclus glahosus which is found in Florida and P. tanneri, in Utah.
long.

In

this species the chelicerae are

Fig-. 168.

Fig. 169.

Fig. 170.

The brown mite, Bryobia praetiosa. (Greatly enlarged.) (After Sorenson, by permission of the Utah Agricultural Experiment Station.) Fig. An the Utah Agricultural Tetranychus adult female, red spider, (After Knowlton, by permission of Experiment Station.) Fig. An adult mite, Eriophyes (After Sorenson, by permission of the Utah Agricultural Experiment Station.)
Fig.
168.
169.
telarius.

170.

blister

pyri.

For a discussion

of the other spider families

found

in this country,

the student should consult the most valuable treatise on this subject,

The Spider Book, by Professor John H. Comstock. Order Acarina. Mites and ticks are mostly small creatures with the cephalothorax and abdomen fused solidly together. They are, in the main, ectoparasites or endoparasites however, some of the aquatic species are able to shift for themselves, living upon various small

water animals.
n;^^nphs.

All mites lay eggs Avhich hatch into six-legged After some growth and moulting they develop into adults
legs.

with eight

298

TEXTBOOK OF ZOOLOGY
group of mites belonging
to the

family Tetranychidae,

known

as

brown mites and red

spiders, are widely distributed, feeding

upon
mite,

practically all kinds of cultivated plants.

The

clover or

brown

Bryohia praetiosa, is cosmopolitan in distribution. It feeds upon fruit and shade trees, garden plants, and common annual and perennial plants.

The common red

spider, Tetranychus telarius,

which

is

common
shade

in this country, attacks field plants, fruit trees, forest trees,

trees,

species of plants.

and shrubs. It is reported that it attacks over 250 Another common mite pest is the leaf blister mite,
to

Eriophyes pyri, which belongs

the

family Eriophyidae.

This

species attacks the leaves of the pear

and apple.

leaves results in the development of


defoliation.

The damage to the brownish blotches and partial

Those belonging Argasidae attack only warm-blooded animals. After taking a meal of blood, they leave the animal and go into hiding. At times they are a serious pest to poultry. Ticks of the family Ixodidae attach themselves to their host, suck blood, and grow to many times their original size. Texas cattle fever and Rocky Mountain spotted fever are dreaded diseases that are carried by ticks.
ticks are all fairly large parasitic species.

The

to the family

The order Scorpionida consists of large-sized arachnids with pedipalpi which resemble the chelipeds or pinchers of the crayfish; also
specialized stinging organ.

with a flattened body and elongated abdomen terminating in a They are nocturnal, hiding during the day under rocks and burying themselves in the sand. They feed upon
insects

and

spiders.

They are viviparous; the mother takes care of the young, protectthem by carrying them around on her back and by helping them to catch their prey. They breathe by means of lung books and have a
ing
direct development like the spiders.
called the pectine
is

peculiar comblike structure,

found in the ventral part of the second abdominal segment. Four families are represented in the United States, and they are found only in the southern and western states. Hadrurus hirsutus, and Vejovis mexicanus of the family Vejovidae are

common

species.

Another interesting little group of arachnids is the order Solpugida, found in the same territory as the scorpions and represented by
twelve species contained in three genera.

Eight of the species be-

long in the genus Eremohates.

PHYLUM ARTHROPODA
The Phalangida, commonly

CLASS

ARACHNIDA

299

called

harvestmen and daddy longlegs,

can be distinguished from other arachnids by their body which is composed of a broadly fused cephalothorax and abdomen, the abdomen consisting of nine segments; long legs; and the presence of

The reproductive organs, an and a penial organ in the male, are located on the obscure division between the cephalothorax and abdomen. The respiratory organs consist of tracheae which open through abdominal spiracles. The harvestmen do not have silk glands and therefore do not construct cocoons for the eggs, which they lay under stones and under the bark of trees. There are about seventy species
only two eyes on the cephalothorax.
ovipositor in the female
in the United States, representing six families.

Species of the family


distributed.

Phalangididae are the most

common and widely


crab,

is represented by only one living Limulus polyphemus is the American species found along the Atlantic Coast from Maine southward. Because of its shape and resemblance to the crabs, it has been called the horseshoe crab. The body consists of two regions; the cephalothorax and the abdomen. There are six pairs of appendages on the cephalothorax. The basal parts of the appendages situated around the mouth are used for crushing the food, which consists mainly of worms. On the abdomen are six pairs of appendages, the last five pairs bearing book-gill structures used in respiration. The males are The a little smaller than the females, but similar in appearance. eggs are deposited in the summer in shallow water in small sandy depressions w^here they are then fertilized by the male.

The order XipJwsura, king

primitive genus and five species.

CHAPTER XXI

PHYLUM ARTHROPOD A
CLASS INSECTA

(CONT'D)

(By Vasco M. Tanner, Brigham Young University)


Insects are the most abundant creatures on the earth today. There

which have never no doubt, is because insects exist in every type of habitat known. They are found in sea water along the shore; in fresh water that ranges in temperature from 50 C. to ice cold; in the soil; in dry desert conditions; on the vegetation of plain and swamp; from the tundra
is

said to be over 650,000 living species,

many

of

been seen by the great majority of mankind.

This,

of the north to the tropical

as well as man,

many

of

pampas; in trees; on and in animals, which are carriers of disease. They ravage
In short,

our crops and damage our stored foods.


insects are omnipresent.
is is

we may say

that

One noted entomologist has said that this an age of insects, and to this we may add that every man's farm "no man's land" and that the contending forces are insects and

man.
This great class Insecta has been upon the earth from the Pennslyvanian times, of the late Paleozoic era, to the present.

This means

that for probably one hundred million years these arthropods have been adjusting to a changing environmental complex, and the success with which they have met the challenge is quite evident today.

Various explanations have been advanced to account for the great adaptability^ of insects in filling practically every niche in nature.

The Russian

biologist, S. S. Chetverikov, argues that the chitinous

in that it has

exoskeleton has been of great value in the evolution of this group, permitted them to develop strong appendages, unlimited external features,
entirely

which has opened up an Dr. C. H. Kennedy, however, has pointed out that there are advantages as well He says: as disadvantages to the possession of an exoskeleton.

and a small

size

new

place in the world of living animals.

300

PHYLUM ARTHROPODA
"The exoskeleton has made
in fully as

CLASS

INSECTA

301

possible very definite advances in the

evolution of insects, but at the same time has limited their evolution

many

other ways."

Aside from the ehitinous exoskeleton, other distinctive characteristics, such as power of flight, which is possessed by no other invertebrate animal a tracheal system, which keeps the hemolymph or blood from becoming impure and finally their great variability and power to reproduce, have made the insects, no doubt, the sucThis leads us to wonder how cessful creatures they are today. successful man will be in his evolution during the next fifty million
;

years.

ment as the

Will he be able to meet the demands of a changing environinsects have?

INSECT CHARACTERISTICS
body is divided into three The head, which consists of six segments, bears a single pair of antennae, the eyes, and the mouth parts; the thorax consists of three segments and is the region which bears three pairs of legs and two pairs of wings, when they are present in the nymphs and adults; the abdomen bears a variable number of segments in the various groups of insects, also the genital apertures which are situated near the anus at the posterior end of the body.
Insects are Arthropoda in which the
regions, the head, thorax,

and abdomen.

Head
The head consists of a number of immovable plates or sclerites forming the head capsule, to which are attached the paired appendages.
visible,

In

many

insects

the sutures separating the sclerites are

and these plates and depressions have been given definite names. The paired appendages furnish much evidence that the head has resulted from the fusion of several segments. The eyes, antennae, mandibles, maxillae, and labium are considered as developing on distinct somites.
six anterior

Evidence concerning the fusion of the segments of the body in the formation of the head comes through an embryological study of the insect.
There are two kinds of eyes
eyes.
:

ocelli,

or simple eyes,

and compound

The simple eye

is

a small area consisting of a single cornea.

the

Simple eyes are generally found in varying numbers along with compound eyes in adult insects; they are, however, usually lack-

302
ing in beetles.

TEXTBOOK OF ZOOLOGY

The compound eye

consists of

many

facets,

which

are the hexagonal-shaped corneal ends of structures called ommatidia. The facets are convex, and insects are short-sighted.
It has

been shown by experimentation that

many

insects are able

to detect colors.

Some students

of the insects

maintain that the

A.

B.
Fig.

Western lubber grasshopper, Brachypephus magnus. female. The form found on the plains. (Photographed
171.

A, male, and B,
T. Murray.)

by Leo

compound eyes function


ocelli are

mainl}^

in

detecting movements,

while

used

to detect light intensities.

There
size,

is

shape, and position on the head

but one pair of antennae or feelers, and they vary in among the various groups of

PHYLUM ARTHROPODA
insects.

CLASS

INSECTA
in

303
insects they

The
;

feelers function in

many ways;

some

are tactile
or

in others they are respiratory or olfactory, or auditory,

may

be used to hold the female during copulation.

Antennae are

useful structures in the classification of insects.

26 V,'

Z7 28
; ; ;
;

2, Fig. 172. The external features of a grasshopper. 1, maxillary palp; mandible 3, labrum i, clypeus 5, frons 6, compound eye 7, ocellus 8, vertex 13, spiracle, thoracic; 12, wing, mesothoracic 9, antenna; 10, gena 11, pronotum first abdominal segment; 15, auditory apparatus; 16, wing, metaIJi, spiracle, thoracic n, supra-anal plate; 18, podical plate; 19, cercus 20, ovipositor; 21, labial palp; 22, femur, prothoracic leg; 23, coxa, mesothoracic leg; 2i, trochanter; 25, femur; 26, tibia; 27, tarsus; 28, femur, metatlioracic leg; 29, spiracle; 30, sternum 31, tergum. (After Turtox key card, courtesy General Biological Supply House.
;

Fig. 173.

Detail

of

ommatidia, magnified.
lished

by The

C. V.

(From White, General Biology, pubMosby Company.)

The mouth parts are chitinous structures and are represented in two distinct types: mandibulate or biting, and suctorial or sucking mouths. Some of the insect orders which possess mandibulate mouth parts are Coleoptera, Odonata, Neuroptera,
the insects by

304

TEXTBOOK OF ZOOLOGY

Mallophaga, Dermaptera, Isoptera, and Orthoptera. The Lepidoptera, Diptera, Heteroptera, Homoptera, and Siphonaptera are suc-

knowledge of the mouth parts is very useful if No other insects are to be effectively controlled and classified. group of organs within the insect body vary in form as do the
torial orders.

labial palp; M., mandible; Me., Pa., palpifer; Sin., submentum


;

Ga., Fig. 174. Mouth parts of Bhomaelia microptera. G, cardo Gl., clypeus L-i"; I/t., ligula galea; L, labium; La, labium (second maxilla); Lac, laclnia mentum MP., maxillary palp; ilfa;. first maxilla; (From White, General Biology.) /St., stipes.

;
;

mouth

parts.

The mandibulate mouth parts are the type from

which the suctorial type has been evolved. In the mandibulate mouth parts there is a labrum or upper lip which is attached on its upper border to the clypeus and extends down over the mandibles. The mandibles or jaws are true appendages which move in a trajis-

PHYLUM ARTHROPODA
verse plane.

CLASS

IXSECTA

305

They are hard, thick plates of chitin with toothed edges adapted for cutting or crushing food. In some beetles the mandibles become greatly enlarged and apparently worthless. The
maxillae or second pair of jaws
in a similar plane.
lie

under the mandibles and move


is

much more complex.

The The cardo

maxillae, which consist of sclerites, are

the piece which hinges the

maxillae to the head; attached to the distal portion of the cardo is the stipes, which bears three sclerites the lacinia, galea, and palpus.

The lacinia is provided with teeth or spines which aid in holding and chewing the food. The palpus is composed of four or five segments and is sensory in function. The labium or lower lip is formed by the fusion of what is believed to have been a second pair of maxillae. The labium is hinged to the head by the mentum from which extends one or two pairs of
lobes, the ligula.

Projecting from the

mentum on
;

each side

is

palpus which consists of one to four segments

it

functions as a sen-

soiy organ, probably detecting senses similar to our


taste

own

senses of

and

smell.

of the mouth,

The hypopharynx or tongue arises from the labium into the cavity and bears the opening of the salivary duct.
In the typical sucking insect the mouth parts consist of two pairs

of sicklelike or styletlike structures which are modified mandibles


maxillae, as

and

found in the mandibulate orders discussed above. The labium forms a long sheathlike structure in which the styletlike manThe upper lip or labrum covers over the dibles and maxillae lie. proximal portion of the beak. Food is taken in a liquid form by
being sucked up through the labial sheath.

In the mosquito the hypo-

pharynx

long and slender like the mandibles and maxillae; the salivary duct extends throughout the entire length of the food channel. The saliva causes an irritation, and if the mosquito is inis

fected with malarial organisms they are introduced into the blood

stream of man.

Thorax
The thorax
is

composed of three segments, which are called the

prothorax, mesothorax, and metathorax.

pair of jointed legs

is

attached to each segment and most adult insects bear a pair of wings

on the mesothorax and metathorax.

The dorsal or back surface

of

306

TEXTBOOK OF ZOOLOGY
is

the segments of the thorax


surface, the sternum,

called the tergum, the ventral or


side, the

under

and each

pleurum.

The

legs are

made

main segments: the coxa, trochanter, femur, tibia, and up The coxa forms the joint by means of which the leg is attarsus. tached to the body. The trochanter is small, while the femur, or thigh, and tibia are large, forming the greater part of the leg. The tarsus or foot is composed of five smaller segments and a pair of
of five
claws.

In insects the tarsal segments

may

differ in

size,

length,

Fig. 175. Jumping leg or third thoracic appendage of Rhomaelia microptera. (From White, C, coxa; F, femur; P, pulvilli Ta, tarsus; Ti, tibia; Tr, trochanter. General Biology.)
;

shape,

and number, and are useful in

classification.

The

legs are

greatly modified for obtaining food, running, walking about, swimming, and jumping. They are also modified for the production or reception of sound, for the collection of food, such as pollen, and for
copulation.
acters.

In some species they also exhibit secondary sexual char-

The wings are thin

folds of the skin,

veins in various ways.

shaped and strengthened with The presence of wings is one of the most

PHYLUM ARTHROPODA

CLASS

INSECTA

307

characteristic features of the insects.


tion,

Because of their great variawings are of much value in classification. The wing is com-

posed of a network of thickened lines called veins and thin areas

between the veins called cells. The number, arrangement, and character of the veins and cells are an aid in grouping insects into The majority of insects possess two families, genera, and species. there are some, however, that have but one pair and pairs of wings some groups are wingless.
;

Fig.

17G.

(From Henderson, permission

Right

wings of a grasshopper.

A, the fore of Utali Agricultural

wing; B, the hind wing. Experiment Station.)

Abdomen
The segments of the abdomen are usually simple, but the number There are only ten present in many insects, yet in the embryo of the insect there are eleven. The jointed appendages have been almost entirely lost in adult insects. On the eighth and ninth segments of the female and the ninth of the male are paired structures forming the genitalia which are the external organs of reproduction. Within the abdomen are found the respiratory, digestive, and genital systems.
varies greatly in different insects.

308

TEXTBOOK OF ZOOLOGY

Body Wall
Another distinctive feature found in the arthropods is the chitinous body wall, which provides the only rigid support for the body. The exoskeleton consists of three layers known as the cuticula or outer layer which is impregnated, more or less, with calcareous matter, the hypodermis or intermediate layer, and the 'basement memhrane. The hypodermis has its origin in the ectoderm and is the active growing layer of the body wall. Chitin is a substance found in

many
is

parts of the insect body, but Chitin


is

it

especially serves to give firm-

ness to the cuticula.

not destroyed by caustic potash. It a most interesting organic substance, resembling horn in some

physical ways.
All the tubercles, spines, setae,

and

scales of the

body wall are

formed by the

cuticula.

These structures are of importance in the

identification of insects.

Metamorphosis
Metamorphosis includes the alterations which an insect undergoes after hatching from the egg, and which alters, extensively, the
general form and life of the individual. All the changes which are undergone by a butterfly in passing from egg to adult each change from egg to larva, from larva to pupa, and from pupa to adult constitute metamorphosis.

There are four types of development or metamorphosis first, metamorphosis; second, pauroametabolous or development without metaholotis or gradual metamorphosis third, hemimetaholous or incomplete, and fourth, holometaholous or complete metamorphosis. The ametabolous insects are the Thj-sanura, Collembola, Mallophaga, and Pediculidae, which after hatching, grow through a number of instars, remaining practically the same form as the adult insect during all the development. This is development without metamorphosis. In the following orders: Orthoptera, Hemiptera, Homoptera, Isoptera, Thysanoptera, and Dermaptera, there is a type of development in which the nymphs gradually increase in size and the rudimentary wings and genital appendages become adult structures. This is known as paurometabolous development.
:

In the Odonata, Ephemerida, and Plecoptera, the newly hatched

naiads pass through an incomplete metamorphosis.

All

of the

PHYLUM AKTHROPODA
hemimetabola naiads
istence.
live

CLASS
life

INSECTA

309

an aquatic

which necessitates changes


in the adult aerial ex-

and physiological adjustments not required

In these orders there are greater changes during developin

ment than are found

gradual metamorphosis.

The Holometabola, in which the larva hatched from the egg bears no resemblance to the adult, goes through a complete metamorphosis.

The holometabolous insects include the following orders

Tri-

choptera, Neuroptera, Coleoptera, Lepidoptera, Siphonaptera, DipThe larva is variously called the maggot, tera, and Hymenoptera.

grub, or caterpillar.

It

eats almost constantly since this

is

the

an insect's life history. After molting several The pupa to rest and prepares for the pupal stage. gradually takes on the adult form and after a few days or even months, the adult or imago emerges.

growth period times it comes

in

The remarkable adaptation of the immature stages of insects to their food supply has undoubtedly had much to do with their great success as a group. Their food habits, minute size, use of flight in locomotion, and rate of multiplication, along with other distinctive characteristics mentioned above, have made possible the development of this dominant group.

CLASSIFICATION
Because of the important role insects play in the life of man it is worth while to be able to recognize some of the common orders. The characters most used for the separation of the orders of insects are the structures of the wings and mouth parts, and the type of development, or metamorphosis. The number of orders recognized in this class varies considerably, depending on the authority followed. It has been divided into the subclasses: Apterygota, the two wingless orders Thysanura and Collembola, and Pterygota,

which includes
lowed here
is

all

the other orders of insects.

Since practically

all

the orders fall into the subclass Pterygota, the arrangement folthat of discussing

them according

to their development.

Subclass Apterygota.
phosis.

Ametabola

are insects without metamor-

Order Thysanura. The members of this order have retracted mouth parts, elongated rather flattened bodies, long antennae, and abdominal appendages. They are soft-bodied small insects, com-

310
moil in

TEXTBOOK OF ZOOLOGY

They feed largely upon dead plant tissue, and beneath stones, dry leaves, and loose and One of the commonest species of this order is the "tish bark. moth" or " silverfish, " which attacks book bindings, the paste of wall paper, and starched clothes. It is about half an inch long ajid has three bristles extending from the tip of the abdomen.

warm

climates.

are found in the soil

locust,

A, Red-legged 177. Grasshoppers common to western United States. Melanoplus feniur-rubrum DeGeer, male B, Haldeman's locust, Hippiscus Schistocerca shoshone Thomas, female coralUpes Hald. C, Slioshone grasshopper, D, two striped Mermiria, Mermiria hivittata Serville E, western meadow grass(From Henderson, Utah Agricultural hopper, Conocephalus vicinus Morse, female. Experiment Station.)
Fig.
;
; :

The following are some of the families and species found in the western United States: Family Lepismidae. Lepisma saccharina Linn. The silverfish moth; Machilidae, Machilis orhitalis Packard;

PHYLUM ARTHROPODA

CLASS
;

INSECTA

311

Campodeidae, Campodea folsomi Silvestri and Entry chocampa wilsoni Silvestri; aud Japygidae, Japyx huhhardi Cook and Evalljapyx
sonoranus Silvestri.

Fig. 177.

(Continued.)
with
retracted

Order

Collenihola.
;

Small
;

insects

mandibulate

mouth parts simple eyes antennae with four segments in most genera; abdomen with six segments, which often carries three ap-

312

TEXTBOOK OF ZOOLOGY

Tlie body is often cylindrical. pendages modified for jumping. About one thousand species have been described, some of the common Family Poduridae. Podura aquatica L. Achorutes species follow Family Entomobryidae Folsomides decemoculatus maturits Fols.
: ; ;

Fig. 178.

Western or Mormon Henderson, permission

cricket, Anabrus simplex Haldeman, female. of Utah Agricultural Experiment Station.)

(From

Fig. 179.

(From cricket, Stenopelmatus fasciatus Thomas. Sand mission of Utah Agricultural Experiment Station.) Henderson,
;

per-

JtlilLs

Tull.

Isoioma dongaia Mac G. 1. titusi Fols and Tomocerus vulgaris Family Sminthuridae, Sniinthiirus niger (Lubbock) S. eisenii
; ;
;

Schott

and Papirius maculosus Schott and Family Neelidae, Mega;

totJiorax incestoides Mills.

All the springtales listed above are found

PHYLUM ARTHROPODA
in

CLASS

INSECTA

313

many

of the states of western America.

Mills reports that the

Collembola are of some economic importance, damaging, in the main,


tender plant tissue. Subclass Pterygota.

Paurometabola
following are

are

insects

with

gradual

metamorphosis. The paurometabolous orders.

some

of the

more important

Fig

ISO.

riglit).

and Oriental Cockroaches American German (ventralofview), Agricultural Experiment Station.) to Utah (From Knowlton, permission
(left

Fig.

181. Praying mantis, Stagmomantis

sp.

It is

named

for

its

pose.

Order Orthoptera.

This

order contains the grasshoppers, katy-

and walking sticks. Members of this order usually po-ssess two pairs of wings some species, however, have their wings greatly modified or reduced, and some do not possess wings at all. The mouth parts are of the biting type. The legs are highly developed for use in getting and holding food and
dids, crickets, cockroaches, mantids,
;

Fig. 182.

(Legend

on opposite page.)

PHYLUM ARTHROPODA
for jumping.

CLASS INSECTA

315

The abdomen
is

and an ovipositor

usually provided with jointed eerei, generally present. About seventeen thousand
is

species have been described.

Some

of the

common

families

and species found

in

many

of the

western states are, first, the Locustidae, or grasshoppers; the species of this family are widely distributed and are of economic imThe portance, doing great damage to crops and forage plants. Tryxalinae, family is divided into three subfamilies, Locustinae,

and Oedipodinae.

The red-legged locust, Melanoplus f emur-nibrum Haldeman's locust, Hippiscus comllipes Hald; (Fig. 177) (DeG.) the Shoshone grasshopper. Schist ocerca sliosJione (Thomas) the twoand the western striped Mermiria, Mermiria hivittata (Serville)
;

meadow

grasshopper, Conocephalus vicinus Morse, are of considerable economic importance and are widely distributed throughout the

western states. The family Tettigonidae consists of the katydids, cave crickets, camel crickets, and sand crickets. The Mormon cricket, Analjrus simplex Hald,

one of the most destructive insects found in this order. It has attracted much attention since the Mormon pioneer days of 1848 when it overran the fields of the pioneers and would have deis

stroyed

devoured them in great quantities and so reduced their numbers that the growing grain was saved. A sea gull monument commemorating this event has been
all

the grain crops,

had the

sea gulls not

erected on the temple grounds in Salt Lake City (Fig. 178).

The katydids, Scudderia furcata Brunner and Microcentrum retinerve (Burmeister) are widely distributed in the western states. Hubbell in his classical study of cave crickets and camel crickets reports more than eighty species. The following species Ceutliophilus utahensis Thomas; C. agassizi Scudder; C. conicaudus Hubbell; and
C. nodidosus

Brunner are

fairly

common.

The sand cricket, or "child of the earth," Stenopelmatus fasciatus (Thomas), is also a very interesting member of this family. The family Blattidae is represented by such common species as the American cockroach, Periplaneta americana (L.) the German roach,
;

Blatella germanica

(L.)

the oriental cockroach, Blatta orientalis L.


a

and Orenivaga erratica (Rehn), United States (Fig. 180).

native

species

of the

western

Some common Hemiptera. 1, adult box-elder bug, Leptocoris trivitFig. 182. tatus Say 2, adult false chinch bug S, nymph or immature false chmch bug i, adult Nnhis ferns L; 5. male bedbug; 6. female bedbug, Giniex lectulanus L. (From Knowlton, permission Utah Agricultural Experiment Station.)
; ;
;

316

TEXTBOOK OF 2oOLOGY

Fig. 183. Some common Homoptera. i. Spring- migrant of rosy apple aphid 2, adult beet leafhiopper S, cottony maple scale, Pulvinaria vitis L., ventral view of body i. Baker's mealybug, Pseudococcus maritimus Elirli, ventral view of body 5, wingless female black cherry aphid 6, lateral view of adult female potato psyllid 7, San Jose scale, Aspidiotus perniciosus Comst., pygidium 8, purple scale, Lepidosaphes becki Newman, pygidium; 9, pine scale, Chionaspis pinifoliae Fitch,
; ;
;

PHYLUM ARTHROPODA

CLASS

INSECTA

317

/iSV

/2.

pygidium; 10, beet aphid, adult wingless female; 11, adult of one of the varieties 01 grape leafhopper common in Utah, Erythronewa siczac Walsh 12, adult male of Emvoasca filamenta De L. (Nos. 1, 2, 5, 6, 10, 11, 12 from Knowlton, permission Utah Agricultural Experiment Station. Nos. 3, 4, 7, 8, 9 from Jorgensen, courtesy Utah Academy of Sciences, Arts and Letters.)
;

318

TEXTBOOK OF ZOOLOGY
species of Pliasmiclae
is

A common
cler)

is

Parabacillus coloradus (Scud-

and

of the Mantidae

Litanseutria ohscura Scudder (Fig. 181).

Order Eemiptera.

This order includes the true bugs, insects with


;

piercing and sucking mouths

the winged species with the front wings

leathery and hard near the base and

Over twenty thousand species of widely distributed, and many are of considerable economic importance. The following represent some of the common families and
species:

membranous over the outer half. bugs have been described. They are

Family Pentatomidae, stink bugs, Chlorochroa sayi Stal, and the harlequin bug, Murgantia histrionica (Hahn) are common The family Coreidae is represented by the squash bug, species. Anasa tristis (DeG.) and the family Corizidae by the box-elder bug,
;

Leptocoris trivittatus
(Schilling), is a

(Say).

The

false chinch bug, Nysius ericae

common

species of the family Lygaeidae.


is

The

lace

bug,

CorytJiucha distincta Osborn and Drake

a handsome Tingiis

tidae; the

common damsel bug NaUs


distributed.

ferus

(L.)

typical of the

family Nabidae.

The members of the family Miridae are numerous

and widely
striders,

(L.) is one of the

The tarnished plant bug, Lygus pratensis commonest mirids in the United States. The water Gerridae; the back swimmers, Notonectidae and the giant
;

water

bug-s,

Belostomatidae are familiar to


of streams and ponds.

all

who

are acquainted

The bedbugs belong to the family with the life Cimicidae, and Cimex lecUdarius L. is an example of a bloodsucking species which is world-wide in distribution (Fig. 182).
Order Homoptera. Many of the most serious insect pests belong Insects to this order, also some species that are beneficial to man. such as the cicadas, with membranous wings and sucking mouths, The aphids, leaf hoppers, and scale insects, constitute this order.
plant lice or aphids, belonging to the family Aphididae, are probably one of a half dozen species of insects known by all. The rosy apple
aphid. Aphis roseus (Baker)
tive apple aphids in the
is

one of the most

common and

destruc-

West. The beet root aphid. Pemphigus hetae the black cherry aphid, Myzus cerasi (Fab.), are destrucDoane, and The potato psyllid, Parafriozoa cockerelli (Sulc), is one tive species. destructive Chermidae. The family Coccidae is a small of the very obscure group of insects, yet they are very destructive and hard to

Baker 's mealy bug, Pseudococcus maritimus Ehrh. the cottony maple scale, Pulvinaria vitis L. the San Jose scale, Aspidiotus
control.
;
;

'!

PHYLUM ARTHROPODA

CLASS

INSECTA

319

Sympe^ 1. Nymph of Rubicund f^^gonfly Fig. i84.-Mayflies and dragonflies. prickleback of the trum ntbicundultim; 2. nymph of Lestes uncatus; 3,. nymph spines on back. ^. ^P n^f Ephemerella grandis. dorsal view. A side view showing n'a>fl>. on abdomen of smaller "prickleback" 4, the "trailer ^P^t^Jil'I'ern bunch? wesiern "uncii 5, nymph ot (drawing by C. H. Kennedy) tn liberate her efcgs
; ;

of 'little ^^'^]^:Cvrr^''VZ'%hvistensen (From Needham and Chnstensen, adult Of the "big curler." Pteronarcrjs sp. 7 permission Utah Agricultural Experiment Station.)

gni'^%Lfcciden^aZis/6f nymph

320
perniciosus Comst.

TEXTBOOK OF ZOOLOGY
;

the purple scale, Lepidosaphes becki

(Newman)

and the pine scale, Chionaspis pinifoliae Fitch are important scale
insect pests.

The Cicadellidae or
:

leaf

hoppers are represented by the

following insect enemies

the sugar-beet leaf hopper, Eutettix tenellus


;

(Baker)
are

Delong's leaf hopper, Empoasca filomenta DeL.

and the

grape leaf hopper, Erythroneura comes (Say).


all

Insects of this order


;

plant feeders, and they are very numerous

over sixteen thou-

sand species have been described (Fig. 183).

Order Isoptera. More than five hundred species of termites, often wrongly called white ants, have been named. Termites are white, They feed principally upon wood, soft-bodied, mandibulate insects.

and in the tropics they are one of the most destructive insects known. Termites are social in habits, forming large colonies which are used for years and contain as many as five hundred thousand to a million individuals. The Nevada termite Termopsis nevadensis (Hagen)

and the western and destructive.

termite, Iteticulitermes Jiesperus Banks, are

common
dis-

More

will be said of these insects

under the

cussion of social insects, later in this chapter.


insects, not more than They are mostly plant feeders, sucking the juices from the plants. The banded thrip, Aeolothrips fasciatus (L.) and the onion thrip, Thrips idbaci Lindeman are common insect pests. About five hundred species of thrips are known.

Order Thysanoptera.

Thrips are very small

two

to three millimeters in length.

Order Dermaptera.

-The earwigs are small terrestrial, mandibulate


appendages at the
tip of the ab-

insects with a pair of forcepslike

domen. The winged species have a short leathery anterior pair of wings which resemble the elytra of some beetles. The small earwig. Labia minor (L.) and the toothed earwig, Spongovostox apicedentatus (Caudell) are species commonly found in the western United
;

States.

Hemimetabolous Insects With Incomplete Metamorphosis


Order Odonata.
large

The

dragonflies

and damsel

flies

are insects with

compound

eyes,

mandibulate mouth parts, four membranous

The wings that are finely veined, and a long slender abdomen. modinaiads are aquatic and possess a labium which has been highly
fied.

It

can be greatly extended for the catching and holding of

PHYLUM ARTHROPODA
prey.
sects.

CLASS

INSECTA
predaceous

321
in-

The adults are swift

flying, brightly colored,

Their food consists of mosquitoes, gnats, Much has been written on the dragonflies of the United States. About twenty-eight hundred species have been deof flying insects.

and many other kinds

The order is divided into the suborders, Zygoptera (damsel There are two families of and Anisoptera (dragonflies). damsel flies, the Agrionidae and CaenagTionidae also two families The beautiful ruby of dragonflies, Aeschinidae and Libellulidae. spot, Hetaerina americana Fabr; and the stalked-winged, Lestes The uncatus Kirby are damsel flies that are widely distributed.
scribed.
flies),
;

dragonflies Lihellula pulchella

Drury and Sympetrum ruMcund^dum


United States (Fig.

are

common west

of the Mississippi River in the

184).

Order Ephemerida. The Mayflies are aquatic insects, with mandibulate naiads, but since the adult stage lasts but a day, the mouth parts are vestigial. The adults have well-developed wings and two or three long abdominal cerci. The life cycle occupies from one to three years. The food of the naiads consists of small aquatic plants and organic matter which is obtained from the rocks and mud on the bottom of streams and along the shores of lakes where they live. They serve as food for larger insects and fishes. The prickleback,
Ephemerella grandis Eaton; and the western bunchgill, Siphlurus occidentalis Eaton are common species. Order Plecoptera (Stone flies). The stone flies are found near

streams, flying low over the water.


parts, four

They have mandibulate mouth

wings that are not so thickly netted with veins as are They the Odonata, but with longer antennae than the Odouata. where they pass their are found on stones along lakes and streams naiad stage. They require running water that is well aerated. Their food consists largely of aquatic insects, such as May flies. They are sometimes used as bait for trout. There are four families the little curler, Pteronarcella hadia Hagen and Perla modesta Banks
;

are representative species.

HOLOMETABOLOUS INSECTS WiTH COMPLETE METAMORPHOSIS


The following are some
group.
of the important orders that fall within this

(Caddis flies). This order includes about eighteen hundred species of "case flies" or "rock rollers," as they

Order

Trichoptera

322
are sometimes called.

TEXTBOOK OF ZOOLOGY

The adults are

less

than an inch long, with

mouth parts since they probThe larvae inhabit the bottoms of lakes, ponds, rivers, and creeks, and as a means of protecting their soft bodies they build cases or tubes of small rocks, shells, bits of wood, and The larvae feed upon plant tissue and small animals which plants.
well-developed wings, but with vestigial

ably take no food.

they capture in
case.

little

nets that are placed near the entrance to their

The adults lay their eggs About eighteen families are recogThe species Hydropsyche partita Banks and H. scalaris nized. Hagen of the net-making family Hydropsychidae, and Platyphylax
Pupation takes place in the water.
in the water on sticks or stones.

designata (Walker) of the family Limnophilidae are

common

in the

western

states.

Fig.

185. Larva

(From Needham of net-making caddis worm, Hydropsyche. Cliristensen, permission of Utah Agricultural Experiment Station.)

and

Order Neuroptera (Nerve Winged Insects). This order contains the doodlebugs, lacewings, snake flies, dobson flies and mantispids. It
is

probably the most heterogeneous order of insects;

all

the species,

however, have biting mouths and two pairs of net-veined membranous wings. The larvae are both terrestrial and aquatic, and feed mainly

There are thirteen families, but probably the families Raphidiidae, snake flies Chrysopidae, lacewing flies and the Myrmeleonidae, doodlebugs or ant lions contain insects most gener-

upon other

insects.

ally encountered.

The lacewing or golden beneficial and widespread

eyes,

Chrysopa
psyllids.

calif ornica Coquillett, is a

species.

It feeds in the larval stage

upon

aphids, thrips, scale insects,

and

Order Coleoptera (Beetles). The beetles are world-wide in their distribution and contain the largest number of species of any order They are adapted for an almost unlimited in the animal kingdom. of conditions, living on plants and animals, on land, and in variety the water. They have biting mouth parts, and the first pair of wings, the elytra, are leathery or hard. They feed on all possible kinds of

PHYLUM ARTHROPODA

CLASS

INSECTA

323

c.

Three species of Coleoptera, A, adult Colorado potato beetle, LeptvnoFig. 186. C, spotted B, larva or slug of Colorado potato beetle tarsa decimlineata Say D, common blister beetle, Epicmita jmnctiblister beetle, Epxcauta maculata Say
; ;

collis

Mann.

(From Knowlton and Sorenson, permission Utah Agricultural Ex-

periment Station.)

324
food.

TEXTBOOK OF ZOOLOGY

species do an enormous amount of damage, while in some of the most beneficial insects are beetles. In the United States, north of Mexico there are one hundred and nine families and Over 200,000 species twenty-four thousand species recognized. described. from all parts of the world have been
contrast,

Many

Some
beetles,
beetles,

of the families

are the leaf beetles, Chrysomelidae


;

which contain the most destructive species the long-horned wood-boring Cerambycidae the click beetles, Elateridae; the June
;

Scarabaeidae

the metallic wood-boring beetles, Buprestidae;

lower left, Above, larva Alfalfa weevil, Phytonomus porticus. Fig. 187. pupa; lower right adult. (From Knowlton and Sorensen, permission Utah Agricultural Experiment Station.)
;

and the
main,
beetles,

weevils, Curculionidae.

The following families


beetles,

are, in the
;

very beneficial:

the

tiger

Carabidae

ladybird beetles, Coccinellidae

beetles, Silphidae.

The cotton

boll weevil,

ground and the carrion Anthonomus grandis, and


Cicindelidae
;

the alfalfa weevil, Phytonomus posticus, have done millions of dollars'

worth of damage. Other groups of weevils of which the following are typical do considerable damage the billbugs, Calendra mormon Chitt. Rhynchites hicolor var. cockerelU Pierce and Apion pro: ;

dive Lee. (Fig. 188).

PHYLUM ARTHROPODA

CLASS

INSECTA

325

Order Lepidoptera (The Butterflies and Moths).


tera the larvae have biting

In the Lepidophave a
of various

mouth

parts, while the adults

highly specialized suctorial structure.

The antennae are

shapes and

sizes.

The two pairs

of wings are covered with scales,


species.

which are highly colored in many

Common weevils. 1, the bill-bug, Calendra mormon Chitt 2, Apion Fi^. 188. (Drawproclive Lee. 5, the rose weevil, Rhynchites bicolor var. cockerlli Pierce. ings by Tanner.)
; ;

This
five

Approximately ninetythousand species are recognized, of which about eight thousand


is

the second largest order of insects.

are found in the United States.

The order

is

divided into the sub-

orders Rhopalocera, butterflies, and Heterocera, the moths.

The larvae or caterpillars are among our most destructive insect They attack the foliage and fruit of the forest, orchard, pests. field, and garden; also, stored food and animal products.

326

TEXTBOOK OF ZOOLOGY

At left, larva of Capitophorus potentillae (Walker) right, strawFig. 189. (From Knowlton berry leaf roller, Ancylis comptana var. fragariae (W. and R.) and Smith, courtesy of Utah Academy of Sciences, Arts and Letters.)
;

Representative of order Lepidoptera. Above, tomato fruitworm (or Fig. 190. (From corn-ear worm) below, adult tomato fruitworm moth, Heliothis obsoleta. Sorensen and Knowlton, permission Utah Agricultural Experiment Station.)
;

PHYLUM ARTHROPODA

CLASS

INSECTA

327

The following are some examples of common species: the monarch butterfly, Danails menippe (Hubner), is widely distributed through the United States, parts of Canada, and south into the This species is typical of the family Danaidae which is one tropics.
of the nine families of butterflies in this country.

Fig. 191. Insects tree borer, Aegeria male moth of peach cultural Experiment

of the order Lepidoeptera. Above, adult female moth of peach below, adult exitiosa; center, cocoon and empty pupal case borer. (Pi-om Sorensen and Knowlton, permission Utah AgriStation.)
;

Some
moths.
species.

of the most destructive species of this order are


(millers)
is

among

the

The Noctuidae The corn-ear worm (Fabr.), feeds upon many plants, a few of which are tomatoes, corn, the green bolls of cotton, squash, strawberries, cabbage, and at times alfalfa (Fig. 190). The gooseberry fruitworm, Zophodiu grossulariae The clearRiley, is a pest belonging to the snout moths or Pyralididae.

a large family of injurious cotton bollworm, Heliothis ohsoleta or

328

TEXTBOOK OF ZOOLOGY

wing moths, Aegeriidae, a rather distinctive family, are represented by the peach-tree worm, Aegeria exitiosa Say, a serious enemy of the peach in most parts of the United States (Fig. 191). The strawberry

Fig, 192.

Life historylished by Thebutterfly. (Prom White, of monarch Mosby Company.)


C, V.

General Biology, pub-

leaf roller, family


it

feeds on both wild

raspberries

and

is

is an imported species from Europe; and cultivated strawberries, blackberries, and found in many parts of the United States,

Eucosmidae,

PHYLUM ARTHROPODA

CLASS

INSECTA

329

Order Siphonaptera (Fleas). Fleas have strong jumping legs, piercing and sucking mouth parts, laterally flattened bodies, but no They are world-wide in distribution; about four hundred wings. species have been described. All of the species in the adult stage They are are external parasites on warm-blooded vertebrates. pests on cats and dogs and known to be carriers of bubonic plague.

m^^^-

the water Fig. 193. Life history of the mosquito. 1, mosquito eggs floating S, mosquito pupa or tumbler; (slightly magnified) 2, mosquito larva or wiggler and Community Health, published by The C. V. J,, adult (From Turner, Personal
;

Mosby Company,

after Turner

and

Collins.)

Order Dipt era (Flies and Mosquitoes).

The Diptera may be char/

acterized as insects with mouth parts specialized for sucking, in some species for piercing; and with only two wings, the halters or second

pair being vestigial structures.

330

TEXTBOOK OF ZOOLOGY

of the most useful insects are found in this order. The robber flies, Asilidae; the syrphids, Syrphidae; the bee flies, Bombyliidae; and the taehinids, Tachinidae, contain many species that are

Many

valuable to mankind.

On

the other hand, the mosquitoes, Culicidae

Fig.

194.

Rowe, and Madsen, by permission

Adult

female sheep

tick,

of the

(From Knowlton, Me.lophaous ovinus L,inn. Utah Agricultural Experiment Station.)

Fig. 195. Life history of the housefly, Musca domestica L. A. A, adult; B, D, eggs. (From Knowlton, Rowe, ture larva; C, pupa inside puparium Madsen, by permission of the Utah Agricultural Experiment Station.)
;

maand

the fruit

Trypetidae; the houseflies, Muscidae; the botflies, Oestridae; and the sheep tick, Hippoboscidae, damage food and
flies,

spread disease and suffering. The larvae of some families are called maggots. Some larvae are parasitic, others predacious, or seaven-

FHYLUM ARTHROPODA

CLASS

INSECTA

331

There are over fifty thousand species of Diptera, ten thougers. sand of which are known to occur in the United States. The suborder Pupipara is a most interesting group, containing the bloodsucking ectoparasites which live upon bats, birds, and mammals. The sheep tick is a fairly common species.

Order Hymenoptera (Bees, "Wasps, and Ants). The Hymenoptera named because of their membranous wings; the word hymen means membrane. In the winged species there are two pairs of wings, the second pair being smaller than the first pair. The mouth
are so

Fig. 196. root aphids.

(From Knowlton, Rowe, and Madsen, by permission


periment Station.)

Chloropisca glabra Below, Above,western green-headedMeig. horsefly, adult


Flies.

Its

of the

maggots feed upon beet Tabanits phaenops O. S. Utah Agricultural Ex-

parts are both biting and sucking, and the females are provided

with ovipositors that have become greatly modified.

mon

In the ichneucomposed of long slender bristlelike structures, which are used for drilling through the bark of trees and depositing their eggs upon insect larvae under the bark. The ants,
flies,

the ovipositor

is

mutillids,

depositing eggs.

and bees use their The pigeon

ovipositors for stinging as well as for

horntails bore into trees, causing con-

siderable damage.

332

TEXTBOOK OF ZOOLOGY
of the

!
of great value
flies,

Many

in biological

Hymenoptera live as parasites and are control work. The braconids, ichneumon
group of parasites.

and

chal-

cid flies are examples of this

number

of the

Hymenoptera are not

beneficial, since

they feed upon the leaves of

Station.) Rowe and Madsen, by permission of the Utah Agricultural Experiment Knowlton, Below, female big-headed fly, Pipunculus subvirescens Loew. (From courtesy of Utah Academy of Sciences, Arts, and Letters.)

Fig

197.

Above,

adult

female

Simulium

vittatum

Zett.

(From Knowlton.

our garden, orchard, and forest vegetation. There are many species that are gall makers, attacking a wide variety of plants. Many species are highly developed as far as social organization is concerned, thousands of individuals living in a single colony.

The

PHYLUM ARTHROPODA
ants,

CLASS

INSECTA

333

honey bees, and social wasps are examples. The Hymenoptera found in this country are divided into three suborders, twentyeight families and about twelve thousand species. The honey bees and silkworms are the only really domesticated insects.

Other Orders
Other orders than the ones discussed above are included in the notable treatises on entomology. These are in the main, however, rare and little known insects. Professor Comstock in his An Introduction to Entomology, recognizes twenty-five orders the Zoraptera, insects resembling termites in many respects, and consisting of but
:

Fig-. 198. The common wasp, or yellow-jacket, Vespula pennsylvamca Saussure. (Prom Sorensen and Knowlton, by permission of the Utah Agricultural Experiment

Station.)

six

known

species in a single genus Zorotypus; the


lice;

Corrodentia,

psocids

and book

the Mallophaga, wingless ectoparasites of

birds; the Embiidina, a small group of about seventy species found in the warmer parts of the world, living under stones and in the
detritus of the soil; the Anoplura, the true lice, an order consisting of sixty-five species of blood-sucking parasites found on the mammals; the Strepsiptera, a group of small twisted- winged insects that
live as parasites

and the Mecoptera, commonly called scorpion a group of about forty American Brues and flies, in addition to the eighteen orders discussed above.
within the body of other insects
species,
;

Melander in their

Classification of Insects recognize thirty-four or-

334

TEXTBOOK OF ZOOLOGY

ders; while Imms, the noted English entomologist, has included

twenty-three orders in his

General Textbook of Entomology.

In

this

elementary consideration of insect classification we have

tried to include information and illustrations which will be of value in interesting the student in the thousands of insects of our environ-

ment.

E, A, female; B, female antenna; C, male antenna; D, eggs (greatly enlarged) H, worker of the anterior view of right mandible; F, larva; G, pupa, (enlarged) (From Sorensen and Knowlton, permission Utah Agricultural Experiblack ant.
; ;

Fig.

199.

Hymenoptera.

Alfalfa-seed

chalcis-fly,

Brucophagus funetris How.

ment

Station.)

SOCIAL LIFE
The great majority of
out any cooperation or
filial

AMONG THE INSECTS

insects live an individual existence, with-

relationship existing between parents The processes that have ever been operative have emphasized the importance of the individual in the scheme of prog-

and

offspring.

PHYLUM ARTHROPODA
ress.

CLASS

INSECTA

335

Despite this, Wheeler, the great authority on insect societies, pointed out that at least twenty-four different times communism or societies have appeared in the class Arthropoda. He reports that
social life occurs in six families of Coleoptera, fifteen families of

Hymenoptera, and in the Dermaptera, Embiidina, and Isoptera. Let us look at some of the ways social life has manifested itself.
In the beetle family, Scarabaeidae, we find a number of species in which there is a cooperation between the male and female for the
perpetuation of their offspring.
rolls

A common
down

species,

Canthon sim-

plex var. corvinus Harold, which the writer has ofttimes observed, up small spheres of fresh cow manure, and then excavates beroll,

neath the

letting it gradually

into a hole in the ground.

and cover over the sphere of manure upon which the female has deposited an egg. The French naturalist and entomologist, J. H. Fabre, reported many interesting observations relating to the preparation of manure pellets for the deposition of eggs by several different scarabaeids. Another beetle, Passalus cornutus, in the family Passalidae, lives in rotten logs. The developing larvae feed upon wood that has been prepared by the adult beetles. The colony is kept together by audible noises made by the mature beetles. The ambrosia beetles of the families Scolytidae and Platypodidae form colonies by making their burrows into the wood of both living and dead trees. Each species of beetle grows a species of fungus which is fed to the developing larvae by the adult beetles.

The male helps

to dig

No

The beetles are probably the least social of all the orders listed. castes have been developed, and the males take but little part
life.

in colony life.

In the Hymenoptera are found varying stages of social


the solitary wasps, the female digs a

In
is

burrow

in the
cell.

ground which

provisioned and then an egg

is

sealed in the

No

other atten-

tion is given to the developing

young and the new generation never

knows the old. The following excerpt from a study of the nesting habits of Odynerus dorsalis Fabr. made by Mr. Edwin Vest gives a good picture of the activities of this solitary wasp.
''

Odynerus

dorsalis is a solitary

wasp

in that each female builds

a separate nest, yet there are often several nesting individuals in

the same vicinity forming a kind of community.

The labor

of dig-

336

TEXTBOOK OF ZOOLOGY

ging the hole for the nest and gathering the provisions is apparently done entirely by the female. At no time was the male seen to engage in any part of this work. After the nesting is begun the females spend the night in the burrows with the head uppermost,
while the males roost upon nearby herbs or shrubs.

teresting.

"Their attempts at copulation are very amusing as well as inBeginning about one or two o'clock in the afternoon the males become very active. They fly rapidly back and forth over the community usually from six to eight inches above the ground. They often alight on a female as she is working about the nest or returning to the nest with food and knock her to the ground. One female was resting on the ground when a male flew down and alighted on her back as if attempting to copulate; another male
attacked with such vigor that the female flew
other male in pursuit.

away with

still

an-

"The ground where the nests are made is hard, dry, and composed principally of clay. In order to penetrate it the female fills a thin pouchlike sac, located within the second segment of the abdomen, with water and uses this to moisten the ground. With her mandibles she digs the dirt out in small pellets, varying in size from These pellets are carried a short 2 mm. in diameter to 6.8 mm. distance away from the hole. This work is continued until the hole There is as deep as desired, the depth varying from 48 to 110 mm.
are usually one or two, rarely three, cells constructed in the tunnel for the deposition of eggs.

The bottom of the hole is enlarged The slightly into a cell and is made very smooth on the inside. cell might be lined with a secretion from the body which forms a cementlike protection to the larva during the winter. The average size of the cells is 23 by 14 mm. In general they are ovoid-elliptical
in shape.

"Each
The wasp

cell is

provisioned with from

five to

twelve Pieridae larvae.

by grasping them with her mandibles just back of the head and supporting them somewhat with her two front legs. Desiring to learn how Odynerus handled the larvae before putting them in the nest, the writer attempted to induce several wasps to pick up worms that were dropped on the ground about the nests. Favorable results were obtained in two cases. When the wasp found the worm she applied her mandibles to various places on the body but spent most of her time biting just back of the head as if
carries these larvae

PHYLUM ARTHROPODA
trying to cut
it off.

CLASS

INSECTA

337

This

is

of paralyzing the victim.

probably a part at least of the process These paralyzed Pieridae larvae have

been kept in the laboratory in bottles for two weeks in warm weather before there began to be any change in their appearance. After that time they began to decompose rapidly. "After the cell is provisioned with the Pieridae larvae the female attaches the egg to the upper part of the cell by a short hairlike process 1.8 mm. in length with the point of attachment to the cell wall concave and about 2 mm. in diameter. Only one egg is deposited in each cell. The cell is then sealed over by wetting the soil at the surface and then carrying it down to be moulded into an

and water-tight compartment. In order to observe the writer used a small pocket mirror to reflect the this process, light down into the hole. This did not seem to interfere with the
apparently
air-

activity of the wasp.

of the nests observed in this study consisted of two cells, with single-celled nests ranking second and three-celled nests third in frequency. The writer was not successful in hatching out all the

"Most

individuals of any three-celled nest dug from the ground but those containing one or two cells were often hatched successfully. Of

those individuals successfully reared in the laboratory it was found that in the case of the one-celled nests the individual invariably developed into a female, while with the two-celled nests the larva in
the lower cell always developed into a female and the upper individual into a male. No successful observations were made on the The facts of the case would seem to indicate three-celled nests.

that the male develops more rapidly than the female, since the egg in the lower cell is laid before that in the upper cell. It was noted that the wasp in the lower cell did not emerge until three days
after the top cell

had been vacated.

primarily to two-celled nests, although


the three-celled types.

The above condition applies it might be equally true of

evident from this study that the eggs laid in July and August hatch and remain in a late larval instar throughout the winter. On August 2 a number of larvae were collected and placed

"It

is

in glass vials.

During the warm weather they were kept moistened by placing a few drops of water on blotting paper covering the cells. About the middle of September they were placed in a north room of the writer's home where they were left throughout the

338

TEXTBOOK OF ZOOLOGY

winter. Some of the larvae spun their cocoons in the vials while others had already done this before being removed from the ground.

The room

in

which they were kept was

cold, the

temperature some-

About the last of May the specimens were removed to the Brigham Young University where they were kept on the writer's desk. The adults emerged fully developed about the middle of July. One female was kept in a breeding cage and fed on a syrup of cane sugar and
times going slightly below the freezing point of water.
distilled water.

thought that under natural conditions the insects emerge earlier in the summer than was indicated by the artificially reared specimens since they have been observed to be very active even during the early part of May. It seems evident that
"It
is

somewhat

these early wasps build their nests in the spring and that young emerge during the same season. Only the individuals

their nest-

ing in the late

summer spend

the winter in the larval stage."

and Polyhia, of the family Vespidae, start new colonies each spring from overwintering queens. After the nests are built and the eggs begin hatching, the queen feeds the larvae until they are completely deThese workers then come to the aid of the exhausted veloped. founder of the colony by taking over the enlarging of the nest and the feeding of the larvae and the queen. The queen's only duty now It will be noted that the Vespidae attend their young is to lay eggs.

The

social wasps, belonging in the genera Vespula, Polistes,

by gathering food and feeding them; also that in turn the adults may feed upon the saliva of the larvae. Wheeler believes that the exchange of food in many of the social insects, which he chooses to
call

"trophallaxis," has been the source of the social habit.

In the family Bremidae, the bumblebees also start a colony in the spring by overwintering queens seeking out an unoccupied mouse The queen gathers hole or some other suitable hole in the ground. pollen and nectar with which she fills a few cells. She then deposits an egg in each cell and waits for them to hatch and develop into
workers.

The workers

assist

in building

and feeding the colony.

When

the winter comes on, the queen, workers, and males die, leaving only the females, which developed late in the summer and which hibernate, to carry on the life cycle. All this is very similar to the
life

habits of the social wasps.

PHYLUM ARTHROPODA
In the honey
bees, ants,

CLASS

INSECTA

339

and

termites, social life is carried to its


is

highest state of perfection.

In these groups the colony

probably

Some ant and termite queens live from ten to fifteen years, building up large colonies consisting of Other queens take up the job fifty to eighty thousand individuals.
perpetuated for hundreds of years.
of continuing the colony.

A well-developed
social insects.

caste system, also

polymorphism,

is

found in these

In a swarm of bees there are three kinds of individThe workers are females that are uals, males, females, and workers. undeveloped sexually. Ants and termites have many different forms
of individuals in each species.
castes.

In a termite colony there are

many

The principal kinds are perfect males and females, or the

royal stock, the fecund pair of the colony; a less fully developed

sexual caste, with rudimentary wings; a worker caste, of fairly


small, sterile, wingless individuals; a soldier caste, morphologically

and which are small Both individuals with the head produced into a kind of snout. males and females are found in the various castes of termites. There is also an interesting symbolic relationship existing between numerous intestinal protozoa and the termites. The wood eaten by the termites is made soluble by the infusoria found in their digesdistinct

from other individuals because of

their large heads

strong jaws; and finally a caste

known

as nasuti,

tive tracts.

Ants are world-wide in their distribution they are also very numerous as individuals and species, since about four thousand species are known today. Wheeler believed that ants are the most highly developed as well as the dominant group of social insects. The Formicidae have a highly developed caste system and usually the workers and at times the males and queens are polymorphic.
;

Guests

There are many species of insects that live in the nests of the myrmecophiles when found with ants, and termitophiles when with the termites. Wheeler reports that fully two thousand species of myrmecophiles and one thousand termitophiles have been described. Many of the guests have become so dependent upon living with ants or termites that
social insects; these guests are called

they are never found outside of the colonies.

Aphids and mealy

340

TEXTBOOK OF ZOOLOGY

bugs are kept as guests for the droplets of lioneydew which they excrete when stroked by the antennae of the symbiont. Dr. S. A.

Forbes has reported most interestingly upon the activitiv'^s of the cornroot aphid, Aphis maidi-radids Forbes and the brown ant, Lasius
in October

The little ants gather the aphid eggs and take care of them during the winter. In the spring before the com commences to grow, the aphids, after hatching, are placed upon the roots of smartweed and some of the grasses. As soon as the corn has started to grow the agamic female aphids are transHere many generations are produced parferred onto the roots. thenogenetically. Then in later September or October wingless males and females are produced. After mating, eggs are laid, which are gathered and stored for the winter by the ants. The ants are repaid for the care they bestow on the aphids by receiving a honeydew given off by the aphids, which they greedily feed upon.
niger var. americanus Emery.

Many
atus

of the insect guests are beetles, Histeridae, Staphylinidae,

Pselaphidae, and Scarabaeidae.

The two

histerids,

Hetaerms

tristri-

Horn and H.

zelus Fall are fairly

common

in ant nests in the

states west of the

Rocky Mountains.

Several species of Xenodusa,

members

of the family Staphylinidae, are

found in ant

hills in the

United States and Mexico.


Reichenl)achia, pselaphids,
C.

and and Cremastocheilus angularis LeC. and KnocJii LeC, scarabaeids, are found in the colonies of several of

number

of species of Batrisodes

the

mound

ants.

Some Diptera

are also guests in ant colonies.

ECONOMIC RELATIONS
Insects attack all kinds of growing crops
struction of plants

V
plants.

and

The deto

and

their products valuable to

man amounts

over a billion dollars annually.


of the
their "place in the sun."

This great loss goes on because

unabated and persistent struggle of the insects to maintain


Plants are not only eaten and

damaged

by

many plant diseases are spread by them. Animals and man suffer greatly from the attacks of insects. Many
insects,

but

species live as endoparasites or ectoparasites on animals

and man, most dreaded diseases known to man are carried by insects. Because of this there has recently developed a new branch of entomology known as "medical entomology." Some of the most notable progress during and
in so

doing also spread disease.

Some

of the

PHYLUM ARTHROPODA

CLASS

INSECTA

341

the past thirty or forty years has been

made

in the field of medical

entomology.

Diseases such as malaria, yellow fever, typhus fever,

African sleeping sickness, bubonic plague. Rocky Mountain spotted fever, tularemia, and elephantiasis are now known to be insect borne.

Much remains to be done in this new entomological field. After man has produced his crops and harvested them he finds many insects ready to take their toll from these
trated products.

for use,

concen-

The "board bill'* of the insect pests of stored foods annually amounts to about four times the cost of all higher
institutions of learning in this country.

Insects belonging to the

orders Coleoptera and Lepidoptera are the main offenders. The pea

and confused flour beetle feed and seeds and their upon and damage products. Much damage is also done to the same products by such species as the Mediterranean flour moth and the Indian meal moth.
weevil, bean weevil, granary weevil,

practically all kinds of grains

Practically all pests of stored foods are world-wide in their distribu-

which makes it difficult to ship food products long distances them for future use without running the hazard of insect damage.
tion,

or store

Many

insects

his upholstered furniture, clothing, furs,

have taken up their abode with man, living upon and rugs. Great losses are

suffered annually

by the producers

of clothing, as well as in the

homes, due to clothes moths.


of dwellings, even furniture

Termites also attack the wooden parts

and books.

drugstore beetles live upon tobacco products,

The tobacco beetles and home furniture, and

many

drugs.

Useful Insects

Fortunately not
allies of

all

insects are our enemies.

Many

species are

as in

man in the struggle many other ways.

against the injurious insects, as well

Everyone knows that honey is produced by the honey bee and by the silk moth, but there are many people who do not know that certain insects produce shellac, the pigment cochineal, tannic acid, formic acid, cantharidin or "Spanish fly," inks and dyes, and
silk

beeswax.

In India a small scale insect, Tachardia lacca Kerr, lives

on trees and produces a secretion that forms a layer over the branches. This substance, shellac, is removed by the natives in

342

TEXTBOOK OF ZOOLOGY

Shellac

various ways, millions of pounds being sold throughout the world. is used for making varnishes and polishes, as an electrical

insulating material, in airplane construction, and


Insects serve as food for

many

other ways.

many

fishes,

amphibians, reptiles, birds,

and mammals, including man. It is important that insects be recogWithout the nized as playing a major role in this connection. insects the food habits of many of the vertebrates would be entirely
changed.
Finally,

many

the blossoms.

plants depend upon insects to assist in pollenizing Only as the insect helps in transferring the pollen

from plant
plant
is
it

to plant or

from the stamens to the pistil of the same possible for some fruits, seeds, vegetables, and orna-

mental plants to develop.

CHAPTER XXII
REPRESENTATIVE INSECTS
(By Vasco M. Tanner, Brigham Young Universitt)

THE LOCUST
being

one of the most common insects, very few boys and girls grow up without having some experience with a grasshopper. They are widely distributed throughout the world, living on grass and low-growing plants of the fields and open country. In the United States many destructive species are found. As early as 1743

The locust or grasshopper

is

known

to practically all people, because

Mr. Smith reported the damaging activities of Melanoplus atlanis in the New England states, and from 1855 to 1877 many outbreaks of grasshoppers were reported in the western United States. Even today the national government is expending large sums annually
to

keep down the activities of the many destructive species. The grasshopper is a typical insect, and along with the beetles and bees, to be discussed later in this chapter, may serve to illus-

trate the general structure of the class Insecta.

The insect body


the segments are

is

divided into a series of rings, or segments, and

known

These plates are of hardened plates. and the depression between the plates is called a suture. The hardness of the plates is due to the deposition of a horny substance called ckitin. In many places two or more of these rings have

made up

as sclerites,

gro^\Ti together, or are fused.

Again, in certain regions of the body,


lost.

tion in this respect,

Eegardless of the amount of variawe find that the segments are always grouped into three regions, known as the head, thorax, and abdomen. The head is made up of a number of segments, which are fused
parts of the segments

may

be

together, forming a boxlike structure

known

as the epicranium.

This boxlike piece which surrounds the eyes and forms the basis of attachment for the movable parts of the head extends down the front of the head, between the eyes, to the transverse suture, and down the sides of the head to the base of the mouth parts. The sides of the epicranium below the compound eyes are known as the genae, or cheeks, while the front of the head between the eyes is
called the frons.
343


r
C.'p.

; ;

i.e.

c-.-.w

^'

r.-cj.

1J^V'

n^.i.
Fig. 200. 1, The external structure of the grasshopper, Dissosteira spurcata. al.. Hind angle of lateral lobe cm., crest of the metazone c.p., crest of the prozone g., gena g.g., genal groove I.e., lateral carina of the metazone m.p., maxillary palpus t.L, transverse incision. 2, Front view of the head of the grasshopper, Dissosteira spurcata, a.g., Antennal groove; ant., antenna; c.c, lateral carina c.e., compound eye c.f., central foveola e.g., carina of the antennal groove cl., clypeus c.o., central ocellus fas., fastigium of the vertex f.e., frontal costa la., labrum carina of the fastigium I.e., lateral l.p., labial palpus g., gena inan., mandible in.e., median carina of the fastigium in.p., maxillary palpus O.C., ocellus; s.e., sulcation of the frontal costa; t.f., tempora, temporal foveola; ver., vertex. (From Henderson, by permission of the Utah Agricultural Experi;
; ; ; ;

ment

Station.)

REPRESENTATIVE INSECTS

345

The grasshopper has both compound and simple eyes. The compound eyes are situated upon the upper portion of the sides of the head, and are large, oval areas with smooth, highly polished surfaces. If the eye is examined with a dissecting microscope, the surface will be seen to be made up of a number of hexagonal areas, which are known as facets. The simple eyes or ocelli consist of One of the ocelli is three small, almost transparent, oval areas. situated on the front of the head, just beloAV the margin of the impression which contains the bases of the antennae, and in contact with the upper portion of the compound eye. The antennae or feelers are two threadlike processes situated median to the compound eyes. Each consists of about twenty-six segments. On the front of the head there is a short rectangular piece, called the clypeus, which is attached by its upper edge to the epicranium, and on the lower edge to the labrum. The mouth parts
is

consist of a

number

of separate parts attached

to the ventral region of the epicranium.

The

first

noticeable part

the Idhrum, or upper

lip,

a flaplike piece attached to the lower

line.

edge of the clypeus. The free edge is deeply notched on the median Just beneath the labrum are the mandibles, or first pair of jaws. Each mandible consists of a single piece which is notched on the inner grinding surface to form a number of ridges or teeth. A second pair of jaws, the maxillae, may be exposed by the removal
of the mandibles.

Each maxilla

is

composed

of a

number

of parts,
;

consisting of the cardo or proximal hinge part of the structure

the

stipes, the lacinia, a sclerite which bears some teeth on its terminal end; the outer lobe or galea; and the maxillary palpus. The caudal part of the mouth parts is the lower lip or laliium, which is composed of the siibmentum which acts as a hinge on the epicranium above; a mentum; labial palpi, and two large outer flaps, the ligulae (Fig.

200).

The prothorax

is

the segment to which the head

is

attached.

It

may

be divided into two regions, the dorsal part known as the pronotum and the ventral portion known as the sternum. The pronotum is a
saddle or bonnetlike piece extending over the dorsal and lateral
regions of the prothorax.
It is made up of a fusion of four plates, which are indicated by the transverse sutures. The sternum or ventral side of the pronotum is also made up of separate plates, or sclerites. The anterior sclerite bears a spine on the median line.

346

TEXTBOOK OF ZOOLOGY

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<D ra
tJ3^ cd o) -^ ^1

bo

ri

o o S o

c .5
^
to

aoK

J^

s^oj;^!^

Eh

c3
!-i

"
aj

aa
r

a'O

<p

Sg

t?a.J
+->

<D

oT

t. .. a ? o c> 2i S ^,j2 c o 5 o

So-I

^t^CLj
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-73
I

!-o y

a>

cq
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,-

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REPRESENTATIVE INSECTS

347

The next two segments, the mesothorax and metathorax, are made up of sclerites that are intimately associated, and their structure The mesothorax is joined to the prowill be discussed together. thorax by a membrane which permits of more or less movement. Posteriorly the metathorax is joined immovably with the first abdominal segment. The mesothorax and metathorax form a strong, boxlike structure for the support of the wing and leg muscles. Like the prothorax these segments are made up of separate plates, held together by a tough, connecting membrajie. These plates may, however, be divided into three groups: the terguni, or dorsal region; the sternum, or ventral region; and the pleuron, or lateral region. On the dorsal and ventral regions of the body the sutures separat-

ing the mesothorax from the metathorax are not very distinct. On the sides of the body, however, there is a very distinct line, or

from the posterior border of the attachment of the second pair of legs toward the dorsal part of the body. This suture divides the mesothorax from the metathorax. The pleura of each of the posterior thoracic segments are again divided by transverse
suture, running
sutures, so that each pleuron consists of

two

sclerites.

pair of legs arises from the lateral and ventral portions of each

Each leg is composed of five parts. The coxa is the first segment and is attached to the thorax by a tough elastic membrane. The next segment, the trochanter, is a very short
of the segments of the thorax.
piece which
is is

hard

to distinguish except in the first pair of legs.

The femur

the third

and

largest

segment of the

leg,

and

in the

case of the metathoracic leg contains the muscles used in jumping. The fourth seg-ment, the tibia, is slender, but about the same length

which is made with the other. The segments up of three segments, each movable bear a series of pads, which terminate on the last one in a large
as the femur.

The

last division of the leg is the tarsus

suckerlike disc

known

as the pulvillus.

There are two pairs of wings. The first pair or wing covers, also called tegmina, is attached to the dorsal region of the mesothorax. They are leathery in texture and do not fold fanlike over the abdo-

men. They are strengthened by many veins and cross veins. The second pair of wings is attached to the metathorax. They are membranous, with many veins to strengthen them, and fold fajilike over The metathoracic wings are used the abdomen when not in use.
in flight.

348

TEXTBOOK OF ZOOLOGY

The last main division of the insect body is the abdomen. It is composed of eleven segments. The seven anterior segments are similar in both the male and female. In the male the first abdominal segment is made up of a curved dorsal shield, the tergum, which
terminates just above the attachment of the third pair of legs. This
piece partially" surrounds the tympanic

membrane, or

ear,

which

is

a large, crescent-shaped area covered with a semitransparent brane.

mem-

The ventral part of the first segment, the sternum, is not attached to the tergum, owing to the large size of the attachment The pleura are entirelj'- absent. The second to the of the legs.
eighth segments are
joins the sternum.
all

quite similar, consisting of a dorsal tergum,


to

which extends laterally


it

The pleura, or

near the ventral part of the body, where side pieces, noted in connection
In

with the thorax, have been inseparably fused to the tergum.

the ninth and tenth segments the terga are partially fused together, the union of the two being indicated by the presence of a transverse The sterna of these two segments are entirely fused and suture.

much

modified, forming a broad, platelike piece.


is

The eleventh

segment

represented only by the tergum, which forms the termi-

nal, dorsal, shield-shaped piece (Fig. 201).

The cerci constitute a pair of plates attached to the lateral posterior border of the tenth segment, and extending back, past the end of the eleventh tergum. The podical plates lie directly beneath the cerci
plates,

and ventral to the eleventh tergum. and the genital chamber lies
to the ninth

The anus opens between these


directly below them.

Attached

sternum

is

the subgenital plate which forms the most

posterior ventral plate of the body.

In the female the eighth segment resembles the other segments, except that the sternum is nearly twice as long, and known as the

and eleventh segments are essensegments nine and ten being partially fused, and tergum eleven forming the terminal, dorsal shield. The plates called cerci and podical plates are similar to those in the male, except that the podical plates are much more
subgenital plate.

The

ninth, tenth,

tially like those of the male, the terga of

prominent.

The ovipositor consists of three pairs of movable plates. The dorsal pair lies just ventral to the eleventh tergum and each plate is long, lance-shaped, and with a hard, pointed tip. The ventral pair
arises just dorsal to the eighth

sternum and resembles the dorsal

REPRESENTATIVE INSECTS
pair.

349

When

these four pieces are brought together, their points


in

are in contact, forming a sharp organ by

male bores the holes in the ground The third set of plates are known as the egg guides. These are much smaller and are located median to the plates of the true
ovipositor.

means of which the fewhich to deposit her eggs.

There are ten pairs of spiracles, or openings in the respiratory system on the body of the grasshopper. Two pairs of these liplike structures are situated on each side of the thorax on the anterior margin of the pleural plates. The mesothoracic spiracle is concealed by the posterior edge of the pronotum. The metathoracic spiracle is located just dorsal to the mesothoracic leg, near the

There is another spiracle just attachment of the metathoracic leg, but this belongs dorsal to the From the second to the eighth to the first abdominal segment. abdominal segments there is one pair of spiracles located on the anterior margin of each segment near the union of the sternum and tergum. The spiracles are one of the most useful sets of structures for determining the segmentation of an adult insect body. This is because there are never more than eight pairs of abdominal spiracles Air passes through the present in any fully developed insect. spiracles into the tracheae and is carried to the tissues of the body. This unique system of breathing enables the insect to keep the body tissues well aerated and the carbon dioxide eliminated from the
suture separating the two segments.

body.

The circulator}^ system consists of a single dorsal tube, or heart, which extends along the length of the median dorsal part of the body. In the abdomen of the fully developed insect this vessel is divided into a number of chambers with side valves, which allows the blood to enter but not to escape, except through the vessel toward the head. Due to the pulsating of this portion of the tube, which has been called the heart, the blood is forced to the anterior part of the body where it flows out into the body cavity and slowly returns to the abdominal region. In this process the tissues are supplied with nourishment from the food materials carried in the
blood. It will be noted that the circulatory system has practically nothing to do with the carrying of oxygen to the tissues.

The digestive system


straight tube extending

of the grasshopper consists of a practically

from the mouth

to the

anus through the

350

TEXTBOOK OF ZOOLOGY

central portion of the body. The food after being ground up by the mouth parts passes into the mouth or pharynx where it is mixed with the salivary mucin and the action of the enzyme, invertase, begins. From the mouth the food is conveyed through the esopha-

\-/A

's-.e./

5t.

-A\.X

H'lnt.

n
-to.
-R;.

J.A
202. Digestive system of Rhomaelia microptera. A, anus; O, crop; Co., colon Int., intestine M, mouth M.T., Malpighian tubules G.C., gastric caeca (From White, General Biology. Oe., esophagus; R. rectum; Sal., salivary glands. The C. V. Mosby Co.)

Fig.
;

REPRESENTATIVE INSECTS

351

gus to the crop and gizzard which are dilatations of the tract filling a great portion of the thorax. The gizzard is muscular and lined with chitinous ridges which strain the coarse particles of food and prevent their entering the next division of the system, the stomach.

\ Ab

Nervous system of Rhomaelia microptera. Ah., first abdominal Fig 203 Su., Sp., supraesophageal ganglion ganglion C, circumesophageal commissure subesophageal ganglion. (From White, General Biology. Tiie C. V. Mosby Co.)
;

acted upon in the stomach by the secretions of the gastric caeca, which are glandular bodies opening into the anterior end of the stomach. They secrete a weak acid which helps in the

The food

is

352
emulsification of fats
tones.

TEXTBOOK OF ZOOLOGY

and the conversion


is

of albuminoids into pep-

absorbed into the hemolymph from the stomach. Between the stomach and the intestines is a pyloric valve which permits the contents of the system to pass in only one direcIn the intestine, which is divided into the ileum, colon, and tion. rectum, absorption of food continues, especially in the ileum. Just back of the stomach many threadlike tubes enter the intestine. These tubes are the excretory organs, known as Malpighian tulules, and perform a similar function to the kidneys of higher animals. The rectum has thick muscular walls with six-surface rectal glands. The feces are expelled from the rectum to the outside of the body through

Much

of the food

the anus.

Fig-. 204. Anterior aspect of brain (supraesopliageal ganglia) of Rhomaeha microptera. (Magnified.) 1, nerve to paired ocellus; 2, nerve to eye, showing fibers to ommatidia S, nerve to antenna; 4 and 5, nerves to mouth parts; 6, nerve to unpaired ocellus; 7, circumesophageal commissure. (From White, General Biology. The C. V. Mosby Co.)
;

commissures or connecting nerve fibers lying along the ventral body wall. Five ganglia are located in the abdomen. Since there are at least eleven segments in the abdomen of the adult grasshopper, it is apparent that the ganglia of some of the segments have fused together. In the larvae of insects there is usually a ganglion to each segment. Three large, well-developed ganglia are found in the thorax the anterior one is connected with the subesophageal ganglia which in turn are connected with the brain or supraesophageal ganglia by nerve fibers which pass on
set of
;

The nervous system connected by a double

consists of a series of ganglia or nerve cells

Nerves pass from the brain to the eyes, The subesophageal ganglia supply the mouth parts with nerves. The legs and wings are coordinated
each side of the esophagus.
antennae, and palpi of the head.
in their

movements by the thoracic

ganglia.

In the vertebrates the

REPRESENTATIVE INSECTS

353

nervous system is dorsal to the digestive tract, and the foreshadowing of this evolutionary change is initiated in the insects by the development in the cephalic region (Figs. 203 and 204). The grasshopper is dioecious; the abdominal structures separating the two sexes are distinctive. The external genital structures have been discussed above. The male organs consist of testes located dorsal to the intestines. The sperms are borne in ducts which

communicate with the penis, which consists of chitinous styles used in copulation with the female. In the female there are two ovaries,

Fig. 206.

Male reproductive organs of Rhomaelia microptera. Te., testes; Y.D-, Fig. 205. vas deferens. (From White, General Biology.) Fig. 206. Female reproductive organs of Rhomaelia microptera. C.S., copulatory sac; O.T. ovarian tube with eggs; Ov., oviduct; Va., vagina. (From White, General Biology. The C. V. Mosby Co.)

which when mature fill the major portion of the abdomen. The oviducts convey the eggs to the vagina, a duct made by the union of the two oviducts, which discharges the eggs through the opening at the base of the egg guide to the outside of the body. The eggs are fertilized by the sperms from the spermatheca, which is dorsal to the vagina and which is connected by means of a sperm duet. The female is able to dig a hole in the ground with the ovipositor

354

TEXTBOOK OP ZOOLOGY
to the

and deposit the eggs

depth of an inch or more.

are covered with a frothy substance which protects

The eggs them from


in

moisture and, to some extent, from the frost. the fall and hatch in the spring of the year.
the grasshopper
is

The eggs are laid The development

of

by gradual metamorphosis.

THE JUNE BUG


The June bugs or May
beetles are

members

of the family Scara-

baeidae, a very large and important family of beetles.

More than

one hundred and twenty-five species of these beetles have been reported as occurring in the United States and Canada, the majority of them being considered as pests. The larvae or white grubs live

underground, destroying the roots of grain, cereal, truck, and garden crops, as well as great tracts of pasture and grasslands. The adults live upon the leaves of many kinds of trees and shrubs, often completely defoliating the trees. Because of the general distribution of these beetles, they have been selected as a type to illustrate the characteristics of Coleoptera, the largest order of arthropods. An examination of a specimen of the genus Phyllophaga reveals that there are three body regions: the head, thorax, and abdomen. The rather small, retracted head bears antennae of nine or ten
joints

and a club composed

of three elongate leaflike joints.

The

antennae are located just beneath the lateral edge of the prominent clypeus. The compound eyes are on the sides of the head near the prothorax. There are no ocelli. The mouth parts are of the biting
type, similar to those of the grasshopper.

The thorax consists of three segments. The metathorax is fused with the first abdominal segment and with the mesothorax, leaving the prothorax free and movable. Attached to the dorsal portion of the mesothorax are the fore wings that are modified into horny sheaths, or elytra, which cover and protect the back of the thorax and abdomen. The hind wings are membranous and folded under
the elytra.

The

legs are well developed, the prothoracic ones being

adapted for digging in the ground.


yellow setae.

The thorax

is

provided with

The abdomen, which


spiracles

is

broadly fused with the metathorax, consists

of eight external segments.

When

the

elytra

are

removed, the

may

be seen in the lateral margins of the dorsal surface of

the abdomen.

The

genital organs of both sexes are simple.

REPRESENTATIVE INSECTS

355

356

TEXTBOOK OF ZOOLOGY
carefully removing the

By

membranous

tergites of the

abdomen

the heart can be seen to consist of a thin-walled dorsal vessel with

paired lateral openings into the body cavity. The blood is forced forward through the heart chambers by the pulsations of the heart walls. There are no arteries and veins, which means that the heart serves mainly as an agitator of the body fluids, helping to distribute the absorbed food to the tissues.

The tracheal system


There are
as
it

is

well developed for carrying the air from

the spiracles to all parts of the body.

many changes

in the digestive

passes from the larval stages to the imago.

system of the June bug The alimentary

tract of the larva consists of a straight tube, except for a

bend

in

the colon.

It is

much

greater in diameter than in the later stages

due to the nature of the food, which consists of roots, humus, and some soil. The food passes from the mouth or buccal cavity into the esophagus and then into the crop. At this point there is a valve between the crop or gizzard and the mid-intestines. Two rows of gastric caeca are present on the anterior end of the midintestines. This is a very unique feature, as it is rarely met with in larval stages of other insects. The large saclike stomach or mid-intestine of the larva is transformed into an elongated coiled stomach in the adult, without the two rows of gastric caeca. At the posterior end of the midintestine and in front of the pyloric valve are ten pairs of pyloric caeca. The hind intestine consists of the ileum, colon, and rectum. There are four Malpighian tubules connected to the hind intestine. In the pupal stage the gastric caeca have disappeared, and the tract is becoming much elongated and coiled. In the adult the excretory organs, the Malpighian tubules, arise in the ileum just posterior to the pyloric valve. They extend into the body and then end blindly at the junction of the colon and rectum.

The nervous system

consists of a ventral nerve chain, a brain, or

supra-esophageal ganglion, a nerve ring which connects the brain and the foremost or infra-esophageal ganglion. There are eight ganglia in the ventral nerve chain, four in the thorax and four in
the abdomen.

four years, depending upon a number of ecological factors.

from three to The adult females dig into the ground and deposit from a hundred to two hundred eggs. The larvae are commonly known as ''white
The
life histories of

the June bugs vary in length

REPRESENTATIVE INSECTS

357

grubs."

and

live

The adults come forth in great numbers in May or June from one to two weeks, feeding upon the foliage of many

plants.

THE HONEY BEE


The honey bee belongs to the order Hymenoptera, composed of insects with two pair of membranous wings, well-developed biting or sucking mouth parts, and the females usually with a stinging organ. Many of the Hymenoptera, such as the honey bee, live a
social life, developing colonies consisting of three types of bees:

queen, drones, and workers.

The worker bee is provided with large compound eyes on the sides of the head and three small ocelli near the median part of the frons. The antennae are attached to the anterior surface of the head. The mouth parts are adapted for both sucking up nectar and chewing. The lahrum is attached to the lower edge of the chjpeus. A The manlittle organ, the epipharynx, is just below the upper lip. dibles are attached to the ends of the labrum and lie over it. Beneath the mandibles is the proboscis made up of several separate structures (1) the glossa or long tongue; (2) the laMal palps; and (3) the maxillae, lateral to the labial palps. The maxillae and labial palps are used in sucking the nectar from the flowers. The thorax is divided into the prothorax, mesotlwrax, and metaThe wings are borne thorax. Each segment bears a pair of legs. upon the mesothorax and metathorax. The legs are very well adapted for the work of the hive. The first pair of legs are provided with
hairs adapted for various uses.

On

the tibia are the curved bristles,

known

and the large spinelike structure, the velum, which is associated with the antenna comb. The metathoracic There are also legs have the tibia modified to form a pollen basket. modified spines and structures on the last pair of legs known as the The modifications found in the pecten, auricle, and pollen combs.
as the pollen Irush,
legs of the bee are remarkable adaptations for the specialized life of
this
insect.

The abdomen is composed of six external segments consisting of a dorsal tergum and a ventral sternum. At the end of the abdomen
is

a highly specialized organ, the sting. Associated with the sting are the poison glands, which secrete a substance composed of an acid

and an

alkali.

358

TEXTBOOK OF ZOOLOGY

REPRESENTATIVE INSECTS

359

digestive tract is well adapted for the specialized life of the bee, that of gathering and feeding upon the nectar of flowers. study of the digestive sj'stem of the larva, pupa, and adult of the

The

solitary wasp,
is

Odynerus

dorsalis,

by Mr. Edwin Vest reveals that

it

optera.

very similar to that of the honey bee, as well as many other In Odynerus or the honey bee the digestive tract may be divided into the fore intestines, mid-intestines, and hind intestines, as

Hymen-

in the

June bug.

The

divisions of fore intestines are

buccal cavity, esophagus; water sac or Jioney The mid-intestine consists of the stomach, while the hind intestine may be divided into the ileum, rectal glands, rectum, and
ventriculus.

mouth or stomach; and the pro:

the

In the larval and early pupal stages the mid-intestine is a thin but in the adult it has developed into a convoluted, looped stomach. The number of Malpighian tubules increases from the
anus.
flat tube,

larval stage to the adult.

in the lar\^a, while there are


is also

Only four Malpighian tubules are found around one hundred in the adult. There

a marked change in the length of the esophagus during metamorphosis. In the adult the esophagus extends from the buccal cavity through the thorax into the first abdominal segment where it enters
the water sac, or in the honey bee, the honey stomach.

The body of the bee nected with two pairs abdomen. The nervous system
brain
is

well filled with tracheae, which are conof thoracic spiracles ajid eight pairs on the
is

is

similar to that of the grasshopper.

The

a ganglion in the

head above the esophagus.

It is con-

nected by a nerve rmg with the subesophageal ganglion, which is in the head but below the esophagus. The two ganglia of the head are connected with two in the thorax and four in the abdomen.

The queen bee when fully developed mates with a drone on the virgin flight. By means of the copulatory organ the male transfers
a supply of sperms to the seminal receptacle of the queen. Just how the queen is able to regulate the laying of eggs that are fertilized by the sperms from the seminal receptacle or those that are not fertilized is not fully known. The fertilized eggs develop into

workers and the unfertilized eggs into drones.

The

life

history of the bee,


its

life

in

the hive, the gathering of


use, as well as

nectar and

development into honey for table

swarming and the rearing of a queen, are fascinating subjects dealt with in the many books devoted exclusively to a study of the honey bee.

CHAPTER
Phylum Chordata
spicuous, better

XXIII

PHYLUM CHORDATA
(kor da'
ta,

cord)

is

made up

of the group of

animals which includes

man

himself and in general the more con-

known

animals.
Characteristics

There

is

a rather wide range of variation as to form and size in

the group.

It includes minute sessile forms, small colonial forms, mud-burrowing forms, and on up to the largest and most complex of living animals. All individuals classified in the phylum possess three distinctive characteristics that are most conspicuous in certain primitive forms. The three features clearly distinguish the phylum from all others and bind together individuals which are widely separated in appearance but characterized by certain traits peculiar to this group alone. These three characteristics are: (1) noiochord, a flexible rod extending from anterior to posterior in the longitudinal axis of the body, lying dorsal to the digestive tube and

ventral to the nerve cord; (2) pharyngeal clefts or gills, a series of paired slits in the wall of the pharynx and in the body wall of some
(3) dorsally located tiibular nerve cord, extending the length of the

body dorsal

to the notochord

and other organs. and


is

The notochord
life of

serves as a stiffening rod


It is present as

the foundation axis

for the endoskeleton.

such at some time during the


it is

every cliordate animal.


of the vertebrae.

In the adult vertebrate

replaced

by the centra
time in the
gills

The

gill clefts

are present at some

life of all individuals placed in this phylum. Although the become modified to form other structures in the adult terrestrial chordates including man, they have had rather typical ones as embryos. The pharyngeal clefts or gills provide a more effective mode of respiration for aquatic animals than that used by most non-

chordates because the


passes through them.

gills

are thus interposed directly in the

course of the circulation,

and the entire blood supply of the body The central nervous system is derived from
first

the ectoderm along the middorsal line of the embryo,

as a

plate, then as a groove, and finally a tube which results in the spinal

360

PHYLUM CHORDATA
cord and brain.

361

In higher forms the anterior end of the tube beexpanded and modified to form the brain. The continuous comes tubular nerve cord is at the apex of the development of centralization in the nervous system, and allows for an increase in number of nerve cells, increased accessibility, and more intimate association
of ganglionic masses to furnish better coordination. These are all advances in both structure and function when compared with other groups. The chordates possess segmentation (metamerism), but it is progressively obscure as one proceeds from simpler to more complex forms. There is a tendency toward fusion of metameres and The internal skeleton of this group shifting of superficial muscles.

compared with the external one of others studied does not give as
great a leverage for the muscles, but
cal freedom allowed and this
is
it greatly increases the mechania distinct advantage as well as an

advance in structure.
Classification

There are approximately 40,000 different species in this phylum which is divided into four established subphyla as follows: Hemichorda (hemikor'da, half cord) or sometimes known as Enteropneusta (en ter op nus'ta) includes order Balanoglossida with its four families, ten genera and twenty-eight species, and order Cephalodiscida with its two genera C ephalodiscus and Bhahdopleura. These
are
all

small wormlike animals.


tail cord),

Urochorda (u r6 kor'da,
cludes the tunicates,
all

or Tunicata (tunika'ta) in-

of

which are marine and mostly small.


it is

Adults show a high degree of degeneration so

the larvae only

There are that exhibit distinctive characteristics of the phylum. three classes: (1) Larvacea, so named because it retains the larva

form throughout
colonial,

life.

Genus Appendicidaria

is

an example.

(2)

Ascidiacea, the sea squirt, either free-swimming or sessile, simple or

may

reproduce sexually or by budding.


(3)

Molgula, Cynthia,

and Ascidia are common examples.


eration.

Thaliacea, free-swimming,

pelagic, solitary or colonial forms, usually exhibit alternation of gen-

Salpa and Doliolum are the most common examples. 16 kor'da, head cord) includes approximately twenty-eight different species of marine, shore-loving, fishlike forms

Cephalochorda (sef a

of which Aniphioxus (Branchiostoma lanceolatus)

is

the most

common

representative.

362
Yertehrata (ver

TEXTBOOK OF ZOOLOGY
te bra'ta, jointed)

animals with backbone

frog,
will be

man.

These are the larger,

more conspicuous animals and

discussed at length in later sections of the book.

Phylogenetic Advances of Chordata

Notochord and endoskeleton, (2) pectoral and pelvic girdles with limbs, (3) development of dorsally located nerve cord with anterior brain, (4) development of five senses, (5) pharyngeal gills
(1)

and lungs for respiration, (6) voice production, and coordination of muscles.

(7)

specialization

Protochordata (Lower Chordates)


Until relatively recent years the two subphyla, Hemichorda and

Urochorda were not classified as Chordata; the former was with Annelida and the latter was independent. With the exception of the value as biological specimens and the use of amphioxus as food by Chinese, this group is of no economic importance.

SUBPHYLUM HEMICHORDATA
One
of the species of Balanoglossus or Dolichoglossus koivalevskii
will serve as

an example. They are wormlike animals which burrow sand along the seashore. They range from 6 to 10 inches in length. Others of the subphylum may be as short as one
into the

mud and

body are

The three portions of the and a segmented trunk. The proboscis, as well as the collar, is hollow and serves as a water chamber. The cavity of the proboscis is filled with water which is dra"wn in and expelled through a proboscis pore or vent located on its dorsal Supporting the base of the side and just anterior to the collar. is a short skeletal process which is stiff and extends anteproboscis riorly from the roof of the mouth region and assists in burrowing.
inch or
still

others as long as four feet.

proboscis, a ringlike collar,

This process, called the diverticulum,

is

usually referred to as the

rudimentary notochord.
in a peculiar position.

However,

it

is
it

very poorly developed and


has the relationship to the

Nevertheless,

digestive tube which

is

characteristic in the embryonic development

of the notochord for certain higher chordates. The mouth opens on the ventral side just anterior to the collar and leads into the straight alimentary canal which extends to the posterior end of the body

PHYLUM CHORDATA
and ends
in the anus.
it lives

363

mud

in

which

as nutriment.

Like the earthworm, this animal utilizes the for food, absorbing the organic matter from it Balanoglossus has numerous paired gill slits, located

in the lateral walls of the anterior (supposedly pharyngeal) position of the digestive tube. In some of the other representatives the gills

numbers or are lacking. Where gills are present, water is passed through them for respiratory purposes, oxygen being absorbed and carbon dioxide being discharged from the blood here. There is no differentiation of a distinct pharynx.
are

much reduced

in

Fig.

209.

External
Glomerulus

features

of

DoUchoglossits

kowalevskii.

(Courtesy

of

Denoyer-Geppert Company.
Pericardium
/
Collar

Nerve cord
Dorsal vessel

Proboscis coelom

Heart /

Trunk

Proboscis

Notochord

Mouth

Ventral vessel

Gill slits

Alimentary canal

Diagram of a sagittal section through anterior portion of DolichoFig. 210. glossus. (From Hegner, College Zoology, published by The Macmillan Company, after MacBride.)

The circulatory system is rather rudimentary. It includes a sinuslike heart which is held in a pericardial sac located in the basal part of the proboscis. A dorsal vessel extends posteriorly from the heart to the posterior end of the trunk. At the collar it is joined by
lateral connectives

vessel extending posteriorly

which encircle the body to connect with a ventral below the intestine. Sinuslike branches
supply various parts of the body.
is

of these

main

vessels

composed of a dorsal cord which is tubular in the region of the collar and extends the length of the trunk, a more or less concentrated center of nerve cells in the collar, and a

The nervous system

364

TEXTBOOK OF ZOOLOGY

ventral cord rimning longitudinally on the floor of the trunk. ventral cord certainly

The

is not a chordate characteristic, but the dominance and hollow structure of the anterior portion of the dorsal one, represent features which are homologous to the central nervous

system of higher chordates.


Excretion seems to be accomplished by a mass of vascular tissue (glomerulus?) located in the proboscis just anterior to the heart. The excreted materials are received by the water in the proboscis
pelled.

cavity (coelom) and pass out the pore with the water as it is exThese animals are dioecious, with gonads in the form of a
genital ridge extending leng-thwise along each side of the anterior

portion of the trunk.

The mature germ

cells

escape through the body

wall, are fertilized in the water, hatch out


Apical plate

and become tornaria

Proboscis

ccelum

Mouth

Anus
Fig. 211.

Tornaria by TheofMacmillan Company, after Metchnikoff larva Hemichorda. (From Hegner, College lished
.

Zoology, pub-

larvae,

which are globular in shape and form a pattern of


In this
respect and

ciliated

bands over the body.

in habit of life these

larvae resemble the larvae of the echinoderms.


theoretical relationship has been proposed.

On

this basis a

Until relatively recent

times this larva was mistaken for a form of adult nonchordate animal and went under the genus name of Tornaria. Dolichoglossus and its subphylum, though lacking in complete

conformity to chordate characteristics, is classified here because of the diverticulum supposedly representing a rudimentary notochord, the gill clefts in the alimentary canal, and the dominance and

grooved structure of the dorsal nerve cord. The group includes Cephalodiscus and Rhaldopleura which are colonial forms living in deep sea.

PHYLUM CHORDATA

365

SUBPHYLUM UROCHORDA, MOLGULA


Subphylum Urocliorda includes
a

number

of

common

represen-

tative marine forms, such as Salpa, Cynthia, Ciona, Clavelma, AsThe latter genus represented by M. manhatt encidia, and Molgula. given particular consideration here. This animal is comsis will be

monly known
of the adult
is

The body as sea lemon, sea peach, or sea squirt. and averages about one inch in diameter. In saclike

this condition it

would be an outcast among chordates because as an adult it has no notochord, and no dorsally located, tubular nerve cord. However, it does present pharyngeal gill slits.

Incurrent siphon Excurrent i/phon Mantle Tunic


Qanqiion

Ana5
^

Genital duct
Testis

Ovary r - Digestive glands -- Esophagus

--Intestim

-Stomach

Branchial fold

- - End05tyle
/Atrium

- - PharynK

Fig. 212. Diagram of Molgula manhattensis from the left side to show the structure with the courses of water and food through the body indicated by arrows.

It is

saved to the chordates by the presence of

all

three of the
is

characteristic features in the larval stage.

The larva
is

free-swim-

ming and shaped


in most of the

like

a tadpole, while the adult

globular and sessile

common forms. Some are brilliantly tinted with color. The adult is covered externally by a cellulose coat or tunic (test), which is secreted by the cells of the underlying mantle. Inside the
mantle
is

the extensive atrial cavity.

On

the dorsal (unattached)

side of the

body are two funnellike siphons.

The anterior one

is

the

Iranchial siphon (oral funnel, incurrent siphon or mouth) and the other is the atrial siphon (atrial funnel, excurrent siphon, or atriopore).

When

the tunic of Molgula

is

removed, one

may

see

most of

366

TEXTBOOK OF ZOOLOGY

the internal organs through the transparent mantle.


this

Upon viewing

from the left the large saclike pharynx may be seen continuing ventrally and posteriorly from the branchial siphon, finally narrowing at its dorsoposterior extremity to become the small tubular esophagus which turns sharply downward and anteriorly to become the
stomach.
digestive gland.
it

The esophagus is partially embedded in a dark-colored The stomach continues anteriorly and upward where

becomes intestine, which turns ventrally on itself in a U-shape. It ends with the anus which opens into the atrial cavity shortly below the atrial siphon. A current of water carries food into the digestive system and oxygen for respiratory purposes. The water enters the branchial siphon, passes into the sievelike pharynx, and
finally

from here passes through the


the surrounding atrial cavity, the atrial siphon.

gill slits

or stigmata in its wall into

and

finally leaves the

body by way of

the stigmata.

absorbed by the blood in the walls of The animal's food consists of minute organisms which
is

Oxygen

style,

mucus secreted by a glandular groove, the endowhich extends from the branchial siphon along the ventral midline of the pharynx to the esophagus. This food mass passes into the esophagus and out through the alimentary canal where digestion and absorption occur. The heart is a contractile tube which pulsates. It lies ventral to the stomach and forces the blood in one direction by a series of contractions and then in the opposite direction by another
are entangled in
series.

Vessels extend in one direction to the pharynx, primarily,

and

in the opposite direction to other organs

and the body

wall.

These animals are hermaphroditic or monoecious. Each has two compound sets of gonads, one on the left side in the loop of the intestine

and the other on the right


closed
tral

tunicates, as Molgula, reproduce

Some of the sessile by budding. There is an oblong, excretory sac which may be seen from the right side. The censide of the body.
is

tween the siphons in the dorsal portion.

reduced to a nodulelike ganglion located beNerves branch from this to history of the tunicate is one the various parts of the body. The life of interest. Cross- fertilization is the rule; that is, spermatozoa from
nervous system
one individual usually
be exceptions to
this.

fertilize

ova from another however, there


;

may

water outside the body. The eggs hatch to produce larvae somewhat similar to amphibian tadpoles which are free-swimming. The larva possesses the typical notochord, gills, and nerve cord of Chordata. For some reason
fertilization occurs in the

The

PHYLUM CHORDATA
it

367

then settles on the bottom and attaches

itself

by adhesive papillae

Some authors express it by larva settles on its "chin." It now undergoes regressive saying this changes involving loss of tail, notochord, and posterior portion of
located in the anteroventral position.

nerve cord.
glion.

The anterior portion of the cord becomes a simple ganThe paired eyes and otocysts (ear structures) also disappear.
This places

The

dorsal side shortens while the ventral side leug-thens.

the mouth in a dorsoanterior position, the anus in the dorsoposterior The numposition, and bends the alimentary canal into a U-shape.
trhqa.
tnt.
/

;e.

d.n.c.

epw.

^'9-3-

cie.

at.oji

Fig.

B,

cerebral vesicle
; ;

e., eye ciliarv funnel d.n.c, dorsal nerve cord g.s., ga., ganglion fixation papillae cardium est., fix., h,., heart m., mouth ncli., notochord st., stomach stat., int., intestine (From Borradaile and Potts, The Invertebrata, ganglion. trk.ga., trunk
;

ad~ga.,

Metamorphosis of an ascidian lari'a. A, larva ready for fixation. 213. an intermediate stage of metamorpliosis. G, completion of metamorphosis. ce.ves., at.oj}., atrial opening at., rudiment of atrium adult ganglion
;

ci.f.,

endostyle
;

eiric, epigill slits

statolith

published

by The Macmillan Company.)

ber of

gill slits

increases greatly.

The

atrial cavity is

formed by

in-

foldings from the exterior on each side which surround the pharynx

and meet each other. The external opening of this cavity is the atrial siphon. The outer wall of this newly formed cavity is the mantle. Later the tunic is secreted by the mantle to become a protective, celluThis process of metamorphosis has caused an active lose covering.

368

TEXTBOOK OF ZOOLOGY

respectable ehordate to become a lazy, stationary form which is not

much more than

a water-bag whose level of development has degen-

erated almost to that of a sponge.

Certain of the sessile forms, which reproduce also by budding, develop colonies with a common tunic. This form is one of the few colonial ehordate animals. In a few
instances tunicates reproduce one generatioji sexually,
is
is

and the next

produced by budding (asexually).


another retrogressive feature.

This alternation of generation

SUBPHYLUM CEPHALOCHORDA, AMPHIOXUS


group which There are four species on American shores Bra7ichiostoma virginiae, B. floridae, B. lermudae, and B. calif orniense. Amphioxus or the lancelet, Branchiostoma lanceolatus, the European form, is an admirable representative of the subphylum and has become classical in its use. However, it is likely that B. virginiae or B. floridae is more commonly studied in the United States. It is a small, fishlike, marine animal whose average adult length is about two or three inches. In its adult
listed twenty-eight species in this

There are usually

are rather locally distributed over the world.


:

form it represents clearly the phylum in a simple condition. It


essentials.

distinctive
is

characteristics

of

the

a ehordate, possessing only rare

It is usually referred to as a close ancestral relative of

Vertebrata.

Habitat.

It is

of the subtropical

found in shore water and on the sandy beaches and tropical portions of the world. These ani-

mals are found along our Atlantic Coast as far north as Chesapeake Bay, at certain points in the Gulf of Mexico, and on the southern Pacific Coast. They may be found along the shores of the Mediterranean Sea, the Indian Ocean, and along the southern coasts of
China.

Habits and Behavior. It burrows rapidly, head first, in the sand by means of a vibratory action of the entire body, but comes to rest with the anterior end exposed to the water. At times, particularly at night and during breeding season, the animal leaves the burrow and swims about like a fish by means of lateral strokes of
the posterior portion of the body.

External Structure.
small lance, the
tail

The

body

of this animal

is

shaped
it is

like a

being the point.

In general,

similar to

Cerebral
vesicle

Oral cirri

Velum
Velar tentacles

Spinal cord

Atriopore
Intestine

Ventral

fin

Anus
Caudal fin

Fig. 214.

Diagram of Branchiostoma
to

(Amphioxus) lanceolatus from the right show the structure.

side

370
a small

TEXTBOOK OF ZOOLOGY

fish, but it does not have a distinct head. The mouth opens on the ventral surface of the anterior portion of the body. It is beneath a rostrumlike projection and is nestled well up in an oral hood which is shaped like an inverted funnel. This hood is fringed with sensory fingerlike oral tentacles. There is a median fin along

the dorsal side, continuing around the tail as the caudal fin and anteriorly about one-third of the length of the body as the ventral

median

fin.

There are no clearly defined lateral

fins,

but a pair of

skin structures, the metapleural folds, extending along the anterior


Dorsal fin
F/n ray

Epidermis

Neurocoek
NotochordoL _ sheath
'/

'/^-^

.^

Spinal

\
^j

nervz Nerve cord

Nobochord
S

Myoto;ne muscle

\
>]

Myocomma
Dorsal Aorta

Coelom
Atrial cavity-

^K =

A^-Epibranchial qroove

.Nephridium
Atriaicavity
^

Pharynx
Gil/rod
Gill

_ _ Liver

bars

Qonad
Hypobranchial qroove
Ventral aorta.

Endostyle

Coelom
Metapleural fold

S. Fig. 215.

Cross

section of

Amphioxus

thirough the level of the posterior portion of the pharynx.

two-thirds of the ventral surface of the body are thought to be their

forerunners.

The ventral and dorsal

fins are

supported by small

vertical rodlike fin rays.

On

the ventral side, just posterior to the

metapleural folds, is an opening, the atriopore, and beside the ventral margin of the caudal fin is the anus. The segmental divisions of the muscles are apparent on the body wall. There are from fifty-eight to sixty-four of them on each side in B. lanceolatus but sixty-nine in B. calif orniense and they are known as myotomes. The myotomes on the two sides are not paired, but alternate with each other. Adjacent ones are separated by a myocomma or myoseptum.

PHYLUM CHORDATA

371

Buccal cirri

QUI slit inwall of phorynx

Afferent branch-

Jal arteries
Ventral aorta
Dorsal aorta

Notochord

_ -Spinal cord

-Distribution throuqh liver

^
..HepaticV.

Subintestinal vein

-Atriopore

-Vcntro- intestinal

V.

.Dorso-intestinai A.

Anus
Caudal vein

Caudal artery

Fig. 216.

Diagram of the circulatory system of Amphioxus.

if

372

TEXTBOOK OF ZOOLOGY

Internal Structure and Metabolic Activities. In small cleared specimens the internal organs are easily observed. The notochord

extends the length of the body as a slender rod of vacuolated

cells

which are
body.

filled

with fluid to give


is

it

turgor or

stiffness.

Immediately

dorsal to this rod

the nerve cord, which also runs the length of the

It has a small central canal or neurocoele


it

extending length-

and is dilated at the anterior end to form the cerebral vesicle or rudimentary brain. A mass of dark pigment is located at the anterior end which is known as the eyespot. There are smaller pigment bodies distributed along the length of the cord. These are thought to be sensitive to light. The nerve cord gives off nerves to the organs of the body. The two anterior ones are paired, but those behind the cerebral vesicle alternate on the two sides. There are dorsal sensory nerves going to the skin and ventral motor nerves
wise through

going to the myotomes.


tentacles,

There are sensory

cells

in the skin, oral

and velar

tentacles.

The circulatory system does not include a heart, but the blood moved by the contractions of a ventral aorta, which branches to form the afferent branchial arteries to the gills. Here these vessels
is

branch into capillaries, providing aeration for the blood. These capillaries converge to form the efferent branchial arteries which lead dorteriorly to the tip of the

The dorsal aorta extends posnumerous branches to myotomes and internal organs along the way. The posterior direction of
sally to join the paired dorsal aortae.

body giving

off

the flow of the blood

is

just opposite to that in the dorsal vessel of the

from the inand continues anteriorly to the liver as the hepatic portal vein. The hepatic vein collects from the liver and leads forward as the ventral aorta. The blood in the subintestinal and hepatic portal veins is laden with dissolved nutriment. The blood in these ventral veins flows from posterior toward the anterior (Fig. 216).
subintestinal vein receives the blood
testine

earthworm.

The

Digestive System. A current of water is carried into the mouth by the ciliated bands on the inner surface of the oral hood. These cilia form what is called a wheel organ because of their rotary motion. Surrounding the mouth is a membranous velum to which are attached twelve velar tentacles, which fold across the mouth and serve as a strainer to hold back the coarser particles, as well as being sensory. The mouth leads to the large, barrel-shaped pharynx. The gill slits are clefts in the lateral walls of the pharynx. The number of clefts

PHYLUM CHORDATA
varies,

373

ranging between fifty and ninety pairs. These open into the atrial cavity which surrounds the pharynx and other visceral organs. In the midline of the roof of the pharynx is an inverted trough, the
hyperhrancliial groove, which
is

is ciliated.

In the

floor of the pharjTix


Its

another ciliated groove, the JiypohrancJiial groove.

glandular

walls, which are capable of secreting mucus, constitute the endostyle. The strings of It functions on the same plan here as in tunicates.

mucus entangle the food particles and are moved anteriorly, and then by two peribranchial grooves are carried dorsally to the hyperbranchial groove. The cilia here move the mass back to the intestine.

blind, fingerlike diverticulum of the intestine, the liver or hepatic

caecum, extends anteriorly from its connection on the anterior part of the intestine to lie on one side of the pharynx. This organ is a digestive gland and empties a digestive juice containing enzj^mes into the intestine. The intestine extends posteriorly to the anus as a relatively straight tube. The food is digested in, and absorbed from, the
intestine.

Respiratory System and Respiration.


in passing capillaries there

As stated above, the water

through the gill slits delivers oxygen to the blood in the and absorbs carbon dioxide from it. The water then passes back through the atrial cavity and out through the atriopore. The blood then distributes the oxygen to all tissues of the body. The gill-'bars, which separate the slits, contain the blood The gills are on the vessels, and are supported by chitinous rods. faces of the gill bars and are covered with cilia which help move
the water through
its course.

Excretory System and Excretion.

Ciliated

nephridia similar to

those of the earthworm lead from the dorsal portion of the coelom to the atrial cavity. The coelomic cavity is reduced in the pharyngeal region to a narrow space surrounding the dorsal aorta above the pharynx and a narrower one around the ventral aorta below. Between the posterior end of the pharynx and the atriopore, the

coelom consists of a narrow space surrounding the intestine with


a thin

membrane separating
it

it

from the

atrial cavity.

Behind the

atriopore

is

relatively larger.

Reproductive System and Life Cycle. This animal is dioecious with each mature individual possessing 26 pairs of (31 to 33 pairs in B. calif orniense) nodular gonads embedded in the body wall near the base of the metapleural folds. When the germ cells mature,

374

TEXTBOOK OF ZOOLOGY

|;

they break through the wall of the gonad into the atrial cavity and
pass out through the atriopore with the water.
in the water.

Fertilization occurs

Early summer

is

the breeding season, and at that time

the animals are quite active during the evenings

and

nights.

Fol-

lowing fertilization comes a series of cleavage divisions which are This is followed by the infolding of one side of the total and equal.

form the gastrula and this in turn becomes a freeswimming larva which reaches adult condition without metamorphosis, only to begin bashfully burying itself in the sand.
spherical
to

body

CHAPTER XXIV
THE VERTEBRATE ANIMAL: SUBPHYLUM VERTEBRATA
In this group to which
vidual.

man

himself belongs are found the dis-

tinctive chordate characteristics at

some time

in the life of the indi-

In terrestrial forms there are certain modifications to pro-

duce other structures.

universal characteristics

symmetry are The segmented vertebral column and other supporting structures form an endoskeleton (internal skeleton) which is the basic support of the body. Paired appendages are usually present at some time in the life of the individual. The majority have two pairs of fins or limbs in adult conThere is a ventrally located heart which is divided into dition. chambers. The Mood contains hemoglohin hearing red corpuscles and amoeboid white corpuscles. In the vertebrate body is a well-developed coelom, which encloses advanced systems of organs for digestion, excretion, circulation, reproduction, and in terrestrial forms, respiration. Cephalization is developed in all vertebrates and along with this they possess a hollow, five-lobed brain located in the more or less distinct head. The sense organs are in an advanced state of development. The body is divided into head, trunk, and tail. The tail is a posterior prolongation of the body behind the anal opening and is found in some degree in all vertebrates. The nech which is a constricted region between trunk and head is conspicuous in terrestrial forms. The appendages are usually arranged with one pair attached to the anterior, pectoral portion of the trunk and one situated at the posterior, pelvic region. This arrangement is less consistent in the aquatic types where the weight of the body is buoyed up by the water and the limbs are used less for support and locomotion. In
Metamerism and
bilateral

among

vertebrates.

different types of vertebrates there are various modifications of

pectoral appendages as arms, wings, pectoral


flippers.

fins,

forelegs,

and

The same

is

generally true for the pelvic limbs.

acteristic

The body wall is composed of the skin, which usually has chartegumentary outgrowths, such as scales, nails, shells, feathers, and hair, as the outer layer, beneath which is the muscular
375

376

I'EXTBOOK OF ZOOLOGY
is

coat and internal to this

the

membranous peritoneum.
In

In

all

vertebrates, except mammals, the coelom consists of only two parts:

the pericardial cavity and the general abdominal cavity.

mam-

mals

it is

further divided into pericardial, thoracic, and aldominal

cor

muscle n, nonmeduUated nerve nm', nm", nm'", nina, and nmb, myelinated nerve fibers P, papilla of hair Sc, hair shaft, u, fat tissue v and w, external and internal sheaths (From Maximow and of hair root; x and y, endings of nonmyelinated nerve fibers. Bloom, Histology, published by W. B. Saunders Co.)

Fig.

217.
;

D, dermis

section of Diagrammatic gland M, sebaceous


gs,
; ;

the

human

skin.

Cor, stratum
;

corneum
;

Malpighian
;

layer

niu,

cavities.

The

first

contains the heart, the second the lungs, and the

third the organs of the excretory, reproductive,

and

digestive systems.

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

377

The vertebrate animal is covered by an integument or skin which serves as a protective and sensorj- organ. It also helps in excretion through the sweat glands, mucus glands, and oil glands as well as Such exoskeletal strucfacilitating temperature regulation in some. tures as scales, nails, hoofs, claws, feathers, hairs and enamel of
teeth are produced

by the

skin.

The integument

is

composed of an

outer stratified epithelial epidermis which consists of several layers of cells, few nerves, and no blood vessels, and the inner fibrous dermis
or corium, which consists of areolar connective tissue, nerves, nerve endings, integumental glands, blood vessels and

lymph

spaces.

The

membrane

type of bone
of

is

developed in the dermis.

The maintenance

several functions which

any living body requires the cooperation of will attain similar fundamental results

wherever in living material they occur. The principal functions performed by the structures in the animal body are: (1) support and protection, (2) movement and locomotion, (3) digestion, (4)
respiration,
(5)

circulation,

(6)

excretion,

(7)

reproduction,

(8)

reception and conduction of stimuli, and (9) internal regulation. These functions merge into one living process which involves the

building up of protoplasm, transformation of energy, and reproDuring the execution of these activities energy is conduction. stantly being changed from the potential to the kinetic form.

Metabolism.

The collective term metabolism

is

employed when

re-

ferring to all of the interactions involved in the living process of protoplasm. It includes the processes concerned with conversion of food
into protoplasm, release of energy through oxidation, production of heat, movement, elimination of wastes or, in other words, these proc;

esses are chiefly

Ingestion, digestion, egestion, absorption, transporta-

tion, respiration, oxidation,

and

elimination.

The processes concerned

with the conversion of food material into protoplasm (building up) Included constitute the phase of metabolism known as anaholism.
here are ingestion, digestion, absorption, transportation, and assimilation. The oxidation of materials of the protoplasm to liberate energy, and the elimination of wastes incidental to it, is known as cataholism
or the "breaking

down"

phase.

one of the fundamental features of all protoplasm, therefore, all physiology, since it is a study of the functions of living organisms, must be concerned with metabolism. It includes all

Metabolism

is

378

TEXTBOOK OF ZOOLOGY

of the chemical changes

and transformations by which energy

is

supplied for the activities of the protoplasm. The skeleton is quite well developed in the vertebrates and serves them quite efficiently for support, stature, protection, and muscle
attachment.
It is

composed of cartilage entirely in some of the

simpler forms and of bone and cartilage in higher types. It is divided into an exosJceleton which is superficial and an inner endoskeleton

which includes all of the deeper skeletal parts. The exoskeleton is a rather minor part in vertebrates and consists of nails, claws, scales, The endoskeleton includes the hair, feathers, and other outgrowths. appendicular portions. The first is composed of the skull, axial and
vertebral column, ribs,

portion

is

pairs of limbs.

composed of In their development bones either replace cartilage

and in some a sternum. The appendicular the anterior and posterior girdles and two

to be called cartilage I ones or they develop in the connective tissue

of the dermis, to be
is

known

as

membrane

composed of segmental

divisions,

The vertebral column the vertebrae, and is divided into


hones.

five

regions as follows: cervical vertebrae of the neck, thoracic vertebrae of the chest, lumbar vertebrae of the small of the back, sacral,

vertebrae of the hip region, and the caudal vertebrae of the tail Bone is a firm, hard tissue consisting of abundant matrix, region.
pocketlike lacunae in the matrix.

and the bone cells which are held in The outer membranous covering The mineral part of the bone consists of bone is called periosteum. They give it chiefly of calcium phosphate and calcium carbonate. firmness and rigidity. The animal matter is composed of the bone cells and cartilage which serve to give the bone life and resilience.
composed of inorganic
salts,

acid, such as the acetic acid in vinegar, will dissolve the matter of bone if allowed sufficient time, in which case mineral the bone will lose its rigidity. Caustic solutions will destroy the animal matter and make the bone brittle. The following outline

weak

presents a

summary

of the principal parts of the terrestrial verte-

brate skeleton.
Divisions of Skeleton of Terrestrial Vertebrate
I.

Axial Skeleton
(a)

Skull
1.
2. 3.

Cranium
Sense capsules Jaw apparatus
(Visceral arches)

THE VERTEBRATE ANIMAL


(b) Vertebral column
1.

SUBPHYLUM

VERTEBRATA

379

Cervical vertebrae (neck)

2. 3. 4.
5.

Lumbar

Thoracic vertebrae (chest) vertebrae (small of back)

Sacral vertebrae (hip)

Caudal vertebrae
Eibs (paired)

(tail)

(c)

Thoracic basket
1.

2.

Sternum (breastbone)

II.

Appendicular Skeleton (girdles and limbs)


(a) Pectoral (anterior)
1. 2.

Girdle:

Limb:

scapula, clavicle, procoracoid and coracoid Humerus (upper arm), radius and ulna (forearm), carpals

(wrist), metacarpals (palm), phalanges (bones of digits)

(b) Pelvic (posterior)


1.

Girdle:

2,

ilium, pubis, and ischium Limb: Femur (thigh), patella (knee cap),

tibia

and

fibula (shank),

tarsals (ankle), metatarsals (sole), phalanges (bones of toes)

no very elaborate adaptation toward a skeleton. The presence of a cuticle in some and the secretion of a hard shell in others seem to be the particular developments related to these special functions in this group. Arcella, Difflugia, the Foraminifera, and Radiolaria exemplify this adaptation.
In Protozoa there
is

The skeleton and integumentary structures serve the Metazoa primarily for a support and protection. The corals of the phylum Coelenterata secrete a calcareous or horny skeleton around the exThe sponges, as a rule, each ternal surface of the body proper. have a calcareous, siliceous (glassy), or horny skeleton extending throughout the body. Such forms as snails, crayfishes, beetles and
representatives of their respective phyla secrete a well-developed exoskeleton as an external cover over most of the other tissues of The muscles and other tissues are attached within. the body.

There are special cells of the epidermis which function primarily The echinoderms, including in production of this skeletal material. animals like the starfish, possess calcareous skeletal plates which are essentially similar to exokeleton except that they are principally beneath the skin.

no well-developed endoskeletal structure known in nonchordate animals but the endophragmal structures extending into the thorax of some Crustacea are thought to be the forerunner of the endoskeleton. A number of exoskeletal modifications are used for
There
is

380

TEXTBOOK OF ZOOLOGY

protection and temperature regulation in most of the groups of vertebrates. Such structures as scales, shells, feathers, hair, nails, horns,

and even enamel

of teeth are of this type.


is
it

Primitively the notocJiord


the chordate group.

the original endoskeletal structure of


are developed the basic structures of

Around

Cranium
Ska//
Orbit

Mandib/e
Ceri//ca/

vertebrcxe

C/av/c/e

Scapu/a

Sternum
Thoracic

Humerus
Rib

vertebrae

Lumbctr
vertebrae
Pe/vis

Sccrum
Radius Utna

^Carpa/s
Mefacarpa/s
Hc^nd

Tibia

Metatarsals

Fig.

218.-

-Human

skeleton.

(From Wolcott, Animal Biology, published by the McGraw-Hill Book Company.)

the vertebral column which functions as the principal axial support The sternum, girdles, and paired limbs have of all vertebrates.

developed with the terrestrial for locomotion on land.

life of

vertebrates and the necessity

THE VERTEBRATE ANIMAL

SUBPHYLUM VERTEBRATA

381

The muscular system represents a system of cells highly specialized in contractility. The muscles are usually attached to the skeleton or occasionally to other muscles by fibrous cords called tendons.
Voluntary muscles are usually connected with the skeleton; those of the visceral organs, e.g., intestine, are involuntary. Cardiac muscle
is

the highly specialized involuntary muscle which makes

up the wall

of the heart.

almost a characteristic of Contractility as a property of all protoplasm is the animal life. fundamental basis for all animal movement. The adult forms of cer-

Independent power of movement

is

tain animals, such as sponges, corals, oysters, barnacles, and others, are sessile however, they all pass through a free, active larval stage.
;

Most of them retain the power


condition.

to

move separate parts

in

adult

Simpler forms of locomotion have already been seen in Protozoa which move from place to place by means of pseudopodia, cilia, or
flagella.

In ciliary movement the numerous small strands of protoplasm beat rhythmically with a stroke in one direction, so timed that the beat passes in a wavelike progression from one end of the ciliated
area to the other (metachronous rhythm).

The stroke

of a eilium

consists of a vigorous bend in one direction and a very deliberate recovery in the other. In many Protozoa the entire body is covered

with

while in Metazoa the entire body may be covered where they are used for locomotion; but more often they cover only areas
cilia

of free surface of epithelium, particularly the linings of passages. Here they serve to move materials along and keep the surface free of foreign material

and

excess mucus.
cells,
is

The development
such as muscle
sists of sets of
cells,

of a high degree of contractility in special

makes

possible muscular

movement which

the

principal kind in higher animals.

A muscular locomotor

system conis

opposing muscles.

In muscular contraction there

a cycle of rapid chemicophysical rearrangement in the cells. Oxidation and heat production are involved in the process. Carbohydrates
in the

form of glucose are oxidized (burned) in the


is

reaction.

During

the shortening of the muscle there

a hydrolysis or absorption of water by the protein product, creatine-phosphorie acid. By-products of the process include carbon dioxide, lactic acid, urea, creatinine,

and phosphoric

acid.

382

TEXTBOOK OF ZOOLOGY

In animals without a skeleton muscle bands are arranged in both


circular

and longitudinal directions. The contraction of the circular group tends to lengthen the body, and the shortening of the longitudinal strands draws the body along. The pressure exerted on the coelomic fluid is thought to be a factor in bringing about an even extension of the body by this means. In echinoderms with the water
vascular system the pressure
is

exerted on water in a system of tubes

which extend
is

to

make

contact with the surface over which the animal

moving.

The

Dig"estive

System.

The

digestive

system

is

typically

straight tube extending through the length of the trunk of primitive


vertebrates.

In the higher forms there are

many

outgrowths, such

and respiratory organs. The anterior region of the digestive tube is the mouth cavity which contains teeth on the jaws, a tongue, and receives saliva from salivary glands. Following the mouth is the pharynx or throat region which receives the internal nostrils, the Eustachian tubes from the middle ears and opens into the esophagus posteriorly. The esophagus is usually tubular and propels the ''swallows" of food posteriorly by consecutive waves of contraction, a process known as peristalsis. It leads to the saclike stomach,
as digestive glands

containing enzymes

whose walls possess gastric glands for secretion of a digestive fluid The (ferments) and weak hydrochloric acid. peristaltic contractions continue along the wall of the stomach to help digestion by churning and mixing the food with digestive juices. At the posterior end a pyloric valve in the form of a sphincter muscle guards the entrance to the small intestine which follows. This is a convoluted tube in most of the advanced forms of vertebrates and is divided into the anterior duodenum, middle jejunum, and
longer than the body and therefore produce digestive enzymes from glands and the liver and it receives digestive juices from two other glands the pancreas. The small intestine serves both as a digestive organ and as the principal absorptive organ of the body. Its internal lining is provided with numerous fine fingerlike projections which The digested increase the inner surface and enhance absorption. food is taken up by the lymphatic spaces and by blood vessels which
posterior ileum.
it is

It is usually

coiled.

Its walls

are

embedded

in the wall just outside of the lining epithelium.

The
It

liver is the

largest organ in the

body of most vertebrates.

secretes the bile

which

is

stored in the thin-walled gall Madder,

THE VERTEBRATE ANIMAL


which
is

SUBPHYLUM
The

VERTEBRATA

383

attached to one of

its lobes.

liver also serves to convert

ener^

carbohydrates to glycogen (animal starch) and store it for future It is also in the liver that protein wastes are production.
converted into urea and uric acid in order that they
the blood in the kidneys.

may

be excreted

is shorter from It villi or digestive glands. than the small intestine possesses no In many receives the fecal matter and delivers it to the anus.

The large

intestine

which

forms of vertebrates the posterior portion of the large intestine is modified to become a cloaca, which receives also the products from
tlie

urinary and reproductive organs.


chief function of this entire system
is

The

that of dissolving and

converting complex food materials into a form which

may be

absorbed

and assimilated by the protoplasm of cells throughout the body. The materials commonly used for foods have large molecules, usually called colloidal in nature. Digestion then must serve to break up these large molecules into smaller ones, thus forming solutions of substances in order that they will readily diffuse through membranes.

Digestive enzymes are responsible for placing the food


So, proteins are converted to soluble
to maltose

materials in solution.
acids, starches

to

and sugars fatty acids and glycerin.


is

and

finally glucose,

amino and fats

an organic substance which by its pres\vill cause or hasten chemical reaction between other substances without itself being consumed. The enzymes are formed in the protoplasm of cells and their action is similar to that of a catalyst, since they accelerate chemical action. There are different types of enzymes each capable of producing speIn general, an enzyme
ence under certain conditions
cific

kinds of reactions. There are oxidizing enzymes (oxidases) capable of bringing about oxidation reducing enzymes (reductases) which produce reduction in tissues; coagulating enzymes (coagu;

lases)

which cause clotting or coagulation; and hydrolysing enzymes by causing a reaction between a substance and water. Most of the digestive enzymes fall in this latter class. Most enzymes consist of a parent substance or precursor (zymogen) which becomes active only in the presence of a certain other substance, termed activating agent or coenzyme. As an example, the precursor of pepsin is pepsinogen which becomes activated in the presence of dilute
(hydrolases) act
acid.

384

TEXTBOOK OF ZOOLOGY
Classes of Digestive Enzymes

1.

Diastases or diastatic
(a) Ptyalin in saliva (b)
(c)

enzymes

split

carbohydrates

Amylase

in pancreatic juice

Glycogenases liver and muscles Converts glycogen to glucose

2.

3.

Lipase or lipolytic enzyme splits fats (a) Steapsin in pancreatic juice Inverting enzymes convert disaccliarids to the less complex monosaccharids
(simple sugars)
(a)

intestinal

juice

Maltase (b) Lactase (c) Sucrase (invertase)


4.

Proteases or proteolytic enzymes


(a) Pepsin in gastric juice (b)
(c)

split

complex proteins
in small intestine

5.

Trypsin in pancreatic juice Erepsin in intestinal juice Clotting or coagulating enzyme


(a) Eennin in gastric juice

functions

In higher Metazoa digestion is accomplished principally extrathrough secretion of enzymes by certain groups of cells. Such systems consist of: (1) an alimentary canal proper; and (2) The associated glands which discharge digestive juices into it. relative length of this canal varies considerably depending on the habitual diet of the organism. In carnivores (flesh-eaters), such as
cellularly

from three to five times as long as the body; while in herbivorous forms (plant-eaters), such as horses and cows, The length of the it is over twenty times as long as the body.
cats

and dogs,

it is

human

approximately ten times the length of the body. The relative proportion of the internal absorptive surface of the alimentary canal to the external surface of the body is significant. In carnivorous animals it is about one-half the area of the skin while in herbivorous animals it is about twice the area of the
digestive tract
is

skin.

man is quite well understood, and and general because of the omnivorous food habits. The action of the several enzymes produced by different glands is a very essential part of the process. The digestion of all organic food materials is brought about by hydrolysis in the same
The process
is fairly

of digestion in

it

typical

kind of chemical change.


tein,

carbohydrate, or fat

simpler products.

In hydrolysis the large molecules of procombine with water and then split into Some foods may require more than one such
first

THE VERTEBRATE ANIMAL


splitting.

SUBPHYLUM VERTEBRATA

385

The

splitting of the disaccharide, maltose, will serve as


this process:

an example of

(Malt sugar)

(Water)

(Glucose)

The two molecules of glucose formed are


absorption.

in

a form for ready

Gastric Digestion.

The

tubular gastric glands located in the


secrete the acid gastric juice

which is a and two important solution of 0.2 to 0.5 per cent hydrochloric acid enzymes, pepsin and rennin. The pepsin when present in the acid medium brings about the splitting of complex proteins into intermediate proteoses and peptones. Be^inin causes the casein in milk This is the first step in its digestion. It is claimed by to coagulate. some that emulsified fats, such as cream, are partially digested by a The digesting mass or chyme in the stomach is congastric lipase. tinually churned and mixed by muscular activity of the walls. "When it becomes saturated (0.4 per cent) with acid and has been reduced

mucous layer of the stomach

to the consistency of soup,

it is

discharged through the pylorus.


the

Intestinal Digestion.

When

chyme

is

ejected through the

pylorus into the duodenum, the hydrochloric acid stimulates certain cells of the intestinal lining, causing them to secrete into the

blood a substance of hormone nature, known as secretin. Upon reaching the pancreas this secretin stimulates it to secrete the digestive fluid, pancreatic juice, into the small intestine

by way of the
also stimu-

pancreatic ducts.

There

is

some evidence that secretin

lates secretion in the liver.

Pancreatic juice
inorganic salts

is

(carbonates, etc.)

a clear, watery, alkaline solution containing and three enzymes; the protease,

and the lipase, steapsin. These act and peptones, starches and sugars, and fats. respectively on proteins This protease is in the form of trypsinogen until it reaches the intestine and is activated by an intestinal enzyme, enterokinase. Trypsin completes the work begun by the pepsin in that it converts proteoses and peptones into amino acids, but it also digests complex proteins which have escaped the action of pepsin. It acts more rapidly and There are nineteen amino acids that efSciently than does pepsin. are regarded as hmlding stones of the protein molecule. In a complex protein like casein, as many as sixteen of these amino acids will
trypsin, the diastase, am.ijlopsin,

386
be found.

TEXTBOOK OF ZOOLOGY

The

tissues of the

animal body must not only have avail-

able a wide range of amino acids but

must

also select in the

proper

proportion the ones needed to reconstruct their specific protein constituency.

Amylopsin (amylase)

is

the pancreatic diastase, and

it is

able to of

bring about hydrolysis of carbohydrates in the alkaline


the intestine without activation.
It

medium

(malt sugar). The pancreatic lipase,

produces dextrin and maltose steapsin, brings about the splitetc.

ting of fats into glycerin (glycerol) and one or

such as stearic acid, oleic acid, butyric acid,

more fatty acids, The alkaline salts

which are introduced by the bile, combine with these fatty acids to form soaps which help in emulsifying the remaining fats, thus making them more readily
split.

Intestinal secretions or succus entericus which are produced by

glands in the mucous membrane of the small intestine include

five

enzymes.
sin,

Enterokinase, which activates trypsinogen to form trypErepsin, the intestinal protease,

has been mentioned already.

supplements the activity of trypsin by converting proteoses and peptones into amino acids.
dextrose.
levulose.

Maltase converts maltose and dextrin into

Invertase changes sucrose (cane sugar) into dextrose and

Lactase converts milk sugar (lactose) into galactose and

dextrose, both simple sugars.

The undigested residue


others) attack

passes into the large intestine where prob-

ably no enzyme digestion occurs.

Certain bacteria

(B.

coli

and

any undigested protein and bring about putrefactive Products of this action may be absorbed; some of them are frequently toxic and must be eliminated in either the urine or the feces. Certain other bacteria here feed upon cellulose and may produce some sugar from it. When the chyme reaches the large
fermentation.
intestine it is about the consistency of thick cream, but
it

becomes

more and more

solid

by absorption of water here

until finally only

concentrated fecal matter remains.

Functions of the Liver.


discharged into the

The secretion of the liver is bile and is duodenum of the small intestine by way of the
is

an alkaline solution which serves to help as it comes from the stomach. This with the pancreatic juice brings about the emulsification of fats mentioned above. Cholesterin and two pigment materials are excreted in
bile duct.

common

This

neutralize the acidity of the

chyme

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

387

The Digestive Enzymes and Their Functions


ENZYME

388

TEXTBOOK OP ZOOLOGY

Absorption and Utilization of Food Materials.

The soluble prod-

ucts of digestion are absorbed through the semipermeable epithelial


lining of the intestine into the blood of the adjacent capillaries, or in

the case of fats into the lacteal IjTnphatics and from here into the subclavian vein.

hepatic portal vein

The blood supplying the intestine and delivered to the liver.


of proteins in the

is

collected

by the

body are to rebuild debiliand energy to the body by oxidation. They serve first and best for the purpose mentioned first. Carbohydrates and then fats are more economical and efficient as sources of fuel for production of heat and energy. Oxidation of protein requires the disposal of much more waste products. The com:

The two functions

tated protoplasm; and help supply heat

parative heat production values of the three are as follows

One gram of protein z= One gram of carbohydrate z= One gram of fat =

4.100 Calories* 4.100 Calories 9.305 Calories

Some portion

of the dextrose

is

distributed
it

and oxidized

directly

for immediate energy, but

much

of

is

transformed into glycogen


This

by the enzyme glycogenase


tissues as needed.

in the liver.

may

be stored here or

by the Normally there is a constant supply of dextrose (0.1 to 0.15 per cent) in the blood and this level must be maintained. The final oxidation products of carbohydrates in the body are heat, The last two are diskinetic energy, water, and carbon dioxide. charged from the body as waste products. Fat is converted to dexIt is usually trose and oxidized to produce heat and kinetic energy.
in the muscles to be reconverted into dextrose for oxidation

stored as a reserve fuel supply in adipose tissue over the body.

Car-

bohydrates in excess

may

be converted to fat, and stored.

fats,

Vitamins and Their Functions. Besides proteins, carbohydrates, inorganic salts, and water there is another indispensable class of food material, the vitamins. They are natural substances found
general, their function
is

number of different foods. In They are recognized usually through the abnormal condition brought on by their deficiency.
in relatively small quantities in a

regulatory.

There is little danger of vitamin deficiency for adults living on a balanced and mixed diet. Much of our knowledge concerning the symptoms brought on by lack of different substances has been
A Calorie equals the amount of heat necessary to raise one degree centigrade under standard conditions.
liter

of water one

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

389

gained by feeding experiments on different kinds of laboratory animals and results applied to human beings. The following outline will give much of the essential information concerning vitamins.

The Vitamins and Their Characteristics


I.

Vitamin

(C20H30O)

antixerophthalmicfat

soluble.

(a) Sources:

carotene (CjoHBe) a yellow pigment in green plant leaves, carrots, and such plant tissues. Transformation of this pigment into the vitamin which is especially stored in shark, cod, halibut or other
fish liver oil,

egg yolk, and milk.

(b) Functions:

Influences efficiency and acuity of vision, important factor in regeneration of visual purple of retina, strengthens and pro-

motes hardiness in epithelial


(c) Effects of Deficiency:

tissue.

Xerophthalmia (lack of tear secretion and dry "Nutritional" roup in cornea), and "night blindness" in human.
birds.

II.

Vitamin B* "Complex."
1.

Bj or Thiamin (Ci2Hi,ON4S)
(a) Sources:

Antineuritic.

Germ

of wheat and other cereal grains, peanuts, liver, and

egg yolk.
(b) Functions:
essential for
(c)

Promotes tone in alimentary tract, stimulates appetite, normal growth, essential for carbohydrate metabolism.
Beri-beri (neurodigestive disturbance following

Effects of deficiency:

diet of polished rice), loss of tonus


tract.
2.

and muscular activity of digestive

Cessation of growth.

Polyneuritis develops in birds.

Riboflavin (C^H^oOoN^).
(a) Sources:

Eggs,

liver, milk,

green leaves, yeast.

(b) Functions:
witli

Necessary for growth, active relation to several enzymes intermediate metabolism of food.
of
in

(c) Effects

deficiency:

Irritation

mouth
3.

human

(cheilosis).

"Yellow liver" of

and inflammation at corners of dogs. "Curl-toe"

paralysis of chickens.

Dermatitis of turkeys.

Nicotinic Acid (CgHsNOz)


(a) Sources:

antipellagric.
egg yolk, green
leaves,

Meat,

liver,

wheat germ, yeast.


cel-

(b) Functions:

Produces active "coenzymes" (I and II), balances


deficiency:

lular function,
(c)

Effects

of

Pellagra in primates

(man and monkeys).

Black-tongue in dogs.
4.

Swine pellagra.

Be or pyridoxine (CgHuOaN).
(a) Sources:

Milk, liver, cereals, yeast.

(b) Functions:
(c)

Necessary for growth.

May

influence oxidation of food.

Effects of deficiency:
still

Paralysis in chickens.

There

are

other recently discovered fractions of Vitamin B. whose func-

tions are specific.

390
5.

TEXTBOOK OF ZOOLOGY
Pantothenic aeid (CgHiTOgN).
(a)

Sources:

Liver, milk,

egg yolk, yeast, molasses, peanuts.


Graying
in

(b) Functions:
(c)

Essential for growth.

Effects of deficiency:
chickens.

black rats.

Dermatitis in rats and

6.

Biotin (doHieOsNjS). (a) Sources: Egg yolk, yeast, cereal grains, molasses.
(b) Functions:
(c)

Essential for growth.

Effects of deficiency:

Thickening of skin and dermatitis in clucks

and
III.

rats.

Vitamin C or Ascorbic Acid (CoHsOe)


(a) Sources:

antiscorbutic-water-soluble.
mammals
except pri-

Citrus fruits, tomatoes, turnips (most

mates and guinea pig can synthesize this vitamin). (b) Functions: Maintains structure of capillary walls. (c) Effects of deficiency: Scurvy in human and guinea pig (bleeding mucous membranes, beneath skin and into joints).
IV. Vitamin

in

(C2SH44O)

antirachitic.
cod-fish
liver oils.

(a) Sources:

Tuna and

Exposure of skin to

ultra-

violet radiation.

(b) Functions:

Eegulation of calcium and phosphorus metabolism.


Soft,

KeSoft

quired for normal growth and mineralization of bone.


(c)

Effects of deficiency:

deformed bones in young

(rickets).

bones (osteomalacia) especially in


V. Vitamin
(a)

women

of the orient.

or Tocopherol (CjaHjoO.)-

antisterility.
cell proliferation

Sources:

Wheat germ

oil,

green leaves, other vegetable fats.

(b) Functions:
(c)

Promotes rapid

Effects of deficiency:

spermatogenesis.

and differentiation. male fowls and rats. Failure of Death of rat embryos in uterus.
Sterility in

VI. Vitamin
(a)

(C3,H4e02)

antihemorrhagic.
necessary for blood clotting).

Sources:

Green leaves, alfalfa, also certain bacteria of the "intestiInfluences the production of prothrombin by the liver
is

nal flora."
(b) Functions:

(prothrombin
(c)

Effects of deficiency:

Blood

fails to clot.

The Respiratory System. The respiratory system


part an outgrowth of the digestive canal.
brates respiration
is

is

at least in

In most aquatic verte-

accomplished by drawing water through gill Air-breathing, terrestrial forms slits in the wall of the pharynx. have developed the trachea (windpipe) and lungs as another outgrowth of the pharynx. A certain amount of respiration takes place through the skin. The respiratory process is composed of two phases: exterTial respiration which includes the exchange of the gases, oxygen and carbon dioxide, between the external environmental medium and the blood and internal respiration which is the
;

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

391

exchange of the gases between the blood and the protoplasm of the Much of the carbon dioxide given up by the cells over the body. cells becomes carbonic acid and carbonates which may be transported by the plasma
(fluid) of the blood.

Respiration has been defined as the process involving the ex-

change of gases between the protoplasm of an organism and its environment. All living protoplasm must be provided with a means of receiving oxygen and giving up carbon dioxide. In protozoa and
simple metazoa, such as sponges, coelenterates, flatworms, round-

worms, and even some annelids, this gaseous exchange is made by almost direct diffusion through the cell membranes to the surrounding medium. This movement of gas through the cell membranes depends on the partial pressure of the particular gas on the two sides of the membrane. Gas will flow in the direction toward the least
pressure.

In the larger and more complex animals where the volume of is such that a more active interchange of gases is required than the general body surface will permit, special organs or moditissue

fications of the surface

must be provided. Also the possibilities of oxygen absorption are greatly increased by the development of respiratory pigments like hemoglobin and hemocyanin, which are
These pigments readily unite with oxygen to form oxyhemoglobin in the case of the former. Thus the blood is enabled to absorb far more oxygen than an equal quantity of ordinary liquid. When the oxygen pressure of the surrounding tissue is sufficiently low, the oxyhemoglobin releases its oxygen rapidly. Carbon dioxide accumulates in excess in the tissues and diffuses from the cells to the lymph, thence to the plasma where much of it combines with sodium Small amounts of CO2 combine with the as sodium carbonate.
carried in a blood vascular system all over the body.

hemoglobin.

The

gills of

most aquatic forms are richly supplied with a

capil-

lary supply of blood and then

membranous surfaces

are directly

absorbed.

exposed to surrounding water from which the dissolved oxygen is In many aquatic worms the gill filaments are outgrowths of the sides of the body wall. Likewise, the more or less plumelike
In gills of crayfish are pocketlike outpushings of the body wall. a number of aquatic insects, worms, fishes, and turtles, the rectum serves as an accessory respiratory organ.

392
Aerial respiration
is

TEXTBOOK OF ZOOLOGY

accomplished in terrestrial animals through In insects a special internal surfaces which must be kept moist. system of branched tubes called tracheae, which open through spiracles along the sides of the body, distribute oxygen to and receive carbon dioxide from all of the cells of the body. In pulmonate

an invagination of the skin, as are also the tracheae of insects. The real lung is a development found in the terrestrial vertebrate, and it is a specialized surface derived from the anterior or pharyngeal portion of the digestive tube. In higher vertebrates, such as birds and mammals, they are extensively lobed, and made spongy by the innumerable small air sacs which provide the enormous respiratory surface necessary. It has been estimated
snails the "lung*' is simply

that

if all

of the average

of these pitlike alveoli of the internal lining of the lungs human being were spread out in an even surface, the

area of it would be more than 100 square yards. The mechanism for the accomplishment of breathing in the cat and other mammals

by the use of the diaphragm and thoracic wall is described in the chapter on mammals. The muscles which control these actions are automatically stimulated through the nervous system to contract when the carbon
dioxide level of the blood reaches a certain point. A respiratory center, located in the medulla oblongata, is affected by the carbon
dioxide and determines the rate of respiratory movements. There are also nerves from the lungs themselves which extend to this center and contribute to the maintenance of the proper rhythm.

Abundance
tory action.

of venous blood stimulates an increase of the respira-

moisture and give

In addition to exchanging gases the lungs also discharge off a certain amount of heat.

The Circulatory System. The circulatory system is a closed system of vessels supplying all parts of the body with blood and a system of spaces, sinuses, and vessels collecting lymph from the various organs to return it to the blood vessels. The blood circulatory system centers in a contractile heart from which tubular arteries lead out to various organs of the body where they branch into minute vessels or capillaries. The capillaries converge as they carry the blood away from the organs to form the veins which carry the blood back to the heart. This is a closed system of vessels. The blood is composed of the clear fluid, plasma, and the Uood corpuscles. The red corpiiscles contain the red pigment matter, hemoglobin, which

THE VERTEBRATE ANIMAL


was mentioned
in connection

SUBPHYLUM

VERTEBRATA

393

with respiration.

Due

to this sub-

stance, the cells have oxygen-carrying power. The white corpuscles or leucocytes are of several varieties and they are amoeboid in character.

These

cells

may make

their

way among

cells of

other tissues

Veins from upper.


part of Bcxjy

Arteries

to upper*

part of Body

Lymphatics

Thoracic duct ^uporiop vena cava


'PulmonaP3^ artary

Pulmonapy vein

Ui^ht aupicla

Left auricle

Infcpiop vena cavavcntpiclG


- Left ventpjclc

fli'^ht

Lacteal^

Hepat ic vain.

Veins from lowcp papt of Body

ArtGplo3 to lowcp
par>t of

Body

Lympfiaticj

Diagram of circulation of the blood in a mammal. The oxygenated Fig. 219. blood is shown in black the venous blood in white. The lymphatics are the black (From Pettibone, Physiological Chemistry, published by The C. V. irregular lines.
;

Mosby Company.)

where they engulf bacteria and foreign matter. Upon exposure to air the dissolved fibrinogen in the blood becomes fibrin and forms a clot which is semisolid and blocks flow of blood from most wounds. The remaining' fluid after the blood clots is called serum. Lymph

394
is

TEXTBOOK OF ZOOLOGY

the capillaries in the various organs, and


corpuscles.

a fluid similar to plasma which has seeped through the walls of it carries amoeboid white

Certain of them are produced in the lymph glands. The spleen is a lymphoid organ in which debilitated red corpuscles
are disintegrated and the products placed in the blood.
Circulation.

Transportation of materials through the protoplasm


and from
cell to cell of
is

of a single cell or a single-celled organism

the metazoan

a fundamental function

among

living things.

In

most Protozoa there is no special arrangement for this function, but the necessary exchange and movement of food materials, waste sub-

and gases is accomplished by simple diffusion of materials. In a few forms, however, of which Paramecium is an example, there This is known is a definite course of movement by the endoplasm. as cyclosis, and it serves to circulate the food vacuoles.
stances,

In double-walled, simple, saccular forms like hydra there is no proan exchange of the water in the gastrovascular cavity. In flatworms, such as planaria, the necessity of increased food distribution is cared for by branching of the gastrovascular
vision necessary except

cavity into diverticula.

transporting materials.
contractile parts
is

In sponges the wandering cells assist in distinct system of tubelike vessels with

developed in the annelid worms, as was studied in the earthworm. Here a closed system of vessels forms a complete In circuit to carry a circulating medium to all parts of the body. because it bears no red this group the fluid is known as hemolymph

The hemoglobin is borne in the fluid. The vertebrate system is closed, and the blood is circulated by the action of a single heart. The hemoglobin, an iron compound, is carried in the red blood In molluscs and some crustaceans there is a similar corpuscles.
corpuscles.

respiratory pigment carried in the plasma, which


cyanin.

is

called hemo-

Instead of iron, copper

is

the principal constituent of this

pigment.
rials

Vertebrate blood

is

largely water carrjdng dissolved mate-

and suspended corpuscles. The fluid part is known as plasma. The amount of blood in a mammal is approximately one-twentieth of the body weight, or in the average man a little more than a gallon. The plasma contains enough inorganic salts to taste slightly salty. When the body Its salt content is about equal to that of sea water.
is

One-fourth is active, the blood is very unequally distributed. always in the heart, large arteries, veins, and lungs. Another fourth
is

held in the hepatic portal system, the liver and

its

sinuses; the

THE VERTEBRATE ANIMAL

SUBPHYLUM
;

VERTEBRATA

395
is

skeletal muscles require another fourth

and the remaining fourth

contains distributed through all of the other organs. Human blood 5,000,000 red corpuscles (erythrocytes) per cubic normally about

millimeter of volume in the male and about 4,500,000 in the female. The average person, weighing 150 pounds, then, would possess ap-

proximately 20,000,000,000,000 (20 trillion) of them. Each erythrocyte is essentially a little capsule filled with hemoglobin which is a

compound

peculiarly fitted to unite with atmospheric oxygen.

When

united with oxygen

it is known as oxyhemoglobin, which is readily reduced to give up the oxygen to the cells when the blood reaches The carbon dioxide given off by the cells is collected the tissues. principally in the plasma and returned to the lungs.

The leucocytes or white corpuscles are quite variable in form and number from 6,000 to 10,000 per cubic millimeter. They are amoeboid and therefore not confined to the blood vessels. One of their chief functions is the destruction of bacteria and other foreign mateThis process is known as phagocytosis. The acrial in the tissues.

companying table summarizes


cells.

essential information concerning blood

The plasma
antibodies.

of the blood contains a group of substances called

These have been produced by various tissues of the body upon contact with certain foreign proteins. Since bacteria and pathogenic Protozoa react as foreign protein, they stimulate the body tissues to the production of specific protective antibodies and physicians have come to make use of these antibodies in sterile serum for preSome of these antigen vention and treatment of several diseases. substances bring about the clumping or agglutination of foreign bacteria, others dissolve

precipitated.

the bacteria, and still others cause them to be The chemical nature of these bodies is not yet known. There are individuals known as hemophiliacs or bleeders whose blood will not clot, and any wound is likely to be fatal. The plasma normally contains a soluble protein, called fibrinogen and calcium in
solution.

Howell 's theory of coagulation of blood holds that there is also an inert substance, antithrombin, which prevents the activation of the prothrombin of the plasma to become thrombin. When blood is shed and exposed to air, the blood cells and platelets produce a substance, cephalin, which, in the presence of calcium, neutralizes the antithrombin, allowing the formation of thrombin. Thrombin reacts

with fibrinogen to produce fibrin, the solid fibers of the

clot.

The rate

396

TEXTBOOK OF ZOOLOGY
Average Characteristics of

Human Blood

Cells

KINDS OP CELLS AND

THE VERTEBRATE ANIMAL


as a mesonephros.
is

SUBPHYLUM VERTEBRATA

397

The former

is

seldom functional, but the latter

up to and including the and salamanders. The metaiiephros is the Amphibia, higher developed kidney as found in reptiles, birds, and mammals. The ureter is the excretory duct which leads from the metanephric
the functional organ in vertebrates
as in frogs

history of these animals as individuals includes successive stages as follows: the pronephros, the sole kidney for a time; followed by the mesonephros which is the dominant func-

kidney.

The

life

tional excretory organ

when

in its glory; and, finally, the develop-

of the others. This is an illustration of the Theory of Recapitulation which says that each individual in its development lives through abbreviated stages of

ment of the metanephros with retrogression

the history of the development of the race.

Excretion.^A certain
olism
is

result of the oxidation necessary for metab-

the production of end-products which are not only of no further use to the protoplasm but may be a distinct menace to the welfare of the organism because of their toxic effects. The substances are usually dissolved and removed as a waste liquid or occasionally as crystals

by

special parts of the body.

In Protozoa this function is performed by general diffusion through the plasma membrane and in many forms by the contractile vacuoles. The quantity of water which passes through the protozoan in twentyfour hours is several times the volume of the animal itself. Among sponges and coelenterates diffusion of liquid wastes through the general surfaces of the body to the surrounding water serves for
excretion.

In an animal like the flatworm, planaria, excretion is accomplished by a system of canals which begins in numerous capillary-sized tubules whose blind ends are composed of individual cells called flame cells. These flame cells are irregular in shape and each bears a tuft of cilia extending into the end of the tubule. The flickering movement of the cilia in the cell gives the appearance of a flame and

moves the accumulated excretion down the tubule. The waste liquid of the surrounding tissues diffuses into this cell. The main excretory ducts open to the surface of the body by excretory pores. This arrangement is sometimes called a protonephridial system. The nephridial system is found in Annelida and has been studied in connection with the earthworm. Here a coelomic cavity is present, and a series of segmentaUy arranged pairs of coiled tubes or

398

TEXTBOOK OF ZOOLOGY

nephridia extend through the wall to the exterior.


wastes accumulate in the eoelomic cavity and are

nephridia through the ciliated funnellike internal

The excreted moved into the end, known as the


to the outside

nephrostome.

This eoelomic fluid

is

drawn

into the canal of the

nephridium by the beating of the cilia and is delivered of the body at the nephridiopore of the next segment.

The green glands of

crayfish are

much more

concentrated, although
of unit organs,

they are modified nephridia.

They function as a pair

each opening by a duct on the basal segments of the antennae.


mollusks there are both nephridia,

In

known

as pericardial glands,

and

the special cells formed from the eoelomic epithelium.

The echino-

derms make use of direct diffusion as well as intracellular excretion by which excreted materials are taken up from the eoelomic cavity by the numerous phagocytic, amoeboid cells of the eoelomic fluid. These cells wander out into the cavities of the respiratory organs where they coalesce into large masses, and finally with their enclosed granules are cast out through the membranes of the respiratory
papillae.

Soluble materials in solution also diffuse through the

memtaken

branous walls of these structures.


care of

In the insects excretion

is

by the Malpighian tubules, which are considered modified nephridia. They are bunched in the posterior part of the body cavity and discharge excretions into the intestine at its junction with the
rectum.

Kidneys.

The

chief excretory organs of vertebrates are called

kidneys, and they are thought

by some authors

to

have developed by

modification and condensation from segmentally arranged nephridial tubules. The fact that in vertebrate embryos as well as in lower
chordates, even the frog, these tubules open into the coelom as

nephrostomes, makes it seem possible that in vertebrates as well The as in annelids the coelom was once important in excretion. substances from essential structures of the kidney for taking waste
the blood and delivering
it

to the exterior of the

body are the

Malpighian corpuscles, each made up of a glomerulus and a Bow-

man*s

capsule,

and the

coiled uriniferous tubules

which discharge

the excretion through collecting tubules into the ureter at the pelvis This canal leads to the cloaca in most vertebrates of the kidney.

below

mammals

(excepting some fish), or to a urinary bladder in the

mammals.

THE VERTEBRATE ANIMAL


The wall
of each

SUBPHYLUM VERTEBRATA
is

399

very thin and readily permits diffusion of water and dissolved materials from the blood into the cavity of the uriniferous tubule on the opposite side of the mem-

Bowman's

capsule

The glomerulus carries arterial blood from the afferent arThe terial branch and discharges it into the efferent arterial branch. soon spreads into a capillary network which surrounds the latter convoluted portions of the uriniferous tubule. Water constitutes the largest volume of materials to be excreted in most animals, except in some desert forms where water is conserved and the excretion is in crystalline form. Water is eliminated by lungs, skin,
brane.

alimentary canal, and kidneys.

In

man

the quantity of sweat dis-

two or three liters a day. In the dog, glands, the water eliminated by the lungs, which has few sweat through panting, is proportionately greater than in man. The kidneys are the most important organs in the excretion of water, and the amount they eliminate is inversely proportional to the amount
charged

may amount

to

excreted by the skin. Most of the water to be excreted from the blood in Malpighiaji corpuscles.

is

taken

Some monium
most

of the nitrogenous wastes are excreted in the form of amHowever, salts and some free or combined amino acids.

of the ammonia which results from protein metabolism is converted into urea in the liver and is carried in that form to the kidneys where it is removed from the set of capillaries ramifying over the convoluted tubules by a process of true secretion. According to this idea, the urine which consists of urea, various salts, other

soluble materials,

uriniferous tubule.
this organ.

and water is excreted by different parts of the The substances which are excreted by the kidney are not formed there, but are merely removed from the blood by

The Nervous System. The nervous system in this type of animal is composed of a hrain and spinal cord forming the central nervous system; nerves extending to all parts of the body, ganglia which are groups of nerve cell bodies outside the central nervous system, and the sense organs which serve for receiving stimuli are usually grouped together under the name peripheral nervous system. A portion of this latter division, consisting of two longitudinal trunks
with ganglia distributed along them, lies parallel to the spinal cord, and constitutes the sympathetic system. Each ganglion has a connection with the adjacent spinal nerve or cranial nerve as the case

400

TEXTBOOK OF ZOOLOGY
be.

might

This system controls the involuntary muscles.

The pe-

ripheral system includes ten to twelve pairs of cranial nerves from

the brain, and ten to thirty-one pairs of spinal nerves in different

forms of vertebrates.
joins the spinal cord.

Each

spinal nerve has

two roots where

it

dorsal root receives fibers from sensory end-

ings and therefore conducts impulses toward the cord.

This root

has a spinal ganglion located on

it.

The ventral

root of each of these

Fig'. 220. Cross section of spinal cord and roots of spinal nerves, sliowing a simple reflex circuit. 1, sensoi-y surface of skin 2, afferent nerve fiber with S, its 5, efferent i, cut end of spinal nerve root of spinal nerve 8, ventral root of spinal nerve dendrites of motor nerve cell body in gray matter of the cord. 9, (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, after Morat.)
;

cell of origin, located in the spinal ganglion nerve fiber 6, voluntary muscle 7, dorsal
; ;

nerves carries fibers extending from the motor


the motor end plates on the voluntary muscle
therefore, pass

cells

in the cord to

The impulses, from the spinal cord to the muscles over these roots. The reflex arc, which is the simplest kind of a nerve conduction circuit, is set up by the connectives from the sense organ or receptor to the gray matter of the cord and then the return connection from the motor nerve cells over the ventral root to the muscles. In gencells.

eral,

the relationship of parts in regard to function

is

similar to

what has already been seen

in the higher nonchordate animals.

THE VERTEBRATE ANIMAL

SUBPHYLUM VERTEBRATA

401

ing, smell, taste,

high development of sense organs for the senses of sight, hearand touch is characteristic of vertebrates. The organs

are receptors and they are stimulated by changes in external environ-

mental conditions, such as light, sound waves, chemical changes, and contact. The eye, which is the organ of sight, is a highly developed organ. It is constructed on the plan of a camera with the eyeball

forming the light-tight box.


transparent front, the cornea.

The wall of

this is

composed of an

outer fibrous sclera (white of eye) which continues anteriorly as a

mented and vascular


as the
iris,

Beneath the sclera is a black, pigwhich continues anteriorly the colored part of the eye. The iris is like a curtain
layer, the chorioid,

C.Vw

crjo.-y\/.

Fig. 221. Diagram of the eyeball; c, cornea; a, aqueous humor; I, lens; v, vitreous humor; sc, sclerotic coat; ch, chorioid coat; r, retina: /, fovea centralis; i, iris; s.L, suspensory ligaments; c.p., ciliary process; cm., ciliary muscles; op.n., optic nerve. (From Zoethout, Textbook of Physiology, published by The C. V.

Mosby Company.)

surrounding a space at the anterior surface of the eye and this space between its medial margins is the pupil. The pupil appears black because there is no light behind it. Behind the pupil is a. transparent lens whose surfaces are curved to bend the rays of light in such a way as to focus them on the sheetlike retina behind. The retina is a lateral extension of the brain and is the sensory part of the eye. It lies as a lining of the inside of the posterior half of the cavity of the eye and is connected directly with the brain by the optic nerve.

The general cavity

of the eyeball is divided into

some chambers. The

402

TEXTBOOK OF ZOOLOGY

external or aqueous chamber between the cornea and the lens, with

the iris extending into


is

it,

is filled

with aqueous humor.


is

This cavity

subdivided by the

iris.

Behind the lens

the large internal or

vitreous

chamber which

is filled

with a jellylike vitreous humor. The


This makes possible an adjust-

curvature of the lens can be controlled by the action of the ciliary

muscle which encircles

its

margins.

ment

and far objects and particularly so in higher As people This power is known as accommodation. vertebrates. they tend to lose this accommodation because of loss of get older The tension on it due to the attachment of elasticity in the lens. the inside of the eyeball by the ciliary process tends to hold it out
of the eye to near

Fig. 222. Diagram of a section through the right ear. B, semicircular canal P, tympanic cavity a, external auditory meatus; o, oval window (fenestra ovale) r, round window Pt., scala tympani containing the three auditory ossicles Eustachian tube; 8, cochlea; T, membrana (fenestra rotunda) below r is seen the tympani; Yt, scala vestibuli. (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, after Czermak.)
;
;

in a flattened condition.

This focuses the eyes very well on distant

objects but does not provide the necessary curvature of the lens
to bring near objects in focus.

Eyeglasses are used by older people

to supply this lost phase of accommodation. A ray of light enters the eye by passing through the cornea, then aqueous humor, pupil, lens, vitreous humor and then to the retina where the sensory cells

are stimulated and the impulse carried to the brain by the nerve
fibers of the optic nerve.

THE VERTEBRATE ANIMAL

SUBPHYI.UM

VERTEBRATA

403

The ear structures provide most classes of vertebrates with facilities for two functions: hearing and equilibrium. This organ consists of an external ear, which serves in catching and directing sound waves within, a middle ear or tympanum, containing ossicles, and the inner ear, which contains the sensory cochlea with its organ of Corti for hearing, and the semicircular canals, which are concerned with equilibrium rather than hearing.
brates while the cochlea

The

latter are

common

to all verte-

is limited to Amphibia and higher classes. The external ear is still further limited to reptiles, birds, and mammals. The middle ear is a space beneath a tympanic memhrane which separates it from the external auditory canal. In this cavity are three bony ossicles, the malleus, incus, and stapes, which transmit the sound vibrations from the tympanic membrane to the membrane The memover the fenestra ovalis leading into the internal ear.

branous labyrinth
inner ear.

is

the

name

often applied to the chambers of the


is

Its ventral

chamber

the sacculus connected with the


is

organs of hearing, and the dorsal portion


related to equilibrium.

the utricidus which

is

forms and the three in

The two semicircular canals in simpler higher, join the body of the utriculus in as
In the higher forms

many

different planes as there are canals.

there are two vertical canals, one anterior


their planes at right angles to each other,

and one posterior, with and one horizontal canal.

At one end

of each canal there

is

a bulblike swelling or ampulla

which contains sensory hairs. When the position of the head is moved, the fluid in the canals changes its level and position to stimulate the sensory hairs, thus giving a sense of position. The sound waves which stimulate the sensory cells of hearing enter the external ear and set up vibrations in the tympanic membrane. These are in turn transmitted by the ossicles to the fluid endolymph within the labyrinth. The vibrations of the fluid extend through the cochlea, in which the sensory cells are supported on the organ of Corti stretched across it. These cells are connected with the brain by way of the auditory or eighth cranial nerve (Fig. 222).

The sense
chamber.
rial

of smell

is

centralized in the epithelial lining of the nasal

Special olfactory cells are stimulated by particles of mateair dissolving


cells.

from the

on this membrane and making contact


of taste
is

with the sensory


is

The sense

similar except that

it

located in sensory cells in taste buds on the tongue, epiglottis,

and

404
lips

TEXTBOOK OF ZOOLOGY
(and barbels of some vertebrates).
of food

The

particles

come

in

by

way

and drink and as the material

dissolves, it reaches

the taste

cells.

and pressure sense organs are located just A few of the pressure sense organs are found in certain of the internal structures of the body. The lateral line system in fishes is sensory to vibrations carried in the water and is quite important to aquatic animals

Most

of the tactile

beneath the skin over different parts of the body.

of this type.

Nervous Function Reception and Conduction. Irritability and conductivity are fundamental functions of all protoplasm, whether The responsiveness of it be in the body of an Amoeba or a man. organisms to change of conditions both externally and internally
determines their behavior. but
it

Living protoplasm
to record or store

is

not only excitable,


effects of previ-

possesses the

power

up the

ous stimuli.

In the

final analysis, the

perceptions and reactions of

man

are but expressions of these primitive functions in a

more

specialized organism.

The protozoan organism has only neuromotor apparatus and depends largely on the primitive properties of irritability and conductivity to guide its activities.

In the simpler Metazoa, such as the co-

elenterates, there are scattered nerve cells connected with each other

form a nerve net. The neuroepithelial or neurovmiscular which make up this continuous net through the body are the forerunners of the typical neurone and are called protoneurones by Parker. A protoneurone transmits in every direction while a true neurone transmits in only one. In the net system there is no central exchange and no specific path of conduction. Every part of the receptor surface of such an organism is in physiological continuity
fibers to
cells

by

with every other part of the body.

ladder.

Next comes the linear type of nervous system in the form of a It is composed of an organization of neurones into a double

chain of ganglia, each cord lying lateral to the digestive tract with
transverse connectives and predominant ganglia at the anterior end.

Such a system was studied


the nervous sj'stem
is

in planaria.

In Annelida and Arthropoda

a modified ladder type in which the two longiganglia have fused along most of the midventral tudinal cords of Toward the anterior end, the cords separate at a paired ganline.

THE VERTEBRATE ANIMAL


glionic enlargement,

SUBPHYLUM

VERTEBRATA

405

the

siil)

esophageal ganglion, and encircle the

alimentary canal to join on the dorsal side as the pair of siipraesophageal ganglia or ''brain."

In Arthropoda the ganglia of the In Echinodermata, the

thorax have undergone considerable fusion.


starfish for

example, the central group of ganglia makes up the circumoral nerve ring around the mouth, and radial branches extend

into each arm.

Branches from these communicate with the sensory

structures of the skin and tube feet.

Concentration of the tissue of the nervous system into definite

organs
brates

is

carried farther in vertebrates than in the less highly or-

ganized forms.
is

The

fact that the central nervous system of verte-

dorsally located and hollow has been brought out previously.

Even within
the

the group of vertebrates, the nervous system shows a

progressive increase in complexity.

The highly developed brain

of

mammal

is

the climax of this tendency.

The neurones have been referred to before as the units of structure and function of the higher type of nervous system, from worms to man. Each neurone is a nerve cell with processes extending from it, and each of these units must conduct nerve impulses in its normal function. The exact nature of the nerve impulse is still somewhat
of a question.
It is

thought to be transmitted as a metabolic change


This
is

passing along the nerve fiber (axone or dendrite).

at least
cer-

partially a chemical change in which oxygen is necessary


tain

and a
is

amount of carbon dioxide

is

produced, but since there


it

only

slight increase in

temperature during the change,


;

seems not to be

a typical metabolic oxidation process


not to fatigue the nerve
fiber.

furthermore, the activity seems

An

electrical
it

of activity along the nerve fiber, but

charge follows the wave apparently accompanies the


itself.

impulse or

is

a result of

it

rather than the impulse

The speed
of different

of electrical transmission has been measured in a

number

animals and nervous transmission

is

much

slower than electrical.

At

room temperature the

sciatic

nerve of a frog will transmit a nerve

impulse at the rate of about 100 feet per second. Conduction over nonmedullated fibers of invertebrates is much slower than this. On the other hand, measurements of the rate of conduction in man show a velocity of about 400 feet per second.

The

reflex arc

and

reflex actions illustrate the simple

form of nervis

ous conduction circuit.

In

its

simplest form the reflex arc

com-

406

TEXTBOOK OF ZOOLOGY

posed of one motor and one sensory neurone; however, it is usuallymore complex. The classical example involves the spinal cord and a spinal nerve. This is known as a reflex of the first level, because
it

returns the motor impulse over the motor fibers of the same nerves which brought in the sensory impulse. The motor axone carrying
cell in

the impulse from the motor nerve

the gray matter usually


is

ends in a muscle cell or a gland. There between the axone of the sensory neurone and the dendrite of the motor, for these come in contact only by a synapse which brings them It has been found experimentally that nervous in close proximity.
be conducted in either direction by the fiber but can cross a synapse only from axone to dendrite, thus serving like a valve in a pipeline. Reflex actions may be in the form of motion, as with-

no protoplasmic union

impulses

may

drawal from unexpected pain, or shivering or formation of goose flesh, or the contraction of the pupil of the eye with increased light
intensity.
Still

other reflex actions include secretion by glands,


of speech, individual actions included in walk-

breathing, movements ing, and others.

Functions of the Spinal Cord.


reflex centers

This organ serves as a system of

which control the actions of glands of the trunk, visceral organs, and skeletal muscles. The spinal cord is also a nervous pathway between the brain and numerous organs of the body. It is said that more than half a million neurones join the cord
through the dorsal roots of the spinal cord.
Functions of the Divisions of the Brain. Conscious sensations and intelligence are centered in the gray matter or cortex of the This section controls voluntary actions and provides cerebrum.

memory

The diencephalon serves as a center for sponThe midbrain is one of the centers of coordinated taneous actions. movement which has to do with posture and eye muscles. The cereassociations.

another center of coordinated movement, particularly with reference to equilibrium. The impulses from the muscles, tendons, joints, and semicircular canals of the ear are coordinated so that in a movement or posture the proper muscles may be contracted to the proper extent at the proper time. Below and behind the cerebellum is the medulla oblongata which controls breathing and may be an
bellum
is

inhibitor on heart action.

It

also regulates digestive secretions,

movements

of digestive organs,

and vasomotor activity of the blood

THE VERTEBRATE ANIMAL


vessels.

SUBPHYLUM VERTEBRATA

407

As a

Avhole, the brain serves as the

organ of coninuinication
is

between the sense organs and the body and


bodily activities.

the coordinator of the

The Reproductive System. The vertebrate reproductive system shows a fairly high degree of development. The sexes are almost universally separate, with the exception of some cyclostomes. The distinct gonads develop to produce special germ cells. The male gonads are testes, and they produce spermatozoa which are carried from the gonads by the vasa deferentia. The female gonads are ovaries, and they produce ova or eggs. They are carried from the body by oviducts. The males of some classes possess for use in copulation certain accessory organs which tend to insure fertilization. The vertebrates which lay eggs are spoken of as being oviparous; in those in which the egg is retained in the body and the embryo develops there, feeding on the yolk of the egg, and is later born alive, the condition is known as ovoviviparous, and in the forms in which the fertilized ovum is retained in the uterus, the embryo being nourished by diffusion of nutriments from the blood of the parent, the condition is said to be viviparous, and here too the young are born alive. In vertebrates the possible offspring produced each season by a single individual varies from one to thousands.
Reproductive Function.
rial

A living

organism

is

in

numerous ways

similar to a machine, but reproduction of

new

units of living mate-

is hardly comparable to any mechanical our industries. New organisms all arise from preexisting organisms of the same kind. The process of cell divi-

by existing organisms

processes

known

in

sion

is

the fundamental basis for all reproduction.

For centuries

before the invention of the microscope it was commonly believed that living things arose spontaneously from nonliving material, or

from the dead bodies of plants and animals.

Certain old books carry directions for the artificial generation of mice or bees. Louis Pasteur did as much as anyone to discredit this idea of spontaneous
generation.

Our present conception

is

that the protoplasmic sub-

stance of the

but a continuation of the specific protoplasm peculiar to an earlier individual or in sexual reproduction to two individuals. Therefore, under ordinary circumstances the structural and physiological complexities which arise through
individual
is

new

embryonic development must be generally similar to those of the


predecessors.

408

TEXTBOOK OF ZOOLOGY

In most of the single-celled organisms reproduction

may

occur

by such equal division

of the

protoplasm (binary

fission) that the

new
cell

individuals cannot be distinguished as parent and offspring.

may reproduce also by sporulation, by which process the forms a protective cyst and by a series of simple divisions (fragmentation) the internal protoplasm breaks into a number of smaller Following this the cyst ruptures and releases these new units units.
Protozoa
as independent individuals.

For the most

part, reproduction

among

protozoans
lished

is

taken to be asexual, but according to a recently pubin

work by Sonneborn, a distinct sexuality exists Examples of asexual reproduction by budding and

Paramecium.
have
al-

fission

ready been pointed out in the studies of reproduction of sponges,


hydra, planaria, and even in tunicates.

Sexual Reproduction.

In certain of the colonial Protozoa, volvox


may
reproduce for several generations by
cells

for example, the colony

asexual division of the individual

but sooner or later the

cells

of the colony become specialized into conjugating individuals.

In

some forms this goes to the extent of certain cells becoming distinct gametes with male and female characteristics. In such forms it is possible to see foreshadowed sexual reproduction as it is known in
Metazoa. In the simplest of Metazoa, as in sponges, there are no specially
organized gonads for the production of germ
cells,

but as a rule the

germ cells are produced in such organs set apart for this purpose. The ovary produces mature or nearly mature ova and the testis produces mature spermatozoa.

Hermaphroditism

is

the condition in which the same individual

produces both ova and spermatozoa. It occurs principally in the simpler Metazoa, a few higher ones, and rarely among normal vertebrates. Previous studies made on the reproduction of hydra have brought out that the gonads are temporary, both being formed by aggregations of formative or interstitial cells between the ectoderm and endoderm. After the seasonal production of germ cells is completed, the gonads disappear. In flatworms and annelid worms the

gonads are permanent structures of the mesoderm. Both ovaries and testes are present. Even in these true hermaphrodites cross-fertilization

insured by copulation or union in such a way that the spermatozoa of one individual fertilize the ova of another. In certain
is

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

409

other hermaphroditic forms (as some cyclostomes) the spermatozoa

and ova of a particular individual are usually not mature at the


same
time.
is

Bisexual reproduction

the form of reproduction


all

common

to

many

groups of the higher invertebrates and nearly


distinct,

vertebrates.

Here the sexes are

each with functional gonads abd ac-

cessory structures capable of producing only one kind of

germ

cells.

In some of the types of animals already studied individuals of the

two sexes have simply deposited the mature germ cells in the same Under the sections on revicinity and at about the same time. (fish) such a procedure has production in starfish and the bullhead been described. In animals like the toads and frogs, a special provision
is

made

to

bring the individuals of the opposite sexes to-

male clasps the female and sheds sperm over the eggs as they are expelled from the cloaca. This act is known as amphiplexus. It will be remembered that the first and second pairs of abdominal appendages of the male crayfish are modified for transferring spermatozoa into the seminal receptacle of the female, where
gether in that the

they remain until the eggs are


or dioecious animals

laid.

This represents a beginning in

the development of a copulatory organ.

The majority of bisexual


fertili-

make

still

greater provision to insure

zation of the ova by copulation or coitus.

At the time

of breeding

the mature spermatozoa are delivered to the cloaca or vagina of tha

female,
female.

and the ova are

fertilized within the genital tract of the

In birds and most reptiles after the addition of nutritive and protective coats the

egg passes to the outside to develop and hatch (ovipall

arous animals) but in


the

mammals, except monotremes,


less

it is

retained

within the uterus during the period of embryonic development, and

young are

bom

as

more or

developed individuals (vivipa-

rous).

In the females of viviparous mammals the posterior portions

young The internal wall of the uterus and the external embryonic membranes (serosa and allantois-chorion) cooperate to form a placenta through which food, metabolic wastes, and respiratory gases diffuse between parental and embryonic blood. The blood does not pass from parent to embryo
of the two oviducts are modified into a uterus within which the

are retained and nourished until ready for birth.

410
or vice versa but
tlie

TEXTBOOK OF ZOOLOGY
necessary materials are allowed to diffuse
dis-

through the tissue of the placenta in which both systems are


tributed.

Parthenogenesis.

In

some species

of invertebrates,

sexual re-

<

production

may

lapse for considerable periods of time, during which

period no males are developed.

The female produces ova which


as parthenogenesis.

develop into
fall of the

new

individuals like herself without fertilization for


is

a whole season.

This

known

Usually in the

year males are developed, and fertile eggs, provided with protective hard shells, are produced by the females of this generation After winter is over such fertile eggs to live through the winter. hatch into parthenogenetic females for the next season.
is

This process

common

in

many

smaller Crustacea, aphids, scale insects, some ants,

bees, wasps, thrips,

a few moths, and

rotifers.

Artificial partheno-

genesis

may

be induced in

many mature

eggs by change of osmotic

pressure due to change of salt content in the surrounding medium. Fatty acids, saponin, solanin, bile salts, benzol, toluol, chloroform,
ether,

and

alcohol are other substances which will induce

it.

Electric

and change of temperature are also used. Such methods have produced artificial parthenogenesis in eggs of sea urchins, starfish, molluscs, annelids, moths, and frogs. The immediate cause of the development by an egg thus stimulated is not known. In normal fertilization of an egg by only one spermatozoon, it has been found that the rate of oxidation then increases from 400 to 600 per cent. There are indications that this is also the case in
stimulus, mechanical pricking,

be the cause of the development in the ovum. Fertilization, where it occurs, has a dual function: (1) that of stimulating the egg to develop, and (2) that of introducing the properties of the male parent.
artificial

parthenogenesis.

This oxidation

may

Classification

In most recent classifications this subphylum is divided into seven classes however, the second is sometimes found as a subclass under
;

the third.

These classes are as follows:

fish

Cyclostomata (si klo sto'ma ta, circle and mouth). Round-mouthed with only median fins, unsegmented notochord, and jawless.

Lampreys and Hagfish.

THE VERTEBRATE ANIMAL

SUBPHYLUM

VERTEBRATA

411

Elasmobranchii (e las mo bran'ki i, metal plate and gills). Pish with jaws, cartilaginous skeleton, persistent notochord, and plaeoid scales. Sharks, Rays, and Chimaeras.
Pisces (Pis'es, fishes).
tion,

True fish with bony skeleton, gill respirawith jaws and paired lateral fins. Catfish, Perch, Bass.

Amphibia (am fib'i a, both lives). Cold-blooded, nonscaled aquatic and terrestrial vertebrates with five-fingered, paired appendages. Most of them breathe by gills in the larval stage and by lungs in the adult. Toads, Frogs, and Salamanders.
Reptilia (rep
til'i

a,

crawling).

Cold-blooded forms which are

fundamentally
ing by lungs.

terrestrial, usually possessing

Turtles, Lizards, Snakes,


.

a scaly skin and breathand Crocodiles.


feathers.

Aves (a'vez, birds) Warm-blooded, erect forms possessing The forelimbs have become wings. All birds.

Mammalia (mama'lia, mammary


tebrates with hair and with
Cats,

or breast).

Warm-blooded

ver-

mammary

glands for suckling the young.

Men, Monkeys, Whales,

Seals, Bats, etc.

CHAPTER XXV
CYCLOSTOMATA*
Because of the absence of jaws this group is sometimes known as Agnathostomata (ag nath o sto' ma ta). This name is in contrast

Gnathostomata (jaw mouth) which includes all other vertebrates. The mouth of the cyclostomes is round, jawless, and suctorial. There are some exoskeletal teeth located on the roof and floor of the mouth and on the tongue. The body is slender and eel-like in shape. It is covered with a slippery, smooth skin and has only dorsal and
to

ventral median

fins.

Classification

The group is divided into two subclasses (or orders according to some authors) distinguishable by presence or absence of tentacles around the mouth, number of gill slits, and the number of semicircular canals. These subclasses are Myxinoidea (Hyperotreti) including the hagfishes; and Petromyzontia (Hyperoartii) including

lamprey (or i