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INSTRUCTOR'S COPY

Compliments
of

THE C. V. MDSBY COMPANY

ST. LOUIS. MO. and SAN FRANCISCO, CAL.

SENT AT THE REQUEST
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MR. FRANK A. VOLK

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TEXTBOOK ^

ZOOLOGY

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

THE C. V. MOSBY COMPANY

1947

Printed in U. S. A.

Press of

The C. V. Mosby Company

St. Loviis

DEDICATED TO
PROFESSOR FRANK A. STROMSTEN

A friend and an inspiration
to the student

PREFACE TO SECOND EDITION

The present edition represents a revision of certain parts of sev-
eral chapters, such as those dealing with Annelida, Genetics, Eugenics,
Internal Regulation and Endocrines, Physiology, and Phylogenetic
Relations of Animals. A brief section on Mammalian Development
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),
for numerous useful suggestions. Many valued suggestions have
been received from individuals in a number of other institutions where
the first edition has been in use. The author's indebtedness and
appreciation is also 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 co-
operation in numerous ways to assist in making this revision.

George E. Potter.
College Station, Texas

f-i

PKEFACE TO FIRST EDITION

The important problems of life are common to all animals (includ-
ing 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. It is exceedingly difficult to
strike the ideal balance between the necessity of presenting sufficient,
factual, "type" material in order that the student will have the
requisite knowledge of classification, structure, function, development,
and organography to appreciate the discussion of principles, and the
opposite temptation to go into endless discussions of theories and
rules, the comprehension of which is unquestionably beyond the
capacity of the student 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. Ap-
parently 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 cer-
tain of these principles. In order to bring this out there must be a
rather close coordination between the studies of the laboratory and
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 upon particular phases of the topics taken up in the course. The
anticipation of this and limitations of space have reduced the volume
of detailed information included.

Many animals from west of the Mississippi River are featured in
this 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
utilized. It is hoped this wiU make the book more useful and mean-
ingful to students in these regions, as well as more teachable.

The introduction of chapters on Animal Anomalies, Animal Re-
generation, Biological Effects of Radiation, Marine Zoologij, and
Wildlife Conservation is a slight departure from the usual textbook
outline, but each of these seems to the author to have enough of
special value and current interest to warrant presentation. The chap-
ters on Regulatory Glands, Animal Distribution, The Animal and Its
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 some-
what 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
the book the genus and species names have been italicized, and many
names of structures and functions have also been italicized the first
time they occur.

The author is indebted and extremely grateful for the cooperation
of several teachers and specialists who have contributed manuscript
for chapters in their fields. For this service acknowledgment is made
to: J. Teague Self, University of Oklahoma, Annelida; Elmer P.
Cheatum, Southern Methodist University, Mollusca, and assisted with
Marine Zoology; Vasco M. Tanner, Brigham Young University,
Arthropoda; Mary Fielding, Public Schools, Waco, Texas, collabora-
tion on Elasmohranchii; Rose Newman, Baylor University, collabora-
tion on Pisces; Ottys Sanders, Southwestern Biological Supply Co.,

8 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 credit for the excellent
art work he has put into this edition. Dr. Titus Evans, of the
University of Iowa, Medical College, has also been of great service
with his excellent talent in creating illustrations. Mrs. Ruth M.
Sanders, Miss Joanne Moore, and Mr. Edward O'Malley have each
assisted by contributing certain illustrations. The drawings used in
Chapter XLII on Genetics and Eugenics were made by Miss Betty
R. Smith of Cornell College. The author is grateful to all of these
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

CHAPTEE I PAGE

Introduction ___________________ 17

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 An-
imals and Plants, 27; Attributes of Life, 30; Balance in Nature, 31;
Zoology as Belated to Man, 33; Agriculture and Zoology, 34; Fish-
eries and the Application of Zoologj-, 34.

CHAPTER II
History op Zoology _________________ 36

CHAPTER III
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 _____________ 65

Characteristics, 65; Classification, 65; Colonial Protozoa, 75; Tropisms
and Animal Reaction, 77; Economic Relations of Protozoa, 77.

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.

CHAPTER VI

Amoeba op Ck<vss Sarcodina ______________ 85

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

CHAPTER VII

Paramecium of Class Inpusoria _____________ 93

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

6164.3

10 CONTENTS

CHAPTER VIII PAGE

Metazoan Organization __________----- 103

General Characteristics, 103; Cellular Differentiation, 104; Cellular
Organization, 105; Development of Sexual Reproduction, 111; Meta-
zoan and Ontogeny, 113.

CHAPTER IX
Phylum Porifera _____-___-------- 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 Protozoa, 129.

CHAPTER X
Phylum Coelenterata ______---------- 130

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

CHAPTER XI

Phylum Ctenophora ______---------- 157

Habitat and Behavior, 157; Anatomy, 157.

CHAPTER XII

Phylum Platyhelminthes ____---------- 160

Classification, 160; Planaria, 163; Habitat and Behavior, 163; Ex-
ternal Anatomy, 165; Internal Anatomy, 165; Metabolism, 170; Re-
production and Life History, 170; Regeneration, 173; Economic Rela-
tions of the Phylum, 174; Phylogenetic Advances of Platyhelminthes,

174.

CHAPTER XIII

Phylum Nemathelminthes _____---------175

Classification, 175; Asearis, A Representative Roundworm, 179; Habi-
tat and Behavior, 179; External Anatomy, 179; Internal Anatomy,
181; Reproduction and the Life Cycle, 183; Relations to Man, 183.

CHAPTER XIV

Molluscoida, Trochelminthes, and Chaetoonatha ___---_ 184
Molluscoida, 184; Bugula, 184; Trochelminthes, 188.

CHAPTER XV

Phylum Annelida (By J. Teague Self) ___-__----- 194
Earthworm, 199; Internal Anatomy, 201; Reproductive Organs, 202;
Digestive System, 203; Circulatory System, 205; Respiratory System,

CONTENTS 11

PAGE
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.

CHAPTER XVI

Phylum Echinodermata _______________ 21 7

Classification, 217; Starfish of Class Asteroidea, 226; Habitat and Be-
havior, 226; External Anatomy, 226; Internal Anatomy, 227; Repro-
duction and Life Cycle, 234; Regeneration and Autotomy, 234;
Economic Relations, 235.

CHAPTER XVII

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 ; Reproduc-
tion 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.

CHAPTER XVIII
Phylum Arthropoda ________________ 263

Classification, 263 ; Crayfish of Class Crustacea, 266 ; Habitat and
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 (Cont'd) (By Vasco M. Tanner) ______ 287

Onychophora and Myriapoda, 287; Onychophora, 287; Myriapoda,
288.

CHAPTER XX

Phylum Arthropoda (Cont'd) (By Vasco M. Tanner) ______ 292

Arachnida, 292; Spiders, 292; Classification of the Arachnida, 295.

CHAPTER XXI

Phylum Arthropoda (Cont'd) (By Vasco M. Taimer) ______ 300

Class Insecta, 300; Insect Characteristics, 301; Head, 301; Thorax,
305; Abdomen, 307; Body Wall, 308; Metamorphosis, 308; Classifica-
tion, 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.

12 contp:nts

CHAPTEE XXII p^^j^

Representative Insects (By Vasco M. Tanner) ________ 343

The Locust, 343; The June Bug, 354; The Honey Bee, 357.

CHAPTER, XXIII
Phylum Chordata _______-____-_--- 360

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

CHAPTER XXIV

The Vertebrate Animal: Subphylum Vertebrata _______ 375

Classification, 410.

CHAPTER XXV

Cyclostomata __________________ 412

Classification, 412 ; Economic Relations of the Class, 413 ; The Lamprey,
413; Habitat, 413; Habits and Behavior, 415; External Structure,
415; Internal Structure, 415.

CHAPTER XXVI

Elasmobranchii __________________ 422

Classification, 422; Economic Relations of the Class, 425; The Spiny
Dogfish, 426; External Features, 426; Muscular System, 427; Skeletal
System, 427; Digestive System, 430; Circulatory System, 431; Respira-
tory System, 435; Nervous System, 435; Urinogenital System, 437;
The Eonnethead Shark, Reniceps (Sphyrna) Tiburo Compared to
Squalus, 439.

CHAPTER XXVII

Pisces, True Fish _________________ 442

Classification, 445; Economic Relations of the Class, 455; Typical
Bony Fish — 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) ___________ 472

Classification, 482; A List of Families of the Amphibia in the United
States, 484; Order Caudata (Urodela) (Tailed Amphibians), 484;
Order Salientia (Anura) (Tailless Amphibians), 485; Economic Im-
portance, 486; Necturus Maculosus, the Mud Puppy, 487; Food and
Digestive System, 489; Circulatory System, 490; Respiratory Sys-
tem and Breathing, 491; Urinogenital System, 492; Skeletal System,

CONTENTS 13

PAGE
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 Ex-
cretion, 515; Skeletal System, 517; Muscular System, 523; Nervous
System, 525; The Sense Organs, 528; Reproductive Organs, 531;
Embryology, 532; The Toad, 538; Habitat, 538; External Features,
539; Internal Structure, 541; Respiratory and Digestive Organs,
541 ; Urinogenital Organs, 541 ; Blood Vascular System, 542 ; Skeleton
and Muscles, 543 ; Nervous System and Sense Organs, 543 ; Em-
bryology, 544.

CHAPTER XXIX

Reptilia (By Leo T. Murray and James E. Blaylock) ______ 545

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-
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.

CHAPTER XXX

AVES _____________________ 582

Classification, 584; Economic Relations, 596; Domestic Chicken, 598;
Habits and Behavior, 598; External Structure, 599; Digestive Sys-
tem, 601; Respiratory System, 603; Circulatory System, 604; Excretory
System, 606; Nervous System, 607; Skeletal System, 609; Muscular
System, 612; Reproduction and Life History, 613.

CHAPTER XXXI

Mammalia _________-__-__----- 616

Classification, 616; Economic Relations, 637; The Cat, A Representa-
tive Mammal, 639; External Structure, 639; Skeleton, 641; Muscu-
lar System, 644; The Digestive System, 646; Circulatory System
648; Respiratory System, 649; Nervous System, 650; Excretory
System, 650; Reproduction and Life History, 652.

CHAPTER XXXII

Animal Anomalies _________________ 654

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

14 CONTENTS

CHAPTER XXXIII „.„„

PAGE

The Endocrine Glands and Their Functions _________ 666

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

CHAPTEE XXXIV

Regeneration (By T. C. Byerly) _____________ 681

Introduction, 681; Regenerative Capacity, 681; Protozoa, 681; Porif-
era, 682; Coelenterata, 682; Platyhelminthes, 683; Annelida, 684;
Mollusca, 686 ; Arthropoda, 686 ; Echinodermata, 686 ; Chordata,
687; Amphibia, 688; Reptilia, 690; Aves, 690; Mammalia, 690;
Basis for Regeneration, 692; Adaptability and Regeneration, 695;
Summary, 696,

CHAPTER XXXV

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

^ mental Action of Roentgen Radiation, 707; Biological Action of
Radium, 708; Effects of Other Radiations, 709; Summary, 710.

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) ______ 719

The Principal Biotic Formations, 724; Adaptation, 727; Succession,
727; Animal Populations, 730; Seasonal Changes, 733; Summary, 733.

CHAPTER XXXVIII

Animal Parasitism (By Sewell H. Hopkins) _________ 735

Social Relations of Animals, 735; Origin of Parasitism, 736; Degrees
of Parasitism, 736; The Successful Parasite, 737; Means of Infec-
tion and Transmission, 740; Parasitism and Host Specificity, 741;
Parasites and the Groups in the Animal Kingdom, 742; Some Repre-
sentative Parasites, 747.

CHAPTER XXXIX

Marine Zoology -.._______-________ 766

CONTENTS 15

CHAPTER XL

PAGE

Wildlife Conservation (By Walter P. Taylor) ____----- 78-1
The Abundance of Wild Animals, 784; The Natural Range of Wild
Animals, 789; The Coming of Civilization and a Declaration of Inde-
fensibles, 792; The Problem of Restoration, 794.

CHAPTER XLI
Comparative Embryology (By A. Richards) _____-__- 798

CHAPTER XLII

MAMMALLA.N DEVELOPMENT __________--_-- 812

Organs and Systems, 817.

CHAPTER XLIII

Genetics and Eugenics (By Frank G. Brooks) ______-- 821

The History of 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;
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
Done? 844; Some Eugenic Measures, 844.

CHAPTER XLIV

Animal Behavior (By Ina Cox Gardner) ____-_---- 846
Introduction, 846 ; Tropistic Behavior, 849 ; Reflex Behavior, 850 ;
Chain Reflex Behavior, 851; Habitual Behavior, 852.

CHAPTER XLV
Paleontology (By W. M. Winton) _______----- 854

CHAPTER XLVI

Phylogenetic Relations op Animal Groups and the Theory of Evo-
lution _____-___-------- 863

Colony Formation in Certain Protozoa, 864; Development of the Gas-
trula, 865; Trochophore Larva, 865; Peripatus and the Wormlike
Ancestry of Arthropoda, 865; Eehinoderms and Their Larval Rela-
tions, 866; Ancestry of the Vertebrates, 866; Basis for the Theory
of Evolution, 870; Darwin and Studies of Evolution, 885,

TEXTBOOK OF ZOOLOGY

CHAPTER I
INTRODUCTION

In whichever direction we turn or wherever we go, whether in
the air, on land, or in the sea, we are surrounded by living creatures.
Their very presence presents problems and fills us with curiosity.
We ask questions. From whence do they come? What is the
source of their energy? Why are there so many different kinds?
What is our relation to other living things? What is life? Such
questions and endless numbers of similar ones kindle the interest
of every thinking person. The constant endeavor on the part of
man to answer these questions and solve the problems of the origin
and nature of life has given us the field of study known as biology.

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

The Biological Point of View

Nature is ever inviting investigation ; her forces are in constant
operation about us, but she hides the truth. The biologist looks
upon himself as a seeker after truth, as one striving to get a glimpse
into the mysteries of life. As he succeeds in obtaining these
glimpses, he soon realizes the existence of certain fundamental
features common to the structure and function of all living forms.
He 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 it provides a true working basis
for further investigation.

18 TEXTBOOK OF ZOOLOGY

Science and the Scientific Method

"Trained and organized common sense" was the definition of
science given 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 con-
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 with frequent repetition and by numerous observers.
Science lays its foundation on accurate observations and depends
on the ability of the senses to reveal the truth. Established facts
rei^resent truth, and the scientist respects truth while to him tradi-
tion or mere opinion counts for little as such.

There is nothing mysterious in the scientific method, although the
steps are often tedious and much involved. The method is sim-
plicity itself; to observe, to identify by comparison, to experiment,
to coordinate, to deduce, to conclude. The scientist is not a ma-
gician Avho can draw his conclusions from thin air as Thurston
seemed to produce almost anything imaginable. Complete and ac-
curate 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 develop-
ment of the microscope and many other instruments. Experiments
are devised to bring to light the features not readily revealed by
direct observation.

After the facts are thus established, the qualities of the thing ob-
served 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 pre-
viously established facts, and, on the basis of the relationships of
qualities, deduction may be made of the principles involved.

It is 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, ob-
servation, and experimentation, the hypothesis becomes a theory.

INTRODUCTION

There is always a considerable volume of evidence which supports the
theory and gives all indication that it is a true statement. Finally,
the theory advances to a principle or law after it has been so thor-
oughly and critically tried as to be generally accepted and assumed
as a truth. This process requires the accumulation of the combined
restilts of numerous investigators over a long period of time. To
many people truth is absolute, not relative, and a conclusion once
drawn is fixed and may not be withdrawn for any reason. In
science conclusions are always subject to modification or even
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
principles have remained valid. However, at any time sufficient
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
animals. The natural sciences, as distinguished from the social
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, which are concerned with living organisms. Zoology and
botany together constitute the science of biology. The expression
animal hiologij is often used as a synonym for zoology. A person
who specializes in the study of zoology is known as a zoologist. There
was at one time an erroneous popular impression that zoologists were
simply "bug-hunters." This conception of the field has been greatly
expanded until now it is considered one of the valuable and serious
fields of science.

The Subdivisions of Zoology

Although zoology is only one of the divisions of the general field
of biological science, it is such a broad field in itself that it is neces-
sary to subdivide it into several divisions for convenience in study.
It has been a relatively short time since all of the known biology,
geology, and related subjects were studied under the head of natu-

TEXTBOOK OF ZOOLOGY

ral history. But now the subject matter of zoology alone has grown
to such 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
bodies of animals. Its organization as a special field of study oc-

Fig. 1. — Divisions of study in the field of biology. (Modified from several authors.)

curred at about the beginning of the nineteenth century. It is now
further divided into several branches.

A. Gross anatomy, which literally means cutting up, includes all
that 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 study of the form and structure of the other animals and these
in turn are compared, finally, with the human anatomy. The dis-
section, 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. The histologist
studies the relationship and arrangement of the cells as they cooper-
ate to comprise the substance of the organism.

C. Cytology is the study of the minute structure of the cells which,
we will learn, are the units of structure of all living matter. Cytol-
ogy, 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-
tion or orderly arrangement of organisms according to their natu-
ral relationships. This field is often spoken of as systematic zool-
ogy. The number of described species of animals as given by dif-
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
extinct forms. It can readily be seen that a system for putting
these large numbers of different kinds of animals into a known
order is one of the first prerequisites for dealing with them. On a
much smaller scale, the department store is systematized for some
of the same reasons. One can see that it would be next to impos-
sible 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
on its floors at 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
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

price are all orderly arranged. In this 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 similari-
ties, 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 king-
dom based on certain general characteristics. Each of these divi-
sions is known as a phylum and is divided into classes, each class
is 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 authorita-
tive description of the particular species. This always gives a
double name to a kind of animal, and for that reason it is the
hinomial system of nomenclature. This system was originated by
Linnaeus. The names are in Latin instead of common vernacular
because Latin is a constant 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
Russian as it is in English.

3. Physiology is the study of the functions of the various parts
of the organism as well as its living process as a whole. It involves
a consideration of metabolism, growth, reproduction, sensitivity,
and adaptation. In this field is included the study of many special
functions, such as digestion, circulation, respiration, excretion,
glandular secretion, nervous activity, muscular contraction, and

INTRODUCTION

others. Many of the processes which occur in the developing em-
bryo are also included here. Much of the present study referred
to as cytology is physiological. Physiology, like morphology, is an
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.

4. Pathology is the study of the abnormal structures and abnor-
mal functioning of life processes. It is really the science of disease
in all of its manifestations. There is a fundamental similarity in
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.

5. Embryology is a study of the origin and development of the
mdividual. It usually involves the changes occurring in the or-
ganism from the time of fertilization by the union of two cells, one
derived from each parent, through the numerous cell divisions,
growth, organization, and differentiation leading to the adult con-
dition. This process includes both morphological and physiological
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.

6. Genetics is the division which deals with the study of varia-
tions, resemblances, and their inheritance from one generation to
the next or from parent to offspring. The characteristic features
of an animal or plant may be transmitted to the offspring somewhat
independently of one another, bringing about a variety of combina-
tions in the progeny. Fairly definite laws governing this inherit-
ance 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 and relationships of the
different groups and races of organisms. It is based on the results
of studies of morphology, embryology, genetics, zoogeography, and
paleontology.

24 TEXTBOOK OF ZOOLOGY

8. Ecology is a study of the relation of the organism to its en-
vironment. Many adjustments in structure and function have been
made by animals to bring them into harmony with the conditions
of the environment. Such conditions as the relation of the organ-
ism to the medium in which it lives to temperature, to light, to
food, to competition, to enemies, to mating, and many other fac-
tors, all become a part of an ecological study. This study usually
draws somewhat upon a knowledge of all branches of biology. This
branch of the field has become prominent in comparatively recent
3"ears.

9. Zoogeography or geographical distribution of animals is con-
cerned with the extent of the regions over Avhich species are dis-
tributed and the association of species in individual regions. In
some respects this field is closely related to ecology. It is con-
cerned with the regions in which species exist and with the factors
affecting their distribution. 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 known as barriers. Oceans, mountains, forests,
deserts and land are all 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 occupy
a suitable region usually means that it has been unable to reach
that region, perhaps because of the topography of the region, its
geological history, or the remoteness of the place of origin of the
species. The English sparrow, which originated in Europe, was not
found in America until after it was introduced by man, and in
relatively few years it became a dominant bird.

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
the rock strata in which they are found. Many of the probable
lines of descent of animals have been discovered by studies of the
fossils. Much concerning the facts and the fate of extinct species
has been learned through this field of study. Paleozoology is ordi-

INTRODUCTION

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 10,000,000. To date, approximately 840,000 species have been

Cbordata
(Frog)

Annelida
{Lumbricus)

Bryo^oa
(Bugulaj

Rotifera

Platyhelminthes
(Tapeworm)

/^rthropoda
(Crayfbh)

MoUusca
(Snail)

Echinodermata
{Starfish)

NemathMnthes
{Ascahs)

Fori f era
(5cypha)

Coelenterafca
(Hydra)

Ctenophcra

(Be roe)
sea walnut

Protozoa
(Paramecium)

ANIMAL KINGDOM

Fig. 2. — Phylum relations in the animal kingdom.

named and described. In order that different forms of animals may
be known and definitely recognized, a scheme of grouping related
kinds has been devised.

The entire kingdom is divided into two subkingdoms: Protozoa,
or all single-celled animals, 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,
family, genus, and species. The principal phyla are listed and de-
scribed briefly:

Phylum Protozoa. — Individuals consist either of a single cell or of
aggregates of cells, by each of which are performed all the essential

26 TEXTBOOK OF ZOOLOGY

functions of life. They are mostly microscopic in size and largely
aquatic in habit. Some live in the ocean, some in fresh water, others
in soil water, and still others as parasites in man and other animals.
About 15,000 are known.

Phylum Porifera (Sponges). — Aquatic metazoans which live at-
tached. Most of them are marine. The body is supported by
fibrous, calcareous, or siliceous spicules, and the body wall is per-
forated by many pores. There are approximately 3,000 known
species.

Phylum Coelenterata (Jellyfish). — All are aquatic and most of
them are marine. They possess radial symmetry, a single gastro-
vascular cavit.y, and tentacles provided with stinging bodies,
nemato cysts. The described species number at least 4,500.

Phylum Ctenophora (Sea "Walnuts or Comb Jellies). — Free swim-
ming, delicate, marine 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 Platyhelminthes (Flatworms). — These are flat, unseg-
mented, bilaterally symmetrical, triploblastic worms. "Flame
cells" are characteristic excretory structures. These animals may
be free-living or parasitic. Tapeworms, liver flukes, and the free-
living, aquatic Planaria are commonly known. Approximately
6,500 species have been described.

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 para-
sitic. The hookworm, ascaris, and the "horsehair worm" are com-
mon representatives. About 3,500 species are known.

Phylum Echinodermata. — Marine animals which have a spiny
skin and the body wall usually supported with calcareous plates.
They are radially symmetrical and have tube feet as organs of loco-
motion. The common representatives are starfishes, sea urchins,
sea cucumbers, and sea lilies. There are about 4,500 known living
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

Phylum Arthropoda. — The representatives of this group may be
aquatic or terrestrial. 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-
closed in a calcareous shell. The single muscular "foot" is a char-
acteristic structure. Common forms include clams, snails, slugs,
and octopuses. About 78,000 species have been recognized.

Phylum Chordata. — Segmentally constructed animals with bilat-
eral 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,
turtles, rats, birds, horses, sheep, cows, monkeys, and men. Ap-
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.
Many authors have dignified each of these as a phylum. They are :
Nemertinea, — nearly unsegmented, contractile, wormlike forms;
Trochelminthes — unsegmented and frequently similar 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 —
sessile 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
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 distinc-
tion. Of course, it is easy to recognize the extremes. Anyone

TEXTBOOK OF ZOOLOGY

Chiorophyll
Sunlight

\nt(zrmedigte
decomposition
products

Bacteriai
decomposition

/ formoldehyde

Living ^/Carbohydrates ,
animals \ fats (and proteins)

Respiration
K ^

Dead
organisms

Pig. 3._The carbon cycle and the fundamental living process.

Inyalra -^rnn fO-)

Hrnnnir- food

a AnimQ/

Sll /~^trf i-.t of Oa

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

INTRODUCTION

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

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existence. All organic animal food is derived either directly or
indirectly from the process of photosynthesis carried on by green
plants to manufacture starch which is stored in plant tissue. Green
plants by effect of their chlorophyll (green pigment) in sunlight

30 TEXTBOOK OF ZOOLOGY

cause carbon dioxide, a gas of the air, and water to unite in the
formation of a simple carbohydrate. This is the basic food material
of plants as well as of animals. During daylight hours while photo-
synthesis is in progress, oxygen is discharged into the air as a prod-
uct of the process. This oxygen adds to the atmospheric supply
and is used by animals in respiration. The carbon dioxide dis-
charged by respiration of plants and animals is made use of by
plants in this synthesis of material. The excretory products of ani-
mals contain nitrogen which is easily transformed into a soluble
form and absorbed by plants to be combined with the simple carbo-
hydrate, already described, to produce protein.

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 libera-
tion of energy, and the by-products excreted are again incorpo-
rated in the earth to be available to other plants.

Attributes of Life

Most of us think we know what life is, but if asked to define it,
we find ourselves confronted by an almost hopeless task. The ques-
tion. What is Life?, is the greatest riddle in the biological world.

The term life is an abstraction with no objective reality except as
it 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 organi-
zation tends to adjust itself to a constantly varying environment.
Numerous attributes of living material may be given. Living mate-
rial has the ability to carry on active chemical reactions without
losing its body form. It is responsive to changes in the environ-
mental conditions ; therefore, it is said to be adaptive. Living
material is able to sustain and reproduce itself under favorable con-
ditions. The size of living organisms varies within definite limits.
Much more will be said of living material in the following chapter.

INTRODUCTION 31

Balance in Nature

The influence exerted by one animal or one group of animals on
another can hardly be estimated until one of them leaves the pic-
ture. In an established animal community which might be said to
be balanced, all groups are held in boimds by their enemies. Bal-
anced 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, Conse-
quently, 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.

The story of the rabbit in Australia is likewise an interesting ex-
ample 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, we have an example of the effect of the natural agents
of repression. The Japanese beetle which was recently intro-
duced in the United States hy accident has ravaged the vegeta-
tion 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 be-
coming with the potential danger of interfering with the natural
balance, that even when some irritating pest is under discussion,

TEXTBOOK OF ZOOLOGY

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INTRODUCTION 33

whose extermination is easily possible, they will advise against it
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 nor-
mal 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:
cultural 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 accomplish-
ments 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
profited enormously. In fact, these fields are in themselves applied
biology. Most of the great discoveries as to the nature and control
of disease, the manner of inheritance of human characteristics, and
the knowledge of 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 human lives. One of the most obvious
uses of other animals is as a source of food supply. All of the phyla
and classes of larger animals furnish at least a few species that find
places on our menu cards, particularly mammals, birds, turtles,
frogs, fish, crabs, lobsters, clams, oysters, and even snails.

Many animals are important because of their destructive tend-
encies in regard to articles valued by man, or to the health and life
of man. Most of the predaceous animals today are not a menace
to man directly, but they do destroy many domesticated as well as
useful wild animals. It is likely 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 at-
tempting to learn what, when, and how much a little boll weevil
eats or when, where, and how it lays its eggs ; and yet, the discovery
of such information may influence the activities of our entire cotton
industry. A recent instance of the economic importance of zoo-
logical knowledge 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
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 insects. The knowledge and
application of parasitology, which is a field of zoology, would avoid
this loss.

Agriculture has benefited greatly from the application of the
principles of heredity to plant and animal breeding. Much funda-
mental knowledge has come from the extensive studies on the
genetics and breeding of the common fruit fly, Drosophila. It is
easily kept in the laboratory and mated. It produces a new gen-
eration 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
in the fishing industry. 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 re-
source. Even with this work and that of all the State Fisheries
Departments, the natural fish life does not flourish as it might, had

INTRODUCTION 35

our public more appreciation of conditions necessary for a fish to
live. 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 and cultural endowments which zoology
has given man are no less valuable than the tangible gifts. To
understand something of the orderly conduct of Nature and to see
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 knowl-
edge growing out of biological study.

CHAPTEE II
HISTORY OF ZOOLOGY

This brief chapter is organized to afford a slight preview of the
works and lives of a selected few of the historic pioneers of zoology.
This is not an attempt to give a complete history of the subject.
The works of numerous pioneers in special fields are being con-
sidered throughout the text rather than in a given chapter.

There were individual persons interested in and studying natural
history long before there was any orgajiized field of study recog-
nized under the name of natural history or the more limited divi-
sions of it, including zoology. Some of the translations from the
early Egyptians and later from the Greeks indicate that there had
been some concern for the problems of life as well as medicine a
number of centuries before Christ. Some of the early Greek
scholars believed that the ocean supported all of the original life.
Hippocrates, a Greek living 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
credit for establishing the scientific method of study which is based
on gathering facts from direct observations and drawing conclusions
from a study of these facts. His observations on the structure and
development of embryo sharks, chicks, and many other animals, as
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 inter-
ested in the development of scientific endeavor. He made a grant
of 800 talents ($200,000 or more) for use by Aristotle in his investi-
gations. Thus even in those times endowments were being set up
for the support of research. The other Greeks who followed Aris-
totle added 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 37-
volume work in which much of the scientific knowledge of the time

HISTORY OF ZOOLOGY

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.

Galen (a.d. 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. — Aristotle (384-322 B.C.), father of naturalists. From a bas-relief
found In tlie collection of Fulvius Ursinus. (Visconti, Iconographic grecque.)
(From Locy, Growth of Biology, published by Henry Holt and Company, Inc.)

swine. During his time it was strictly against the law to make
dissections of the human body so he 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.
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

TEXTBOOK OF ZOOLOGY

Fig. 8. — Galen (A.D. 131-200), anatomist. Acta Medicorum Berolinensium, Vol.
5. 1719. (From Locy, Groivth of Biology, published by Henry Holt and Company,
Inc.)

HISTORY OF ZOOLOGY

young student and was not satisfied to accept the authority of
Galen's textbook. Therefore, after beginning his medical education
at Brussels, he transferred to Padua where human dissection was

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

then allowed. He later became professor of surgery there. He was
the first, since the time of Aristotle and Galen, to prove that direct

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 medi-
cine following his time.

William Harvey (1578-1657). — Following closely upon the epoch-
making 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. — William Harvey (1578-1657), father of physiology. (From Garrison,
History of Medicine, published by W. B. Saunders Company.)

suspected a circuit of the blood from the heart to the lungs and
return, but Harvey was the first to demonstrate circulation, and the
first to arrive at an idea of a complete circulation of all of the blood
through a closed system of vessels. This new idea was presented
in 1628. He also did notable work in embryology.

Marcello Malpighi (1628-1694) was a famous Spanish anatomist,
histologist, 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. Numerous organs of the human
body are named for this renowned scientist of his time. Like other
early microscopists, he had to build his own microscope.

Antonj van Leeuwenhoek (1632-1723) lived almost contemporane-
ously with Malpighi and like him made many contributions to the

Fig. 11. — Leeuwenhoek (1632-1723), pioneer micromotist. (From a painting by
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
of 419 lenses, most of which he had ground. Further study on
capillary blood circulation, first descriptions of spermatozoa, ex-
tended observations on bacteria and microscopic animals, and his

TEXTBOOK OF ZOOLOGY

valuable contributions to the development of the microscope are
the enviable accomplishments of this man.

Carolus Linnaeus (1707-1778) was a very eminent Swedish biolo-
gist, who, like many early students of this subject, was educated
as a physician. He followed somewhat in the footsteps of Ray
(1628-1705), 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 dis-
tinguishing 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 in-
variability 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 43

first 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. Lin-
naeus 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
and that no living species is absolutely fixed. Much later, in 1866,
Ernst Haeckel organized the modification of this system as used
in modern times.

Georges Cuvier (1769-1832) is credited with establishing the field
of comparative anatomj^ He was of French ancestry and largely
self-educated by his studies at tlie seashore. A number of anatomi-
cal structures bear his name.

Karl Ernst von Baer (1792-1876), a Russian biologist, is one who
really established 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 is explained elsewhere, came as a result
of his 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.

Louis Agassiz (1807-1873) is 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.

TEXTBOOK OF ZOOLOGY

Charles Darwin (1809-1882), an Englishman, made extensive
studies on the problem of the manner and means by which new
species of organisms arise. He very effectively developed the thesis
that they originate by a process of natural selection. This was based
on the idea that no two individuals are exactly alike, that new varia-
tions are constantly appearing, and finally that those individuals
or groups best suited to their environment would be the ones to
persist and produce progeny. His conception of the factors and

P^ig.

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

limitations determining the development of new species, pictures a
constant struggle for existence among organisms, with those whose
natural variations happen to fit them best to the changing features
of the environment persisting as dominant species and others being
crowded out. Those least fitted to the environment would naturally
become extinct.

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. It was the vast accumulation of facts covering
a period of twenty years which commanded the attention of sci-
entists 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. 14. — Charles Darwin (1809-1882), the author of Origin of Species. (From
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
as Mendel's Laws of Heredity.

TEXTBOOK OF ZOOLOGY

■,(S,

C^^f^e^a^-^

Fig. 15. — Greg-or Mendel (1822-1S84), the Austrian monk who pioneered In
studies of heredity. Plaque by Theodor Charlemont based on "Fuschia picture"
made between 1861 and 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 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
preexisting life. He showed that living organisms cause fermenta-
tion 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

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 in-
dustry 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 paleon-
tology 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

an outstanding scholar in the fields of heredity and embryology.
He is best known for his theory, that there is continuity of germ
plasm from generation to generation.

Hugo DeVries (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-
den, though small, transformations. He is widely known for his
experimental studies in plant breeding and genetics, particularly
with evening primrose.

E. D. Cope (1840-1897) was one of the greatest comparative anat-
omists of America. He 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 dur-
ing the current years that it is difficult even to catalogue the im-
portant contributions. It is an extremely active field, particularly
in the realm of the experimental endeavors. The printed program
for the annual meeting of the American Zoological Society, which
is made up largely of titles and abstracts of new papers to be pre-
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. This book is extremely interesting to
read.

CHAPTER III
PROTOPLASM AND THE CELL

Living- Matter, or Protoplasm

Little is known concerning the origin of living matter, or proto-
plasm, as it is called, but more and more is being learned about its
nature, characteristics, structure, and activities. Living matter is
ahvays active in some degree, and this activity attracted the atten-
tion of scholars at a rather early date, but serious study of the
material was not begun until approximately one hundred years ago.
A Frenchman by the name of Dujardin, in 1835, realized that 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 liv-
ing things.

The Cell Principle

Cells had been seen and even superficially described during the
latter part of the seventeenth century and numerous times during
the eighteenth century, but their significance was not realized.
Hooke, an Englishman, in 1665 in observing cork with the micro-
scope he had made, saw the spaces in it and called them cells be-
cause 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

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 no discredit to this theory or these men that they and many
other biologists of the time had erroneous ideas concerning the
essential features of the cell. Although Brown had recently dis-
covered the nucleus, the cell wall was thought to be the essential
part, though now we know it is not a universal structure of all
cells since practically 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

aterial produced by the cells, the cell prmciple soon came to be
stated thus : all living things are composed of cells and cell products.
With the years, the conceptions of the nature of the nucleus, the
cell membranes, and the composition of protoplasm itself have all
added their contributions to the present understanding of the mean-
ing and application of the cell principle. The cell is now regarded
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 sum of the activities of its
cells.

With the embracing of the functional activity of the cell as a part
of the principle underlying living processes comes also the inclusion
of heredity and development. Cell division, growth, tissue forma-
tion, migration of cells, formation of cell products, chromosome rela-
tionships 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 respon-
sible, there is developed a new individual which usually resembles
its parents quite closely.

The influence of the cell theory on biological thinking and progress
as well as its effect on fundamental thinking generally, can hardly
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 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-
sally true. This conception focused the thinking of all biologists
in the same direction and therefore it had a great unifying influ-
ence. 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. An understanding of the permeability
of cellular membranes, the transformation of energy by chemical
reaction within cells, the roles of electrolytes in living substance,
and the principles of heredity are some of the results of this new
conception of life embraced in the cell theorj^

PROTOPLASM AND THE CELL

General Characteristics of Protoplasm and the Material of the Cell

To begin with, it may be said that this substance has a variable
degree of fluidity under different conditions. The range of this
variation may be from semisolid to semiliquid. It is viscid and
gelatinous in consistency. It is more or less granular, nearly color-
less, and 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
viscosity.

WT.

Km / •"
•• '■"■ ..' *'■ ■

J'y- '''■■.

: i->. -v ♦•"•<.*. '-^J'.' S ■■■•■■ .'.•■..■■

' ... i- --•■.-.,■.■ -- f

. >v^^._.^.J^"^-?'^•%^,^

* ' * - .

Fig. 19. — structure of living protoplasm as seen in tlie starfish. (From Wilson,
The Cell, published by The Macmillan Company.)

Protoplasm is in a colloidal state of the emulsoid type. A colloid
is 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
reversible. With loss of water from the dispersion medium the dis-
persed particles of the colloid become congested by loss of general
fluidity. This condition is known as the gel state. When there is
increased water in the dispersion medium and the particles move with
greater ease in the more fluid medium, the colloidal state tends to be-

54 TEXTBOOK OF ZOOLOGY

come sol. This transfer of water may be due to chemical changes in
the dispersed particles or in the dispersion medium of the colloid.
The ability of protoplasm, because of its colloidal nature, to change
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 described briefly a number of important activities common to
all protoplasm. These properties are:

1. Irritability, which refers to the capacity present in all proto-
plasm for responding to changes in environmental conditions, or
external stimuli.

2. Conductivity refers to the fact that the impulses produced by
stimuli or irritations at one point in protoplasm are conducted to
other parts of not only a single cell but also to adjoining cells.

3. Contractility, which is the power of contraction and relaxation
that is common to the substance of every cell.

4. 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.

5. Growth is 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
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 repro-
duction among animals.

Consciousness, which refers to the awareness of one's own exist-
ence, is frequently given as a property of protoplasm. It is certain
that some protoplasm possesses consciousness, but evidence of this
quality is rather intangible. Spontaneity is also considered a prop-
erty of protoplasm by some. To be certain that the activity and
source of all reaction comes from within is likewise rather difficult
of definite proof, so this is simply mentioned here as another prop-
erty which is often listed.

PROTOPLASM AND THE CELL 55

Physical Nature of Protoplasm

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). Instead of being
in the form of a true solution like salt in water, it consists of sus-
pensions of relatively large molecular aggregations varying roughly
between 0.0001 and 0.000001 of a millimeter in diameter. These
particles keep 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 (characteristic of colloidal substances). Protoplasm dif-
fuses slowly or not at all through animal membranes. It changes
from a fluid or sol state to a more solid or gel state and may return
in the other direction. Ordinarily the viscosity of the continuous
phase or supporting liquid is only three or four times that of water,
while with the dispersed particles included it is only eight or ten
times that of water. The viscosity of the nuclear fluid is only twice
that of water. Since glycerin has a viscosity about a thousand times
as great as water, it will be realized that most protoplasm is quite
fluid in its active state. Changes in viscosity accompany and are
essential to the activity and functioning of it.

Protoplasm is not a single compound; it is a colloidal system of
a number of chemical compounds existing together. Colloidal systems
are 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
one centimeter its total surface will be 12.6 square centimeters.
Now, if the same volume of material is in colloidal particles of the
average size given above, the total surface of these will be approxi-
mately 7,000 square meters. This increase in surface is one of the
significant effects of colloidal organization of substances, because
many important reactions occur at these surfaces. By the presence
of salt ions in the continuous phase and these becoming adsorbed

56 TEXTBOOK OF ZOOLOGY

upon the surfaces of the colloidal particles, they acquire an electric
charge. Protoplasm exhibits these several phenomena because of
its colloidal nature.

Chemical Nature of Protoplasm

Up to the present time, protoplasm has eluded complete and
exact chemical analysis. 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, cal-
cium, sodium, chlorine, magnesium, iron, potassium, iodine, and fre-
quently others like silicon, aluminum, copper, manganese, bromine,
and fluorine. The most abundant of these are found named in the
first part of the list. A few of them are usually given as constitut-
ing approximately the following percentages of protoplasm : oxy-
gen 65 per cent, carbon 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 car-
hohydratcs, fats, proteins, and also enzymes. The inorganic com-
povmds consist of several inorganic salts and water.

The carbohydrates, which include starches and sugars, are com-
pounds 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 CeHioOe-
This is actually built into some parts of the cell, but its chief func-
tion 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 six molecules of v.'ater (H2O) and six molecules of carbon
dioxide (CO2). Glucose is 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
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 57

to combine with more oxygen in oxidation and therefore release
more energy. 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
of the structure of the cell and, secondly, the storage of food.

Proteins constitute the bulk of the foundation or framework of the
cellular structure and are the most abundant organic constituents.
They are composed of carbon, hydrogen, oxygen, and nitrogen, with
the frequent addition of traces of sulphur, phosphorus, magnesium,
and iron. All of the proteins have large molecules, each being com-
posed of thousands of atoms ; as an illustration, take hemoglobin of
the red blood corpuscles with its formula C7i2Hii3oN2i40245FeS2. Pro-
teins have a slow rate of diffusion, high resistance to electric cur-
rent, and 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 numer-
ous 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
entering into the reaction. The well-known example of catalysis
is the effect of a small amount of platinum in increasing the rate
at which hydrogen and oxygen combine to form water. A particu-
lar 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 gen-
eral 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

58 TEXTBOOK OF ZOOLOGY

types of reactions going forward in active protoplasm, it is seen
that there must be numerous enzymes present in the cells of every
organism.

Water constitutes 60 to 90 per cent of protoplasm and maintains
many substances in solution. Water is not only a very efficient
solvent; but it is important to protoplasm because of its compara-
tively high surface tension, because its presence gives the proto-
plasm a consistency compatible with the range of variation neces-
sary for metabolism, and because of its high specific heat. This
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 rela-
tion to other materials of the protoplasm vary in different cells and
in different species.

The inorganic salts are present in considerable numbers but in
relatively small amount. 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
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 more complex than any other sub-
stances. In a sense, protoplasm is quite unstable in that it changes
its 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
all of the necessary adjustments of living matter to its environment.
On account of the extreme complexity of protoplasm it is not sur-
prising that the chemistry of all of its activities is not yet com-
pletely understood.

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 any
cells known. 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, may exceed this size.
Some of the nerve cells, though of less mass, may be several feet
in length. Muscle cells are relatively long 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 cell.

The shape of the typical cell is spherical; but due to the effects
of mechanical pressure, specialized functions, and unequal growth
almost all cells are far from this shape. They vary greatly in shape
and include platelike, cubical, columnar, polygonal, and spindle-
shaped forms. The particular form of any cell is not a haphazard
matter but strictly controlled by morphological and functional
necessities.

60 TEXTBOOK OF ZOOLOGY

A cell consists of a mass of jellylike cytoplasm surrounding a
nucleus. The outer surface of the cytoplasm is modified, the proto-
plasm having more density here to form the plasma membrane, or
cell membrane, which is the outer covering of the animal cell. This
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, structure-
less fluid, hyaloplasm, and the interspersed fibrillar substance known
as spongioplasm. Within the cytoplasm, lying near the nucleus, in
most animal cells is the centrosome. Its substance is known as kino-
plasm and is made up of two parts, the larger ce7itrosphere, enclosing
a (two if divided) centriole. Vacuoles are often present as small

lis I

h '

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 chromone-
mata, 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.)

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

The nucleus, which is usually round and centrally located, is sur-
rounded 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 61

portion of this is haryolymph, or nuclear sap, while the meshwork
of fine fibers extending through 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 in-
to 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 chroma-
tin 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 structure 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 catabo-
lism goes on until the cytoplasm is depleted.

Cell Division

The cell is limited in its size, as is the complete organism. This
limit 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 mate-
rial, 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 por-
tion with its share of the nucleus. This direct method of division
was called amitosis. Its actual occurrence is quite rare. The usual
method of cell division is far more complex and less direct. There
are several preliminary changes or phases which must occur before
the actual cleavage of the cell into two new ones. This is mitotic

TEXTBOOK OF ZOOLOGY

cell 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.

Although the process of mitosis is a continuous series of changes,
for convenience in stud3% these changes will be set out as six phases.

CEL-l-

MEfv»BRANE
CENTROSPHERE
tvlUCUEAR
MEMBRANE

CHROMATIN
NUC!_EOI_US
CYTOPLASM

SPINDL.E

ASTRAU

RAYS

C H ROMOSO M E

CENTBIOLE

EARL-Y prophase:

EQUATOR r Al_
P1_ATE

ASTER

LATE PROPHASE

MEXAPHASE

ADJACENT
CEI_i_

INTER ZONAU
Fl BE RS

EARLY ANAPHASE

LATE ANAPHASE

CL-EAVAQE
FURROW

TELOPHASE

DAUGHTER CELLS

Figr. 22. — The stages in typical mitosis (indirect cell division) as .shown in
fertilized Ascaris eggs. They follow each other in order : resting cell, early pro-
phase, late prophase, metaphase, early anaphase, late anaphase, telophase, an'^
daughter cells. (Drawn by Titus Evans.)

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 fibers or threads. (Some hold that
the chromatin retains its linear arrangement from one cell genera-
tion to the next.) Each consists of a double linear series of chromatin
bodies like two chains bound closely together to form the thread.
A single one of these two series in the thread is known as a chromo-
nema (pi., chromonemata) . The chromatin bodies comprising the
chromonemata are often called chromomeres (Fig. 21).

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. Ac-
cording 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
changes, the nuclear membrane begins to degenerate. 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
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
requires about eight minutes. The nuclear membrane now has al-
most completely disappeared.

The metaphase follows with no interruption. The chromosomes,
still 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.

64 TEXTBOOK OP ZOOLOGY

As 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
the telophase stage. 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 sepa-
rate from the group, although certain ones still clump together.
The chromosomes progressively lose their identity and their stain-
ing qualities. The nucleus resumes its granular appearance of the
resting cell. One or more nucleoli soon become evident. The for-
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. The universality
of this process in all types of organisms, both plant and animal,
and the regularity of the occurrence of the phases of the process
suggest that it is of vital significajice. The great precision with
which the chromatin is divided between the two cells seems to
indicate that this is a most significant step. Chromatin is recog-
nized as the material which makes possible the inheritance of quali-
ties 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
identical hereditary qualities. 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
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 IN GENERAL

The animals included in this group are usually said to be the
first to 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 cell. Protozoa are univer-
sally placed first 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
of 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
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 of the animal kingdom. In spite of the exceedingly large
number of species and microscopic size, the phylum has been quite
systematically classified and is divided into classes, orders, families,
genera, and species. The phylum is usually divided into four classes,
each characterized by a distinctive locomotor structure or by the
total lack of such features, as in one of the classes.

TEXTBOOK OF ZOOLOGY

1. Class Mastigophora (mas ti gof '6 ra) which means whip bear-
ers, includes forms that possess one or more whiplike extensions of
the cytoplasm, or flagella. The number of flagella is limited, and
they serve the animal as its means of locomotion. In some species
they serve the organism in feeding. The flagellum is a contractile
structure. There are some species in which exist both flagellate
and amoeboid stages. This seems to show a rather close relation
of this class to the next. This class also has a close relationship
with plants in that many of its representatives possess chlorophyll.

Cercomonas

111 _ \>asis
Monosiga

Cht lononas

Codonosiga.

Phaous

Trachelmonas

Peranema Maatigamoeba

Fig. 23. — Group of representative Mastigophora. (Reprinted by permission
from Curtis and Guthrie, Textbook of General Zoology, published by John Wiley
and Sons, Inc.) (Figure of Chilomonas modified.)

These forms are frequently classified as plants b}^ botanists. The
class 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. Holozoic refers to forms which ingest and digest food
material. 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

A larg-e number of Mastigophora live in quiet streams, ponds, lakes,
and in the ocean, Euglena is a very common!}^ studied fresh-water
form. Noctiluca is 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
square rods. When stimulated, these animals are luminescent and at
night frequently give up an attractive greenish or bluish white light ;
Uroglena is a fresh-water form which is often found in water supply
basins and causes a pungent, oily odor and unpleasant taste in the
water. Giardia (Fig. 386), Trichomonas, Chilomastix, Retortamonas
and Enteronomas are all genera with representatives occurring in the
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
feet) which are temporary structures and can be withdrawn. The ani-
mal is able to accomplish locomotion by extending the protoplasm into
these pseudopodia. The representatives of this class include many free-
living forms as well as numerous parasitic ones. A number of the rep-
resentatives of class Sarcodina secrete an external shell of lime, silicon,
chitin, cellulose, or some bind in sand or other solid substances with one
of the secretions. 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
studied. Endamoeba histolytica is the most common parasitic form,
Arcella, which secretes its shell, and Difflugia, which constructs its

TEXTBOOK OF ZOOLOGY

Shell of sand cemented together by a secretion, are two of the most
commonly observed shell-bearing forms, (b) Foramimera is an
order of shelled forms whose pseudopodia are very slender a^d
reticnlar. The pseudopodia are extended through small pores m the
sheU Only a very few of this group live m fresh water. The vast

^^iijA^l^

"s :.M

iU- e

Fig. 25.-Life cycle of one of the Foraminifera PoZysfomeZZa crw^^A^
spheric individual; B amoeboid 9^11^ .f.^^P^^^ from it ^ youn^^^
dividual developing from amoeboid cell ;D,nmcrosphericin^^^^ Parker and

gametes escaping from it ; F, union of gametes. Kearawn
Haswell, Zoology, published by The Macmillan Co.)

PHYLUM PROTOZOA IN GENERAIv

majority are marine, and Glohigerina is 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

Fig. 26. — Shells of different Foraminifera. A, Rhahdamina abyssorum (X4.5) ;
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. They live quite commonly in masses
of decaying vegetable material upon which they feed, (d) Heliozoa
is a group with thin, radially arranged, threadlike, unbranched
pseudopodia. Actinophrys sol is a common one found in fresh-water
streams and ponds, (e) Badiolaria is a marine group with fine, ray-

TEXTBOOK OF ZOOLOGY

like pseudopodia and a shell composed largely of silica. The pseudo-
podia extend through the relatively large apertures in the shell.

3. Class Infusoria (infuso'ria, crowded).— This group includes
those single-celled animals covered with small hairlike, cytoplasmic
processes known as cilia. They occur in both fresh and marine
waters as free-swimming organisms. There are a few parasitic forms,
notably Balantidium coli. Paramecium, Stentor, and Vorticella are

-ZT^ffflara 0i^» — ■

Didinium

Coleps

STEHTOl^

VOKTlceiLA.

Fig. 27. — Group of typical Infusoria. (Courtesy of General Biological Supply

House.)

the commonly studied infusorians. The class is now divided into two
subclasses, Ciliata and Sudoria. The first, Ciliata, is composed of
four orders, (a) Holotrichida, possess cilia of equal length over the
body, or they are restricted to particular regions in specialized forms ;
a cell-mouth is present in most forms. Paramecium is our most com-
mon genus living in fresh water. Didinium, Frontonia, Chilodon,
and Coleps are other common forms. Opalina is a well-known para-

PHYLUM PROTOZOA IN GENERAL

sitic genus which inhabits the large intestine of the frog, (b) Hetero-
trichida possess a well-developed undulating membrane in the cyto-
pharynx. The body cilia are small or partially absent, but the cilia
of the oral region are well developed. In some forms this oral region
possesses membranelles. Stentor, Halteria, and Bursaria are common
fresh- water genera while Balantidnim (Fig. 389) is a parasite in the
intestine of man and some other mammals, (c) Tlypotrichida possess
cirri or structures formed by fusion of cilia ; these are found prin-
cipally on the ventral side. The cell is flattened dorsoventrally and

Prorodon

From ton I a

iupiom

StyhnyoMa

Laorymaria

i^m

lionotw

Fig. 28. — Representatives from class Infusoria. (Reprinted by permission from
Curtis and Gutlirie, Textbook of 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 TEXTBOOK OF ZOOLOGY

the most common living genus. Epistylis and Carchesium are well-
known colonial genera. Vorticella and Carchesium have contractile
stalks while Epistylis is attached by noncontractile branching stalks.

The second subclass, Suctoria or TentacuUfera, as it 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
in their early stages are often amoeboid, but in the completed life
history locomotor structures are wanting. During the life cycle there
is a spore stage. The animals of this class are entirely parasitic

^^(^^''f^^T^ Ep i th e Hum
; i<m^l . :,; hfH-Early sta§e

.Intermediate sta^e

-natarc 5ta^e

Fig. 29. — Gregarina attached to an epithelial cell of host's Intestine. Other
stages of its development are shown within adjacent cells.

and they are usually transmitted to other animals in the spore
stage. 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
dividing into fragments while 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 dis-
solved upon entrance to a host and liberates the organisms.

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

PHYLUM PROTOZOA IN GENERAL 73

cated. 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, (a) Gregarinida (Fig. 29), commonly
called gregarines, inhabit the cells (cystozoic) of earthworms, cock-
roaches, other insects, and occasionally vertebrates in their early
stages, but later they may become free in the cavities of the host. They
may attain considerable size, (b) Coccidia are minute monocysted
forms which are permanent intracellular parasites of molluscs, arthro-
pods, 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)
Haemosporidia. The representatives of this order live chiefly in the
red blood corpuscles of vertebrates. 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
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 fila-
ment. There are two orders : (a) Myxosporidia are found chiefly as
fish parasites, but occur occasionally in reptiles and amphibia. The
gall-bladder, uriniferous tubes, 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
characteristic genera, (b) Microsporidia have in each spore a single
polar capsule. This group is parasitic chiefly in arthropods, and
occasionally in other invertebrates, fish and amphibia.

The third subclass Acnidosporidia includes forms which produce
simple spores. Again there are two orders: (a) Sarcosporidia. As
the 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 not known, but the saclike, encysted form in muscles of mammals
is known as Miescher's tube or sac. (b) Haplosporidia are single cells,
each with a single nucleus, and they have a relatively simple struc-
ture. 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
of certain of these forms. Life cycles in which there are primary
and intermediate hosts are quite common among parasites. This
example will illustrate also the relationship of insects to disease-
producing organisms. There are three species of human malaria-
causing organisms: (a) Plasmodium vivax, which causes tertian
fever, is characterized by an attack each forty-eight hours, (b) Plas-
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) may live in the blood of man by a series of asexual
generations which may continue throughout the 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 cor-
puscles 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-
gametes which develop from the male gametocyte. If a female
Anopheles mosquito bites and sucks blood from this person, the mos-
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
motile, 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 ex-
pense 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
coalesce, while slender, spindle-shaped sporozoites develop within,
each with a chromatin dot as a nucleus. The capsule of this oocyst
is 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 IN GENERAL

this development to go on in the mosquito. These little parasitic
sporozoites make their way to the salivary gland of the mosquito
where they may remain for weeks. When tliis mosquito ' ' bites ' ' a man,
some of the saliva with sporozoites flows into the wound, and the
process of asexual multiplication begins over again in the red cor-
puscles of this person as a new host.

Colonial Protozoa

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

Codonosi^a

Carchejium

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
Joanne Moore.)

of these forms only two cells adhere, but in others the cells may
remain attached after many divisions, with the result that thou-
sands 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. Pan-
dorina 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-

TEXTBOOK OF ZOOLOGY

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

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.)

in 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

PHYLUM PROTOZOA IN GENERAL 77

distributed, so that certain groups carry out a definite portion of the
entire metabolism. They are classified into general body (somatic)
cells and reproductive (germ) cells. 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-
rents, and electric currents. The response to a stimulus may be
either positive or negative. 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
whole. Tropisms are named with respect to the stimulating agent,
and the common ones usually recognized are :

a. Phototropism, response to light

b. Thigmotropism, response to contact

c. Chemotropism, response to chemical changes

d. Thermotropism, response to temperature

e. Geotropism, response to gravity

f. Rheotropism, response to mechanical currents

g. Electrotropism or galvanotropism, response to electric currents

If the animal is attracted to the source of the stimulation and turns
toward it, the response is said to be positive. If the organism is
repelled by the stimulus, the response is negative. It has not been
thoroughly determined why an animal responds to a specific stimulus
in a certain way. The minimum strength of stimulus which is neces-
sary to get a response is 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 be-
havior 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
lion, but many of them are parasites. Many diseases of man and
animals are caused by Protozoa. Most of the diseases of this origin
are more prevalent in the tropical and subtropical regions of the
earth. Such diseases may attain sufiScient importance to render
large portions of continents uninhabitable 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
sleeping sickness in Africa. There are other Protozoa that render
water unfit for drinking or help fertilize the soil.

Amoebic Dysentery. — Ulcers on the inside of the walls of the
intestine of man are caused by this disease. There results from
this, severe diarrhea and dysentery. From the intestine the infec-
tion, if allowed to continue, will be carried to the liver where
serious abscesses are formed. The infection is usually obtained
directly through drinking water or eating food which has been
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. Some other Amoebae have been found
in human beings, but, so far as known, they are not pathogenic. End-
amoeba coli, Endolimax nana, and Endamoeba gingivalis are such
examples.

Foraminifera which is an order in class Sarcodina has some eco-
nomic 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 repre-
sentatives has a complicated skeleton of silica. From their skeletal
remains comes an ooze on the sea floor sometimes hundreds of feet
deep. From this is formed quartz or flint.

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

PHYLUM PROTOZOA IN GENERAL 79

is called trypanosomiasis for the genus name of the animal that
causes it, 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 col-
lect 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
death usually within three or four months, or it may be extended
into years. Such animals as antelope, cattle, and some wild game
are susceptible to the disease and may serve as carriers. This com-
plicates the control of it. The disease has been considered abso-
lutely fatal, but 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
related 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
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-
tributed 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,
Dr. Laveran found a curious parasite in the blood of malaria pa-
tients. Several years later Laveran and IManson independently sug-
gested that the organism might be transmitted by some blood-
sucking 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

of necessary relations for production of the disease is broken. Mos-
quitoes are destroyed by draining swamps which serve as breeding
places, 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 physi-
cian 's direction. Quinine is a specific drug for this disease.

Texas Fever. — The small sporozoan, Babesia higemina, causes this
disease in cattle by destroying red blood corpuscles. The red cor-
puscle count of the host may be reduced from an average of 7,000,000
per cu. ml. to less than 1,000,000 per eu. ml. The disease is trans-
mitted from cow to cow by the cattle tick and its young.

Nagana, similar to African sleeping sickness in man, dourine, a
sexual disease of the horse, and surra, are all 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. Such diseases as Rocky Mountain spotted
fever, transmitted by the Rocky Mountain spotted fever tick and
fatal to man ; dengue or breakbone fever, a very unpleasant and un-
comfortable disease, transmitted by the yellow fever mosquito Aedes
(Stegomyia) ; as well as perhaps rabies, scarlet fever, typhus fever,
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 eco-
nomic 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, slug-
gish 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 pro-
portions of a cigar with a blunt anterior and a sharp posterior end.
At the anterior end, attached near the mouth, it bears a very
slender, almost transparent, whiplike filament, the flagellum. This
is an extension of the cytoplasm. The superficial layer of the cell or
ectosarc (ectoplasm) is covered by an extremely thin portion, the
cuticle. Most of the euglenoid forms have spiral markings (stria-
tions) 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 cyto-
pharynx. 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 lo-
cated the nucleus. It is usually obliterated from view by the abun-
dant chloroplasts. Small contractile vacuoles empty from the endo-
plasm into the reservoir.

Food and Assimilation

The food problem among Euglenae as a group is interesting from
the biological standpoint. It seems that some Euglenae are able
to ingest other small organisms through the mouth and cytopharynx

TEXTBOOK OF ZOOLOGY

to 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. Others, like E. gracilis, are able to
assimilate dissolved nutriment by absorption through the general cell
surface (saprophytic nutrition). In fact, this species has been main-
tained for more than two years in a nutrient solution in darkness.

-Fhgellum

:0'».

'Mouth

-Stigma

Contractile
Vacuole

'§■

■)■■<■.

■©.

1^5

A\^

^Reservoir

■ Ctirom at opt) ore

-Nucleus

Fig. 32. — Euglena viridis Ehrenberg. A chlorophyll-bearing flagellate.

Those forms like E. viridis that are abundantly endowed with chloro-
phyll obtain their food largely by photosynthesis as does the green
plant. This process utilizes water, carbon dioxide, dissolved mineral
salts, 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 83

process is 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 pro-
duced 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 vesicle communicating
with the exterior.

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 divi-
sion, and a new one is developed in each daughter cell. During ad-
verse 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 cyst.
On rare occasions two individual Euglenae come together side by
side and fuse permanently into a single cell. This is somewhat
similar to the zygote formation in sexual reproduction.

84 TEXTBOOK OF ZOOLOGY

Behavior

Euglena usually lives 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
will be a negative response. A medium light is optimum for it.
There is naturally an attraction to light in those forms which utilize
it in the manufacture of food by photosynthesis. Direct, intense
sunlight, however, is injurious to them. When Euglena swims
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
reactions this cell exhibits the irritability that is characteristic of all
protoplasm.

Locomotion and Flag'ellar Movement

Contractions and expansions take place in Euglenae when they
are not actively moving about. These movements resemble waves
of contraction (peristaltic contraction) passing over the cell. Some
of the larger species 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
of the whiplike movements of the flagellum through the water. A
spiral path is followed due to the continuous turning of the body.
The flagellum is made up of an elastic outer sheath which encloses
an axial filament composed of one or more contractile fibrils.

CHAPTER VI

AMOEBA OF CLASS SABCODINA

It is likely that no microscopic organism has attracted so much
attention and popular interest as Amoeba. Amoeba is recognized
by the public generally as a simple and low form of life. Even the
writers of fiction speak of the range of the span of complexity of
animal life as extending "from Amoeba to Man." The pedigree of
Amoeba is probably as long as that of any of the animals we know
and involves hundreds of times as many generations as many of
the common animals ; yet Amoeba remains in a relatively primitive
and simple state. Little or nothing is known about the real an-
cestry 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

The many kinds of Amoeba live in fresh water, marine water,
soil, 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, spring pools, drain ditches, abandoned tajining pits, in
streams where the water runs over rocky ledges, and wherever
there is abundant aquatic vegetation. It is often found on the sur-
face of submerged lily pads. A 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.

The general appearance of this animal is that of a slate-colored,
lustrous, irregular mass of gelatinlike substance with slowly-mov-
ing, fine particles within. When it is active, the outline is con-
stantly changing.

86 TEXTBOOK OF ZOOLOGY

Structure

Amoeba proteus is one of the largest of the fresh-water forms.
Its average diameter is about M.00 iJ^ch (0.25 mm.), while its ex-
treme 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.
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
into 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

FOOD
'fj VACU01_E

I ^^ " '^T^'- '^ VACUOLE

I 1 Z-^Z-Z^^^> ^ NUCl_EUS

> f '.'©**« V-''^^S? -^ PSEUDOPODIUM

U",

■^-

^i^ ECTOPL.ASM

Fig. 33. — Drawing to show the appearance and structure of a living Amoeba

proteus.

inclusions as well as the nucleus. The larger bodies in the cytosome
are food vacuoles, single, shiny, contractile vacuoles containing
watery fluid 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, plas-
masol 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

phenomena are the same as those found in the highest forms of life
but reduced to very simple terms. Here we may study the entire
metabolic cycle in progress within the confines of a single cell. Its
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-
tous algae. Bacteria may be used to some extent and rotifers (small
Metazoa) are sometimes devoured.

Ing-estion. — Amoeba has no definite mouth but the food is taken
into the 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.

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. The function of digestion
is to convert complex materials into a soluble, absorbable form. It
is 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
on the process. A 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 debris that has been ingested
with the food is carried to the surface of the cell and cast out or
egested by simply being left behind as the animal moves away.

Assimilation. — This is the process of transforming the digested
food material into protoplasm. In Amoeba the digested food mate-
rial 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 con-
tact with the dissolved food.

Respiration. — This is a process whereby the gas, carbon dioxide
(CO2), leaving the protoplasm, is exchanged for oxygen (O2) en-
tering 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
dissolved oxygen in order that this diffusion may 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
of the decaying vegetation present. Amoebae may live several
hours in water from which the oxygen is removed before asphyxia-
tion occurs. The contractile vacuole likely assists in discharging CO2.

Catabolism or Dissimilation. — 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,
and carbon dioxide are residual products of this process.

Excretion. — These by-products of metabolism in the form of waste
liquids must be disposed of. 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 con-
tractile vacuole along with some carbon dioxide. The contractile
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 ap-
parently 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 by diffusion through the cell surface. There is
likely some excretion by this means in all Amoebae.

Growth. — If there is increase in the volume of a body, this is
spoken of as growth. In all living organisms growth is accom-
plished by addition to the protoplasm. If food is plentiful, more
material is added to the protoplasm than is used up in the oxidation
which produces active energy. In other words, growth occurs
when the rate of anabolism exceeds the rate of catabolism in the
organism.

Reproduction and Life Cycle

The life history of the niajiy-celled animals to be studied later
includes a series of changes from egg, through embryo state, to
adult. In Amoeba the cycle is likely only partly known, because it

Nucleus

Contractile
iracaole

Fig. 35.

-Diagram to show fission in amoeba. A, Beginning of the process; B,
fission nearing completion. (Drawn by Joanne Moore.)

is difficult to maintain cultures in perfectly normal conditions for
sufficiently long periods to get this complete story. Ordinarily, the
animal grows when conditions are favorable until it attains a cer-
tain size; 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
that influence it. It will be recalled that all materials used by a
cell 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

90 TEXTBOOK OF ZOOLOGY

its diameter; while its surface increases only according to the
square of its diameter. In other words, the amount of material in
a growing cell increases approximately twice as fast as the plane
surface needed to surround it. It is logical, then, to assume that a
point may be reached when the surface area will not be sufficient
for the passage of necessary materials into and out of a cell. There
is, however, considerable variation in the size of cells; hence it
seems there must be other factors besides volume and surface rela-
tion in operation. Modified surface and difference in the rate of
metabolism certainly would be factors affecting the size of the
organism. When Amoeba reaches the limit of size, a division oc-
curs. Binary fission, by which two new individuals are produced,
has been definitely established, and some other methods of reproduc-
tion have been presented. Calkins, an authority on Protozoa, states

New cells /Nuclear fragment

Pig. 36. — Diagram to show amoeba encysted and undergoing the process of sporu-

lation. (Drawn by Joanne Moore.)

that Amoeba starts out as a tiny pseudopodiospore which has only
one pseudopodium. It then passes through a growth period and
increases in complexity until it reaches the full-grown condition. It
then divides by binary fission into two daughters. When each daugh-
ter has 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-
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 91

Behavior

All of the activities of an animal which come in response to in-
ternal or external stimuli make up the "behavior." The activities
of the animal under discussion include the formation of pseudo-
podia, ingestion of food, locomotion, and others. Amoeba proteus
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
IV. To physical contact, it responds positively if the impact is
gentle ; otherwise the response is negative. It responds negatively to
strong light and finds its optimum in a moderately reduced light.
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 chemi-
cals which have been tried. Amoebae have an optimum tempera-
ture 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.
A 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 be-
havior 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
in 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.

92 TEXTBOOK OF ZOOLOGY

Successive pseudopodia are formed in the moving Amoeba proteus as
it goes in a given direction. The pseudopodia are temporary loco-
motor structures. Most zoologists explain this movement as being
due to the contraction of the more viscous ectoplasm, particularly in
the "posterior" region. This brings about a forward movement in
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 endo-
plasm (plasmagel). This process continues, pushing the advancing
tip farther and farther forward. At the opposite side, the plasma-
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 up-
per side of the body ; it continues to move forward to the tip of the

- - Particle -"" i

Pseudopodium

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 station-
ary portion. If the specimen has several pseudopodia, one or more
may be developing while others are receding. In the latter, the
flow of plasmasol is back through the centers of the pseudopodia
toward the main mass. Temperature and other environmental fac-
tors affect the rate of locomotion.

Dillinger mounted some of the animals on the edge of a slide in
a groove formed 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.

CHAPTER Yll

PARAMECIUM OF CLASS INFUSORIA

This animal has been the subject of much study and the victim
of considerable experimentation. 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.

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 collect-
ing 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 is sometimes described as being slipper-shaped. The
anterior portion, which is blunt but generally narrower, represents
the heel part ; while the posterior portion, which is generally broader
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

TEXTBOOK OF ZOOLOGY

right from this view. The body is covered with fine hairlike cilia
which are of even length except in the oral groove and at the pos-
terior extremity, where they are noticeably longer. The cilia within
the gullet are fused together into a sheet, forming the undulating
membrane.

CON-r RACTIL.E
VACUO l_E

ORAl_ GROOVE

MACRO-
NUCI_E.US

CONTRACTILE
VACUOLE

TRICHOCYSTS

PEI_l_ICl_E

Fig. 38. — Diagram showing the structure of Paramecium, much enlarged.

by T. C. Evans.)

(Drawn

The cell is divided into the outer, tough, nongranular ectosarc
which is 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 ecto-
sarc. There are a great many spindle-shaped cavities located in the
ectosarc with their long axes perpendicular to the surface. These
structures, trichocysts, are filled with a semifluid substance and each
opens to the outside through the pellicle. The endosarc, composed of

PARAMECIUM OF CLASS INFUSORIA

endoplasm, is within. It contains food vacuoles, two contractile
vacuoles, macronucleus, and other granular masses. The numerous
food vacuoles are formed, one at a time, at the inner end of the gullet
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
called cydosis. The contractile vacuoles are located near each end
of the animal. 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 micro-
nucleus is located in the curved surface of the macronucleus and is
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

M Anus

-Food vacuole

Fig. 39. — Cydosis in Paramecium, showing the course of the food vacuoles through
the endoplasm while digestion is in progress.

Food. — Smaller protozoans, bacteria, and particles of debris con-
stitute the principal items on the menu for Paramecia.

Ingestion. — This animal hunts its food, and when it locates a re-
gion where food is abundant, it settles down and becomes relatively

96 TEXTBOOK OF ZOOLOGY

quiet. The food is swept through the oral groove by the beating
action of the cilia, 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-
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.

Excretion of the waste products of metabolism in solution is by
means of the alternate filling and expelling of fluid by the two con-
tractile vacuoles, or it may occur to some extent by diffusion
through the entire cell membrane.

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

The actual reproduction is by transverse binary fission which in
itself is asexual. The cell divides transversely into individuals, and
this is repeated for long series of generations, one after another.
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 divi-
sion ranges between thirty minutes and two hours, depending on en-
vironmental 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 car-
ried some cultures of P. caudatum through a long series of genera-
tions 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
usually small, rather unhealthy appearing individuals. Shortly after
the adherence of the conjugants the nuclei of each undergo changes.
The micronucleus enlarges and divides, forming two micronuclei,
while the macronucleus undergoes disintegration and final disap-
pearance. 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 en-
large and become macronuclei; three of the others degenerate, and
one remains as a micronucleus. This micronucleus divides, and al-
most 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 macro-
nucleus 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" micro-
nucleus of the other conjugant is compared to fertilization.

A 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 in-
tervals of about every forty or fifty generations, the macronucleus

TEXTBOOK OF ZOOLOGY

Fig. 40. — Conjugation and subsequent divisions in Paramecium, showing activi-
ties of the micronucleus. Circles are micronuclei and crescents are macronuclei.
The shaded ones have been resorbed. The divisions for micronuclei actually occur
within the cells instead of outside, as figured for convenience. (From "White,
treneral Biology.)

PARAMECIUM OF CLASS INFUSORIA

disintegrates, and the micronuclei undergo two divisions which pro-
duce 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.

■■'. '.'■'■ ',\ ->:.'. • "
• '- '."' ','■.•' • • . - ' ■'.
■ '• ".'.■'/•'''.",■• • •• "'- * ' ■

■ -

I » . . •

* ^^^■^^- — Reaction of paramecia to a drop of 0.5 per cent NaCl. A, Introduction
of the drop beneath the cover glass; B, four minutes later. (Prom Jennings. Be-
havior of the Lower Ojffanisms, published by The Columbia University Press.)

There is still difference of opinion as to the exact function of con-
jugation and endomixis, but the chief result of the processes seems
to be the reorganization of the nuclear substance. This may allow
for variations in the fundamental constitution of the race. Accord-
ing to some authors these processes rejuvenate or rencAv the vitality
of the individuals. In recent years, not only sexual reproduction but
also district sexes have been described for Paramecium.*

•Sonneborn, Science News Letter, Aug. 21, 1937.

100

TEXTBOOK OP ZOOLOGY

Behavior

This animal is an active swimmer and necessarily shows ready
response to environmental factors. Its behavior consists of its
spiral course in locomotion, avoiding reactions, responses to food
material, contact and other minor reactions. Its reactions to stimuli
are somewhat similar to those described for Amoeba; however, it
seems not to be affected by ordinary light. It reacts either posi-
tively or negatively to contact, change of chemical constitution,
change in temperature, to gravity, and to electric current. The re-
sponse to contact is positive, negative to ultraviolet light, negative

——7 = 9 ^ ^ :; ^ - s 1 ^ / /^

' ^ 9 "

JS'

19-

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

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

Be-

ss-

Fig. 42. — Reactions of paramecia to temperature, a, Paramecia are in a
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 Para-
mecia 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.). Gravity causes the anterior end to point upward, and when
placed in moving water, the animals will swim upstream. If Para-
mecium comes in contact with a solid object when it is moving, it
will back away, swing on its posterior end to a slightly different

PARAMECIUM OF CLASS INFUSORIA

101

direction and try again. This may be repeated, and is known as
the "avoiding reaction/' Such a reaction really involves simply
one or more negative responses. These animals are constantly sam-
pling the water and avoiding the conditions which are least favor-
able. 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
is 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 con-
tact with the water and form a mass of fine threads. These threads
will entangle many of the aquatic enemies of these animals.

Locomotion

The beating action of the cilia against the water serves as the
principal 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 swim-
ming. 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
elastic sheath and a fibrillar core. Contraction of the protoplasmic
substance on one side, bends the cilium in that direction. The re-
verse stroke is much more passive. The movement of one tier of
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 many cells functioning
as a unit are called metazoans. In most respects the vital activities
of Metazoa are similar to those of Protozoa. Since Metazoa are
more or less like compound Protozoa with some degree of inter-
cellular differentiation, it is thought by many authorities that they
arose through organization of single-celled organisms. In some forms
of compound or colonial Protozoa, only two cells adhere together after
cell division, but in others the cells may remain attached after many
di\^sions. The size of different colonies may range from two to two
thousand similar cells. In the most complicated protozoan colonies
there may be several different types of cells. The representatives
of class Mastigophora are the most likely ancestral forerunners
of Metazoa. The colonial forms, such as Gonium, Pandorina,
Eudorina, Pleodorma, and Volvox, are rather plantlike in character-
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 all of the strictly many-celled animals. The
cells are definitely organized and classified morphologically as well
as physiologically. There is a well-regulated division of labor.
Among the single-celled animals each cell, like primitive man, is
largely independent of its 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, and as a result, a
special group is able to care for a particular function necessary to
the life of the entire organism. In return, other special groups care
for other functions. In this way each exchanges the products of
its labor for the products of the labors of the other groups. In
human society this becomes more and more complicated as civiliza-

10.1

104 TEXTBOOK OP ZOOLOGY

tion advances; so it is Avith development of complexity in meta-
zoans. Another characteristic of Metazoa is the presence of a defi-
nite center of control localized in a particular group of cells which
becomes the nervous system in higher forms.

Cellular Differentiation

In Protozoa there is seen fair development of intracellular dif-
ferentiation, making it possible for one part of a cell to perform a
particular function, 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
of as intercellular differentiation. The modification of metabolic

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.
Certain groups of cells become specialists in a particular phase of the
metabolic activity. Some become protective surface cells, others se-
crete special enzymes, still others specialize in excretion, and so on.
The entire metazoan body is usually divided into germ cells, which
are specialized for reproduction, and somatic cells or body cells,
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
gonads of the two sexes. 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 be-
come separated from the body and may give rise to a new generation.

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
cells as well. 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
immortal.

The protoplasm of the somatic cells is known as somatoplasm. This
is rebuilt with each generation, and when the individual dies, all of
the somatoplasm perishes. In final analysis, the somatoplasm serves
as a means of conveyance for the germ plasm through the current
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.

A tissue is an organization of similar cells into a group or layer
for the performance of a specific function. A certain amount of
intercellular substance is characteristic 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. — A sheet of cells that covers external or in-
ternal surfaces of the body is known as an epithelium. The epi-
dermis or outer layer of the skin and the layer of column-shaped
cells lining the inside of the intestine are good examples. Accord-
ing to function, this type of tissue can be classified as protective
epithelium, glandular epithelium, and sensory epithelium. The epi-
thelium which covers external surface of an organism usually de-
velops various protective structures in the different groups of ani-
mals : 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 all tissues whose func-
tion is to bind together or support the various parts of the body.
Connective tissue is, in most cases, composed of slender cells with
an abundance of intercellular material. This tissue is almost uni-
versally present in the various organs throughout the body. Ten-
dons, 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. Bone and cartilage, which make
up the framework of the body and support the other tissues, are
called supporting tissues. In crayfishes and grasshoppers the sup-
porting tissue is chitin instead of bone or cartilage. Cartilage is 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 solid salts.
The scattered cells are present as bone cells.

Muscular or Contractile Tissue. — This is distinctive because of
its ability to 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, volun-
tary, skeletal, as found in the muscle of the arm ; and striated, in-
voluntary, cardiac, as found in the wall of the heart. Skeletal,
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 lo-
cated. 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

107

this tissue is rather difficult, but the fibers are faintly 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 recog-
nizable : (a) the axo7ie, usually a single unbranched fiber except for
infrequent collateral branches; and (b) dendrites, frequently much

Q O

Fig. 45. — Typical cells and tissues from vertebrate animals. 1, Squamous
epithelial cells ; Z, section through a portion of bone showing Haversian canal
(in center), bone cells, lacunae, canaliculi, and matrix; 3, section of hyaline
cartilage showing cartilage cells in lacunae, and matrix between lacunae ; i, sec-
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.)

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 impulses are conducted toward the cell body over
the dendrites 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. In this way an impulse can be trans-
mitted from one part of the body to other parts. The chief function
of the nervous tissue is to relate the organism to its environment.

Vascular Tissue. — This is fluid tissue consisting of cells known as
corpuscles in a fluid medium called plasma. The cells are the red
corpuscles (erythrocytes) and white corpuscles (leucocytes), while
the plasma or fluid is the intercellular substance. Blood and lymph
are the two common vascular tissues. Lymph has no red corpuscles.
In the blood of mammals the red corpuscles are without nuclei ;
while in fish, frogs, turtles, and birds these cells are nucleated. The
chief function of this tissue is 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 all of the different types of tissues just de-
scribed. For example the stomach is an organ with an internal
cavity. It is 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.

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

a. The Integumentary System is composed of the skin and its out-
growths, such as hair, nails, scales, horns, hoofs, and similar struc-
tures. Its principal 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
body. 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 pro-
tection of the internal, vital organs.

METAZOAN ORGANIZATION

109

c. The Muscular System consists of muscles, the voluntary, stri-
ated 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. The general form of the system is that of a tube, and
it is frequently called the alimentary canal. The functions of in-

110 TEXTBOOK OF ZOOLOGY

gestion, digestion, egestion, absorption, secretion, and very little ex-
cretion are performed by this system. In general, it puts the food
in solution so that it may be absorbed by the blood.

e. The Respiratory System consists of structures capable of de-
livering 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
Metazoa, and lungs in the terrestrial vertebrate forms ; accessory to
the lungs are the nasal passages, pharynx, larynx, trachea, and
bronchi.

f. The Circulatory or Vascular System is a very extensive one con-
sisting of the heart, arteries, veins, capillaries, lymph spaces, lymph
nodes, and lymphoid glands. The general functions are: (1) to dis-
tribute 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
matter.

g. The Excretory or Urinary System is made up of tubular struc-
tures and accessory parts, such as flame cells, nephridia, Malpighian
tubules, green glands, and kidneys. In the mammals, the ureters,
urinaiy bladder, and urethra are accessory to the kidneys. The kid-
neys withdraw liquid waste products of metabolism from the blood
and deliver them to the outside of the body. The nitrogenous sub-
stances, 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-
ical substances, known as hormones, directly into the blood. The
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
typical examples of these organs. They go under the names of
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 sensa-
tions, correlate them, and coordinate the activities of the parts of the
body. By the function of the sensory portion of the system, the ani-
mal becomes aware of the environment and relates itself to it. 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
to cells to 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
the study of specific animals.

Development of Sexual Reproduction

Keproduction makes great advances among the metazoans. The
simple fundamental process of reproduction by cell division or
binary fission has been studied already. This is not possible for
most metazoan animals, but, in general, this type of animal be-
gins life as a single cell resulting from the fusion of two sex cells,
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: (a) the ordinary, nutritive individuals, whose means of
reproduction is fission and (b) reproductive individuals or gametes
of two forms : the large, egglike, inactive macrogametes and the
smaller, motile microgametes. In reproduction these two types of
cells unite to form a single zygote, from which a new colony arises by
repeated divisions. In a number of the Sporozoa, both sexual and
asexual generations occur. The zygotes, which are formed in the
sexual phase or generation, produce a number of spores which de-
velop sporozoites (already studied under Plasmodium.). These be-
come 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 primi-
tive 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

hermaphroditic or monoecious. Most of the types of animals in the
phylogenetic scale, up to and including the worms, are normally
hermaphroditic.

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,
the gonads of the other sex having degenerated in that individual.
The sexes are separate under such conditions and are said to be
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 as parthenogenesis. The case of the ordi-
nary aphids or plant lice, known to every gardener, is a good ex-
ample. In the spring an egg which was fertilized and laid the pre-
vious fall hatches to produce an individual known as a stem-mother.
This individual feeds on the sap of the particular plant on which she
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 fer-
tilized 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 of a number of animals, such as frogs, molluscs, worms,
sea urchins, and others have been artificially stimulated to continue

METAZOAN ORGANIZATION 113

development by application of chemical, electrical, or mechanical
agents. This goes under the name of artificial parthenogenesis.

Metagenesis is a phenomenon occurring in the life history of a
number of scattered species of Metazoa, including the coelenterate,
Ohelia; two or three marine worms; and Salpa, the tunicate (a chor-
date animal). This process is an alternation of production of sexual
individuals in one generation and asexual in the next. The offspring
in each case differs from its parents. This is spoken of as alternation
of generation. In Ohelia, a coelenterate related to Hydra (to be
studied shortly), there 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 com-
plete 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 Ontogeny-
Ontogeny refers to the development and life history of the indi-
vidual 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 cells or gametes are produced in their
respective gonads as previously described. They are in a very im-
mature state when they are first differentiated, and are called pri-
mordial germ cells.

The maturation (gametogenesis) or development of the germ cells
occurs while they are still 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
modified at one point to bring about a fusion and subsequent reduc-
tion 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,
having the normal number of chromosomes for the somatic cells of
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 pri-
mordial cell divides by mitosis to form two oogonia. Each of these
divides similarly. As is typical of mitotic division, each chromosome
divides with the division of the cell. This series of divisions con-
stitutes the multiplication period of the maturation process. In some

Oogenesis

Primordial _ f^

qermcell WC

Spermatoqemsis

Primary -(fi\\

oocyte \'^(/

Secondary f. i v

oocyte "\ /

Nature //T

ovum { f j

[it. polar body
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.

Primordial
qerm cell

J'permafco-
■ gonia

--Zbromosome

Primary
ipermatocybe

_ Secondary
spermato-
cyte

/(• jSpermatid

Mature
spermato-
zoon

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 primary oocyte. Each
of these oocytes divides by meiosis, the fused chromosomes dividing
as though they were single ones in 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 cyto-
plasm does not divide equally; nearly all of it goes to one of the

METAZOAN ORGANIZATION 1]5

cells in each case. This large cell is called the secondary oocyte and
the small one is the first polar body. Each of these cells has four
chromosomes. Following this the secondary oocyte divides to form
the mature ovum and another polar body. Occasionally the first
polar body divides, but none of them have any further significance
after cariying away half of the chromosomes. They now degenerate,
and their protoplasm is reabsorbed by the surrounding tissue. The
series of divisions and changes following the primary oocyte stage
constitute the maturatio7i period of the process. The ovum contain-
ing the haploid number of chromosomes is now prepared to unite
with a mature spermatozoon in fertilization.

Spermatogenesis is completed within the tubules of the testis, and,
like oogenesis, is a series of mitotic cell divisions. The primordial
germ cells divide by mitosis to form spermatogonia, and this process
continues just as it does in oogenesis, until the division of the pri-
mary spermatocytes which have developed during the growth period.
When the primary spermatocytes divide, the division is an equal one
and 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
number or, in this ease, four chromosomes. The change from sper-
matid to spermatozoa does not involve a cell division but simply rear-
rangement. 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.

The maturation process is very significant for at least two impor-
tant reasons. First, during the fusion and subsequent divisions of
the cells, there is given opportunity for variation of the genetic com-
position. Secondly, the number of chromosomes is reduced to half
in each mature germ cell, thereby making it possible for the germ
cells to 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 sper-
matozoon 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
to form the fusion nucleus, and the fertilization is complete. The
significance of fertilization is largely centered around two important
functions. First, it is the impetus for the development of an embryo
from the egg under most normal circumstances; however, partheno-
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 ; 5, the sixteen-cell stage ; 6, morula
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 en-
doderm. (Drawn by T. C. Evans.)

the means for inheritance of characteristics from two different lines
of ancestry. This union also restores the diploid 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. In eggs where the
yolk material is scant and evenly distributed, the ensuing cleavage

METAZOAN ORGANIZATION 117

divisions extend completely through the zygote, forming nearly
equal cells. If the yolk is concentrated in one end of the egg, the
divisions of the developing embryo are unequal. During the early
divisions all of the cells of the body divide at so nearly the same
rate that it appears as if the zygote were being cut with a knife or
cleaver into smaller parts. This process provides for the rapid in-
crease in the number of cells 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.

As 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
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
formed. The blastula does not simply increase in circumference,
but there comes a time when the wall on one side pushes in (in-
vaginates), 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 divi-
sion 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.

In higher forms, immediately following gastrulation, a third germ
layer, the 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,
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
is the coelom or future body cavity. From each of the germ layers,
particular parts of the body are derived.

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 infold-
ings, 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
the nervous system; the mesoderm furnishes the muscles, skeleton,
circulatory system, blood, excretory, and reproductive systems be-
sides 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 phylum, Porifera (p6 rif'er a), means ''pore-
bearers," and this, 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 sessile in habit, being fastened to piers, pilings, shells,
rocks, etc., for life. There is entire lack 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
piercing the walls and making connection between the internal
cavity and the outside. 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
material as a skeleton.

The organization of the sponges is a loose one, and the interde-
pendence 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
a common ancestor with the choanoflagellate type of colonial Pro-
tozoa. 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
like endoderm develops into the external layer. The so-called ecto-

119

120

TEXTBOOK OF ZOOLOGY

derm comes to line the internal cavities and its function is circu-
lating the water. The 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 modi-
fied diploblastic (two germ layers) form. Because of these pecu-
liarities, some authors have called sponges Mesozoa or Parazoa.

Classification

Class Calcispongiae. — Single, shallow-water, marine forms, char-
acterized 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. — Simplest type, possessing a very thin body wall
with pores as perforations in individual cells. The internal cavity
is lined with choanocytes. Leucosolenia.

PHYLUM PORIFERA

121

Order Heterocoela. — Moderately complex wall. Choanoeytes in
radial canals. ScypJia (Grantia).

Class Hyalospongiae. — Sponges which possess siliceous spicules
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.

Class Demospongiae. — Forms which have either nontriaxial sili-
ceous 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.

Order Tetraxonida. — These are ordinarily attached to the bottom in
deep water. Thenea.

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
•^ '**-••/•* J to jorm gemmates

inWrgzd^zrnmuka

K"'-'^ Mi-

«,'^-*i-

Fig. 50. — Spongillaj showing reproduction. (Courtesy General Biological Supply

House.)

Fresh-Water Sponges

In the southwestern part of the United States, at least in central
Texas, there are four species of fresh-water sponges: Spongilla
fragilis, TrocJwspongilla horrida, Asteromeyenia plumosa, and Ephy-
datia crater if ormis. Of the four, Spongilla fragilis seems to be
the most abundant in this area. Most of the colonies of this
species are irregular in shape, averaging approximately % of an
inch in diameter; but there are some as large as 6 inches by 2^
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 (particularly Trochospongilla
horrida and Ephydatia crater if ormis) grow abundantly in muddy
ponds and in muddy streams whose turbidity equals 110 parts of
solid matter per million. In this region again the growth of the
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
temperature in the spring. These gemmules are ordinarily de-
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
at least a dozen different ones. The representatives of this order have
skeletons of spongin and are found in subtropical and tropical marine
waters. Euspongia, the bath sponge.

Order Myxospongida. — These sponges are entirely devoid of skele-
ton. Haliscara.

THE SIMPLE SPONGE

Scypha coronata* (Ellis and Solander) has been mistakenly called
"grantia," the European form, by most textbooks for years. This
is a commonly studied representative of the phylum. It is available
and is also 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 attached by the basal or proximal end; the
opposite end is free or distal. A colony may be formed by budding.
Water is 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 open-
ing at the top. The osculum and ostia maj^ be closed and there

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

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
cell in a primitive fashion. This results in very slow transmission
of impulses and is called neuroid transmission.

External Anatomy

The average length of Scypha is about three-fourths of an inch.
It is 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 Anatomy

Internally there is a large central or gastral cavity which is simply
a water cavity and is not comparable to a stomach or enteron. In

Dctfnal

EpiiKctium

•1

k«l>M«n

FlaftlUitd Oitmbc
(Radul Canal) \

10%

4^ 11... cu.-
I ■■■"«

DiAfftftm of Akod Sponge

S-' . : ■ : S , Caiiral Co.ihcl.um

. InkaltAI Canal

Charr.bcr

Stereogram to illuatntc ample Leucen Sponge

Dermal Oatta

^Subdarmal Cavity

Ch^lcnf Cnal

FUstlUied Cham^r

StetcogrKiT) to illustiate Sycon Sponge

Diagram L, S, of rSagon (Icucon) type of canal 3truc:ure such «9 occurs
in the Demospongiae

Fig. 52. — Structure of different types of sponges, shown diagrammaCcally.
(Courtesy Pacific Biological Laboratories.)

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 be-
tween 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 PORIFERA

125

carry the water, but it substantially increases the surface area of the
animal. 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, identi-
fied as the ascon, sycon, and rliagon types of which the first is the
simplest, the second intermediate, 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 classi-
fication of sponges. Some have a skeleton of calcareous spicules,
others of siliceous spicules, others of the fibrous spongin, and still
others have no skeleton. Spongin of the ordinary bath sponge, which
is simply the skeleton of one of these animals, resembles silk chemi-
cally. It is formed by some special cells called spongioblasts. The

honaxon

.raxon

Triaxon
Hon ax on Jriradiatc

Fis

53. — Types of calcareous skeletal spicules found in different sponges. (Drawn

by Joanne Moore.)

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 com-
posed of simple, flat, epithelial cells, contractile cells (myocytes),
gland cells 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

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TEXTBOOK OF ZOOLOGY

spongioblasts are located in this layer. The incurrent canals are
lined with flat pavement cells. At each prosopyle there 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 cen-
tral cavity have similar histological structure since the former are
outpouchings of the latter. The cells here are primarily special
shaped ehoanocytes, peculiar to sponges, interspersed with scat-

Jpicule —

Sckroblast

. Dermal cells

-flaciei

lum

collar

AirchaeccYbe.
Co/lencyte

DertnalceU —

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.)

tered, flat, epithelial cells. 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
drive the suspended food particles into the funnellike mouths of
the collar cell where a food vacuole is formed in the cell in amoeboid
fashion. Of course the spicules appear in a histological section.
The entire arrangement is quite similar to a large colony of semi-
independent cells which do not function as integral parts of a tis-
sue 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. From these
living cells, if kept in favorable conditions, a new sponge will
reorganize.

Metabolism

A sponge obtains food from the water which is continually pass-
ing by way 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
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
some authors. 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
by diffusion from cell to cell. This may be augmented by the
amoeboid wandering cells. Respiration is carried on 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, per-
haps assisted by the wandering cells. Egestion is probably accom-
plished 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. Bud-
ding 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 re-
maining 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 in-
filtrated with siliceous materials. They are usually formed during
adverse conditions and can withstand desiccation and other severe

128

TEXTBOOK OF ZOOLOGY

circumstances. 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 dis-
cussions. Sponges are usually hermaphroditic, but the germ cells

Sexual I^eprodaction

Fig. 55. — Methods of reproduction in Scypha. (Courtesy General Biological Supply

House. )

Qranular cells
(Dermal epithelium)

Segmentation cavity

Ragellafced cells
{Qastral epithelium)

Osculum breaks thru here

-Dermal epithelium

■ Gasbral epithelium

Gastral cavity
(future cloaca)

--Dloitopore

Typical f ree-5W/mming
Awphiblastula

Typical /Amphiblaifcu/a
at time of attachment

Fig. 56. — Diagrammatic sections of Scypha larvae.

of the male usually mature before those of the female. The repro-
ductive cells are produced in the jellylike middle layer. Fertiliza-
tion 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 129

Economic Relations

Many sponges are beneficial to man, and there are a few wliich
are detrimental. Oysters and some other Mollusca are injured or
destroyed by certain sponges which attach themselves to the mol-
lusc's body or by others which bore through its shell and thus
kill it.

Of positive importance, lesser items include the large flint de-
posits from siliceous spicules of some species and those used for
ornaments. 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
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

This group appears to be somewhat advanced over the Choano-
flagellata of the Genus Proterospongia in Class Mastigophora. In
sponges the cells are partially organized into layers and are differ-
entiated for separate functions. Sexual reproduction is developed
here in a simple way. The group has advanced so little that little
else 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 possess-
ing a single large cavity in the body. There is a single opening to
this cavity, and it functions as both mouth and anus. 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
species are marine, but there are a few fresh-water forms. The
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, cal-
careous cases around themselves. In several species there is the
typical alternation of generations of attached and free-living forms.
Most coelenterates are attached or very sedentary for at least a
part of the life span.

The radial symmetry is correlated with an attached habit of life.
A 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. A limited amount of the
digestion, however, takes place within the endoderm cells after par-
ticles of partially digested food have been engulfed by these cells.
This is called intracellular digestion. Excretion and respiration are
carried on by the general surfaces of the body. Asexual reproduc-
tion is 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

131

readily available, easily collected and handled, and is representative
of multicellular animals of simple formation. 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 is divided into three classes, each with three or four
orders.

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. — Structure of Gonionemus. ad. Adhesive pad ; g, gonads ; li, lithocyst ;
m, mouth ; mn, manubrium ; n, nematocyst ; ra, radial canal ; re, ring canal ; st,
stomach; t, tentacle; ve, velum. (From "White, General Biology.)

Order Leptolina — a group which has a sedentary or sessile polyp
stage. 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 repre-
sentative of the phylum, but since Gonionemus and Ohelia are com-
mon marine forms, a brief description of them may be included
here.

132 • TEXTBOOK OF ZOOLOGY

Gonionemus is a small jellyfish form, measuring about a centi-
meter 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 cello-
phane. The exumhrella is the convex upper, or aboral side while the
suhumhrella is the concave, lower, oral side. A short stalklike part,
the manuhiHiim hangs down from the center of the subumbrella. At
its distal end is the mouth, bordered by four oral lobes. The mouth
is 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 endo-
derm, 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 is a marine, colonial type resembling a branched plant in
appearance. The individuals are attached to each other in the
colony, and it is 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 gen-
erations. From the hydrorhiza there is an upright stem, the hydro-
caidus. 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 is the third type of individual connected with an Obelia
colony. The perisarc, which is composed of chitin, covers the colony.
In some parts this is ringed, and it expands at the base of the

PHYLUM COELENTERATA

133

Obelia habit

Mouth ■?

tb/drotheca-

Coelenteron ■''

hntoderm

tctoderm

Qonotheca
MedU5Q-bud
B\a5bo5t\/ie -

Radiol canal ^^
Repiroductive

Moubh 7^^-tirf^

Jcatocyjfc— -
Tenbacles-

Kedusa

Obelia

Fig. 58. — 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

hydranth 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 cover-
ing is often called the gonangium. Fibrous processes connect the
perisarc to the soft, inner parts (coenosarc). The cavity of the
hydranth is continuous with that of the hydrocaulus, and is, there-
fore, a part of the gastrovascular cavity.

Medusa e x

J Sperm -from

J another

J medusa

^__ ..Ferbilucd eqq

\*" "^ *;> Cleavacfe
^ cell stage. ^"^

Mature,
qonancjlcim.^^g

BJastula.

^ \arva ^

Position of
mature colony

Fig. 59. — Life cycle of Ohelia, illustrating polymorphism and metagenesis. Adult
hydroid colony with mature gonangium gives rise to sexual medusa which is pro-
duced 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,
just beneath the perisarc, 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 at-
tached around the basal margin of the hypostome. The hydranth cap-
tures 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

PHYLUM COELENTERATA

135

in the internal cavity. With the exception of reproductive proc-
esses, a single hydranth of Obelia will be 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) ; /, nettling cells. (From Van Cleave, Invertebrate
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. The medusae of this sort are spoken of as hydromedusae
to distinguish them from the scyphomedusae or jellyfishes of Class
Scyphozoa.

136

TEXTBOOK OF ZOOLOGY

Order Trachylina. — This order includes two suborders of hydro-
medusae which come from the egg- directly with no polyp stage.
Campanella and Liriope are generic examples.

Order Hydrocorallina. — This group resembles the corals by produc-
ing strong calcareous skeletons. They have extensive, branched hy-
drorhiza 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. — Physalia, the Portuguese man-of-war a floating colonial coelenterate.
(From Hegner, College Zoology, published by The Macmillan Company.)

Order Siphonophora. — This is a pelagic order of colonial coelen-
terates with extreme polymorphism. A common tube of the coenosarc
unites the five kinds of individuals of the colony, and this cavity is
continuous from one individual to another. The blind end of the
coenosarcal tube is an air-filled, bladderlike float (pneumatopJiore)

PHYLUM COELENTERATA 137

with a superior crest. The polyps hang down into the water beneath
this float. The types of polyps include : gastrozooids (nutritive or
feeding), dactylozooids -with nests of nematocysts and having long
tentacles (tactile and protective), gonozooids which are male, repro-
ductive zooids, and others which produce ova-bearing medusae. Swim-
ming bells (nectocalyces) often occur just below the pneumatophore.
Most of the individuals are specialized to such a degree that they
care for only limited functions. This specialization and diversity of
forms is such that the entire colony appears as a single individual.
Physalia, the Portuguese man-of-war, is a typical example. Its sting
is quite poisonous ; bathers coming in contact with the trailing ten-
tacles, which bear batteries of nematocysts, suffer severe pain.

Class Scyphozoa. — The coelenterates belonging here are large
jellyfishes 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.
Several of the representatives of this class are thought by some
zoologists to exist generation after generation only as medusae, but
it 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.

Order Stauromedusae. — Conical or vase-shaped medusae which usu-
ally lack marginal sense bodies (tentaculocysts). The tentacles are
distributed perradially and interradially. Lucernaria and Haliclystus
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 ten-
taculocysts perradially and interradially distributed on the margin

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TEXTBOOK OF ZOOLOGY

of the bell. Tentacles are usually present also on the margin of
the bell. This is the most numerous and extensively distributed
group of Scyphozoa. Aurellia and Stomolophus are common examples.
Aurellia* is the typical example, and, like most jellyfishes, is
composed largely of water. When they are dried, only a thin film
remains. This is a common one and ranges from New England to
the Gulf of Mexico. It may reach a foot in diameter.

Cut surface of body wall

Stomach

Qonad

^ Jub-gsnital pit

Upper portion of

manubrium

-^r — 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 in
the Gulf of Mexico. Bisected to show internal structure.

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. A suhgenital pit lies in each quadrant of this
side of the animal. The mouth leads through a short passageway
into the angular gastrovascular cavity which in turn has four lateral
gastric pouches containing the fringelike gonads. There is 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

139

Lonq tentacles --
(in Chryiaoro)

Larqe subqenibal pit (as in Chrysaora)

TentaculocyJt

Admdial canal

Perradial canal

Jhort5f'mp/e oral
- arm (Aurelia)

■ -^^ >-:, -Intenadial canal

■ ;■ . • ■ • . .'iv. - •*. -iy ■ :T*L^5=f'^^^"""V C Admdial canal

■i^-:- ■:■■■ lIfJ&;^-#- vV-i^^:?-----^'"'?^"^'

'S^_]_; V^ _ _ Aon(7 ribboh-like oral
' ^ ) j "^ arm (in Chrysaom)
- 'Small 6iibgenibal
/^C"""^ p/fc (aj /n Aurelia)

^ '^ Qastric pouch

'^ Short tentacles
(Aurelia)

Fig. 63.

Marqinal lobes (as in Chrysaora)

-Aurellia and comparative structure of jellyflshes. (Modified from figure
in Pacific Biological Laboratories' catalogue.)

Stages in the
development
of the scyphis
toma

Planula MM
larva ^^

Stages in the
-development
^of the jbrobila

Zygote

Sperm from
separate adult

Fig. 64. — Life cycle of Aurellia aurita, showing staj^es from germ cells to the
ephyra stage which precedes the adult condition.

140 TEXTBOOK OF ZOOLOGY

canal. The eight tentaculocysts are symmetrically located at eight
points ou the margin, each between marginal lappets. The tentaculo-
cysts 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.

Class Anthozoa. — All animals in this class conform to polyp or-
ganization and may be colonial or solitary. They have an ecto-
dermal esophagus and longitudinal partitions called septa (mesen-
teries) incompletely dividing the gastrovascular cavity. Muscular
tissue bands are found in the septa. The mesogloea is quite abun-
dant and contains a good many cells that resemble primitive con-
nective tissue cells. Many of these animals produce a calcareous
external skeleton called coral. Both sexual and asexual reproduc-
tion are common.

Subclass Zoantharia. — This group has numerous paired septa,
typically occurring in multiples of six, and plain tubular tentacles.
It includes sea 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
the water near shore, even in tide pools. They average about three
or four inches in height and two or two and a half inches in diam-
eter. 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 "seooch-
ing" 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

PHYLUM COELENTERATA

141

ciliated groove, the siphonoglyphe, through which water is constantly
carried into the gastrovascular cavity for respiratory purposes. The
gastrovascular cavity is divided into radially arranged compart-
ments 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 sec-
ondaries which do not quite reach the wall of the gullet, hence their
medial ends are free in the cavity. Between these and the pri-
maries 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
is interspersed among the others. There is a band of muscle run-
ning 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.

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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
disc is practiced by this animal. Occasional longitudinal fission may
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,

Hollcw
Tentacle

Vf)i.tfsl ^'

Siphonogtyph

SpSincler

Diagrammatic T. S. of Anthozoan Polyp fi\ level A-A

EnJoceet, cKamter between

two metenteriei of ,

the ume pair "-^^

Cxaeoel. cKsmber
between pair* ^
o( meicnteriei ' ■^

Directive*
(Ventral bet of
Primary Meccnteriea)

Otagrammatic T S. of Anthozoan Polyp at level B-B

Fig. 66. — Diagrams to show the structure of the anthozoan, Metridium, (Courtesy

of Pacific Biological Laboratories.)

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
external limestone skeleton ; most of them are colonial. The indi-
viduals of colonies communicate with each other by coenosarcal con-
nections. Otherwise they are similar to anemones. Astrangia, Madre-
pora, and Oculina are examples.

PHYLUM COELENTERATA 143

Astrangia is the common coral polyp, and it is quite similar to a
small sea anemone to which calcium carbonate has been added by
secretion from the ectoderm cells as well as having budded to form
a colony of numerous individuals. Coral poljqDS vary in size from
one-sixteenth of an inch to several inches in diameter. In time
continually growing colonies of these animals can produce enormous
stony barriers (reefs) in the sea. One such reef is over 1,100 miles
in length 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 typi-
cal examples.

srri,.^^.^

•Trv^.- ^»*

"^^^

Fig. 67. — Common coral, Astrangia danae. A, Stone produced by the animals
when cleaned ; B, polyps in natural habitat.

Subclass Alcyonaria. — The features of this division include eight
hollow, feathered tentacles, eight mesenteries, and one siphonoglyphe.
Colonial and pol^'morphic forms are not uncommon.

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 sym-
metrical. The common sea fan, Gorgonia, as well as the precious
Corallium rubrum are well known examples.

Order Pennatidacea. — Another colonial form whose body is modi-
fied so that one portion is submerged in the substratum. The colony

144 TEXTBOOK OF ZOOLOGY

takes a bilateral form, and the individuals are born on a disc or axial
stem which is supported by a hard skeleton. There may be dimor-
phism of zooids within the colony. Renilla and Pennatula, sea pens
and sea feathers, are typical examples.

HYDRA

Habitat and Behavior

Hydra (Chlorohydra) viridissima is likely the most common hydra
of the Southwest. It is the small green hydra which is very
active and has short tentacles. This species has the green color
because of the presence of a unicellular alga, Chlorella vulgaris, in
the 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
called symbiosis.

Most of the hj^dras are found in cool fresh water, attached to the
surface of plant leaves, smooth sticks, debris, or even the surface film
of the water. The brown hydras, such as H. americana, H. carnea,
and Pelmatohydra oligactis, are sluggish and have longer tentacles
than the green ones.*

Hydra is a sedentary kind of animal and may remain stationary
for a considerable period of time if living conditions are uniformly
good. When the environmental conditions are changing, and the
animal is in 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 nemato-
cysts. 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-

♦Recent taxonomic information concerning Hydras of the United States may be
found in the papers of Libbie H. Hyman, published in tlie Transactions of the
American Microscopical Society, Vols. 48, 49, and 50.

PHYLUM COELENTERATA

145

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

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

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.)

intensity, the animal will contract all of the tentacles and the body
also. If the stimulus is restricted to one locality and is not too
strong, the animal will contract in the affected area, by the with-
drawal of one tentacle. The movements of the animal are per-
formed by contraction and relaxation of the contractile fibers con-
nected with certain of the cells. The activities come in response
to internal as well as external stimuli.

146 TEXTBOOK OF ZOOLOGY

The common tropisms, which have been described previously, are
present in hydras. They respond to light and will find an optimum
intensity which varies with the different species. Green hydras
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 tempera-
ture 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 re-
mains 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 slip-
ping 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 reat-
tached. This process is consecutively repeated and is called "loop-
ing." Occasionally 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
bubble of gas at the basal disc and floating back to the top on that.

External Anatomy

Hydra is a macroscopic animal, but it is relatively small. Its body
is quite contractile, being able to extend from a contracted length
of two or three millimeters to a length of eighteen or twenty milli-
meters. 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. viridis-
sima 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
fingerlike tentacles, which extends free in the water. Tentacles may

PHYLUM COELENTERATA

147

Stretch out to be slender threads five to seven centimeters in length.
They are very useful either singly or as a group in capturing and
delivering food to the mouth. 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.

Mouth

Tentacle
Hypostome

Battery of
Nematocysts

Bud

Basal Disc

Fig. 69. — Hydra showing external features.

From the side it appears simply as an indentation or notch in the
conical end of the hypostome. The proximal or attached end termi-
nates 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 occasion-
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

are seasonal reproductive organs. Both ovaries (female gonads) and
testes (male gonads) may 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-
maries; if 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 hypo-
stome, which is essentially equivalent to what is called the ventro-
dorsal axis of more advanced forms. All of the metazoans have a
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 diplo-
blastic 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. Interspersed
occasionally among these cells are the larger cnidohlasts in which
develop the neniato cysts, stinging cells or nettle cells. These are dis-
tributed over all the body except the basal disc, but they are much
more numerous near the distal part of the column and on the ten-
tacles. The nematocysts are usually contained in little raised
tubercles in the ectoderm. Each tubercle contains a large barbed one
and several of a smaller variety. Four different kinds have been de-
scribed. Since the large barbed type is the most conspicuous, it will
be described here. In the cnidoblast the nematocyst appears as a
sac of fluid within which is inverted a barbed stalk with a coiled
thread attached. Projecting out of the superficial surface of the
cnidoblast is a triggerlike process called the cnidocil, which when
chemically stimulated causes the cnidoblast to discharge the nemato-

PHYLUM COELENTERATA

149

cyst. Chemicals, such as weak iodine, acetic acid, or methyl ^een,
when added to the water, will bring this about. Contact will not.
In this reaction the stalk and thread are everted, probably by de-
velopment of pressure. This type of nematocyst produces a hypno-
toxin which anesthetizes the animals into which it is discharged. In
another form the sac is small, the stalk is barbless, and the thread is
elastic ; it becomes coiled around the object against which it is dis-
charged, and thus impedes locomotion of the victim.

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 undis-
charged 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
cell, 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
near their bases. As a nematocyst is discharged, the entire cnido-
blast is replaced by an interstitial cell migrating into position, A
damaged or spent cell of the body may be replaced from the inter-
stitial cell. Besides these three types, there are the scattered, ir-

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TEXTBOOK OF ZOOLOGY

TEMTACLE

— MOUTH

•TESTI S

GASXRO-
VASCUl_AR
CAVI TY

ECTODERM
MESOSLOE A
EfslDODERM

OVARY

BASAU DISC

Fig. 71. — Diagrammatic longitudinal section of hydra, showing mature gonads and
typical cell layers. (Drawn by Titus C. Evans.)

ECTODERM

EPITHEl_IO-
MUSCUL. AR
CELL.

I NJTER-
STI Tl Al_
CEI_U

NEMATOCYST
C N I DOBL. AST

MESOGUOEA

DIC3ESTIVE

CEUUS

G l_ A IM D

CEI_U

Fig. 72. — Cross-section through the column of hydra. The central space is the
gastrovascular cavity or enteron. (Drawn by Titus C. Evans.)

PHYLUM COELENTERATA 151

regular, slender, neuroepithelial cells which are joined into a net
by intercellular processes. These cells fit between the others and
are either sensory or motor in function, thus receiving external
stimuli and also causing contraction of the contractile cells 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, par-
ticularly for the fibrils of the epitheliomuscular cells. In some of the
other coelenterates, this layer is exceedingly thick and heavy.

The inner, thicker cell layer of the wall is the endoderm which lines
the lumen of the gastrovascular cavity. The most conspicuous cells
here are the nutritive-muscular cells which are long, vacuolated struc-
tures attached to the 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 by reducing its circumference. These
cells often possess flagella at 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
themselves between the nutritive-muscular cells and secrete what
is probably a digestive fluid into the gastrovascular cavity. Neuro-
epithelial 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 em-
bryo of more complex metazoans.

Metabolism

The food of hydra consists of small insect larvae, minute worms,
small bits of organic matter in the water, water fleas, and other
small Crustacea. Ingestion of the food has been described already.
Upon entering the mouth the morsel of food is moved some distance
down 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
enteron digestion takes place. The wall possesses many more of the
gland cells in the endoderm, 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 dis-

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TEXTBOOK OF ZOOLOGY

solution of the food by the enzymes is augmented by the churning
effect of the contractions of the body. The flagella present on the
nutritive-muscular cells create currents of water which also hurry
the process. The dissolved material is presumably absorhed by the
cells of the endoderm, and by diffusion the nutrient solution reaches
the ectoderm cells just outside. Small particles of the partially di-
gested substance are engulfed by the free ends of many of the
nutritive-muscular cells by virtue of their amoeboid activity. These
particles are taken in food vacuoles, and the digestion is completed
there just as it is in an amoeba or Paramecium. This illustrates
something of the primitive organization of hydra as a metazoan.

Fig. 73. — Hydra with body turned inside out in attempting: to Ingest a piece
of meat. (From Curtis and Guthrie, Textbook of General Zoology, published by
John Wiley and Sons, Inc.)

As will be remembered, this process of converting the digested food
into 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
or circulation of nutriment. This dissolved material reaches the
remote parts of the protoplasm by diffusion through the membranes
and protoplasm generally. The gastrovascular cavity has the dual
function of digestion and circulation.

PHYLUM COELENTERATA 153

Many of the animals used as food have hard skeletal parts that
will not digest. These indigestible portions are ejected from the
cavity through the mouth by reverse peristalsis, and the process is
known as egestion. Eespiration furnishes the necessary exchange of
oxygen and carbon dioxide by diffusion through the plasma mem-
branes. The dissolved oxygen in the water in which the animal lives
is 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
to transform potential energy there to kinetic energy and heat.
Accompanying this oxidation there are produced some waste by-
products in solution including urea, uric acid, and water which
must be expelled from the body. In hydra this excretion is accom-
plished by diffusion through the general surface of the body. There
is some indication that there may be accumulation of waste prod-
ucts 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
animals.

The Nervous System and Nervous Conduction

The neuroepithelial cells are distributed among the other cells of
the germ layers. There is a greater abundance of them on the hypo-
stome, 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 primi-
tive 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
organized type of nervous system but not a very efficient one.

Reproduction and Life Cycle

Reproduction is both asexual and sexual. Asexual reproduction
is accomplished very efiiciently and quite rapidly. This process is
essentially reproduction by somatic cell division. Nutrient mate-

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TEXTBOOK OF ZOOLOGY

rial accumulates at some point near the middle of the column. The
bud first appears as a slight superficial bulge. The cell division at
this point is very rapid, involving considerable activity in inter-
stitial 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 posi-
tion 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
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, matura-
tion, 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 inter-
stitial cells which accumulate at a certain place between the ectoderm
and endoderm, where they multiply by division to form oogonia in

PHYLUM COELENTERATA

155

the female gonad and spermatogonia in the male. All phases of
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. A single sperm unites with
the mature ovum, and this zygote 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 TEXTBOOK OF ZOOLOGY

outer layer is 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
portion of one. Complete animals may be formed from very small
pieces (% mm. in diameter) of a hydra. 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 phe-
nomenon was first discovered in animals from studies on Hydra in
1744 by Trembly.

Economic Relations of the Phylum

The entire group is not worth much in dollars and cents to man
directly. A 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
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-
defined gastrovascular cavity with one opening, the mouth; (3)
presence of tentacles with (4) nematocysts or sting-bodies ; (5)
continuance of sexual reproduction; (6) distinct radial symmetry
and, (7) a nerve net.

CHAPTER XI

PHYLUM CTENOPHORA

This is a group of exclusively marine animals, most of which are
pelagic (float near the surface). There is a limited number that
lives and moves about on the bottom. Ctenophora (te nof '6 ra —
comb-bearing), because of their similarity to coelenterates, are often
classified as a class in this phylum. There are only twenty-one Amer-
ican species representing this phylum, and they are commonly called
sea walnuts or comb jellies. Most of them swim by means of eight
rows of fused cilia, called swimming plates or combs. These animals
are quite clear and transparent, with a faint tint of pink, purple, or
blue. They are often phosphorescent. There are two classes in the
phylum: (1) Tentac^data, with a pair of tentacles present either in
the larva stage or throughout life. Mnemiopsis leidyi is a lumines-
cent, 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
example.

Habitat and Behavior

These are primarily surface-living forms with rather wide distri-
bution but most abundant in tropical seas. They 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.

Anatomy

The size of different individuals of this group ranges from five
millimeters to four feet in length, and the shape may be spherical,
pear-shaped, ribbonlike or cylindrical. The symmetry is said to be
biradial since there are eight rows of radially arranged paddles or
plates which are equally distributed on each side of the median

157

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TEXTBOOK OF ZOOLOGY

line. 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
Left CAitrovfticular Cansl

Tentaeulftt 'NviVrC^j

Sheath ""~-;,.r

Tentacle

■ -S

A '-i

Bilobed Stomodaeum

Right CastrovBicular Canal
lUdial CanaU

^^ Intcrradial Cant,

i^j.^ ;,'• ■ »j^r).iT. 'i — Tranaverse or
'Yvi^^—--''' • Infundibular Plane

/.'/ ■ ;■.-• "^ Aboral 5cn«e Orgon

■.■..■•■•;■',:;'•■•'""" — ~ - Plate row with

Infundibular Canal underneath

Diagram looking down on aboral pole

Pleurobrachia bachei

Aboral End

InfunJibuTum

^ ^ Slomodaeuni

Inlerradial
Canal

Plate Row with '
branch of infundibular
canal ayilem undetnealh

'C^ Radial
\ Canal

o.. Bilobfff
Stomo<faeuin

■ Mouth

Perspective! Drawing a}ong infundibulai plane

Oral End
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 with endoderm; undigested food is
egested through the mouth. The six canals mentioned above are
called excretory canals. 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

159

tentacular pouches connected with the outside ; one lies on each side
of the infundibulum. 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 in-
ternal calcareous ball (statolith) rolls as the level of the body changes.
These animals are monoecious (hermaphroditic) ; the ova are formed
in ovaries along one side of each meridional canal and spermatozoa

Fig. 77. — Sea walnut or comb jeUy, Beroe ovata, from Atlantic Ocean and Gulf of
Mexico. (Courtesy of General Biological Supply House.)

along the other. The mature germ cells rupture into the infundibu-
lum 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 com-
pared 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 XII

PHYLUM PLATYHELMINTHES

The representatives of 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 sym-
metrical 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.

The representatives of the two parasitic classes have, for the most
part, quite 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 — little stirring). — This class con-
sists of a group of soft-bodied, elongate and usually free-living
forms. The surface layer or epidermis is ciliated in patches, and
there is a plentiful supply of secreting cells in this layer. The
digestive tube is single, three-branched, or many-branched. 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 161

Class 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 — girdle form). — This group is also char-
acterized by a heavy cuticular cover, and a long, ribbonlike body
divided into sections called proglottides.

These tapeworms each have a knoblike "head" or scolex on the
anterior proglottid. This structure is supplied with suckers for at-
tachment 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 mus-
cular tissue of several different animals. The class includes five
orders: Pseudophyllidea, Cyclophyllidea, Tetraphyllidea, Trypano-
rhyncha, and Heterophyllidea. Taenia (Figs. 400, 401, and 402),
Diphyllohothrium, Ilymenolepis are examples.

Class Nemertina. — It seems difficult to know where to classify
this 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
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 fre-
quently 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 may contract to less than two feet in length.
Locomotion is accomplished by the cilia and the contractility of
the body. The proboscis is a very characteristic organ which is

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TEXTBOOK OF ZOOLOGY

in the form of a hollow tube turned back through the body inside
of a cavity called the proboscis sheath. By contracting the saclike
sheath, the proboscis 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. 79.

Fig. 78. — A nemertine worm, Lineus socialise with the body coiled. Natural
length about 15 cm. (Redrawn and modified from Hegner, College Zoology,
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
by The Macmillan Company, after Coe.)

pouches. Both eggs and sperm are discharged from respective in-
dividuals through a dorsal pore and fertilization occurs in the sur-
rounding water. Following cleavage there is a helmet-shaped larva
called pilidium. Cilia develop on the lappets at the lower margins
of the body and on a patch at the opposite pole or apical plate. This
plate is the principal nerve center of the animal in this stage. The

PHYLUM PLATYHELMINTHES

163

adult appears after metamorphosis. In some forms there is a creep-
ing larva known as Desor's larva. The 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 cen-
tral nervous system consists of two ganglia and three longitudinal
cords passing through the body. 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. Pros-
toma, Cerebratulus, Tetrastemma are representatives.

Georaqe cavity

L'lji^^i^— -Mesenchymal cell

Stomach

Lctodermal /

invagination

EsophacjUi

.fctodermaf
invagination

Ventrolateral lobe

Fig. 80. — Structure of pilidium larva of the nemertlne worm in partial section.
(Redrawn and modifled from Wolcott, Animal Biology, published by McGraw-Hill
Book Company, Inc.)

PLANARIA

Habitat ajid Behavior

This free-living, fresh-water, flatworm thrives beneath the rocks,
logs, leaves, algae, or debris at the bottom of shallow spring-fed
brooks and pools. They must have pure, clear, cool water. These

164 TEXTBOOK OF ZOOLOGY

animals are rather gregarious and when at rest will group together
beneath objects where the light is not intense. They respond nega-
tively to bright light. They usually feed upon minute plants and
animals, dead animal bodies, and living forms, such as small arthro-
pods 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
the surface of fresh meat, apparently sucking up the nourishing
fluids from the meat. If very minute quantities of meat juice are
liberated in the water at specific points, the planarians are at-
tracted to those points.

Eye Genital pore

Side of head

PharjTix sheath Proboscis

Fig. 81. — Entire planaria with pharynx extended in position for feeding. (From
Hegner, College Zoology, published by The Macmillan Company, after Shipley and
McBride.)

The locomotion is accomplished in an easy gliding fashion by the
action of the beating cilia and muscular contractions of the body.
The ability to move along in this way is enhanced by the secretion
of slippery mucus which essentially lays a 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 behavior of this animal is of a reflex or automatic type. The
receiving or receptor sensory cell transfers the impulse produced 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 form of a lappet or "ear," called an auricle.
These are sensitive to touch and chemical stimulations but not to
sound. 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 accom-
plish locomotion. The average length of fully developed active P.
maculata is about three-fourths of an inch.

Internal Anatomy

The ectoderm covers the outer surfaces of the body and composes
the 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

166

TEXTBOOK OF ZOOLOGY

PHYLUM PLATYHELMINTHES

167

buccal cavity which it nearly fills; a three-branched enteron or in-
testine, 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 many lateral, blind extensions or pockets called diver-
ticula which greatly increase the surface exposure of the organ and
project among most of the other tissues of the body. The whole
arrangement represents a complicated gastrovascular cavity whose
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
of the bod}', which receive 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
long cilia that are continually beating in a direction toward the
tubule, the movements appearing something like a flickering flame.

168 TEXTBOOK OF ZOOLOGY

The cellular walls of the tubules as well as the flame cells arise in
the mesoderm. Under strict definition, some authors object to calling
this arrangement a system.

Another mesoderm organization is 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 at the anterior is the central portion
of the system. It is known as the cephalic ganglion and gives
branches to sensory areas of the head, auricles, etc., besides joining
the longitudinal nerve cords. The transverse commissures connect
the two longitudinal cords at from 15 to 20 points like the rungs of
a ladder. At each point where a transverse commissure meets a
longitudinal cord, is a small ganglion composed of a few nerve cell
bodies. The branch nerves extend to the surrounding tissue from
these points.

The reproductive system is fairly well developed in most species
except P. dorotocephala which rarely develops sexual organs. Its
reproduction is entirely by asexual fission. The sexual reproduction
of other planarians is hermaphroditic, which is 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 common cavity called the genital atrium or geiiital
cloaca, which opens externally at the genital pore. Some authors
describe glands which pour a seminal fluid into the system. The
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

169

by way of the vagina. There are numerous yolk glands joining each
oviduct along its length ; a glandular structure of questionable func-

— Auricle

CerebraL
(jancjlion

LorKjitudinaL
nerve cord

Testis /

Vasa

efferenbia

Lateral

nerve

Vos l_(

deferens

Mouth \_^^

lumen of

pharynx

Seminal

vesicle

5eminal

receptacle

Intestine

K-/rax^ Oviduct

•H : -— ^ / Pharyngeal

chamber

Penis

Genital pore

Fig. 84. — Reproductive system of the planarian worm. Male organs shown on one

side only.

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 of this phylum pos-
sess 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 diges-
tive system responsible for nourishment of the individual, and this
followed by the nervous system which relates the organism to its
surroundings.

Metabolism

The food is principally animal tissue with some plant matter, and
ingestion takes place through the proboscis. The food may be par-
tially digested by a fluid produced in the pharynx. The principal
process of digestion occurs in the cavity of the enteron. Here the
process is similar to that of Porifera and Coelenterata, being both in-
tercellular 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 mem-
branes of adjacent cells. Since the diverticula of this system pene-
trate all parts of the body, and the diffusion of materials supplies all
other cells, no circulatory system is necessaiy to transport nutriment.
There is no anus, so all indigestible material is egested by way of the
mouth. Respiration is accomplished through the general surface
epithelium, and oxygen is distributed by diffusion through the proto-
plasm and fluid-filled spaces of the parenchyma. Catabolism or dis-
similation 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
or male germ cells mature in the testes, then pass through the vasa
efferentia and vasa deferentia, to the seminal vesicles where they

PHYLUM PLATYHELMINTHES

171

are stored in advance of copulation. Here they become organized
into pockets known as spermatophores. The ova mature in the
ovaries, pass 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

Fig. 85. — Planarian cocoons and the young hatching.

Vitelline cells

Wandering
cells

Endodenn

Provisional
pharynx

Ectoderm

Wandering
" cells

Endoderm
-Primitive

gut
Wandering
— cells

Vitelline
ceUs

Mouth

Fig. 86. — Development of Planaria lactea. 1, E!gg surrounded by yolk; 2, four
blastomeres from segmented egg ; S, later stage ; if, still later, after blastomeres
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. )

they are thought to be stored. Cross-fertilization is practiced by these
animals. Planarians have been observed to copulate with an apparent
exchange of spermatozoa in the form of spermatophores. In copula-

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TEXTBOOK OF ZOOLOGY

tion the cirrus or penis is protruded from the genital pore to enter
the genital pore and extend into the uterus of the other copulant. In
this way spermatozoa may be transferred from each animal to the
other. Spermatozoa have been found along the oviduct as far as
the anterior portion, so fertilization likely occurs somewhere along
this tube. At breeding time zygotes are found in the atrium, and
each is surrounded by a large number of yolk cells (nurse cells).
Each yolk cell contributes its store of nourishment to the egg cell
to which it is attached. From one to several zygotes, surrounded by
many thousands of yolk cells, become enclosed in a capsule-like shell

Fig. 87. — Fission as it occurs in Planaria dorotocephala.

secreted by the genital atrium and known as a cocoon. These are
expelled from the atrium and each is attached by a stalk to the under
sides of submerged stones or vegetation in the water. In the cocoon
the embryo passes through cleavage divisions, blastula stage, gastru-
lation 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 con-
stricts and then divides into anterior and posterior portions each of
which forms the missing parts by rapid cell division. The axial

PHYLUM PLATYHELMINTHES 173

orientation of the tissue is retained ; i.e., an anterior portion develops
in the position of the original anterior portion, and a posterior por-
tion at the original posterior position. This process is 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. The animal pos-
sesses a well-defined axial organization in which the "head" por-
tion as usual has the highest metabolic activity of the body. Be-
ginning at the anterior there is a gradient of decreasing metabolic
activity until a level just posterior to the mouth is reached, and here
a sudden increase occurs. 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
anterior end of the second individual. This seems to indicate a
kind of zooid organization in the animal. In larger, older individ-
uals there may be other such points of increased metabolism pos-
terior to this first one. 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. When the animal is young, it is relatively short and
the entire body, but particularly the ''head," carries on a high rate
of metabolism. The head at this time holds a dominance over the
length of the organism. As the animal grows older, it becomes
longer, 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 muti-
lated 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 fission. A piece from the middle of the animal will regener-
ate a head portion at the anterior margin and a tail portion at the
posterior margin. A more complete discussion of this phenomenon
will be found in a later chapter 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
in higher vertebrate animals, including man. Such groups as the
intestinal flukes, liver flukes, lung flukes, blood flukes, pork tape-
worm, beef tapeworm, margined tapeworm of dog, gid worm, hy-
datid worm, common tapeworm of dog, chicken tapeworm, dwarf
tapeworm, sheep tapeworm, tapeworm of horse, and fish tapeworm
are all serious parasites. They cost many thousands of dollars and
much debility each year. A more complete discussion of this topic
will be found in the chapter on Animal Parasitism.

Phylogenetic Advances of Platyhelminthes

(1) Anteroposterior principal axis, (2) bilateral symmetry, (3) a
distinct third germ layer, the mesoderm, (4) an excretory system
of flame cells, (5) central nervous system extending with the axis
of the body, (6) specialized gastrovascular cavity, and (7) perma-
nent sexual reproductive organs.

CHAPTER XIII

PHYLUM NEMATHELMINTHES

This group is known as the unsegmented roundworms or thread-
worms. Some of the Nemathelmiuthes (nem a thel min'thez, thread-
worms) 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
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 is a total absence of cilia. They
also lack respiratory and circulatory systems, true coelom, and
definite locomotor organs. The group is very widely distributed
and is deserving of considerable attention. Some of the better
known forms are Ascaris (pigworm 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 occupy-
ing almost every possible habitat capable of supporting life. There
are many free-living, fresh water, marine, and soil-inhabiting spe-
cies, and large numbers of parasitic forms living at the expense of
other animals and plants. This is a very important class parasiti-
cally. 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 com-
mon in the free-living forms. The skeletonlike cuticle, common to
all, is shed periodically like the molting of arthropods. The nervous

175

176 TEXTBOOK OF ZOOLOGY

system is composed of a circumpharyngeal ring from which cords
extend posteriorly. It is a sensory-neuro-muscular system. The
structure of the free-living forms is generally more complex than
that of the parasitic forms. They are adapted to a wide variety of
habitats and can withstand many rigors of natural adversity, such
as freezing, high temperatures, droughts, and other unfavorable con-
ditions. Large numbers of free-living forms have not been named
and described. Representatives of this class have an intestine but
no proboscis.

Order Ascaroidea. — It includes both parasitic and free-living
forms. Ascaris (Fig. 90), the common intestinal worm, is the most
abundant. Enterohius vermicularis, the human pinworm; Strongy-
loides stercoralis, 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 in
birds by obstructing the windpipe are all 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
elephantiasis (Fig. 390) through occlusion of blood and lymph vessels.
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
in the kidneys, body cavity, and alimentary canal of mammals and
birds. The genus Dioctophyme includes the largest roundworms,
some reaching more than three feet in length.

Order Trichinelloidea. — This parasitic group has a peculiar cuticle
lining the esophagus, outside of which is a single layer of epithelial
cells. The common trichina (Fig. 396) and the whipworm are well-
known examples.

PHYLUM NEMATHELMINTHES

177

Class Gordiacea (gor di a'she a, a knot). — Superficially these ani-
mals resemble the nematodes, but the fundamental structure is quite
different, and therefore, it is likely proper to give them the rank of
a class. 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

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

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TEXTBOOK OF ZOOLOGY

in the parenchyma; in the adult female the two lateral spaces are
lined with peritoneum. The nervous system is composed of a mid-
ventral cord with a nerve ring at the anterior end. There are
rudimentary eyespots and scattered sensory cells over the body.

Fig. 89. — Structure of Acanthocrphala. Internal structure of the genital organs
of a young female nematode, Neoechinorhynchus emydis. (From Van Cleave,
Invertebrate Zoology^ first edition, second impression, published by McGraw-Hill
Book Company, Inc.)

Sexually the group is dioecious with the gonads opening into the
posterior end of the digestive tract. Fertilization takes place within
the body of the female. Gordius aquaticus and Paragordius varius
are the common examples of the group.

PHYLUM NEMATHELMINTHES 179

Class Acanthocephala (a kan th6 sef 'a la, thorn head) includes a
group, known as "spmy-headed worms," which is absolutely para-
sitic in its habits. The adults are from a few millimeters to fifty milli-
meters in length and have an elongated, flattened body when found in
the intestine of a vertebrate but become distended to a cylindrical
shape when removed to some solution outside the body. The protru-
sible proioscis is a peculiar and characteristic structure located at the
anterior end of the body. It bears numerous recurved hooks or
spines, and in many species it is capable of receding into a proboscis
receptacle or sheath. There is 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 con-
stitutes the central nervous system.

The male reproductive organs are the testes and a set of cement
glands joining the cirrus which is held in the copulatory bursa at
the posterior portion of the body. 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 con-
siderable number of embryos in the body cavity. Finally the em-
bryos 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
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
of men and hogs. It is entirely dependent on its host for furnish-
ing suitable food and environment. The 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.

External Anatomy

This is one of the largest nematodes, females commonly reaching
a length of from eight to fourteen inches and males averaging six

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TEXTBOOK OP ZOOLOGY

to twelve inches. Males are always more slender and have a curled
tail instead of the blunt tail of the female. The mouth is guarded
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
cylindrical with tapering ends. The smooth surface is marked by
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

181

extremity. 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

The body wall is composed of the thin, outer, smooth cuticle, the
epidermis, whose cells run together, and a thick layer of longitudi-
nal muscle fibers, whose medial margins are rather baggy. There

/?70uth circumesopfiagea/

aterc/s oyan'es ^

ov/c^ucts I \ 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

psei/cfocoe/

cut/c/e 1 intestine

ep/c/er/T}/5\ body ivcr//

^em/'na/ ves/c/e
seta/ soi
recta/r?

pseadocoeJ

musc/e )
cutic/e
inteet/'ne

ej(cretoru cana/

pe/ifcfl_
o//dact setae:

oyan/
ner/e oord

a/7i/:

'/Vi/sc/e
at/c/e
ep/der/nh

Fig. 91. — Internal anatomy of Ascaris Iwmbricoides. A, Diagram of lateral view
of dissection of female ; B, cross-section of tlie midregion of the body of female ;
C, longitudinal section of posterior portion of male ; D, reproductive system of
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 lateral lines. The body cavity
of this animal is a primitive or false coelom which is lined externally
by the mesoderm of the body wall and internally by the endoderm

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TEXTBOOK OF ZOOLOGY

of the intestinal wall. Ordinarily the coelom, when fully developed,
is lined both laterally and medially with mesodermic peritoneum.
This is the simplest type of animal in which the body cavity or
coelom is found. In higher forms the outer coat of the intestine is
mesodermic. The alimentary canal is quite straight and simple and
lies in the dorsal part of the body cavity. There is no need for
great specialization of the digestive system since the food is taken
from the digested material in the intestine of the host. A contrac-
tile 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 ovum, 4., and amoeboid spermatozoa, B, of Ascaris lumbri-
coides. (From Curtis and 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 around the pharynx which
gives off a large dorsal longitudinal nerve and a large ventral
longitudinal nerve. There are usually four other smaller longitudi-
nal nerves and some connectives. In the males the testis is a thread-
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 183

Reproduction and the Life Cycle

The animals copulate, and at this time the spermatozoa are intro-
duced 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 pro-
duce a crop of eggs in excess of two thousand per gram of feces.
Based on this figure, the daily production is computed to be some-
thing 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 sus-
ceptible 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
liver, thence to the heart, and thence to the lungs. By burrowing
out from here, these larvae make their way to the throat, esophagus,
and back to the 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
the host.

Relations to Man

Heavy infestation in man may cause severe hemorrhages and set
up pneumonia that is often fatal. Anemia is 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
to remove them. The toxic substances from these parasites may
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

These groups are rather conveniently considered in the same
chapter, because they are more or less isolated, small groups of the
unsegmented worm type.

MOLLUSCOIDA

This is the name of a group composed of two classes, as they are
treated here. It is usually considered a phylum name, but many
authors prefer to give each of the classes phylum rank. The justi-
fication of the latter plan may be questionable.

Class Bryozoa (bri 6 zo'a — moss animals) includes a group of co-
lonial animals often called Polyzoa, which are similar to colonial
hydroids in their manner of growth and forms. It is true that their
structure distinguishes them very readily. Nearly all bryozoans
are marine, although there are a few fresh water forms. In ex-
ternal appearance they resemble certain of the corals and hydroids.
It 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
enclosed in this area. Bugula is an example of a treelike type of this
subclass. 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.

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 soft
parts are known as polypide. They are within the primitive coelomic
cavity, the wall of which is the zooecium. The presence of retractor
muscles make it possible for each zooid to be withdrawn into the
vaselike part of the chitinous skeleton. There are some smaller
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

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.

stitute another modified type of zooid, are filamentous, whiplike
appendages. They are thought to be variations of the avicularian
modification.

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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 arrange-
ment is known as the lophophore and has the shape of a horseshoe
when expanded. Within, the digestive tube is U-shaped and termi-

liaublt

K rXUAVO'ELLA,

PECTIMATELLA

Kabi-1- " CKI^TATELLA

Fig. 94. — Three forms of fresh-water Bryozoa showing the habit of life for each.
(Courtesy of General Biological Supply House.)

nates at the anus, which is located just outside the lophophore. The
digestive tract is held in place by strands of mesenchymatous tissue
extending from the wall of the coelom. Special strands of this tis-
sue are termed the funiculus. The body is triploblastic and there-

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 187

fore composed of ectoderm, eudoderm, 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 make their appearance either in the funiculus
or in the lining of the coelom and fertilization occurs in the body
cavity. 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 indi-
vidual, the zooid of which will form a neAv colony by budding.

The branching Plumatella, which is supported by a secretion of
calcium carbonate, and the slimy Pectinatella, whose skeleton is
in the 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 stimu-
lated by it. So far as is known this group has little if any eco-
nomic 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 mol-
luscan 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 struc-
ture of this type of animal. This organ is composed of two coiled
appendages which bear numerous ciliated tentacles. The cilia pro-
duce 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

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TEXTBOOK OF ZOOLOGY

tract ends blindly in many bracliiopods. The entire tube is lined
internally with ciliated epithelium. A segmented, true coelom is
present, but the septa are a little bit difficult to distinguish. Exten-
sions of the coelom enter the arms and mantle of this type of animal.
About two pairs of nephridia are connected with the coelom and
serve in excretion. The coelomic cavities produce the gonads also.
The sexes are distinctly separate and mature germ cells are dis-
charged into the coelom, thence to nephridia and outside. Fertili-
zation takes place in the water, and a free-swimming, ciliated larva
hatches from the egg.

Digestive gland
Stomach
Heart

Adductor muscle
I

^ophophore
/ i Dorsal valve

' Dorsal mantle

Intestine
Nephridium

Muscle

Mouth

Pig. 95. — Diagram of a sagittal section of a brachiopod to show internal organs.
(From Hegner, College Zoology, published by The Macmillan Company.)

Magellania flavescens and M. lenticularis are commonly studied
forms. They are entirely marine and represent an old line of ani-
mals. There are relatively few modern forms in existence. The
group is of little economic significance.

TROCHELMINTHES

The rotifers (Rotifera) are common examples of this group known
as the Trochelminthes (trok el mm'thez — wheel worm). In early
times they were called "wheel animalcules." There is very little
difference between the trochophore larva of this group and the
adult animal. Gastrotricha constitutes another small division of
this group but will not be discussed in detail here.

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 189

Rotifers are plentiful in fresh water, and a few of them inhabit
the sea. They are microscopic in size, and they are often associated
with Protozoa. They are very resistant to adverse conditions pro-
duced by drought and may be distributed in dry form.

The body of a rotifer is bilaterally symmetrical and can be di-
vided into head, trunk, and foot. It is covered externally by a
cuticle. The so-called head is rather largely a troclial disc com-
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
wheels. They are responsible for obtaining food and for locomo-
tion. The 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 adhe-
sive glands which assist the animal in adhering to most surfaces.
The foot as a whole serves in locomotion, pushing the animal along.

The internal organs include several systems which lie in the rather
extensive body cavity or false coelom. The digestive system begins
anteriorly at the mouth which receives other small organisms as
food. It is a cavity leading to the pharynx. Inside the pharynx is
a mill-like organ or mastax, composed of chitinous jaws, which mas-
ticates the particles of food. 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
the anus. Nearly the entire internal surface of the alimentary
canal is lined with cilia which aid the movement of the food mate-
rial through it. The stomach and intestine are lined internally
with endoderm.

The excretory system is well developed and consists of a number
of flame cells, 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 and also receives products of the urino-
genital system. It 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

TEXTBOOK OF ZOOLOGY

respiration 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

.Brain

—Eye

Tooth
"Mastax
Salivary

gland

Stomach-
intestine

- Vitellarium

-Egg

Blasendarm"

_ Contractile

bladder
-Rectum

— Anus
- Foot-gland,

-r\ — Pharynx

Salivary
gland

Flame cell

Excretory
canal

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. The body wall is composed of an outer
cuticle over a thin layer of ectoderm. Under this layer is the meso-
dermal tissue which includes mesenchymatous cells and muscle fibers.

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA

191

This group of animals is bisexual, and dimorphism (striking dif-
ferences in form of the two sexes) is present. The 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
yolk gland. Rotifers may be oviparous (lay eggs), ovo viviparous,
or even a few are viviparous. The eggs produced during the sum-
mer are thin-shelled, of two sizes, and develop parthenogenetically.

Large Egg
Females

Female -

/
Small Egg

Males
Sperm

Winter Eggs
(Fertilized)

Late SuKiner

Pass Winter
in thick shell

Females Females

/ ^
Eggs'" Eggs

/ ^

Parthenogenesis Parthenogenesis k Svunner

Many Generations Manv Generations f season

I "i

Females Females

i i ^

Large Eggs Small Egg

Fig. 97. — Life cycle of the rotifer, Hydatina.

The smaller type produce males. The eggs produced during the
winter are thick-shelled, produce females only, and require fertili-
zation. 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 per-
forate 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

192

TEXTBOOK OF ZOOLOGY

of the oviduct. They then lie dormant and inactive in the water
for a period before hatching. There is considerable similarity be-
tween the adult rotifer and the trochophore larva of some annelids,

Spines

-4

-Hooks

. 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. This resemblance has prompted
the theory that the above groups are rather closely related to the
rotifers.

MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 193

Class Chaetognatha (ke tog'na tha — horse's mane, jaw). — These
small marine worms are often called arrowworms, and they are well
adapted to livmg at the surface of the ocean. Horizontal fins sup-
port the animal at the surface and also make it possible for it to
move about rapidly. 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
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. These animals are lacking in circulatory
or excretory structure.

Each individual is capable of producing both ova and sperma-
tozoa, that is, the hermaphroditic condition prevails. The ovaries
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 sper-
matozoa are discharged into this cavity and delivered to the ex-
terior 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 pres-
ence 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. These worms are found in almost every
type of free-living habitat where moisture is present. There are
many forms which live in the ocean, either swimming freely, bur-
rowing in the sand, or living in especially prepared tubes. Fresh-
water streams and ponds are inhabited by numerous forms of an-
nelids, 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 num-
bers. In the process of adaptation the annelids have become diver-
sified in their anatomical features until only a very few characters,
such as those mentioned in the beginning of this chapter, are com-
mon 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 classes:

Class I. Chaetopoda

Order 1. Polychaeta

Order 2. Oligochaeta
Class II. Archiannelida
Class III. Hirudinea
Class IV. Gephyrea

Order 1. Echiuroidea

Order 2. Sipunculoidea

Class Chaetopoda (ke top'O da, hair and foot). — This class includes
the most commonly known forms of the phylum. There are marine,

194

PHYLUM ANNELIDA

195

fresh-water, and terrestrial forms; and they all possess setae (chaetae),
or bristlelike appendages on the body segments. The setae are
chitinous and are embedded in pits of the integument. They bear
muscle attachments which make them movable and therefore useful
in locomotion. The coelom, which surrounds the straight digestive
tract, is divided between the segments by partitions known as septae.

Fig. 99. — Representative annelids. From left to right, Arenicola cristata, lug
worm ; 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 angle-
worm. (Courtesy of Denoj-er-Geppert Company.)

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 out-
side by means of a nephridiopore. The nephridia remove nitrogenous
waste materials from the coelomic cavities and from the blood.

196 TEXTBOOK OF ZOOLOGY

The inner body wall of each segment is made up of an inner longi-
tudinal layer and an outer circular layer of muscle. Segmental nerves
which are derived from segmental nerve ganglia innervate the meta-
meres 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 gan-
glion joined together by a pair of commissures. The brain, however,
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-
terior region of the body by a varying number of paired, segmental
hearts or connectives. The dorsal vessel exhibits wavelike contrac-
tile 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 chlorocru-
orin 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.

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. It possesses many setae (chaetae) located in
fleshy parapodia. In this case the parapodia with their setae con-
stitute the 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

197

the parapodial muscles. A dorsal and a ventral cirrus are usually-
present. The 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
specialization of the anterior segments. It is composed of a prnsto-
mium, which bears a pair of tentacles, a pair of palps, and two pairs
of eyes. The peristomium 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

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, para-
podium; 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 in capturing small
organisms and crushing anything which is to be swallowed. The suc-
ceeding segments are all alike 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 pro-
hoscis. The pharynx leads into the relatively narrow esophagus

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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 diges-
tive tract is a straight intestine which continues to the last segment,
where it opens to the outside.

Prostomlum

Prostomlal
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 is composed principally of a dorsal and a
ventral blood vessel joined in each segment by a pair of connecting
vessels. The blood is forced anteriorly through the dorsal vessel and

PHYLUM ANNELIDA 199

passes posteriorly through the ventral one. Its 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.

Each segment of the body except the peristomium has two nephridia
opening directly 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
sexes are separate and there are gonads in all the segments except
those in the anterior end of the body. The sex cells arise from the
walls of the coelom and when ripe pass to the outside, fertilization
taking place in the water. The fertilized egg develops into a trocho-
phore larva, which metamorphoses into the adult animal.

In the central nervous system there are two suprapharyngeal gan-
glia dorsal to the pharynx. These are connected by means of com-
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 body to
the anal segment. Lateral nerves from the ventral nerve chain in-
nervate the various organs of the worm. Two eyes receive nervous
connections from the brain and the animal is apparently able to
detect moving objects.

Order Oligochaeta. — The best known example of the order Oligo-
chaeta 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 it is so well
known and because its features represent so well those common to
the entire order.

EARTHWORM

The body of Lumbricus terrestris varies from six to fourteen inches
in length and gives the appearance of a number of rings joined in a
linear arrangement. The rings are the body segments, or meta-
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

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TEXTBOOK OF ZOOLOGY

remains constant, while the number posterior to this varies. This
is because growth is accomplished by the addition of segments poste-
rior to the clitellum.

The prostomium is a sort of knoblike lobe at the anterior end,
projecting out over the mouth. It is not considered a true meta-
mere. The first segment is incomplete due to the opening of the

Pros'f'omi'u/n

XXVI-

xxxrr-i

xxxvir-

Openjngf
ofoviducf

^Open!n0 of
"^Mpis deferens

^Sem/'ncfl
% groove

, C//fe//u/7i

^Sefae

A'lus

Fig. 102. — External anatomy of earthworm, ventral • view, segments in roman
numerals. (Prom Wolcott, Animal Biology, published by McGraw-Hill Book
Company, 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 201

ments. The openings of the oviducts through -which the eggs pass
to the outside are seen as minute pores, one on each side of segment
XIV. 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
convey sperms to the outside, are located, one on each side, in the
anterior part of segment XV. In sexually mature worms, segments
XXX, XXXI, or XXXII to segment XXXVII are swollen to form
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 chitin-
ous 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. A 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

The body of the earthworm, if cut open along the mid-dorsal
line, 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 com-
partments. These coelomic divisions communicate with each other
by means of pores in the septae so that the clear fluid which fills
the coelom can circulate freely. 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
thin layer of cells known as peritoneum (mesothelium).

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TEXTBOOK OF ZOOLOGY

Reproductive Organs

The earthworm is hermaphroditic, the organs of both sexes being
present in every animal. The seminal receptacles, oviducts, and
ovaries are female organs, and the testes and seminal vesicles are male
organs. The seminal vesicles are three pairs of light-colored bodies
located in segments IX, XI, and XII. In sexually mature individ-
uals they 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 seg-
ments 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 seg-
ments 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
of small white bodies located in segments IX and X. The ovaries
are two minute bodies located one on each side of segment XIII.

Digestive System

The mouth cavity extends through segments I to III and leads
into the bulbous, muscular pharynx which extends through segment
V. The pharynx plays the part of a sucker in securing food for

Fig. 104. — Diagram of dorsal dissection of an earthworm in region of segments
I to XXI. be, buccal cavity ; eg, calciferous glands ; cr, crop ; dv, dorsal blood
vessel ; eo, esophagus ; g, gizzard ; n, nephridium ; sb, subpharyngeal ganglion ; st,
stomach-intestine ; pe, peristomium ; II-XXI, somites ; pJi, pharynx ; pr, prostomium.
(From White, General Biology.)

the animal. The esophagus is a straight narrow tube extending from
the 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. A thin-walled intestine extends to the anus, which opens to
the outside through the last segment.

The intestine is not a simple tube but has a large fold, the typhlo-
sole, protruding into its lumen from the dorsal side giving it more
absorptive surface for the assimilation of food. The coelomic side of

•<lv

Fig. 105. — Cross section of the earthworm through a posterior segment, ch,
chloragogue cells ; cir, circular muscle fibers ; coe, coelom ; cti, cuticle ; dv, dorsal
blood vessel ; ep, epidermis ; int, intestine ; la, lateral neural vessel ; Id, lateral
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 any kind of organic
matter which may pass through its digestive tract. The animals
remain in the soil during the daytime and work their way through

I
I I

; I

I I

PHYLUM ANNELIDA 205

it 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 mat-
ter, such as leaves. Food is drawn into the mouth by suction pro-
duced by the muscular pharynx. In the pharynx it receives the
secretions 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.

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 intestino-
integumentary vessels. The dorsal vessel conveys the blood an-
teriorly and forces it along by waveiike contractions. The paired
hearts receive the blood from the dorsal vessel and by pulsating move-
ments force it into the ventral vessel which distributes it to the body
wall, the uephridia, and the intestine. In the intestine food is taken
up ; in the skin gaseous exchanges are made with the water in the
moist soil ; 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, and then returns to the dorsal vessel by way of
the parietal connectives. The blood flows from the intestine through
the typhlosole into 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 vessel carries it anteriorly. The circulatory system
is equipped with numerous valves which keep the blood from flow-
ing in the wrong direction.

fieart

Dorsal vessel

Intostlno-tegxuueniai;
-— — vessel
,- Ventral vessel

Sub-neural vessel

Septa
Dorsal vessel

Septa jl IX Septa

Intestlnotegtimentary
vessel

VIII
Dorsal vessel

CEsopbagus

Typhlosolar

vessel

Ventral vessel

Sub-neural vessel £

Efferent Intestinal vessel;

Kephridium
Lateral-neural vessel

Afferent intestinal vessel

Ventral
vessel

Sub-neural 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 X ; 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
in a moist environment, this type of respiration is possible.

Excretory System

The function of excretion is cared for principally by the paired
nephridia, which are found in each segment except the first two and
the last one. A single nephridium consists of a ciliated funnel (the
nephrostome) , a thin coiled tube, and a nephridiopore. The cilia of
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

^...^.'S-. 107.— structure of a nephridium from earthworm, a, ampulla between the
ciliated and nonciliated portions of the intracellular canal ; 6il, ciliated canal coe
nn^Mr'^f'^^^'^ ^''T' coelomic epithelium; ext, external opening ^nephridiopore) Zc^
fneT ?;/**in^ter.^iii T^'' septum between somites; nst, nephrostonie^ (internal open-
mjWikherf h^ r^^t'^^^"" ^anal. (From Parker and Haswell : Textbook of Zoology,
published by The MacmiUan Company, after Meisenheinmer, after MazE^irski.)

208

TEXTBOOK OP ZOOLOGY

The Nervous System

The "brain" of the earthworm consists of the suprapharyngeal
ganglion, two circumpharyngeal connectives, and the suhpharyngeal
ganglion. 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 portion of the nervous system of an earthworm from dorsal

view.

Each ganglion is really the fusion of two, a deviation from the con-
dition found in many annelids and arthropods where there are two
ganglia in each segment and the nerve cord is double. The supra-
pharyngeal ganglion lies dorsal to the pharynx in the third segment
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 modi-
fied in the ventral ganglia and sent to the responding organs by
efferent neurons. Nerve impulses then have the nature of a simple
reflex except that the ventral ganglia are connected by association
neurons which conduct stimuli from one to the other. Because of
this arrangement a stimulus applied to any part of the body will
cause responses to occur in a wavelike manner in both directions
from the point of stimulation. Located in the dorsal part of the
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

As has already been described, the earthworm is hermaphroditic.
Self-fertilization does not occur, 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-
tions and the ventral surfaces of their bodies in close contact from

Apertures of
5eminal receptacles

Aperture of
Vas deferens

C /it el lam

5eminal droove Pore of oviduct

Dorsal
blood vessel

body wall

Intestine

5eminal
groove

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. With their bodies in close contact a
closed passage is formed between the genital openings of the two in-
dividuals. Sperms pass from the testes out through the seminal
vesicles and vasa deferentia to the closed passage and move through it

blastocoef

t^eso blast
cell

Endoderm

Coelomk
cavities

Hesodermic
tissue

Ectoderm

Fig. 110. — Development of the earthworm. A, blastula, surrounded by a mem-
brane ; B, section of a blastula showing blastocoele and one of the mesoblast cells
(primary cell) of mesoderm layer; G and D, showing stages in the beginnmg of
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
archenteron ; G, later stage showing cavities in the mesoderm ; H, gastrxila in
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

211

to the seminal receptacles of the mate, where they are stored. In
the meantime, the clitellum of each individual secretes a band
which binds them together at these two points. After each has re-
ceived 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 re-
leased into it, and as it passes the openings of the seminal recep-
tacles sperms which came from the reproductive mate are released.
Both ends of the band close, forming a cocoon in which fertiliza-
tion and development take place.

Fig. 111. — Regeneration and grafting in earthworm. A, anterior five segments
regenerated from posterior part of a worm ; B, tail regenerated from the posterior
portion of a worm ; C, tail regenerated from an anterior piece of a worm ; D,
union of three pieces to make a long worm ; E, grafting of two pieces to form a
double bodied worm ; F. anterior and posterior pieces grafted together to form a
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, and the cells resulting from its divisions
move into the cleavage cavity, and will form the mesoderm. As the
mesoblast cells 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 experi-
ments 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 re-
sults. When any part of the tail region is cut off, the lost parts
readily regenerate. Numerous grafting experiments have also been
performed on earthworms. Almost any part of an individual
grafted to the cut surface (if properly located) of another will fuse
to it and grow. In this way numerous unusual forms of earth-
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. They are very small and lack both
setae and parapodia. Internally they are very similar to the earth-
worm. The best known example of this group is Polygordius,
which has a long cylindrical segmented body with a pair of ten-
tacles on the prostomium. Two ciliated pits are present as a reten-
tion of juvenile characters. The troehophore larva is common to
the entire group.

Class Hirudinea (hlr u din'e a, leech). — These animals are com-
monly known as the leeches. They are usually flattened dorsoven-
trally, possess both an anterior sucker and a posterior sucker, have
characteristically thirty-two segments and possess no external ap-
pendages. 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,
there is a muscular pharynx, a short esophagus, midgut or crop,

PHYLUM ANNELIDA

213

intestine, and ectodermal rectum. Blood which is sucked from an-
other animal receives a ferment from the salivary glarids of the
pharynx, which prevents it from coa^lating. It is then stored in
the diverticulae of the crop. The animal is capable of ing'esting three
times its own weight in blood, and, since several months may elapse
before it is all digested, frequent feedings are not necessary. The

Fig. 112. — Development stages of Polygordius, an archiannelld. A, blastula ;
B, gastrula ; 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, eye-
spots, and tentacles; H, adult worm, showing faint external segmentation. (From
Newman, Textbook of General Zoology, published by The Macmillan Company,
after Whitman.)

214

TEXTBOOK OP ZOOLOGY

coelom is very much reduced due to the excessive development of the
mesodermal tissue. Each animal is hermaphroditic. Sperms are
placed on the skin of another leech, and they apparently work
through it into the ovaries, where fertilization occurs. Development
takes place in a cocoon produced by the clitellum. Two nephridia are
present. The nervous system is typical of the annelids.

Fig. 113. — Glossiphonia fusca, a leech, showing annulation, segmentation, and
internal organs. I to XXVII, somites ; 2-70, annuli ; an, anus ; dt. ej., ductus
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
possess a trochophore larva. They are usually comparatively large
and live in shells, crevices, and such other places as will afford protec-
tion. In this class, the representatives of the order Echiuroidea have
a well-developed prostomium, used in capturing prey and in loco-
motion. In Bonellia, the female is the normal individual, while the

PHYLUM ANNELIDA

215

male has no proboscis, is ciliated, and lives in the segmental organ of
the female. Representatives of the order Sipunculoidea have no
prostomium in the adult.

Nephrostomo

Male genital pore

Alimentary canal

Anus

Fig. 114. — Bonellia viridis. Female (above) has bifurcated proboscis ; male
(below) is ciliated over the surface and much smaller. (From Hegner, College
Zoology, published by The Macmillan Company.)

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 them to decay.
Their continuous burrowing through the soil also makes it porous,
a necessary condition for plant growth.

Earthworms have also less desirable qualities. They serve as
secondary hosts for parasites of several animals. Most of the para-
sites having the earthworm as a secondary host live as adults in
birds, pigs, and other animals which use the worms as food.

They have created a 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
did not appear to be at all numerous, but with the presence of water
they have become very abundant.

216 TEXTBOOK OF ZOOLOGY

The medicinal leech was once used in the bleeding of individuals
as a treatment for various ailments. Various forms of leeches live
as parasites on turtles and other forms 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
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

(1) 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
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
a rather 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 charac-
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 sys-
tem 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 primi-
tive radial symmetry while the Echinodermata have only a second-
ary 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 animals as starfishes, brittle stars, sea urchins, sea cucum-
bers, and sea lilies.

Class Asteroidea. — The general features of the body include a
central disc usually with five arms or rays radiating from it. There
are some species which do not adhere to this pentamerous condition

217

218

TEXTBOOK OF ZOOLOGY

and have up to forty rays. The rays are not usually sharply con-
stricted from the central disc. An ambulacral groove is present in
each ray. This groove is formed by a particular arrangement of
skeletal plates. Skeletal plates also support the aboral side of the
central disc as well as the region around the mouth. Surrounding
the spines on the skin are distributed some pincherlike calcareous

^^«SlK*n,

Fig. 115. — A brittle star, oral view.

organs called pedicellariae. These vary considerably in different
species. The anus and madreporic plate are located aborally. The
mouth is located on the oral or ventral side of the central disc, and
the radiating parts of the vascular, ambulacral, and nervous, sys-
tems lie orally along the ambulacral groove of each arm. Asterias,
Astropecten, Pisaster, Solaster, Oreaster and Echinaster are repre-
sentative genera.

PHYLUM ECHINODERMATA 219

Class Ophiuroidea. — Brittle stars and serpent stars. There is a
small central disc with five long, slender rays which are clearly
marked off from the disc. The rays are lacking in ambulacral
grooves. The tube feet do not serve in locomotor functions but are
tactile only. Both the madreporite and mouth are located ven-
trally. On the oral side of the central disc, five interradial sets of
skeletal plates project toward the mouth in the formation of jaws
which are operated by muscles in chewing food. The anus is not
present. The viscera are confined to the central disc. The brittle
stars and serpent stars are quite active and live in the shore waters.
At low tide they may be found under rocks and debris, but they
move about and feed during high tide. The ability of autotomy

Fig. 116. — Oral view of a basket star belonging to class Ophiuroidea. (By courtesy

of General Biological Supply House.)

(self -mutilation) is so well developed here that arms will become de-
tached by merely grasping them. This makes it difficult to collect
entire animals alive. Ophioderma, Ophiura, Ophiothrix and Gorgono-
cephalus are common Atlantic and Gulf Coast genera.

Class Echinoidea. — Sea urchins and sand dollars are representa-
tives of this group the members of which have lost the rays but
still retain the pentamerous (five division) condition of the body.
The sea urchins are globular or hemispherical, while the sand dol-
lars 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

TEXTBOOK OF ZOOLOGY

of plates can be compared to a starfish with its arms turned up over
its body until the tips all touch each other. The surface of the
skeleton bears processes which support movable spines. Tube feet
may be thrust out through perforations in the plates of the ambu-
lacral rows. These rows correspond in position to the ambulacral
grooves of the starfish. The plates of the inter-ambulacral rows

Madrepori'fe

W':-

ItT{-eranjbu/acra( ^
p/afes *^

Anybu/cfcraf
p/afes

Fig. 117. — Dried test of the sea urchin, Arhac'xa. A, shows arrangement of the
plates on the aboral side; B, oral view showing mouth and perioral area. (From
Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.)

are not perforated. 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

PHYLUM ECHINODERMATA

221

removing algae from rocks for food. This arrangement constitutes
Aristotle's lantern, and the esophagus leads internally from its
aboral part. Aborally, each ambulacral area ends in a single ocular
plate at the periproct. A series of genital plates interspace the
ocular plates of the periproct, and one of these genital plates has
become the madreporite. The anus is a small pore located near the
center of the periproct.

Jtomac/?

flmbalacral
Ridge

Fig. 118. — A portion of the test of the sea urchin has been removed to show the

internal organs.

Upon first view, a dissected sea urchin seems to be almost entirely
gonads, internally. In each pentagonal section there is a mass of
the gonadal structure which is held in place by a band of tissue
known as the genital rachis extending from the aboral wall. Small
genital ducts lead 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 mad-
reporite to a circumoral or circumesophageal canal which encircles
the mouth. A radial vessel extends from the circumoral canal through
the length of each ambulacral arch. In each ambulacral plate are
found two pores for a tube foot which is a lateral branch of the

222 TEXTBOOK OF ZOOLOGY

circumoral canal. Each foot is connected with a bulblike ampulla.
These tube feet along- with some mobile spmes constitute the loco-
motor 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. From it the intestine leads in an opposite
direction and in sea urchins finally opens externally by the median
aboral anus. In the sand dollar the intestine passes along the poste-
rior ambulacral plate to the anal opening which is near the margin
of the disc-shaped body. A branch from the esophagus runs par-
allel to the stomach and finally joins it. This tube is heavily

Fig. 119. — Thyone, tlie common sea cucumber.

ciliated internally and is known as the siphon. Its function is con-
jectured to be either respiratory or a means of washing refuse from
the intestine. The principal organs of respiration are the inter-
radial pouches and the tube feet. The nervous system is composed
of a circumoral ring with radial cords extending into the ambula-
cral areas. Strongylocentrotus, Arbacia, Tripneustes, Clypeaster,
and Echinarachnius (sand dollar) are representative genera of the
group.

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

223

color. There is some remnant of the pentamerous condition in that
there are five double rows of tube feet extending lengthwise on
five sides of the body of some forms, others have less or none. The
expanded body of a holothurian is soft like a bladder partly filled
with liquid and the body wall is very muscular. The madreporite
is located internally. The row^s of tube feet serve as structures of

■Oral fen-f-ac/es

R/n0 cai/7a/

Po//<frtf
ves/c/es

Afnpa//ae

/nusc/es

/nusc/e ba/7cf

Coviencrr7

Soncrcf

cr— -\V^/r?^esf/'^e

!_'- b/ooc/ yessef

bfood vessel

t^^^fBi fiespira-^ory

/?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. Some of them, next to the mouth,
assist the tentacles in 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

The digestive system of most sea cucumbers consists of a short
esophagus which is supported by a skeletal structure at the point
where it enters the body cavity. This structure serves as attach-
ment for the tentacles and retractor muscles. Following the esopha-
gus is a short but rather inflated stomach which leads to the long,
coiled intestine. This tube is partially supported by a mesentery
which is attached to the midventral line of the body wall. The in-
testine 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 ambu-
lacral 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 cucum-
ber, 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 character-
istic of the group. Representative genera of this class include ;
Thyone, Holothuria, Cucumaria, Leptosynapta, Aphelodactyla and
Caudina.

Class Crinoidea. — Most of the sea lilies live attached by long
stalks, but a few are free. At the free end of the stalk are located
the five, many-branched arms which make up the calyx. The branches
of the anus are called pi^inules. In most forms some lateral pro-
jections, called cirri, are distributed at regular intervals along the
stalk. The mouth is located in the uppermost center of the calyx
and is surrounded by the anus. The anus 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 ciliated amhidacral grooves which
serve to transport food to the mouth. Modified tube feet are present,
but they serve more as tentacles than as feet. They lack ampullae

PHYLUM ECHINODERMATA

225

and are lactile 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-

Fig. 121.

^TALKZD CKIWOID

-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. A few
scattered individuals may be found on muddy or sandy shores, but
they are quite scarce. They are often found clinging to pilings,
old boats, and other objects in the water. By action of the tube
feet they are able to cling very tenaciously to almost any solid
object. At low tide they 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
habits. They ordinarily live and move about with the oral side
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

The body is composed of a central disc and some (usually five)
radiating arms or rays. 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 ar-
ranged somewhat in rows. Between the spines the exposed skin
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
plate with which the jaws articulate. There are large and small
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

227

constitute the hivium. The other three arms and their adjacent por-
tions of the central disc compose the trivium. On the oral side sur-
rounding the mouth is a perioral membrane or peristome. An amhu-
lacral groove, containing rows of tube feet, radiates from this along
the oral side of each arm. A reddish pigment spot in the end of
each arm is called an eye. The spines are longer and stronger around
the mouth and along the margins of the ambulacral grooves than
elsewhere.

Fig. 122. — The ochre starfish, Pisaster ocJiraceus, an abundant form along- the
Pacific coast (XVi). (Johnson and Snook, Seashore Animals of the Pacific Coast,
published by The Macmillan Company.)

Internal Anatomy

The body wall is relatively strong and hard without being per-
fectly rigid. This condition is due 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.
They lie in a flat position in the aboral portions of the body wall.
The skeleton of the ambulacral grooves consists of four rows of
articulated, 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
plates. The outer rows of plates, forming the margin of the groove,

Fig. 123. — Dissection of the starfish, Asterias. The aboral wall has been re-
moved from 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,
rectal gland ; T.F., tube feet : T.F.2, arrangement of tube feet in skeletal ray.
(From White, General Biology, published by The C. V. Mosby Company.)

PHYLUM ECHINODERMATA

229

are shorter and are known as adamhulacral plates. Five flat oral
ossicles 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
this cavity are located the organs of most of the systems. The
digestive system is a modified tube extending vertically from the
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 esopha-
gus 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 ;
amb, ambulacral ossicle ; coe, perivisceral coolom ; db, dermal branchia ; hca,
hepatic caeca ; musj muscle ; oSj ossicle ; pd, pedicellaria ; ph, perihemal space ; ra,
radial canal ; rn, radial nerve ; rv, radial blood vessel ; sp, spine ; Sp, septum m
radial blood vessel (rv) ; tf, tube feet; v, valve between tube foot and radial
canal. (From White, General Biology.)

as hepatic or pyloric caeca, is located in each arm, and each pair
joins the pyloric pouch by a duct which seems to be a continuation
of this pouch. These glands and possibly the pyloric pouch pro-
duce digestive enzymes in solution. The fluid secreted by the wall
of the cardiac portion probably does not contain enzymes. A short
rectum or intestine leads aborally from the pyloric pouch to the pore-
like anus at the exterior surface of the central disc. Two brown,
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

TEXTBOOK OF ZOOLOGY

over it. The tube feet are attached to the shells, and, by coopera-
tive activity, an enormous pull is exerted on the valves of the
shell. After the shell is open, the stomach of the starfish is everted
through its mouth and is spread over the tissues of the prey which
are digested in situ. An abundance of digestive fluid secreted over
the food causes the mollusk to be digested in its own shell and this
material is then taken into the stomach of the starfish. It is reported
that between four and five dozen clams may be eaten by a single star-
fish in a week. It has also been shown that a starfish may survive
after months of fasting. After feeding, the stomach is withdrawn
into the body cavity by five pairs of retractor muscles, one pair ex-
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 'pores from them are located in the crevice be-
tween adjacent arms.

Fig. 125. — Longitudinal section through the central disc and one ray of a star-
fish, a, anus ; am, ampulla ; car, cardiac stomach ; coe, perivisceral coelom ; ey,
eyespot ; hca, hepatic caeca ; i, intestine ; m, mouth ; tnp. madreporic plate ; nr.
nerve ring ; oe, esopliagus ; os, ambulacral ossicle ; py, pyloric sac ; ra, radial
canal ; re, ring canal ; rca, lectal caeca ; sc, stone canal ; sp, spine ; tf, tube feet.
(From White, General Biology.)

The water-vascular system is composed of the madreporite, stone
canal, circumoral or ring canal, radial canals, Tiedemann's bodies,
lateral canals, ampullae, and tube feet. Water is taken in through
the sievelike madreporite on the aboral side of the central disc and
is conducted by the S-shaped, calcareous stone canal (hydrophoric
canal) to the ring canal, which encircles the mouth. The movement
of the water through the madreporite and stone canal is 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.
These bodies produce amoeboid cells. The five radial canals extend
distally, 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

PHYLUM ECHINODERMATA

231

foot. The ampulla is bulblike ajid is located above the roof of the
ambulacral groove in the coelom. It is connected through its
ambulacral pore with the contractile tube foot which hangs down
into the ambulacral groove. The distal or free end of the foot
has a slightly inverted, suckerlike shape. The proximal pair of

Radial canal J^

tiadreporite
Stone canal

Tiedemanri's body

Tube foot

Fig. 126. — Diagram of water-vascular system of the starfish.

Fig. 127. — Starfish "walking" on glass. Notice the extended tube feet. (Courtesy

of General Biology Supply House.)

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 am-
pullae contracting to force water into their respective tube feet

232

TEXTBOOK OF ZOOLOGY

TU A.

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 show-
ing 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 short-
ened. Q, Aboral view of a young starfish showing the developing spines.

(Legend continued on opposite page.)

PHYLUM ECHINODERMATA 233

to extend them. The walls of both ampullae and tube feet are mus-
cular. 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
drawing itself. When the pressure is released by the ampulla, the
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.

A 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
coelom and bathes all of the organs, serves as a circulatory medium
in that it absorbs the digested food and distributes it. This fluid
bears amoebocytes which are cells capable of picking up particles
of waste material and carrying them to the dermal branchiae, where
they pass through the membrane to the exterior. These dermal
branchiae are pouches of the coelomic wall which extend outward
between the skeletal plates and have the additional function of
respiration. When these pouches are completely extended, they
nearly cover the exterior surface of the animal, and thus expose
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
extent also. There is a certain amount of diffusion of dissolved
wastes through the dermal branchiae and the walls of the tube feet.

The nervous system is radially arranged about the oral ring which
encircles the mouth just orally to the ring canal. From the oral
ring, a radial nerve extends the length of each arm and ends in the
pigmented eyespot. These nerves lie in the roof of the ambulacral
grooves. The aboral surface is supplied by a less conspicuous aboral

a, anus ; ac, anterior coelom ; ad, anterodorsal arm ; b, brachiolar arms ; ci,
adoral ciliated band ; dr, dorsal surface developing rays ; es, esophagus ; f, point of
fixation ; int, intestine ; I, lateral arm ; Ic, left coelomic pouch ; m, mouth ; md,
median dorsal arm ; mp, madreporic pore and water tube ; pad, posterodorsal arm ;
po, postoral ciliated band ; pr, preoral ciliated band ; re, right coelomic pouch ; sp,
spines; st, stomach; w. five water tubes of the water vascular system. (Modified
from Wilson and McBrlde. By permission, The Macmillan Co.)

234 TEXTBOOK OF ZOOLOGY

nerve which extends from an anal nerve ring. Branches of these
nerves extend to the numerous nerve cells distributed in the epi-
dermis above the nerve cords. The pigment eyespots at the tips
of the arms are photosensitive and sensitive to touch. The pedi-
cellaria and tube feet are also sensitive to touch. There is little
centralization except in the oral ring and radial cords, still there
is sufficient centralization for the necessary coordination exercised
by the animal.

Reproduction and Life Cycle

The starfish is dioecious ; i. e., the sexes are separate. The repro-
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 •

arms. Mature eggs produced in ovaries of females and mature :

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 sur-
face 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 hipin-
naria in which there are several extended symmetrical processes,
is known as the hrachiolarian stage (Fig. 128). Following this condi-
tion is a metamorphosis during which many processes are formed, and
the radial 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
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
quite well developed with regard to its arms. Any or all of the
arms of the starfish may be lost and the missing parts regenerated.
(Fig. 367.) An arm with a small portion of the central disc will
regenerate the missing parts under favorable conditions. A muti-
lated arm or one caught in the grip of some enemy may be cast off

PHYLUM ECHINODERMATA 235

by breaking loose at the constricted point where it joins the central
disc. This ability of self -mutilation is known as autotomy. Follow-
ing autotomy there is regeneration of a new part.

Economic Relations

Compared with many other animals the echinoderms are relatively
unimportant economically. The sea cucumbers of several different
species are used as food by the Chinese and other oriental people.
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. This dried product is semileathery
and gelatinous. It is quite expensive and is 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 con-
tains a considerable quantity of roe at the season just before
spawning. Production of "sea eggs" has become quite an industry
in the Orient, Italy, and the West Indies. The Barbados are par-
ticularly noted for their production of this commodity.

Perhaps the starfish is the most important of the group, but its
relationship is almost entirely of negative importance. It is one
of the worst enemies of clams, oysters, and snails. The starfish
grows in enormous numbers around the oyster beds of the Atlantic,
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
to dry. Salted or smoked starfish roe (eggs) are considered a
delicious food by many people.

The brittle stars and crinoids have little value except as geologi-
cal indices and biological specimens. Their skeletal parts contribute
to the formation of limestone.

CHAPTER XVII

PHYLUM MOLLUSCA
(By Elmer P. Cheatum, Southern Methodist University)

GENERAL CHARACTERS

The phylum Mollusca includes such familiar animals as the snails,
clams, oysters, and cuttlefish. Even though they appear different
externally, all are soft-bodied, unsegmented, usually bilaterally
symmetrical, and most of them produce a shell composed princi-
pally^ of calcium carbonate. A muscular foot is present which may
be modified for different functions. In the snail it is used for
creeping ; in the clam for plowing through the substrate, and, in the
nautilus or squid for seizing and holding prey. 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 respira-
tion. 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
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 are sluggish animals and occupy a
diversity of habitats, occurring abundantly on land, in fresh water,
ajid in the sea. Although most of the species live in moist sur-
roundings, a few inhabit arid regions. Some species, such as the
cuttlefish, are strictly 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 vari-
ous marine forms bears close resemblances to the trochophore larva
of the annelids. Whether or not they are direct descendants of the

236

PHYLUM MOLLUSCA

237

annelids is a matter for conjecture since some morpboJogists regard
this similarity in larval forms as an example of adaptive parallelism
in a similar type of environment. Certainly morphological evi-
dence shows a close relationship.

THE SNAIL

(Detailed description based on Helix)

Habitat and Behavior

Snails occupy a variety of habitats. They occur abundantly in
fresh water, salt water, brackish water, and thermal springs; they
live in the arid sections of the country and occur abundantly in the
tropics where certain arboreal forms are found. Some species belong-
ing to the genera Caecilianella and Helix live underground, feeding

[Mesodermal

band

EsopbaS
Head kidney.
Qtocfst

Mesenchyme

Apical or^an
Eye

Stomach

Preoral
ciliated
ring

Blastocoelz

_, ■MS

Rnal vesicle

Apical or^an

Endoderm
Embryonic mMcle

Prototroch

Mesoderm

Jelotroch

Fig. 129. — A, Trochophore larva of Eupomatus (a polychaete annelid), side
view. (After Shearer.) B, Veliger larva of Patella (a marine snail) frontal sec-
tion. (After Patten. (Drawn by Joanne Moore.)

on roots of plants ; many other species live deeply embedded in moist
humus. Certain species, such as Helix hortensis and Helix aspersa,
excavate holes in rocks and live in them. Although most snails are
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 palustris, Physa gyrina and Helisoma trivolvis, when
frozen gradually, can live at least several weeks in solid cakes of ice.
Land 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 ac-
tivity in the woodland may show only nocturnal activity in an open,

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TEXTBOOK OF ZOOLOGY

exposed habitat. Movements of most land snails appear to be co-
incident with moisture rather than darkness. Preceding prolonged
periods of cold, land snails may move to protected places, such as
beneath dead logs, dense mats of humus, crannies in or under

Lymnoea

buUmoides

techelia

Physa
bumerosa

Tropicorbis
liebmanni

Lymnaea .
stagnalis

(Lymnaeidae)

Physa
anatina
( Fhysidoe)

Menetus
dllatatus

Lymnaiza
palusbris

Ferrissia
excenthca
(Ancylidaej

HelisoiriQ

tnvolvis

lentum

(Planorbidac)

Fig-. 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

239

Water snails are active all four seasons, 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 move-
ments of Lymnaea, Physa, or Relisoma may be observed through the
ice. During periods of dry weather when ponds and creeks dry up,
snails embed themselves in moss and mud, and in this manner are
able sometimes to survive long periods of drouth. During this con-
dition epiphragms may be formed in certain species {Lymnaea
palustris), the same as in land snails; these structures probably func-
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 ob-
serv^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
roemen

bulimulus
dealbatus
liquabilis

Rurnina
decoUata

Fig. 131. — Common terrestrial snails.

The life span seems to 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
years old.

Parasitism and commensalism are both exemplified by certain
species of snails. A commensalistic relationship exists between the
rare mollusk Lepton squamosum and the crustacean Gehia stellata.
The former feeds on secretions produced by the latter. A few species
of sponges, echinoderms, annelids, and mollusks are parasitized by
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,

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TEXTBOOK OF ZOOLOGY

some shells are shaped like house roofs (Patella that lives in the sea
or Ferrissia, a fresh-water form). The worm shell (Vermetus
spiratus) is so loosely coiled that it superficially resembles a worm.
Some shells, such as those belonging to the genus Murex, may have
long peculiarly curved spines extending out from the main shell
body that give to the shell a grotesque appearance. In the sea and
land slugs the shell is either rudimentary, internal, or absent.

Polyqyra ^ Polyqyra
texasiana dorfeuiHiana

Helicina

orb'iculata

tropica

f?etinella
indentata
paudiirata

_ .... Euconulus

Zonitoides chersinus

orboreus trochulus

Strobilops

lobyrinthica

texasiana

(qostrocopta
armifera

Succinea Zuqfandina Pupo'ides Phiiornycu^
Qvam sincjleyana marq'inatus carolinensis

Fig. 132. — Common terrestrial snails.

If the shell is held with the aperture toward the observer and the
aperture is on the left, the shell is said to be sinistral; if on the right,
the shell is dextral. Most species are normally dextral, but occa-
sionally a reversal occurs which has been found to be inherited.

The shell which is largely composed of carbonate of lime is
secreted by the mantle and usually consists of three layers. Em-
bedded within the latter may be pigments that give the occasional
brilliant colors to certain species. The thickness of the layers is
dependent on the richness of lime salts in the environment; thus,
snails living in an acid bog have thin transparent shells, whereas
the same species inhabiting an area rich in lime salts have thicker,

PHYLUM MOLLUSCA

241

perhaps opaque shells. Certain species, such as Polygyra roemeri,
P. alholahris and P. texasiana are capable of repairing broken shells
if the damage is not too severe.

Body. — 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

Vemetus spirgtus
{Worm shelf)

Limacina ausMb
(Ptefopod)

Muxextenuispina
[Venus's comb)

, Urosalpinx ,
{Oyster drill)

. Aeolis
(sea slug)

Teredo navalis.
(Ship worn)

Fie:. 133. — Marine mollusks.

true tentacles which are probably sensitive to contact and smell, and
a pair of stalked "eyes" which can possibly detect different light
intensities, but are not sight organs. Our common genera of water
snails (Lymnaea, Physa, Helisoma) have their eyes situated at the

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TEXTBOOK OF ZOOLOGY

base of the tentacles. Just in front of and below the tentacles is a
mouth. Located on the side of the head is the genital pore. The
broad muscular foot is covered with a mucus-secreting integument.
Just ventral to the mouth is the opening of the pedal gland which
deposits a highway of mucus over which the snail usually glides;

Respiratory
aperturTz

Velum

i I Mouth

Genital aperture ' ^V^,
Tentack

Respiratory aperture

Jtaihed eye

Edqe of wantle
Foot

~1'
Genital aperture

Fig, 134. — Fresh-water and land snails with bodies expanded. A, fresh-water
snail, Lymnaea; B, land snail, Humboldtiana.

the gliding movements are scarcely perceptible. In some marine
snails the surface of the foot is covered with cilia, the latter facili-
tating movement. The visceral hump, which encloses the digestive,
circulatory, respiratory, excretory, and reproductive systems, is
protected by the shell which is lined with the mantle. A thick

PHYLUM MOLLtJSCA

243

collar is produced where the mantle joins the foot, and just beneath
this mantle-collar is the respiratory aperture ; back of the latter
is the anal opening.

Internal Morphology

Digestion. — Just within the mouth of a snail is a rounded organ
known as the buccal mass. The latter is 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
/ / VoQina
/ /Solvarv 0/ane/

/Penis

/Crop
'' ^-Tentacle

enilal Pore

Anierior

''Tentacle

-Pharynx
'Mouth

^\ ^Cerebral Ganglion
^Sa/ifo. ry

•.bra I Canal,
yry Qucr

Fig. 136. — Internal anatomy of Helix. Shell removed.

minute recurved teeth, the radula, supported and moved by con-
nective tissues and muscles. On the roof of the mouth is a horny
jaw which pulls food into the mouth cavity. It is then rasped by
the radula into fine particles and mixed with saliva which flows
into the buccal cavity from salivary glands that lie on each side of

244 TEXTBOOK OF ZOOLOGY

the crop. The masticated food is then passed into the esophagus
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 con-
vert starches into glucose, and, in the case of Helix, the ferment is
powerful enough to dissolve the cellulose of plant cells, thus releas-
ing the protoplasm so that it may be utilized. From the crop, food
enters the stomach and is passed on into the intestine where absorp-
tion takes place. Feces are discharged to the outside through the
anus.

Respiration

Land and most fresh-water pulmonate snails breathe by a fold of
the richly vascularized mantle which has been modified into a
primitive lung, whereas the branchiate snails breathe by true gills.

In all probability pulmonate snails that inhabit the deep water
of lakes use the pulmonary sac as a gill and breathe like the bran-
chiates. When the water is cold, it is not necessary for aquatic pul-
monate snails to make periodic trips to the surface in order to re-
new their air supply, 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
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
to ascend to the surface by flotation. Such movements are prob-
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

The blood of the snail consists of a plasma which is usually color-
less, 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 245

ventricle through a common aorta which divides into two branches,
one of which supplies the head and foot, and the other carries blood
to the visceral hump. The terminal branches of these arteries com-
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
three pairs of ganglionic swellings: the cerebral ganglia, situated
above the esophagus, supply nerves to the anterior regions of the
body; the pleural, pedal, and visceral ganglia lie below the esopha-
gus and are connected to the cerebral ganglia by commissures.
From them, nerves extend out to the visceral hump and basal parts
of the body. The arrangement of ganglia and their connectives is
of taxonomic importance.

Excretory

The kidney is a yellow gland situated near the heart. Its ureter,
a thin-walled tube, parallels the rectum and opens near the anus.

Reproduction and Life Cycle

Most fresh-water and terrestrial pulmonate snails, as well as the
sea slugs, are hermaphroditic. The majority of the marine shelled
gastropods and our fresh-water branchiates, such as Pleurocera,
Goniohasis, and Amnicola are unisexual. The reproductive system
of a unisexual snail is relatively simple but is exceedingly complex
in the hermaphroditic species.

In bisexual (hermaphroditic) snails cross-fertilization ordinarily
occurs. The ova, as well as spermatozoa, are produced by the ovo-
testis. 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 ovo-
testis 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. — Genitalia of Helix aspersa; act of union. (Modified, after Cooke, Cam-
bridge Natural History. By permission of The Macmillan Company.)

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-pro-
tected places, such as in rotten wood or beds of humus. The eggs

PHYLUM MOLLUSCA

247

'©

©OO

Fig. 138. — "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.

Qoniobasis comolenfis
(Pleuroceridae)

Coivpeloma decisum
(Vwiparldoiz)

Amnicola comolensis

CochliopQ texana
(Amnicolidae)

Fig. 139. — Some common fresh-water branchiate snails.

248

TEXTBOOK OF ZOOLOGY

are covered with thin shells which prevent undue water loss. In
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 mus-
cular foot which is protruded from between the two valves at the

Anodonto
^tevjortiana

Qaadrula
forsheyi

Proptey-Q
purpurata

Amblema
costota

Leptodea Carunculina .Musculium

fraqilis texasen5is rejrnsst

Fig. 140. — Some common fresh-water bivalves.

anterior end of the shell they plow their way slowly through the
stream or pond bed, feeding on the microscopic organisms in the
water. At the posterior end of the shell are two openings : the
ventral siphon which pulls in food and water, and the dorsal siphon
through which wastes and deoxygenated water are eliminated.

PHYLUM MOLLUSCA

249

Movement is varied among the pelecypods. Scallops may move
rapidly by suddenly contracting the valves, thus ejecting a jet of
water. Oysters are motile in their larval stages but in the adult
stage are attached to rocks and other objects. Many marine mus-
sels are attached to objects on the bottom or along the shore. At-
tachment 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 esti-
mated that Anodonta, one of our common genera of fresh-water
clams, attains its maximum growth in twelve to fourteen years.

External Features

Shell. — Unlike the snail whose shell is of one piece, the clam shell
is 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 -^ j^ ^^p„_^»-i^^ ^

Dorsal siphc

, , Growth linns

foot

Anterior
protractor
&. retractor
muscle

Pallial \inz 6

Fig. 141. — External (A) and Internal (B) shell features of Lampsilis anodontoides.

The oldest part of the shell is the umho which is usually a rounded
protuberance near the top of the valves and is 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

The shell is covered by a horny, pigmented periostracum. Under-
lying this is 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 sur-

Manble cut free

Perkaniial cavity
' AuHcle

Rectum

Ant retractor

Anbsyior
adductor 1

Poit retractor
Post, adductor
I Ex siphon

Protractor Ext . labial palp

Left qill plate

Fig. 142.

-Lampsilis anodontoides with the left mantle partially removed and
turned back to expose the underlying organs.

face of each shell may be seen the curved pallial line which extends
between the two adductor muscles and indicates the partial attach-
ment 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 for-
ward to the mouth which lies between ciliated labial palps. Upon

(

PHYLUM MOLLUSCA

251

entering the mouth, food is passed through a short esophagus into
the saclike stomach. Here it comes in contact with a digestive fer-
ment produced by the digestive gland which is discharged into
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
facilitates the digestion of carbohydrates. The food, having been
mostly digested and partly absorbed in the stomach, is passed on

ryjfcalline .style
1-* t^ucous qiands

%-—Intest\r)e

Fig. 143. — Cross section through the style sac and intestine of Lampsilis anodon-

toides. (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
siphon.

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.
Oxygen is absorbed by the capillaries and carbon dioxide passed
into the water where it is discharged to the outside through the
dorsal siphon.

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TEXTBOOK OF ZOOLOGY

Circulation

The heart which is composed of a ventricle and two auricles lies
in the pericardium. The ventricle, a muscular organ, surrounds
the rectum and drives blood forward through the anterior aorta
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-
fication in the gills it is returned to the auricles by way of the
efferent hranchial veins. 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 gan-
glion, the two ganglia being connected by means of a cerebral com-

Perlcardial wall Reno 'pericardial pore

Post, aorta
Vertical ^°^^- odductor M. 1
water tubes \ kidney I

txhalQnt '' ^^^^
Siphon

Ventricle \ Excretory pore

Auricle

Ant.aorta Liver
i -Stomach j Cerebral

commisjure

Ant .adductor
muscle

Inhalant

siphon , Qji, ,

Mantle •^'^e" \ \

Visceral Q. Qonad

Labial
Mouth P'^'P'

I root

Intestine 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, terminat-
ing in a visceral ganglion which is usually located just ventral to
the posterior adductor muscle. The visceral as well as the pedal
ganglia are united.

The sensory organs of the clam are primitive. Covering each
visceral ganglion is a patch of sensory epithelium called the os-
phradium, the function of which may be to test the purity of the
water brought in through the respiratory system. A short distance
back of each pedal ganglion is a statocyst which functions in equi-
librium. It is composed of a small calcareous concretion, the
statolith, which is surrounded by sensitive cells. In addition to
the sensory organs named, there are many sensory cells distributed
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 peri-
cardium. Each consists of a glandular portion which excretes
waste, and a thin-walled bladder that is connected with an ex-
cretory pore through which wastes are discharged to the outside.

Reproduction and Life Cycle

The small bivalves belonging to the family Sphaeriidae (Sphae-
rium) 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
gills the eggs are fertilized. Thus, the gills serve as brood pouches
or marsupia and may become greatly distended due to the tremen-
dous number (as many as three million) of developing embryos.

The small bivalve larva, which ranges in size from about 0.05 to
0.5 millimeter in diameter, is called a glochidium and has a single
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. In most clams the glochidia are discharged
to the outside through the dorsal siphon. They fall to the floor of

254

TEXTBOOK OF ZOOLOGY

the river, pond, or lake, and lie with their jaws agape, or snap
their jaws on any object. If the soft filament of a fish's gill or a
fin of the fish comes in contact with the glochidium, it will close

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down upon it and remain attached if the fish is the suitable host
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attachment of glochidia produce by proliferation new cells which

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. It shortly breaks loose from
its host, drops to the stream or pond bed, and leads an independent
life. The rapid dissemination of mussels in a river system can be
accounted for by the movements of their fish-hosts.

Economic Relations of the Phylum

Mollusks have been used as food by man from the beginning of
civilization. Oysters, clams, scallops, snails, and the arms of cuttle-
fish are found in the menus of peoples all over the world. It has
been estimated that the oyster industry along the Atlantic Sea-
board 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 be-
comes lodged between the mantle and the shell. Iridescent protec-
tive layers of mother-of-pearl are deposited around the foreign par-
ticle, thus producing the pearl. The Japanese have been success-
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 medium of
exchange. The wampum of the eastern coast of North America
consisted of strings of cylindrical beads made from brightly colored
clam shells. Shells have always been and still 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

(Fasciola hepatica) whose intermediate host is the small fresh-water
snail, 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; arrange-
ment 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. This class ap-
pears to be the most primitive in the phylum, and its members
have departed least from the ancestral condition. Bilaterally
symmetrical body; tentaculess head, eyes absent; shell, if present,
consists of eight overlapping plates. Most species have a flattened
foot but other species are slender and wormlike Ischnochiton con-
spicuus.

Class II. Pelecypoda

Includes the bivalve moUusks, such as the oysters, clams, scallops,
and cockles. More than ten thousand species have been described,
of which approximately four-fifths live in the ocean. Division of
the class into orders is based on giU characters.
Order 1. Protobranchiata

Marine species; gills consist of short, flattened leaflets; dis-
tribution along the Atlantic and Pacific Coasts.
Order 2. Filibranchiata

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

257

Order 4. Pseudolamellibranchiata

Marine species; gills plaited into vertical folds; shell fre-
quently inequivalve. The oyster (Ostrea) and Pecten illus-
trate this order.

ischnochiton

Dental ium

Loliqo brevipennis Polypus bimaculatus (Octopus )

Fig. 146. — Repre-sentatives of three classes of mollusks. Class Amphineura,
Ischnochiton; Class Scaphopoda, Dentalium ; Class Cephalopoda, Loligo brevipennis
(squid) and Polypus bimaculatus (octopus).

Class III. Gastropoda

Includes the snails and slugs. Approximately fifty-five thousand
species have been discovered and described. Shell, if present, uni-
valve.

258

TEXTBOOK OP ZOOLOGY

Order 1. Prosobranchiata

Mostly marine, but fresh-water and land forms are repre-
sented. As the name implies, the gills are situated in the
mantle cavity anterior to the heart. This order embraces
such animals as the limpets, abalones, and periwinkles, all
of which live in the sea; also a few fresh- water genera, such
as Goniohasis, Campeloma and Pleurococera; Helicina orbicu-
lata, a terrestrial southern species which is 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
into two fins which are used in swimming. Some of the
heavier types have broad cephalic discs, adapted for burrow-
ing in the sand. Many are found in coral beds and in sea-
weeds, 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 eyes,
and one pair of retractile tentacles; shell in form of
elevated or depressed spire, rudimentary and concealed,
or absent.
Class IV. Scaphopoda

Marine. Mantle edges grown together along ventral side forming
tube, with a shell of same shape and open at both ends. Commonly
known as tooth shells. Approximately 300 kno-u-n living species.
(Dentalium.)
Class v. Cephalopoda

Marine. 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.)

Order 1. Tetrabranchiata

The chambered nautilus (Nautilus) is a representative of
this order. The animal inhabits the last chamber of a flat-
tened spiral calcareous shell. As the name Tetrabranchiata
implies there are four gills; also four primitive kidneys and

PHYLUM MOLLUSCA

259

■ Si ph uncle

1 __ - ' ^ Jepta

Fig. 147. — Sectional view of internal structure of Nautilus.

Cyrtoceracone

Orthoceracone

Qyroceracom

Goniatlte

Ceratite

Ammonite

Fig. 148. — Evolution of the cephalopods.

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
conical shells). This latter group in all probability gave rise to the entire
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
or highly complex line. This line is called the suture and in fossil shells whose
outer shell coating is lost, it stands out rather conspicuously. The suture line
is used as a taxonomic character for the group.

Order 2. Dibranchiata

Octopods and squids are representative types. Shell internal
or absent; two gills and two primitive kidneys; ink sac pres-
ent; mouth surrounded by 8 to 10 tentacles which are
furnished with suckers. This order includes the largest of
all moUusks, the giant squid (Architeuthis princepsj which
may attain a total length, including arms, of over fifty feet. |

The squids and octopods are noted for their ability to change
color by the rapid contraction or expansion of chromatophores
in their skin. Their juovements are rapid and are produced
by expelling water from the mantle cavity through the mus-
cular siphon with such force that the animal is jerked back-
ward. In the squids, fins along the sides of the body
facilitate locomotion.

Loligo hreviyennis is the small squid found along the Gulf coast.
Wheu taken out of the water it is usually a mottled red or tan.
The visceral mass and mantle cavity are enclosed by a thick mus-
cular mantle. Beneath the skin along the back is a primitive endo-
skeleton in the form of a feather-shaped shell. The squid is preda-
tory, feeding on almost any animal it can capture. Within the
pharynx are two large jaws moved by powerful muscles. The
pharynx connects with an esophagus which in turn terminates in a
muscular stomach. Digestive juices from the liver and pancreas
are emptied into the stomach, and after the food is partially di-
gested, it is passed into a thin-walled cecum where digestion is com-
pleted and absorption takes place. Wastes are discharged through
the anus which opens near the base of the siphonal fold. The blood

PHYLUM MOLLUSCA

261

(. Sucker

Hectocotylhed arm.

Cartilage

Siphon

Anus

Muscle

Esophaaus

Rectum

Inkiac

Anb. aorta

5yitemic heart— -
Pen

^ Lt. post cava

J Spermabophoric sac

rl\ stomach

:-il Pen -

Skowach-

po'uch

-Cub edge of
body wall

.Fir)

Fig. 149. — Dissection of squid to sliow internal anatomy.

262

TEXTBOOK OF ZOOLOGY

system, which is closed, is composed of arteries, veins, and two
branchial hearts. 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 repro-
ductive 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.

-- 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 spe-
cialization when compared with the nervous system of other mol-
lusks. 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
complicated than those described for the clam, are situated near
the brain mass. Ciliated pits which are supposed to be olfactory
in function 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 triplo-
blastic, segmented, bilateral, and covered by a chitinous exoskele-
ton. The coelom is modified by a marked reduction as a result of
specialized vascular spaces. The segmentation or metamerism of
the body is expressed in a high degree in this phylum and there
is a definite relation of appendages to segments. The segments
have undergone greater specialization and greater regional differ-
entiation 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
as found in most arthropods is spoken of as heteronomous. This
group has fairly distinct head, thorax, and abdomen. The append-
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 un-
yielding to growth, it becomes necessary for the arthropod to shed
the skeleton periodically during its growing periods. This molting
or ecdysis, as it is called, is quite characteristic of many of the divi-
sions of this phylum.

The circulatory system is of the ojyen type, since there are large
sinuses or spaces surrounding most of the organs instead of a con-
tinuous 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

Section I. Branchiata (brankia'ta, gill) gill-breathing, aquatic
forms for the most part.

Class I. Crustacea, craj^fish, crab, pill bug, barnacle, water flea,

etc.

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Subclass Entomostraca, fairy shrimps, water fleas, and barnacles.
Order Branchiopoda, fairy shrimp (Branchipus), water flea
(Daphnia) .
Order Ostracoda, Cypris.

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 as subclasses, thereby dispensing with Entomostraca.)

Subclass Malacostraca, pill bugs, sow bugs, sand fleas, lobsters,
craj^fish, 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.

Class II. Onychophora, Peripatus, the wormlike arthropod.

Division B. Antennata. More highly specialized forms with one
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 flies.

Order Plecoptera, stone flies.

Order Emhiidina, embicls. (Texas, California, Florida.)

Order Orthoptera, crickets, grasshoppers, roaches.

Order Isoptera, termites or "white ants."

Order Dermaptera, earwigs.

Order Coleoptera, weevils and beetles.

Order Strepsiptera, stylopids (parasites in insects),

Order Thysanoptera, thrips.

266 . TEXTBOOK OF ZOOLOGY

Order Corrodentia, book lice.

Order Mallophaga, bird lice.

Order Anoplura, body lice ("cooties"), crab louse.

Order Hemiptera, true bugs, as squash bug.

Order Romoptera, plant lice, scale insects, cicadas.

Order Neuroptcra, aphis lions, ant lions.

Order Trichoptera, caddis flies.

Order Lepidopiera, butterflies and moths.

Order Mecoptera, scorpion flies.

Order Dipt era, true flies, mosquitoes.

Order Siphonaptera, fleas.

Order Hymenoptera, wasps, ants, bees.

Division C. Arachnoidea (ar ak noi'de a, spiderlike). A group
without antennae but Avith tracheae, book lungs or book gills, and
four pairs of thoracic appendages.

Class V. Arachnida, spider, mite, scorpion, king crab, etc.

Order Scorpionida, scorpions.

Order Pedipalpi, vinegarroon and tarantula.

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 of the classification of the phylum has been placed
early in the chapter in order that the student may realize the mag-
nitude of its size and 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
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
species for a 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
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 is sufficient 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.
Some species carry air tunnels vertically from the original hori-
zontal 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

Fig. 152.— Cavibarus clarkii, the swamp crayfish, a very common species in the
swamps ol: the Southern States. (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
this is commonly sufficient to avoid the enemy. The daytime is
usually spent in hiding under objects or in the mouth of the bur-
row. Crayfishes may at times desert their aquatic habitat and go
foraging out over swampy land. In some localities certain species
build their burrows down to the subterranean water table right
out in the fields and become important pests. Sight, touch, and
chemoreception are important senses in this animal.

The crayfish captures other animals, such as tadpoles, small
fish, and aquatic insects, by waiting in hiding and suddenly seizing

268 TEXTBOOK OF ZOOLOGY

them. The crayfish is quite well protected, due to its protective
color which matches the background, its chitinous skeletal cover-
ing, and its pinchers. In spite of this, they are captured by water
snakes, alligators, turtles, fish (such as bass and gars), frogs, sala-
manders, herons, and raccoons in particular. Many have been ex-
terminated 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
into head and thorax. The boundary between these is marked by
the oblique cervical groove on each side of the region. The shell-
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. The
anterior end of the cephalothorax is drawn out to almost a point, and
this portion is called the rostrum. 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
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. It serves in reproduction for the
receipt and storage of spermatozoa.

The portion posterior to the thorax, which is frequently called
"tail" by fishermen, is really the aldomen, and the tail proper is at
the posterior end of this. The abdomen is 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
lateral plate, the pleuron; and the slender ventral sternum in the
form of a narrow bar extending from side to side. A thin arthro-
podial membrane extends between successive sterna and allows for
movement of the segments upon one another.

PHYLUM ARTHROPODA — CLASS CRUSTACEA

269

Endopodite
Protopodite.:."

External ope'ning
of nephtidium.

..3 Protopodite

A.l

Exopodite

Endopodite

Mx.l

Exopodite
Endopodite

Mp.l

Epipodite

Endopodite
Protopodite,.,.

Mp,3

Eindopodite..,

^.Endopodite

Endopodite

Exopodite

Mx,2

•Epipodite

Mv.2

Exopodite

Protopodite

Epipodite-

Endopodite
Protopodite k

Epipodite

.ChitinouB thrrada

Fig. 153. — Examples of cephalic and thoracic appendages of the crayfish, ventral
view. A. 1, Antennule ; A. 2, antenna ; L. i, fourth walking leg ; M, mandible
Mp. 1. fir.st maxilliped ; Mp. 2, second maxilliped : Mp. 3, third maxilhped ; Mx. 1,
first maxilla; Mx. 2, second maxilla, (From Newman, Outlines of General Zoology,
published by The Macmillan Company, after Kerr.)

270 TEXTBOOK OF ZOOLOGY

The appendages are paired, with one pair attached to each typi-
cal segment. There are nineteen such pairs. They are all de-
veloped on the same plan from the typical biramous (two branched)
appendage. The five anterior pairs of abdominal appendages are
quite typical of the primitive 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 con-
sisting of a basal protopodite composed of coxopodiie, joining the
body and the hasipodite; the exopodite or lateral branch and the
endopod'ite or medial branch each have many joints. The first two
are much reduced in the female, but in the male the protopodite
and endopodite are fused and extended to serve as an organ for
transfer of spermatozoa. The posterior pair of swimmerets, at-
tached to the sixth abdominal segment, are broadened into fanlike
structures for swimming. They are known as uropods and have
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 composed
of the two joints of the protopodite and five of the endopodite. Join-
ing the coxopodiie (first segment of protopodite) is a sheetlike struc-
ture which supports a gill and some chitinous threads. The three
anterior walking legs possess pinchers or chela which are formed by
the terminal segment being set on the side of the second segment.
The walking legs are used in locomotion, ofi'ense, and defense. The
three anterior segments of the thorax bear three pairs of biramous
maxillipeds. The parts are quite typical in most respects. Each has
an epipodite joining the basipodite and all except the first bear gills.
These appendages are used in getting food to the mouth. |

To the segments of the 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
the first. They are both leaflike and modified. The epipodite and
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
to a leaflike protopodite and small endopodite. The jawlike man-
dible at each side of the mouth is composed of hard protopodite

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 some-
times called "feelers." They consist of the protopodite of two
parts, a long many-jointed, filamentous endopodite and a relatively
short, fan-shaped exopodite. Anterior to these are the antenmiles
which are biramous and feelerlike. The exopodite and endopodite
are similar in these.

The principle of homology is excellently illustrated by the ap-
pendages 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 each of the
appendages of the crayfish is similar to all others. Some become modi-
fied with development. Other illustrations of homologous structures
are the human arm and the bird 's wing. In organisms like crayfish
where the appendages of successive segments are homologous to each
other, the condition is spoken of as serial homology. Homologous
stiiTctures are found in many animal groups and are used in establish-
ing relationships. It ha.s been suggested that the parapodia of Nereis
represent possible forerunners of crustacean legs. They are both typi-
cally biramous and both take about the same position on the body,
as well as having a similar segmental distribution. There is also con-
siderable similarity in their structure.

Internal Structure

Beneath the shell-like, chitinous exoskeleton there is a very rep-
resentative set of systems. As in most higher animals the segmen-
tation is retained in the muscular system, nervous system, and to
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.

Respiratory System.^ — Under the branchial areas of the carapace
may be found the paired, feathery gills held in the gill cavity or
branchial chamber. There are three types of gills present here:
pleurohranchiae, attached to the sides of the thorax; podohranchiae,
arising from the epipodites of the thoracic appendages; and arthro-

272

TEXTBOOK OF ZOOLOGY

branchiae, which arise from the coxopodites of the thoracic append-
ages. Several of the segments have lost the pleurobranchiae. The
scaphognathite moves in such a way over the external surface of
the gills as to move the water in an anterior direction. The water
is 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.

Carapace
removed

Hasc/e ^

Ventral thoracic
artery

Wntral sinus

Pericardial sinus
Heart
Ostium
hasck

Gonad

Intestine
Digestive $land
Efferent vessel

Gill

Nerve cord
Carapace

Fig. 154. — Diagram of cross section throug-h the posterior thoracic region of a
crayfish. Arrows indicate flow of blood.

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 seg-
ment. 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 — CLASS CRUSTACEA

273

meut of bristles which serve as a strainer that allows only properly
masticated food to pass through. The pyloric chamber is consider-
ably smaller and curves downward to continue posteriorly as the
tubular intestine which extends almost directly posteriorly through
the center of the abdomen to the anus in the last segment. Large
digestive glands (hepato-pancreas) lead into the pyloric chamber
through hepatic ducts. The secretion of these glands contains diges-
tive enzymes.

The vascular system consists of a heart, the pumping organ; the
arteries, definite vessels; the sinuses, a series of blood spaces; and
the Nood which circulates. It consists of the fluid plasma containing

5 6 7 g <? 10 u

Fig. 155. — Lateral view of a dissection of the crayfish to show many of the
internal organs. 1, supraesopliageal ganglion ; 2, circumesophageal connective ; S,
ophthalmic artery ; J/, stomach, cardiac portion ; 5, lateral teeth ; 6, median teeth :
7, antennary artery; 8, testis; Oj, hepatic artery; 10, ostiuin ; 11, heart; 12, dorsal
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
31, anu9. (Modified from Turtox Key Card of Crayfish. Courtesy General Bio-
logical Supply House.)

white corpuscles but without red ones. The hemocyanin which ab-
sorbs oxygen is dissolved in the plasma. Fresh blood is almost clear
and colorless, but it takes a blue color after standing in the air for a
short time. 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,

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 pos-
teriorly through the abdomen just dorsal to the intestine. It sup-
plies the intestine and muscles of the body wall. The sternal artery
is a large branch arising from the dorsal abdominal artery just after
it leaves the heart. It passes ventrally through the nerve cord and
divides into a posterior, ventral ahdominal artery and an anterior,
ventral thoracic artery. These branches carry blood to the ventral
portions of the body. Besides the pericardial sinus already men-
tioned, there are others returning the blood to this one. The sternal
sinus is the main one, and it is located beneath the thorax. From it
several branches lead into the gills. This provides for a course
through the gills. From them blood is collected by branchio-cardiac
canals and delivered to the pericardial sinus. A 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.

The excretory system consists principally of a pair of large
bodies located in the ventrolateral portion of the head. These are
richly supplied with blood 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 longi-
tudinal 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 supra-
esophageal ganglion which is located anterior to the esophagus and is
joined to the cord by two circumesophageal commissures or connec-
tives, one passing on each side of the esophagus. From this dorsal
ganglionic mass, nerves pass to the eyes, antennae, and antennules.
The most anterior portion of the ventral cord receives these com-

i
i

PHYLUM ARTHROPODA — CLASS CRUSTACEA

275

missures. 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.

eyes. The antennae are tactile organs (sensitive to touch), the endo-
podite of which is a relatively long jointed filament. The exopodite
is much shorter and fan-shaped. The basipodite and coxopodite 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. In addition to these slender jointed proc-
esses each antennule has a saclike statocyst in its coxopodite. This
structure is an infolding from the outside and is lined with exo-
skeleton and sensory hairs. 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 stimu-
lations the crayfish is able to determine its orientation in space,
i.e., it knows whether it is in 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 ob-
jects in the water in which a crayfish lives during molting, there
will be no statoliths in the statocysts and the ajiimal has an im-
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.

The eyes, which are of the compound type, are mounted on movable
stalks, one on each side of the head region. They are described as
compound because each one is composed of a large number of in-
dividual sight units, each of which is essentially an eye. Each of
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
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-
other lenslike structure. Surrounding the latter are sensory cells
making up the retinula. The wall of the ommatidium possesses pig-
ment 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

PHYLUM ARTHROPODA — CLASS CRUSTACEA

277

the possibility of reflection within the ommatidium is reduced. In
dim light the pigment is concentrated partly toward the outer and
partly toward the basal portion of the ommatidia which allows more
refraction of rays by the crystalline cones and a combination of
images in several adjacent units. In brighter light only the ray from

Cornea ~

- Corneagen cells

. Crystalline cone"

Distal retinal
pigment cells

^^^

-f1

Proximal retinal
pigment cells

Rhabdome-- —

Basement
•membrane-

-Mm

■' Nerve fibers ■

Fig. 157. — Longitudinal section of ommatidia from eye of crayfish, a, position
of pigment when light is present ; b, position of pigment when in the dark. Notice
in the latter the distal pigment is in the outward position and the proximal pig-
ment is concentrated inwardly. (From Hegner, College Zoology j published by The
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
the optic nerve. The type of vision produced in the compound eye

278 TEXTBOOK OF ZOOLOGY

is "mosaic" in that there is registered only a single image by the
eye. Each ommatidium which is in focus on the object registers
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
fewer ommatidia will be stimulated. The crayfish eye is often
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 pyloric portion. The digestive juices possess enzymes which
convert the food into soluble form, and as it passes along the in-
testine, it is absorbed by the blood and distributed to the tissues
over the body. This conversion of food material into protoplasm is
assimilation. The external phase of respiration has put oxygen in
the blood, and it is distributed throughout the protoplasm of the
cells. The energy stored in the food material is released or converted
to kinetic form by union Avith the oxygen (oxidation) in the proto-
plasm. From this union there is excess heat produced. Mechanical
and chemical activity is 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 collected by the blood. The green glands relieve the blood
of these 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

These animals are dioecious (sexes separate) 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
in the fall may not be laid before spring.

PHYLUM ARTHROPODA — CLASS CRUSTACEA

279

Pi.

Fig. 158. — Development of the crayfish. A, Toun^ crayfish clinging to swim-
merets 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. A 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.)

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, particu-
larly 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
located beside the pyloric chamber of the stomach 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
in the coxopodite of the third walking leg (pereiopod) of each respec-
tive side. The ova develop in follicles in the ovary. The maturation
divisions (oogenesis) take place here and, when mature, the eggs
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 ex-
tend posteriorly and ventrally to open externally on the coxopodite
of each fifth walking leg. During copulation (mating) the sperm
cells are transferred by the two pairs of anterior swimmerets (pleo-
pods) 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 two sides of the body. The fertilized eggs are fastened to the
swimmerets by a secretion and appear much as small bunches of
shot-sized grapes hanging there. The later development continues
here, and they are aerated by movements of the swimmerets through
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
months. The larvae grasp the swimmerets with their chela and re-
main with the mother for about a month. Two or three days after
hatching they pass through the first molt or ecdysis; that is, they

PHYLUM ARTHROPODA — CLASS CRUSTACEA 281

shed the outer cuticle. 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. Mutilated or
lost legs or mouth parts are readily restored.

The genus Cambarus has the ability to allow a walking leg to
break off at a certain line or joint if it is caught or injured. A new
leg will develop from tliis stump. This phenomenon is called autot-
omy. There are special muscles to help in this and a membranous
valve stops the passage of blood through the leg, thus preventing ex-
cessive bleeding. Bleeding will stop more quickly if the break oc-
curs at such a point than it would otherwise. Autotomy often
makes it possible for the animal to sacrifice a leg to save its life.

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
at several million dollars annually in the United States. The
numerous smaller genera, like Daphnia, Cyclops, Cypris, Gam-
marus, Asellus, and Euhranchipus, comprise a large part of the food
of many of our food fish either directly or indirectly. The more
minute ones also feed many clams and oysters and finally end in
human consumption. The shrimp and crab fisheries are the most
important of the Crustacea on the Texas coast of the Gulf of Mexico.
In the Mississippi valley and on the Pacific Coast the crayfish is
used extensively as a food. It 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
fish or potential food of such fish,

282 TEXTBOOK OF ZOOLOGY

Characterization of Other Crustacea

Besides crayfish the order Decapoda includes lobster, shrimp, and
crab. 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
in that the cephalothorax is broader than it is long, the abdomen
is poorly developed, and folded sharply beneath the thorax. Crabs
of different kinds vary in diameter from a few millimeters to sev-
eral inches. There are four species of swimming crabs in the Gulf
of Mexico, of which the hlue or edihle crab {Callinectes sapidus,
Fig. 408) is the most important and best known. The lady crab and
calico crab are also interesting species. When the blue crab is cap-
tured at molting time it is called the soft-shelled crab. At other
times it is 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-
face and lifted out in a dip-net. 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 in-
activity the abdomen has become soft and partly degenerate. The
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
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 occa-
sionally 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
in two groups, which point in opposite directions. Respiration is
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. They are a garden pest in
that they eat leaves of delicate plajits. 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-
tom in fresh water. Gammanis is the best known fresh-water form.
The legs of representatives of this order are divided into two groups,
with the legs of each group pointing toward each other. These are
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 fresh-water crustacean. (Courtesy of General Bio-
logical Supply House.)

swim with the ventral side up and their long, leaf-like appendages
hang from 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 con-
stitute an important common group. Daphnia is one that is en-
closed 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
one-tenth of an inch in length. Cyclops is another common fresh-
water 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 be-
come greatly modified.

The ostracods are small, swimming, bivalve forms that are some-
times 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 little resemblance to
other Crustacea that they are usually overlooked as such by the
layman. 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. After attachment, the legs become modified into feather-
like bristles which 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

A 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 come at this point. This theory maintains that certain devel-
opmental stages or structures of the individual are related to an-
cestral conditions. That is, the individual in its development tends
to repeat in an abbreviated fashion the history of the development

PHYLUM ARTHROPODA — CLASS CRUSTACEA

285

of the race. Briefly stated ontogeny recapitulates pliylogeny. There
is still some doubt as to the validity of this generalization in direct
application.

A classical example which is frequently cited is that of the devel-
opment of the shrimp, Penaeus, which hatches out as a nauplius larva,

Nftuplius «t*c{<«

Fig. 160. — Nauplius stage of the barnacle, Balanus. (Courtesy of General Bio-
logical Supply House.)

Fig. 161,

iSchizopod

Fig. 162.

Fig. 161. — Zoea and Megalops stages of developing Crustacea. Crabs include
these stages in their development. (Courtesy of General Biological Supply House.)

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
adult Mysis and therefore is called the Mysis stage. Gammarus is
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
of these developmental stages. This idea generally has served as
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 respira-
tion, (5) dioecious reproduction, (6) development of eyes and
other sense organs, (7) green glands and malpighian tubules (in-
sects) 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, now considered as the class
Onychophora, is restricted to the more tropical and semitropical
regions of the earth south of the Tropic of Cancer. These primitive
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 as soft-bodied, wormlike, ter-
restrial forms with internally segmented bodies. The body may be
divided into a head and abdomen. On the head is one pair of an-
nulate antennae and a pair of jaws. The body bears many pairs of
legs which are not distinctly jointed, but are provided with trans-
verse 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. The nervous system is
ventral, consisting of separate longitudinal nerve cords connecting
a number of ganglia. A pair of eyes is located at the base of each
antenna. The body wall consists of 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 classes in its tracheae, dorsal vessel with
paired ostia, lacunar circulation, mouth parts, and salivary glands.
These resemblances are by no means close, however, and Peripatus

287

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 papillae.
Many of the species are viviparous, a single female producing as many
as thirty living young in a season.

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; Macro-
peripatus perrieri (Bouvier) is found at Vera Cruz, Mexico; while
the species of the family Peripatopsidae are confined to New Guinea,
Australia, Tasmania, New Zealand, Cape Colony, and Chile.

The classes Diplopoda and Chilopoda are considered by some au-
thors as orders of the class Myriapoda. The more recent students
of these groups, however, have adopted the plan of classification fol-
lowed here.

Fig-. 163. — Peripatus capensis. Natural size. (After Moseley from Folsom's
Entomology. Redrawn by Nelson A. Snow.)

Myriapoda

The Diplopoda are terrestrial arthropods commonly called mille-
pedes. 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
tergite, the collum, considered by some students of this group as a seg-
ment which has played an important role in the formation of the head
in some of the other groups of arthropods. A single pair of legs seem,
however, to belong to this first segment, as does a single pair of legs
to the three following segments. These four segments are said to
constitute the thorax. Ducts from the reproductive organs open at
the base of the second pair of legs on tlie third body segment.

The abdomen consists of an indefinite number of segments, each
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 seg-
ments have resulted from the fusion of two segments.

The legs are jointed very much as in the insects. The tarsus con-
sists of the three segments.

The heart is a dorsal structure with side valves and an anterior
tube to the head similar to the arrangement in the insects. The di-
gestive tract consists of a mouth, esophagus, stomach, and intestines.
The excretory wastes of the body are removed by two or four pairs
of Malpighian tubules which discharge their excretions into the in-
testines.

Fig. 164. Julus terrestris. A common millepede. Side view of anterior end.
a, antenna; ab. abdomen; colj collum (first thoracic segment) ; e, a group of
ocelli ; g, gnathochilarium ; go, genital opening ; h, head ; I, labium ; m, mandible ;
th, thorax. (After Borradaile and Potts, Invertebrata. Redrawn by Nelson A.
Snow.)

The millepedes feed upon vegetable matter, decaying as well as
living plant substance. They are slow-moving, wormlike creatures,
living in dark, moist places. When disturbed they usually roll them-
selves up into a little coil. The eggs are laid in damp earth, and
when the 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-

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TEXTBOOK OF ZOOLOGY

atus are fairly 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 legs, and by their long antennae. The Chilopoda,
as a rule, move faster than millepedes.

The Chilopoda are carnivorous, preying upon adult as well as im-
mature insects, also upon spiders and Mollusca. The mouth parts
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 first body segment, the last two seg-
ments are legless, and each of the other body somites bear a pair of
seven-jointed walldng legs.

The circulatory system is well developed ; it extends the full length
of the 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
parts of the head.

PHYLUM AETHROPODA — CLASS ONYCHOPHORA

291

The digestive system is fairly simple, consisting of the esophagus,
stomach, and intestines. A pair of ]\Ialpighian tubules empty into
the anterior part of the intestines. The tracheae are branched, being
connected to a pair of spiracles 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 Scolo-
pendridae. The species S. heros is a large dark greenish colored
species found in Kansas, Arizona, Southern California, and Texas.

CHAPTER XX
PHYLUM ARTHROPODA (CONT'D)

ARACHNIDA

(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, and several other less common orders, such
as the Pentastomoids 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 mem-
bers 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 ap-
pendages, the chelicerae and pedipalpi; four pairs of walking legs,
each consisting of seven joints; lung books, which are remarkable
respiratory organs; simple eyes, that are definite in number and ar-
rangement in the various families ; abdominal appendages which have
been modified into spinning organs; and no metamorphosis, the de-
velopment being direct.

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
are to be found upon this part of the body. The first pair, the
chelicerae, are two-jointed, consisting of a basal part or mandible
and a terminal claw. The mandible is stout and covered on the in-
ner 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. The pedipalpi, consisting of six joints,
are located in front of the legs. They are used in handling the food,

292

PHYLUM ARTHROPODA — CLASS ARACHNIDA

293

as feelers, and in the males the terminal segments are used as copu-
latory organs. These palpal organs are useful in the classification
of many of the families of spiders. The basal segments, or maxillae,
are used in chewing the food. The four following pairs of appendages
are the legs. Each leg is made up of seven segments; namely, the
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,

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
clearly shown. (From Knowlton, by permission of the Utah Agricultural Experi-
ment Station.)

are the front pair of breathing openings or slits which communicate
with the lung books. Near these is the opening of the reproductive
organs, which in the females is protected by a plate called the
epigynum. Near the end of the abdomen are three pairs of spin-
nerets. In front of these is an opening to the tracheae and just pos-
terior 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

and the anterior portion of the stomach, which is called the sucking
stomach. 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 or heart, arteries,
and veins, located above the intestine. The heart receives its blood
from the body by means of three pairs of ostia. It is then forced
anteriorly through the aorta into the cephalothorax and also pos-
teriorly into the abdomen.

The nervous system is composed of a large ganglion in the cephalo-
thorax 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 a pair of branching air tubes or tracheae. This system of breathing
is found only in the arachnids.

The reproductive organs consist of the ovaries in the female and
the testes in the male. 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 fe-
male 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 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, develop-
ing from over-wintering eggs in the spring, growing to maturity dur-
ing 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
five of the common orders of the class Arachnida : (1) Araneida, the
true spiders; (2) Acarina, the mites and ticks; (3) Scorpionida, the
scorpions; (4) Phalangida, the daddy longlegs; and (5) Xiphosura,
the king crabs.

According to a recent study, A Natural Classification of Spiders, by
Dr. A. Petrunkevitch, the order Araneida is made up of sixty-two
families. Species of many of these families are rarely encountered,
and some of them are not found in America. 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
of food. The females carry the cocoons attached to their abdomens.
The eyes are arranged in three rows of four, two, and two. The
species, Lycosa gulosa, L. kochii, L. frondicola, and Allocosa parva
are common species in the western states.

2. Theridiidae, the comb-footed spiders. The spiders of this fam-
ily are found on low growing vegetation, fences, buildings, and at
times under boards and rocks. They build rather loose webs from
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 abdomen is scarlet in color in the live spider.

3. Thomisidae, crab spiders. These crablike spiders do not con-
struct webs, but lie in wait in the flowers for insects that visit them.
They are often highly colored for protection. They possess eight
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 common species.

4. Drassidae. The drassids live mainly on the ground, under stones,
or in silk tubes on shrubs and grass. The spinnerets are generally
long enough to extend a little behind the abdomen. The eyes are eight

296 TEXTBOOK OF ZOOLOGY

in number and arranged in two rows, but in this family there are
only two tarsal claws. They differ from the wolf spiders in that the
body is much longer than wide and flattened slightly on the back.
This is one of the large families of spiders. Drassus negledus is a
representative species.

5. Attidae, jumping spiders. Members of this family attract atten-
tion by their jumping and bright coloration. They live on fences,
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,
occupying an area on the cephalothorax known as the ocular quad-
rangle. The eyes on the front row are the largest. The males and
females differ considerably in size and structure. This family is
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.

6. Argiopidae, the orb-web spiders. The spiders of this family build
rather large typical webs for the purpose of capturing their prey.
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 Tetrag-
natha lahoriosa; T. extensa; Metargiope trifasciata; Neoscona ben-
jamina; and Aranea gemma.

Most of the species of Tetragnatlia are found in moist places ; they
build their webs over running water. Their bodies are round and
long. Metargiope trifasciata is the common garden spider, found in
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.

7. Aviculariidae, the tarantulas. Representatives of this family are
confined to the south and southwestern United States. They are
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 common tarantulas.

8. Pholcidae, the pholcids, are spiders with long legs and six or
eight eyes. They build loose webs in dark places, such as cellars and
protected corners of buildings. There are six genera in the United
States. Pholcus phalangioides is a large species. The abdomen is
elongated in form, and the legs are one and one-half to two inches
long. In this species the chelicerae are used to carry the eggs. Three
species are found in the genus Physocyclus : two of them are Physo-
cyclus glahosus which is found in Florida and P. tanneri, in Utah.

Fig-. 168.

Fig. 169.

Fig. 170.

Fig. 168. — The brown mite, Bryobia praetiosa. (Greatly enlarged.) (After
Sorenson, by permission of the Utah Agricultural Experiment Station.)

Fig. 169. — An adult female, red spider, Tetranychus telarius. (After Knowlton,
by permission of the Utah Agricultural Experiment Station.)

Fig. 170. — An adult blister mite, Eriophyes pyri. (After Sorenson, by permission
of the Utah Agricultural Experiment Station.)

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. All mites lay eggs Avhich hatch into six-legged
n;^^nphs. After some growth and moulting they develop into adults
with eight legs.

298 TEXTBOOK OF ZOOLOGY

A group of mites belonging to the family Tetranychidae, known as
brown mites and red spiders, are widely distributed, feeding upon
practically all kinds of cultivated plants. The clover or brown mite,
Bryohia praetiosa, is cosmopolitan in distribution. It feeds upon
fruit and shade trees, garden plants, and common annual and peren-
nial plants. The common red spider, Tetranychus telarius, which is
common in this country, attacks field plants, fruit trees, forest trees,
shade trees, and shrubs. It is reported that it attacks over 250
species of plants. Another common mite pest is the leaf blister mite,
Eriophyes pyri, which belongs to the family Eriophyidae. This
species attacks the leaves of the pear and apple. The damage to the
leaves results in the development of brownish blotches and partial
defoliation.

The ticks are all fairly large parasitic species. Those belonging
to the family 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.

The order Scorpionida consists of large-sized arachnids with pedi-
palpi which resemble the chelipeds or pinchers of the crayfish; also
with a flattened body and elongated abdomen terminating in a
specialized stinging organ. 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, protect-
ing them 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
direct development like the spiders. A peculiar comblike structure,
called the pectine is found in the ventral part of the second ab-
dominal segment. Four families are represented in the United States,
and they are found only in the southern and western states. Had-
rurus 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 — CLASS ARACHNIDA 299

The Phalangida, commonly 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 ab-
domen consisting of nine segments; long legs; and the presence of
only two eyes on the cephalothorax. The reproductive organs, an
ovipositor in the female 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 ab-
dominal 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
in the United States, representing six families. Species of the family
Phalangididae are the most common and widely distributed.

The order XipJwsura, king crab, is represented by only one living
primitive genus and five species. Limulus polyphemus is the Ameri-
can 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 cephalo-
thorax 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
a little smaller than the females, but similar in appearance. The
eggs are deposited in the summer in shallow water in small sandy
depressions w^here they are then fertilized by the male.

CHAPTER XXI
PHYLUM ARTHROPOD A (CONT'D)

CLASS INSECTA

(By Vasco M. Tanner, Brigham Young University)

Insects are the most abundant creatures on the earth today. There
is said to be over 650,000 living species, many of which have never
been seen by the great majority of mankind. This, 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 con-
ditions; on the vegetation of plain and swamp; from the tundra
of the north to the tropical pampas; in trees; on and in animals,
as well as man, many of which are carriers of disease. They ravage
our crops and damage our stored foods. In short, we may say that
insects are omnipresent. One noted entomologist has said that this
is an age of insects, and to this we may add that every man's farm
is "no man's land" and that the contending forces are insects and
man.

This great class Insecta has been upon the earth from the Penn-
slyvanian 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 suc-
cess 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
exoskeleton has been of great value in the evolution of this group,
in that it has permitted them to develop strong appendages, un-
limited external features, and a small size which has opened up an
entirely new place in the world of living animals. Dr. C. H. Ken-
nedy, however, has pointed out that there are advantages as well
as disadvantages to the possession of an exoskeleton. He says:

300

PHYLUM ARTHROPODA — CLASS INSECTA 301

"The exoskeleton has made possible very definite advances in the
evolution of insects, but at the same time has limited their evolution
in fully as many other ways."

Aside from the ehitinous exoskeleton, other distinctive character-
istics, such as power of flight, which is possessed by no other in-
vertebrate 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 suc-
cessful creatures they are today. This leads us to wonder how
successful man will be in his evolution during the next fifty million
years. Will he be able to meet the demands of a changing environ-
ment as the insects have?

INSECT CHARACTERISTICS

Insects are Arthropoda in which the body is divided into three
regions, the head, thorax, and abdomen. 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 re-
gion 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 pos-
terior end of the body.

Head

The head consists of a number of immovable plates or sclerites
forming the head capsule, to which are attached the paired append-
ages. In many insects the sutures separating the sclerites are
visible, 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 de-
veloping on distinct somites. Evidence concerning the fusion of the
six anterior segments of the body in the formation of the head
comes through an embryological study of the insect.

There are two kinds of eyes : ocelli, or simple eyes, and compound
eyes. The simple eye is a small area consisting of a single cornea.
Simple eyes are generally found in varying numbers along with
the compound eyes in adult insects; they are, however, usually lack-

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TEXTBOOK OF ZOOLOGY

ing in beetles. The compound eye consists of many facets, which
are the hexagonal-shaped corneal ends of structures called om-
matidia. 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

I
I

Fig. 171. — Western lubber grasshopper, Brachypephus magnus. A, male, and B,
female. The form found on the plains. (Photographed by Leo T. Murray.)

compound eyes function mainl}^ in detecting movements, while
ocelli are used to detect light intensities.

There is but one pair of antennae or feelers, and they vary in
size, shape, and position on the head among the various groups of

PHYLUM ARTHROPODA — CLASS INSECTA

303

insects. The feelers function in many ways; in some insects they
are tactile ; in others they are respiratory or olfactory, or auditory,
or may be used to hold the female during copulation. Antennae are
useful structures in the classification of insects.

26 V,'
Z7 28

Fig. 172. — The external features of a grasshopper. 1, maxillary palp; 2,
mandible ; 3, labrum ; i, clypeus ; 5, frons ; 6, compound eye ; 7, ocellus ; 8, vertex ;
9, antenna; 10, gena : 11, pronotum ; 12, wing, mesothoracic ; 13, spiracle, thoracic;
IJi, spiracle, first abdominal segment; 15, auditory apparatus; 16, wing, meta-
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. (From White, General Biology, pub-
lished by The C. V. Mosby Company.)

The mouth parts are chitinous structures and are represented in
the insects by two distinct types: mandibulate or biting, and suc-
torial or sucking mouths. Some of the insect orders which possess
mandibulate mouth parts are Coleoptera, Odonata, Neuroptera,

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Mallophaga, Dermaptera, Isoptera, and Orthoptera. The Lepidop-
tera, Diptera, Heteroptera, Homoptera, and Siphonaptera are suc-
torial orders. A knowledge of the mouth parts is very useful if
insects are to be effectively controlled and classified. No other
group of organs within the insect body vary in form as do the

Fig. 174. — Mouth parts of Bhomaelia microptera. G, cardo ; Gl., clypeus • Ga.,
galea; L, labium; La, labium (second maxilla); Lac, laclnia ; I/t., ligula ; L-i";
labial palp; M., mandible; Me., mentum ; MP., maxillary palp; ilfa;. first maxilla;
Pa., palpifer; Sin., submentum ; /St., stipes. (From White, General Biology.)

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 — CLASS IXSECTA 305

verse plane. 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 lie under the mandibles and move
in a similar plane. The maxillae, which consist of sclerites, are
much more complex. The cardo is 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 be-
lieved 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 a
palpus which consists of one to four segments ; it functions as a sen-
soiy organ, probably detecting senses similar to our own senses of
taste and smell.

The hypopharynx or tongue arises from the labium into the cavity
of the mouth, 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 and
maxillae, as found in the mandibulate orders discussed above. The
labium forms a long sheathlike structure in which the styletlike man-
dibles and maxillae lie. The upper lip or labrum covers over the
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 is 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 in-
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. A 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

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TEXTBOOK OF ZOOLOGY

the segments of the thorax is called the tergum, the ventral or under
surface, the sternum, and each side, the pleurum. The legs are made
up of five main segments: the coxa, trochanter, femur, tibia, and
tarsus. The coxa forms the joint by means of which the leg is at-
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
claws. In insects the tarsal segments may differ in size, length,

Fig. 175. — Jumping leg or third thoracic appendage of Rhomaelia microptera.
C, coxa; F, femur; P, pulvilli ; Ta, tarsus; Ti, tibia; Tr, trochanter. (From White,
General Biology.)

shape, and number, and are useful in classification. The legs are
greatly modified for obtaining food, running, walking about, swim-
ming, and jumping. They are also modified for the production or
reception of sound, for the collection of food, such as pollen, and for
copulation. In some species they also exhibit secondary sexual char-
acters.

The wings are thin folds of the skin, shaped and strengthened with
veins in various ways. The presence of wings is one of the most

PHYLUM ARTHROPODA — CLASS INSECTA

307

characteristic features of the insects. Because of their great varia-
tion, wings 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 char-
acter of the veins and cells are an aid in grouping insects into
families, genera, and species. The majority of insects possess two
pairs of wings ; there are some, however, that have but one pair and
some groups are wingless.

Fig. 17G. — Right wings of a grasshopper. A, the fore wing; B, the hind wing.
(From Henderson, permission of Utali Agricultural Experiment Station.)

Abdomen

The segments of the abdomen are usually simple, but the number
varies greatly in different insects. 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.

308 TEXTBOOK OF ZOOLOGY

Body Wall

Another distinctive feature found in the arthropods is the chitin-
ous 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 mat-
ter, the hypodermis or intermediate layer, and the 'basement mem-
hrane. 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 parts of the insect body, but it especially serves to give firm-
ness to the cuticula. Chitin is not destroyed by caustic potash. It
is 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 under-
goes 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 — con-
stitute metamorphosis.

There are four types of development or metamorphosis : first,
ametabolous or development without metamorphosis; second, pauro-
metaholotis or gradual metamorphosis ; third, hemimetaholous or incom-
plete, 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 — CLASS INSECTA 309

hemimetabola naiads live an aquatic life which necessitates changes
and physiological adjustments not required in the adult aerial ex-
istence. In these orders there are greater changes during develop-
ment than are found in gradual metamorphosis.

The Holometabola, in which the larva hatched from the egg bears
no resemblance to the adult, goes through a complete metamorpho-
sis. The holometabolous insects include the following orders : Tri-
choptera, Neuroptera, Coleoptera, Lepidoptera, Siphonaptera, Dip-
tera, and Hymenoptera. The larva is variously called the maggot,
grub, or caterpillar. It eats almost constantly since this is the
growth period in an insect's life history. After molting several
times it comes to rest and prepares for the pupal stage. The pupa
gradually takes on the adult form and after a few days or even
months, the adult or imago emerges.

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 fol-
lowed. It has been divided into the subclasses: Apterygota, the
two wingless orders Thysanura and Collembola, and Pterygota,
which includes all the other orders of insects. Since practically all
the orders fall into the subclass Pterygota, the arrangement fol-
lowed here is that of discussing them according to their development.

Subclass Apterygota. — Ametabola are insects without metamor-
phosis.

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-

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TEXTBOOK OF ZOOLOGY

moil in warm climates. They feed largely upon dead plant tissue,
and are found in the soil and beneath stones, dry leaves, and loose
bark. One of the commonest species of this order is the "tish
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.

Fig. 177. — Grasshoppers common to western United States. A, Red-legged
locust, Melanoplus feniur-rubrum DeGeer, male ; B, Haldeman's locust, Hippiscus
coralUpes Hald. ; C, Slioshone grasshopper, Schistocerca shoshone Thomas, female ;
D, two striped Mermiria, Mermiria hivittata Serville : E, western meadow grass-
hopper, Conocephalus vicinus Morse, female. (From Henderson, Utah Agricultural
Experiment Station.)

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.)

Order Collenihola. — Small insects with retracted mandibulate
mouth parts ; simple eyes ; antennae with four segments in most
genera; abdomen with six segments, which often carries three ap-

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TEXTBOOK OF ZOOLOGY

pendages modified for jumping. Tlie body is often cylindrical.
About one thousand species have been described, some of the common
species follow : Family Poduridae. Podura aquatica L. ; Achorutes
maturits Fols. ; Family Entomobryidae Folsomides decemoculatus

Fig. 178. — Western or Mormon cricket, Anabrus simplex Haldeman, female. (From
Henderson, permission of Utah Agricultural Experiment Station.)

Fig. 179. — Sand cricket, Stenopelmatus fasciatus Thomas. (From Henderson, per-
mission of Utah Agricultural Experiment Station.)

JtlilLs ; Isoioma dongaia Mac G. ; 1. titusi Fols ; and Tomocerus vulgaris
Tull. ; 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 — CLASS INSECTA

313

in 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 are insects with gradual
metamorphosis. The following are some of the more important
paurometabolous orders.

Fig ISO. — American German (ventral view), and Oriental Cockroaches (left to
riglit). (From Knowlton, permission of Utah Agricultural Experiment Station.)

Fig. 181.— Praying mantis, Stagmomantis sp. It is named for its pose.

Order Orthoptera. — This order contains the grasshoppers, katy-
dids, crickets, cockroaches, mantids, 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

Fig. 182. — (Legend on opposite page.)

PHYLUM ARTHROPODA — CLASS INSECTA 315

for jumping. The abdomen is usually provided with jointed eerei,
and an ovipositor is generally present. About seventeen thousand
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 spe-
cies of this family are widely distributed and are of economic im-
portance, doing great damage to crops and forage plants. The
family is divided into three subfamilies, Locustinae, Tryxalinae,
and Oedipodinae. The red-legged locust, Melanoplus f emur-nibrum
(Fig. 177) (DeG.) ; Haldeman's locust, Hippiscus comllipes Hald;
the Shoshone grasshopper. Schist ocerca sliosJione (Thomas) ; the two-
striped Mermiria, Mermiria hivittata (Serville) ; and the western
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 sim-
plex Hald, is 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 de-
stroyed all the grain crops, had the sea gulls not 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
erected on the temple grounds in Salt Lake City (Fig. 178).

The katydids, Scudderia furcata Brunner and Microcentrum reti-
nerve (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. ;
and Orenivaga erratica (Rehn), a native species of the western
United States (Fig. 180).

Fig. 182. — Some common Hemiptera. 1, adult box-elder bug, Leptocoris trivit-
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, permis-
sion Utah Agricultural Experiment Station. Nos. 3, 4, 7, 8, 9 from Jorgensen,
courtesy Utah Academy of Sciences, Arts and Letters.)

318 TEXTBOOK OF ZOOLOGY

A common species of Pliasmiclae is Parabacillus coloradus (Scud-
cler) and of the Mantidae is 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 membranous over the outer half.
Over twenty thousand species of bugs have been described. They are
widely distributed, and many are of considerable economic impor-
tance. The following represent some of the common families and
species: Family Pentatomidae, stink bugs, Chlorochroa sayi Stal,
and the harlequin bug, Murgantia histrionica (Hahn) are common
species. The family Coreidae is represented by the squash bug,
Anasa tristis (DeG.) ; and the family Corizidae by the box-elder bug,
Leptocoris trivittatus (Say). The false chinch bug, Nysius ericae
(Schilling), is a common species of the family Lygaeidae. The lace
bug, CorytJiucha distincta Osborn and Drake is a handsome Tingi-
tidae; the common damsel bug NaUs ferus (L.) is typical of the
family Nabidae. The members of the family Miridae are numerous
and widely distributed. The tarnished plant bug, Lygus pratensis
(L.) is one of the commonest mirids in the United States. The water
striders, Gerridae; the back swimmers, Notonectidae ; and the giant
water bug-s, Belostomatidae are familiar to all who are acquainted
with the life of streams and ponds. The bedbugs belong to the family
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
to this order, also some species that are beneficial to man. Insects
with membranous wings and sucking mouths, such as the cicadas,
aphids, leaf hoppers, and scale insects, constitute this order. The
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) is one of the most common and destruc-
tive apple aphids in the West. The beet root aphid. Pemphigus hetae
Doane, and the black cherry aphid, Myzus cerasi (Fab.), are destruc-
tive species. The potato psyllid, Parafriozoa cockerelli (Sulc), is one
of the very destructive Chermidae. The family Coccidae is a small
obscure group of insects, yet they are very destructive and hard to
control. Baker 's mealy bug, Pseudococcus maritimus Ehrh. ; the cot-
tony maple scale, Pulvinaria vitis L. ; the San Jose scale, Aspidiotus

PHYLUM ARTHROPODA — CLASS INSECTA

319

Fig. i84.-Mayflies and dragonflies. 1. Nymph of Rubicund f^^gonfly Sympe^
trum ntbicundultim; 2. nymph of Lestes uncatus; 3,. nymph of the prickleback
Ephemerella grandis. dorsal view. A side view showing spines on back. ^. ^P n^f
on abdomen of smaller "prickleback" ; 4, the "trailer n'a>fl>. ^P^t^Jil'I'ern bunch?
tn liberate her efcgs (drawing by C. H. Kennedy) ; 5, nymph ot wesiern "uncii
gni'^%L™fcciden^aZis/6f nymph of '■little ^^'^]^:Cvrr^''VZ'%hvistensen
7 adult Of the "big curler." Pteronarcrjs sp. (From Needham and Chnstensen,
permission Utah Agricultural Experiment Station.)

320 TEXTBOOK OF ZOOLOGY

perniciosus Comst. ; 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) ; Delong's leaf hopper, Empoasca filomenta DeL. ; and the
grape leaf hopper, Erythroneura comes (Say). Insects of this order
are all 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,
soft-bodied, mandibulate insects. They feed principally upon wood,
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 termite, Iteticulitermes Jiesperus Banks, are common
and destructive. More will be said of these insects under the dis-
cussion of social insects, later in this chapter.

Order Thysanoptera. — Thrips are very small insects, not more than
two to three millimeters in length. 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 com-
mon insect pests. About five hundred species of thrips are known.

Order Dermaptera. — -The earwigs are small terrestrial, mandibulate
insects with a pair of forcepslike appendages at the tip of the ab-
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 apiceden-
tatus (Caudell) are species commonly found in the western United
States.

Hemimetabolous Insects With Incomplete Metamorphosis

Order Odonata. — The dragonflies and damsel flies are insects with
large compound eyes, mandibulate mouth parts, four membranous
wings that are finely veined, and a long slender abdomen. The
naiads are aquatic and possess a labium which has been highly modi-
fied. It can be greatly extended for the catching and holding of

PHYLUM ARTHROPODA — CLASS INSECTA 321

prey. The adults are swift flying, brightly colored, predaceous in-
sects. Their food consists of mosquitoes, gnats, and many other kinds
of flying insects. Much has been written on the dragonflies of the
United States. About twenty-eight hundred species have been de-
scribed. The order is divided into the suborders, Zygoptera (damsel
flies), and Anisoptera (dragonflies). There are two families of
damsel flies, the Agrionidae and CaenagTionidae ; also two families
of dragonflies, Aeschinidae and Libellulidae. The beautiful ruby
spot, Hetaerina americana Fabr; and the stalked-winged, Lestes
uncatus Kirby are damsel flies that are widely distributed. The
dragonflies Lihellula pulchella Drury and Sympetrum ruMcund^dum
are common west of the Mississippi River in the United States (Fig.
184).

Order Ephemerida. — The Mayflies are aquatic insects, with man-
dibulate 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. They have mandibulate mouth
parts, four wings that are not so thickly netted with veins as are
the Odonata, but with longer antennae than the Odouata. They
are found on stones along lakes and streams where they pass their
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 of the important orders that fall within this
group.

Order Trichoptera (Caddis flies). — This order includes about
eighteen hundred species of "case flies" or "rock rollers," as they

322 TEXTBOOK OF ZOOLOGY

are sometimes called. The adults are less than an inch long, with
well-developed wings, but with vestigial mouth parts since they prob-
ably take no food. The 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
plants. The larvae feed upon plant tissue and small animals which
they capture in little nets that are placed near the entrance to their
case. Pupation takes place in the water. The adults lay their eggs
in the water on sticks or stones. About eighteen families are recog-
nized. The species Hydropsyche partita Banks and H. scalaris
Hagen of the net-making family Hydropsychidae, and Platyphylax
designata (Walker) of the family Limnophilidae are common in the
western states.

Fig. 185.— Larva of net-making caddis worm, Hydropsyche. (From Needham and
Cliristensen, permission of Utah Agricultural Experiment Station.)

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
upon other insects. 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-
ally encountered.

The lacewing or golden eyes, Chrysopa calif ornica Coquillett, is a
beneficial and widespread species. It feeds in the larval stage upon
aphids, thrips, scale insects, and psyllids.

Order Coleoptera (Beetles). — The beetles are world-wide in their
distribution and contain the largest number of species of any order
in the animal kingdom. They are adapted for an almost unlimited
variety of conditions, living on plants and animals, on land, and in
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

Fig. 186. — Three species of Coleoptera, A, adult Colorado potato beetle, Leptvno-
tarsa decimlineata Say ; B, larva or slug of Colorado potato beetle ; C, spotted
blister beetle, Epxcauta maculata Say ; D, common blister beetle, Epicmita jmncti-
collis Mann. (From Knowlton and Sorenson, permission Utah Agricultural Ex-
periment Station.)

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food. Many species do an enormous amount of damage, while in
contrast, some of the most beneficial insects are beetles. In the United
States, north of Mexico there are one hundred and nine families and
twenty-four thousand species recognized. Over 200,000 species
from all parts of the world have been described.

Some of the families which contain the most destructive species
are the leaf beetles, Chrysomelidae ; the long-horned wood-boring
beetles, Cerambycidae ; the click beetles, Elateridae; the June
beetles, Scarabaeidae ; the metallic wood-boring beetles, Buprestidae;

Fig. 187. — Alfalfa weevil, Phytonomus porticus. Above, larva ; lower left,
pupa; lower right adult. (From Knowlton and Sorensen, permission Utah Agri-
cultural Experiment Station.)

and the weevils, Curculionidae. The following families are, in the
main, very beneficial: the tiger beetles, Cicindelidae ; ground
beetles, Carabidae ; ladybird beetles, Coccinellidae ; and the carrion
beetles, Silphidae. The cotton boll weevil, Anthonomus grandis, and
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). — In the Lepidop-
tera the larvae have biting mouth parts, while the adults have a
highly specialized suctorial structure. The antennae are of various
shapes and sizes. The two pairs of wings are covered with scales,
which are highly colored in many species.

Fi^. 188. — Common weevils. 1, the bill-bug, Calendra mormon Chitt ; 2, Apion
proclive Lee. ; 5, the rose weevil, Rhynchites bicolor var. cockerlli Pierce. (Draw-
ings by Tanner.)

This is the second largest order of insects. Approximately ninety-
five thousand species are recognized, of which about eight thousand
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
pests. They attack the foliage and fruit of the forest, orchard,
field, and garden; also, stored food and animal products.

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Fig. 189. — At left, larva of Capitophorus potentillae (Walker) ; right, straw-
berry leaf roller, Ancylis comptana var. fragariae (W. and R.) (From Knowlton
and Smith, courtesy of Utah Academy of Sciences, Arts and Letters.)

Fig. 190. — Representative of order Lepidoptera. Above, tomato fruitworm (or
corn-ear worm) ; below, adult tomato fruitworm moth, Heliothis obsoleta. (From
Sorensen and Knowlton, permission Utah Agricultural Experiment Station.)

PHYLUM ARTHROPODA — CLASS INSECTA

327

The following are some examples of common species: the mon-
arch butterfly, Danails menippe (Hubner), is widely distributed
through the United States, parts of Canada, and south into the
tropics. This species is typical of the family Danaidae which is one
of the nine families of butterflies in this country.

Fig. 191. — Insects of the order Lepidoeptera. Above, adult female moth of peach
tree borer, Aegeria exitiosa; center, cocoon and empty pupal case ; below, adult
male moth of peach borer. (Pi-om Sorensen and Knowlton, permission Utah Agri-
cultural Experiment Station.)

Some of the most destructive species of this order are among the
moths. The Noctuidae (millers) is a large family of injurious
species. The corn-ear worm or cotton bollworm, Heliothis ohsoleta
(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
Riley, is a pest belonging to the snout moths or Pyralididae. The clear-

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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 history of monarch butterfly. (Prom White, General Biology, pub-
lished by The C, V. Mosby Company.)

leaf roller, family Eucosmidae, is an imported species from Europe;
it feeds on both wild and cultivated strawberries, blackberries, and
raspberries and is found in many parts of the United States,

PHYLUM ARTHROPODA — CLASS INSECTA

329

Order Siphonaptera (Fleas).— Fleas have strong jumping legs,
piercing and sucking mouth parts, laterally flattened bodies, but no
wings. They are world-wide in distribution; about four hundred
species have been described. All of the species in the adult stage
are external parasites on warm-blooded vertebrates. They are
pests on cats and dogs and known to be carriers of bubonic plague.

m^^^-

Fig. 193. — Life history of the mosquito. 1, mosquito eggs floating m the water
(slightly magnified) ; 2, mosquito larva or wiggler ; S, mosquito pupa or tumbler;
J,, adult (From Turner, Personal and Community Health, published by The C. V.
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. /

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Many of the most useful insects are found in this order. The rob-
ber flies, Asilidae; the syrphids, Syrphidae; the bee flies, Bombyli-
idae; and the taehinids, Tachinidae, contain many species that are
valuable to mankind. On the other hand, the mosquitoes, Culicidae ;

Fig. 194. — Adult female sheep tick, Me.lophaous ovinus L,inn. (From Knowlton,
Rowe, and Madsen, by permission of the Utah Agricultural Experiment Station.)

Fig. 195.— Life history of the housefly, Musca domestica L. A. A, adult; B, ma-
ture larva; C, pupa inside puparium ; D, eggs. (From Knowlton, Rowe, and
Madsen, by permission of the Utah Agricultural Experiment Station.)

the fruit flies, Trypetidae; the houseflies, Muscidae; the botflies,
Oestridae; and the sheep tick, Hippoboscidae, damage food and
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

gers. There are over fifty thousand species of Diptera, ten thou-
sand of which are known to occur in the United States. The sub-
order Pupipara is a most interesting group, containing the blood-
sucking ectoparasites which live upon bats, birds, and mammals.
The sheep tick is a fairly common species.

Order Hymenoptera (Bees, "Wasps, and Ants). — The Hymenoptera
are so 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

Fig. 196. — Flies. Above, Chloropisca glabra Meig. Its maggots feed upon beet
root aphids. Below, adult western green-headed horsefly, Tabanits phaenops O. S.
(From Knowlton, Rowe, and Madsen, by permission of the Utah Agricultural Ex-
periment Station.)

parts are both biting and sucking, and the females are provided
with ovipositors that have become greatly modified. In the ichneu-
mon flies, the ovipositor is composed of long slender bristlelike struc-
tures, which are used for drilling through the bark of trees and de-
positing their eggs upon insect larvae under the bark. The ants,
mutillids, and bees use their ovipositors for stinging as well as for
depositing eggs. The pigeon horntails bore into trees, causing con-
siderable damage.

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Many of the Hymenoptera live as parasites and are of great value
in biological control work. The braconids, ichneumon flies, and chal-
cid flies are examples of this group of parasites. A number of the
Hymenoptera are not beneficial, since they feed upon the leaves of

Fig 197. — Above, adult female Simulium vittatum Zett. (From Knowlton.
Rowe and Madsen, by permission of the Utah Agricultural Experiment Station.)
Below, female big-headed fly, Pipunculus subvirescens Loew. (From Knowlton,
courtesy of Utah Academy of Sciences, Arts, and Letters.)

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 con-
cerned, thousands of individuals living in a single colony. The

PHYLUM ARTHROPODA — CLASS INSECTA

333

ants, honey bees, and social wasps are examples. The Hymenoptera
found in this country are divided into three suborders, twenty-
eight 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 Intro-
duction 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 Corrodentia,
psocids and book lice; 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 mam-
mals; the Strepsiptera, a group of small twisted- winged insects that
live as parasites within the body of other insects ; and the Mecoptera,
a group of about forty American species, commonly called scorpion
flies, in addition to the eighteen orders discussed above. Brues and
Melander in their Classification of Insects recognize thirty-four or-

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TEXTBOOK OF ZOOLOGY

ders; while Imms, the noted English entomologist, has included
twenty-three orders in his A 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.

Fig. 199. — Hymenoptera. Alfalfa-seed chalcis-fly, Brucophagus funetris How.
A, female; B, female antenna; C, male antenna; D, eggs (greatly enlarged) ; E,
anterior view of right mandible; F, larva; G, pupa, (enlarged) ; H, worker of the
black ant. (From Sorensen and Knowlton, permission Utah Agricultural Experi-
ment Station.)

SOCIAL LIFE AMONG THE INSECTS

The great majority of insects live an individual existence, with-
out any cooperation or filial relationship existing between parents
and offspring. The processes that have ever been operative have
emphasized the importance of the individual in the scheme of prog-

PHYLUM ARTHROPODA — CLASS INSECTA 335

ress. 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. A common species, Canthon sim-
plex var. corvinus Harold, which the writer has ofttimes observed,
rolls up small spheres of fresh cow manure, and then excavates be-
neath the roll, letting it gradually down into a hole in the ground.
The male helps to dig 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 re-
lating 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 beetles are probably the least social of all the orders listed.
No castes have been developed, and the males take but little part
in colony life.

In the Hymenoptera are found varying stages of social life. In
the solitary wasps, the female digs a burrow in the ground which is
provisioned and then an egg is sealed in the cell. 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 pic-
ture 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 appar-
ently 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.

"Their attempts at copulation are very amusing as well as in-
teresting. Beginning 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 away with still an-
other male in pursuit.

"The ground where the nests are made is hard, dry, and com-
posed principally of clay. In order to penetrate it the female fills
a thin pouchlike sac, located within the second segment of the ab-
domen, with water and uses this to moisten the ground. With her
mandibles she digs the dirt out in small pellets, varying in size from
2 mm. in diameter to 6.8 mm. These pellets are carried a short
distance away from the hole. This work is continued until the hole
is as deep as desired, the depth varying from 48 to 110 mm. There
are usually one or two, rarely three, cells constructed in the tunnel
for the deposition of eggs. The bottom of the hole is enlarged
slightly into a cell and is made very smooth on the inside. The
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 cell is provisioned with from five to twelve Pieridae larvae.
The wasp carries these 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 be-
fore 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

PHYLUM ARTHROPODA — CLASS INSECTA 337

trying to cut it off. This is probably a part at least of the process
of paralyzing the victim. 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 de-
posited 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
apparently air- and water-tight compartment. In order to observe
this process, the writer used a small pocket mirror to reflect the
light down into the hole. This did not seem to interfere with the
activity of the wasp.

"Most 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
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 de-
veloped into a female, while with the two-celled nests the larva in
the lower cell always developed into a female and the upper in-
dividual into a male. No successful observations were made on the
three-celled nests. The facts of the case would seem to indicate
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. The above condition applies
primarily to two-celled nests, although it might be equally true of
the three-celled types.

"It is 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
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-
times going slightly below the freezing point of water. 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
distilled water.

"It is thought that under natural conditions the insects emerge
somewhat earlier in the summer than was indicated by the arti-
ficially reared specimens since they have been observed to be very
active even during the early part of May. It seems evident that
these early wasps build their nests in the spring and that their
young emerge during the same season. Only the individuals nest-
ing in the late summer spend the winter in the larval stage."

The social wasps, belonging in the genera Vespula, Polistes, 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 de-
veloped. These workers then come to the aid of the exhausted
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
is to lay eggs. It will be noted that the Vespidae attend their young
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
hole or some other suitable hole in the ground. The queen gathers
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, leav-
ing 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 — CLASS INSECTA 339

In the honey bees, ants, and termites, social life is carried to its
highest state of perfection. In these groups the colony is probably
perpetuated for hundreds of years. Some ant and termite queens
live from ten to fifteen years, building up large colonies consisting of
fifty to eighty thousand individuals. Other queens take up the job
of continuing the colony.

A well-developed caste system, also polymorphism, is found in these
social insects. In a swarm of bees there are three kinds of individ-
uals, males, females, and workers. The workers are females that are
undeveloped sexually. Ants and termites have many different forms
of individuals in each species. In a termite colony there are many
castes. 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
distinct from other individuals because of their large heads and
strong jaws; and finally a caste known as nasuti, which are small
individuals with the head produced into a kind of snout. Both
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 diges-
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
social insects; these guests are called myrmecophiles when found
with ants, and termitophiles when with the termites. Wheeler re-
ports 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
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
niger var. americanus Emery. The little ants gather the aphid eggs
in October 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 trans-
ferred onto the roots. Here many generations are produced par-
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.

Many of the insect guests are beetles, Histeridae, Staphylinidae,
Pselaphidae, and Scarabaeidae. The two histerids, Hetaerms tristri-
atus 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. A number of species of Batrisodes and
Reichenl)achia, pselaphids, and Cremastocheilus angularis LeC. and
C. KnocJii LeC, scarabaeids, are found in the colonies of several of
the mound ants. Some Diptera are also guests in ant colonies.

ECONOMIC RELATIONS

Insects attack all kinds of growing crops and plants. The de-
struction of plants and their products valuable to man amounts to
over a billion dollars annually. This great loss goes on because
of the unabated and persistent struggle of the insects to maintain
their "place in the sun." Plants are not only eaten and damaged
by insects, but many plant diseases are spread by them.

Animals and man suffer greatly from the attacks of insects. Many
species live as endoparasites or ectoparasites on animals and man,
and in so doing also spread disease. Some of the 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

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 for use,
he finds many insects ready to take their toll from these concen-
trated products. 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
weevil, bean weevil, granary weevil, and confused flour beetle feed
upon and damage practically all kinds of grains and seeds and their
products. Much damage is also done to the same products by such
species as the Mediterranean flour moth and the Indian meal moth.
Practically all pests of stored foods are world-wide in their distribu-
tion, which makes it difficult to ship food products long distances
or store them for future use without running the hazard of insect
damage.

Many insects have taken up their abode with man, living upon
his upholstered furniture, clothing, furs, and rugs. Great losses are
suffered annually by the producers of clothing, as well as in the
homes, due to clothes moths. Termites also attack the wooden parts
of dwellings, even furniture and books. The tobacco beetles and
drugstore beetles live upon tobacco products, home furniture, and
many drugs.

Useful Insects

Fortunately not all insects are our enemies. Many species are
allies of man in the struggle against the injurious insects, as well
as in many other ways.

Everyone knows that honey is produced by the honey bee and
silk 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
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

various ways, millions of pounds being sold throughout the world.
Shellac is used for making varnishes and polishes, as an electrical
insulating material, in airplane construction, and many other ways.

Insects serve as food for many fishes, amphibians, reptiles, birds,
and mammals, including man. It is important that insects be recog-
nized as playing a major role in this connection. Without the
insects the food habits of many of the vertebrates would be entirely
changed.

Finally, many plants depend upon insects to assist in pollenizing
the blossoms. Only as the insect helps in transferring the pollen
from plant to plant or from the stamens to the pistil of the same
plant is it 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

The locust or grasshopper is one of the most common insects,
being known to practically all people, because 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
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 is divided into a series of rings, or segments, and
the segments are made up of hardened plates. These plates are
known as sclerites, 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
gro^\Ti together, or are fused. Again, in certain regions of the body,
parts of the segments may be lost. Eegardless of the amount of varia-
tion in this respect, we 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
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

C.'p.

U i.e.

c-.-.w

■A

r.-cj. L

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 grass-
hopper, 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 ;
g., gena ; la., labrum ; I.e., lateral carina of the fastigium ; l.p., labial palpus ;
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 com-
pound eyes are situated upon the upper portion of the sides of the
head, and are large, oval areas with smooth, highly polished sur-
faces. 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
three small, almost transparent, oval areas. One of the ocelli is
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 consist of a number of separate parts attached
to the ventral region of the epicranium. The first noticeable part
is the Idhrum, or upper lip, a flaplike piece attached to the lower
edge of the clypeus. The free edge is deeply notched on the median
line. 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 ven-
tral 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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REPRESENTATIVE INSECTS 347

The next two segments, the mesothorax and metathorax, are made
up of sclerites that are intimately associated, and their structure
will be discussed together. The mesothorax is joined to the pro-
thorax by a membrane which permits of more or less movement.
Posteriorly the metathorax is joined immovably with the first ab-
dominal 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, how-
ever, 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
suture, running 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
sutures, so that each pleuron consists of two sclerites.

A pair of legs arises from the lateral and ventral portions of each
of the segments of the thorax. 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
piece which is hard to distinguish except in the first pair of legs.
The femur is 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
as the femur. The last division of the leg is the tarsus which is made
up of three segments, each movable with the other. The segments
bear a series of pads, which terminate on the last one in a large
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 mem-
branous, with many veins to strengthen them, and fold fajilike over
the abdomen when not in use. The metathoracic wings are used
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 mem-
brane. The ventral part of the first segment, the sternum, is not
attached to the tergum, owing to the large size of the attachment
of the legs. The pleura are entirelj'- absent. The second to the
eighth segments are all quite similar, consisting of a dorsal tergum,
which extends laterally to near the ventral part of the body, where
it joins the sternum. The pleura, or side pieces, noted in connection
with the thorax, have been inseparably fused to the tergum. In
the ninth and tenth segments the terga are partially fused together,
the union of the two being indicated by the presence of a transverse
suture. The sterna of these two segments are entirely fused and
much modified, forming a broad, platelike piece. The eleventh
segment is 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
and ventral to the eleventh tergum. The anus opens between these
plates, and the genital chamber lies directly below them. Attached
to the ninth 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
subgenital plate. The ninth, tenth, and eleventh segments are essen-
tially like those of the male, the terga of segments 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
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 349

pair. When these four pieces are brought together, their points
are in contact, forming a sharp organ by means of which the fe-
male bores the holes in the ground in which to deposit her eggs.
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.

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 con-
cealed by the posterior edge of the pronotum. The metathoracic
spiracle is located just dorsal to the mesothoracic leg, near the
suture separating the two segments. There is another spiracle just
dorsal to the attachment of the metathoracic leg, but this belongs
to the first abdominal segment. From the second to the eighth
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
present in any fully developed insect. Air passes through the
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
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 of the grasshopper consists of a practically
straight tube extending 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.

-to.

-■R;.

J.A

Fig. 202. — Digestive system of Rhomaelia microptera. A, anus; O, crop; Co.,
colon ; G.C., gastric caeca ; Int., intestine ; M, mouth ; M.T., Malpighian tubules ;
Oe., esophagus; R. rectum; Sal., salivary glands. (From White, General Biology.
The C. V. Mosby Co.)

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

Fig 203 Nervous system of Rhomaelia microptera. Ah., first abdominal

ganglion ; C, circumesophageal commissure ; Sp., supraesophageal ganglion ; Su.,
subesophageal ganglion. (From White, General Biology. Tiie C. V. Mosby Co.)

The food is acted upon in the stomach by the secretions of the gas-
tric caeca, which are glandular bodies opening into the anterior
end of the stomach. They secrete a weak acid which helps in the

352

TEXTBOOK OF ZOOLOGY

emulsification of fats and the conversion of albuminoids into pep-
tones. Much of the food is 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 direc-
tion. In the intestine, which is divided into the ileum, colon, and
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
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.)

The nervous system consists of a series of ganglia or nerve cells
connected by a double set of 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
each side of the esophagus. Nerves pass from the brain to the eyes,
antennae, and palpi of the head. The subesophageal ganglia supply
the mouth parts with nerves. The legs and wings are coordinated
in their movements by the thoracic ganglia. In the vertebrates the

REPRESENTATIVE INSECTS

353

nervous system is dorsal to the digestive tract, and the foreshadow-
ing 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 separat-
ing the two sexes are distinctive. The external genital structures
have been discussed above. The male organs consist of testes lo-
cated 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.

Fig. 205. — Male reproductive organs of Rhomaelia microptera. Te., testes; Y.D-,
vas deferens. (From White, General Biology.)

Fig. 206. — Female reproductive organs of Rhomaelia microptera. C.S., copula-
tory 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

and deposit the eggs to the depth of an inch or more. The eggs
are covered with a frothy substance which protects them from
moisture and, to some extent, from the frost. The eggs are laid in
the fall and hatch in the spring of the year. The development of
the grasshopper is 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 gar-
den 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 distribu-
tion 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. The thorax is provided with
yellow setae.

The abdomen, which is broadly fused with the metathorax, consists
of eight external segments. When the elytra are removed, the
spiracles 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

By carefully removing the 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 is well developed for carrying the air from
the spiracles to all parts of the body.

There are many changes in the digestive system of the June bug
as it passes from the larval stages to the imago. 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 disap-
peared, 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.

The life histories of the June bugs vary in length from three to
four years, depending upon a number of ecological factors. The
adult females dig into the ground and deposit from a hundred to
two hundred eggs. The larvae are commonly known as ''white

REPRESENTATIVE INSECTS

357

grubs." The adults come forth in great numbers in May or June
and live 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: a
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 chew-
ing. The lahrum is attached to the lower edge of the chjpeus. A
little organ, the epipharynx, is just below the upper lip. The man-
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 meta-
thorax. Each segment bears a pair of legs. The wings are borne
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 as the pollen Irush, and the large spinelike structure, the
velum, which is associated with the antenna comb. The metathoracic
legs have the tibia modified to form a pollen basket. There are also
modified spines and structures on the last pair of legs known as the
pecten, auricle, and pollen combs. The modifications found in the
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

The digestive tract is well adapted for the specialized life of the
bee, that of gathering and feeding upon the nectar of flowers. A
study of the digestive sj'stem of the larva, pupa, and adult of the
solitary wasp, Odynerus dorsalis, by Mr. Edwin Vest reveals that it
is very similar to that of the honey bee, as well as many other Hymen-
optera. In Odynerus or the honey bee the digestive tract may be
divided into the fore intestines, mid-intestines, and hind intestines, as
in the June bug. The divisions of fore intestines are : the mouth or
buccal cavity, esophagus; water sac or Jioney stomach; and the pro-
ventriculus. The mid-intestine consists of the stomach, while the hind
intestine may be divided into the ileum, rectal glands, rectum, and
anus. In the larval and early pupal stages the mid-intestine is a thin
flat tube, but in the adult it has developed into a convoluted, looped
stomach. The number of Malpighian tubules increases from the
larval stage to the adult. Only four Malpighian tubules are found
in the lar\^a, while there are around one hundred in the adult. There
is also a marked change in the length of the esophagus during meta-
morphosis. 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 is well filled with tracheae, which are con-
nected with two pairs of thoracic spiracles ajid eight pairs on the
abdomen.

The nervous system is similar to that of the grasshopper. The
brain is 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 fer-
tilized 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, life in the hive, the gathering of
nectar and its development into honey for table use, as well as
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 XXIII

PHYLUM CHORDATA

Phylum Chordata (kor da' ta, cord) is made up of the group of
animals which includes man himself and in general the more con-
spicuous, better 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 cer-
tain 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.

The notochord serves as a stiffening rod and is the foundation axis
for the endoskeleton. It is present as such at some time during the
life of every cliordate animal. In the adult vertebrate it is replaced
by the centra of the vertebrae. The gill clefts are present at some
time in the life of all individuals placed in this phylum. Although the
gills become modified to form other structures in the adult terrestrial
chordates including man, they have had rather typical ones as em-
bryos. The pharyngeal clefts or gills provide a more effective mode
of respiration for aquatic animals than that used by most non-
chordates because the gills are thus interposed directly in the
course of the circulation, and the entire blood supply of the body
passes through them. The central nervous system is derived from
the ectoderm along the middorsal line of the embryo, first as a
plate, then as a groove, and finally a tube which results in the spinal

360

PHYLUM CHORDATA 361

cord and brain. In higher forms the anterior end of the tube be-
comes expanded and modified to form the brain. The continuous
tubular nerve cord is at the apex of the development of centraliza-
tion 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 com-
plex forms. There is a tendency toward fusion of metameres and
shifting of superficial muscles. The internal skeleton of this group
compared with the external one of others studied does not give as
great a leverage for the muscles, but it greatly increases the mechani-
cal freedom allowed and this is a 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 En-
teropneusta (en ter op nus'ta) includes order Balanoglossida with its
four families, ten genera and twenty-eight species, and order Cephalo-
discida with its two genera C ephalodiscus and Bhahdopleura. These
are all small wormlike animals.

Urochorda (u r6 kor'da, tail cord), or Tunicata (tunika'ta) in-
cludes the tunicates, all of which are marine and mostly small.
Adults show a high degree of degeneration so it is the larvae only
that exhibit distinctive characteristics of the phylum. There are
three classes: (1) Larvacea, so named because it retains the larva
form throughout life. Genus Appendicidaria is an example. (2)
Ascidiacea, the sea squirt, either free-swimming or sessile, simple or
colonial, may reproduce sexually or by budding. Molgula, Cynthia,
and Ascidia are common examples. (3) Thaliacea, free-swimming,
pelagic, solitary or colonial forms, usually exhibit alternation of gen-
eration. Salpa and Doliolum are the most common examples.

Cephalochorda (sef a 16 kor'da, head cord) includes approximately
twenty-eight different species of marine, shore-loving, fishlike forms
of which Aniphioxus (Branchiostoma lanceolatus) is the most common
representative.

362 TEXTBOOK OF ZOOLOGY

Yertehrata (ver te bra'ta, jointed) animals with backbone — frog,
man. These are the larger, more conspicuous animals and will be
discussed at length in later sections of the book.

Phylogenetic Advances of Chordata

(1) 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
and lungs for respiration, (6) voice production, (7) specialization
and coordination of muscles.

Protochordata (Lower Chordates)

Until relatively recent years the two subphyla, Hemichorda and
Urochorda were not classified as Chordata; the former was with An-
nelida 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
into the mud and sand along the seashore. They range from 6 to 10
inches in length. Others of the subphylum may be as short as one
inch or still others as long as four feet. The three portions of the
body are proboscis, a ringlike collar, and a segmented trunk. The
proboscis, as well as the collar, is hollow and serves as a water cham-
ber. 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
side and just anterior to the collar. Supporting the base of the
proboscis is a short skeletal process which is stiff and extends ante-
riorly from the roof of the mouth region and assists in burrowing.
This process, called the diverticulum, is usually referred to as the
rudimentary notochord. However, it is very poorly developed and
in a peculiar position. Nevertheless, it has the relationship to the
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

363

and ends in the anus. Like the earthworm, this animal utilizes the
mud in which it lives for food, absorbing the organic matter from it
as nutriment. 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
are much reduced in numbers or are lacking. Where gills are pres-
ent, 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.

Fig. 209. — External features of DoUchoglossits kowalevskii.

Denoyer-Geppert Company. )

(Courtesy of

Proboscis coelom
Proboscis

Pericardium
Glomerulus / Collar

Heart /

Nerve cord

Dorsal vessel

Trunk

Notochord Mouth

Ventral vessel

Gill slits

Alimentary canal

Fig. 210. — Diagram of a sagittal section through anterior portion of Dolicho-
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 which encircle the body to connect with a ventral
vessel extending posteriorly below the intestine. Sinuslike branches
of these main vessels supply various parts of the body.

The nervous system is 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

364 TEXTBOOK OF ZOOLOGY

ventral cord rimning longitudinally on the floor of the trunk. The
ventral cord certainly is not a chordate characteristic, but the domi-
nance 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
cavity (coelom) and pass out the pore with the water as it is ex-
pelled. These 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 and become tornaria

Apical plate

Mouth

Proboscis
ccelum

Anus

Fig. 211. — Tornaria larva of Hemichorda. (From Hegner, College Zoology, pub-
lished by The Macmillan Company, after Metchnikoff . )

larvae, which are globular in shape and form a pattern of ciliated
bands over the body. In this respect and in habit of life these
larvae resemble the larvae of the echinoderms. On this basis a
theoretical relationship has been proposed. Until relatively recent
times this larva was mistaken for a form of adult nonchordate ani-
mal 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, As-
cidia, and Molgula. The latter genus represented by M. manhatt en-
sis will be given particular consideration here. This animal is com-
monly known as sea lemon, sea peach, or sea squirt. The body
of the adult is saclike and averages about one inch in diameter. In
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 struc-
ture 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
characteristic features in the larval stage. The larva is free-swim-
ming and shaped like a tadpole, while the adult is globular and sessile
in most of the 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 atrio-
pore). When the tunic of Molgula is removed, one may see most of

366 TEXTBOOK OF ZOOLOGY

the internal organs through the transparent mantle. Upon viewing
this from the left the large saclike pharynx may be seen continuing
ventrally and posteriorly from the branchial siphon, finally narrow-
ing at its dorsoposterior extremity to become the small tubular esoph-
agus which turns sharply downward and anteriorly to become the
stomach. The esophagus is partially embedded in a dark-colored
digestive gland. The stomach continues anteriorly and upward where
it becomes intestine, which turns ventrally on itself in a U-shape. It
finally 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
from here passes through the gill slits or stigmata in its wall into
the surrounding atrial cavity, and finally leaves the body by way of
the atrial siphon. Oxygen is absorbed by the blood in the walls of
the stigmata. The animal's food consists of minute organisms which
are entangled in mucus secreted by a glandular groove, the endo-
style, which 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
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 com-
pound sets of gonads, one on the left side in the loop of the intestine
and the other on the right side of the body. Some of the sessile
tunicates, as Molgula, reproduce by budding. There is an oblong,
closed excretory sac which may be seen from the right side. The cen-
tral nervous system is reduced to a nodulelike ganglion located be-
tween the siphons in the dorsal portion. Nerves branch from this to
the various parts of the body. The life history of the tunicate is one
of interest. Cross- fertilization is the rule; that is, spermatozoa from
one individual usually fertilize ova from another ; however, there may
be exceptions to this. The fertilization occurs in the water outside
the body. The eggs hatch to produce larvae somewhat similar to am-
phibian tadpoles which are free-swimming. The larva possesses the
typical notochord, gills, and nerve cord of Chordata. For some reason

PHYLUM CHORDATA

367

it then settles on the bottom and attaches itself by adhesive papillae
located in the anteroventral position. Some authors express it by
saying this larva settles on its "chin." It now undergoes regressive
changes involving loss of tail, notochord, and posterior portion of
nerve cord. The anterior portion of the cord becomes a simple gan-
glion. The paired eyes and otocysts (ear structures) also disappear.
The dorsal side shortens while the ventral side leug-thens. This places
the mouth in a dorsoanterior position, the anus in the dorsoposterior
position, and bends the alimentary canal into a U-shape. The num-

d.n.c.

trhqa.
tnt. / ;e.

epw.

^'9-3- cie.

at.oji

Fig. 213. — Metamorphosis of an ascidian lari'a. A, larva ready for fixation.
B, an intermediate stage of metamorpliosis. G, completion of metamorphosis.
ad~ga., adult ganglion ; at., rudiment of atrium ; at.oj}., atrial opening ; ce.ves.,
cerebral vesicle ; ci.f., ciliarv funnel ; d.n.c, dorsal nerve cord ; e., eye ; eiric, epi-
cardium ; est., endostyle ; fix., fixation papillae ; ga., ganglion ; g.s., gill slits ;
h,., heart ; int., intestine ; m., mouth ; ncli., notochord ; st., stomach ; stat., statolith ;
trk.ga., trunk ganglion. (From Borradaile and Potts, The Invertebrata, 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, cellu-
lose covering. This process of metamorphosis has caused an active

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, and the next
is produced by budding (asexually). This alternation of generation
is another retrogressive feature.

SUBPHYLUM CEPHALOCHORDA, AMPHIOXUS

There are usually listed twenty-eight species in this group which
are rather locally distributed over the world. There are four species
on American shores : Bra7ichiostoma virginiae, B. floridae, B. lermu-
dae, 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
form it represents clearly the distinctive characteristics of the
phylum in a simple condition. It is a ehordate, possessing only rare
essentials. It is usually referred to as a close ancestral relative of
Vertebrata.

Habitat. — It is found in shore water and on the sandy beaches
of the subtropical 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 Mediter-
ranean 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, particu-
larly 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. — The body of this animal is shaped like a
small lance, the tail being the point. In general, it is similar to

Cerebral
vesicle

Oral cirri

Velum

Velar tenta-
cles

Spinal cord

Atriopore
Intestine

Ventral fin

Anus

Caudal fin

Fig. 214. — Diagram of Branchiostoma (Amphioxus) lanceolatus from the right side

to show the structure.

370

TEXTBOOK OF ZOOLOGY

a small 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

.^ Spinal nervz

\ Nerve cord

^j Nobochord

S Myoto;ne muscle

\ Myocomma

>] Dorsal Aorta

^K— =■■ A^-Epibranchial qroove
.Nephridium
^ Atriaicavity

_ _ Liver

Neurocoek '/^-^

NotochordoL _ '/
sheath

Coelom

Atrial cavity-
Pharynx

Gil/rod

Gill bars

Qonad

Ventral aorta.

Hypobranchial qroove

Endostyle

Coelom

Metapleural fold

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

1 Caudal artery

Fig. 216. — Diagram of the circulatory system of Amphioxus.

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 filled with fluid to give it turgor or stiffness. Immediately
dorsal to this rod is the nerve cord, which also runs the length of the
body. It has a small central canal or neurocoele extending length-
wise through it 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
going to the myotomes. There are sensory cells in the skin, oral
tentacles, and velar tentacles.

The circulatory system does not include a heart, but the blood
is moved by the contractions of a ventral aorta, which branches to
form the afferent branchial arteries to the gills. Here these vessels
branch into capillaries, providing aeration for the blood. These capil-
laries converge to form the efferent branchial arteries which lead dor-
sally to join the paired dorsal aortae. The dorsal aorta extends pos-
teriorly to the tip of the body giving off numerous branches to myo-
tomes and internal organs along the way. The posterior direction of
the flow of the blood is just opposite to that in the dorsal vessel of the
earthworm. The subintestinal vein receives the blood from the in-
testine and 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).

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

373

varies, 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 ciliated. In the floor of the pharjTix
is another ciliated groove, the JiypohrancJiial groove. Its glandular
walls, which are capable of secreting mucus, constitute the endostyle.
It functions on the same plan here as in tunicates. The strings of
mucus entangle the food particles and are moved anteriorly, and then
by two peribranchial grooves are carried dorsally to the hyper-
branchial groove. The cilia here move the mass back to the intestine.
A 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 rela-
tively straight tube. The food is digested in, and absorbed from, the
intestine.

Respiratory System and Respiration. — As stated above, the water
in passing through the gill slits delivers oxygen to the blood in the
capillaries there 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
vessels, and are supported by chitinous rods. The gills are on the
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 pharyn-
geal 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 from the atrial cavity. Behind the
atriopore it 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. Fertilization occurs
in the water. 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
total and equal. This is followed by the infolding of one side of the
spherical body to form the gastrula and this in turn becomes a free-
swimming larva which reaches adult condition without metamor-
phosis, only to begin bashfully burying itself in the sand.

CHAPTER XXIV

THE VERTEBRATE ANIMAL: SUBPHYLUM

VERTEBRATA

In this group to which man himself belongs are found the dis-
tinctive chordate characteristics at some time in the life of the indi-
vidual. In terrestrial forms there are certain modifications to pro-
duce other structures. Metamerism and bilateral symmetry are
universal characteristics among vertebrates. The segmented verte-
bral 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 indi-
vidual. The majority have two pairs of fins or limbs in adult con-
dition. There is a ventrally located heart which is divided into
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, ex-
cretion, 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
different types of vertebrates there are various modifications of
pectoral appendages as arms, wings, pectoral fins, forelegs, and
flippers. The same is generally true for the pelvic limbs.

The body wall is composed of the skin, which usually has char-
acteristic tegumentary outgrowths, such as scales, nails, shells,
feathers, and hair, as the outer layer, beneath which is the muscular

375

376

I'EXTBOOK OF ZOOLOGY

coat and internal to this is the membranous peritoneum. In all
vertebrates, except mammals, the coelom consists of only two parts:
the pericardial cavity and the general abdominal cavity. In mam-
mals it is further divided into pericardial, thoracic, and aldominal

cor

Fig. 217. — Diagrammatic section of the human skin. Cor, stratum corneum ;
D, dermis ; gs, sebaceous gland ; M, Malpighian layer ; niu, muscle ; n, non-
meduUated 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
of hair root; x and y, endings of nonmyelinated nerve fibers. (From Maximow and
Bloom, Histology, published by W. B. Saunders Co.)

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
facilitating temperature regulation in some. Such exoskeletal struc-
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 is developed in the dermis.

The maintenance of any living body requires the cooperation of
several functions which 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 repro-
duction. During the execution of these activities energy is con-
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 pro-
toplasm. 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)
constitute the phase of metabolism known as anaholism. Included
here are ingestion, digestion, absorption, transportation, and assimila-
tion. 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.

Metabolism is one of the fundamental features of all protoplasm,
therefore, all physiology, since it is a study of the functions of liv-
ing organisms, must be concerned with metabolism. It includes all

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,
hair, feathers, and other outgrowths. The endoskeleton includes the
axial and appendicular portions. The first is composed of the skull,
vertebral column, ribs, and in some a sternum. The appendicular
portion is composed of the anterior and posterior girdles and two
pairs of limbs. In their development bones either replace cartilage
to be called cartilage I ones or they develop in the connective tissue
of the dermis, to be known as membrane hones. The vertebral column
is composed of segmental divisions, the vertebrae, and is divided into
five regions as follows: cervical vertebrae of the neck, thoracic verte-
brae of the chest, lumbar vertebrae of the small of the back, sacral,
vertebrae of the hip region, and the caudal vertebrae of the tail
region. Bone is a firm, hard tissue consisting of abundant matrix,
composed of inorganic salts, and the bone cells which are held in
pocketlike lacunae in the matrix. The outer membranous covering
of bone is called periosteum. The mineral part of the bone consists
chiefly of calcium phosphate and calcium carbonate. They give it
firmness and rigidity. The animal matter is composed of the bone
cells and cartilage which serve to give the bone life and resilience.
A weak acid, such as the acetic acid in vinegar, will dissolve the
mineral matter of bone if allowed sufficient time, in which case
the bone will lose its rigidity. Caustic solutions will destroy the
animal matter and make the bone brittle. The following outline
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. Cranium

2. Sense capsules

3. Jaw apparatus
(Visceral arches)

THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 379

(b) Vertebral column

1. Cervical vertebrae (neck)

2. Thoracic vertebrae (chest)

3. Lumbar vertebrae (small of back)

4. Sacral vertebrae (hip)

5. Caudal vertebrae (tail)

1. Eibs (paired)

2. Sternum (breastbone)

II. Appendicular Skeleton (girdles and limbs)

(a) Pectoral (anterior)

1. Girdle: scapula, clavicle, procoracoid and coracoid

2. Limb: Humerus (upper arm), radius and ulna (forearm), carpals
(wrist), metacarpals (palm), phalanges (bones of digits)

(b) Pelvic (posterior)

1. Girdle: ilium, pubis, and ischium

2, Limb: Femur (thigh), patella (knee cap), tibia and fibula (shank),
tarsals (ankle), metatarsals (sole), phalanges (bones of toes)

In Protozoa there is no very elaborate adaptation toward a skele-
ton. 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 Foram-
inifera, and Radiolaria exemplify this adaptation.

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 ex-
ternal surface of the body proper. The sponges, as a rule, each
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 body. The muscles and other tissues are attached within.
There are special cells of the epidermis which function primarily
in production of this skeletal material. The echinoderms, including
animals like the starfish, possess calcareous skeletal plates which
are essentially similar to exokeleton except that they are princi-
pally beneath the skin.

There is no well-developed endoskeletal structure known in non-
chordate 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

380

TEXTBOOK OF ZOOLOGY

protection and temperature regulation in most of the groups of ver-
tebrates. Such structures as scales, shells, feathers, hair, nails, horns,
and even enamel of teeth are of this type.

Primitively the notocJiord is the original endoskeletal structure of
the chordate group. Around it are developed the basic structures of

Ska//

Ceri//ca/
vertebrcxe

Scapu/a

Sternum

Thoracic
vertebrae

Lumbctr
vertebrae

Sccrum

Mefacarpa/s

Cranium
Orbit

Mandib/e

C/av/c/e

■Humerus
Rib

Pe/vis

Radius
Utna
^Carpa/s

Hc^nd

Fig. 218.-

Tibia

Metatarsals

-Human skeleton. (From Wolcott, Animal Biology, published by the
McGraw-Hill Book Company.)

the vertebral column which functions as the principal axial support
of all vertebrates. The sternum, girdles, and paired limbs have
developed with the terrestrial life of vertebrates and the necessity
for locomotion on land.

THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 381

The muscular system represents a system of cells highly special-
ized in contractility. The muscles are usually attached to the skele-
ton 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.

Independent power of movement is almost a characteristic of
animal life. Contractility as a property of all protoplasm is the
fundamental basis for all animal movement. The adult forms of cer-
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 to move separate parts in adult
condition.

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 proto-
plasm 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 cilia while in Metazoa the entire body may be covered where
they are used for locomotion; but more often they cover only areas
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.

The development of a high degree of contractility in special cells,
such as muscle cells, makes possible muscular movement which is the
principal kind in higher animals. A muscular locomotor system con-
sists of sets of opposing muscles. In muscular contraction there is
a cycle of rapid chemicophysical rearrangement in the cells. Oxida-
tion and heat production are involved in the process. Carbohydrates
in the form of glucose are oxidized (burned) in the reaction. During
the shortening of the muscle there is 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 longi-
tudinal 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 to make contact with the surface over which the animal
is moving.

The Dig"estive System. — The digestive system is typically a
straight tube extending through the length of the trunk of primitive
vertebrates. In the higher forms there are many outgrowths, such
as digestive glands 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 contrac-
tion, a process known as peristalsis. It leads to the saclike stomach,
whose walls possess gastric glands for secretion of a digestive fluid
containing enzymes (ferments) and weak hydrochloric acid. The
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
posterior ileum. It is usually longer than the body and therefore
it is coiled. Its walls produce digestive enzymes from glands and
it receives digestive juices from two other glands : the liver and
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
increase the inner surface and enhance absorption. The digested
food is taken up by the lymphatic spaces and by blood vessels which
are embedded in the wall just outside of the lining epithelium. The
liver is the largest organ in the body of most vertebrates. It
secretes the bile which is stored in the thin-walled gall Madder,

THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 383

which is attached to one of its lobes. The liver also serves to convert
carbohydrates to glycogen (animal starch) and store it for future
ener^ production. It is also in the liver that protein wastes are
converted into urea and uric acid in order that they may be excreted
from the blood in the kidneys. The large intestine which is shorter
than the small intestine possesses no villi or digestive glands. It
receives the fecal matter and delivers it to the anus. In many
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.

The chief function of this entire system is 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, usu-
ally 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 mem-
branes. Digestive enzymes are responsible for placing the food
materials in solution. So, proteins are converted to soluble amino
acids, starches and sugars to maltose and finally glucose, and fats
to fatty acids and glycerin.

In general, an enzyme is an organic substance which by its pres-
ence under certain conditions \vill cause or hasten chemical reaction
between other substances without itself being consumed. The en-
zymes 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 spe-
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
(hydrolases) act 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 acti-
vating agent or coenzyme. As an example, the precursor of pep-
sin is pepsinogen which becomes activated in the presence of dilute
acid.

384 TEXTBOOK OF ZOOLOGY

Classes of Digestive Enzymes

1. Diastases or diastatic enzymes — split carbohydrates

(a) Ptyalin in saliva

(b) Amylase in pancreatic juice

2. Lipase or lipolytic enzyme — splits fats
(a) Steapsin in pancreatic juice

3. Inverting enzymes — convert disaccliarids to the less complex monosaccharids
(simple sugars) — intestinal juice

(a) Maltase

(b) Lactase

4. Proteases or proteolytic enzymes — split complex proteins

(a) Pepsin in gastric juice

(b) Trypsin in pancreatic juice — functions in small intestine

5. Clotting or coagulating enzyme
(a) Eennin in gastric juice

In higher Metazoa digestion is accomplished principally extra-
cellularly through secretion of enzymes by certain groups of cells.
Such systems consist of: (1) an alimentary canal proper; and (2)
associated glands which discharge digestive juices into it. The
relative length of this canal varies considerably depending on the
habitual diet of the organism. In carnivores (flesh-eaters), such as
cats and dogs, it is from three to five times as long as the body;
while in herbivorous forms (plant-eaters), such as horses and cows,
it is over twenty times as long as the body. The length of the
human digestive tract is 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 signifi-
cant. 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
skin.

The process of digestion in man is quite well understood, and
it is fairly typical 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
kind of chemical change. In hydrolysis the large molecules of pro-
tein, carbohydrate, or fat first combine with water and then split into
simpler products. Some foods may require more than one such

THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 385

splitting. The splitting of the disaccharide, maltose, will serve as
an example of this process:

(Malt sugar) (Water) (Glucose)

The two molecules of glucose formed are in a form for ready
absorption.

Gastric Digestion. — The tubular gastric glands located in the
mucous layer of the stomach secrete the acid gastric juice which is a
solution of 0.2 to 0.5 per cent hydrochloric acid and two important
enzymes, pepsin and rennin. The pepsin when present in the acid
medium brings about the splitting of complex proteins into inter-
mediate proteoses and peptones. Be^inin causes the casein in milk
to coagulate. This is the first step in its digestion. It is claimed by
some that emulsified fats, such as cream, are partially digested by a
gastric lipase. The digesting mass or chyme in the stomach is con-
tinually churned and mixed by muscular activity of the walls. "When
it becomes saturated (0.4 per cent) with acid and has been reduced
to the consistency of soup, it is discharged through the pylorus.

Intestinal Digestion. — When the chyme is ejected through the
pylorus into the duodenum, the hydrochloric acid stimulates cer-
tain 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 diges-
tive fluid, pancreatic juice, into the small intestine by way of the
pancreatic ducts. There is some evidence that secretin also stimu-
lates secretion in the liver.

Pancreatic juice is a clear, watery, alkaline solution containing
inorganic salts (carbonates, etc.) and three enzymes; the protease,
trypsin, the diastase, am.ijlopsin, and the lipase, steapsin. These act
respectively on proteins and peptones, starches and sugars, and fats.
This protease is in the form of trypsinogen until it reaches the intes-
tine 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
efSciently than does pepsin. There are nineteen amino acids that
are regarded as hmlding stones of the protein molecule. In a com-
plex protein like casein, as many as sixteen of these amino acids will

386 TEXTBOOK OF ZOOLOGY

be found. 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 con-
stituency.

Amylopsin (amylase) is the pancreatic diastase, and it is able to
bring about hydrolysis of carbohydrates in the alkaline medium of
the intestine without activation. It produces dextrin and maltose
(malt sugar). The pancreatic lipase, steapsin, brings about the split-
ting of fats into glycerin (glycerol) and one or more fatty acids,
such as stearic acid, oleic acid, butyric acid, etc. 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 mak-
ing 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. Enterokinase, which activates trypsinogen to form tryp-
sin, has been mentioned already. Erepsin, the intestinal protease,
supplements the activity of trypsin by converting proteoses and pep-
tones into amino acids. Maltase converts maltose and dextrin into
dextrose. Invertase changes sucrose (cane sugar) into dextrose and
levulose. Lactase converts milk sugar (lactose) into galactose and
dextrose, both simple sugars.

The undigested residue passes into the large intestine where prob-
ably no enzyme digestion occurs. Certain bacteria (B. coli and
others) attack any undigested protein and bring about putrefactive
fermentation. 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
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. — The secretion of the liver is bile and is
discharged into the duodenum of the small intestine by way of the
common bile duct. This is an alkaline solution which serves to help
neutralize the acidity of the chyme as it comes from the stomach. This
with the pancreatic juice brings about the emulsification of fats men-
tioned above. Cholesterin and two pigment materials are excreted in

THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA

387

The Digestive Enzymes and Their Functions

ENZYME

REGION OF DI-
GESTIVE TRACT*

DIGESTIVE
JUICES

FOODS

AFFECTED

SUBSTANCES
PRODUCED

Ptyalin

Mouth

Saliva

Starch
(carbo-
hydrates)

Maltose

Pepsin

Stomach

Gastric juice
from gas-
tric glands

Proteins in
acid
medium

Proteoses
and pep-
tones

Bennin in
mammal

Stomach

Protein of
milk

Coagulated
to form
paracasein

Gastric
lipase

Stomach

Emulsifies
fats

Glycerol and
fatty acids

None

Liver

Bile

Emulsifies
fats

None

Amylase or
amylopsin

Produced in
pancreas

Pancreatic
juice pro-
duced in
pancreas
but acting
in small
intestine

Carbohy-
drates

Maltose

Steapsin

Produced in
pancreas

Intestinal

Lipins
(fats)

Glycerol and
fatty acids

Trypsin

Produced in
pancreas

Proteoses
and pep-
tones in
alkaline
medium
with enter-
okinases

Polypeptids

Erepsin

Small
intestine

Polypeptids

Amino acids

Maltase

Small
intestine

Maltose

Glucose
(dextrose)

Lactase in
mammals

Small
intestine

Lactose

Glucose and
galactose

Invertase or
s^icrase

Small
intestine

Sucrose

Glucose and
levulose

*The esophagus and colon do not secrete any enzymes.

the bile. Bile is secreted all of the time, but between meals it is stored
in the gall bladder and supplied in quantity at meal time.

Besides these digestive and excretory functions the liver serves
in another capacity. It is a storehouse for carbohydrates which it
converts to glycogen (animal starch) by enzyme action. This sub-
stance is also stored in the voluntary muscles. It is easily recon-
verted to dextrose for ready oxidation. Most of the protein by-
product iirea (and uric acid in some forms) is formed in the liver
and carried by the blood to the kidneys for excretion.

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 sub-
clavian vein. The blood supplying the intestine is collected by the
hepatic portal vein and delivered to the liver.

The two functions of proteins in the body are : to rebuild debili-
tated protoplasm; and help supply heat and 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 pro-
tein requires the disposal of much more waste products. The com-
parative heat production values of the three are as follows :

One gram of protein z= 4.100 Calories*

One gram of carbohydrate z= 4.100 Calories
One gram of fat = 9.305 Calories

Some portion of the dextrose is distributed and oxidized directly
for immediate energy, but much of it is transformed into glycogen
by the enzyme glycogenase in the liver. This may be stored here or
in the muscles to be reconverted into dextrose for oxidation by the
tissues as needed. 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,
kinetic energy, water, and carbon dioxide. The last two are dis-
charged from the body as waste products. Fat is converted to dex-
trose and oxidized to produce heat and kinetic energy. It is usually
stored as a reserve fuel supply in adipose tissue over the body. Car-
bohydrates in excess may be converted to fat, and stored.

Vitamins and Their Functions. — Besides proteins, carbohydrates,
fats, inorganic salts, and water there is another indispensable class
of food material, the vitamins. They are natural substances found
in relatively small quantities in a number of different foods. In
general, their function is regulatory. They are recognized usually
through the abnormal condition brought on by their deficiency.
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 liter of water one
degree centigrade under standard conditions.

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 A (C20H30O) — antixerophthalmic — fat 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 fac-
tor in regeneration of visual purple of retina, strengthens and pro-
motes hardiness in epithelial tissue.

(c) Effects of Deficiency: Xerophthalmia (lack of tear secretion and dry
cornea), and "night blindness" in human. "Nutritional" roup in
birds.

II. Vitamin B* "Complex."

1. Bj or Thiamin (Ci2Hi,ON4S) — Antineuritic.

(a) Sources: Germ of wheat and other cereal grains, peanuts, liver, and
egg yolk.

(b) Functions: Promotes tone in alimentary tract, stimulates appetite,
essential for normal growth, essential for carbohydrate metabolism.

(c) Effects of deficiency: Beri-beri (neurodigestive disturbance following
diet of polished rice), loss of tonus and muscular activity of digestive
tract. Cessation of growth. Polyneuritis develops in birds.

2. Riboflavin (C^H^oOoN^).

(a) Sources: Eggs, liver, milk, green leaves, yeast.

(b) Functions: Necessary for growth, active relation to several enzymes
witli intermediate metabolism of food.

(c) Effects of deficiency: Irritation and inflammation at corners of
mouth in human (cheilosis). "Yellow liver" of dogs. "Curl-toe"
paralysis of chickens. Dermatitis of turkeys.

3. Nicotinic Acid (CgHsNOz) — antipellagric.

(a) Sources: Meat, liver, egg yolk, green leaves, wheat germ, yeast.

(b) Functions: Produces active "coenzymes" (I and II), balances cel-
lular function,

4. Be or pyridoxine (CgHuOaN).

(a) Sources: Milk, liver, cereals, yeast.

(b) Functions: Necessary for growth. May influence oxidation of food.

♦There are still other recently discovered fractions of Vitamin B. whose func-
tions are specific.

390 TEXTBOOK OF ZOOLOGY

5. Pantothenic aeid (CgHiTOgN).

(a) Sources: Liver, milk, egg yolk, yeast, molasses, peanuts.

(b) Functions: Essential for growth.

6. Biotin (doHieOsNjS).

(a) Sources: Egg yolk, yeast, cereal grains, molasses.

(b) Functions: Essential for growth.

III. Vitamin C or Ascorbic Acid (CoHsOe) — antiscorbutic-water-soluble.

(a) Sources: Citrus fruits, tomatoes, turnips (most mammals except pri-
mates and guinea pig can synthesize this vitamin).

(b) Functions: Maintains structure of capillary walls.

IV. Vitamin D (C2SH44O) — antirachitic.

(a) Sources: Tuna and cod-fish liver oils. Exposure of skin to ultra-
violet radiation.

(b) Functions: Eegulation of calcium and phosphorus metabolism. Ke-
quired for normal growth and mineralization of bone.

(c) Effects of deficiency: Soft, deformed bones in young (rickets). Soft
bones (osteomalacia) especially in women of the orient.

V. Vitamin E or Tocopherol (CjaHjoO.)- — antisterility.

(a) Sources: Wheat germ oil, green leaves, other vegetable fats.

(b) Functions: Promotes rapid cell proliferation and differentiation.

VI. Vitamin K (C3,H4e02) — antihemorrhagic.

(a) Sources: Green leaves, alfalfa, also certain bacteria of the "intesti-
nal flora."

(b) Functions: Influences the production of prothrombin by the liver
(prothrombin is necessary for blood clotting).

The Respiratory System.— The respiratory system is at least in
part an outgrowth of the digestive canal. In most aquatic verte-
brates respiration is accomplished by drawing water through gill
slits in the wall of the pharynx. Air-breathing, terrestrial forms
have developed the trachea (windpipe) and lungs as another out-
growth 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 environ-
mental 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
cells over the body. Much of the carbon dioxide given up by the
cells becomes carbonic acid and carbonates which may be trans-
ported 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 surround-
ing medium. This movement of gas through the cell membranes de-
pends 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
tissue is such that a more active interchange of gases is required
than the general body surface will permit, special organs or modi-
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
carried in a blood vascular system all over the body. These pig-
ments 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 oxy-
gen pressure of the surrounding tissue is sufficiently low, the oxy-
hemoglobin releases its oxygen rapidly. Carbon dioxide accumu-
lates in excess in the tissues and diffuses from the cells to the
lymph, thence to the plasma where much of it combines with sodium
as sodium carbonate. Small amounts of CO2 combine with the
hemoglobin.

The gills of most aquatic forms are richly supplied with a capil-
lary supply of blood and then membranous surfaces are directly
exposed to surrounding water from which the dissolved oxygen is
absorbed. In many aquatic worms the gill filaments are outgrowths
of the sides of the body wall. Likewise, the more or less plumelike
gills of crayfish are pocketlike outpushings of the body wall. In
a number of aquatic insects, worms, fishes, and turtles, the rectum
serves as an accessory respiratory organ.

392 TEXTBOOK OF ZOOLOGY

Aerial respiration is accomplished in terrestrial animals through
special internal surfaces which must be kept moist. In insects a
system of branched tubes called tracheae, which open through
spiracles along the sides of the body, distribute oxygen to and re-
ceive carbon dioxide from all of the cells of the body. In pulmonate
snails the "lung*' is simply 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
that if all of these pitlike alveoli of the internal lining of the lungs
of the average 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 stimu-
lated 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 of venous blood stimulates an increase of the respira-
tory action. In addition to exchanging gases the lungs also discharge
moisture and give off a certain amount of heat.

The Circulatory System. — The circulatory system is a closed sys-
tem of vessels supplying all parts of the body with blood and a
system of spaces, sinuses, and vessels collecting lymph from the vari-
ous 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 min-
ute 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 — SUBPHYLUM VERTEBRATA

393

was mentioned in connection 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 char-
acter. These cells may make their way among cells of other tissues

Veins from upper.
part of Bcxjy

Lymphatics

Thoracic duct -

^uporiop vena cava
'PulmonaP3^ artary

Ui^ht aupicla —

Infcpiop vena cava-

fli'^ht vcntpiclG

Lacteal^ —

Hepat

ic vain. ■

Veins from lowcp
papt of Body

Lympfiaticj —

Arteries to upper*
part of Body

Pulmonapy vein

— Left auricle

- Left ventpjclc

ArtGplo3 to lowcp
par>t of Body

Fig. 219. — Diagram of circulation of the blood in a mammal. The oxygenated
blood is shown in black ; the venous blood in white. The lymphatics are the black
irregular lines. (From Pettibone, Physiological Chemistry, published by The C. V.
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 TEXTBOOK OF ZOOLOGY

is a fluid similar to plasma which has seeped through the walls of
the capillaries in the various organs, and it carries amoeboid white
corpuscles. 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
of a single cell or a single-celled organism and from cell to cell of
the metazoan is 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-
stances, and gases is accomplished by simple diffusion of materials.
In a few forms, however, of which Paramecium is an example, there
is a definite course of movement by the endoplasm. This is known
as cyclosis, and it serves to circulate the food vacuoles.

In double-walled, simple, saccular forms like hydra there is no pro-
vision necessary except an exchange of the water in the gastrovascu-
lar cavity. In flatworms, such as planaria, the necessity of increased
food distribution is cared for by branching of the gastrovascular
cavity into diverticula. In sponges the wandering cells assist in
transporting materials. A distinct system of tubelike vessels with
contractile parts is developed in the annelid worms, as was studied
in the earthworm. Here a closed system of vessels forms a complete
circuit to carry a circulating medium to all parts of the body. In
this group the fluid is known as hemolymph because it bears no red
corpuscles. 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
corpuscles. In molluscs and some crustaceans there is a similar
respiratory pigment carried in the plasma, which is called hemo-
cyanin. Instead of iron, copper is the principal constituent of this
pigment. Vertebrate blood is largely water carrjdng dissolved mate-
rials 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.
Its salt content is about equal to that of sea water. When the body
is active, the blood is very unequally distributed. One-fourth is
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

skeletal muscles require another fourth ; and the remaining fourth is
distributed through all of the other organs. Human blood contains
normally about 5,000,000 red corpuscles (erythrocytes) per cubic
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 erythro-
cyte 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 tissues. The carbon dioxide given off by the cells is collected
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 amoe-
boid and therefore not confined to the blood vessels. One of their
chief functions is the destruction of bacteria and other foreign mate-
rial in the tissues. This process is known as phagocytosis. The ac-
companying table summarizes essential information concerning blood
cells.

The plasma of the blood contains a group of substances called
antibodies. These have been produced by various tissues of the body
upon contact with certain foreign proteins. Since bacteria and patho-
genic Protozoa react as foreign protein, they stimulate the body tis-
sues to the production of specific protective antibodies and physicians
have come to make use of these antibodies in sterile serum for pre-
vention and treatment of several diseases. Some of these antigen
substances bring about the clumping or agglutination of foreign bac-
teria, others dissolve the bacteria, and still others cause them to be
precipitated. 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 sub-
stance, 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

AVERAGE NUMBER

STRUCTURE

PER CUBIC MILLI-

COLOR WITH

METER OP BLOOD

WRIGHT'S STAIN

SOURCE

FUNCTION

Erythrocytes

Nonnucleated, circu-

Endothelium of

Transport oxygen;

(red blood cells)

lar, biconcave ;

capillaries of

remain in blood

5,000,000 (males)

orange buff ; 7.5

bone marrow

vessels

4,500,000 (fe-

to 7.7 microns in

males)

diameter

Leucocytes

Colorless in life

(white cells)

1. Granulocytes

Nucleus of lobes

Eeticuloend othelial

Amoeboid ; can

6,000 to 10,000

joined by thread ;

cells outside

leave blood ves-

stains dark lilac,

capillaries of

sels and enter

cytoplasm pale ;

bone marrow

tissues

blue with gran-

Defend against in-

ules; 9 to 12 mi-

fection

crons in diameter

a. Neutrophile

Granules stain

60 to 70%

weakly

b. Eosinophile

Granules few, eosin

2 to 4%

(red)

c. Basophile

Granules deep blue

0.5 to 1.5%

2. Lymphocytes

Nucleus single,

Lymphoid tissue.

Nonmotile ; related

20 to 30%

large, round, deep

spleen and

to immunity

blue ; scant cyto-

lymph glands

plasm, clear blue

4 to 10

3. Monocytes

Nucleus single.

Spleen and bone

Very motile ;

5 to 10%

large, round, deep
blue ; much cyto-
plasm, muddy
blue; 12 to 20

marrow

phagocytic

Platelets

Small, ref ractile, no

Bone marrow

Provide substance

200,000 to 400,000

nucleus ; dark
blue to lilac ;
2 to 3

needed in clotting

(Reproduced by peri«is.«ion from Textbook of Zooloay by Storer, copyrighted
1943, by McGraw-Hill Book Co., Inc.)

of the heartbeat for an average adult man at rest is about 72 times
per minute. The contraction phase of the heartbeat is called the
systole and the relaxation phase is the diastole. It has been estimated
that an average circuit of the circulation of blood in man can be com-
pleted in twenty-three seconds, with about two seconds of this time
being spent in capillaries.

The Excretory System. — The excretory system of vertebrates con-
sists of kidneys, excretory ducts, and often a urinary bladder. The
kidneys serve to remove from the blood, waste nitrogen products
and excess salts in solution as well as to dispose of excess water.
In simpler vertebrates there is a pronephros type of kidney as well

THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA

397

as a mesonephros. The former is seldom functional, but the latter
is the functional organ in vertebrates up to and including the
Amphibia, as in frogs and salamanders. The metaiiephros is the
higher developed kidney as found in reptiles, birds, and mammals.
The ureter is the excretory duct which leads from the metanephric
kidney. The life 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-
tional excretory organ when in its glory; and, finally, the develop-
ment of the metanephros with retrogression 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
the history of the development of the race.

Excretion.^A certain result of the oxidation necessary for metab-
olism is 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 sub-
stances are usually dissolved and removed as a waste liquid or occa-
sionally 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 twenty-
four 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 ar-
rangement 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. The excreted
wastes accumulate in the eoelomic cavity and are moved into the
nephridia through the ciliated funnellike internal end, known as the
nephrostome. This eoelomic fluid is drawn into the canal of the
nephridium by the beating of the cilia and is delivered to the outside
of the body at the nephridiopore of the next segment.

The green glands of crayfish are much more concentrated, although
they are modified nephridia. They function as a pair of unit organs,
each opening by a duct on the basal segments of the antennae. In
mollusks there are both nephridia, 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 mem-
branous walls of these structures. In the insects excretion is taken
care of 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
as in annelids the coelom was once important in excretion. The
essential structures of the kidney for taking waste substances from
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
of the kidney. This canal leads to the cloaca in most vertebrates
below mammals (excepting some fish), or to a urinary bladder in the
mammals.

THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA

399

The wall of each Bowman's capsule is very thin and readily per-
mits diffusion of water and dissolved materials from the blood into
the cavity of the uriniferous tubule on the opposite side of the mem-
brane. The glomerulus carries arterial blood from the afferent ar-
terial branch and discharges it into the efferent arterial branch. The
latter soon spreads into a capillary network which surrounds the
convoluted portions of the uriniferous tubule. Water constitutes
the largest volume of materials to be excreted in most animals, ex-
cept in some desert forms where water is conserved and the ex-
cretion is in crystalline form. Water is eliminated by lungs, skin,
alimentary canal, and kidneys. In man the quantity of sweat dis-
charged may amount to two or three liters a day. In the dog,
which has few sweat glands, the water eliminated by the lungs,
through panting, is proportionately greater than in man. The kid-
neys are the most important organs in the excretion of water, and
the amount they eliminate is inversely proportional to the amount
excreted by the skin. Most of the water to be excreted is taken
from the blood in Malpighiaji corpuscles.

Some of the nitrogenous wastes are excreted in the form of am-
monium salts and some free or combined amino acids. However,
most of the ammonia which results from protein metabolism is con-
verted into urea in the liver and is carried in that form to the kid-
neys 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, and water is excreted by different parts of the
uriniferous tubule. The substances which are excreted by the kidney
are not formed there, but are merely removed from the blood by
this organ.

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 por-
tion 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 connec-
tion with the adjacent spinal nerve or cranial nerve as the case

400

TEXTBOOK OF ZOOLOGY

might be. 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. Each spinal nerve has two roots where it
joins the spinal cord. A 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
cell of origin, located in the spinal ganglion ; i, cut end of spinal nerve ; 5, efferent
nerve fiber ; 6, voluntary muscle ; 7, dorsal root of spinal nerve ; 8, ventral root of
spinal nerve ; 9, dendrites of motor nerve cell body in gray matter of the cord.
(From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company,
after Morat.)

nerves carries fibers extending from the motor cells in the cord to
the motor end plates on the voluntary muscle cells. The impulses,
therefore, pass from the spinal cord to the muscles over these roots.
The reflex arc, which is the simplest kind of a nerve conduction cir-
cuit, 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 gen-
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

A high development of sense organs for the senses of sight, hear-
ing, smell, taste, and 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. The wall of this is composed of an
outer fibrous sclera (white of eye) which continues anteriorly as a
transparent front, the cornea. Beneath the sclera is a black, pig-
mented and vascular layer, the chorioid, which continues anteriorly
as the iris, the colored part of the eye. The iris is like a curtain

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 it, is filled with aqueous humor. This cavity
is subdivided by the iris. Behind the lens is the large internal or
vitreous chamber which is filled with a jellylike vitreous humor. The
curvature of the lens can be controlled by the action of the ciliary
muscle which encircles its margins. This makes possible an adjust-
ment of the eye to near and far objects and particularly so in higher
vertebrates. This power is known as accommodation. As people
get older they tend to lose this accommodation because of loss of
elasticity in the lens. The tension on it due to the attachment of
the inside of the eyeball by the ciliary process tends to hold it out

Fig. 222. — Diagram of a section through the right ear. B, semicircular canal ;
a, external auditory meatus; o, oval window (fenestra ovale) ; P, tympanic cavity
containing the three auditory ossicles ; Pt., scala tympani ; r, round window
(fenestra rotunda) ; below r is seen the Eustachian tube; 8, cochlea; T, membrana
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 equi-
librium rather than hearing. The latter are common to all verte-
brates while the cochlea is limited to Amphibia and higher classes.
The external ear is still further limited to reptiles, birds, and mam-
mals. 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
over the fenestra ovalis leading into the internal ear. The mem-
branous labyrinth is the name often applied to the chambers of the
inner ear. Its ventral chamber is the sacculus connected with the
organs of hearing, and the dorsal portion is the utricidus which is
related to equilibrium. The two semicircular canals in simpler
forms and the three in higher, join the body of the utriculus in as
many different planes as there are canals. In the higher forms
there are two vertical canals, one anterior and one posterior, with
their planes at right angles to each other, 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 of smell is centralized in the epithelial lining of the nasal
chamber. Special olfactory cells are stimulated by particles of mate-
rial from the air dissolving on this membrane and making contact
with the sensory cells. The sense of taste is similar except that it
is located in sensory cells in taste buds on the tongue, epiglottis, and

404 TEXTBOOK OF ZOOLOGY

lips (and barbels of some vertebrates). The particles come in by
way of food and drink and as the material dissolves, it reaches
the taste cells.

Most of the tactile and pressure sense organs are located just
beneath the skin over different parts of the body. 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 vibra-
tions carried in the water and is quite important to aquatic animals
of this type.

Nervous Function — Reception and Conduction. — Irritability and
conductivity are fundamental functions of all protoplasm, whether
it be in the body of an Amoeba or a man. The responsiveness of
organisms to change of conditions both externally and internally
determines their behavior. Living protoplasm is not only excitable,
but it possesses the power to record or store up the effects of previ-
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 de-
pends largely on the primitive properties of irritability and conduc-
tivity to guide its activities. In the simpler Metazoa, such as the co-
elenterates, there are scattered nerve cells connected with each other
by fibers to form a nerve net. The neuroepithelial or neurovmiscular
cells 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
with every other part of the body.

Next comes the linear type of nervous system in the form of a
ladder. 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 in planaria. In Annelida and Arthropoda
the nervous sj'stem is a modified ladder type in which the two longi-
tudinal cords of ganglia have fused along most of the midventral
line. Toward the anterior end, the cords separate at a paired gan-

THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 405

glionic enlargement, the siil) esophageal ganglion, and encircle the
alimentary canal to join on the dorsal side as the pair of siipra-
esophageal ganglia or ''brain." In Arthropoda the ganglia of the
thorax have undergone considerable fusion. In Echinodermata, the
starfish for example, the central group of ganglia makes up the cir-
cumoral 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 is carried farther in vertebrates than in the less highly or-
ganized forms. The fact that the central nervous system of verte-
brates is dorsally located and hollow has been brought out previously.
Even within the group of vertebrates, the nervous system shows a
progressive increase in complexity. The highly developed brain of
the 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
passing along the nerve fiber (axone or dendrite). This is at least
partially a chemical change in which oxygen is necessary and a cer-
tain amount of carbon dioxide is produced, but since there is only
slight increase in temperature during the change, it seems not to be
a typical metabolic oxidation process ; furthermore, the activity seems
not to fatigue the nerve fiber. An electrical charge follows the wave
of activity along the nerve fiber, but it apparently accompanies the
impulse or is a result of it rather than the impulse itself. The speed
of electrical transmission has been measured in a number of different
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 nerv-
ous conduction circuit. In its simplest form the reflex arc is com-

406 TEXTBOOK OF ZOOLOGY

posed of one motor and one sensory neurone; however, it is usually-
more 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
the impulse from the motor nerve cell in the gray matter usually j
ends in a muscle cell or a gland. There is no protoplasmic union
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
in close proximity. It has been found experimentally that nervous
impulses may 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-
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,
breathing, movements of speech, individual actions included in walk-
ing, and others.

Functions of the Spinal Cord. — This organ serves as a system of
reflex centers which control the actions of glands of the trunk, vis-
ceral organs, and skeletal muscles. The spinal cord is also a nerv-
ous 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
cerebrum. This section controls voluntary actions and provides
memory associations. The diencephalon serves as a center for spon-
taneous actions. The midbrain is one of the centers of coordinated
movement which has to do with posture and eye muscles. The cere-
bellum is 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
inhibitor on heart action. It also regulates digestive secretions,
movements of digestive organs, and vasomotor activity of the blood

THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 407

vessels. As a Avhole, the brain serves as the organ of coninuinication
between the sense organs and the body and is the coordinator of the
bodily activities.

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 copu-
lation 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 be-
ing nourished by diffusion of nutriments from the blood of the par-
ent, 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. — A living organism is in numerous ways
similar to a machine, but reproduction of new units of living mate-
rial by existing organisms is hardly comparable to any mechanical
processes known in our industries. New organisms all arise from
preexisting organisms of the same kind. The process of cell divi-
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 new individual is but a continuation of the specific
protoplasm peculiar to an earlier individual or in sexual reproduc-
tion to two individuals. Therefore, under ordinary circumstances
the structural and physiological complexities which arise through
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 individuals cannot be distinguished as parent and offspring.
Protozoa may reproduce also by sporulation, by which process the
cell forms a protective cyst and by a series of simple divisions (frag-
mentation) the internal protoplasm breaks into a number of smaller
units. Following this the cyst ruptures and releases these new units
as independent individuals. For the most part, reproduction among
protozoans is taken to be asexual, but according to a recently pub-
lished work by Sonneborn, a distinct sexuality exists in Paramecium.
Examples of asexual reproduction by budding and fission have al-
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
for example, the colony may reproduce for several generations by
asexual division of the individual cells 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 pro-
duces 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 verte-
brates. 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 com-
pleted, 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-fertiliza-
tion is insured by copulation or union in such a way that the sper-
matozoa of one individual fertilize the ova of another. In certain

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.

Bisexual reproduction is the form of reproduction common to
many groups of the higher invertebrates and nearly all vertebrates.
Here the sexes are distinct, 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
vicinity and at about the same time. Under the sections on re-
production in starfish and the bullhead (fish) such a procedure has
been described. In animals like the toads and frogs, a special pro-
vision is made to bring the individuals of the opposite sexes to-
gether in that the 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 trans-
ferring spermatozoa into the seminal receptacle of the female, where
they remain until the eggs are laid. This represents a beginning in
the development of a copulatory organ. The majority of bisexual
or dioecious animals make a still greater provision to insure fertili-
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, and the ova are fertilized within the genital tract of the
female.

In birds and most reptiles after the addition of nutritive and pro-
tective coats the egg passes to the outside to develop and hatch (ovip-
arous animals) but in all mammals, except monotremes, it is retained
within the uterus during the period of embryonic development, and
the young are bom as more or less developed individuals (vivipa-
rous). In the females of viviparous mammals the posterior portions
of the two oviducts are modified into a uterus within which the young
are retained and nourished until ready for birth. 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

410 TEXTBOOK OF ZOOLOGY

or vice versa but tlie necessary materials are allowed to diffuse
through the tissue of the placenta in which both systems are dis-
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
develop into new individuals like herself without fertilization for
a whole season. This is known as parthenogenesis. Usually in the
fall of the year males are developed, and fertile eggs, provided with
protective hard shells, are produced by the females of this generation
to live through the winter. After winter is over such fertile eggs
hatch into parthenogenetic females for the next season. This process
is 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
stimulus, mechanical pricking, 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
artificial parthenogenesis. This oxidation may 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.

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:

Cyclostomata (si klo sto'ma ta, circle and mouth). Round-mouthed
fish 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). True fish with bony skeleton, gill respira-
tion, with 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 terrestrial, usually possessing a scaly skin and breath-
ing by lungs. Turtles, Lizards, Snakes, and Crocodiles.

Aves (a'vez, birds) . Warm-blooded, erect forms possessing feathers.
The forelimbs have become wings. All birds.

Mammalia (mama'lia, mammary or breast). Warm-blooded ver-
tebrates with hair and with mammary glands for suckling the young.
Cats, 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
to 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
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 semi-
circular canals. These subclasses are Myxinoidea (Hyperotreti) in-
cluding the hagfishes; and Petromyzontia (Hyperoartii) including
lamprey (or improperly, lamprey eel to some).

Myxinoidea or hagfish are all included in one family Myxinidae
which is divided into three genera : Myxine of the Atlantic and Pacific
Oceans, Bdellostoma and Paramyxine of the Pacific. These each
have specific characteristics, but they all agree in having a terminal
nostril, four tentacles on each side of the mouth, ability to produce
enormous quantities of mucus, and the lack of the oral funnel or
sucker. They all possess twelve or more pairs of gills, only one
semicircular canal in the inner ear, and a functional pronephros.
The development of the hagfish does not include a metamorphosis.
They usually live in the mud of the sea bottom except when they
are feeding either on the dead body of a fish or attached to a live
one. They frequently enter the mouth or gills of fish caught in
nets or those dead from natural causes and devour all of the inter-

•If the frog or toad Is to be used as the laboratory animal representing the typical
vertebrate, and the instructor so desires. Chapters XXV, XXVI, and XXVII may be
omitted until after the study of Chapter XXVIII and then assigned if time permits.

412

CYCLOSTOMATA 413

nal organs and flesh. They frequently attack living fish which have
been otherwise injured.

Subclass Petromyzontia likewise includes only one family, Petro-
myzontidae, which follows the type name. There are several genera
including Petromyzon, the common Atlantic form, Ichthyomyzon
of the lakes and streams and Entosphenus of the Pacific coast.
Entosphenus tridentatus trident atus is the northern form and E.
tridcntatus ciliatus is the southern form. The lampreys live in both
salt and fresh water, and they are quite predaceous, attacking fish
of considerable size. The characteristics of the group will be brought
out under the discussion of Lamprey as a typical representative.

Economic Relations of the Class

In a general way lampreys are both beneficial and injurious. They
all serve as excellent fish food and fish bait when they are in the
larval stage. Brook lampreys are classed as wholly beneficial since
they feed on microscopic organisms while larvae and do not feed
as adults. Sea lampreys and lake lampreys are both valuable as
human food, especially just preceding the spawning season. The
sea lamprey, for the two or three years it spends in the ocean, lives
at the expense of marine fish. It attaches itself and rasps a hole
in the side of a fish about once a month, and through the hole thus
formed, it sucks the fish's blood. One will remain to a single fish
for about five days, get its fill, and release itself. The fish frequently
dies as a result. Since the sea lamprey does not feed after it starts
up stream, it does little harm to fresh-water fish except as the
newly matured ones are making their trip to sea. The lake lamprey
is similar except that it spends its entire life in fresh water. They
are very destructive to lake fish since they are predaceous and
spend their adult lives in the lakes.

THE LAMPREY

Habitat

All live on or in the muddy bottoms of fresh-water streams dur-
ing larval stages. In adult life the sea lamprey goes to the open
sea and the lake lamprey goes to the deep water of the lakes. Both
return to the fresh-water streams to spawn a few years later.

414

TEXTBOOK OF ZOOLOGY

SUCTORIAL MOUTH

NASAL OPENING

\- EYE

-GILL CLEFT

•DORSAL FIN

-URINOGENITAL
PAPILLA

CAUDAL FIN

Fig. 223. — Lateral view of the Pacific lamprey, Entosphenus tridentatus. (Drawn

by Titus C. Evans.)

CYCLOSTOMATA

415

Habits and Behavior

The animal is a rather inebriate type of swimmer because it is
long and slender and does not possess paired fins. It winds its way
through the water and occasionally comes to rest by attaching
itself to a rock by means of an oral funnel.

External Structure

In most respects the Atlantic lamprey, Petromyzo7i marinus, and
the Pacific form, Entosphenus trident aUis, correspond quite closely
in structure and make excellent representatives for study of the
group. The following account will fit them generally. They may
reach a length of three feet and three inches. The color is rather
variable but might be expressed as being a variegated olive brown.
Along the length of the lateral axis of the body are distributed
sensory organs. There are two dorsal fins and a caudal fin, but no
paired fins. At the anterior end of the animal is the mouth with
the luccal funnel extending from it. This funnel is provided with
ehitinous teeth used in rasping through the body wall of the host
fish. The annular cartilage supports the margin of the funnel and
holds it open. Along the margin is a fringe of papillae. The mouth
lies at the bottom of the funnel. In the floor of the mouth is a
plungerlike tongue supported by a cartilage and bearing teeth.
There are seven uncovered gill slits along each side of the anterior
portion of the body. In front of the gills on each side of the head
is a poorly developed eye. It has no lid, simply being covered with
transparent skin. In a middorsal position on the head is located
the single nostril which leads into an olfactory chamber, and on
ventrally as a pituitary pouch or caecum. The anus is located in
the midventral line a short distance anterior to the tail. Immediately
behind it is the urinogenital opening at the tip of a papilla. The
papilla is larger in the male specimens.

Internal Structure

The muscular system is quite primitive. It is principally a series
of zig-zag myotomes along the length of the body very similar to
those in Amphioxus. A large lingual muscle is differentiated for
moving the tongue, and several bundles of muscular tissue radiate
through the wall of the funnel to expand and contract it.

416

TEXTBOOK OF ZOOLOGY

-BUCCAL FUNNEL
-TONGUE
-NASAL SAC

-OESOPHAGUS
-PITUITARY POUCH

BRAIN

PHARYNX

NOTOCHORD

-INTERNAL GILL
PORE

-EXTERNAL GILL
PORE

-GILL POUCH

-GILL LAMELLAE

■BRANCHIAL

CARTILAGE
•PERICARDIAL

CARTILAGE

-LIVER
-MYOTOMES

-INTESTINE
-KIDNEY

-TESTIS

-PRONEPHRiC
DUCT

ANUS

-URINOGENITAL
APERTURE
URINOGENITAL
SINUS

Fig 224 — Lateral view of dissection of Entosphenus to show principal organs.

(Drawn by Titus C. Evans.)

CYCLOSTOMATA 417

The skeletal system is cartilaginous, developed around a nonseg-
rnented notochord along each side of which are paired cartilages
called neural arches. At the anterior end is a skull whose floor and
sides are cartilaginous, while the roof is membranous, except for a
transverse bar. There are two auditory capsules near the posterior
part of the skull. The buccal funnel is supported by the annular
cartilage already mentioned and three sets of labial cartilages. The
branchial area is supported by the cartilaginous hranchial basket
which is composed of a i)air each of dorsal and ventral longitudinal
bars, two pairs of sinuous, lateral bars, and nine much-curved, dorso-
ventral bars. The anterior one of these is not in contact with a gill
aperture. The cartilaginous pericardium joins the branchial basket
at the posterior end.

The digestive system is not very highly developed because the
adult lives entirely on blood and lymph of other fish, obtained by
rasping a hole through the body wall and sucking it out. They
take a meal about once in three or four weeks. The blood is passed
from the mouth down the esophagus which continues into the in-
testine at the level of the posterior end of the branchial region.
The intestine is slender and almost straight, but it has a slight
internal fold which extends spirally through its length. This is
called a typhlosole or spiral valve, and it tends to increase the absorp-
tive surface. The intestine ends posteriorly at the anus. The liver
is found in the anterior part of the body cavity.

The circulatory system consists of a heart with two principal
chambers, arteries, capillaries, veins, and lymphatic spaces. The
posterior and anterior cardinal veins located just lateral to the lower
side of the notochord collect blood from the body wall and head
region, and empty it into the common cardinal vein which extends
ventrally to the sinus venosus. The sinus venosus receives also the
single inferior jugular and the hepatic vein from the ventral region.
The blood then passes through the sinuauricular valve to the single
auricle, thence by the auriculoventricular aperture to the single ven-
tricle, thence through hulhus arteriosus to the ventral aorta. Six
pairs of afferent branchial arteries carry the blood to the gills where
capillaries supply the gill lamellae. The efferent branchial arteries
collect this blood, carry it dorsally to join the dorsal aorta which is
made up by their convergence. A carotid branch of this artery
supplies the brain region, and the main aorta passes posteriorly.

418

TEXTBOOK OF ZOOLOGY

giving branches to the viscera and body wall. There is no renal
portal system; the caudal vein simply divides, giving one part to
each posterior cardinal vein.

— DORSAL AORTA

5TH.R. EFFERENT
BRANCHIAL A.

4TH. R. AFFERENT
BRANCHIAL A.

VENTRAL AORTA

INFERIOR JUGULAR V.

R. JUGULAR VEIN

VENTRICLE

SINUS VENOSUS

ATRIUM

HEPATIC V.

R. POST CARDINAL V,

RENAL A.

RENAL V,

INTESTINAL V.

INTESTINAL A.

CAUOAL VEIN
CAUDAL ARTERY

Fig. 225. — Diagram of oblique ventrolateral view of heart, arteries, and veins
of lamprey. Arrows indicate direction of flow of blood. (Drawn by Titus C.
Evans. )

The seven pairs of gills and respiratory tube constitute the prin-
cipal features of the respiratory system of this animal. When the

CYCLOSTOMATA

419

animal is not attached to a host, water may be drawn through the
mouth, under the velum, through the respiratory tube, through the
paired gills and to the outside through the seven pairs of external
apertures. The blood in the gill capillaries is aerated from the
oxygen carried in the water as it passes over the gill lamellae.
While the lamprey is attached to a host fish, the water is drawn
into the respiratory tube through the gill slits and then discharged
through them.

— OLFACTORY SAC
OLFACTORY LOBES

PINEAL EYE
RIGHT CEREBRAL
HEMISPHERE

OPTIC NERVE

RIGHT GANGLION
HABENULAE

DIENCEPHALON
PITUITARY POUCH

OCULO-MOTOR N.

MIDDLE CHOROID
PLEXUS
OPTIC LOBES
TRIGEMINUS N,
CEREBELLUM
FOURTH VENTRICLE

AUDITORY SAC
AUDITORY NERVE
MEDULLA OBLONGATA

NOTOCHORD
VAGUS N.

Fig. 226. — Brain of lamprey.

Lateral view ; dorsal view.
Evans.)

(Drawn by Titus C.

The nervous system shows the development of a small, primitive
brain, which possesses all five principal divisions of a vertebrate
brain. From anterior to posterior it is composed of olfactory lobes,
cerebral hemispheres closely fused to preceding, single dicncephalon
with its dorsal epiphysis, midbrain with a pair of optic lobes, insig-
nificant narrow bandlike cerebellum just behind the optic lobes, and
the medulla just posterior to it. This continues directly posteriorly
as the flattened spinal cord. The roof of the brain is rather mem-
branous, as it is not entirely closed over. The sense organs include

420

TEXTBOOK OF ZOOLOGY

the single nasal chamber which is located immediately anterior to
the brain. Extending ventrally from the nasal chamber and project-
ing beneath the brain to end blindly just above the esophagus is the
pituitary pouch. As it passes beneath the diencephalon it makes
contact with the infundibulum. The eyes of this animal are not
highly developed, and sight is not used extensively by it. The audi-
tory organ, which does not include an organ of hearing, is only for
equilibrium ; it consists simply of a vestibule and two vertical semi-
circular canals. The sense of taste centers in taste buds located in
the respiratory tubes between the gill slits and possibly near the
inner margin of the buccal funnel.

The urinogenital system shows only fair development. The rib-
bonlike kidneys lie, one at each side of the notochord and just dorsal

P 0 AU HB OA SC N PfJ VN OE MM M

LP-.

OH OP

cc vf a;jc t c

VA V A G L F

AN CF

Fig. 227. — Ammocoetes larva of the lamprey, Entosphentos tridentatus. A,
auricle of heart; AN, anus; ANC, anterior end of notochorcl ; AU, ear; BA, bran-
chial arteries (afferent) ; G, duct connecting pharynx and thyroid ; CC, cranial
cartilage, extending from tip of upper lip to a point slightly anterior to end of
notochord, where it divides to form two lateral rods ; CF, caudal fin ; DA, dorsal
aorta ; F, folds in intestinal wall, suggesting a possible origin of the spiral valve ;
FB, forebrain ; G, gall bladder ; HB, hind brain ; I, intestine ; L, liver ; LP, upper
lip, supported by cranial cartilage; M, body muscles (myotomes) ; MB, midbrain;
MA'', position in which mesonepliros will develop; N, notochord; O, eye; OE, esorh-
agus ; OH, oral hood; OL, olfactory organ; OP, oral papillae; P, pineal body;
PC, pericardial cavity ; PN, pronephros, showing pronephric tubules with their
ciliated funnels (nephrostomes) ; SC, spinal cord; T, thyroid; UL, under lip; V,
ventricle of heart; VA, ventral aorta; VE, velum; VN, hepatic vein. (Courtesy
of Albert E. Galigher, Inc.)

to the peritoneal lining of the body cavity. A mesonephric duct ex-
tends posteriorly along the free edge of each to join the small urino-
genital sinus. This is located just posterior to the rectum and opens
externally by the urinogenital papilla just behind the anus. The single
gonad is rather large and is suspended by a peritoneal fold into the
coelom. The sexes are presumably separate, but hermaphroditic con-
ditions are occasionally found. Germ cells when mature are dis-
charged from the gonad into the body cavity and go by way of two

CYCLOSTOMATA 421

genital pores into the iirinogenital sinus, then out through the papilla
to the environmental water where fertilization occurs.

The life history may be summarized as follows:

a. The eggs which contain considerable yolk in the vegetal por-
tion and are about one millimeter (%5 inch) in diameter are laid in
fresh-water streams, usually between March and June for all kinds
of lampreys. The eggs first stick to objects, then fall in the sand.
They are fertilized in the water almost immediately after laying.
Cleavage follows, in about six hours when the optimum temperature
of 22.5° C. prevails. At 20° C, this division requires nine days.

b. The adults spawn but once and then die.

c. A tadpolelike larval form, ammocoetes or mud lamprey, hatches
from the egg and lives from four to five years in the mud along the
streams where the eggs are laid.

d. At the end of four or five years the ammocoetes undergo meta-
morphosis to become adult. They remain under the mud from July
or August to February or March while undergoing this transforma-
tion to adult condition,

e. The sea lamprey then migrates to the ocean and the lake lamprey
moves down stream to a large fresh-water lake. They both become
parasitic on other fish and continue this existence for from one and
one-half to three and one-half years, when they return to fresh-water
streams to breed again.

CHAPTER XXVI

ELASMOBRANCHII*

Unlike the cyclostomes, the Elasmobranchs are covered with scales
and have two sets of paired fins on the ventrolateral surfaces of the
body. In addition to these, there are unpaired or median fins. The
gill apertures, except for the first, or spiracle, are slitlike instead of
circular, as seen in the lamprey. The gills are supported by gill
arches, and the mouth has an upper and lower jaw. The skeleton is
entirely cartilaginous, there is a partially persistent notochord, and
the exterior is covered and protected by placoid scales. The males
have a modification of each pelvic fin known as a clasper which is
used as a copulatory organ.

The mouth is not right at the anterior end of the body but is ven-
tral or subterminal. There is present in the ileum of the small in-
testine a spiral valve which increases the internal surface, thus add-
ing absorptive area. The Elasmobranchs have no operculum or air
bladder.

Classification

The class is divided into two rather easily distinguished sub-
classes. The first group is very common to American shores and
the second is rarely seen in our waters.

Subclass Selachii. — This group includes the sharks which are
cylindrical in shape, possess laterally located gill slits, and are active
swimmers; and the rays, which are dorsoventrally flattened, possess
ventrally located gills, are less active, and dwell on the bottom of
the sea. This subclass is usually divided into two orders.

Orders Euselachii and Cyclospondyli. — The sharks make up these
orders. The dogfish sharks (Squalus acanthias and others), tiger
shark (Galeocerdo arcticus [Faber]), cub shark {Carcharias platydon
[Poey]), shovelhead or bonnethead shark {Reniceps tiburo [Linn],
and man-eater shark {Carcharodon carcharias [Linn]) are forms com-
monly found. The majority of sharks are carnivorous and active,
but they rarely attack man unless the person is already wounded.

•In collaboration with Miss Mary Fickling.

422

ELASMOBRANCHH

423

The average length of most sharks commonly observed ranges be-
tween three and six or eight feet. Their natural food consists prin-
cipally of Crustacea, small fish, squids, and refuse. In the Gulf of
Mexico and other warm seas the so-called man-eater may occasion-
ally reach a length of thirty feet and is sometimes charged with

Fig. 228. — Southern sting ray, Dasyatis americana, a common form in the Gulf of

Mexico.

eating human beings. The shovelnose (bonnethead) and hammer-
head sharks are very interesting forms. The shape of the head of
each is about the shape ascribed to it by the common name. The
former has been considered sufficiently interesting to warrant fur-
ther discussion of it as an example of the class.

424

TEXTBOOK OF ZOOLOGY

Order Batoidei — Skates and Rays. — This is a group of depressed or
dorsoventrally flattened fishes in which the gill slits are located on
the broad, flat, ventral side. "These fish lack the anal fin and the
caudal is absent or reduced. The saw-fish, Pristis pectinatus is a
sharklike ray with a long tooth-bearing rostral process or snout that
resembles a double-edged saw. These animals may reach a length of
fifteen or twenty feet, with a saw five feet in length.

The skates are distributed along our Atlantic shores and are ovip-
arous. The eggs are enclosed in dark brown cases or capsules,
quadrate in outline and of considerable size. They have hornlike
processes extending from each corner. There are about six species
of skates, of which Raja erinacea, B. diaphanes, and B. ackleyi are
common ones.

Fig. 229. — Butterfly ray, a common bottom feeder.

The rays are of similar shape, but they bear their young alive and
tend to have a smoother skin. The rays are more numerous in the
warmer waters. The torpedo ray of family Torpedinidae, has at-
tracted considerable attention because of its ability to generate and
store electrical energy in the muscles of the bases of the broad pec-
toral fins. These electric organs are capable of discharging suffi-
cient current to paralyze other animals, ring a doorbell, or light a

ELASMOBRANCHn 425

flashlight bulb. The sting ray or stingaree is very common in the
Gulf of Mexico. The average width of those usually seen is from
eighteen inches to two feet. They have a long, slender, whiplike
tail with a strong spine or sting on the dorsal side of its proximal
third. Dasyatis sahina and Dasyatis americaiia are two common
forms. The butterfly ray, Pteroplatea micrura, is a broad-bodied
form with an exceptionally short tail. They, too, are quite common
in the water of the Gulf of Mexico. The sting is usually obsolete in
this form. It is called butterfly ray because of the manner of flap-
ping the lateral expansions about as a butterfly moves its wings in
flight.

Subclass Holocephali. — This group contains an order with three
modern genera. Psychichthys affinis (or Chimaera affinis, as often
called) is the only species taken from the waters of the coasts of
North America, and then only rarely. Chimaera monstrosa is another
species which is found in South American waters.

Economic Relations of the Class

Many of the smaller sharks, like Squaliis acanthias and Mustelus
canis are very destructive to lobsters, crabs, shrimp, squid, and valu-
able fish which they use for food. They also damage much fishing
gear by tearing through nets. It is estimated that the damage done
in this way averages $400,000 in Massachusetts alone. Along the
coasts of California and in the Gulf of Mexico both sharks and rays
are a nuisance to the seining fisherman.

The sting rays or "stingarees" which are armed with the barbed
stinging spine on the proximal portion of the tail are generally
common in most of the warmer fishing waters. "With a sudden
swing of the tail one can inflict an ugly and extremely painful
wound. Some people become severely ill as the result of such a
sting. Bathers particularly dislike "stingaree" infested beaches as
well as those infested with the less common torpedo ray.

The skins of certain sharks and skates, which have the sharply
pointed, toothlike scales, are used as a polisher of wood and other
materials and is called shagreen. Shark skins are now being manu-
factured into leather on a commercial basis. Large quantities of
oil are extracted from some of the sharks, as the cub shark for ex-
ample. This oil is used in currying leather in the tanning industry.

426 TEXTBOOK OF ZOOLOGY

Shark liver oil is of high vitamin content and has an important
medicinal use.

In many countries, particularly of the Orient, these fish are com-
monly used as food. It is said that small sharks and skates are
offered for sale right along with other fish in the markets of China.
They are also salted and dried. In the United States there is an
unfounded prejudice against eating these fish, but dogfish are now
being canned and sold under the name of "grayfish." The wing-
like fins of skates and rays make delicious steaks. Sawfish steaks
are quite desirable and the saws are preserved as ornaments. The
flesh of sharks and rays is also ground up and used extensively as
fertilizer. In some parts of the world the fins of sharks are used
in the manufacture of gelatin. A good many dogfish and bonnet-
head sharks are sold for purposes of study in zoology laboratories.

THE SPINY DOGFISH

This shark is the most commonly studied representative of the
Elasmobranch group. Squalus acanthias is the scientific name ap-
plied to the common form taken along the Atlantic coast and
Squalus suckleyi is the name given the similar one of the Pacific
coastal waters. The average length of Squalus is between two and
one-half and three feet. It is a strong swimmer and is frequently
seen as a scavenger in harbors as well as going out to sea for ex-
tended periods. It apparently makes a spring migration northward
along the coast and a return movement in the fall. Because of the
ventral location of the mouth, these fish find it necessary to turn
ventral side up to eat morsels of food from the surface of the water.

External Features

The body is generally spindle-shaped (fusiform) tapering at both
head and tail. There are two pairs of fins, the anterior pectoral
and the posterior pelvic, or ventral fins. In addition to the paired fins,
there are two unpaired, median, dorsal fins, each with a spine at its
anterior margin (hence spiny). Male individuals may be distin-
guished from females by the fingerlike extensions, or claspers on the
pelvic fins. The dorsal and ventral lobes of the caudal fin, or tail,
are unequal and based on this, the tail is described as heterocercal.

There are six pairs of uncovered gill clefts in the walls of the
pharynx. The anterior one, which is dorsally located and greatly

ELASMOBRANCHII 427

modified, is called the spiracle. It contains a rudimentary gill struc-
ture. The mouth aperture is somewhat the shape of an inverted U,
located on the ventral side of the head, and supplied with sharp
teeth on the jaws. These teeth are developed by modification of
the placoid scales which cover the skin over the body generally.
The placoid scales are primitive exoskeletal structures with a hasal
plate embedded beneath the skin and a spine projecting on the sur-
face. This spine has a pulp cavity, surrounded by dentine, which
is covered on its surface by enamel. This structure is considered
to be homologous to the vertebrate tooth. The paired nostrils are
openings on the ventral side of the snout, anterior to the mouth.
The eyes, the lids of which are immovable, are situated on the sides
of the head. The cloacal aperture is located between the bases of
the pelvic fins.

Muscular System

The segmental arrangement of myotomes, separated by myo-
eommas, is fairly complete along both sides of the body. The
principal specializations of independent muscles are found in the
form of myotome modification in the region of the mouth gills and
paired appendages. The trapezius found above the branchial area;
the superficial constrictors extending from the head to beyond the
gill slits and assisting in their operation; and the adductor man-
dihularis, connected with the lower jaw, are all examples of special
developments.

Skeletal System

The endoskeleton of the sharks is composed of cartilage. It con-
sists of axial skeleton (skull and vertebral column) ; visceral skeleton
(jaw and gill arches) ; and, appendicular skeleton (pectoral girdle
and fins, pelvic girdle and fins). The vertebral column and skull are
much more developed than in the cyclostomes. The notochord has
become segmented and partially replaced by cartilage. The centrum,
which has replaced a considerable portion of the notochord in each
vertebra, is deeply concave at each end, and is said to be amphicoelous.
Some of the remains of the notochord fills these interstices between
vertebrae.

The skull is laid on a foundation of the ventral hasal plate. The
dorsal side is fairly well enclosed with cartilage. The anterior ex-
tension of the skull is the rostrum and the depression in its dorsal

428

TEXTBOOK OF ZOOLOGY

side is the anterior fontanelle. The nasal capsules are rounded, car-
tilage-encased cavities, one at each side of the base of the rostrum.
These capsules house the olfactory sense organ in life. The orbits
are laterally located, spherical depressions which normally hold the
eyes. Each orbit is guarded anteriorly, dorsally, and posteriorly by
slight extensions of the cartilage known as preorhital process, supra-
orbital crest, and postorbital process respectively. The orbits are
laterally located spherical depressions in which the eyes are set.

aosTuuM

OLFACTORY
CAPSULE

EPIPHYSIAL
FORAMEN

LATERAL
LINE FOR^M^^»A

ENDOLYMPHATIC
DUCT
ORblT

OPT\C

FORAMEN

PTERYOO-
QU ADR ATE
MECKELIAN
CARTIlAOE

CERATOHYOID
HYOMANDl&ULAR

CERATO- ,

&RANCH lALS

EPI-
BBAMCHIALS

CAROTID
FORAMEN

PHARYNGO-J
BRANCH I ALS \.
OTIC
CAPSULE

Fig. 230. — The skull and visceral arches of the dogfish shark, Sqimlus acant.hias.
Dorsal view above, ventral view below. Latei'al aspect on the riglit. (From
Atwood : Introduction to Vertebrate Zoology, published by The C. V. Mosby
Company. )

The visceral skeleton consists of the upper jaw (palatopterygoid
or quadratopterygoid cartilages), lower jaw (Meckel's cartilages),
hyoid arch (hyomandibular, ceratohyal, basihyal), and five branchial
arches (each typical one has pharyngobranchial, epibranchial, cera-
tobranchial, hypobranchial, and basibranchial cartilages).

ELASMOBR AN C HH

429

NOSTWU

OILL

CONUS ARTERIOSUS

VENTP.\C\.E

TESTIS

UVER

GALL feL^DOER

STOMACH

B\LE DUCT
DORSAL AORTA
. COELIAC ARTERY

PANCREAS

- &PLE.EN

MESENTERIC
ARTSRV

RECTAL GLANO
ARTERY

RECTAL OLANO
SEMINAL VESICLE

INTESTINE

A60OMIMAU PORE

PELVIC FIH
CLASPtR

Fig. 231. — Ventral view of the visceral anatomy of the dogfish sharlc, Squalus
acanthias, male. (From Atwood: Introduction to Vertebrate Zoology, published
by The C. V. Mosby Company.)

430

TEXTBOOK OF ZOOLOGY

The pectoral girdle is composed of the ventral coracoid bar and
the dorsolateral scapular process at each end of it. The fin consists
of three flat basal cartilages (propterygimn, mesopterygimn, and
metapteryginm), a series of radial cartilaginous rays, and a series of
exoskeletal dermal rays. The pelvic girdle is made of one cartilagi-
nous bar (ischiopubis) with a fin joining at each end. The basals of
this fin are fused into one cartilaginous plate.

Fig. 232, A. — Dissection of the valvular portion of the small intestine. A, the
bonnet-head shark to show the spiral valve; B, spiral valve of Raia. (B re-
produced by permission, from General Zoology by Wieman, copyrighted 1938 by
McGraw-Hill Book Co., Inc.)

Digestive System

Most of the organs of this system and other viscera lie in the
pleuroperitoneal portion of the coelomic cavity. Anterior to this the
pericardial portion of the coelom contains the heart. The digestive
organs are in the form of an alimentary canal with accessory glands.

ELASMOBRANCHH 431

The canal begins anteriorly witli the mouth, which leads directly into
the pharynx, in whose lateral walls are the gill slits (and spiracle).
Following this is the short tubular esophagiis, which leads into the
cardiac end of the stomach. This organ, which is somewhat broader
than the esophagus, is rather U-shaped. The posterior, or pyloric,
portion of the stomach is provided with a sphincter muscle, the
pylorus, which controls the passage of food materials into the in-
testine. The duodenum is the short anterior portion of the intestine
which follows the pyloric portion of the stomach and leads into the
valvular portion of the intestine (ileum). This section of the small
intestine is of considerably greater diameter than the duodenum and
contains internally a spiral valve, which is a spirally arranged in-
folding of the mucous lining. This arrangement serves to slow the
passage of food and increases the absorption surface. The principal
absorption takes place through this part of the intestine. The val-
vular portion leads to the short, narrow, large intestine, which emp-
ties into the cloaca (Figs. 231 and 232).

The liver, pancreas, and rectal gland are accessory glands con-
nected with this system. The liver is a large, three-lobed organ
with the saclike gall bladder located just dorsal to the junction of the
right and middle lobes. The gall bladder stores bile produced by the
liver and delivers it to the duodenum through the hile duct. The
pancreas is divided into two lobes, an oval ventral and a slender dorsal
lobe. Ducts lead from it to the duodenum. The rectal gland is a
spindle-shaped gland leading into the large intestine directly. The
reddish spleen, which is a lymphoid rather than digestive organ, lies
around the greater curvature of the stomach.

The digestive tract and adjacent organs in both species are sus-
pended from the body wall by mesenteries, which are extensions from
the peritoneum, or membranous lining of the coelom. The mesentery
supporting the stomach is the mesogaster, the one extending between
the spleen and stomach is the gastrosplenic, and the mesorectum sup-
ports the large intestine and rectal gland.

Circulatory System

This centers in the heart, which is located ventrally at about the
level of the posterior pair of gills, and consists of two principal
chambers and two accessory chambers. The pericardium, a mem-
branous extension of the peritoneum, encloses the heart. The two

432

TEXTBOOK OF ZOOLOGY

VENTRAL CAROT\0 A

AFFERENT E.RANCH\ALS

TRUNCUS ARTERIOSUS
ATRIUM

VENTRICLE
DUCT OF CUVIER

HEPATIC SINUS
CARDINAL SINUS

INTERNAL CAROTID A.
POSTERIOR CAROTID A.

VENTRAL AORTA
EFFERENT E.RANCH\ALS

ANTERIOR CARDINAL SINUS
JUGULAR V.

SINUS VENOSUS

HEPATIC PORTAL V

HEPATIC A
GASTRIC A

HEPATIC V.

LATERAL
ABDOMINAL V

PANCREATIC A

INTESTINAL V
INTESTINAL A.

RENAL PORTAL V
\L1AC V.
\LIAC A-

CAUDAL A
CAUDAL V

SUBCLAVIAN V.
SUBCLAVIAN A.

OVARIAN A.
GENITAL V.
COELIAC A.
OVIDUCAL A.
GASTRIC V.
DORSAL AORTA
POST- CARDINAL V.

SPLENIC INTESTINAL V.

PANCREATIC V.

SPLENIC A.
INTESTINAL A.

RECTAL GLAND A.
RECTAL GLAND V.

Fig. 233. — The circulatory system of the spiny dogfish, Squalus acanthias.
(From Atwood: Introduction to Vertebrate Zoology, published by The C. V.
Mosby Company.)

ELASMOBRANCHII

433

Internal carotid
Ventral carotid

Hyoidcan artery

1st afferent branchial A

Ventral aorta

Ventricle

Subclavian V<
Subclavian A.

Lateral abdominal V. -

Spleen

ZQd qui slit

Ant. cardinal W.

Efferent branch-
ial A.

Dorsal aorta

Duct of Cuvier

Coeliac A.

LPoit. cardinal
Ventral gastric A.

Ventral pancreas

Qastro-splenic A.

Sup. mesenteric A.
Kidney

A- Gonad

Post- mesenteric A
Intestine

Pelvic fin
Uiac A:

Inf. mesenteric A.
Rectal qiand

Cloaca
Caudal A.

Pig. 234. — Diagram of lateral view of the circulatory system and other organs of
bonnet-head shark. (From dissections by Mary Fickling.)

434 TEXTBOOK OF ZOOLOGY

main chambers are the single, more dorsal auricle, and the ventral,
muscular ventricle. Leading into the auricle is the sinus venosus,
which receives blood from the veins of the body. A bulblike en-
largement at the base of the ventral aorta, where it leaves the ven-
tricle, is the conus arteriosus. The ventral aorta leads anteriorly
from the conus and gives off three pairs of afferent branchial arteries
which branch to the five pairs of gills. The blood spreads from these
by capillaries through the gill lamellae for oxygenation. The pre-
trematic and postreniatic branches of the efferent branchial arteries
which form four efferent branchials leave the gills and join the dorsal
aorta, which is formed by them in the dorsal midline. The hyoidean
extends from the ventral portion of the first pretrematic to the
spiracle where it spreads in capillaries. The ventral carotid leads for-
ward from the spiracle to the internal carotid. Extending anteriorly
from the dorsal part of each first efferent branchial is a common
carotid which supplies arterial blood to the head and brain. The
dorsal aorta extends posteriorly from the junction of the efferent
brachial arteries, soon giving off the subclavian arteries to the pectoral
fins; coeliac to stomach, liver, and pancreas; gastrosplenic to stomach
and spleen; superior mesenteric to the valvular intestine to be-
come the posterior mesenteric artery there; renal arteries to the
kidneys; the inferior mesenteric to the rectal gland and large intes-
tine; and the iliac to the pelvic fins and cloaca. The subclavian
artery leaves the aorta more anteriorly, coming off ahead of the pos-
terior efferent branchial ; the coeliac is farther back and sends a pan-
creaticomesenteric artery above the duodenum, through the ventral
pancreas, and along the valvular intestine a gastric to the stomach and
the hepatic artery to the liver ; the gastrosplenic and superior mesen-
teric arise very near each other.

The systemic veins all return venous blood to the sinus venosus,
which empties into the auricle by way of the sinu-auricular aperture.
Hepatic veins lead directly from the liver to the sinus venosus. The
ducts of Cuvier collect blood from the anterior cardinals of the head
region and posterior cardinals of the trunk region and empty it into
the sinus. A hepatic portal system collects from the stomach, in-
testines, pancreas, and spleen and empties into the liver. The two
renal portal veins bring blood to the kidneys from the single caudal
vein of the tail. This blood spreads through the capillaries of the
kidneys and is collected into the postcardinals through the renal

ELASMOBRANCnn 435

branches. There is also a system of lymph spaces, which supple-
ments the blood circulatory system.

Respiratory System

The gills in the wall of the pharynx are constantly bathed in
water forced through from the mouth. An exchange of carbon
dioxide for oxygen is made by the blood as it passes through the
capillaries of the gills. This is made possible by diffusion of these
gases through the membranes of the gill lamellae. The gills are
supported by cartilaginous gill arches.

Nervous System

The central nervous system consists as usual in vertebrates of the
hrain and spinal cord. The brain includes two large olfactory lobes
at its anterior, followed by two cerebral hemispheres, a depressed
diencephalon, a pair of large optic lobes, a well-developed cerebellum,
and behind this the tnedulla oblongata. There is a very slight con-
striction between olfactory lobes and cerebrum. The cerebellum is
divided into quarters by a longitudinal groove and a transverse
groove. It covers a part of the optic lobes as well as the anterior
portion of the medulla oblongata. On the lateral walls of the medulla
are located the acusticolateral areas, including the earlike auricles.

The cavity within the medulla, which opens to the dorsal surface
beneath and behind the cerebellum, is the fourth ventricle. There
are ten pairs of cranial nerves which are numbered and named from
anterior to posterior: I, olfactory; II, optic; III, oculomotor; IV,
trochlearis; V, trigeminus; VI, abducens; VII, facial; VIII, audi-
tory ; IX, glossopharyngeal ; X, vagus. The olfactory nerves extend
from the olfactory lobes ; optic from diencephalon and optic lobes;
oculomotor from ventral side of optic lobes or midbrain; trochlear
from dorsal side of optic lobes between them and cerebellum; the
trigeminus, abducens, facial, and auditory all from the anterior
portion of the medulla oblongata; the glossopharyngeal from a
more posterior part of the sides of the medulla. These last two
supply the gills, lateral line, and certain viscera.

The spinal cord is tubular and somewhat flattened. It extends the
length of the vertebral column and gives off paired spinal nerves seg-
mentally.

436

TEXTBOOK OF ZOOLOGY

The sense organs include the eyes, the olfactory organ, internal ear,
and the lateral line system. The eyes are in the orbits, one on each
side of the cranium. They are quite typical of the vertebrate eye
described in the general chapter on phylum Chordata. The olfactory

OUPACTORV
e>ULB

OUFACTORV
TRACT

CEREBRUM

MAX>LUAR.*S

OPTIC LO&E

OPHTHALMICUS
SUPERFICIALIS

OPHTHALMICUS
PROFUNDUS

MANDI&ULARIS

TRIGEMINAL

FACIAL
PALATINUS

AUDITORY

HYOMANDIBULARIS

OLOSSOPHARYNGEAL

VAOUS

TERMINAL

OPTIC
TROCHLEAR

OCULOMOTOR
A60UCENS

CORPU&
RESTIFORMUS

MEDULLA

Fig. 235. — Dorsal view of the brain and cranial nerves of the dogfish shark,
Sgualus acanthias. (From Atwood : Introduction to Vertebrate Zoology, published
by The C. V. Mosby Company.)

organ consists of a pair of nasal sacs on the ventral side of the ros-
trum which open by nostrils. The nasal chambers are blind sacs and
lined with a sensory lamellated olfactory membrane in which the
olfactory nerve ends. The internal ears are composed of a vestibule

ELASMOBRANCHII

437

and three semicircular canals. An endolymphatic canal leads from
the dorsal exterior into the lower part of the vestibule the sacculus.
A posterior pouch of the sacculus is the lagena, which is considered
the foreranner of the cochlea of higher vertebrates. The ear serves
the sense of equilibration in the fish.

A canal extends along the side of the body in the lateral line and
forward onto the head, lying just beneath the skin. This is the
lateral line system. On the head there are some pores with tubes
extending beneath the skin to small bulbs called ampullae of Loren-
zini. The function of the lateral line system ajid these ampullae
is perception of water pressure and vibrations.

Endolymphatic duct

Anterior semi-
circular canal

Utriculus

Posterior semi-
circular canal

Horizontal semi-
circalar canal

Recessus utnculi L^ \^^^^ '-'"?^'^

Sacculus
Fig. 236. — Diagram of lateral view of left internal ear of Reniceps tiburo.

Urinog-enital System

The kidneys are thin, slender organs extending along the dorsal
body wall, one on each side of the vertebral column. The posterior, or
caudal, portion functions in excretion. There is an accessory meso-
nephric duct embedded in each kidney which carries urine to the
urinogenital sinus of the male, and the Wolffian duct serves as the
urinary duct of the female, emptying into the urinary sinus. A
papilla leads from the sinus to the cloaca in both. In the male sper-
matozoa are produced in the testes, carried by several vasa efferentia
through the mesochorium to the convoluted cranial portion of the
Wolffian duct, the epidid^Tnis of each side, which continues poste-
riorly as the vas deferens. This tube enlarges to become the semincH
vesicle and continues into the inflated sperm sac which is directly con-
nected with the urinogenital sinus. The spermatozoa then pass out
through the papilla to the cloaca, thence to the outside by way of

438

TEXTBOOK OF ZOOLOGY

the anus. During copulation they are transferred to the cloaca of
the female by use of the claspers. They swim by their own motility
into the uteri and oviducts.

Fig. 237. — Urogenital systems of Sgualus acanthias; A, female; B, male.

The ova of the female are produced in the ovaries which are located
one on each side of the median dorsal line in the anterior portion of
the coelom; Each gonad is suspended in a mesentery, the mesovarium.
Mature ova rupture from the ovary into the body cavity and enter

ELASMOBRANCHII

439

the funnellike ostium of the oviduct, which is held by the falciform
mesentery at the anterior end of the peritoneal cavity. As the ova
pass down the oviduct, they receive a covering which is secreted bj''
the shell gland in the wall of the duct. Fertilization occurs in the
oviduct and the embryo develops in the uterus which is the expanded
lower portion of the oviduct. The embryo is nourished by the large
yolk mass of the egg.

THE BONNETHEAD SHARK, RENICEPS (SPHYRNA) TIBURO

COMPARED TO SQUALUS

The bonnethead (or shovel-nosed) shark is common in the At-
lantic along the coast of the Southern States and in the Gulf of
Mexico. It occurs abundantly along the Louisiana and Texas Gulf
coast during May and June. It averages about the same size as
Squalus. In many respects it is similar to the smooth dogfish,
Mustelus canis, and the ground shark, Carcharhinns.

Epiphyseal foramen

,. Anterior fontanel/e

m^''"m^ .,..-|-°'^°^torycap5a/e

^^^^

Supraorbital process

-Preorbital process
- PosbDrbitcil process

• Endolymphatic fossa

Foramen maqnum'

Endolymph i

Fig. 238. — Dorsal view of the skull of bonnet-head shark. (From dissections by

Mary Fickling.)

The peculiar shovel-shaped head with the eyes out on the lateral
margins is one of the striking features of Reniceps by which it dif-
fers from the others mentioned. In Reniceps there are no spines in
front of the dorsal fins and a single anal fin is present on the ventral
side between the anus and tail. The spiracle is absent, leaving only
the five pairs of gill slits. The other external features are similar
to those of Squalus.

The skeleton of the skull is shaped considerably different from that
of Squalus. This is brought about by lateral extension. Each olfac-

440

TEXTBOOK OF ZOOLOGY

toiy capule is extended far to the lateral of the base of the rostrum
instead of lying beside it. The orbit with the modified supraorbital
crest, preorhital process, and bladelike postorhital process are also

Ventral
pancreas

Cardiac
stomach

H\dr)ey
SmaW'mtzstme -
Recta/ qloDd - -

Larqe intestine

Spleen
stomach

Mesoneplinc duct
-- Cloaca

Fig. 239. — Internal anatomy of bonnet-head shark. Reniceps tiburo, from ventral
view. (From dissections by Mary Fickllng.)

projected at the terminal portion of this arm of cartilage. The pos-
terior part of the skull is somewhat narrowed and flattened but
otherwise similar to that of Squalus.

ELASMOBRANCHII 441

There are a few differences in the digestive systems of the two.
In the pharynx there are no spiracles. The stomach of Reniceps is
J-shaped instead of U-shaped and the long slender pyloric portion is
armlike. The spiral folds of the spiral valve are more telescoped
into each other than in Squalus. There are only two lobes in the
liver of bonnethead and the gall bladder is nearly embedded in its
tissue. In the circulatory system, Reniceps has five afferent branchial
arteries branching from the ventral aorta while Squalus has only
three, two of which branch. The branching of the coeliac artery is
somewhat different in the two animals.

The brain has the same general parts as it does in Squalus but
they are quite modified. The olfactory lobes are broader and almost
completely fused to each other. The cerebrum is a somewhat smaller
single lobe just posterior and dorsal to the olfactory lobes. There
is no line of demarcation between the hemispheres. The diencepha-
lon is entirely hidden from dorsal view by the cerebrum and cere-
bellum. The latter is large, irregularly divided into three lobes, and
convoluted. It covers not only the diencephalon but also most of
the optic lobes (midbrain) and much of the medulla oblongata. The
medulla has well-developed acousticolateral areas.

The nasal chambers of Reniceps are quite large and kidney-shaped.
They contain extensive folds or lamellae of the olfactory membrane.

The shape of the testes in male Reniceps is much longer and more
slender than in Squalus. In addition to this, there is a long glandu-
lar body, the epigonad, which extends from the level of the gonad
proper to the region of the cloaca.

Copulation in Reniceps probably occurs during May and June in
the Gulf of Mexico, at the time when they are so numerous in the
shore waters. Fairly mature "pups," ^s the developing young are
called, have been found in the uteri of specimens collected off shore
in Texas Gulf waters in late August and early September.

CHAPTER XXVII
PISCES, TRUE FISH

This important class includes quite an extensive variety of dif-
ferent forms. They are aquatic and possess the usual adaptation of
gills for respiration, and paired fins as well as median fins to assist
in locomotion. Most forms within the class have scales as an exo-
skeletal covering of the skin. The endoskeleton is primarily bony.
Pectoral and pelvic girdles are developed to support the paired fins,
but the pelvic girdle is usually small. The fins are supported by
fairly well-developed fin rays. The majority of families in this class
possess a swim bladder. The typical shape of the fish's body is
fusiform or spindle-shaped, with all of the original features of
stream-lining. The shape assists in dividing the water as the fish
moves through it. As the water passes over the thicker part of the
body, it rushes in to push forward on the posterior slopes of the
spindle form of the body. This is an adaptation for easy production
of speed. The sedentary forms of fish usually tend to lose this shape
and become flattened or otherwise modified. The shape of the body
varies from that of the long slender eel to that of the globe-shaped
box-fish and inflated puffers which can float like balloons. The
sea horse is one of a group of very peculiarly shaped forms. Still
another peculiar adaptation is the flying fish. The fins of fish are
found singly in the form of a dorsal median fin, sometimes divided
into two ; a single caudal fin over the tail ; a ventral median anal fin
in most species; a pair of pelvic or ventral fins which are quite
variable in position and in some forms rudimentary, and the paired
pectoral fins. These paired fins are supported by bony girdles. The
pelvic fins of the perch are located almost immediately ventral to
the pectoral fins, while in the bullhead catfish they are just anterior
to the anus. In this catfish there is a second dorsal, which is com-
posed entirely of skin and is called an adipose fin. The structure of
the caudal fin and posterior end of the vertebral column is distinctive
and has been classified. The most primitive type of tail is the
diphyceroal in which both the cutaneous and osseous parts are
equally divided between dorsal and ventral regions. The hetero-
cercal tail is asymmetrical and the tip of the vertebral column ex-

442

PISCES, TRUE FISH

443

tends into the dorsal lobe as has already been seen in the dogfish.
Still another type of tail is the liomocercal, which is internally un-
balanced but externally symmetrical. The original notochord turns

Fig-. 240. — Spiny boxflsh, Chilomycterus schoepfli, from Gulf of Mexico and Atlantic

Ocean.

Fig. 241. — Diagram showing some peculiar bony fish. A, common eel ; -B, sea
horse; C, flying fish. (From Krecker, General Zoology, published by Henry Holt
& Company, after Jordan.)

into the dorsal lobe, but the lobes stroke the water with about equal
surface and force. It forces the fish through the water in a hori-
zontal plane and is correlated with a terminal mouth.

444

TEXTBOOK OF ZOOLOGY

There are three principal types of scales which cover and protect
the body of most true fish (a notable exception is the catfish to be
described later). These are: ganoid, cycloid, and ctenoid. The first
are usually rhombic or oval in shape and are covered by a dentinelike
substance called ganoin. Such fish as gar pikes and bowfins possess
this type. Cycloid scales are rather disc-shaped with conspicuous
concentric lines. They are usually imbricated on the skin, like a
shingle roof. The third type is similar to the cycloid except that
the free edge of the scale bears some spiny projections or cteni.

Fig. 242. — The different types of fish scales. 1, cycloid; 2, ctenoid; 5, ganoid;
i, placoid. (From Krecker, General Zoology, published by Henry Holt and Company,
after Her twig.)

Cycloid scales are found on the carp while the ctenoid are charac-
teristic of the perch and sunfishes. The age of many fish can be
determined by the distribution of the concentric lines on the scales.
The lines formed during nongrowing periods fuse closely together,
thus indicating seasonal periods on the scale.

The skeleton includes, besides the paired iins and girdles already
mentioned, the amphicoelous (concave in both ends) vertebrae and
bony cranium, which is complete and independent of the visceral
skeleton. This latter portion consists of seven arches, the jaw
structures, and five gill arches. The bones of the operculum arise
as a part of this division.

The digestive tract is in the usual form of a canal with out-
growths. Food ranging from vegetation, insect larvae, Crustacea,
clams, and snails to small fish and amphibia is utilized. It passes

PISCES, TRUE FISH 445

through the toothed mouth, pharynx, esophagus, stomach, duodenum,
ileum, and large intestine during the process of digestion. Teeth
are located on the jaws, roof of the mouth, and walls of the pharynx,
and are used primarily for holding prey. Gastric glands in the wall
of the stomach supply some of the digestive juices. Pyloric caeca
which join the anterior portion of the duodenum increase the
absorptive and digestive surface in many fish.

The respiratory system consists of the mouth, gills, and, to some
extent, the swim bladder in certain fish. Water is drawn in or
inspired through the mouth and forced out or expired through the
gill slits. The mouth and pharynx form a water-tight pumplike
arrangement with the help of the flaplike oral valves just inside the
lips and the IrancJiiostegal memhrane at the margin of the operculum.
The exchange of oxygen and carbon dioxide between the blood in
the capillaries of the gills and the water occurs as the water passes
over the gill lamellae. Oxygen is absorbed by the blood, and carbon
dioxide is discharged to the water.

The circulation in most fish is, in general, similar to that described
for the lamprey, except for certain specializations and phylogenetic
developments. The system includes the paired anterior and poste-
rior cardinal veins meeting in the duct of Cuvier which joins the
sinus venosus, the hepatic portal vein leading to the liver, the hepatic
vein from liver to sinus venosus, the two-chambered heart with its
accessory sinus venosus and lulhus arteriosus, ventral aorta, bran-
chial arteries, dorsal aorta, and the various branches.

Excretion is accomplished by a pair of dorsally located meso-
nephric kidneys, each of which is connected by a mesonephric duct
to a urin<iry sinus or bladder. This bladder opens to the exterior by
an aperture located just posterior to the anus.

Classification

Few students of the fish are in complete agreement on the classi-
fication of all groups of fish, but a general scheme of the most
acceptable plan will be presented. There are something like 20,000
species of fish described, of which more than 3,300 species occur in
North America. There are two subclasses. An abridged summary
of their orders and families is included here.

446

TEXTBOOK OF ZOOLOGY

Subclass Teleostomi. — True Fishes. In this group are found all
fresh-water fishes and many of the marine fish which frequent our
waters and shores, except lamprey. In this general division of the
group are four orders in the world, each including subdivisions of
either families or in the larger orders, suborders and families.

Order Crossopterygii (Family Polypteridae) . — Polypterus, an Afri-
can fish using the swim bladder as an accessory respiratory organ.

Fig. 243. — Polydon spathula, spoonbill cat or paddle fish and its associates. (Cour-
tesy of American Museum of Natural History.)

Order Chondrostei. — Paddlefishes and Sturgeons. A ganoid type
of fish with abundant proportion of cartilage in the skeleton and with
bony ganoid scales.

Family Polyodontidae. — Paddlefishes or Spoonbills. Polydon spath-
ula is found in the Mississippi Valley. It has a smooth skin and
long, flat paddlelike snout, with a few ganoid scales on the tail.

Family Acipenseridae. — Sturgeons. There are only three genera
usually described for this country. The body is covered with five
rows of keeled, ganoid shields and the tail is distinctly heterocercal

PISCES, TRUE FISH 447

with the upper lobe quite extended and slender. There are no teeth
in the mouth. Acipenser fulvescens is the Mississippi Valley form,
and, though it was once abundant, it can now be found only occa-
sionally. Both families of this order furnish both flesh and roe
(caviar) as food for man.

Order Holostei. — Gar pikes and Bowfins. This is another ganoid
type but with a more complete bony skeleton. In most of the repre-
sentatives of the order, the scales are of the enamellike ganoid
variety, but in a few they are cycloid and imbricated.

Family Amiidae. — Bowfin or Fresh-water Dogfish. In Amia calva,
the only species in existence, we find the cycloid scales and another
form capable of accessory respiration by means of swim bladder.
They inhabit the fresh-water lakes and sluggish streams as far
southwest as east Texas.

Family Lepisosteidae. — Gar pikes (garfishes). The long-nosed
gar and alligator gar are the most common southwestern forms,
while the long-nosed and short-nosed are the common species of
the Middle West. These are covered with a strong armor of
rhombic, enamellike, ganoid scales. The pelvic fin is abdominal and
the dorsal fin is far to the posterior. The snout is conspicuously
extended, and the tail is heteroeercal.

Order Teleostei. — True Bony Fishes. A group which includes the
majority of existing fishes. It is thought to have descended from
the ganoid type. Some are soft-rayed fishes with open connection
between swim bladder and alimentary canal (Physostomi), and
others are spiny-rayed fishes with no air duct from swim bladder
(Pkysoclisti). Their skeletons are highly ossified. They have cycloid,
ctenoid, or no scales and their type of tail is usually homocercal.

Suborder Isospondyli. — Tarpon, Herring, Salmon, etc.
Family Elopidae. — Ten-pounders. Found in warm marine waters.
Flops saurus is the typical representative.

Family Megalopidae. — Tarpons. This is a very active game type
of fish with large scales and an extended filament from the dorsal
fin. It is very abundant throughout the Gulf of Mexico and is a
famous game fish all along the Texas-Louisiana coast.

Family Hiodontidae. — Mooneyes. Fish of this family have greatly
compressed bodies which are covered with large, silvery, cycloid

448 TEXTBOOK OF ZOOLOGY

scales. There are three species in our western streams and in the
Great Lakes. Hiodon tergisus is the form found in the Mississippi
Valley.

Family Albulidae. — Ladyfishes. A small group of a few species
found in warm marine waters.

Family Dussumuriidae. — Round Herrings. This is a scarce, small,
bluish fish, with a rounded belly outline.

Family Clupeidae. — Herrings. This family includes a large num-
ber of species, and it is thought that there are more individuals in
this group than in any other. They are found in most seas, and
many species enter fresh water to spawn, but there are very few
fresh-water species. The family includes many valuable food fishes
of which Clupea harengus is the most important.

Family Dorosomidae. — Gizzard Shads. This is an abundant, widely
distributed, prolific, but almost inedible, group of fish. The body
is quite flat from side to side, and the edges are thin. There is a
filament extending from last ray of the dorsal fin.

Family Engraulidae. — Anchovies. These fish have elongated, small,
compressed bodies. They are abundant in warm seas and swim in
large schools. Anchoviella (Anchovia) mitcMlli is one of the most
common American forms.

Family Coregonidae. — Whitefishes. There are about twenty fresh-
water lake species. This is an important commercial fish. Ciscos
are included in this group also.

Family Salmonidae. — Salmon and Trout. This is a group of
elongate, moderately compressed, large-mouthed fish with fine scales,
a lateral line and, internally, a large number of pyloric caeca. They
are principally northern and abound in clear waters north of lati-
tude 40°. They are noted as game and food fish.

Family Thymallidae. — Graylings. These fish resemble the previ-
ous group in many ways, but they have a large dorsal fin. The
Michigan Grayling, Thymallus tricolor, is a typical species. It is
a northern fish.

Family Osmeridae. — Smelts. These differ from Salmonidae chiefly
in the presence of a blind saclike stomach. The esophagus and
pylorus join close together.

PISCES, TRUE FISH

449

Family Argentiuidae. — Argentines. This is a small group of
northern or deep-sea species.

Suborder Apodes. — Eels, etc.

Family Anguillidae. — True Eels. Fishes with elongated slender
body and no pelvic fins. The scales are embedded in the skin. They
are among the most voracious of fishes and are quite hardy. Eels
come several hundred miles inland from the Gulf of Mexico in the

Fig. 244. — European carp, Cyprinus carpio. a, scale carp; b, leather carp.
(From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger,
after Smith, Fishes of North, Carjlina.)

streams of the Southwest. They go out to sea to spawn and the
young make their way back up the streams. It is thought that the
female spawns once and then dies.

Family Congridae. — Conger Eels. A group of marine eels, dis-
tantly related to the Anguillidae, the common eel. They are with-
out scales, and the body is often entirely black.

Family Ophichthyidae. — Snake Eels. These eels are also without
Bcales and have the tongue more or less adherent.

450 TEXTBOOK OF ZOOLOGY

Family Muraenidae. — Moray eels. In this type there is an absence
of pectoral, as well as pelvic, fins. The skin is thick, leathery, and
scaleless.

Suborder Eventognathi. — Suckers, Carp, and Minnows.

Family Catostomidae. — Suckers. This is a large and important
group in fresh water. These fish have elongated bodies covered with
cycloid scales, toothless jaws without barbels, and a round sucking
mouth. The air bladder is large and connected with the alimentary
canal. They feed on plant tissue and small animals. There are 60
species in the fresh waters of North America.

Family Cyprinidae. — Carp, Dace, and Minnows. A group of fish
whose air bladders are usually large and commonly divided into
anterior and posterior lobes. As in the suckers, the jaws of this
group are toothless. They range from small minnows, two inches
in length, to large carp of two feet in length. Some of the most
abundant species of general distribution fall in this family.

Suborder Nematognathi. — Catfish.

Family Ameiuridae. — Fresh-water Catfishes. These fish are de-
void of scales, have barbels on the lips, and a large, tough air blad-
der. As a group, they are excellent commercial and food fish, and
all are tenacious of life. The bullhead catfish and channel catfish
are very well known and almost universally distributed over the
United States, east of the Rocky mountains.

Family Ariidae. — Sea Catfish. These, too, are scaleless and have
barbels. Bagre marina, the Gaff-topsail Catfish, and Oaleichthys
felis, common sea cat, are very common along the shores of the south-
ern Atlantic and the Gulf of Mexico.

Suborder Iniomi. —

Family Synodontidae. — Lizard Fishes. The body of this type of
fish is elongate and covered with cycloid scales. The head is de-
pressed and the mouth is quite wide with strong teeth.

Suborder Haplomi. — Mud Minnows, Pikes, and Pickerels.

Family Umbridae. — Mud Minnows. These carnivorous minnows
live in the mud of sluggish streams. They have no lateral line and
are covered with cycloid scales. There are at least three American
species.

Family Esocidae. — The Pikes or Pickerels. These fish have a
slender body, a large mouth with the lower jaw projecting, and a

PISCES, TRUE FISH 451

dorsal fin far to the posterior, near the base of the forked tail. Esox
lucius, the common pike, and Esox masquinongy, the muskellunge,
are familiar northern forms. Esox vermiculatus, the mud or grass
pickerel, and Esox niger penetrate the Southwest.

Suborder Cyprinodontes. — Killifishes, Mosquito Fish, and Cave
Fish.

Family Cyprinodontidae. — Killifishes. This group of minnows,
well represented by Funduhts heteroclitus, the common killifish, or
Zygonectes notatus, top minnow, are found commonly in shallow
estuaries or fresh-water streams. F. heteroclitus is a much used labo-
ratory animal along the Atlantic coast.

Family Poeciliidae. — Mosquito Fishes. This is a very common
mosquito-eating fish of the South. Ganibusia is a widely distributed
genus.

Family Amblyopsidae. — Cave Fishes. There are a few species of
these small fish living in subterranean streams of the central and
southern United States.

Suborder Synentognathi. — Garfishes and Flying Fishes.

Family Belonidae. — Sea Garfishes. These are long-bodied, vora-
cious fish of the warm seas. They range from Maine to the Texas
Coast of the Gulf of Mexico.

Family Exocoetidae. — Flying Fishes. Some of these peculiar fishes
are able to leave the water and glide through the air for several
yards. The pectoral fins are greatly developed and serve as planing
surfaces. There are about sixty different species inhabiting the
tropical seas.

Suborder Thoracostei.- — Pipefish, Sea Horses, and Sticklebacks.

Family Syngnathidae. — Pipefishes and Sea Horses. A group of
very peculiarly-shaped fishes whose bodies are quite long and slender.

Family Gasterosteidae. — Sticklebacks. Most representatives of
this family have prominent spines just anterior to the dorsal fin.
They are principally northern in distribution.

Suborder Anaca7ithini. — Codfishes.

Family Gadidae. — Codfishes. These are large-mouthed fish with nu-
merous teeth and pyloric caeca. They are food fish of northern seas.

Suborder Heterosomata. — Halibut Flounders, Soles, and Tongue
Fish. (Flat-fishes.)

452

TEXTBOOK OF ZOOLOGY

Family Hippoglossidae. — Halibuts. The Halibut family is prin-
cipally marine and is used a great deal for food.

Family Pleuronectidae. — Flounders. These fish have a laterally
flattened body and both eyes on the right side of the head.

Family Achiridae. — Broad Soles. These are found* quite com-
monly along the Gulf coast. Like the other flounders they are greatly
compressed, lie on the left side, and have both eyes on the right.

Family Cynoglossidae. — Tongue Fish. In these the eyes are on
the left side of the flat body and the entire body resembles a tongue
in shape.

Fig. 245.

-White mullet, Mugil curema. (From Metcalf, Textbook of Economic
Zoology, published by Lea and Febiger, after Smith.)

Fig. 246. — Spanish mackerel, Scomheromorus maculatus. (From Metcalf, Textbook
of Economic Zoology, published by Lea and Febiger, after Smith.)

Suborder PercomorpM. — Mullets, Mackerels, Pompanos, Perches,
Basses, Snappers, etc.

Family Mugilidae. — Mullets. There are numerous species of com-
mon forms in the estuaries and river mouths along our southern
shores. (Fig. 245.) This is an esteemed food fish along Atlantic
coast, but it is ignored in the Gulf States.

Family Atherinidae. — Silversides. Most of these fish are small,
compressed and covered with even cycloid scales. They live in
schools, particularly along the coast in the warmer regions.

PISCES, TRUE FISH

453

Family Spliyraenidae. — Barracudas. This group includes vicious,
voracious, pikelike fishes of the warmer seas. There are 15 species.

Family Poljaiemidae. Threadfins. This common name comes from
the pectoral filaments. These are found among collections from the
Gulf of Mexico.

Family Istiophoridae. — Sailfishes. Among this group are found
some of the swiftest of fishes. The dorsal fin projects high above the
body.

Pig-. 247. — Large-mouthed black bass, Micropferus salmoides. The prize fresh-
water game fish. (From Metcalf, Textbook of Economic Zoology, published by Lea
and Febiger, after Smith.)

Fig. 248. — Marine croaker, Micropogon undulatus. (From Metcalf, Textbook of
Economic Zoology, published by Lea and Febiger, after Smith.)

Family Scombridae.— True Mackerels. These fish are a pretty,
metallic blue in color and are very important commercial fish of the
Atlantic. They live in great schools.

Family Cybiidae. — Spanish Mackerel. These mackerels have a
scaly body. The tail is usually quite widely forked. They are fine
food fish and are very abundant in the Gulf of Mexico (Fig. 246).

454 TEXTBOOK OF ZOOLOGY

Family Thunnidae. — Tunnies and Bonitos. There is not a wide
difference between these and the other mackerels. They reach large
size and are quite numerous on the high seas.

Family Carangidae. — Pompanos. This family is another which is
related to the mackerels. These are tropical fish and several species
of them are choice food fish.

Family Centrarchidae. — Sunfishes and Fresh-water Basses. This
is one of the widely distributed families with numerous species and
abundant individuals. The bodies of most of the sunfishes are about
the shape of a person's hand. They all build nests and are desirable
game fish.

Family Etheostomidae. — Darters. A brilliantly colored group of
small fish found in clear, swiftly-moving streams.

Family Percidae. — Perches. These fish have rather small fusiform
bodies with ctenoid scales. Perca flavescens, the yellow perch of the
northern states, is almost a classical classroom form.

Family Serranidae. — Sea Basses. There are several important
food fish included in this group.

Family Lutianidae. — Snappers. These fish, particularly Lutianus
campechanus, the red snapper, are abundant in the deep waters off
the Gulf coast. They are prized commercial fish.

Family Sciaenidae. — Drumfishes. In this group the air bladder is
usually large and constructed to enable the fish to make a grunting
or drumming sound. They are carnivorous, and common on sandy
shores of warm seas. Aplodinotus, the gaspergou or drum is found
in fresh water.

Suborder Cataphracti. — Sea Robins.

Family Triglidae. — Gurnards or Sea Robins. The bodies of mem-
bers of this family are usually covered with rough scales. They are
common in all warm seas.

Suborder Discocephali.- — Remoras.

Family Echeneidae. — Remoras or Shark Pilots. The spinous dor-
sal fin of this fish is modified to form a sucker on top of its head. The
remora attaches itself to the ventral side of a shark and is carried
about with it.

Suborder Gobioidea. —

Family Gobiidae. — Gobies. These small, carnivorous fish are found
creeping about on sandy bottoms along shores and in mouths of
rivers. There are at least 500 species, chiefly of tropical seas.

PISCES, TRUE FISH 455

Suborder Jugulares. —

Family Batrachoididae.— Toadfishes. This group includes a num-
ber of species which have large mouths and somewhat the appearance
of toads.

Suborder Pledog^iathL—Trigger^hes, Filefishes, and Porcupine
Fishes.

Family Balistidae. — Triggerfishes. The spines of the dorsal fins of
these fish are long and saw-toothed.

Family Monacanthidae.— Filefishes. The body of this type of fish
is much compressed and covered with rough, velvety skin. The fins
are poorly developed.

Family Diodontidae.— Porcupine Fishes. The fixed spines on the
bony plates in the skin are characteristic of the group.

Suborder Pediculati. —

Family Lophiidae. — Anglers. A heavy appearing, broad-bodied
fish. The first spine of the dorsal fin is extended and enlarged at the
tip to hang over the mouth as a bait for other fish.

Subclass Dipnoi.— Lungfishes. These fish have long, slender, paired
fins and a well-developed median fin. The air bladder is connected
with the pharynx and is modified to serve as a lung. They possess
characteristics which apparently place them intermediately be-
tween fishes and Amphibia. There are only three living genera.

Family Ceratodontidae.— Australian Lungfish. There is only one
species in this family and it is Neoceratodus fosteri, found in stag-
nant waters of Australia. This species is able to breathe air.

Family Lepidosirenidae. — Lungfishes of South America and Africa.
Proiopterus is the genus found in the marshes of Africa. They feed
on insects, worms, crustaceans, and smaller vertebrates. They go
into aestivation by burrowing into the mud of the marshes during the
dry summer season. Here they secrete a cocoon of slime for protec-
tion. Respiration is carried on by two lungs formed from a modified
air bladder. Lepidosiren paradoxa is the single form found in South
America.

Economic Relations of the Class

Fish have been one of the stable sources of food for man all
through history, and it is in this respect that they are most impor-
tant to man in present times. H^idreds of millions of dollars

456 TEXTBOOK OF ZOOLOGY

annually are yielded by the commercial fisheries. The annual catch
of salmon alone is estimated at $17,000,000. Besides salmon, cod-
fish, halibut, herring, shad, mackerel, mullet, red snapper, buffalo
fish, carp, catfish, trout, ciscoes, and pike perch are all important
food and commercial fish. In addition to the food value of the
flesh, the eggs of several species, such as sturgeons and paddlefish,
are in great demand as caviar. Several of the food fish are also
greatly prized as game fish. Many of the game fish, such as bass,
crappie, trout, and the pikes, are choice food. Some highly desirable
game fish, such as the tarpon, are almost worthless as food.

Several groups of fish have a distinct negative value. The sharks
destroy many other fish, lobsters, and crabs; damage perhaps one-
half million dollars' worth of fishing gear per year, as well as the
larger ones, injuring or even killing a human being occasionally.
Certain rays, such as Dasyatis sahina and D. americana, have a
poisonous spine on the tail, and the pectoral fins of catfish have a
barbed poisonous spine. Either of these fish can inflict an ugly, pain-
ful wound which is dreaded by all fishermen.

The United States government through the U. S. Fish and Wildlife
Service and most states through their Fish and Game departments are
making continuous studies of the fishing industry and are conduct-
ing fish culture on a rather large scale.

The gar pike is decidedly a predator, living largely on other fish.
The value of the sport of fishing is usually underestimated. Besides
having recreational value, there is much money spent on the trips,
tackle, clothing, etc., inspired by this sport.

There are still several other valuable relations of fish to the welfare
of man. Such oils as cod-liver oil, haliver oil, shark-liver oil, espe-
cially, are much used as medicinal products for production of vitamin
D which is a preventive for rickets. Fish scrap, which is left after
the oil is extracted, is used as a fertilizer and is also put up in the
form of fish meal and sold as protein feed for other animals. The
hides of sharks and some other fish make fine leather when tanned.
Ganibusia, a top minnow of the South, called the mosquito fish, is
an asset because of its appetite for mosquito larvae. It is used to
control the spread of mosquito-borne diseases, such as malaria, yellow
fever, and dengue fever, by destroying mosquitoes. A liquid, known
as pearl essence, is extracted from certain fish scales in an ammonia

PISCES, TRUE FISH 457

solution. "When glass beads are coated with wax and covered with
this solution they become artificial pearls.

TYPICAL BONY FISH— YELLOW BULLHEAD AND SOME
COMPARISONS WITH YELLOW PERCH*

The yellow bullhead catfish, Ameiuriis natalis, is widely distrib-
uted through the fresh waters of the United States. Its distribution
did not originally reach the Pacific coast, but during recent years
it has been successfully introduced. The natural range extends
throughout the Middle West, South, and well into the Southwest.
These fish will be the principal subject of this description, but there
will be some comparisons made with yellow perch, Perca flavescens.

The bullhead inhabits nearly all sluggish streams, ponds, and
lakes. It lives along the muddy banks and around submerged rocks
and logs in the water. It is a very hardy fish and is able to thrive
in almost any aquatic condition. Perch lives in clear lakes and ponds.

External Features

The body is stout, the head is short and broad, and the mouth is
wide. There is a relatively small dorsal fin located anteriorly, an
adipose fin back near the tail, and a rounded caudal fin forming the
tail. Just anterior, to the caudal fin on the ventral side of the body
is a single, broad, bladelike anal fin. Anterior to this is a pair of
ventral or pelvic fins. Just posterior to the gills and in a lateral
position are the pectoral fins. The skin of Ameiurus is smooth and
without scales whereas the skin of perch is covered with ctenoid
scales. There are two pairs of nostrils on the head, as well as eight
feelerlike harhels. These are distributed, two dorsally, one attached
to the maxillary process at each side of the mouth, and four pale-
colored ones on the skin of the lower jaw. Perch has none. Ex-
tending along each side of the body is a lateral line. The eyes are
relatively small and without lids. On each side of the head is a
flaplike operculum, which covers the gills. The tail is homocercal.
The colors of the upper parts of the bullhead range from yellowish
green to dark brown. The sides are a lighter waxy yellow or yel-
lowish brown, and the ventral side is yellow. The dorsal barbels
are brown, while those ventral to the mouth are pinkish cream.
There is a darker longitudinal band running lengthwise of the anal

•In collaboration with Rose Newman.

458

TEXTBOOK OF ZOOLOGY

fin. The general shape of the body is fusiform or spindle-shaped
and hence offers little resistance to the water. In fact, the body
splits the water as it passes through it, and the water closes in on
the posterior slopes of the spindle shape to help force it forward.

ESOPHAGUS

UIVER

6AL_U

Bl_ADDER

DUODENUM

7t'!l\ CABDI AC

STOMACH
PYLORUS

:iT4-FUNDIC

STOMACH

SPl_EEN

I_EUM
Al R
BUADDER

Kr DNEV

LARGE
INTESTINE

TEST I S

ANUS
^BL-ADDER
I — VAS

DEFERENS

MESO-

NEPHRIC

DUCT

UROGENI TA1_
PAPIUL.A

Fig. 249.-

-Digestive system and other visceral organs of Ameiurus natalis. (Drawn
by Titus Evans from dissections by Rose Newman.)

Digestive System and Digestion

Except for some habits of a scavenger, the food of the bullhead is
similar to that of the yellow perch, which includes crayfishes, water
snails, insect larvae, as well as insects, and other small fish. The
dead bodies of almost any animal Avill be eaten by the bullhead.
The digestive system consists of the mouth, pharynx, esophagus,
stomach, intestines, and anus. The mouth is large and has teeth

PISCES, TRUE FISH 459

at the front supported by the maxillary jaw above and the mandibu-
lar jaw below. The teeth serve to hold the prey or food in the mouth.
The tongue, which is supported by the hyoid hone, has a row of
papillae running along its midline posteriorly into the pharynx. The
pharynx is rather funnel-shaped and has four gill slits in each lateral
wall. The bones in the roof of the pharynx bear superior tooth pads
which are round or oval in shape. The esophagus is a straight mus-
cular tube near the posterior end of which enters the ductus pneu-
maticus from the air bladder. As is generally the case in fish, diges-
tion begins in the stomach which is saclike and continues directly to
the pylorus in the bullhead, but is cylindrical in perch, with the
pyloric portion extending from the side. Gastric glands in the walls
secrete enzjones which start the digestive process. In perch there is
a group of fingerlike pyloric caeca attached to the side of the pyloric
region. The mass of partially digested material passes through the
pyloric valve to the duodenum of the small intestine. The small in-
testine is shorter and less coiled in the bullhead than in the perch.
It receives the hile duct from the liver and possibly some small pan-
creatic ducts from small masses of pancreatic tissue held in the mes-
enteries. There is not a distinct pancreas in either the perch or the
bullhead. Digestion continues in the small intestine and most of
the absorption of food by the blood occurs through the walls of
the posterior portion of the ileum. Following the small intestine the
short broad large intestine leads to the anus where fecal material
is discharged.

Circulatory System and Circulation

The heart lies almost free in the pericardial space at the extreme
anterior end of the body cavity. It is composed of two principal
chambers and two accessor}^ ones. There is a single auricle with the
accessory sinus venosus leading into it, and the single ventricle which
leads into the accessory conus arteriosus. The blood enters the sac-
like sinus venosus from the common cardinal veins and hepatic veins.
It passes through a valve to the auricle, then with contraction of the
auricle to the muscular ventricle through the auriculoventricular
valve. The contraction of the ventricle forces the blood through the
semilunar valve into the conus arteriosus, thence to the ventral aorta
which branches into four pairs of afferent branchial arteries into the
gill arches. Here the blood passes through finely branched capillaries

460

TEXTBOOK OF ZOOLOGY

-^i

5.1c

v'^

•+,

.*^

"Ol

'^)

^\J^

«l) G 'o

ti
to

b
O

!-•
<»
V

<u
bo

o
in

PISCES, TRUE FISH

461

CAROTID

EFFERENT

BRAMCHl Al_

SULBUS

ARTERIOSUS

AFFERENT

BRAtslCMI AU

DORSAL AORTA

SUBCI_AVI AN

TO HEAD

KIDNEY

HEPATIC

COEI_l ACO-

MESENTERIC

PNEUMATO-

CVSTtC

GASTRIC

MESENTERIC

RENAL-

I NTESTINAU

SPLENIC

GENITAU
IL-t AC

HYPOGASTRIC

CAUDAL

Fig-. 251.

i-16. ^ox. — Arteries and most of the visceral organs of Ameiurus natalis Le
Sueur, lateral view. A, auricle; Bl., urinary bladder; H.K., head kidney ;Ht.ii:-,
kidney proper; il., ileiim ; L.I., large intestine; Sto., stomach; T., testis. (Drawn
by Titus Evans from dissections by Rose Newman.)

462

TEXTBOOK OF ZOOLOGY

ANXERlOR

CARD I NA,l_

DUCT OF

CU VI ER

SINUS VEISJOSUS

HERAT I C
HERAT IC
PORTAl_
ACCESSORY
PORTAL.

PtslEUMATI C
GASTRI C

I NTESTINAL.

POST
CARDINAL.

MESENTERIC

INTESTINAL

ABDOMI NAt_

PORTAI_

ll_l AC

RENAU PORTAl_

GEN I TAl_

CAODAU

Fig. 252. — Veins and viscera of Ameiurus natalis Le Sueur, lateral view. H.K.,
head Icldney; II., ileum; Kid., kidney; L.I., large intestine; St., stomach. (Drawn
by Titus Evans from dissections by Rose Newman.)

PISCES, TRUE PISH 463

where it is aerated by absorption of oxygen from the water passing
over the surrounding gills. This blood continues by convergence of
the capillaries into the efferent branchial arteries which lead dorsally
and join in the formation of the dorsal aorta. This is the principal
artery of the trunk of the body and gives subclavian branches to the
pectoral fins, coeliaco-mesenteric artery to the viscera, parietal ar-
teries to the body wall, renal arteries to the kidneys, and finally ends
in the caudal artery supplying the tail. The food substances and
oxygen are supplied to the tissues of the body by means of capillary
branches through them. These capillaries also collect the waste prod-
ucts and converge to form the veins which carry the blood back to
the heart. The 'posterior cardinals return from the posterior portion
of the trunk, the hepatic portal from the visceral organs to the liver,
and the hepatic from the liver to the sinus venosus, while sub-
clavian veins return from the pectoral fin region. The blood consists
of the fluid plasma, oval nucleated red. corpuscles, and amoeboid
white corpuscles.

Respiratory System

The mouth is used in forcing water over the four pairs of gills.
Water is drawn into the mouth by lowering the floor, while the
branchiostegal membranes at the margins of the opercula are closed
down over the gills. The opening of the mouth is guarded by fleshy
flaps or oral valves. When the mouth is filled with water, it is closed,
and these valves prevent water from escaping through this aperture.
The branchiostegal membranes relax and when pressure is applied to
the water in the mouth, it is forced out over the gills. This process
is repeated in rhythmic sequence in order that the blood passing
through the capillaries of the gills will be constantly aerated.

The air bladder likely has some respiratory function. It is a large,
tough sac, taking up almost one-half of the space of the abdominal
cavity. The pneumatic duct extends from its midventral region to
the posterior part of the esophagus. There is likely some exchange
of air through this and the possibility of some degree of diffusion of
gases between this and the blood in its walls. In the perch, which
has no pneumatic duct, it is found that oxygen is secreted into the
closed swim bladder during periods of plentiful supply in the water
then drawn upon by the blood at times when the environmental
supply is scant. Another function of the air bladder is to decrease

464

TEXTBOOK OF ZOOLOGY

the specific gravity of the body of the animal in water and to serve
as a hydrostatic organ. In forms where the bladder is open to the
exterior the volume of air in it can be regulated by direct exchange
and allow the animal to take a definite level in the water without
effort. In the closed type, as in perch, the volume is regulated by
secretion or absorption of oxygen, as the need may be.

Excretory Organs

The kidneys of Ameiurus are similar to those in the perch. They
are located in the dorsal wall of the abdominal cavity, posterior to
the air bladder and just outside of the peritoneum. The head kidney

— head kidney
(pronephros)

pneumato—
cystic duct

a i r bladder

l_EFr OVARY
RIGHT OVARY

KIDNEY

(mesonephro^

Fig. 253.-

URINARY
BLADDER

OVIDUCT

MESONEPHRIC
DUCT

-Urogenital system of bullhead, lateral view. (Drawn by Titus Evans
from dissections by Rose Newman.)

(pronephros) is a paired mass of lymphoid tissue in front of the air
bladder. The functional kidneys are composed of numerous urinifer-
ous tubules supplied with blood capillaries which extract urea,
creatinin, and other wastes from the blood. A slender mesonephric
tube leads from each kidney to the urinary bladder. The urine is
stored in the bladder and finally expelled through the urinogenital
pore just posterior to the anus.

Skeletal System

Since there are no scales on the bullhead, the exoskeleton consists
of the soft fin rays which support the fins. The ctenoid scales of a
fish like the perch are also exoskeletal.

PISCES, TRUE FISH 465

The encloskeleton is principally bony and is composed of skull,
vertebrae, ribs, pectoral and pelvic girdles, and bony fin supports.
The main axis of the skeleton is the vertebral column and skull,
known as the axial portion. The first five vertebrae of the neck
or cervical region are fused together, but the remainder are sepa-
rate and are called amphicoelous because each end of the centrum
or body is concave. The parts of one of the simple vertebrae are
the body or centrum just mentioned, and the neural arch over the
spinal cord which lies in the neural canal. A neural spine extends
dorsally from the neural arch, and the parapophyses extend laterally
from the centrum and support the ribs. There are haemal arches
supporting haemal spines on the ventral side of the posterior ver-
tebrae. The adjacent vertebrae articulate at the centra and are held
in place by ligaments. Vertebrae six to fourteen bear ribs from the
transverse processes.

The skull is very largely bone with some cartilage. The bones are
arranged bilaterally. The skull may be divided into cranium and
visceral skeleton. The cranium encloses the brain and is composed
of the frontal hones, postfrontals, parietals, supraoccipital, exoccipi-
tals, hasioccipitals, 'basisphenoids, alisphenoids, and parasphenoid.
The ethnoids, sphenoids, epiotic, quadrates, pterygoids and nasals pro-
tect and support the auditory and olfactory organs.

The visceral skeleton supports the gills and includes the jaws. The
maxillary arch supplies both upper and lower jaws. The upper jaw
develops from a cartilaginous pterygoquadrate process into the pair
of premaxillae and pair of maxillae bones. The lower jaw is the
mandible. The premaxillae and mandible both have short spinelike
teeth. Just behind the mandible is found the hyoid arch, referred
to as number two. It supports the tongue, floor of the mouth, and
operele. The next five arches support the four gills and are known
as gill or hranchial arches. Each is composed of either four or five
parts and has spiny gill rakers at its anterior margin. They are
located in the lateral wall of the pharynx and are covered by the four
opercular bones of each side which compose the framework of the
operculum.

The skeleton of the paired fins is known as the appendicular skele-
ton. The pectoral girdle, made up of scapula, coracoid, supraclavicle,

466

TEXTBOOK OF ZOOLOGY

POST FRONTAL.

FRONTAL.

PRE MAXIUt-A

DENTARY

IMASAU

MAXIl_I-A

ECTETHMOID

MESETHMOID

QUADRATE

ARTICULAR

ANGUL-AR

HYOMANDIBUI_AR

METAPTERYGOI D

I NTER OPERCULUM
PRE-OPERCUl_UM
BRANCHIOSTEGAL
RAYS

SUB OPERCUL-UM
OPERCULUM
PECTORAL
Gl RDLE
DEFENSIVE
SPINE
PECTORAL
RAYS

PTEROTIC
PHENOTIC

SUPRA-
OCCI PI TAL
CERVICAL
VERTEBRA

PELVIC RAY

DORSAL RAY

TRUNK.
VERTEBRA

ADIPOSE FIN
■CENTRUM
NEURAL spine:

ANAL RAYS
HAEMAL SPINE

CAUDAL
VERTEBRAE

CAUDAL RAY

Fig. 254. — Skeleton of Ameiuru^ natalis Le Sueur, lateral view.
Evans from dissections by Rose Newman.)

(Drawn by Titus

PISCES, TRUE FISH

467

mesoclavicle, and infraclavicle, supports the pectoral fin which con-
sists of a row of hasal elements, and distal to this, a row of radial ele-
ments. The most anterior radial is completely ossified, terminates
in a sharp spine, and has the posterior edge serrated. In addition,
it has a poisonous secretion with which to inflict wounds. These
are much stronger in the bullhead than in the perch. The bullhead
has a rudimentary pelvic girdle, but the perch does not. It consists

Fron tal
Postfrontol
Parasph enoid
Prefrontal

Nasal
Premaxilfa

L acrimal

Suborbital

Ectopteryg old

Maxil/a-

Denfary

A rticular

Angular

Ceratohyoid

Hypohyal
Urohyal

Supraocopital

|— Paris tal

Epiotic

— Po St temporal
Pterotic
-Hyomandibuhr
-Opercular
r-Entopteryyoid
^ Met op terygoid
Quadrate
Symplectic

Preopercular
Subopercular
Brancti/osteyal Pay
Inf er opercular

Pig. 255. — Lateral view of tlie sltull of tlie yellow perch, Perca flavescens. (Drawn

by B. Galloway. )

of two similar ischiopubic plates united in the middle. Posterior to
these in the midline is a platelike fusion of the basals of the fin.
The radials are all fibrous. The anal fin and the dorsal fin are sup-
ported by interspinous bones. The anterior ray of the dorsal is a
bony spine.

Muscular System and Locomotion

Locomotion is not the only function of the voluntary muscular
system, but it is an important one. In addition to this function,

468

TEXTBOOK OF ZOOLOGY

certain muscles are specialized for feeding and others to assist in
respiration. The segmental myotomes divided dorsally and ven-
trally by the lateral line are the chief muscles of locomotion, since
most of the power is delivered by lateral strokes of the tail against
the water. The myotomes are modified in the regions of the paired
fins to supply certain muscles to them. The action of the fins serves

OUFACTORY BULB
OI_FACTORY TRACT

OPTIC N.

TROCHLEAR M.
OCULOMOTOR N.

TRIGEMIMUS N.
PACIAL- fsl.
CEREBRUM
PINEAU BODY
OPTIC LOBE
CEREBELLUM

AUDITORY N.
POSTERIOR LOBES

MEDULLA OBLONGATA

VAGUS IM.
GLOSSOPHARYNGEAL N.

Fig. 256. — Dorsal view of brain and cranial nerves of Ameiurus natalis Le Sueur.
(Drawn by Titus Evans from dissections by Rose Newman.)

to help in equilibration and in guiding the body as it is forced
through the water. The fins act somewhat as a combination keel
and rudder. "Without them the fish is unable to hold its upright
position and guide itself through the water.

In connection with the mouth and feeding process there are sev-
eral distinct muscles. The adductor mandihularis elevates the lower
jaw while the geniohyoid ancf mylohyoid raise the floor of the mouth
and tongue. There are eight different sets of muscles connected with
the respiratory movements of the opereula and gills.

PISCES, TRUE FISH

469

Nervous System

The central nervous system, composed of the brain and spinal cord,
is a little more developed than it is in the lamprey or shark. The
cerebral hemispheres are closely fused with the olfactory tracts which
extend by long tracts anteriorly to the olfactory bulbs (Fig. 256). The
diencepMlon, which is immediately posterior to the cerebrum, is cov-
ered dorsally by the large dome-shaped cerebellum (much smaller in
the perch). Extending dorsally from the roof of the diencephalon is
a slender, fingerlike pineal body (epiphysis) which is the vestige
of a third or median eye. Also partially covered by the cerebellum
is the midbrain which is divided into two rounded optic lobes. The
medulla oblongata lies just posterior to the cerebellum and is quite

Fig. 257.

OI_FACTORV TRACT
OPTIC NERVE C2)
CEREBRUM

OCUL.OMOTOR Ni C35

HYPOPHYSIS
OPTIC UOBES

NFERIOR LOBES
TROCHL-EAR N. 0*5
TRIGEMINIES N. Ca3
AND FACIAL N. CT)
ABDUCENS Nt (6)
MEDUUl_A OBUONGATA

AUDITORY N. fe)
GL_OSSOPHARYNGEA1_ N .(9)

VAGUS N. <'lO'>

-Ventral view of brain and cranial ner^^es of Ameiurus natalis Le Sueur.
(Drawn by Titus Evans from dissections by Rose Newman.)

prominent, due to the large posterior lobes (tubercula acoustica) on
each of its anterolateral positions. Dorsally, between these lobes, is
a diamond-shaped slit which leads into the cavity of the brain. This
is the fourth ventricle. On the ventral side of the diencephalon is
the optic chiasma where the optic nerves meet, and behind this are
the inferior lobes with the stalklike infundibulum joining the glandu-
lar hypophysis. Two peculiarities of the brain of Ameim^us are the
large cerebellum and the large posterior lobes. There are ten pairs
of cranial nerves emerging from various levels of the brain. Three
of these have strictly sensory function, three are strictly motor in
function, and four have both sensory and motor function. The bull-
head has forty-one pairs of spinal nerves arising segmeutally from

470

TEXTBOOK OF ZOOLOGY

the spinal cord. Each has a dorsal raraus, or branch, and a ventral
one extending out to certain parts of the body in the region.

The sense of taste is highly developed and is centered in the numer-
ous and well-developed taste huds which are distributed on the inside
and outside of the lips, in the lining of the first three gill slits, on the
barbels, and in gTOups over the external surface of the body, even to
the tail. The eyes are small and without lids but have fair power of
vision as this sense goes in fish. The focal distance is between twelve
and eighteen inches, and is better for detecting motion than for
recognizing objects. The fish does not have a sense of hearing;

Fig. 258. — Eggs of trout with well-developed embryos, and recently hatched
fry. A, eggs with embryos; B, fry. (Courtesy of General Biological Supply
House.)

the ear structures serve in the sense of equilibrium. Ameiurus, perch,
and other fish have a well-developed pressure and water-vibration
sense centered in the lateral line system. The sense of touch is dis-
tributed over the epidermis but is particularly keen in the lips and
barbels.

Reproduction and the Life History

The bullhead, perch, sunfish and many other common fish build
nests of one sort or another, lay the eggs in the nest, and guard the
nest until the eggs hatch. The details of the reproduction and
breeding are not so well known in Ameiurus natalis as they are in

PISCES, TRUE FISH 471

A. nehulosus, the brown bullhead. This being the case and since the
two are very similar, a brief description will be given for the latter.
The observations were made on a pair in an aquarium in AVashing-
ton, D. C. They made a nest on July 3 by removing with their
mouths more than a gallon of gravel from one end of the tank,
leaving the slate bottom bare. On July 5 about two thousand eggs
were deposited in four masses. Ninety-five per cent of them hatched
in five days with the water at 77° F. The young remained in masses
until six days old; then they began to swim. By the end of the
seventh day they were swimming actively and most of them collected
in a school just beneath the surface, where they remained for two
days, afterwards scattering. It is also reported that they ate finely
ground liver on the sixth day and had enormous appetites after the
eighth day. They were 4 mm. long when hatched and had attained
a length of 18 mm. by the fourteenth day. At the age of two months
their average length was 50 mm. Both parents assume responsibility
in caring for the eggs, keeping them agitated constantly by a gentle
fanning motion of the ventral fins. The egg masses are also sucked
into the mouth and then blown out with some force. These opera-
tions were continued until the fry (newly hatched fish) swam freely.

CHAPTER XXVIII

CLASS AMPHIBIA
(By Ottys Sanders, Southwestern Biological Supply Company)

As there are many vertebrate animals which lead an amphibious
life, it was natural for Linnaeus to group these together under the
class Amphibia. This, of course, was classification based on habits
rather than on structure, and as soon as such animals as the seal
and crocodile were studied structurally they were removed from
the class. Today the name is restricted to a group of vertebrates
which we know as frogs, toads, salamanders, and caecilians. They
are intermediate between fishes and reptiles. Except in caecilians,
they have paired limbs, usually with fingers and toes, and never
have paired fins like fishes. They have a moist, naked skin lacking
the protective hair of mammals or the feathers of birds. The cae-
cilians, none of which has been reported from the United States, are
wormlike burrowing creatures of the tropics. They have small
scales between their transverse body rings, although these are not
usually seen unless a dissection is made. These animals and a few
others such as the large South American frog ceratopharys, which
has dermal bones or ''scales," are the only ones of the class to have
scales. The amphibians are cold-blooded vertebrates, in contrast to
the warm-blooded mammals and birds.

The frogs, toads, and salamanders usually lay their eggs in
water. These develop into tadpoles or larvae breathing with gills
before metamorphosing to become adults which breathe with lungs.
A few species of frogs and salamanders lay their eggs on land and
pass their entire development in the egg. Kicord's frog, Eleuthero-
dactylus ricordii, and the slimy salamander, Plethodon glutinosus, are
examples of species that lay their eggs on land. These land eggs lack
the calcareous shell of reptile and bird eggs.

There are other exceptions to the general characteristics of this
diverse class. A large group of salamanders, the plethodontids, do
not have lungs even as adults, and their respiration takes place in
the mouth cavity and through the skin, both of which are richly
supplied with blood vessels.

472

CLASS AMPHIBIA

473

Size. — While most modem Amphibia are small creatures, paleon-
tological species reached large proportions, as, for example, the
Mastodonsaurus, which had a skull 4 feet long and a total length
of probably 15 or 20 feet. Among living amphibians, the giant
salamander of Japan and China, Megalobatrachus japonicus, grows
to a length of 5 feet. In the Southwest, the largest salamanders
are Siren lacertina, which attains a length of about 30 inches, and
the "hellbender," Cryptobranclnis, which commonly grows to be

Fig. 259. — The caecilian, Ichthyophis glutinosus, adult female, guarding her
eggs on the left, and a larva showing external gills on the right. Partly after
Sarasins. (From Atwood : Introduction to Vertebrate Zoology, published by The
C. V. Mosby Company.)

about 18 inches long and AmpMuma, the Congo eel. The goliath frog
of Africa reaches a body length of nearly a foot, while southern bull-
frogs, larger than their northern relatives, may grow to be over 7i/2
inches in body length, with a total length of 16 to 18 inches when the
legs are extended. The giant toad or marine toad, Bufo marinus, is
the largest of the true toads, and attains a body length of 8% inches.
The smallest frog in the United States is the swamp tree frog, Pseu-
dacris ocularis, which ranges from North Carolina to southern
Florida. Adults measure only % to % of an inch in body length.

474

TEXTBOOK OF ZOOLOGY

As far as is known, the length of life of Amphibia ranges from
ten to fifty-two years. The larger ones, in general, seem to live
longer than the smaller species. Some species of toads may live
about thirty years, frogs probably less.

Coloration. — Amphibians as a group are very colorful. The bright
green tree frog, Eyla cinerea, which makes bell-like calls from the
reeds and cattails in the summer months, the small grayish canyon
toad, Bufo pnnctatus, with its red warts, the varicolored common
tree frog, Hyla versicolor, with its orange groins, are but a few ex-
amples of beautiful species. Amphibians possess considerable abil-
ity to change color, and many of the tree frogs equal or surpass the
chameleon in this respect.

y.:..^^^^Ji^

Fig. 260. — Melanophore from Rana temporia. A. pigment distributed m response
to light; B, pigment contracted. (Redrawn and modified from Noble, Amphibia of
North America published by McGraw-Hill Book Company.)

Their different colors are due primarily to various combinations of
three kinds of pigment cells in their skin. The black melanophores
are branching pigment cells which may contract or expand, and,
when these predominate, the skin appears black or brown. Yellow
or red results from the action of lipophores contained in spherical
cells, and white from the guanophores. Green color results from the
reflection of light from guanin granules wherein all the light rays
escape absorption except the green. Different arrangements of
these pigment cells produce color changes which are initiated by
various stimuli, such as light, temperature, moisture, and the chemi-
cal composition of the frog's habitat. These color chajiges are

CLASS AMPHIBIA

475

directly beneficial to the animal when they help it to resemble more
closely its surroundings and thus avoid capture.

The Skin. — Amphibians have a soft, moist skin which is kept in
that condition primarily by a rich supply of mucous glands. Aquatic

Fig. 261. — Spadefoot toad, Scaphiopus coucliii, showing the shape of the pupil of
tlie eye. (Photograph by Thos. Mebane Jones.)

Fig. 262. — Feet of spadefoot toad, Scaphiopus coucliii, showing the dark-colored,
dartlike spades. (Photograph by Thos. Mebane Jones.)

and forest-inhabiting frogs and toads have a smoother skin than
species which live in drier places. Burrowing frogs and toads, such
as the spadefoot toad, Scaphiopus, also have thin, smooth skins,

476

TEXTBOOK Of zoology

The skin not only protects the underlying tissues from excessive
light but also has other functions. With its pigment it helps to
regulate temperature by transformijig light into heat. A most im-
portant function is its use as a respiratory organ. As previously
mentioned, one large group of salamanders, the plethodontids, lack
lungs and use the skin and buccal cavity for respiration. During
hibernation, practically all of the respiration of frogs is taken care
of through the skin. In Africa there is a frog, with greatly reduced

^ 5^

Pig. 263. — "Hairy frog." (Redrawn and modified from Noble, AmpMiia of North
America, publislied by McGraw-Hill Book Company.)

lungs, which, in the male sex, has developed a strange aid to respi-
ration. It has patches of vascular villosities on the thighs and sides
to such an extent that it has been named the ''hairy frog." These
villosities help provide sufficient oxygen for its increased metab-
olism during the breeding season.

Since amphibiajis have moist skins, they are in constant danger
of drying out, and therefore seek moist places where they may
absorb water through their skins. Most of them are nocturnal in

CLASS AMPHIBIA 477

their habits and therefore can be found during the daytime under
logs, in crevices or burrows in the earth, or in other situations where
they can protect themselves against this constant threat of desiccation.
Food and Feeding Habits. — Adult frogs and toads eat animal
food, while the tadpoles eat either animal or plant food. The food
of the adults consists primarily of living insects, worms, snails,
spiders and other small invertebrate animals. Many large frogs
and a few smaller ones are cannibalistic. Amphibians depend to
a large extent upon their sight in detecting food. While, in gen-
eral, frogs and toads will seize a moving object without much ex-
amination, the toads quite often stalk their prey and inspect it.
If a disagreeable insect, such as a stag beetle with strong mandibles,
is swallowed, it can be disgorged because, fortunately, the toad
has a wide esophagus. Most of the frogs and toads and many sala-
manders utilize their eyeballs in swallowing food. Their eyes can
be retracted into the head and by this action they help to push
food in the mouth cavity toward the esophagus.

Amphibians can go for a long period of time without food. Tad-
poles may live for months, and experiments made on axolotls
(larvae of the tiger salamander) have demonstrated that they may
live for about a year on the food stored in their own tissues. Dur-
ing the hibernation season and breeding season most salamanders
and frogs do not feed.

Enemies of Amphibia. — The enemies of amphibians are many. In
their larval or tadpole stages they are a delicate food for giant
water bugs, dragonfly nj^mphs, larvae of water beetles, and other
aquatic insects. Small crustaceans devour the gills of salamander
larvae, and fish appreciate their good flavor. Snakes, turtles, alli-
gators, birds, and mammals feed upon the adults and young. Man
enjoys the hind legs of frogs, and there is an increasing demand
for these as food. Man also destroys amphibians by polluting the
streams where they breed, and his automobile kills countless toads
and frogs on the highways. Nor are amphibians immune to disease
and gross infestation by parasites.

Powers of Regeneration. — The power of regenerating lost parts
is one way in which Nature aids the group. Young tadpoles may
regrow limbs or tails, although adult frogs and toads are appar-
ently unable to regenerate lost appendages. The axolotl larva of

478 TEXTBOOK OF ZOOLOGY

the tiger salamander, Amhystoma tigrinum, which is found in Texas,
New Mexico, Colorado, and elsewhere, has been used extensively in
experiments for studying the nature of this regeneration.

Means of Defense. — Amphibians have few ways of protecting
themselves from their enemies. Their coloration often blends in
with their surroundings and camouflages them, and their habit of
remaining immobile frequently causes them to be overlooked. Many
species practice death feints and some swell up by inflating their
lungs, making themselves more difficult to swallow. The mucous
glands of frogs and salamanders make them slippery, and, in the case
of salamanders particularly, their writhing and twisting movements
when captured make them hard to hold. A few salamanders have,
in addition, the ability to break off their tails and escape.

One of the most protective weapons that amphibians have, how-
ever, is the secretion of their poison glands. This is especially
effective in the case of toads, many of which have large glands on
their shoulders, known as parotoid glands. An animal that has
attempted to bite or swallow a toad and felt the effects of the poi-
sonous secretion of the parotoids upon the mouth tissues will not
soon forget the experience. The largest known toad of the North
American continent, Bufo marinus, which ranges from Texas to
Patagonia, produces one of the most virulent poisons known among
amphibians. There are records of dogs which have been killed by
its secretions. Glandular secretions of certain South American
toads, Dendrohates, have been used by the Indians of Colombia for
poisoning their ari'ows. The secretions of toads are ordinarily quite
harmless to man, however, unless they happen to get into his mouth
or eyes.

Voice. — The amphibians were probably the first vertebrates to de-
velop a voice. The calls of modern frogs and toads are very distinc-
tive, each species having its own particular call. Most of the
croaking is done by the males, and the primary function of these
calls seems to be to attract females and other males to the pond or
stream. It is during the breeding season that the air resounds at
night with their choruses, although certain species may croak at
other times.

The croaking of frogs and toads is usually done with the mouth
and nostrils closed. The air is forced back and forth between the

CLASS AMPHIBIA 479

lungs and mouth over the vocal cords, causmg them to vibrate.
Vocal sacs, when present usually lying either in the floor or at each
corner of the mouth, puff out to make resonating chambers which
increase the volume of the call. Bullfrogs quite often call while
under water. A few frogs, such as Ascaphus, which lives in the cold
mountain streams of Washington and the northwestern United
States, have given up their voice and reduced their lungs. Appar-
ently voice would not be as useful to this species as to frogs living
in quieter places, for its sound would not carry above the noise of
the mountain streams. So far as is known, none of the salamanders
use voice in attracting mates, and most of them are silent through-
out their existence.

Breeding and Egg-Laying Habits. — Frogs, toads, and salamanders
make periodic migrations to ponds and streams for the purpose of
egg-laying. These periods, called the breeding season, usually occur
during the spring months or, in tropical climates, during the rainy
season. Salamanders often come to the pools much earlier than do
the frogs and toads and may also begin their egg-laying earlier.

Most amphibians are oviparous, and their eggs are fertilized by
the male after they leave the body of the female. Some salamanders
and caecilians, however, have the eggs fertilized before they are
laid. Among salamanders in many species, the males deposit sper-
matophores containing sperm which are picked up by the females
and provide internal fertilization. A few species of salamanders
such as the fire salamander of Europe, Salamandra salamandra, give
birth to living young.

While the majority of amphibians lay their eggs in water, and
the young pass through tadpole or larval stages, there are many
exceptions. The eggs of the Texan cliff frog, Eleutherodactylus
latrans, are laid on land, as are the eggs of its relatives in Mexico,
and the tadpole stage is passed in the egg. Many salamanders lay
their eggs on land. Species in the Southwest, such as Plethodon
cinereus, usually lay their eggs in cracks and hollows in logs. The
slimy salamander, Plethodon glutinosus, lays its eggs in moist places,
often in the walls of caves. Some species of Oriental frogs are
reported to lay their eggs in trees high out of the water. There
is also reported a South African frog, Arthroleptella lightfooti, which
undergoes its entire development on land and cannot swim when
placed in water.

480 TEXTBOOK OF ZOOLOGY

The marsupial frogs of South America, Gastrotheca, carry their
eggs in a dorsal sac or brood pouch which is found in the female.
The Amazonian frogs Pipa and Protopipa carry their eggs and tad-
poles in individual dermal chambers on the back of the female. In
the ease of a small frog (Bhinoderma) in Chile, eggs are carried in
the vocal pouch of the male where they metamorphose and hatch as
fully formed young. Phyllolates and Dendrobates, two frogs from
the northern part of South America and Central America, transport
their tadpoles on the back of the male to the stream where they
pass the rest of their tadpole stage and metamorphose. In the case
of the obstetrical toad of Europe (Alytes olstetricans), the male
carries the eggs wrapped around his legs until they hatch.

Fig. 264. — Adult Ambystoma tigrinunij tiger salamander. (Photograph by Sanders.)

Secondary Sexual Characters. — Secondary sexual characters com-
pose those differences, exclusive of the reproductive organs, be-
tween males and females of a species. These differences may be
both structural and physiological. Familiar secondary sexual char-
acters are the nuptial pads of male frogs, their swollen thumbs
during the breeding season, and, in the male bullfrog, the size of
the tympanum, which is larger than that of the female. These
sexual characters may be various. In some salamanders the teeth
of the male may elongate; in others, glandular masses at the base
of the tail or elsewhere may be present in the males and absent in
the females. One of the most bizarre secondary sexual characters
is found in an African frog (Petropedetes newtoni). In this frog
the male has the columella of the ear pushed through the drum to
form a noticeable projection.

Hibernation. — ^Amphibia are more or less adapted to their en-
vironment; and, when winter comes, bringing low temperatures and

CLASS AMPHIBIA

481

a scarcity of food, most of them hibernate. Frogs crawl into the
mud in the bottom of ponds or other damp spots, dig into the ground
under logs, or crawl into cracks and crevices. Toads burrow into
the ground, the depth to which they go depending on the type of
soil. They may go as far as 18 inches underground in sandy
soil. Salamanders may bury themselves in the mud, under rocks
in running streams, in rotting tree stumps or in burrows in the
ground.

Fig-. 265. — ^Axolotl larva of the tiger salamander, Ambystoma tigrinum.

graph by Sanders.)

(Photo-

'■i'-S*. <«»',•'

"^m^:

'*g! *.«».(*'•

Fig. 266. — Ambystoma texanum, one of the most common salamanders in Texas.

(Photograph by Sanders.)

After establishing itself in hibernation quarters the amphibian
reduces all vital activities to a minimum. Respiration is carried on
entirely through the skin, and the body in its dormant state secures
the slight amount of nutriment needed from the food stored in its
tissues. In some hot countries during the dry, torrid season amphib-
ians aestivate in a protected moist place, reducing their activities
until the severest weather is over.

482

TEXTBOOK OF ZOOLOGY

Classification

There are estimated to be about 1,900 known species of living
frogs, toads, and salamanders in the world, and about 60 species

Fig. 267. — Typhlomolge rathbuni, the blind cave salamander of Texas. (Photo-

grapii by Sanders.)

''^^i.t ' ■ '^M

Fig. 268. — Pseudacris streckeri, Strecker's ornate chorus frog. (Photograph by

Thos. Mebane Jones.)

of caecilians. None of the caecilians have been reported from the
United States. In the United States there occur about 79 species
of salamanders and about 70 species of frogs and toads.* Many of

♦According to the Check List of North America Amphibia and Reptiles by
Stejneger and Barbour, 4th edition.

CLASS AMPHIBIA

483

these species are subdivided into several subspecies. The Southwest
contains a large proportion of all of these.

Some characters used in classifying salamanders are : the presence
or absence of gills, either external or internal; color markings;
shape and appearance of body; length; number of costal grooves;
number of digits ; position of teeth ; presence or absence of a naso-
labial groove ; plantar tubercles ; shape of vertebrae ; form of cranial
bones and cartilages; presence or absence of lungs ; presence or
absence of ypsiloid cartilage.

Some characters used in classifjdng adult frogs and toads are :
color markings; length of body and of hind limb; shape of head;

Fig. 269. — Tree frog, Hyla crucifer. (Photograph by Thos. Mebane Jones.)

nature of skin; presence or absence of parotoid glands and their
shape ; presence or absence of tympanum ; presence or absence of
cranial crests and their shape ; presence or absence of teeth and
their situation; the shape of the vertebrae; shape of the sacrum
and pectoral girdle ; shape of pupil of the eye ; presence or absence
of adhesive discs at the ends of digits.

The student interested in classification and identification of species
should consult appropriate keys for the various groups of Amphibia.
There is appended at the end of the book a list of references deal-
ing with this class of animals.

484 TEXTBOOK OF ZOOLOGY

A List of Families of the Amphibia in the United States

The ranges cited below are not exact but give an idea of the dis-
tribution of the genera.

Order Caudata (Urodela) (Tailed Amphibians)

Suborder Crypto'branchoidea
Family Cryptobranchidae

Cryptohranchus alleganiensis (1 species). This so-called "hellbender"
ranges from the eastern states west to Iowa, south to Louisiana.

Suborder Ambystomoidea
Family Ambystomidae
Amby stoma (13 species). Common species in the Southwest are: the Tiger
salamander {A. tigrinum) ; the Texan salamander {A. texanum) ; and the
Marbled salamander {A. opacum).
Dicamptodon ensatus (1 species). Eegion of San Francisco, Calif.
Bhyacotriton olympicus (1 species). Olympic Mountains, Wash.
Suborder Salamandroidea
Family Salamandridae

Triturus (5 species in the United States). The common newt of the South-
west is Triturus viridescens louisianensis. The other species represented is
T. meridionalis.
Family Amphiumidae
Amphiuma (2 species). A. tridactylum, the three-toed congo eel, ranges
from northern Florida to eastern Texas.
Family Plethodontidae

Gyrinophilus porphyriticus (1 species). Eastern states west to Kentucky,

south to Georgia.
Fseudotriton (2 species). Pennsylvania to Louisiana.
Eurycea (6 species). Range from New England to Texas.
Manculus quadridigitatus (1 species). North Carolina to Texas. This dwarf
salamander has only four toes.

Stereochilus marginatus (1 species). Dismal Swamp, Virginia to Georgia.

Typhlotriton spelaeus (1 species). The blind salamander of the caves of
Missouri and Arkansas.

Typhlomolge rathbimi (1 species). The blind cave salamander of Texas.

Leurognathus marmorata (1 species). North Carolina mountains.

Desmognathus (5 species). Southern Canada to the Gulf of Mexico, eastern
states westward to Illinois. Most common species in Southwest is D.
brimleyorum, Brimley's triton.

Plethodon (15 species). Distributed over almost the entire United States.
Common in the Southwest is P. glutinosus, the slimy salamander.

Hemidactylium scutatum (1 species). Canada to Louisiana. Another four-
toed salamander.

CLASS AMPHIBIA

485

Flethopsis wrighti (1 species). Oregon.

Batrachoseps (2 species). The worm salamander. Both species on the

Pacific.
Ensatina (3 species). All on the Pacific Coast.
Aneides (4 species). On Pacific Coast and in southeastern states.
Eydromantes platycephalus (1 species). Yosemite salamander.

Suborder Proteida

Family Proteidae (with external gills and 2 pairs of limbs)

Necturus. According to a recent revision of the genus by Mr, Percy Viosca,
of New Orleans, describing two new species from Alabama and two new
species from Louisiana, the number of species in the U. S. is increased
from three to seven. The common large Necturus from the Great Lakes
region is N. maculosus; the species which seems to be the most common
in southern states is N. heyeri Viosca, which extends into Texas.

Suborder Meantes

Family Sirenidae (with external gills, without hind limbs)

Siren (2 species). Eastern Virginia to Texas. Both S. lacertina and S.
intermedia are found in the Southwest.
' Pseudobranchus striatus (1 species). South Carolina to Florida.

Order Salientia (Anura) (Tailless Amphibians)

Suborder Amphicoela
Family Liopelmidae

Ascaphus truei (1 species). Washington and a few other points on the
Pacific Coast.

Suborder Anomocoela

Family Pelobatidae (Spadefoots)

Scaphiopus (5 species). One species in the East, one each in Florida and
California; four species in the Southwest. These are the spadefoot toads,
the pupils of whose eyes are vertical when in daylight.

Suborder Procoela

Family Bufonidae (Toads)
Bufo (17 species). Species of Bufo are distributed over the entire United
States. Among common species in the Southwest are B. cognatus, B. com-
pactilis, B. debilis, B. insidior, B. marinus, B. fowleri, B. punctatus, B.
valliceps, and B. woodhousii.
Family Leptodactylidae (Robber Frogs)

Leptodactylus labialis (1 species). Found only in Texas.

Eleutherodactylus (3 species). One species in Texas (Texas cliff frog), one

species in Arizona, one species in Florida.
Syrrhophus (2 species). Both species limited to Texas.
Family Hylidae (Tree Frogs)

Acris gryllus (1 species). The cricket frog, widespread throughout eastern
and central United States, including the Southwest.

486 TEXTBOOK OF ZOOLOGY

Fseudacris (6 species). Throughout the same regions as Acris. Various
subspecies of the swamp cricket frog (P. nigrita) are common in the
Southwest. The recently described P. streckeri Wright, ranging through-
out Texas, is a very colorful species, and its high-pitched staccato chirp is
one of the earliest to be heard at breeding pools in Texas.

Hyla (12 species). Various species in all of the United States. They are
the most colorful of all the frogs. Common species in the Southwest in-
clude: H. arenicolor, H. cvnerea, H. crucifer, H. squirella, H. versicolor.

Suiarder Diplasiocoela

Family Ranidae (True Frogs)

Baria (18 species). Various species occur in all parts of the United States.
Common species in the Southwest are: B. sphenocephala, B. pipiens, B.
catesbeiana, B. clamitans.

Family Brevicipitidae (Narrow-mouthed Toads)

Bypopachns cuneus (1 species). In southern Texas.

Microhyla (3 species). Ranges from Virginia to Texas, northward to Mis-
souri and Indiana.

Economic Importance

The entire group of Amphibia are of considerable economic value
because they feed to such a large extent on insects, thus becoming
valuable aids to the farmer in controlling noxious insects. In the
flooded rice fields of Louisiana, bullfrogs grow fat eating insects,
crayfish, and other small animals.

Frogs are used throughout the world as an article of food by
man as well as by other animals. In the eastern United States,
large quantities of the leopard frog and wood frog are consumed.
In the southern states, bullfrog legs have been a favorite food for
years. Within recent years businesses have developed which are de-
voted to supplying bullfrog legs, and the demands for these from
all parts of the country have been so great that it may become
necessary to afford some protection to prevent the rapid depletion
of these animals. Attempts have been made to operate frog farms
and raise a supply. Most of these attempts have been failures be-
cause of the high overhead cost. The axolotl is used in Mexico as
food; and water dogs, such as our Necturus, are reputed to have
a good flavor.

Dried frogs and toads have been used in China both as a source
of food and for medicinal purposes. It is reported that toad skins
have been used in Japan and elsewhere for making a fine type of

CLASS AMPHIBIA

487

leather. Dried salamanders have been used as a vermifuge. Adult
frogs and salamanders, as well as larval stages, are widely used
as laboratory animals.

Fig. 270. — Bufo valUceps is a common toad. (Photograph by Thos. Mebane Jones.)

NECTURUS MACULOSUS, THE MUD PUPPY

Necturus maculosus, the mud puppy or water dog, is a very com-
mon example of the salamander division of Amphibia found from
the Mississippi basin eastward, and is the one most commonly used
for laboratory study. It lives in ponds and streams, spending most
of the time in the mud at the bottom, but swimming and crawling
about at night. It comes ashore only occasionally. Insect larvae,
crayfish, worms, frogs, and occasionally fish comprise much of its
food.

Fig. 271, A. — Diagram of dissection to show principal organs of Necturus. 1, af-
ferent branchial artery, I ; 2, bulbus arteriosus ; S, afferent branchial artery, II ;
i, afferent branchial artery. III ; 5, ventricle ; 6, hepatic sinus ; 7, subclavian
artery; S, postcaval vein; 0, dorsal aorta; 10, right lung; 11, postcardinal vein;
12, liver; IS, hepatic portal vein; 11,, gastrosplenic vein; 15, pancreas; 16, gall
bladder; It, ventral abdominal vein; IS, testis; 19, kidney; SO, renal portal vein;
21, pelvic vein; 22, caudal vein; 23, mesonephric duct; 21/, gill slit; 25, external
gills; 26, internal jugular vein; 27, left auricle; 28, external jugular vein; 29, sub-
clavian vein; SO, common cardinal vein or duct of Cuvier ; 3/, gastric artery; S2,
left lunng ; SS, pulmonary vein ; Sk, stomach ; S5, spleen ; 36, pylorus ; 37, duodenum ;
S8, mesenteric vein; S9, ileum; J,0, large intestine; Jil, femoral vein; 1)2, urinary
bladder; JiS, vesical vein; H, cloaca. (Courtesy of General Biological Supply
House.)

CLASS AMPHIBIA 489

The group of vertebrates which Necturus represents is of par-
ticular interest because of its transitional position between aquatic
and terrestrial forms. Necturus is aquatic and fishlike in its pos-
session and use of external gills, although it has only three arches.
The body is used like that of a fish in SAvimming ; that is, by lateral
strokes of the tail against the water. It is terrestrial in the de-
velopment of pectoral and pelvic girdles and limbs for crawling.
Also lungs are developed for aerial respiration, although not highly
functional. The sixth or last pair of aortic arches of the primitive
series gives off a pulmonary artery and still retains the connection
to the dorsal aorta. This portion of this arch is known as the duct
of Botallus. The heart has become three-chambered; there are now
two auricles instead of only one as in fish. The posterior cardinal
veins are still present, but their function is partially taken over
by the newly developed post cava.

Necturus remains in a larval condition throughout its life, be-
comes sexually mature, and reproduces without metamorphosis. Such
a condition is referred to as neoteny. The retention of external gills
is a very marked larval feature.

Food and Digestive System

This animal is quite inactive and requires relatively little food. It
does make use of several aquatic inhabitants including crayfish, other
small crustaceans, snails, insect larvae, leeches, some minnows, and
occasionally fish eggs for food.

The mouth is located in the anterior, terminal position and with a
fairly wide gape. Teeth are located on the premaxillae, vomer, and
palato-pterygoid bones of the upper jaw, and the dentary and splenial
bones of the lower jaw. The tongue is broad and only slightly mov-
able. The internal nares enter the mouth cavity as a slit on each
side between the two dorsal rows of teeth near their posterior termi-
nations. More posteriorly, in the lateral walls of the pharynx are the
two pairs of gill slits or pharyngeal clefts. Still more posteriorly
there is a very small inconspicuous pharjoigeal prominence with a
tiny slit, the glottis. The esophagus leads posteriorly from the
pharynx and joins the anterior or cardiac portion of the prominent
stomach. The stomach has the typical shape and appearance of this
organ in the lower vertebrate groups, possessing only the cardiac por-
tion anteriorly and the posterior narrowed pyloric portion. This

490 TEXTBOOK OF ZOOLOGY

leads into the anterior section of the small intestine or duodenum,
which includes the first S-shaped turn of the tube. The more coiled
part of the small intestine following this is the ileum. This empties
into the short but somewhat broadened large intestine, which opens
into the cloaca, the common receptor of faecal matter from the intes-
tine and urinogenital products from the wolffian ducts, urinary blad-
der and miillerian ducts. The urinary Madder is a thin-walled sac
hanging at the ventral side of the cloaca, whose lumen it joins.

The liver is an elongated, dark-colored and somewhat serrated or-
gan lying in the ventral portion of the body cavity. The gall bladder
is a membranous sac attached to the margin of the liver (usually at
the right side). The bladder is connected with the duodenum by a
hile duct which is obscured by a mass of pancreatic tissue. The pan-
creas is divided into slender lobes and lies in the vicinity of the junc-
tion of stomach and duodenum. One lobe extends to the tip of the
spleen which lies dorsolateral to the stomach. Another slender lobe
extends posteriorly in the mesentery that supports the anterior part
of ileum and the mesenteric vein. Both of these lobes join the mass
of pancreatic tissue around the bile duct and the pancreatic ducts
enter the duodenum at this level.

Circulatory System

There is a partial conversion from the straight branchial type of
circulation of the fish to the pulmonary type of the terrestrial verte-
brates, in that the number of functional aortic arches is reduced,
pulmonary vessels are added to supply the lungs, and the atrium of
the heart is divided into two parts to keep the systematic and aerated,
pulmonary blood partially separated. With certain modification of
the situation in fish, the system consists of heart, aortic arches, sys-
temic and pulmonary arteries, as well as systemic, renal portal, hepatic
portal, and pulmonary veins. The heart consists of right and left
atria (auricles) and one ventricle with the two usual accessory
chambers, the sinus venosus which joins the right atrium and the
conus arteriosus which leads from the ventricle to the ventral aorta.
The right atrium receives the systemic blood and the left atrium, the
pulmonary blood by way of pulmonary veins. Blood in passing
through the heart may be traced by entering the sinus venosus from
systemic veins, pass by way of sinu-atrial (sinuauricular) valve to
right atrium, thence through the atrioventricular valve to the ventricle
which it enters simultaneously with aerated blood from left atrium.

CLASS AMPHIBIA 491

The blood is expelled from here through the conus arteriosus into the
ventral aorta. As will likely be remembered, the theoretical, primitive,
and embryonic typical number of aortic arches in vertebrates is six.
This number is modified in most adult vertebrates, usually by reduc-
tion. Even teleost fish have only four branchial arches. In sala-
manders this number may be referred to as four but considerable
modification has occurred. The first (anterior), second, and third
original arches have been rearranged and combined to form the com-
mon external, and internal carotid arteries. The fourth and fifth
supply the external gills with the fourth becoming the systemic arches
which meet dorsally to form the dorsal aorta. The sixth arch is
modified to supply a large pulmonary artery from each side to the
respective lung. The portion of the sixth aortic arch which continues
on dorsally to join the aorta, from the point where the pulmonary
branches off, is known as the duct of BotaUus.

The special modification of the veins is centered around the devel-
opment of the post cava which is formed posteriorly by the junction
of urinogenital veins. The pair of posterior cardinals, which are
characteristic of fish, are retained also but are greatly reduced. They
usually join the post cava some distance anterior to the kidneys and
parallel the aorta to the heart where they enter the ducts of Cuvier,
one on each side. Another modification is the pelvic-ventral abdomi-
nal complex which connects the renal portal arrangement with the
hepatic portal. A pelvic vein branches from the femoral on each side
before it joins the renal portal. The two pelvic veins pass ventrally
to meet each other at the midventral point of the pelvis and this
union forms the ventral abdominal vein which either enters the liver
or a branch of the hepatic portal before it enters the liver, thus pro-
viding a cut-off in the course of the venous circulation in going
anteriorly from the posterior limbs. The lateral veins of sharks form
a similar cut-off but enter the duct of Cuvier instead of the hepatic
portal system.*

Respiratory System and Breathing

The respiration may be divided into cutaneous, performed through
the wet skin ; branchial through the gills ; and pidmonary through the
lungs. There are several parts to the latter arrangement. The small
external nares lead by way of passages to the slitlike internal nares
which open into the mouth between the posterior ends of the two

♦Helpful Illustrations of the circulatoi-y system of Necturus may be found in
Stuart: Anatomy of Necturus maculosus, Denoyer-Geppert Co., Chicago.

492 TEXTBOOK OF ZOOLOGY

dorsal rows of teeth. The mouth is made airtight by the shape and
fitting of the lips. The general portion of this cavity posterior to
the angle of the jaws is the pharynx. Well back in the floor of it,
is the tiny slitlike glottis in the midst of a slightly thickened laryngeal
prominence, the opening of which would receive only an object the
size of the head of a pin. The glottis leads into a recess called the
larynx and the two smooth-walled, saclike lungs extend posteriorly
from this. These saclike lungs have a fairly abundant vasculariza-
tion (blood supply). The air is pumped into and from the lungs by
the movements of the floor of the airtight mouth and change of posi-
tion of visceral organs within the body cavity. Branchial respiration
is accomplished largely by waving the highly vascularized external
gills back and forth in the water. The capillary branching of aortic
arches 4 and 5 provides most of this blood supply to the gills. The pul-
monary artery supplying the lungs is formed by a large branch from
aortic arch number 6.

Urinogenital System

The following organs constitute this composite system : a pair of
mesonephric kidneys, a pair of gonads (testes in male, ovaries in
female) numerous vasa efferentia from testes, one pair of Wolffian
or mesonephric ducts (ducts of Leydig in male), one pair of Miil-
lerian ducts or oviducts (in female, only vestigial in male) single
cloaca, the urinary bladder and the mesenteries (mesovarium, meso-
tubarium, and mesorchium).

The kidneys are somewhat elongated and flat but thicker toward
the posterior, suspended in the dorsal peritoneum and lying dorsal
to the large intestine. The kidney of the female is smaller than
that of the male. The Wolffian duct leads from the lateral margin
of the kidney in either sex and proceeds directly from the posterior
portion of the kidney to make a dorsolateral entrance into the
cloaca. Inside the kidney the Malpighian corpuscles, including
glomeruli, are connected with the uriniferous tubules, which in turn
join the collecting tubules and they lead to the Wolffian duct. After
the urine enters the cloaca it collects in the urinary bladder which
hangs ventrally and serves as a storage reservoir. Upon becoming
filled with urine the bladder contracts and forces the urine back
into the cloaca and from here it passes to the exterior by way of
the anus.

CLASS AMPHIBIA 493

In the male specimen the yellow or brown-colored cylindrical
testes are located one in either side of the dorsal part of the body
cavity and each suspended by a fold of the dorsal peritoneum, the
mesorchium. The vasa efferentia, which are tiny sperm tubules
about the size of very fine threads, and the spermatic blood vessels
are suspended in this mesentery. The vasa efferentia enter the
medial side of the kidney (except at its anterior) and deliver
spermatozoa to a longitudinal Bidder's canal just within. This
canal is connected with the medial ends of collecting tubules and
through them the spermatozoa reach the Wolffian duct, as does the
urine. The Wolffian duct carries them to the cloaca. When the
spermatozoa reach the cloaca and bladder they clump into bundles
called spermatopliores, and are stored until breeding time. A Wolffian
duct (mesonephric duct) which serves both for conveying urine as
well as spermatozoa is called a duct of Leydig.

In the female the pair of ovaries can usually be recognized by the
presence of eggs of some stage of development in them. When fully
mature each ovary seems to be a large sac full of large yellow ma-
ture eggs about the size of small peas. In specimens with immature
ovaries the eggs may be about the size of pinheads. Each ovary is
suspended from the dorsal peritoneum by a mesentery, the meso-
varium. There is a prominent, coiled, white oviduct or Miillerian
tube in the body cavity at each side of the other organs whose an-
terior end is suspended in the anterior portion of the body cavity
and spreads into a wide membranous funnel called the ostium. The
mesentery which supports the oviduct is the jnesotuharium. When
the ova reach maturity inside the ovary they escape by a rupture
in its wall which frees them in the coelomic cavity. Due to the
shape of the body cavity and position of visceral organs these eggs
move to the anterior part of the cavity and the ciliated mouths of
the two ostia receive them one at a time in each. As these ova pass
down the MuUerian tube (duct) they are met by spermatozoa, fertil-
ization occurs, a mucous substance is added as a cover by the glands
in the oviduct. These fertilized cells in a pouchlike posterior part
of each oviduct which is called the uterus and after a few accumu-
late they are deposited by passing from the body by way of the
cloaca and anus. These zygotes (fertilized eggs) are deposited by
attachment to the under sides of rocks, logs, etc. in the water in
small clutches of from 25 to 90 individuals. The embryonic stages
are passed here and the larvae hatch out as tiny fishlike organisms.

494 TEXTBOOK OF ZOOLOGY

At about a year of age they are one and one-half or two inches in
length with a stripe down the side which gives them a peculiar
appearance.

The actual breeding and copulation activities (if any) do not
seem to be very well understood. There is a prevailing idea that
the spermatophores are passed from the male to the female in the
autumn and held in the genital tract of the female until the suc-
ceeding spring when the eggs mature and pass down the oviducts.
The act of transferring spermatophores is described as occurring
in shallow water or on the muddy margin of the pond or stream
by the male, depositing them here while the female follows and
collects them into the cloaca by use of its swollen lips, the papillae
there, and the mucus which is secreted by the cloacal glands that
lie at the sides of the cloacal aperture.*

Skeletal System

The skeleton of these animals is classified as a bony skeleton but
is not completely ossified and a considerable part of it is cartilage.
The axial portion consisting of skull, vertebral column and ribs;
and the appendicular portion, consisting of the two girdles with
limbs constitute the essential parts of this system. The skull is
platybasic (flat and broad) with a marked fusion and loss of primi-
tive bones when compared with the teleost fish. The anterior, dorsal
surface of it is covered by a single, fused frontal bone, posterior to
which, and extending beneath and somewhat lateral to this, is the
large pair of ixirietals. At the anterior tip of the frontal are the
premaxillae which bears teeth. Just posterior to this and somewhat
covered by the frontal is the vomer, which also bears teeth. Both
the nasals and maxillae are absent. The braces, at the side of the
skull, are the palatopterygoid bones, each of which bears a few
teeth; the quadrate cartilage; quadrate hone, which articulates with
the lower jaw ; and the squamosal, which appears more dorsally. The
otic group is represented only by the prootic, a small, irregular one
which lies between the anterior part of the squamosal and the pari-
etal, and another small one, the opistliotic, which is at the postero-
lateral corner of the skull. The foramen viagnum (large opening)
is located at the mid-posterior position and an occipital condyle is
located at each side of it for articulation with atlas, the first vertebra.
The principal part of the floor of the skull consists of the large flat

•Helpful Illustrations of the urinog-enital systems of Necturus may be found
In Stuart: Anatomy of Necturus maculosus, Denoyer-Geppert Co., Chicago.

CLASS AMPHIBIA 495

parasphenoid. The lower jaw is composed of a pair of each, dentary
bones, which bear teeth; splenial bones, which bear the last few teeth;
and the angular bones, devoid of teeth and articulating with the
quadrate of the skull. Necturus usually has forty-six amphicoelous
vertebrae. They articulate with each other by anterior and posterior
zygapopJiy.ses as well as the ends of the centra. There is one cervical
vertebra, atlas, with which the skull articulates. Posterior to this
one are about eighteen thoracolumbar vertebrae each of which bears
a pair of short Y-shaped ribs. Each rib has a double head (bicipital) ,
the dorsal head or tuherculum articulating with the transverse process
of the vertebra and the ventral head or capitulum articulating with
the side of the centrum. Following the thoracolumbar group is a
single sacral vertebra to which the ilium of each side is attached by
way of the sacral rib. The remainder of the series, posterior to this
point, consists of caudal vertebrae.

The pectoral girdle is principally cartilage in structure. The ven-
tral portion is formed by a posterior coracoid cartilage in the muscles
of the body wall, and an anterior procoracoid. Projecting dorsally
and laterally is the third unit of each side, the scapula. The most
dorsal, free margin of this is frequently referred to as suprascapula.
The recess formed at the junction of scapula with the ventral parts
into which the arm articulates is called the glenoid fossa. The skele-
ton of the anterior appendage includes the proximal humerus (in the
brachium), the radius and idna in the forearm (antebrachium), six
carpals in the wrist, four metacarpals in the palm, and four digits
each composed of joints or phalanges.

The pelvic girdle is likewise largely cartilage, but it is fused in the
midventral line. The anterior, ventral part is the puhic plate con-
sisting of cartilage, posterior to this is the pair of iscMa which are
partly ossified. Extending dorsally on each side is a slender ilium
which joins the sacral rib and this in turn the sacrum. In the lateral
position where the ilium meets the two ventral parts of the girdle is
a concave recess into which the head of the femur of the thigh articu-
lates. This recess is called the acetahidum. Distal to the thigh is
the shank with two bones, the tibia and the fihula lying parallel to
each other. There are six somewhat fused tarsals in the ankle. Distal
to this are the four elongated metatarsals and beyond each is the digit,
composed of phalanges.*

•Illustrations of the skeleton of Necturus may be found in Stuart: Anatomy
of Nectwrus maculosus, Denoyer-Geppert Co., Chicago,

496

TEXTBOOK OF ZOOLOGY

Muscular System

The muscles of the body are divided into segmental myotomes with
intervening connective tissue sheets or myosepta. A horizontal septum
along the side of the body divides the muscles into a dorsal, epaxial
portion and a ventral hypaxial portion. The principal sets of super-

Fig. 271, B, — Left lateral view of the muscles of the head and shoulder re-
gion of the salamander, Nectiirus maculosis, (From Atwood, Comparative Yerte-
brate Dissection, The Blakiston Company.)

i— ~ ~ rectus abdominis

myotomes '•

pectineus
I _^ pubofemoralis -■
■p internus '^

ischiofemoralis— ""

rectus exteraus

■-gracilis • *- -

- pubotibialis
femorofibulariS:- -

- ischiocaudalis''

gluteeus maxim us
_/— - semimembranosus
pyrifonnis— ''
flexor commimis -
extensor communis

Fig. 271, C — The muscles of the hind legs of Necturus maculosus ; ventral view
on the left, dorsal view on the right. In the dorsal view tlie ilium has been cut
from the pelvic girdle and deflected downward. 1, 2, and 3 are extensors of the
foot, and 4 is a flexor. (From Atwood, Comparative Vertebrate Dissection, The
Blakiston Company.)

CLASS AMPHIBIA 497

ficial muscles or those of the head and gills, body wall, and the
appendages. Because of the development of the terrestrial limbs,
the latter group is much more complicated in this amphibian than
it was in the fishes. For the detailed information concerning the
specific muscles the student will depend on the accompanying illus-
trations and the laboratory study.

The Nervous System and Sense Organs

Since this system resembles that of the fish Avhich has been studied
already, and is so closely similar to that of the frog, which is de-
scribed in the next section of the book, it seems unnecessary to
describe it here.*

THE BULLFROG

Habitat

The bullfrog is a solitary animal except during the breeding sea-
son. It is strictly aquatic and does not leave the pools as does the
leopard frog. It prefers bodies of quiet water where there are both
shallows and deeper water, such as lagoons, small lakes, and the
cypress ponds of swampy regions. In such a situation, the shore is
protected by low willows or other trees, and the shore waters are
filled with aquatic plants, pickerel weeds, and floating lily pads.
These furnish not only a good hiding place but a good hunting
ground for the crayfish, insect larvae, water beetles, snails, and
other aquatic organisms which make up the bullfrog's diet. This
diet is quite varied and may even include younger frogs.

Bullfrogs are found in North America east of the Kockies from
Canada to Mexico. They have also been introduced into the western
portion of the United States and into various foreign countries.

External Structure

Bullfrogs obtained in the South and Southwest are usually of two
species, Rana catesheiana Shaw, the common bullfrog, or Bana grylio
Stejneger, the southern bullfrog. Individuals of the former species
attain larger sizes, and the giant bullfrogs of the southern swamps
usually are Rana catesheiana. The two species differ not only in size
but also in external appearajice, particularly when alive. However,
they are essentially the same anatomically, and this chapter is based
on a study of Rana catesheiana.

•Illustrations of this system mav be found in Stuart: Anatomy of Necturus
maculosusj Denoyer-Geppert Co., Chicago.

498

TEXTBOOK OF ZOOLOGY

The common bullfrog is ordinarily greenish or olive brown. Un-
derparts are mottled with dark spots on a white background, and
the upper surfaces may be plain or marked with large dark splotches.
The legs are marked with crossbars and other splotches of dark
color. Preserved specimens appear brownish gray with the dark
mottling lighter in color than on the living specimen.

The body of the bullfrog includes the head and trunk. Attached
to the trunk on either side anteriorly are the forelegs and posteriorly
the hindlegs.

The head has two prominent eyes which protrude above its sur-
face. These can be drawn back into their orbits and forced some-

Fig. 272. — External features of the common bullfrog, Rana catesbeiana. (Courtesy
of Southern Biological Supply Company.)

what into the mouth cavity. The lower lid of the frog's eye with
its attached nictitating membrane is drawn up over the eye, not by
independent movement of the eyelid, but as a result of the retraction
of the eye into the orbit. The upper eyelid is immovable. Back
of each eye is a circular oval area, the tympanum or eardrum. In
the females this is about the size of the eye, while in the males it
is larger than the eye. A small fold of skin, the tympanic fold,
runs from the eye around the posterior margin of the tympanum.
The two nostrils or nares are near the anterior part of the head, and
each is guarded by a valve. The mouth reaches from one side of

CLASS AMPHIBIA 499

the head to the other and has an upper and lower jaw. The anus
or vent is at the extreme posterior end of the trunk.

The forelimbs are composed of the upper arm, which joins the
trunk, the forearm, wrist or carpus, and the hand with its four digits.
In the male, particularly during the breeding season, the innermost
digit, or thumb, is enlarged, whereas the thumbs of females remain
apparently the same size. The digits may have tubercles on them,
and their positions in relation to various bones of the hand give rise
to specific names for these tubercles. The forelimbs are used not
only to help support the body but also as an aid in pushing food
into the mouth.

The hindlimbs are long and have powerful muscles. Bullfrogs
ordinarily leap about three feet but can easily cover a distance of
five or six feet. The hindlegs are composed of the thigh, which
joins the trunk ; the shank; and the ankle, or tarsus. Following the
tarsus is the foot with five digits (toes), which are connected by a
web, producing a very efficient swimming organ.

The smooth damp skin, which is soft and loosely attached to the
body except in the head region, is composed of two layers, an outer
epidermis and an inner dermis. The skin is pigmented and very
rich in mucous glands, which aid in keeping it moist. Bullfrogs
moult or shed the superficial layer of epidermal cells of their skin
at varying intervals.

Dig^estive System and Digestion

The mouth cavity, or buccal cavity continues directly into the phar-
ynx with no sharp line of bounjiary between them. The latter narrows
toward the esophagus, which is a short gullet leading directly from the
pharynx to the stomach. The lining of the esophagus has a number
of longitudinal folds and is ciliated. The stomach normally lies on
the left side of the body. It is curved, with the convex side toward
the bullfrog's left. Its anterior or cardiac end is wide, and the
pyloric or posterior end is narrowed and constricted where it joins
the small intestine. The duodenum, or anterior part of the small
intestine, runs forward almost parallel with the stomach. At the
point where the intestine turns back posteriorly the duodenum be-
comes the ileum, which composes the remainder of the small intestine
and is considerably coiled. The large intestine or rectum is sharply

500

TEXTBOOK OF ZOOLOGY

marked off from the small intestine and is wide and short. It passes
directly into a muscular part, the cloaca, which terminates in the anus
or vent.

The buccal cavity has in its roof near the end of the snout two
patches of small conical teeth, called vomerine teeth. In addition, the
upper jaw has a single series of small conical teeth on its edge known
as maxillary teeth. These teeth serve primarily to help hold the cray-
fish, insect, or other animal captured for food, and they may help at
times in crushing it. The tongue is somewhat leaflike in shape and is
deeply notched behind, making it bicornute. Its anterior half is at-
tached to the floor of the mouth just back of the tip of the lower jaw.

Vomerine teeth

Fbor of orbit M/'

Isophaqus M,.^^,

Vocal 5ac

Maxillary teeth
. InLcmol nares

Sulojs marqinalis

_ lustach'ian tuoe
Qlotth

C Tonque

Fig. 273. — Mouth or buccal cavity of the bullfrog.

and its posterior end is free. In order to get the tongue out of the
mouth the posterior part has to somersault over the attached ante-
rior part. The tongue of the bullfrog is somewhat smaller pro-
portionally than that of the grass frog, as might be expected, for
the latter is more dependent on this organ when it hunts insects
inlajid. Taste buds are present on the tongue and palate.

Esophagus, stomach, and intestine have an outer longitudinal and
an inner circular layer of smooth muscle. The peristaltic contrac-
tions of these muscles pass the food through the digestive tract and
aid in mixing it with the gastric juice in the stomach. They may
also be used to regurgitate a disagreeable substance swallowed by
the frog, in which case the stomach turns inside out and protrudes
into the mouth cavity. The stomach can be greatly expanded and
acts as a reservoir for food which may be available only at irregular
intervals and the frog has to take advantage of a food supply when

CLASS AMPHIBIA 501

it is present. The mucosa of the intestines has a number of longi-
tudinal and transverse folds which produce a great absorptive sur-
face through which the digested food can be taken up by the blood
stream and transported to different parts of the body.

The liver lies on each side of and behind the heart. It is three-
lobed, two lobes being on the left and one on the right, connected
by narrow bridges of liver tissue. Between the right and left lobes
is the gall Madder, which receives an alkaline secretion known as bile
from the liver and stores it until needed in the process of digestion.
Bile is carried from the gall bladder to the duodenum by the hile
duct, which passes through the pancreas on its way. The liver is
not primarily a digestive gland, for, while the bile it secretes per-
mits the fats to be more easily digested by a lipase from the pan-
creas, the bile itself contains no digestive enzymes. Although its
function in altering fatty substances is important, of prime impor-
tance is its ability to store glycogen and the fat upon which a hiber-
nating frog lives. It is also concerned in the formation of urea
and in the destruction of red blood corpuscles.

The pancreas lies in the loop between the stomach and duodenum.
It is a long, whitish, irregularly-lobed gland whose alkaline secre-
tion is of considerable importance in digestion, for it contains three
digestive enzymes. This secretion is taken from the pancreas by
pancreatic ducts which empty into the bile duct that passes through
the pancreas before entering the duodenum near its beginning.

Intestines, liver, and pancreas are covered with peritoneum. The
mesenteries which hold the body organs in position and the internal
surface of the body wall likewise are made up of this peritoneal
membrane.

Digestion. — Since frogs live primarily on insects, crayfish, and
other small invertebrate animals, their food is very rich in proteins.
Their vomerine and maxillary teeth are too feeble to do more than
slightly crush their prey, so digestion begins in the stomach. Here
the gastric glands secrete hydrochloric acid and an enzyme, pepsin,
which converts the proteins to peptones. Peristaltic contractions of
the stomach cause a thorough mixing of the gastric juice with the food
and then this partly digested food (chyme) is passed posteriorly into
the small intestine. Here, activated by the acid nature of the food,
the intestinal glands release into the blood stream a substance, secretin,
which on reaching the pancreas causes it to pour forth into the duo-

502 TEXTBOOK OF ZOOLOGY

demim its highly alkaline secretion. In addition, this pancreatic juice
contains three digestive enzymes : trypsin, which continues the diges-
tion begun by pepsin in the stomach, converting proteins to amino
acids; an amylase, annjlopsin, which changes starches into sugars;
and a lipase, steapsin, which, aided by the bile, causes a splitting
of the fats into glycerol and fatty acids. Bile also contributes to the
alkaline condition here.

The process of digestion is completed in the intestine and the
food products are taken up by absorption in its mucosa layer. These
foods in solution are taken by the blood stream and lymph vessels
to various parts of the body where they are utilized for building
tissue or for supplying energy, leaving as by-products urea and
carbon dioxide. Sugars that are not used are stored as glycogen in
the liver and in voluntary muscles. The liver also serves to store
fats and to secrete urea and sugar directly into the blood stream.

Food that is not digested passes to the large intestine where it is
retained for a time and then passed to the outside through the anus
as feces.

Other Glands. — Attached by a mesentery to the wall of the intes-
tine near the anterior end of the rectum is the spleen. It is a small,
reddish, spherical, Ijnnphoid organ, the functions of which are but
incompletely known. The destroying of red blood corpuscles is an
important duty, as possibly also is the formation in its tissues of
lymphocytes, one type of white blood corpuscle. In mammals the
spleen is also believed to accumulate iron freed by the metabolism
of other tissues. This iron is subsequently used in the formation
of hemoglobin.

The two thyroid glands are small and lie in front of the glottis
under the floor of the mouth. There is one on each side of the hyoid
apparatus. The secretion and functions are discussed in the chapter
on Internal Regulation.

A thymus gland lies under the skin behind the tympanic membrane
on each side. It is partly covered with muscle and is small. Further
discussion of it will be taken up in the chapter on Internal Regulation.

Circulatory System

The circulatory^ system comprises the Mood vascular system and the
lymphatic system. The two systems are closely interrelated in that
they both carry to the tissues of the body nutritive material neces-

CLASS AMPHIBIA 503

sary for metabolism aud remove from them to the excretory organs,
waste products of body activity. They differ in several respects;
the lymph neither contains red blood corpuscles for transporting
oxygen nor moves in a continuous closed vascular circuit as does
the blood. Other differences will be noted in the discussion.

The Blood Vascular System. — The blood moves through a closed
system of tubelike vessels of various sizes which distribute it to
all parts of the body. The pump is the heart, which, by its con-
tractions, forces the blood to flow to the tissues. Since the system
is a closed one, the blood eventually returns to the heart.

The blood vessels leading away from the heart are the arteries.
When these reach the tissues, they break up into very small vessels,
the capillaries. The vessels leading back to the heart are the veins.
The arteries and veins are connected by the capillaries.

Blood is comprised of a clear liquid called the 'plasma, suspended
in which are blood corpuscles of three kinds, the red blood corpuscles
or erythrocytes, the white blood corpuscles or leucocytes, and the
spindle cells or thrombocytes. In addition, the blood may contain
dissolved nutritive substances from the digestive system, waste
products from tissue repair aud destruction, hormones being trans-
ported from organs of one part of the body to another, or foreign
substances accidentally introduced.

The capillaries are very small vessels, the walls of which are made
up of endothelium continued from the linings of arteries and veins.
The}^ connect the distal ends of the arteries with the proximal ends
of the veins, but in so doing they branch extensively and anastomose
to form fine networks in the tissues invaded. Through their thin
walls, acting as semipermeable membranes, food products brought
by the arterial blood pass into the tissues, oxygen is unloaded from
the red blood corpuscles, and carbon dioxide and waste products
are taken up to be conducted into the veins. Leucocytes are able
to get out of the capillaries, squeezing their way between the cells
of the capillary walls, and thus become free in the surrounding tis-
sue to engulf bacteria or other harmful objects.

The abundance of the capillaries varies with the activity of the
organ ; the greater the rate of metabolism the greater their abundance.
Examples of such are the various glands and the mucous membrane
of the digestive tract, in contrast, a tendon has few capillaries.

504

TEXTBOOK OF ZOOLOGY

The arteries are large vessels with elastic walls and carry blood
from the heart to the capillary networks in the various organs and
tissues of the body. The arteries arise from the conus arteriosus

1 /

Irtternal care ^^ '

■\uricL:.iari '--■

I .5»> ,-j1

_ ■-; ■

/ <

\nl3 "iC- r

■r,3SinT.:.r:c^:.

PosX L :

— *>

/:. ■ ,

-' ■

:,ta: . ■. ....

Dorsal

aorta

Iliur^

■'.

Fig. 274. — X-ray picture of bullfrog with arterial system and a portion of
venous system injected. All labels indicate arteries except where otherwise noted.
(X-ray courtesy of Dr. Malcolm B. Bowers.)

which divides just above the auricles into a right and left truncus
arteriosus. Each of these trunks splits into three arches going to

CLASS AMPHIBIA

505

each side of the body, the anterior carotid arch, the middle, systemic
arch, and the posterior, pulmo cutaneous arch.

The Carotid Arch. — Each carotid arch divides into two branches.
The more ventral, Ungual artery, or external carotid, passes forward,
giving branches to the thyroid, pseudothyroid, muscles of the hyoid

External c

Auricularis

ral
Occipital
Internal carotid

Cutaneous
Carotid qiand

Conus arteriosus-
Pulmonary.

Systsmic arch

Lateralis
Dorsalis

Cozliaco.rnesenteric

Brachial
Vertc bra I

\^Left qastric

PANCREAS

].Riijht Cjastric
l_Coeliac
Anterior
mesenteric

Splenic

(Irinoaenital

Epigastric
Viae

Recto.vesical
Sciatic

■■RECTUM! pogfer/or mescnter/c
, -'h^^^^^emora I
i h

Fig. 275. — Arteries of the bullfrog from ventral view. (Drawn by Ruth M.

Sanders.)

and tongue, and then extends along the edges of the lower jaw.
The internal branch is larger and is called the internal carotid. It
has at its base a spongy enlargement known as the carotid gland
which by its structure serves to steady the pressure of blood passing
into the artery. This artery follows the side of the neck to the base

506 TEXTBOOK OF ZOOLOGY

of the skull, giving off the palatine artery to the roof of the mouth,
the cerebral carotid which enters the skull and supplies the brain,
and the ophthalmic artery to the eye.

The Systemic Arch. — The systemic arch soon after it leaves the
truncus supplies a small laryngeal artery to the larynx and mus-
cles of the hyoid. It then curves downward and around the esopha-
gus on each side. It gives off an occipitovertehral artery which
sends a small artery to the dorsal side of the esophagus, then branches
at the spinal cord into the occipital artery, running anteriorly on
the dorsal side of the skull to the orbit and tympanum, and the
vertebral artery, turning posteriorly along the spinal column. Imme-
diately posterior to the occipito vertebral artery the large subclavian
artery arises from the systemic arch. It branches to the shoulder and
adjacent body wall and enters the arm as the brachial artery.

The systemic arches from each side, after curving under the ali-
mentary canal, meet near the anterior end of the kidneys and fuse
into a single large artery, the dorsal aorta, which extends posteriorly.
At or just posterior to this meeting point, there arises from the aorta
the large coeliacomesenteric artery which divides into an anterior
branch, the coeliac artery, and a posterior branch, the anterior mes-
enteric artery. The coeliac artery divides into right and left gastric
arteries. The latter runs directly to the dorsal or left side of the
stomach, while the former sends off small pancreatic arteries to the
pancreas ; a larger hepatic artery to the pancreas, gall bladder, and
liver; and continues to the ventral side of the stomach, where it is
distributed. The anterior mesenteric artery gives off the splenic
(lienal) artery to the spleen and then divides into two parallel ves-
sels which send numerous smaller arteries to the small and large
intestines.

The urinogenital arteries consist of about four to six small much-
divided arteries which are given off from the ventral side of the dorsal
aorta to right and left, supplying the kidneys, reproductive organs,
and fat bodies. A few small lumbar arteries arise either as branches
of these or directly from the aorta and go to the body wall on each
side. The small posterior mesenteric artery is given off near the
posterior end of the aorta, passing to a portion of the rectum and,
in the female, to the ovisac. It often anastomoses on the rectum with
descending branches of the anterior mesenteric,

CLASS AMPHIBIA

507

Near the posterior end of the body cavity the dorsal aorta divides
into two iliac arteries going to the hind legs. Each of these gives
off (1) an epigastric artery suppljang the bladder and dorsal and
ventral body walls of the region, and (2) just below it, a femoral
artery passing to the body wall, skin, and proximal muscles of the
thigh. As the iliac artery enters the leg, a rectovesicular artery is sent

Mandibular

Brachial.

Sinus

venos

Cardiac-
Hepatic.

Cutanecus-

!!^lnternal jut^ular
External juijular

. Subscapular

Innominate

-Pre cava I

Posterior
vena cava

Soermatic

Dorso. lumbar-

LIVER
LOWER L^FT ;^^0;v^.^\

I.OBC .>-.V ^^^\ Hepatic
portal

Gastric

Splenic
/.Mesenteric

SPLEEN

Renal

Renal portal.
Vesical

External iliac
Fe moral -

Fig. 276. — Veins of bullfrog from ventral view. (Drawn by Ruth M. Sanders.)

off to the rectum, bladder, and skin on the dorsal surface of the
thigh. In the upper leg the continuation of the iliac, now called the
sciatic, gives off a branch to the right and to the left, supplying the
muscles, and then continues down the leg, sending off several branches
at the knee.

The pulmo cutaneous arch takes blood to the respiratory organs:
the lungs, skin, and buccopharyngeal cavity. The pulmocutaneous

508 TEXTBOOK OP ZOOLOGY

arch on each side divides into a pulmonary artery to the lungs and
a large cutaneous artery, which passes outward to the skin. Impor-
tant branches of the cutaneous are: the auricularis, supplying the
tympanum and adjacent head region ; the dorsalis, supplying the skin
of the back ; and the lateralis, which is distributed to the skin of the
side.

The Veins. — These vessels usually parallel the arteries that brought
blood to the tissues from which the veins are returning it. The walls
of the veins are thinner and not as elastic as those of the arteries.
Many veins, particularly those of the limbs, have semilunar valves
on the internal surface of the wall which open in the direction of
flow and prevent the backflow of blood.

In returning blood to the heart, the venous system carries some of
the blood through the kidneys or through the liver, providing renal
or hepatic filters to eliminate urea and other waste products from the
blood or to alter it chemically. Pulmonary veins from the lungs
carry oxygenated blood, which differs from the type of blood found
in the other veins.

The venous circulation, therefore, may be divided into four main
systems: the systemic, hepatic portal, renal portal, and pulmonary
systems.

The systemic veins carry the greatest load of blood to the heart.
The larger collecting veins of the system consist of two precavals
receiving blood from the anterior parts of the body, except the lungs,
and a single postcaval or posterior vena cava receiving blood from
the posterior parts of the body. The two precavals empty into the
anterior end of the sinus venosus of the heart, and the posterior vena
cava empties into its posterior end.

Each of the two anterior precavals receives blood from three
branches: (1) the external jugular bringing blood from the tongue,
hyoid, thyroid, pseudothyroid, and floor of the mouth; (2) the in-
nominate vein, made up of a fusion of the internal jugular returning
blood from the brain and other parts of the head, and the subscapular
vein bringing blood from the back of the arm and shoulder; and
(3) the subclavian vein, a fusion of the brachial vein, returning blood
from the forelimb, and the large musculocutaneous vein, which forms
an ellipse down the side of the body and extends up into the head
region, returning blood from the skin and outer muscles in these
regions.

CLASS AMPHIBIA 509

The large posterior vena cava originates between the kidneys and
receives blood from each kidney by five or six renal veins, from the
gonads by small spermatic or ovarian veins, and from the fat bodies
by other small branches. Near the heart the vena cava receives two
large hepatic veins from each side of the liver.

The Hepatic Portal System. — This system is comprised of two
chief veins, the hepatic portal vein and the ventral abdominal vein.
These veins, instead of carrying blood directly to the heart, bring
it to the liver to pass through a netAvork of sinusoids (modified
capillaries). It is returned to the systemic system through hepatic
veins that join the postcaval.

Veins from the large and small intestines unite to form the mesen-
teric vein which is joined as it progresses forward by the splenic vein
from the spleen, pancreatic veins from the pancreas, and gastric
veins from both sides of the stomach. The vessel resulting from
these unions is the hepatic portal vein. It passes through the anterior
portion of the pancreas and sends a large branch into the lower left
lobe of the liver. At about this point it often receives a final gastric
branch which has passed on top of the pancreas to join it. It then
continues a short distance to join the abdominal vein just below the
heart.

The abdominal vein arises as follows: Two large veins, the sciatic
and femoral, bring blood from the hindlimbs. The femoral, as it
enters the body cavity, gives off the pelvic vein. The pelvic veins from
each side of the body join in the middle to form the large ventral
abdominal vein. As the abdominal vein runs toward the heart along
the median portion of the ventral body wall, it receives vesicular veins
from the bladder, parietal veins from the body wall and, at its ante-
rior end, a cardiac vein from the heart. In the region of the liver
it leaves the body wall, is joined by the hepatic portal vein, and enters
the right and upper left lobes of the liver by short branches, dis-
charging its blood into sinusoids.

The Renal Portal System. — This system, like the hepatic portal
system, diverts blood to a purifying organ instead of carrying it
directly to the heart. In this ease, the blood is taken to the kidneys.

The outer femoral vein and the medial sciatic vein collect blood
from the hindlegs. The femoral vein, after giving off the pelvic vein,
runs anteriorly and joins the sciatic, to make the renal portal vein.
Near the kidney this vein receives the dorsolumbar vein from the body

510 TEXTBOOK OP ZOOLOGY

wall and, in the female, several vessels from the ovisacs (uteri). The
renal portal vein follows the dorsolateral margin of the kidney, send-
ing numerous transverse branches into the organ, where they break
up into capillaries. Blood which passes through these capillaries is
purified of some of its waste products and then leaves the kidney
through the renal veins which empty into and originate the poste-
rior vena cava of the systemic system.

Pulmonary Veins.— These veins run along the inner walls of each
lung, returning the oxygenated blood to the heart. The right and left
pulmonary veins unite to form a single vessel which empties into the
left auricle on its dorsal side. Other veins which take on oxygen are
those coming from the skin and buccopharyngeal cavity.

The Heart. — The heart is enclosed in the pericardial cavity, which
is lined by a transparent tissue, the pericardium, and is separated
from the remainder of the body by the transverse septum. It is the
rhythmically contracting organ that circulates the blood. It is coni-
cal in shape and in the frog consists of a right and left thin-walled
auricle above a single thick-walled ventricle. On the ventral side
is a muscular tube, the C07ms arteriosus, described with the arteries.
It conducts blood away from the heart. On the dorsal side of the
heart is a thin-walled sac, triangular in shape, the sinus venosus,
which receives venous blood from the systemic veins.

The sinus venosus empties into the right auricle through the sinu-
auricular aperture. This aperture has liplike valves on each side to
prevent the blood from flowing back into the sinus when the auricle
contracts. The smaller left auricle receives oxygenated blood from
the pulmonary vein. Valves are not necessary at this opening, for
pressure on the auricular w^alls tends to close the small oblique aper-
ture when the auricle contracts.

Both auricles pass blood into the ventricle through a common open-
ing, the auriculoventricular aperture, which is divided by the inter-
auricular septum separating the two auricles. This aperture has two
large valves on each side and two small valves at each end which
regulate the discharge of blood into the ventricle and prevent its
backflow.

Blood leaves the ventricle and enters the arterial system through
the conus arteriosus. The opening into the conus is protected by
three pocketlike semilunar valves which open inwardly into the conus
when blood is passing out but are tightly closed at other times. The

CLASS AMPHIBIA

511

proximal portion of the couus is known as the pylangium, and the
distal portion as the synangkim. Running through the length of the
pylangium is a longitudinal spiral valve, one edge attached to the
dorsal wall of the pylangium and the other edge lying free in the
vessel. Upon contraction of the conus this structure is brought into
contact with the ventral wall and helps direct the flow of blood into
the arches.

Near the anterior free end of the spiral valve where it is the
widest, there is a pair of small synangial valves which, together

Carotid A

System.A

Pulmocuta-
neous f\.

R auricle -
Spiral valve

Conus orte-_
nosus

Semilunar vaUz

Truncus arteriosus

'^-Pulmonary aperburz

-Sinu-auricular aper-
ture

—Loft auricle

— Intzrauricular sep-
tum

--Dmtle inpulmo-

cutaneous A.

- -/luricufo- ventricular
valve

- -">:7 Ventricle

Fig. 277.-

-Heart of frog with the ventral wall removed and bristles shown through
the arteries of the truncus arteriosus.

with the end of the spiral valve, separate the pylangium from the
synangium. Just below these valves is an aperture which leads into
the trunk formed by the union of the two pulmocutaneous arteries.
The synangial chamber is very short and gives off almost imme-
diately two large branches, one to the right and the other to the
left. In each of these branches originate the three main trunks or
arches of the arterial system. They are formed by two longitudinal
septa dividing the vessel into three compartments. All three trunks

512 TEXTBOOK OF ZOOLOGY

are therefore enclosed in one large vessel for a short distance before
breaking up into three separate vessels. The carotid arch originates
from the anterior compartment, the systemic arch from the middle
compartment, and the pulmocutaneous arch from the posterior com-
partment. Blood enters the anterior and middle compartments from
the synangium, but enters the posterior compartment, or pulmocuta-
neous arch, from the pylangium.

The heart beats in a wavelike peristaltic manner. The sinus venosus
contracts first, then the auricles (the right auricle preceding the left
by a moment), then the ventricle, and finally the conus.

Venous blood from the right auricle enters the right side of the
ventricle, and oxygenated blood from the left auricle enters the left
side. Muscular ridges of the ventricular wall tend to hold the blood
and reduce mixing. Since the heart's contractions are wavelike,
the ventricle immediately forces the blood into the conus through
the semilunar valve. Venous blood from the right auricle is closest
to the conus, and it passes out first, flowing into the closest open-
ing offering the least resistance. This is the opening in the
pylangium to the pulmonary arch, leading to the lungs. As the
contraction of the ventricle comes to an end, forcing out the re-
maining oxygenated blood, the pylangial part of the conus contracts,
bringing the spiral valve against its ventral wall. This action,
together with that of the synangial valves which are anterior to
the common opening of the pulmonary arches, completely shuts off
the flow of blood into these arches. The blood therefore passes into
the synangium and enters the chambers leading to the systemic
arteries or the carotid arteries. Since the carotid arteries offer
some resistance to blood flow, the blood tends to enter the larger
systemic arteries first. As the systemic arteries fill, they offer more
resistance to the blood, while resistance in the carotid arteries de-
creases due to their emptying into capillaries ; so the last oxygenated
blood from the ventricle passes into the carotids and is conveyed
to the head region.

The heart must beat sufficiently fast and pump a sufficient volume
of blood at each stroke to insure an adequate supply of oxygen and
food to the body tissues, as well as to remove waste products as
they form. The rate of pulsation is influenced greatly by tempera-
ture up to a certain maximum rate, for the activity and metabolism
of the bullfrog are considerably affected by temperature. Blood

CLASS AMPHIBIA 513

pressure is increased by a constriction of the smaller arteries or
arterioles. Their muscular walls may contract from stimuli received
from the nervous system or from hormones.

Blood corpuscles, which are of three kinds, float in the plasma.
The erythrocytes are flattened and elliptical, with an oval nucleus
in the center. They contain a pigment, hemoglobin, which has the
property of absorbing oxygen. The colorless thrombocytes or
spindle cells are not as large as the erythrocytes but resemble them
except for their tapering ends. When these cells contact certain
foreign bodies, they break up, releasing a substance that causes,
upon contact with air, the coagulation of certain proteins in the
blood plasma in which blood corpuscles become entangled, forming
a clot. The insoluble protein strands thus formed are called fibrin
(see chapter on The Vertebrate Animal). After the frog has been
injured, the formation of a clot prevents indefinite bleeding and
makes it possible for the tissues to begin repair.

The white blood corpuscles or leucocytes are of three kinds: lym-
pJiocytes, monocytes, and granulocytes. Their outline is irregular,
due to their amoeboid movement, and the shape of their nuclei
varies greatly. They are much less numerous in the blood stream
than are the red blood corpuscles and spindle cells. Leucocytes
may escape from blood capillaries and engulf bacteria and other
harmful substances in the tissues. They are finally returned to the
venous system by lymphatic vessels. Worn out corpuscles are re-
moved from the blood stream by the liver and spleen. The spleen
seems to be the primary organ concerned in supplying new blood
corpuscles except for a period in the spring when the bone marrow
may produce some. Leucocytes may also increase by fission.

Ljnnphatic System. — The lymphatic system of the bullfrog is an
open system comprised of a series of large irregular sinuses in vari-
ous parts of the body. It collects lymph from the tissues and
eventually returns it to the veins. The lymph is a colorless fiuid
containing leucocytes but no erythrocytes. It is derived from seep-
age of plasma from the capillaries. It bathes all of the cells, col-
lects wastes, and distributes food products. In the region of the
intestinal tract, lymphatics absorb a considerable amount of fat
and are called lacteals. Lymph removes cellular debris and trans-
ports leucocytes which engulf harmful material and cleanse the tis-
sues of the body.

514 TEXTBOOK OF ZOOLOGY

Between the skin and muscle are a series of subcutaneous lymph
sacs; other sinuses are in the mesenteries, around the vertebral
column, and elsewhere. The peritoneal and pericardial cavities are
connected with the lymphatic system. Nephrostomes on the ventral
surface of the kidney convey lymph from the peritoneal cavity into
the renal veins.

Respiratory Organs and Respiration

Air enters through the nostrils, passes into a small olfactory
chamber and then into the mouth cavity through the internal nares,
which open in the roof of the mouth. The mouth is kept tightly
closed in breathing. Air is sucked in by lowering the floor of the
mouth and is then forced into the lungs by raising the floor, the
external nares being closed by valves. This pushes the air through
the slitlike glottis immediately behind the tongue in the floor of the
mouth, thence into a short larynx which connects with the lungs.

The walls of the larynx are reinforced by a framework of cartilage,
and the laryngeal chamber supports two horizontal fleshy folds, the
vocal cords, which extend across the passageway. When a frog
croaks, its mouth and nostrils are kept tightly closed, and the air
is forced back and forth between lungs and mouth cavity, causing
the vocal cords to vibrate. The sound is amplified in male frogs
by the vocal sacs which act as resonating chambers. In the bullfrog
the two internal vocal sacs have openings into the floor of the
mouth at each corner, and, when inflated, they swell out under the
throat and sides of the body in the region of the lungs. Bullfrogs
frequently call under water.

The two lungs lie dorsal to the heart on each side and dorsal to
the liver. They are very elastic sacs with their inner walls raised
into a number of ridges, forming chambers which are called alveoli.
These chambers are richly supplied with a network of blood vessels
for facilitating the oxygenation of the blood. In the bullfrog the
lungs are also important as a hydrostatic organ,

"While the lungs play the major role in respiration, other factors
are of considerable importance. The lining of the mouth of the bull-
frog contains a large number of blood vessels and serves for a type of
respiration known as huccopharyngeal respiration. With the glottis
closed, air is drawn into the mouth cavity and forced out by rhythmi-

CLASS AMPHIBIA 515

cal movements of the throat. Oxygen is taken np by blood vessels in
the lining of the month by diffusion.

The skin of the bullfrog plays a large part in its respiration, and
frogs that are not protected from drying out soon die. Gaseous ex-
change of carbon dioxide and oxygen can take place through the
moist vascular skin, and, since its area is large, it serves effectively
as a respiratory organ. This type of respiration is known as cutane-
ous respiration. During hibernation, practically all respiration of
the bullfrog is of this nature. Even at other times, the skin releases
more carbon dioxide than do the lungs. The functions of respira-
tion are discussed in the chapter on The Vertebrate Animal.

Excretory System and Excretion

The two kidneys lie between the parietal peritoneum and dorsal
body wall in the posterior region of the body cavity. They are
dark red in color, flattened and elongated. They are made up of a
very great number of uriniferous tuhules. A mesonephric duct runs
from the posterior lateral border of each kidney and empties into the
dorsal side of the cloaca. The urinary bladder also opens into the
cloaca but does so on its ventral surface, and the ducts do not join
the bladder. The bladder is a two-lobed sac with very thin walls
which stores the urine collected from the cloaca. When filled, the
bladder contracts and forces the urine back through the cloaca and
outside through the anus. Embedded in the ventral surface of each
kidney is a yellowish red patch, the adrenal gland, which will be dis-
cussed in the chapter on Internal Regulators.

The waste products resulting from the vital processes of destruc-
tion, repair, and growth in the body must be removed if the organism
lives. These are taken from the tissues by the blood and more espe-
cially by the lymph. We have already mentioned the expulsion of
carbon dioxide and water through the skin and lungs. Another prod-
uct of protein metabolism is urea. This soluble crystalline substance,
formed to a large extent in the liver from the nitrogen of protein
metabolism, enters the blood stream and is removed by the kidneys.
The kidneys also remove foreign substances from the blood and pass
these to the outside through their mesonephric ducts and the cloaca.

Frogs and toads excrete considerably more urine per day propor-
tionally than does man, although this may vary considerably, for in
some forms the bladder may act as a filter for water which is used

516

TEXTBOOK OF ZOOLOGY

over and over. It has been estimated that, while man excretes about
one-fiftieth of his weight per day, the frog excretes about one-third
of its weight. During hibernation and aestivation, however, in com-
mon with the slowing down of its other body functions, the kidney
function of the frog is practically stopped.

Ostium --

Postcaval V

Ovaty-

Oviduct —

L.intest'im —
Uterui

Fig. 278.-

Cloaca 1?-

Anus

-Urogenital system of the frog from ventral view. Male organs shown on
one side, female on the other.

The kidney is not only concerned with the elimination of waste
products but also has other functions. One of these is the reabsorp-
tion by its tubules of useful substances, such as some of the salts
and glucose which have filtered out, and their reintroduction to the
blood stream. In their food frogs obtain less sodium chloride than
do mammals, and this is compensated for in part by a retention of
salts from the water taken in, while in mammals water is retained
and the salts are eliminated.

CLASS AMPHIBIA 517

Another function is in maintaining the concentration of body
fluids. Frogs absorb water through their skin at a rather constant
rate, varying with the temperature. The kidney in turn expels
water at the same rate and thus maintains the proper balance. In
addition to its usual function the urinary bladder may be used as a
storage reservoir for water during temporary drought. The water
may be absorbed from it by other tissues until the proper osmotic
equilibrium of the tissues with the blood is produced. The excretory
function is further developed in the chapter on The Vertebrate An-
imal.

Skeletal System

The bullfrog has no exoskeleton, its body being covered by
smooth skin. The endoskeleton may be considered in two main divi-
sions, the axial and appendicular portions. The axial part includes
the skull and vertebral column; the appendicular portion consists of
the bones of the limbs and their supports, the pectoral and pelvic
girdles.

Bones are joined to one another by structures made up of connec-
tive tissue which allow varying degrees of movement between them.
These structures are called joints or articulations. In some cases, as
in the skull, the joints are immovable and the bones are separated only
by a thin sutural ligament of connective tissue. In other cases, the
joints are slightly movable, as in the vertebral column where a plate
of dense tissue and cartilage connect the vertebrae. In still other
cases the bones are freely movable, as in the limbs, and here the bones
are entirely separated, but are held in place by ligaments.

The Axial Skeleton. — The skull, which is composed of cartilage,
cartilage bones, and membrane bones, forms a case for the brain and
capsules for the sense organs. The frog's cranium has considerably
more cartilage than do the skulls of higher vertebrates and less than
those of lower vertebrates. The cartilage bones are so called because
of their origin in cartilage which has subsequently been partly re-
placed by ossified tissue, forming bones separated by sutures. These
cartilage bones are found at various points on the cartilage box that
composes the foundation of the cranium. Cartilage bones are the
sphenethmoids, pro-otics, exoccipitals, pterygoids, palatines, and car-
tilaginous quadrates. The membrane bones develop from ossifications
of membranes which cover the cartilage and cartilage bones. They
are thin and may be separated from the others. The membrane bones

518

TEXTBOOK OF ZOOLOGY

are the premaxillaries, maxillaries, nasals, frontoparietals, quadrato-
jugals, squamosals, parasplienoids, and vomers. The bones enclosing
the brain constitute the cranium.

On the dorsal surface of the cranium, the two frontoparietals
form most of the roof, the pro-otics form the roof of the auditory

— PrzmaxlUary

Moxi//ary

-— -v-%— 5phenethmo(d

^— Frorko-par\eba\

■J- Pterygoid
- - Squamosal
-\\— Pro otic
I- Squamosal

—Quadratojuqal
Columella
Fig. 279. — Dorsal view of the skull and upper jaw of the bullfrog.

- Premaxillary

— Palatine

fronto-parietal

y^- Parasphenoid

-Squamosal

'^ -Quadrabjugal

-Quadrate car-
' $. tilaqe

Fig. 280. — Ventral view of the skull and upper jaw of the bullfrog.

capsule (inner ear capsule), the sphenethmoids form the posterior
wall of the olfactory capsule (nasal chamber), and the two tri-
angular nasal bones lie above. On the ventral surface of the era-

CLASS AMPHIBIA

519

nium are the slender palatines extending laterally on each side
from the anterior end of the sphenethmoid to the upper jaw. The
vomers form the floor of the olfactory capsules, and their ventral
surfaces bear the vomerine teeth. The parasphenoid forms the
floor of the brain case.

At the posterior end of the cranium is a large opening, the foramen
magnum, through which the spinal cord passes. On each side of this
opening are the exoccipital bones. Each bone has a rounded projec-
tion at its base, an occipital condyle, which articulates with the ver-
tebral column.

Dorsal fissure ^ f\ Neural spine

OraymatteK. \ | I , Neuralarch

Zygapophysis ^^^^^_ ?^\^<^ W^C^ ^/^ y^ Tram'^erx process

VJhibzmatten -^^^^"..^^^'^^^^^^^^^^'ssss^livW IXiramaker

Dorsal root _ '^:i^£^!!!!!^^^^^::::::i^^ — Pla mater

Ventral fissum. ^/^^^^^^^ i- \^ Ventral root

I
Centrum

Fig-. 281. — structure of a single vertebra and cross section of the spinal cord.
(Redrawn and modified from Holmes, Biology of the Frog, by permission of The
Macmillan Company, after Howe, Atlas of Zootomy.)

Visceral Skeleton. — The visceral skeleton is that part of the axial
skeleton which consists of the jaws and hyoid apparatus in the
adult. The gill arches of the tadpoles are included in this portion.
These parts originate in cartilage which is later partially replaced
and reinforced by ossifications. The hyoid apparatus is primarily
cartilaginous and serves as a support for the base of the tongue and
the larynx. According to some authors, the jaws and the hyoid
were originally the branched arches supporting the gills, and evi-
dence of this is seen when the frog tadpole breathing with gills
transforms to the frog breathing without gills.

The upper jaw consists of a pair of short premaxillary bones in
front, a pair of long maxillae fonning the sides, and a pair of short
quadratojugals as the posterior portions. The premaxillae and
maxillae each bear a row of small conical teeth.

520 TEXTBOOK OF ZOOLOGY

The lower jaw is formed primarily of a cartilaginous rod known as
Meckel's cartilage. At the extreme anterior tip of the jaw the rod is
ossified to form two small bones, the mentomeckelian bones. It is sub-
sequently covered anteriorly by a dentary bone and posteriorly by an
angulosplenial bone. The jaws are attached to the cranium by a com-
bination of three bones on each side, the squamosal, pterygoid, and
palatine, to form a suspensory mechanism.

The vertebral column is made up of a series of nine typical verte-
brae and a long bone, the urostyle, which includes a fusion of the
vertebrae of the tadpole tail.

In the neck region, there is one cervical vertebra, the atlas, which
articulates with the skull. This is followed by seven trunk vertebrae,
then one sacral vertebra whose processes support the pelvic girdle,
and finally the urostyle, which contains all of the caudal vertebrae
fused into one piece.

The basal portion of the typical vertebra is known as the centrum.
The centrum is concave in front and convex posteriorly, and there-
fore is procoelous except one vertebra which is ampliicoelous in Rana.
Attached to the centrum is a bony arch, the neural arch, which ex-
tends dorsally from the centrum around the spinal cord. The neural
arch has extending from its sides, at the point of union with the
centrum, a pair of riblike transverse processes to which muscles are
attached. A dorsal projection of the neural arch is the neural spine.
In addition, the neural arch has at each its anterior border and pos-
terior border a pair of processes known as zygapophyses by which
the vertebrae are coupled together, the posterior zygapophyses
of one vertebra overlapping the anterior zygapophyses of the succeed-
ing one (Fig. 281). This arrangement furnishes a protected canal for
the spinal cord and a firm axial support which also allows bending of
the body. The spinal nerves emerge between vertebrae through
intervertebral foramina protected by the cartilaginous pads between
the vertebrae.

Appendicular Skeleton. — The anterior portion of the appendicu-
lar skeleton is composed of the pectoral girdle, sternum, and bones
of the forelimbs. The posterior portion has the pelvic girdle and
bones of the hindlimbs.

The pectoral girdle and sternum furnish a support and place of at-
tachment for the forelimbs and their muscles. They also provide a
case to protect the heart, lungs, and other organs in the anterior part
of the body. This girdle is not connected to the vertebral column.

CLASS AMPHIBIA

521

Each side of dorsal part of the girdle is composed of a large flat
bone, the suprascapula, which curves ventrally and joins the scapula,
narrowing as it does so. From the ventro-anterior end of the scapula
two bones extend to the midventral line of the body and would meet
their fellows from the opposite side except that the narrow epicora-
coid cartilage intervenes. The anterior of these two bars is the
clavicle and the posterior one the coracoid. At the junction of cora-
coid and scapula a depression is formed, known as the glenoid fossa,
into which the forelimb articulates. The ventral sternum is separated

^j-^y -^ Lpisternum

s- — Omosbernum

Fig. 282. — Diagram of the ventral view of the pectoral girdle of Rana catesbelana,

natural position.

into two portions by the pectoral girdle. The anterior portion is com-
posed of a bone, the omosternum, to which is attached anteriorly a
rounded plate of cartilage, the episternum. The posterior portion is
composed of a bone, the sternum proper (mesosternum), and a round-
ed cartilage, the xiphisternum, which has a notch at its posterior mar-
gin through which the abdominal vein runs as it leaves the body wall.
The pelvic girdle furnishes a place of attachment and support for
the hindlimbs. Each half of the pelvic girdle is composed of three
bones, the ilium, ischium, and piibis. The more slender ilium is at-

522

TEXTBOOK OF ZOOLOGY

tached anteriorly to the transverse process of the ninth vertebra, and
posteriorly it fuses with the pubis and ischium, forming a dishlike
concavity, the acetahulum, which receives the hindlimb. The pubis
forms the ventral part of the acetabulum, the ischium the posterior,
and the ilium the anterodorsal.

The forelimbs join the body by a ball and socket joint at the
glenoid cavity in the pectoral girdle. The large bone which makes
this articulation is the humerus. The succeeding bone of the forearm
is the radio-ulna, a fusion of two originally distinct bones. The
wrist, which follows, contains six carpal bones arranged in two rows.
Each hand, or manus, contains four metacarpals following the carpals,
and distal to these are four complete digits and an exceedingly small
rudimentary fifth near the thumb, the prepollex, consisting of only

8^? vertebra

Sacral

diapophyses
cylindrical

Uium

Urostyle

Acetabubm

- -Ischium

Fig. 283. — Pelvic girdle of Uie bullfrog, dorsal view.

a single bone. Each of the four digits, or fingers, extends from a
metacarpal bone. This is followed in digits II and III by two
phalanges and in digits IV and V by three phalanges.

The hindlimhs have essentially the same structure as the fore-
limbs. The large bone which joins the girdle at the socketlike
acetabulum is known as the femur. This bone articulates with the
tihiofibula, which, like the bone of the forearm, is a fusion of two
bones. The tarsus or ankle differs from the wrist, being composed
of two long bones, the tihiale and fibidare, and two small tarsals.
There are also two extremely small bones forming the prehallux, or
rudimentary sixth toe. Distal to the tarsals are five long meta-
tarsals. Each foot contains five complete digits, each following a
metatarsal bone. In digits I and II are two phalanges, in digits III
and V three phalanges, and in digit IV four phalanges.

CLASS AMPHIBIA

523

Muscular System

Muscular tissue controls the movements and positions of various
parts of the body of the bullfrog. This it does by contracting, that
is, by shortening and thickening its elements.

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524 TEXTBOOK OF ZOOLOGY

Movements may be under voluntary control, as the skeletal muscles,
involved in moving the limbs, in which case the muscle fibers are
striated and are known as voluntary muscle. Other movements, such
as the heartbeat and the peristaltic movements in the intestines, are
not under control of the will. Muscles concerned in these actions
are known as involuntary and are usually made up of smooth muscle
fibers except in the heart, which contains striated cardiac muscle.

Most voluntary muscles are attached to bones at one end or at both
by specialized connective tissue bands known as tendons. The end of
the muscle which is attached to a relatively fixed and immovable part
is called the origin; the end which is attached to the part which
moves when the muscle contracts is known as the insertio7i. A typical
voluntary muscle is made up of three parts : the tendons attached at
its ends; the membrane surrounding the muscle, known as the fascia;
and the belly, or fleshy part, of the muscle.

The different actions performed by the various skeletal muscles
give rise to descriptive names applied to them. Some of these are as
follows :

Extensor — one that straightens a part, such as extending the foot.

Flexor — one that bends a part, such as a joint.

Adductor — one that draws the limb toward the median ventral line.

Abductor — one that draws the limb away from the median ven-
tral line.

Levator — one that raises a part, such as the lower jaw.

Depressor — one that lowers a part, such as the lower jaw.

Rotator — one that rotates one part on another.

The 'pectoral muscles cover the chest and ventral portion of the
upper body region ; the rectus al)dominis extends along the median
ventral region ; the paired ohliquus externus and internus cover most
of the sides of the trunk. The muscles of the limbs are numerous.
There are some eighteen separate muscles which control various move-
ments of the legs. A detailed description of these and other muscles
of the frog would be confusing to the elementary student and there-
fore is not included. The major muscles of the hind leg are illus-
trated in Fig. 284 and can be clearly understood after a careful dis-
section in the laboratory.

CLASS AMPHIBIA

525

Nervous System

The three divisions in which the nervous system of the bullfrog
may be considered are: (1) central nervous system, (2) peripheral
nervous system, and (3) sympathetic nervous system.

o/facb3ry tracb
.oifflct»rylobe'

Cerebrum

Optic nerve

Pineal body

J)/Gnccphalon

i^tic lobe

Cerebellum

Nedulla oblongata

4Lh ventricle

Jjlossophatynqeal

.Vacjof nerve

J^ spinal nerve

22^ jpinal nerve

Trigeminus

Facia/,
Auditory

Thoracic
enlargement

}

Brachial
plexus

5piT)at cord

Lambar.
enlargement

^ 5'^ jpinal mrve

.+^ .spinal nerve

_ JZalcareoui body
5S! jpinal nerve

,.6t2) ipinal nerve

^ spinal nerve

-_a*-bjpinal nerve

5 — spinal nerve

jotb jpinal nerve

Sciatic plexus

Jdatic nerve

Fig. 285. — Dorsal view of the nervous system of a frogr.

The central nervous system, so called because it comprises the
larger number of nerve centers, consists of the brain and spinal cord.
The peripheral nervous system consists of (1) the paired cranial and
spinal nerves which connect the brain and spinal cord with other

FiJumterminale.

526 TEXTBOOK OF ZOOLOGY

organs of the body and (2) a large number of small nerve centers,
ganglia, distributed throughout the body. The sympathetic nervous
system is a part of the peripheral nervous system. It is made up of
a large number of small ganglia, two rows of which form the sym-
pathetic trunks on each side of the vertebral column and connect with
the spinal nerves. The branches of these sympathetic trunks connect
with numerous small ganglia throughout the tissues of the body.
This system controls and regulates primarily the involuntary move-
ments of such organs as the heart, digestive tract, glands, organs of
respiration, and walls of blood vessels.

Central Nervous System.— The brain is covered with a pigmented
membrane known as the pia mater. The brain has three main divi-
sions, the forehrain, midbrain, and hindbrain. The forebrain consists
of a pair of elongated cerebral hemispheres, separated from each other
by a fissure, and two enlargements at the anterior end of the hemi-
spheres known as the olfactory lohes. These lobes are fused on the
dorsal side but separated by a groove on the ventral side. Immedi-
ately behind the forebrain is the diencephalon. On its dorsal sur-
face is a vestige of the pineal organ which was more developed in the
tadpole. On its ventral surface is the optic chiasma, a crossing of
the optic nerves formed by fibers from the right and left sides, each
crossing to supply the eye of the opposite side. Just behind the optic
chiasma is the inf undibulum, and somewhat behind this is the pituitary
body, or hypophysis. The pituitary is of dual origin, developing in
part from the diencephalon and in part from the roof of the mouth
cavity. The midbrain contains two large rounded optic lobes. The
ventral part of the brain below these lobes is the crura cerebri.

The hindbrain consists of the cerebellum and the medulla oblon-
gata. The cerebellum in the frog is almost rudimentary and consists
of a transverse fold of tissue immediately posterior to the optic lobes.
The cerebellum is in close connection with the large triang-ular
medulla oblongata which constitutes the most posterior part of the
brain and is continuous with the spinal cord.

Internal Organization. — The central nervous system is hollow.
In embryological development the central cavity is large ; but, as
maturity is approached, the walls thicken, and the cavity, particu-
larly in the spinal cord, is much reduced. In the brain these cavi-
ties, known as ventricles, form a continuous channel for the flow of
cerebrospinal fluid. The ventricles are connected one with another

CLASS AMPHIBIA 527

by openings known as foramina. The cavities are large in four
regions: (1) the paired lateral ventricles in the cerebral hemispheres,
(2) the single third ventricle in the diencephalon, (3) the paired
optic ventricles in the optic lobes, (4) the single large triangular
fourth ventricle in the medulla oblongata. Vascular nets of blood
vessels in the much-folded pia mater constitute clioroid plexuses
that form the roofs of the third and fourth ventricles and extend into
the other ventricles somewhat. Most of the cerebrospinal fluid is de-
rived from the blood vessels of these plexuses.

The spinal cord is continuous with the medulla oblongata ante-
riorly, runs posteriorly through the canal formed by the vertebrae,
and finally tapers to a narrow filament which ends in the urostyle. It
is covered by two membranes, an outer dura mater and an inner pia
mater. It is somewhat flattened, and a median fissure occurs on both
its dorsal and ventral sides. The central part of the cord comprising
its bulk is made up of gray matter consisting primarily of nerve
cells. In the center of this gray matter is a small hollow canal, the
neurocoele, which communicates with the ventricles of the brain.
Surrounding the gray matter is white matter consisting chiefly of
nerve fibers.

Peripheral Nervous System. — The peripheral nervous system is
composed of the cranial, spinal, and sympathetic nerves, the last of
which will be considered separately.

The cranial nerves arise from the brain, and there are ten pairs of
them in the bullfrog. Counting from the olfactory lobes backward,
they are as follows: olfactory, optic, ocidomotor, trochlearis, trigem-
inus, ahducens, facial, auditory, glossopharyngeal, and the vagus.
All of these, with the exception of the tenth or vagus nerve, run to
parts of the head. The vagus nerves branch to the heart, lungs, and
digestive system.

The bullfrog has ten pairs of spinal nerves. Each spinal nerve
originates from the gray matter in the spinal cord by a dorsal and
a ventral root. These roots pass out of the vertebral column between
vertebrae through an opening or intervertebral foramen and unite
into a nerve trunk, branches of which extend to the muscles and skin
of the body and limbs. The dorsal root is known as the sensory or
afferent root and has a ganglion; the ventral root is known as the
efferent or motor root and has no ganglion. "Where these roots meet

528 TEXTBOOK OF ZOOLOGY

after leaving the spinal cord, they are covered on the ventral side by
a large calcareous body, the periganglionic gland, or "gland of
Swammerdam. "

The first spinal nerve arises between the first and second vertebrae,
the second between the second and third vertebrae, and so on until
the tenth, which is small and emerges from the urostyle near its an-
terior end. These nerves frequently send branches to preceding or
succeeding nerves to form plexuses. Two large plexuses in particular
are present. Branches from the first and third nerves join with the
large second nerve to form the trachial plexus, which supplies nerves
to the muscles of the forelimbs and shoulder. Nerves number seven,
eight, and nine fuse to form the large sciatic plexus which supplies
the sciatic nerve to the hind leg.

Sympathetic Nervous System. — From the first sympathetic gan-
glion, nerves are given off which form a cardiac plexus on the heart.
Another plexus, formed primarily from nerves of the third, fourth,
and fifth sympathetic ganglia, is the solar plexus on the dorsal surface
of the stomach. In addition, numerous ganglia are scattered through-
out the tissues of the body, all being connected by sympathetic nerve
fibers and finally communicating with the sympathetic trunks. The
cooperation of certain cranial and spinal nerves with the sympa-
thetic in relation to the involuntary actions of a number of the vital
internal organs is referred to as the autonomic function.

The Sense Organs

The olfactory sacs, or nasal chambers, are located internal to the
external nares. The median portion of the nasal chamber is lined
with olfactory epithelium which contains sense cells possessing proto-
plasmic processes known as olfactory hairs on their free ends. These
olfactory hairs are stimulated by chemical substances present in the
air and pass the stimuli received through the olfactory cells to the
olfactory nerves.

The degree to which the sense of smell is used by amphibians is not
known. It is likely, however, that it may cause the frog at times to
approach objects and may serve to test the food substances it takes
into its mouth.

The eyes lie in cavities, or orbits, on the dorsolateral sides of the
head. The exposed portion of the eyeball is covered by a transparent
membrane, the cornea, which is continuous with the opaque connec-

CLASS AMPHIBTA

529

tive tissue sheath covering the remainder of the eyeball and known
as the sclera. Attached to the sclera are several muscles which move
the eye in various directions. The ms of the bullfrog is colorful,
being either golden or reddish bronze, and is clearly visible through
the transparent cornea. In its center is an oval opening, the pupil,
which can be contracted or expanded by the action of muscle fibers in
the iris and, like the shutter of a camera, regulates the amount of
light which enters the inner chambers of the eye. The lens lies
behind the iris and is flattened on its outer surface. It is enclosed in
a membrane and held in place by delicate fibers to the ciliary body.
The space between the cornea and lens is filled with a watery trans-
parent substance, the aqueous humor.

(^^terlorverticafj^

Ampulla

-"-■-^ — Sacculus

Fig. 286. — The right internal ear of the frog, lateral view.

The main cavity of the eye back of the lens is filled with a gelati-
nous tissue, the vitreous humor. The walls of this cavity are made
up of three layers, the outer sclerotic coat, previously mentioned,
then a vascular pigmented chorioid and the innermost layer, the
retina. The anterior portion of the chorioid forms the ciliary body,
which subsequently is continuous with the iris.

The retina contains the photosensitive ceUs of the eye which pass
the stimuli received on to the optic ner\-e. These sensitive cells,
known as the rods and cones, lie embedded in the tissue so that light
has to pass through several layers of nerve fibers, as well as much
supporting tissue, before reaching them. The rods and cones com-
municate with fine branches of the optic nerve, which enters the eye
posteriorly.

530 TEXTBOOK OF ZOOLOGY

Sharpness of vision is dependent on both the proper focusing of
the lens and the proper amount of light reaching the retina. When
the light is too strong, the pupil of the iris contracts and cuts down
the volume. The eye of the frog has little if any accommodation or
focusing of the lens. It therefore has very imperfect vision.

The ear of the bullfrog is covered externally by a membrane, the
tympanum. A Eustachian tube runs between the middle ear and
mouth cavity. The tympanum has attached to it a bony rod, the
columella, the other end of which is joined to a portion of the inner
ear. This rod transmits sound vibrations from the tympanum to the
inner ear.

The inner ear lies in a cavity of the skull known as the auditory
capsule. The structures of the inner ear compose a membranous
labyrinth which is surrounded by a lymphlike fluid, the perilymph.
The labyrinth is formed of a dorsal utriculus concerned with equilib-
rium and a ventral sacculus functioning as an auditory organ. The
utriculus is connected with three semicircular canals which are placed
in planes almost at right angles to one another. Two are vertical
canals, and the third, on the outer side of the utriculus, is hori-
zontal. The sacculus is irregular, pouchlike, and filled with a fluid,
the endolymph. It also contains the nerve endings which receive the
stimuli and convey them to the auditory nerve.

Sound progresses in the following fashion. The tympanic mem-
brane vibrates to sound waves, and these are transported by the
columella to the inner ear. These vibrations are taken up by the
endolymph of the sacculus and are received by the nerve endings
which lead to the auditory nerves. These nerves convey the im-
pulse to the brain, subsequently giving rise to auditory sensations.

In a similar manner, movements of the endolymph in the utricu-
lus affect sensory cells and cause a reaction associated with a sense
of position or equilibration.

Sound and hearing play an important role in the life of frogs,
the calls of the males serving to attract the females and others to
the ponds during the breeding season. They are of prime importance
in the daily life of the terrestrial toad, who is on the alert when
an insect has announced its location by a sound.

CLASS AMPHIBIA 531

Reproductive Organs

The ovoid testes (Fig. 278) of the male bullfrog are attached to each
kidney by a fold of peritoneum. In this fold of peritoneum, running
between the testes and kidneys, are several small ducts, the vasa
efferentia. These ducts connect with the mesonephric duct through
the collecting tubules of the kidney. Spermatic fluid containing the
spermatozoa passes from the testes through the vasa efferentia into
the kidney, then into the mesonephric duct, which opens into the
cloaca, and thence to the outside through the anus. In some species,
this duct is slightly expanded prior to its opening into the cloaca to
form the seminal vesicle, a reservoir for spermatozoa. This is poorly
developed in the bullfrog.

The two ovaries of the female bullfrog, when filled with eggs,
occupy a large part of the body cavity and consist of folded sacs
covered with peritoneum. They originate in about the same posi-
tion as do the testes and lie in a fold of the peritoneum ventral to
the kidneys. The eggs lie in the outer surface of the ovary and
during their growth are surrounded by a network of blood vessels
and follicle cells.

The two oviducts are greatly convoluted white tubes, one on each
side of the body cavity, running from near the base of the lungs to
the dorsal wall of the cloaca. Their anterior ends are funnel-shaped
and open into the body cavity. Their posterior ends are dilated to
form thin-walled ovisacs or uteri which open into the cloaca near the
entrance of the mesonephric duct. They are not connected at any
point with the ovaries.

When the eggs are mature at the breeding season, they break
through the walls of the ovary and its peritoneal covering and are
free in the body cavity. They make their way to the funnel-shaped
opening, or ostium, of the oviduct and, probably by ciliary action or
movements of the female, are squeezed into it. The oviducts contain
a large number of glands which secrete a clear, jellylike material.
As the eggs are forced down the oviduct by ciliary action, they
become coated with the gelatinous material, which swells enor-
mously when it contacts water.

Fertilization in the bullfrog is external, and the spermatozoa of
Die male enter the eggs after they have been laid in the water.

532 TEXTBOOK OF ZOOLOGY

Attached to the anterior end of the testes of the male frog and
to the ovaries of the female are fingerlike projections known as
fat bodies. These serve to store a reserve fat supply which the bull-
frog may draw on during hibernation or at other times. They are
largest before hibernation and smallest after egg lajdng. Recent
experiments have also shown that these fat bodies are essential for
allowing the normal development of the sex organs and for maintain-
ing their health. When they are removed, there is a deterioration of
eggs and sperm.

Embryology

The bullfrog lays its eggs in a large floating mass, forming a sur-
face film on the water, usually among brush or plants near the
pool's edge. This mass may be from 1 to 2^/2 feet in diameter and
may contain ten to twenty thousand eggs. In Texas, bullfrogs may
lay their eggs as early as February, though it is more common for
them to be laid later in the season.

The eggs of the bullfrog are smaller than those of the leopard
frog. They hatch in about four or five days, depending on the tem-
perature. After hatching, the tadpole normally spends about two
years in the water before transforming as a young bullfrog. The
tadpole may grow to be four to six inches long, but the average
body length of the young bullfrog as it metamorphoses is about
1% to 2 inches. It usually takes about three to four years for this
young frog to attain maturity and begin egg laying.

The embryology of the bullfrog does not differ materially from
that of the leopard frog, and the following account is based, except
where otherwise noted, upon the development of the latter.

The egg when laid is a single cell. The upper portion of the
egg has considerable pigmentation, making it black. This part of
the egg is known as the animal hemisphere, and it is thought that
the pigmentation serves to absorb and retain heat necessary for
development. The lower portion is white and is known as the
vegetal hemisphere. The bullfrog egg is surrounded by a layer of
transparent jelly, but does not have an inner envelope of jelly, as
does the leopard frog egg. This jelly protects the egg and helps
it to retain heat. The nucleus of the egg, or germinal vesicle, lies
near the animal pole. The boundary of the egg is known as the
vitelline membrane.

CLASS AMPHIBIA

533

The eggs are fertilized externally by tlie male, who is clasping
the female as the eggs are laid and discharges spermatozoa into the
water. The first spermatozoon to swim to the Q^g and enter it by
piercing the vitelline membrane initiates fertilization. After the
sperm has entered, a fertilization membrane is formed which prevents
the entrance of additional spermatozoa. Only the head of the sper-
matozoon enters, the remainder being discarded. This head, which
is composed primarily of the male spermatozoon nucleus, fuses with
the nucleus of the e^g to complete fertilization and start development.

B/aifomtrts-

2 Ce//

VCe//

6 Cell

/e Cell

8laslul<i

Gasl-rula

{Yolk P/ua)

Fig. 287. — Diagrams of early cleavage stages, blastula and gastrula of the frog.
This is holoblastic, unequal type of cleavage. The upper, shaded portion of each
of the first five diagrams represents the animal hemisphere, and the lower portion
of each, the vegetal hemisphere. The circular plug seen in the gastrula stage is
the yolk plug.

Development begins with cleavage which is a series of mitotic divi-
sions. Cleavage results in the rearrangement of nuclear material in
relation to the cytoplasm. The furrow made by cleavage cuts through
the entire egg, and such cleavage is known as total, or holoblastic.

Cleavage and Blastula Formation. — The first and second divisions
run from pole to pole at right angles to each other and divide the
egg into four equal hlastomeres. The third cleavage is parallel to

534

TEXTBOOK OF ZOOLOGY

the equator of the egg- and somewhat above it. This produces four
cells at the animal pole that are smaller than the four cells at the
vegetal pole. Subsequently the cells at the animal pole (micromeres)
divide more rapidly than the cells at the vegetal pole, for they con-
tain less yolk. Such cleavage is known as unequal, and as divisions
proceed the cells at the animal pole become smaller and more numer-
ous than those at the other pole. The final result of the following
divisions is to form a hollow sphere, the Uastula, whose cavity is
known as the hlastocoele. The blastula is essentially one cell layer
thick, although in reality some cells have been crowded from the sur-
face, giving the appearance of additional layers.

micromeres

m&cromcres

TROGLEGS, CK065 aSCTlOM OF BLASTULA

Fig. 288. — Section tlirough blastula stage of developing frog. (Courtesy of General

Biological Supply House.)

Gastrulation. — Gastrulation begins with the appearance of a
small groove slightly below the equator of the egg. The upper
edge of this groove is known as the dorsal lip of the hlastopore.
Coincident with the appearance of this groove, the pigmented animal
cells begin to grow down over the white vegetal, or yolk, cells; and
the dorsal lip moves downward as the line advances. It also extends
its edges in a crescent shape laterally around the egg until they
finally meet to form a circle. The area enclosed by this circle shrinks
as the cells grow down from all sides and its rim moves downward.
Cells on the side where the groove began advance more rapidly than

CLASS AMPHIBIA

535

the others; and the rim on this side, or the dorsal lip, is forced down
to the vegetal pole and a little beyond on the other side before gas-
trulation is complete. The area enclosed by the circle or blastopore
is finally very small, and the white vegetal cells which fill it are called
the yolk ping.

While the animal cells have been advancing and covering the
vegetal cells, changes have been going on internally in the eg,g. The
appearance of the groove or dorsal lip of the blastopore was caused

Ectoderm —

Endoderm--

Arch(?nfccron
or

primitive

intestine

dorsal Up of blastopore''

Yolk plaq-i -Ventral lip of bIa5topore

Fig. 289. — Gastrula stage of developing frog in section.

by an infolding of outside cells. This ingrowth of cells progresses
around the egg as the crescent groove extends itself. The cells
which consequently come to lie inside are known as endodermal
cells, those on the outside as ectodermal cells.

By this invagination a cavity known as the archenteron is formed,
the walls of which are made up of the invaginated endoderm cells.
At first it is quite flat, but it expands as invagination and the other

536

TEXTBOOK OF ZOOLOGY

processes of gastrulation proceed, and soon it takes up the space for-
merly occupied by the cleavage cavity or blastocoele. Its posterior
end is the blastopore, and this, as previously mentioned, is plugged
with yolk or vegetal cells Imown as the yolk plug. As a final result,
the gastrula forms a two-layered embryo of ectoderm and endoderm
cells, each layer of which may be several cells thick.

Mesoderm Formation. — Before the process of gastrulation is com-
pleted, a sheet of cells forms between the ectoderm and endoderm
cells. This sheet of cells is known as mesoderm. As the mesoderm
grows it splits into two sheets, an outer, or somatic layer, which lies
next to the ectoderm of the embryo wall, and an inner, or splanchnic

otic pit

ectoderm

mural tube

mesoderm

notochord
hypochord
.sonvaic mesoderm
splanchnic mesoderm-^

midjut-

fc>rej;ut

^oik

<Seciion through oiic pit

^eciion ihrou^h mid-^ut

Fig. 290.

FROGL EMBRYO, SflCfWltia nMODER.'A rOLD.5

-Two levels through the body of the neural tube stage of the developing
frog. (Courtesy of General Biological Supply House.)

layer, which lies next to the endoderm cells of the archenteron. The
cavity formed between these two layers is the beginning of the coelom,
or body cavity. From these three layers, ectoderm, endoderm, and
mesoderm, all the body structures are formed.

Formation of Nervous System. — With the reduction of the blasto-
pore to a very small area, there appears on the dorsal side of the
embryo a thickened plate of ectoderm known as the neural plate
which is wide in front but narrows posteriorly. This plate soon is
flanked on each side and in front by ridges known as neural folds.
As these folds arise, the remainder of the plate becomes a longitudinal

CLASS AMPHIBIA 537

groove known as the neural groove. The neural folds or lateral edges
of the plate grow rapidly and curve in to meet in the dorsal midline,
where they fuse to form the neural tube. Anteriorly the neural tube
soon closes to end blindly while posteriorly it remains open to the ex-
terior at the blastopore for a time. Subsequently this communica-
tion with the blastopore closes. The neural tube soon separates from
the ectoderm above, its walls thicken and through other changes it
develops into the central nervous system. The anterior part becomes
the brain and the posterior part the spinal cord.

During these changes, the egg, which previously was spherical, has
elongated in the axis of the neural tube. The blastopore has been
covered by the folding of edges of the neural groove extending to
its borders. Subsequently the embryo takes on form so that defi-
nite body regions can be identified.

Later Development. — In the head region appears an elevated side
plate, the gill plate, where later the gills develop. Anterior to this,
a swelling on each side of the head denotes the beginning of certain
sense organs. A depression on the anterior ventral surface is a fore-
runner of the mouth, and posterior to this a crescent-shaped area
indicates the beginning of a ventral sticker by which the newly
hatched larva may attach itself to objects. At the posterior end of
the body a tail hud appears, and the region of the blastopore becomes
the anus. The embryo soon hatches, branched external gills which
serve as respiratory organs make their appearance, and definite sense
organs can be found on the head. This is the external gill stage.
The intestine becomes coiled ; the tail elongates ; and muscle segments,
or myotomes, can be seen along the sides of the body. Shortly after
hatching, the external gills are absorbed and internal gills take their
place. A fold of skin, the operculum, develops over this region.
There remains on the left side, however, a small opening, the spiracle,
through which water, taken in at the mouth, may pass out after going
over the internal gills. The skeleton is cartilaginous; lateral line
sense organs are present on the sides; the pineal organ is evident; and
the animal is similar to a fish.

The tadpoles feed primarily on plant substances. In the mouth of
the bullfrog tadpole is one row of teeth above and three rows below,
plus a border of projections, known as papillae, for testing food sub-
stances. Between the lips is a horny beak somewhat like that of a

538 TEXTBOOK OF ZOOLOGY

bird's with which the tadpole can scrape thin pieces from leaves of
aquatic plants, or algae and other plant material from sticks and
stones.

Near the end of the larval period (about two years in the bull-
frog), which varies considerably with species and environment, the
tadpole prepares to metamorphose into a frog. First the hind legs
push through the skin at the base of the tail; then the forelegs
appear, forcing their way through the operculum on the right and
the spiracle on the left side. As the lungs develop, the tadpole has
to come to the surface of the water frequently to give out a bubble
of impure air and take in a purer one. The tail is gradually ab-
sorbed, the intestines shorten, the homy beak disappears, the mouth
widens, the gills are resorbed, the legs develop, and the tadpole
becomes a frog.

In general, it may be stated that the ectoderm gives rise to the
nervous structures, the epidermis and its outgrowths. The endo-
derm forms the epithelial lining of the intestine, and outgrowths
of the intestine, such as the epithelial lining of gills, lungs, liver,
pancreas, gall bladder, urinary bladder, etc. From the mesoderm
are formed the muscular, vascular, and skeletal systems. Most of
the organs are formed not from a single germ layer but from a
combination of these tissues. The elementary tissues have been
discussed in the chapter on Metazoan Organization.

THE TOAD

One of the common toads is the American toad, Bufo americamis.
It resembles closely its relatives, the Woodhouse's toad {Bufo wood-
housii Girard) and Fowler's toad (Bufo fowleri), and only by close
scrutiny can they be distinguished. Bufo woodJiousii ranges, in gen-
eral, from Texas to Kansas and Nebraska and westward to Arizona
and southeastern California.

Habitat

In contrast to the bullfrog, the toad is entirely terrestrial except
when it goes to the water during the breeding season to lay its
eggs. Although the toad's skin is tougher and better protected
from drying out, water evaporates through it rapidly, and it can-
not endure dry heat. Most of the toad 's supply of water is obtained

CLASS AMPHIBIA

539

by absorption through the skin. Toads usually hide away in some
crevice, burrow, or moist place during the daytime, coming out in
the late evening to get their meal of insects.

Fig. 291.

-Woodhouse's toad, Bufo woodhoiisii. (From photograph taken by A. H.
Wright from specimen furnished by L. M. Klauber.)

External Features

The toad differs markedly from the bullfrog in external appear-
ance. Its body is broad and thick, and its hind limbs are short.
It reaches a body length of 21/2 to 43^4 inches. Its skin is rough
and covered with warts. Its color is not as drab as a casual glance
would cause one to believe. If its skin is not covered with soil
from its burrow, the upper parts are grayish or a dull yellowish
brown often blotched with darker irregular areas. A light vertebral
stripe runs down the middle of the back from between the eyes to the
vent. The limbs are faintly barred, and the underparts are a light
yellow with or without black markings on the breast. The largest
warts are encircled at their base with a narrow black border. There
is usually only one large wart in a spot of this nature, although this

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TEXTBOOK OF ZOOLOGY

may vary. Individual toads of a species may vary widely in color
and other characteristics.

The single vocal sac is a rounded throat pouch which balloons out
when the toad calls. The toad does not have internal sacs as does the
bullfrog or sacs between the ear and shoulder as does the leopard
frog.

On each shoulder is a large oblong gland known as the parotoid
gland, which is not found in frogs. Anterior to it and on the side
of the head is the vertically oval tympanum. According to Kellogg,
the secretion of the parotoid gland "is an acid irritant causing pain
in cuts and producing a bitter astringent sensation in the mouth."

'^ ^Nr External nares

>^1 VW^V Preorbital crest

cS ' ( V^=^^X- - Postorb'ital crest

" ""^^-V-- Tympanum
J-- Parotoid

Fig. 292. — Dorsal view of head region of a toad to show parotoid glands and

cranial crests.

It may act as a poison or, at any rate, as a powerful repellent when
the toad is seized by other animals. A dog that picks up a toad
lets it go at once and does not soon forget the experience.

The ridges between the eyes and back of them are known as
cranial crests. They are made by the bones of the head and are
variously named according to their position. Those between the
eyes are known as superciliary or supraorbital; the transverse crests
back of the eyes and at right angles to the superciliary crests are the
postorbital crests. A crest that is absent in Woodhouse's toad but
often occurs in other toads is one running from the postorbital crest
to the parotoid gland and known as the preparotoid crest. The paro-
toid glands are usually in contact with the postorbital crests in
"Woodhouse's toad.

The toes are about one-third webbed, the webs being fleshy. There
are two tubercles on the sole of the foot known as metatarsal tu-

CLASS AMPHIBIA

541

bercles. The inner of these is large and has a homy black edge which

is used in digging when the toad burrows into the ground. The

other metatarsal tubercle is small.

The toad, like the frog, has four fingers on each hand and five

toes on each foot. The first finger is slightly longer than the

second.

Internal Structure

The internal organs of the toad are similar to those of the frog,
and the previous description of these is referred to. Only striking
differences will be pointed out. The word ''frog" used subsequently
refers to bullfrogs or leopard frogs.

Respiratory and Digestive Organs

Since the toad is terrestrial and has a thicker epidermis than the
frog, it needs to depend to a larger extent on its lungs; these are
large and well vascularized, being more spongy than those of the
bullfrog or leopard frog.

The motith is large and toothless, lacking the maxillary and vo-
merine teeth of the frog. Two openings, one on each side of the
tongue, are apertures to the single vocal sac. The tongue of the toad
is not notched behind like that of the frog. It is thicker and
rounded with more of its posterior end free. The surface of the
tongue is sticky and holds the captured insect as it is pulled back
into the mouth. The liver is two lobed, in contrast to the three-
lobed liver of the frog.

Urinogenital Organs

The mesonephric ducts from the kidneys, which in the frog opened
separately into the cloaca, unite in the toad and open into it in a
single duet. The urinary bladder in the toad is large and may
function also as a reservoir for water to prevent the animal's dry-
ing out. It is held in place by sheets of peritoneum and has a
sphincter muscle at its mouth which permits its contents to be
emptied rapidly. This discharge may lighten the toad and make
it easier to escape from its enemies.

The testes of the toad are elongated and extend along a good
portion of the length of the kidney. At their anterior ends, between
them and the fat bodies, occurs an irregularly shaped granular

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TEXTBOOK OF ZOOLOGY

body, Bidder's organ, Avhich is thought to represent a rudimentary
ovary. Some experiments on the toad in which the testes were
removed have indicated that this body may develop into a func-
tional ovary. In male toads there also may be found alongside the
mesonephric duct a coiled tube which is the remains of a rudi-
mentary oviduct that is nonfunctional. This rudimentary structure
is not found in the male bullfrog, although it is encountered in the
male leopard frog.

Blood Vascular System

For studying the blood vascular system, as well as some other
systems, the toad is quite superior to the leopard frog. The vessels
are of larger caliber, and the broad interior of the body makes the
dissection more easily examined. The bullfrog, of course, is superior
to either of these.

Scapu la

Coracoid

Epicoracoid
Mesosterrium

rnuTTi

Fig. 293. — Diagram of ventral view of tlie arciferal, pectoral girdle of toad.
(Modified from Kellog, Mex. Tailless Amphibia. U. S. National Museum Bulletin
No. 160.)

Arterial System. — The subclavian artery is larger in proportion
to the size of the animal than in the bullfrog. It sends large
branches to muscles of the pectoral girdle, forelimb, and to the
upper portion of the bulky side muscles. Two or three conspicuous
branches of the vertebral artery run diagonally across the dorsal
wall of the body cavity to the lateral muscles of the body which
they enter. One large femoral branch, instead of two small ones, as
in the bullfrog, is given off from the sciatic artery in the upper leg
and runs ventrally, branching into the surrounding muscles.

Venous System. — The parietal branches of the ventral abdominal
vein are relatively larger in the toad and may extend horizontally
across the ventral body wall to the large side muscles, The dorso-

CLASS AMPHIBIA

543

lumbar vein is often quite large, with branches running the entire
leng-th of the body cavity and others coming from the lateral muscles.

Skeleton and Muscles

The pectoral girdle of the toad is quite different from that of the
bullfrog and leopard frog. In the latter specimens, the two halves
of the girdle grow together in the midventral line, separated by a
cartilage, and the chest is not expansible. This type of pectoral
girdle is known as firmisternal. In the toad, however, the epicoracoid
cartilages separating the two halves of the girdle overlap in front,
and the chest is expansible. This type of pectoral girdle is known
as arciferal (Fig. 293).

2 jy? vertebra

Sacral

diapopbjses

dilated

Ilium

' Urostyle

Ij- Ischium

Fig. 294. — Pelvic girdle and urostyle of the common toad, Bufo americamis.

As in the bullfrog, the toad has nine typical vertebrae and a tenth
which is included in the urostyle. Like the frog's, the vertebrae
are procoelous. Unlike that of the frog, however, the single sacral
vertebra which precedes the urostyle has its transverse processes
(sacral diapophyses) dilated or expanded. In the bullfrog they were
more circular.

The muscles of the legs, pectoral girdle, and body wall are large
and thick. In preserved specimens their origins are clearly out-
lined, and they are more easily separated one from another in dis-
section than those of the leopard frog.

Nervous System and Sense Organs

The large calcareous bodies around the spinal ganglia, seen in the
bullfrog, are not evident in the toad. While both bullfrogs and

544 TEXTBOOK OF ZOOLOGY

toads have poor vision, due to the lack of accommodation in the
eye, the toad appears to see better. Toads also retain sound im-
pulses longer than do frogs. Sound is of greater importance to the
toad, for in his terrestrial hunting ground he is dependent to a
large extent on sound in locating insect food.

Embryology

The eggs pass continuously through the oviduct with the result
that they are laid in long strings or tubes of albuminous jelly. The
egg mass is laid underwater, and the toad moves around during
the egg laying process with the result that the strings may be con-
siderably tangled around submerged vegetation. Shallow creeks
provide a favorite breeding ground.

The length of time taken for the eggs to develop and hatch de-
pends upon the temperature. They usually hatch in from two to
four days. The tadpoles may transform into young toads in from
30 to 60 days after hatching. They measure % to % of an inch
at this time.

Toad tadpoles are black. Usually toad tadpoles can be distin-
guished from frog tadpoles by the position of the anus. The anus
in toad tadpoles is median, while in frog tadpoles it is somewhat
on the right side. The spiracle of toad tadpoles is very small and
is on the left side.

CHAPTER XXIX

REPTILIA
(By Leo T. Murray and James E. Blaylock)

Class Beptilia includes among living- forms, turtles, snakes, lizards,
alligators, and similar animals. These are the only living vertebrates
which are cold blooded, breathe by lungs, and have a single median
occipital condyle. A more complete list of distinguishing charac-
teristics has been given by Gadow as follows:

"1. The vertebrae are gastroceutrous.

"2. The skull articulates with the atlas by one condyle, which is
formed mainly by the basioccipital.

"3. The mandible consists of many pieces and articulates with the
cranium through the quadrate bones.

"4. There is an auditory columellar apparatus fitting into the
fenestra ovalis.

"5. The limbs are of the tetrapodous, pentadactyl type.

"6. There is an intracranial hypoglossal nerve.

"7. The ribs form a true sternum.

"8. The iliosacral connection is postacetabular.

"9. The skin is covered (a) with scales, but (b) neither with
feathers nor with hairs ; and there is a great paucity of glands.

"10. Reptiles are poikilothermos (cold blooded).

"11. The red blood corpuscles are nucleated, biconvex, and oval,

' ' 12. The heart is divided into two atria and an imperfectly divided
ventricle. It has no conus, but semilunar valves exist at the base
of the tripartite aortic trunk.

"13. The right and left aortic arches are complete and remain
functional.

' ' 14. Respiration is effected by lungs ; functional gills are entirely
absent, even during embryonic life.

"15. Lateral sense organs are absent.

"16. The metanephric kidneys have no nephrostomes. Each kidney
has one separate ureter.

"17. There is always a tj^ical cloaca.

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546 TEXTBOOK OF ZOOLOGY

"18. The eggs are meroblastic.

' ' 19. Fertilization is internal, 'and is effected, with the single ex-
ception of Sphenodon, by means of male copulatory organs.

"20. An amnion and an allantois are formed during development.

"Numbers 1, 2, 6, 7, 8, 14, 16, 18, and 20 separate the Reptiles
from the Amphibia. Numbers 9 (b), 10, 12, and 13 separate them
from the Birds and Mammals. Numbers 3, 8, and 11 separate them
from the Mammals."

The majority of living forms are covered by scales. The turtles
have epidermal scutes as an external covering ; and the crocodiles
have a tough skin w^ith osseous plates in it. Many living reptiles
are capable of very rapid movement ; and the Pterosauria, an ex-
tinct order, had wings. Only one lizard and a relatively small
number of snakes are poisonous. It is thought that the poison gland
is a recent development among reptiles.

At present herpetologists place all known reptiles into nineteen
groups or orders. Only four of these orders have living representa-
tives. This indicates that Reptilia is an old class of vertebrates
which is now decadent.

FOSSIL REPTILES

A study of the fossils of reptiles reveals that during the Triassic,
Jurassic, and Early Cretaceous geological periods reptiles were the
dominant animals on earth. They occupied and dominated all types
of habitats then much as mammals do now. Many of these ancient
reptiles were no larger than the common lizards of today, but others
were the largest vertebrates that have ever lived. Brontosaurus,
"the thunder lizard," was nearly seventy feet long and weighed
approximately forty tons. This enormous reptile lived in the shallow
waters of ponds and bays and fed on the plants that grew in the
mud along the margins of the water. Tyrannosaurus, "the king
lizard," reached a length of forty-seven feet and a weight greater
than that of any elephant. Since it was entirely carnivorous in its
feeding habits, it is easily the most terrible animal that has ever
lived on earth. Many variations in form and size have bridged the
gap between the largest and the smallest. The five or six known
families of flying reptiles varied in expanse of wings from a few
feet to as much as twenty-five feet. As a group, they dominated
the air for several million years. A few small wormlike burrowing

REPTILIA 547

lizards less than two feet in length are known from the strata
deposited in the Oligocene time in North America. Though reptiles
had become completely adapted to living on land, many of them
returned to the water. Most living turtles spend the greater part
of their lives in water but must return to land to deposit their eggs.
In the heyday of reptiles there were many other aquatic and marine
forms showing numerous and diverse adaptations to living in a
liquid medium. The ichthyosaurs (fish lizards) were the most per-
fectly adapted to aquatic life, being very fishlike in form.

CLASSIFICATION OF LIVING REPTILES

Many different plans have been proposed for grouping the reptiles.
All of these plans have attempted to express the relationships of
the various groups to each other; and the relations of representa-
tives of a group to other members of the same group. The following
arrangement is adapted from Williston, and includes only those
groups having living representatives.

Class — Reptilia

Subclass — Anapsida. Temporal region of skull not perforated.

Order — T estiidvnata. A single coracoid; ten dorsal vertebrae, their ribs
expanded to meet on the dorsum or a dermal layer of bony plates.
Turtles.

Suborder — Fleurodira. Neck retracted laterally; pelvis united with the

plastron.

No representatives of this suborder are found in North America.
Suborder — Cryptodira. Neck retracted vertically; pelvis not united with

the plastron; carapace with marginal plates.

The majority of the living turtles of North America belong to thi?

suborder.

Suborder — Trionychoidea. Neck retracted vertically; carapace with no
marginal plates.

The ''soft-shelled" turtles make up this suborder.
Subclass — Parapsida. Temporal region of skull with one opening.

Order — Squamata. Quadrate freely articulated proximally (streptostylic)
or secondarily fixed. Lizards and Snakes.
Suborder — Lacertilia (Sauria). Parietals never united to basisphenoid
by descending plates; the brain case more or less membranous an-
teriorly.

The one hundred and seventy-four kinds of lizards known from the
United States and Lower California belong to this suborder.

548 TEXTBOOK OF ZOOLOGY

Suborder — Ophidia (Serpentes). The braincase enclosed by descending
plates from parietals and f rentals; no external limbs present. This
suborder includes all snakes.

Subclass — Diapsida. Temporal region of the skull with two openings.

Order — Bhynchocephalia. Amphicoelous vertebrae, premaxillae with a de-
curved beak; pineal eye present.

Suborder — Sphenodontia. Has the characters of the order. Sphenodon,
the single living genus of this order is famous as being one of the
primitive living reptiles. It is found in New Zealand and on pome
smaller islands of that region.
Order — Crocodilia. Procoelous vertebrae; premaxillae never decurved; no
pineal eye present. Crocodiles and Alligators.
Suborder — EusucMa. Has the characteristics of the order.

The living crocodiles and alligators belong to this suborder.

Order Testudinata (Chelonia)

Suborder Pleurodira. — Turtles of this group are found in South
America, Africa, and Australia. They are known as "side neck"
turtles because they do not retract the head and neck under the
carapace but lay it along the periphery of the shell. Some members
of the group have a pair of bones, the mesoplastra, in the plastron
that is not present in other living turtles, though it was common
among forms now known only as fossils. The bones of the pelvic
girdle are sutured to the plastron as well as to the sacral vertebrae.
This is also a characteristic unique among living forms but more
common in extinct species. Hence, it is thought that members of
this suborder are more primitive in structure than the members of
either of the other two suborders having living representatives.

Suborder Cryptodira. — There are sixty-one species of turtles in
North America north of Mexico or in the oceans that bound the
shores of this region. Fifty-seven of this number belong to this
suborder. Six families are represented.

Family Kinosternidae. — This family includes the turtles commonly
known as the "mud turtles,"' "stinkpots," or "musk turtles."
They are all small, brown or black turtles, sometimes with white
or yellow lines on the head and neck. Many turtles are fre-
quently mistaken for members of this family that belong to
some other family. In California, where no kinosternid turtle is
found, a member of the family Emydidae is called the "mud turtle."
All kinosternids possess musk glands that open through pores on

REPTILIA 549

the margins of the carapace just anterior and just posterior to the
bridge. If a dry musk turtle be disturbed, a drop of yellow liquid
can often be seen to appear at each one of the openings of the
ducts from the four musk glands. This liquid gives off a disagree-
able odor. If it touches hands or clothing, it is very difficult to
remove.

Family Chelydridae. — The common snapping turtles and the alli-
gator snappers are the living members of this family. Both turtles
have rows of raised prominences along each side of the carapace;
and a row of large bony ossicles along the middorsal region of the
tail. The head is large and formidable. The alligator snapper has
a pair of wormlike appendages in the mouth, which it is said are
used to entice fish within reach of its xiowerful jaws. Both turtles
are a plain brown color dorsally and dirty white to black ven-
trally. The alligator snapper may attain a weight of ojie hundred
and fifty pounds, while the common snapper will seldom exceed
forty pounds. Large specimens of either turtle can easily amputate
a finger or possibly a hand. Both kinds of snapping turtles possess
scent glands very similar to those found in the kinosternids. This
family is an excellent example of discontinuous distribution. It
was long thought that it was confined to the Western Hemisphere
but a genus is also found in New Guinea.

Family Emydidae. — Many turtles, diverse in habits and appear-
ance, belong in this large family. All of the "hard-shelled" pond
and river turtles commonly called "sliders," the painted turtles,
the red-bellied turtles, the box turtles and many others fall into
this group. While there is great variation among members of the
family, they all exhibit certain tendencies as well as fundamental
structural similarities. Many species are brightly colored. The
painted turtles have bright red colors patterned with dark green
and black. Different species of the genus Pseudemys possess vari-
ous types of prett}^ colorations. The wood turtle has a somber
carapace, a bright plastron with pleasing black spots, and a rich
reddish-orange skin on the legs and neck. The males of many
species of this family have long straight claws on the toes of the
fore feet. Members of this family are found on all the continents
except Australia. Emys hlandingii, found in the Great Lakes region,
is very similar to Emys orbicularis of Europe.

550 TEXTBOOK OF ZOOLOGY

Family Testudinidae. — Only three species of turtles found in the
United States belong to this family; namely, the gopher tortoise,
Berlandier's tortoise, and Agassiz's tortoise. Members of this family
are numerous in Africa, on the Galapagos Islands, and in other
widely separated localities. How these three species of turtles come
to be in North America is a most interesting problem in animal
distribution. All three species found in this country are dark brown
or black on the carapace, often lighter ventrally. Young individuals
usually show a light area in the center of each dorsal scute. Since
they are all dry-land turtles, they lack the streamlined form of
water dwellers. The most distinct characteristic of our species is
a narrow extension of the plastron into a gular process.

Family Cheloniidae. — This family includes the green turtles, the
shell turtles, and the loggerhead turtles. All are marine animals,
and show modifications for an aquatic existence in the form of the
body and in the modification of feet into flippers. Many members
of this family may weigh five hundred pounds, but most specimens
seen in the markets weigh much less.

Family Dermochelidae. — This family contains but one genus with
two species. They are commonly known as the leatherback turtles,
the trunk turtles, or the harp turtles. Unlike most turtles, they
lack the covering of horny scutes, being covered instead with a
leathery integument. These are the largest of living turtles. Large
individuals may weigh as much as one thousand pounds.

Suborder Trionychoidea. — Members of this suborder are found in
North America, Africa, Asia, and New Guinea. They are among
the most aquatic of all land and fresh-water forms. The only occa-
sion on which they leave the water is to deposit eggs.

Family Trionychidae. — This is the only family of the suborder
having representatives in North America. It is represented by five
species and one subspecies, all in the genus Amy da. All are "soft-
shelled" turtles covered with a soft rubberlike skin instead of the
horny scutes present on most turtles. The color on the dorsal side is
olive brown, while the ventral side is white. Any of these species will
bite viciously when angered and can inflict painful wounds. It was
probably one of these turtles that gave rise to the belief that a turtle
would not loose its hold until it heard thunder. They have a habit
of retaining their grip on a victim very tenaciously.

reptilia 551

Order Squamata

Suborder Lacertilia (Sauria) (The Lizards). — This group contains
more different kinds of living animals than any other suborder
among the reptiles. There are more than 2,500 living species known
on earth. Of this number, about 175 species are found in North
America, north of Mexico. Kepresentatives of nine families are
found among them.

Family GekJconidae. — There are about fifty genera, containing
some 300 species, in this family. They are found around the world
in the tropical and semitropical regions. Seven species are known
from the United States and neighboring regions.

All of our species are small, seldom attaining a length of six
inches; but some tropical forms may be over a foot in length. The
colors vary considerably but are often bright, as is usual in noctur-
nal animals. The scales of the skin are very minute. This gives
the geckos a soft, smooth, appearance unlike that of any of the
other lizards. The eyes usually have vertical pupils and are with-
out lids, though they are covered by a transparent, cutaneous mem-
brane. As in some other lizards, there is nothing obstructing the
auditory passage through the head. It is possible to see through
this passage. Members of this family are among the few lizards
that can make a sound other than hissing. Their characteristic
call sounds like the word "gecko."

In a great many species the toes are flattened on the end to form
adhesive discs. These enable a gecko to walk across the ceiling of
a room with ease. In most species the tail is short and thick.

Geckos sleep during the day but come forth at nightfall in search
of their insect prey, which they capture by means of their short
sticky tongues.

Family Iguanidae. — This family of lizards has more representa-
tives in the United States than any other single family. Of ap-
proximately 175 species of lizards known from this country, 90
species and 19 subspecies belong to this family. Representatives
are found in all parts of the United States except the most northern
portion, as well as throughout Central and South America, and the
West Indies. Two genera are found in Madagascar and one in the
Fiji Islands. As might be expected in so large a group, great varia-

552 TEXTBOOK OP ZOOLOGY

tion in size, form, and coloration occurs. The chameleons (Anolis)
change from various shades of brown to light green in response to
changes in the intensity of the light. Members of the Central Ameri-
can genus Basiliscus are remarkable for erectile middorsal crests. The
horned lizards (Phrynosoma) bear conspicuous osseous spines or
"horns" on the posterior and lateral borders of the head. All the
members of the family, however, have certain structural character-
istics in common ; such as, fleshy tongue, and eyes with round pupils
and well-developed lids. Femoral pores are usually present on the
males. Most of the species found in the United States lay eggs,
though some species are known to be ovoviviparous. Various types
of habitats have been adopted by different iguanid lizards. The
chameleons and many species of the genus Sceloporus are essentially
arboreal, while the homed lizards (Phrynosoma) are strictly ter-
restrial.

Crotaphytus collaris, the collared lizard, is a rather large lizard with
a long tail and a heavy body. It is brightly colored and has a yellow
collar bordered with black. Its distribution is in the Southwest,
from the Plains westward.

Certain tropical species are semiaquatic and one species is semi-
marine. The majority of species are insectivorous, though three
genera are herbivorous.

Family Anguidae (Alligator lizards, "glass snake," joint snake,
etc.). — This interesting group of lizards is represented in the United
States by 10 species. Approximately 40 other species are found in
other regions of the world. Most of these other species are native
to the tropical regions of the Western Hemisphere, though some are
found in Europe, Asia, and Africa. A reduction in the size and
strength of the limbs is common in this family. Many species, such
as our "glass snake," are entirely legless. Other common charac-
teristics are a fold in the skin where the ventral plates join the
body wall ; a long, brittle tail ; eye with a lid ; emarginate, protractile
tongue; and solid teeth. Many of our species have large auditory
openings connected by an unobstructed passage. Our largest species
reaches a maximum length of approximately one foot, while a form
found on the Balkan peninsula may be three feet long. Our alliga-
tor lizards (Gerrhonotus) are ovoviviparous while the "glass snake"
(Ophisaurus) lays eggs. All members of this family feed on animal
food, such as insects, snails, and small mammals.

REPTILIA

553

Family Anniellidae (blindworm, "worm snake," worm lizard). —
This family consists of one genus and two species found in southern
California. These small, legless, wormlike lizards are burrowing in
habit. The ears are concealed and the eyes are covered by translu-

Fig. 295. — Collared lizards, Crotaphytus collaris, female and male. This is a
beautiful lizard of the Southwest. (Courtesy of Ottys Sanders, Southwestern
Biological Supply Company.)

cent skin and poorly functional. The tongue is protractile as in
the members of the Anguidae.

Family Helodermatidae (Gila monsters, beaded lizards). — This
family contains one genus and two species. One species, Heloderma
suspectum, is found in Arizona, New Mexico, and northern Mexico.

554 TEXTBOOK OF ZOOLOGY

The other species, II. horridum, ranges through central and western
Mexico to northern Central America. These lizards may reach a
length of 2 feet though smaller ones are more commonly seen. The
surface of the body is totally unlike that of any other lizard, being
covered by beadlike ossicles or tubercles. The most interesting and
distinctive structural characteristic of these lizards is their grooved
teeth with ducts from poison glands opening at the base of the
grooves. No other family of lizards in America is venomous. The
color of H. suspectum is black marbled with pale pink, salmon, or
flesh. The Mexican species is black with yellow or lemon spots or
bars. The short thick tail becomes more slender when the animals
fast. The natural food of the animals is not known. They take
eggs readily in captivity and thrive on them. Reproduction is
oviparous, the eggs being laid in warm, moist sand where they
hatch in twenty-eight to thirty days.

Family Xantusiidae (night lizards). — This small family contains
only three genera with a total of seven species. Five species, all
of the genus Xantusiidae, are found in southern California, Lower
California, and Arizona. One other genus is found in Central
America ; and a third in Cuba.

These lizards are seldom over six inches in length. The color
changes from dark brown in subdued light to lighter hues in
stronger light. The pupils are vertical, and the eyes are without
lids. The tongue is only slightly extensible.

These little reptiles are strictly nocturnal, hiding by day under
fallen Yucca plants or in crevices between boulders. So far as is
known they are insectivorous in feeding habits. At least one species
is known to be ovoviviparous.

Family Teiidae (striped lizards, race runners, sand lizards). — This
family contains 40 genera with more than a hundred species. Twenty-
two species and subspecies, all belonging to the genus Cnemidophorus,
are found in the warmer parts of the United States. All other
members of the family are found in South America and the West
Indies.

The species of Cnemidophorus are long slender, active lizards cap-
able of surprising speed in running. The ground color is usually
some shade of brown. Lines, bands, or spots of lighter color form
various patterns on different species. The tongue is black, forked,
and protractile.

REPTILIA 555

Our species are found in open, sunny, sandy places. If disturbed
they skim over the ground with great rapidity, but if hard pressed
they take refuge in burrows. Insects make up the bulk of their
food. All the species lay thin-shelled eggs which are deposited in
shallow excavations in the sand to be hatched by the heat of the
sun.

Family Scincidae (skinks, or smooth lizards). — This is one of the
largest families of lizards, being composed of over four hundred
species arranged in thirty genera. Three genera containing sixteen
species are fonnd in the United States. They are more abundant
both in number of kinds and in number of individuals in tropical
regions, especially in tropical parts of the old world and in the
Australian regions. South America has fewer skinks than any other
region in the world.

All the skinks are relatively small lizards, the largest in this coun-
try seldom attaining a length of ten inches. The scales are smooth
and usually shining. The color varies with age. The young are
darker than the adults and color patterns of lines present on the
young often disappear on adults. There is great variation in the
development of the limbs. Most of our forms have one or both
pairs of legs.

The skinks are diurnal, feeding by day and seeking a hiding place
at night. Many old world kinds are burrowers in sand, but of all
American forms only one Florida species is a burrower. The ma-
jority of species are to be found under bark, logs, stones and in
other dark, cool places. Some kinds of skinks have been observed
to guard the eggs by curling about them. Some old world species
are ovoviviparous.

Family Amphishaenidae (worm lizards). — This highly modified
family is represented in the United States by one genus with one
species in Florida and another genus with one species in southern
California. Forty species are known from the American tropics and
others from northern Africa and the Mediterranean region.

These remarkable lizards are all limbless except Bipes hiporus,
the two-footed lizard of Lower California, Avhich has the anterior
pair of limbs well developed. The skin is without scales and forms
numerous rings about the body, suggesting an annelid worm in
appearance. The eyes are absent or reduced. There is usually no
external ear opening.

556 TEXTBOOK OF ZOOLOGY

Both of our species lead a subterranean existence, boring tuimels
in which they move backward and forward with equal ease.

Suborder Ophidia (Serpentes) (Snakes). — No group of reptiles is
of greater natural interest to man than the snakes. Superstitions
and stories relating to snakes are as old as written language. Many-
religions and cults have used the serpent as a symbol of good or
of evil. In many regions of the world today many of the most
poisonous of snakes are venerated and protected by the natives.
The snake dance of the Hopi Indians of our own country is a well-
known example of the symbolic use of snakes.

The ophidians are highly modified vertebrates. Their anatomical
structure indicates that they have been derived rather recently,
geologically speaking, from lizardlike ancestors. Some lizards are
totally limbless, while some snakes of the family Boidae have vestiges
of the posterior pair of limbs. In the structure of the jaws there
is close similarity between some snakes and some lizards.

Upon the basis of structure and arrangement of teeth snakes
have been arranged into the following four groups:

The Aglypha, or those with solid, ungrooved teeth. Our harmless
snakes all have this type of dentition.

The Opisthoglypha, or those having the posterior maxillary teeth
grooved. These snakes are venomous but seldom dangerous to man.
The position of the venom conducting teeth makes it difficult for
the snake to inflict a wound on man. The lyre snakes (Trimorpho-
don) of the Southwest, the black-headed snakes (Tantilla) of the
Southern States, and a few other rare snakes belong to this group.

The Proteroglypha are those that have the anterior maxillary
teeth grooved and often enlarged and elongated. Many of the most
dangerous snakes in the world belong to this group. The coral
snakes, cobras, and sea serpents have this type of dentition.

The Solenoglypha, or those having hollow, hinged fangs in the
anterior part of the mouth. The rattlesnakes, copperheads, water
moccasins and their relatives make up this group. They are all
venomous and dangerous to man.

There are approximately 2,300 known species of snakes. Of this
number, some 225 species are venomous; but 75 of these poisonous
species are so small or rare that there are only 150 to 175 species
that man need fear.

REPTILIA 557

In the United States there are 234 recognized species and subspecies
of snakes. Of this number, 51 species and subspecies are venomous
but 13 of these are too small or rare to be considered dangerous to
man. Hence, there are 38 kinds in our country that we must avoid.

Family Leptotyphlopidae (worm snakes).- — Three species belong-
ing to this family are found in southwestern United States. In
Mexico, Central America, Asia, and Africa, there are about thirty
species. One of these, a Syrian species, is the smallest of all adult
snakes. Those found in our own country are small, seldom attain-
ing a length of more than a few inches. They are all plain flesh,
or various shades of pale pinkish lavender in color. An iridescent,
silvery sheen extends over all. The head is blunt and of the same
diameter as the neck and body. The small eyes are covered by
translucent scales. It is probable that they have very poor powers
of vision. The tail is likewise blunt and very short. There is a
vestige of a pelvis present in some members of the family.

All of the North American species are burrowers, making long
tunnels in which they find insect larvae and worms. They seldom
come to the surface except when forced out of their burrows by
heavy rains.

Family Boidae (boas and pythons). — There are sixty to seventy
species in this family, some of which are found in all tropical parts
of the world. In the United States there are three species, all found
in southern California or neighboring desert regions. Our species
are all small, but the largest of living snakes are members of the
family. There are authentic records of specimens 30 feet long and
weighing approximately 300 pounds. There is usually some ex-
ternal evidence of vestigial limbs present. Though none are venom-
ous, many species have elliptical pupils. All members of the family
are constrictors in feeding habits, preferring warm-blooded animals
as a rule. The females lay eggs and some species are known to
coil about them until the young are hatched.

Family Coluhridae. — This is the largest of all the families of
snakes, containing 90 per cent of the living species. In the United
States more than 100 species of snakes belong to this family. Mem-
bers of the family range farther north and south of the equator
than those of any other family of snakes. Being so numerous and
widespread, it is not surprising that some species should have
adopted every available habitat. Hence, the variety in size, form.

558 TEXTBOOK OF ZOOLOGY

and color is great. The arboreal species are slender and green.
The terrestrial species are heavier in body and varied in color. The
subterranean, the semiaquatic, and the aquatic forms all show
adaptations to their environments. Most of our Coluberine snakes
are nonvenomous, but some are mildly poisonous opisthoglyphs. A
majority of the snakes in this family lay eggs but some bring forth
the young alive.

Family Elapidae (corals, harlequins). — Twenty-nine genera with
about 140 species make up this family. All except two genera are
found in the Old World only. Africa, Asia, Malay Archipelago,
and Australia have representatives of this family. In Australia
there are only a few representatives of other families. In the United
States there are two genera containing one species each. Together
they cover most of the southern half of our country.

All the snakes in this family are deadly poisonous proteroglyph
serpents. The cobras of Asia and Africa kill thousands of persons
every year. This is due partly to superstitions and religious be-
liefs that protect these snakes in those regions. The venom of this
family of snakes is largely neurotoxic in action; i.e., it acts on the
nervous centers. Hence, it usually acts much more quickly than
the slower hemolytic and hemotoxic venoms of the pit vipers. Some
cobras may attain a length of ten to twelve feet. Men have been
known to die in less than an hour after being bitten by such a snake.
However, the venom of the coral snakes, the American representa-
tives of this family, is more deadly per unit volume. The coral
snakes are seldom more than two feet long and are not capable of
injecting such large quantities of venom.

The coral snakes are beautiful little snakes marked with bril-
liant cross bands of red, yellow, and black. There are three harm-
less snakes found in the same parts of our country that have the
same colors in their patterns. None of them, however, duplicates
the sequence of the bands on the coral snakes. In these poisonous
snakes the order of the colored bands is red, yellow, and black.
In the harmless species the order is red, black, and yellow. The
following jingle is a good device for remembering these color
schemes:

Red and yellow

Kill a fellow.

Red and black

Venom lack.

REPTILIA

559

The small, conical head and slender, cylindrical body of the coral
snakes fit them for their subterranean life. They seldom come to
the surface during the day, but may be found at night crawling
about in search of food. They eat other snakes and small lizards.
These snakes lay eggs.

Family Hijdrophidae.— Members of this family are marine relatives
of the Elapidae. Only one species is found in the New World. It
occurs off the west coast of Mexico and has been reported as very
common in some localities at certain seasons. The females come
into shallow coastal waters to give birth to their young. Here the
young have some protection from their enemies and access to small
fish suitable for food. Adults of Old World species have been
sighted one thousand miles from land. All species have the tail
flattened for swimming.

Family Croialidae (pit vipers).— This family is composed of six
genera which contain about eighty species. Members of the family
are found over all the temperate and tropical parts of the Western
Hemisphere. In the Old World they are found in India, China, and
neighboring regions.

Three of the six genera in the family have representatives in the
United States. In fact, all the dangerously poisonous snakes in
this country, except the coral snakes, belong to this family. They
are all solenoglyph snakes. There is a prominent pit on each side
of the head between the eye and the nostril. The rattlesnakes
(Crotahis and Sistrurus) bear rattles on the end of the tail. All
members of the family have elliptical pupils.

The poison glands and highly developed fangs enable these snakes
to capture their food with a minimum of effort on their part. The
venom is injected so quickly and so unexpectedly that the prey has
little chance to avoid it. Most small animals die very soon after
being bitten. The reptile then swallows the carcass at its own
pleasure. Rattlesnakes prefer mammals. In regions where these
snakes abound wild rats and mice are rare. Water moccasins take
frogs and other cold-blooded aquatic animals for food. The cop-
perhead appears to enjoy both warm-blooded and cold-blooded
animals.

All members of this family give birth to living young or lay thin-
shelled eggs which hatch in a very short time, usually less than
an hour.

560 textbook of zoology

Order Rhincocephalia

Suborder Sphenodontia (Sphenodon, Tuatara). — The only living
representative of this order is Sphenodon punctatum, a lizardlike
animal found on a few small islands off the coast of New Zealand.

It is often called a "living fossil" because many of its anatomical
characters are found in no other living reptile. Some of these
characters are old, even in relation to many extinct reptiles. The
entire brain is said to be smaller than one of its eyes. Unlike all
other living reptiles, it has no external copulatory organ. There
are ten separate carpal bones present. This is a primitive number.
Many other skeletal features indicate a close relation to reptiles
of other geological periods.

The adults usually attain a length of about twenty inches. They
are dull yellowish or olive brown in color. A middorsal row of
spinelike scales extends from the occipital region to the end of the
tail. There are other rows of smaller excrescences along the sides.
One of the most interesting features of the animal is its pineal eye.
This "third eye" is located in the center of the head between the
eyes. It is surrounded by a rosette of small scales and covered
by a translucent plate. The nerve from this eye is well developed
and passes to the brain through a foramen in the cranium. There
are a retina and a cornea in the structure of this organ, but the
extent of its function as an eye is unknown.

The habits of the animal are as unique and interesting as its
structure. It lives along the shore in burrows with a small petrel,
a shore bird of that region. It is said that the reptile and the bird
have special sides in the enlarged chamber at the end of the burrow
and neither trespasses on the other's space. The food of these ani-
mals consists of insects, spiders, and crustaceans. In captivity they
have been known to thrive on a diet of earthworms. They are
nocturnal, hiding by day and hunting along the beaches at night.
Reproduction is oviparous, about ten eggs being a usual complement.

Order Crocodilia

Suborder Eiisuchia (crocodiles and alligators). — This order is rep-
resented by about twenty living species. They are found along the
coastal plains in our Southern States, southward through Mexico,
Central America, and in the warmer regions in South America. In

REPTILIA 561

the Old World they occur in tropical Africa, Southern Asia, Java,
Sumatra, and Northern Australia.

In the United States two genera with one species each are found.
One, Alligator mississi'p'piensis , is found in all our coastal states from
the Kio Grande to the Carolinas. The other, Crocodilus acidus, is
found in the southern tip of Florida, ]\Iexico, Central and South
America.

The general appearance of alligators is well known to everyone.
The animals are not covered by scales but are protected by rows
of dermal ossicles in the skin. Formerly a crocodile or an alligator
might be found that was as long as thirty feet, but it is doubtful
whether such an animal can be found today.

The females lay eggs in a nest built of sticks and decaying vegeta-
tion. The heat generated by decomposing vegetable matter aids in
incubating the eggs. It was formerly thought that the growth of
alligators was very slow but specimens in captivity have been
brought to a length of five and one-half feet in five years.

THE HORNED LIZARD

The horned lizard is a convenient reptile to study since one or
more species is found locally throughout southwestern United States.
It is usually found in abundance and is relatively easy to capture.
The ''horns" on the head are unique among living reptiles. The
general plan of structure is sufficiently generalized to illustrate most
of the characteristics typical of reptiles.

Habits and Behavior

These lizards are diurnal animals, feeding by day and at night bur-
rowing into the ground until only their head spines can be seen. Their
food consists of small beetles, flies, smooth caterpillars, moths, ants,
and other small insects. The animals are solitary, not being found
in groups in dens. Each one burrows underground to hibernate
during the cooler season of the year. After mating, a female of
an oviparous species digs a cup-shaped hole and deposits twenty
to twenty-five eggs in it, covering the eggs with several layers of
earth. Females of the ovoviviparous species bring forth the young
alive.

562 TEXTBOOK OF ZOOLOGY

External Structure

The horned lizards are typical iguanid lizards, with broad, flat
bodies covered with horny, strongly keeled scales and spines. There
is at least one row of spines in a marginal fringe at the lateral
edges of the belly. Erect, scattered spines of various shapes and
sizes are apparent on the animal's back. The body is devoid of
a dorsal crest but is covered with small granular scales and with
four rows of enlarged, sharply pointed spines. The ventral parts
are covered with small, smooth, light-colored, rectangular scales.

The body of the animal is divided into three well-defined regions :
head, neck, and trunk. The trunk bears two pair of well-developed
linibs and tapers posteriorly to a broad, short tail.

The head is short and the muzzle descends steeply in profile, but
is not separated from the front by a conspicuous angle. It is
covered with small scales, and is bordered posteriorly by a row of
osseous spines. The most anterior structures are paired, small
nostrils. These are small rounded apertures situated one on each
side of the snout, a short distance from its anterior end. Just
posterior to the nostrils are the eyes. They are situated one on
each side of the head about midway between the nostrils and the
tympanic area. Each eye is guarded by a short, thick upper eyelid
and a thinner lower lid. The lower lid covers most of the eye when
that organ is closed. Within the eyelids and attached to the anterior
comer of the eye is a thin, transparent nictitating membrane which
closes backward over the eye. Above each orbital socket is a bony
structure which forms a posterior superciliary angle by being pro-
duced into a short postorbital horn. Behind the eye and a little
posterior to the mouth is the auditory aperture. A thin tympanic
memhrane is stretched over it. Above the auditory aperture and be-
tween it and the eye are three temporal horns. They form the post-
lateral border of the head. The occipital horns mark the dorsoposte-
rior boundary of the head. The gape of the large mouth begins at
the snout and extends posteriorly to within a short distance of the
auditory aperture. The lower labial scales vary from the small
rounded anterior ones to the prominent, acute posterior spinelike
scales.

The cervical region is thick and stout but well differentiated. A
transverse gular fold is present on the ventral surface of the neck.

REPTILIA 563

A row of enlarged gular scales parallel the gular fold and a small,
single spine may be found on each side at the posterior gular border.
Two longitudinal folds are present on each side of the cervical region.
These folds descend forward and obliquely downward to the plane
of the gular fold.

The trunk is depressed and is broadly fusiform in shape. It is
flattened ventrally and tapers posteriorly toward the pelvis.

The anterior Unibs spring one from each side of the body near the
anterior end of the trunk. They are divided into three divisions :
proximal or hrachium, middle or antehrachium, and distal or manus.
The distal division is terminated by five clawed digits. The first
digit is the shortest. It is designated as the thumb or pollex.

The posterior limbs arise one from each side of the body near the
posterior end of the trunk. They are also divided into three regions :
proximal or thigh, middle or shank, and distal or foot. The foot,
like the hand, ends with five clawed digits. The first digit, or hallux,
is the shortest. The ventral surface of the thigh bears twelve to
fifteen femoral pores, whose function is obscure. They are present
in adult males only.

Between the thighs on the ventral aspect of the body is a slitlike,
transverse vent, or anus. It is the common outlet for the digestive
and urinogenital systems. The short tail is broad at the base but
tapers quickly toward the distal end. In males two genital swellings
lie one on each side of the ventral surface of the broadened proximal
region of the tail just posterior to the vent.

Like most reptiles, these lizards shed their thin epidermal cover-
ing periodically. The variable color pattern is brighter for a time
after this skin is cast.

Digestive System

The alimentary canal is composed of the following organs in order :
mouth, esophagus, stomach, small intestine, large intestine, rectum,
cloaca, anus. Several accessory organs and structures are teeth,
tongue, pancreas, and liver.

The upper and lower jaws form the anterior boundary of the aper-
ture of the mouth, and are each provided with a single row of small,
faintly tricuspid teeth. These teeth are not adapted for mastication
but for seizing and holding prey. The posterior nares are apertures
situated a little behind the end of the snout and separated from one

564 TEXTBOOK OF ZOOLOGY

another by a vertical partition supported by the vomerine bones of
the skull. The Eustachian pits are paired diverticula from the dorsal
wall of the pharynx continuous with the tympanic cavities, which
are closed externally by the tympanic memtranes. The mouth cavity
ends posteriorly as the pharyngeal cavity which is produced poste-
riorly into the horizontal slit which leads to the esophagus. The
glottis, or aperture of the trachea, is located centrally on the summit
of the laryngeal chamher. The tongue is a fleshy organ lying on the
floor of the mouth. It is furnished with many tactile and gustatory
cells, and is usuallj^ covered with a sticky secretion which assists it
in catching and holding prey. It can be extended from the mouth
and is used in capturing insects.

The tubular esophagus opens into the mouth cavity by way of a
horizontal slit posterior to the pharjmgeal cavity. It is deeply pig-
mented anteriorly and lies dorsal to the trachea. The walls are thick
and muscular. It opens posteriorly into the cardiac portion of the
stomach.

The stomach is a cylindrical, muscular organ with relatively thick
walls. When empty it is but little greater in diameter than the
esophagus or the intestine. Since it lies chiefly on the left side of
the body, it is largely concealed by the left lobe of the liver. It is
larger at the anterior or cardiac end and constricted at the posterior
or pyloric end. The pyloric valve is an annular ridge of muscular
tissue which narrows or closes the aperture between the stomach and
the small intestine. The inner surface of the walls of the stomach
is thrown into longitudinal folds or rugae. Distributed over the
entire inner surface are the openings of minute ducts from the numer-
ous gastric glands which secrete the gastric juice. This digestive fluid
normally begins the digestion of proteins in the food.

The intestine forms several coils in the posterior third of the coelom,
and is held in place by the mesenteries. As is commonly the case
in insectivorous or carnivorous animals the intestine is relatively short.
The anterior part of the small intestine is the duodenum. The com-
mon bile duct from the liver and the pancreatic duct from the pan-
creas join this section of the intestine. There is no definite division
between the duodenum and the ileum, the succeeding portion of the
intestine. After several turns the ileum passes into the large intestine
on the right side of the body. An annular ridge of tissue, the ilea-

REPTILIA

565

caecal valve, narrows the aperture between the ileum and the large
intestine. The large intestine can be traced from the right side
of the body downward and medially to a point where it passes out
of sight beneath the pelvis to enter the cloaca. There is a very small

Aorta

■Pulmonary artery

Liver

Qall bladder

Duodenum ,

Pancreas —

Ileum— f^^-

Large intestine --^^

0^/ary

- -Adipose body
Oviduct

— Cloaca

Fig. 296. — Dissection of a horned lizard to show internal organs, ventral view.

blind pouch or cecu7n, if any, at the position of the ileocaecal valve,
where the ileum joins the large intestine. Posteriorly the large in-
testine becomes the rectum before entering the cloaca which is di-

566 TEXTBOOK OF ZOOLOGY

vided into an anterior and posterior chamber. The anus is the pos-
terior end of the alimentary canal.

The pancreas is an elongated branched body of glandular material
located within the first loop of the intestine between the stomach
and the duodenum. It secretes an alkaline digestive fluid which is
emptied into the duodenum.

The liver is a larg'e, bilobed organ divided imperfectly into right
and left lobes. It secretes bile, an alkaline digestive fluid. This fluid
is stored in the gall Madder until food enters the duodenum. It then
passes into the intestine through the common bile duct.

Respiratory System

A basic feature that distinguishes reptiles and other higher ver-
tebrates from amphibians and fish is that they do not breathe by
gills during development. The respiratory system of all lizards is
typical of air-breathing vertebrates. A tubular trachea conducts
air from the mouth to the highly vascularized lungs. This tube is
prevented from collapsing by rings of cartilage. A gaseous ex-
change is effected between inhaled air and the blood. The oxygen
is absorbed and carbon dioxide exhaled. Breathing is effected by
the motion of the ribs. Air passes through the nostrils, or external
nares, into the olfactory chaml)er. From there it passes through the
internal or posterior nares into the mouth cavity to be forced
from there through slitlike glottis into the chamber of the larynx.
The larynx continues posteriorly as the trachea, which lies in the
midventral line of the throat and extends to the lungs. The posterior
end of the trachea bifurcates to form two smaller tubes, the right
and left hronchi. Each bronchus connects with a corresponding lung
a short distance from the apex of the lung. Within the lung each
bronqhus divides into secondary bronchi and these in turn give off
tertiary branches. The smallest tubules, bronchioles, carry the air
into air spaces or alveoli. It is in the alveoli that an exchange of
gases takes place.

The Circulatory System

The circulatory system of reptiles shows distinct advances over
the circulatory system of amphibians. Since respiration is carried
on exclusively by the lungs, changes which are in keeping with this

REPTILIA 567

modification are apparent in the pulmonary circulation. The system
consists of a heart, arteries, capillaries, and veins, forming a continu-
ous or closed set of vessels throughout the body.

The heart lies at the extreme anterior end of the body cavity and
is enclosed in a thin- walled sac, the pericardium. The heart consists
of a dorsal sinus venosus, a right and a left auricle, located anteriorly,
and a posteiior ventricle. The auricles are thin-walled chambers
which communicate with the ventricle by means of the auriculoven-
tricidar aperture. This aperture is guarded by an auriculoventricular
valve which consists of two flaps. The thin vertical inter auricular
septum separates the auricles. The ventricle is a thick, cone-shaped,
muscular chamber. The base is directed anteriorly and is connected
to the auricles at the auriculoventricular aperture. The thick, spongy
walls of the ventricle leave only a small cavity within its chamber just
below the auriculoventricular aperture. It is completely divided into
tw^o chambers by the perforated interventricular septum only w^hen in
a state of contraction.

Three arterial trunks arise from the ventral surface of the ven-
tricle. The pidmonary artery arises from the right portion of the
ventricle. Its aperture is separated from that of the right aorta by
means of a muscular partition.

The pulmonary artery is visible for only a short distance. It origi-
nates from the right side of the ventricle, passes anteriorly, then
turns dorsal to the heart. It soon divides into right and left trunks
supplying the respective lungs.

The right aortic arch arises from the left half of the ventricle and
at first is dorsal to the other two arterial trunks. Soon the pulmonary
artery assumes the dorsal position and the right aortic arch becomes
ventral and passes to the right and anteriorly between the auricles.
At the anterior end of the heart it forks to form a Y. The branch
passing to the left is the left common carotid artery. The right
branch forks again. The anterior branch is the right common carotid
artery, and the other is the continuation of the right aortic arch,
which gives off the two subclavian arteries.

The left aortic arch is the most ventral of the three arterial trunks
and it also arises from the left side of the ventricle. The right aortic
arch is in close relation to the left arch. The left aortic arch then
passes laterally and diagonally across the pulmonary artery and the

568

TEXTBOOK OF ZOOLOGY

right aortic arch, and continues cephalad on the left side of the
inter auricular space.

The aortic arches may be traced laterally and are seen to bend
around the esophagus and unite with one another on the dorsal side
of the esophagus, ventral to the vertebral column, to form the median
dorsal aorta. An esophageal artery may branch from the left aorta.

Rt. interna] ccroti'd_

Rt. aorta^-.^ps=^

Rfc. Subdavian A
Rt aorta

Dorsal aorta

RthepaticA.
Ant. mesenteric A

SpermaUc A

Renal A A A

Kidney_

Femoral A.^fl _ _ ^^ ~^:=;fs:^

Sciatic A-i

Cow.

^Lt. common carotid A.

External carotid A.

internal carotid A.

-Connection, 3''-^h.^^arcli

_3i ^^.L.aorta

Pulmonary A.

-.-LpajponaryA.
-br^-L. iubclavian A.

\ Ventricle

Antqastric

Coeliac A.

Ventral qastric A.

ijTis^ .Hepabo-duodenalA.
y 1 _5up. mesenteric A.
'n htestinat A.

Lumbar A-

Adipose A.

Fig. 297. — Heart and arterial system of horned lizard, ventral view.

The right and left comtnon carotid arteries arise from the right
aortic arch at the point where it leaves the left aortic arch. The first
branch of each is the external carotid which supplies the lower jaw.
Each of the common carotids next gives off an internal carotid artery
which supplies the right and left sides of the head. Now the common

REPTILIA

569

carotids pass laterally and parallel with the corresponding aortic
arches. At the distal end of the common carotids each anastomoses
with the corresponding aortic arch.

The right and left subclavian arteries arise from the right aortic
arch just anterior to its junction with the left aortic arch to form the
dorsal aorta. Each passes laterally ; one to each of the forelimbs.

The dorsal aorta is a median, unpaired, longitudinal artery lying
in the mid-dorsal line ventral to the vertebral column. A little
further caudal small gastric arteries pass to the stomach. Near the
posterior end of the stomach the large coeliac artery is given off. This
soon divides into two trunks. The anterior coeliac artery supplies
the digestive glands, stomach, and duodenum. The posterior branch,
the ventral gastric supplies the ventral surface of the stomach. The
anterior mesenteric is the next unpaired branch of the aorta and it
soon divides into the hepatoduodenal to the left side of the liver and
the duodenum, and the superior mesenteric to the ileum. The right
hepatic artery springs from the dorsal aorta and passes to the right
lobe of the liver. The inferior mesenteric arteries include three small
vessels arising from the dorsal aorta at the level of the pelvis, which
supply the large intestine and rectum. Two pairs of small branches,
the genital and renal arteries, supply the gonads and kidneys re-
spectively. The right and left iliac arteries branch from the aorta
and pass into the respective hindlimbs and bifurcate to become fem-
oral and sciatic artery of each limb. Further posteriorly the dorsal
aorta continues into the tail as the caudal artery.

The veins of the body, with the exception of the pulmonary veins,
enter the sinus venosus. This is a thin-walled chamber located dor-
sally to the right auricle. It communicates with the right auricle by
means of the sinuauricular aperture. This aperture is guarded by
the two-lipped sinuauricular valve.

The pulmonary veins bring blood from the lungs. They unite into
a short common trunk which enters the left auricle on its dorsal wall.

The postcaval vein, which receives the wide hepatic vein from the
liver, extends from the anterior border of the liver to the sinus
venosus. Posteriorly it lies between the gonads, and is formed by
the union of a pair of efferent renal veins which lie parallel to the
epididymis or oviducts. The postcava also receives a spermatic or
ovarian tributary from each gonad.

570

TEXTBOOK OF ZOOLOGY

The hepatic portal vein is formed by the union of the gastrosplenic,
mesenteric, and the ventral abdominal veins. It passes anteriorly and
enters the tissue of the liver. The principal branch of the hepatic
portal is continued posteriorly as the abdominal vein. The gastro-
splenic is a large vessel and lies within the duodenoJiepatic mesentery,

External ju(]u!ar V.

^ Internal juqular V-

Sinus venosus

J'ulmonary V.
_ _^ _ ,^^^y ^.Hepatic V.

/- ' ' branches in liver

i HepaUc portal V.

gastric V.

-Ventral flMomi'na I V.

J Renal V.

I inferior mesentericV

^^ — Femora] V

.Sciatic V.
— -/.Renal portal V.
.Caudal V.

Fig. 298. — Venous system of horned lizard, ventral view.

parallel to the pancreas. It receives gastric veins from the stomach
and small pancreatic veins. It is formed at its distal end by the
union of the splenic and mesenteric veins from the spleen and in-
testines. Several separate gastric portal veins take blood directly
from the stomach to the liver.

REPTILIA 571

The renal portal system drains a considerable portion of the body.
The venous blood from the tail is carried by means of a caudal vein.
It lies ventrally to the caudal vertebrae, and bifurcates at the base
of the tail to form the two lateral pelvic veins. Each of these passes
across the ventral face of the corresponding kidney to contact the
posterior edge of the corpora adiposa. Each receives efferent veins
from these bodies, at the anterior border of which the pelvic veins
empty into the epigastric or anterior abdominal vein. This vein ex-
tends anteriorly along the inner face of the body wall to enter the
left lobe of the liver on the posterior face. The renal portal veins
receive the pelvic veins near the kidneys. The pelvic veins are formed
by the union of the femoral and the sciatic veins from each hindlimb.

The right precava is located in the lateral neck region and passes
posteriorly to the sinus venosus. It is formed by the union of the
internal and external jugulars. For a short distance it is the common
jugular. Then it receives the right subclavian vein which returns
the blood from the right forelimb. From that point on it is the right
precava. The left precava is formed by the union of the left internal
jugular and the left subclavian veins. The left external jugular is
not present. The right and left precavae converge with the postcava
and enter the sinus venosus.

The Urinogenital System

The reptilian urinogenital system shows decided advances over
the types of urinogenital systems found in lower vertebrates. The
mesonephric type of kidney found in lower groups is replaced by
a metanephric type of kidney in the reptiles. The products of the
gonads are conducted through special ducts from these glands and
not through the kidney, as in lower animals. Instead of producing
hundreds of small eggs in a season, as in certain amphibians and
fishes, the horned lizard lays twenty to twenty-five eggs of moder-
ate size. The eggs of all vertebrate classes below reptiles are with-
out shells. This makes external fertilization possible and necessi-
tates an aquatic nesting site. The eggs of reptiles are covered by
a tough, thick shell. Fertilization must occur internally before the
shell is put on; hence, the males have intromittent copulatory or-
gans for introducing spermatic fluid into the females. Such eggs
will develop on land and often in rather dry places. The oviducts

572

TEXTBOOK OF ZOOLOGY

are much shorter in animals producing only a few ova than in
animals producing many.

Certain similarities, however, are evident in the urinogenital sys-
tem of reptiles and of lower forms. The urinogenital products are
voided into the common cloacal chamber as in lower animals.

The kidneys of the horned lizard are reddish brown, paired bodies
lying on each side of the middorsal line, between the large intestine
and the body wall. Technically they are outside of the body cavity
since they are dorsal to the peritoneum. Each kidney has a delicate
excretory duct, the ureter. In the males this duct opens into the

Oviduct —

Apertures
oviduct
and ureter

-Intestine

J4} Kidney

%M Ant. cloaca

M^ Cut oviduct

'^M' Ureter

r Post.Cloaca

^ Anus

Fig. 299. — Urinogenital system of female horned lizard from ventral view.

vas deferens and a common duct leads to the cloaca. In the females
the ureter connects the posterior end of the kidney with the dorsal
wall of the cloaca.

The male reproductive organs consist of a pair of testes, a pair of
eversible hemipenes, and various connecting ducts. The testes are
white oval bodies lying in the posterior part of the body cavity, one
on each side of the mesorectum. Each testis is attached to the dorsal
body wall by a sheet of peritoneum, the mesorchium. The series of
ducts connecting each testis with the cloaca includes the rete testis,
the epididymis, and the vas deferens in order. The vas deferens
passes posteriorly, joins the ureter, and enters the cloacal wall.
The paired, eversible hemipenes, or copulatory organs, are vascular

REPTILIA 573

sacs embedded in the musculature of the ventral side of the base
of the tail. They are elongated, smooth-walled structures which
open into the posterior chamber of the cloaca. During copulation
these organs protrude through the vent of the cloaca by everting
themselves; i.e., turning themselves wrong side out. They are pulled
back into place by the contraction of a retractor muscle attached to
the distal end of each organ. Often only one hemipenis functions
during copulation, but this is not an invariable rule.

The female reproductive organs consist of a pair of ovaries and a
pair of oviducts. Each ovary is attached to the dorsal body wall in
the posterior end of the body cavity by means of a double sheet of
peritoneum, the niesovarium. The folded oviducts lie dorsolateral
to the ovaries. The anterior end of each oviduct is expanded into a
thin-walled ostium; posteriorly, each duct opens into the dorsal wall
of the posterior cloacal chamber. The oviducts are held in position
by a broad fold of the peritoneum, the hroad ligament.

The common cloacal chamber of both sexes receives the products
discharged by the alimentary canal, the kidneys, and gonads. The
cloaca is divided into two regions, an anterior and a posterior, by
an incomplete ridge of mucous membrane.

The Nervous System

The reptilian nervous system shows certain advances over the
nervous system of amphibians. Twelve pairs of cranial nerves are
present, as in birds and mammals. The various regions of the
brain are better developed than in lower vertebrates ; however, the
reptilian brain is still a comparatively simple structure.

The nervous system of horned lizards, as in other vertebrates,
consists of the central nervous system, the peripheral nervous sys-
tem, and the sympathetic nervous system.

The central nervous system consists of a 'brain located within the
skull, and a spinal cord located within the neural canal. The brain
has all the divisions of the typical vertebrate brain. The peripheral
nervous system is composed of the twelve pairs of cranial nerves from
the brain and sixteen pairs of spinal nerves from the spinal cord.
These nerves are arranged segmentally. Typically, there is one pair
to each segment. The sympathetic system includes two conspicuous
white trunks that extend along each side of the vertebral column
and the lateral fibers from these trunks. Each of the sympathetic

574 TEXTBOOK OF ZOOLOGY

trunks is connected with all of the spinal nerves on the same side
of the body by a number of ganglionic enlargements. The usual
sensory and motor functions are carried on by these systems. The
autonomic system helps to regulate involuntary reactions.

The special sense organs of the head are the eyes and the ears.
Each eyelall reposes in an orbital cavity, and is attached to the walls
of the cavity by voluntary muscles which serve to move it in all direc-
tions. The innervation is from the optic lobes and other parts of the
brain by way of the optic nerves. The structure of the eye itself is
similar to that of other vertebrates.

The ear consists of two principal parts: the middle ear, or tym-
panum, and the internal ear, or membranous labyrinth. The cavity
is closed outwardly by the tympanic membrane. It communicates
with the mouth cavity by means of the Eustachian passage. A small
rod of bone and cartilage, the columella, stretches across the cavity
from the tympanic membrane and is fixed internally into the mem-
brane covering the fenestra ovalis. The internal ear is enclosed by
the bones of the auditory region.

The Skeletal System

The reptilian skeleton retains many features of the amphibian
skeleton and forecasts many features found in birds and mammals.

The divisions of the skeleton of the horned lizard are the skull,
the vertebral column, ribs, sternum, the limb girdles, and limbs.

The skull proper is triangular and somewhat pyramidal in form.
The base of the pyramid is represented by the posterior aspect of the
skull, and the apex is represented by the snout. It is more com-
pletely ossified than in amphibians and is composed of a greater
number of bones. There are sharp protuberances and excrescences
that form the skeletal support of the "horns" of horned lizards.
Were these magnified to the size of dinosaurs, they would be equally
imposing. The skull articulates posteriorly with the atlas, or first
cervical vertebra.

The vertebral column consists of several distinct regions and a
number of vertebrae. There is a cervical region composed of seven
vertebrae which form the skeleton of the neck. A thoracicolumbar
region consists of fourteen vertebrae and with the ribs and sternum

REPTILIA

575

make up the skeleton of the trunk, A sacral region of two vertebrae
gives attachment to the pelvis. A caudal region of fourteen vertebrae
supports the tail.

txterriQl narc

Maxilla

5upra orb'ibal

Orbital fossa

Parietal £

foramen

Parietal

j^^^f^ Premoxil/or

Nasal

Pr<Bfror>tal

\^ Juqal

ik___fronta/

iat. temporal
fossa

\ Posterio-

lateral
process

Fig. 300. — Dorsal view of the skull of horned lizard.

Vomer:

Jugal

iupna-_ _
orbital

BasI---
occipital

Internal nare
Teeth

Palatine

Porasphenoid

Pbzryqoidi

Ba 51* sphenoid

Inferior tem-
poral fossa

^Occipital condyle

Fig. 301. — Ventral view of the skull of horned lizard.

The pectoral girdle is composed of bone and cartilage. The two
anterior limbs articulate with it. The limb is a rather generalized ex-
ample of a typical pentadactylous vertebrate limb. The pelvic girdle

576

TEXTBOOK OF ZOOLOGY

is composed of a pair of triradiate bones, or innominate bones. Each
innominate bone consists of three separate bones : the ilium, ischium,
and pudis. These three bones radiate from a triradiate suture at
the acetabulum or hip socket. The pelvic girdle articulates with the
sacral vertebrae by means of the ilia. The ischia are the most pos-
terior bones of the three. The pubic bones extend anteroventrally
and unite at the pubic symphysis. As in the forelimb, the structure
of the hindlimb is of the characteristic vertebrate type.

Cfav//'c/e__
Bpkorocoid.^.

Coracoid fenestra

Epbternum

Sternum

Posterior
cornu

■n Supra scapula

Scapula
-Pre coracoid
-Coracoid
—Humerus

-Vontanelle

- -Trochlea
-.Olecranon
--Radius

Ulna

Styloid P.

--Carpals
Metacarpal
PholarK^es

Fig. 302. — Sternum, pectoral girdle, and disarticulated forelimb of horned lizard,

ventral view.

Muscular System

The reptilian musculature shows few unusual characteristics. In
a general way it may be said to be intermediate between that of
amphibians, on the one hand, and birds and mammals, on the other.

Since the horned lizard is a generalized terrestrial animal, the

musculature follows the typical vertebrate plan rather closely.

REPTILIA 577

THE TURTLE

The pond turtles of various genera belonging to Family Emydidae
have long been used as convenient and useful examples for studies
of reptilian characteristics. While there are many variations in
details, the main features of Troost's turtle, Pseudemys troostii
elegans (Wied), are foundj very little modified, in other members of
the family. This is one of the most abundant species of the Mississippi
Valley region and is veiy frequently the turtle sold by supply houses
for laboratory use.

One of the most interesting characteristics of this species (P.
troostii elegans) has only recently been discovered. The adult males
are frequently melanistic; i.e., have the normal color pattern con-
cealed by a superimposed layer of black pigment. The 3'oung males
have the same coloration as the females and only gradually take on
the melanistic coloring. Hence, all stages from normal coloration to
jet black can be found among them.

Habits and Behavior

These turtles are commonly seen sunning themselves on logs
along the margins of ponds and rivers. When disturbed, they fall
into the water with a splash and swim hastily for the deeper parts
of the body of water. This activity in daylight hours might lead
one to suspect that they are entirely diurnal, but turtles are known
to feed at night. Hence, it seems that they are not as strictly regu-
lated by hours of light and darkness as are most vertebrates. Since
all turtles are cold-blooded animals, they hibernate in winter in
temperate regions. Turtles are often seen in considerable numbers
in a restricted localitj^ but there is no congregating or communal
instinct among them. Each one is totally oblivious of the presence
of all the others, except during the mating season, when the males
seek out the females. Turtles have been observed to mate at all
times from spring to autumn but the chief mating period occurs
soon after the animal comes out of hibernation in the spring. The
eggs are laid soon afterwards in a hole or pit in the earth excavated
by the female for that purpose. When the full complement has
been deposited, she covers them and returns to the water. Turtles
taken in the autumn months are often found to contain a full com-
plement of completely shelled eggs. This indicates that the mating
that commonly occurs immediately prior to the laying of these eggs

578 TEXTBOOK OF ZOOLOGY

does not fertilize the eggs of the year but those of succeeding years.
In experimenting upon this phase of the life history of turtles, the
United States Fish and Wildlife Service has discovered the surprising
fact that a female turtle may lay fertile eggs for as long as four
successive years after a single mating.

External Structure

No vertebrate is so unique in structure as the turtle. No turtle
could be mistaken for any other animal. The arched or dome-
shaped dorsal covering, the carapace, and the plane floor of this
armor, the plastron, set the turtles apart from all living vertebrates.
The head and neck can be completely withdrawn into the shelter
of this covering; and legs, feet, and tail can be partially protected
in this manner.

The body of a turtle may be divided into the following regions:
head, neck, trunk, and appendages.

The head is covered with thin, smooth skin. Various colored lines
and patterns -may be present on the skin of the head and neck.
Troost's turtle has an oblong red patch on each side of the head and
neck unless it has been completely obliterated by black pigment de-
posited over it. The neck is relatively long and slender, and re-
tractile within the shell by vertical sigmoid flexures. The skeletal
elements of the trunk region are completely co-ossified into an im-
mobile boxlike structure. This carapace and plastron are covered by
scutes of epidermal tissue. These scutes are definite in number and
arrangement. Various color patterns are found on or under them.
The legs are typical pentadactyl limbs. The toes are united by
various degrees of webbing in aquatic forms. The tail is relatively
small and short.

Digestive System

The organs of the digestive tract are mouth, pharynx, esophagus,
stomach, small intestine, large intestine, rectum, cloaca, and anus.
Various accessory structures and glands are the cutting edges of the
beaklike jaws, the liver and the pancreas. The size and relative
length of the various parts of the alimentary canal are correlated
with the food habits of the animal. Turtles eating largely vegetable
food have intestines many times the length of the body. Those taking

REPTILIA

579

chiefly animal matter possess intestines only a little longer than the
body. The accessory glands, liver and pancreas, function as in other
vertebrates.

External jugular vein„

Brachial vein

Scapular vein

Subclavian vein

Thyreo-5capularvein.
Right prccavfll vein.

Gall bladder-

Poslcaval vein

SiniiS veno3us

Right auricle

Pancreas

Postcaval vein

Kidnej/

Small intestine..

.Trachea

.Cervical artery

Esophagus

..Lung

Axillaryiartery

.Left aorta

.Thyroid gland

..Subclavian artery

.Brachiocephalic apter_y

.Pulmonary artery

.Left lobe of liver

.Ventricle

.Stomach

.Anterior abdominal vein

_Sapepiop mesenteric artery

_ Ovary

.Dorsal aorta

.Oviduct

.Renal portal vein

_Lar^e intestine

.Openini^ of oviduct

.Cloaca

-tJrinary bladder

_Anu5

Fig. 303. — Dissection to show internal organs of tlie turtle, Pseudemys troostii

elegans, ventral view.

Respiratory System

The respiratory system is adapted for breathing air only, re-
gardless of the fact that turtles spend the greater part of their lives
in water. The organs composing the system are the nostrils,
pharynx, trachea, bronchi, and lungs. An indispensable accessory
structure is the htjoid apparatus. Since the ribs are made fast in
the carapace, breathing cannot be accomplished in the usual vertebrate
manner; hence, the hyoid apparatus, located in the gular region,
functions as a pump and the turtle seems to swallow air. The lung

580

TEXTBOOK OF ZOOLOGY

space is relatively great in some turtles. This enables them to remain
submerged for considerable periods. Accessory urinary bladders also
serve in aquatic respiration.

Marq'inals
Scapula

Humerus

Radius —

Entoplasbron

Epiplastron

Tibia ---rJi
Fibula - ■

)(\pHp\aslrov\

Suprapygal

Fig. 304. — Skeleton of the turtle, Pseudemys troostii elegans, showing ventral view
of carapace and dorsal view of plastron.

Circulatory System

The circulatory system of the turtle consists of the heart, arteries,
connecting capillaries, and veins.

The heart is typically reptilian in structure, having two auricles
and one ventricle with a perforated partition dividing it. The pul-
monary circulation returns blood to the left auricle, but this aerated

REPTILIA

581

blood is partially mixed with venous blood in the ventricle before
being sent into the aortic arches.

Urlnog'enital System

The excretory system consists of two kidneys, their ureters, the
cloaca, and a bilobed urinary bladder. The urine is voided into the
cloaca and stored in the urinary bladder until expelled through the
anus.

The sexes are separate. The male reproductive system is made up
of a pair of testes, a pair of vasa deferentia, and an evertible penis
located on the anterior wall of the cloaca. The female genital organs
are ovaries, oviducts, and cloaca.

The Nervous System

This system is typically reptilian (see page 573). In some turtles
adaptations in structure of nervous tracts to the eye are known to
have occurred, probably in response to feeding habits. That part
of the peripheral nervous system which normally innervates the
musculature of the costal region is absent.

The Skeleton

The skeleton of turtles is the most characteristic of their systems
Embryological studies have shown that in the early stages the
skeleton follows the typical vertebrate plan. Soon, however, the
ribs rise above the limb girdles; those above the pectoral girdle
bend anteriorly and those over the pelvic girdle bend posteriorly.
The process of co-ossification goes on over a period of years and is
not altogether completed until maturity is reached. The result is
a vertebrate animal with the limb girdles enclosed by the ribs. No
other vertebrate follows such a plan.

The Muscular System

This system shows modifications corresponding to those in the
skeleton. The intercostal muscles of the body wall are not present
as such. The muscles of the neck are highly developed and adapted
to moving the head with great rapidity. The muscles of the legs are
little modified, being similar to those of other vertebrates.

CHAPTER XXX

AVES

Class Aves (A' vez), which includes all birds, has several distinc-
tive characteristics. The fundamental structural features of this
class are quite closely related to those of reptiles. The outstanding
characteristics of birds are as summarized in the following state-
ments. (1) The skin is covered with feathers which are exoskeletal
outgrowths of it. Birds are the only animals with feathers. (2) The
jaws are toothless in the adult and are covered with a hard horny
beak. (3) All modern birds are bipedal, the forelimbs being modi-
fied into wings, or undeveloped. (4) The pelvic girdle is securely
anchored to the vertebral column to support the bird adequately on
two legs. These limbs serve the animal in locomotion on land and in
water as well as for perching and climbing. (5) The caudal ver-
tebrae are greatly reduced in number and all except the free an-
terior ones are fused into a single bone, the pygostyle. (6) In most
birds the digestive system is modified to provide a crop for storage of
food, and a muscular portion of the stomach for chewing food. (7)
They are warm-blooded vertebrates.

Most people associate birds with flight, but there are numerous
birds which do not have this power. In fact, several have practically
no wings. The wings of the New Zealand kiwi are so rudimentary
that they are completely concealed by the body feathers.

Many birds are migratory, moving north for the summer season
and south for the winter. This is a very aristocratic habit around
which the living activities are centered. The migration routes are
precisely laid out ; however, birds occupying the same general breed-
ing range do not necessarily spend the winter in the same region or
vice versa, and they do not necessarily travel the same route in the
fall as was traveled in the spring. The birds return to the same
breeding range and winter home, season after season, which seems to
demonstrate a definite homing sense. An example of extreme migra-
tion is the Arctic tern which nests within the Arctic circle and win-
ters within the Antarctic circle, a distance of 10,000 or 11,000 miles.

582

AVES

583

This is traversed fall and spring and largely over water. The aver-
age migrating bird travels only about 23 miles a day on migration,
but this tern makes at least 150 miles a day. Recently the U. S. Army
Signal Corps reported that one of its messenger pigeons had flown 600

Fig. 305. — Migration airways of birds in North America. (From Metcalf, Textbook
of Economic Zoology, published by Lea and Febiger.)

miles in less than 14 hours (42 miles per hour) without the aid of a
tail wind. Several theories attempting to explain bird migration in-
volve such factors as food conditions, temperature, changing length
of days, and hormone control. The explanation is still an open
question.

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TEXTBOOK OF ZOOLOGY

Classification

Birds constitute a relatively uniform group of animals. They are
much more similar throughout the class than any other group to be
studied. The differences between the most dissimilar birds are no
greater than the differences seen among frogs in the order Salientia,
in class Amphibia. Such characteristics as size, color, wing develop-
ment, type of beak and feet, are the bases for distinguishing the

Pig. 306. — African ostrich, Struthio camelus. (From Metcalf, Textbook of Economic
Zoology, published by Lea and Febiger.)

orders of birds. There are four orders of flightless birds which are
sometimes placed in a group designated as subclass Ratitae. This
group includes such birds as ostriches, rlieas, cassowaries, and kiwis.
The sternum has no keel in any of these, the feathers are without
barbules, and the wings are either absent or reduced. All other birds
would be grouped in subclass Carinatae according to this plan. More
than 14,000 species of birds are classified into twenty-five orders.

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585

A brief summary of the orders of birds will be given in the follow-
ing pages. Some of the orders are divided into numerous families.

Struthioniformes (Ostriches). — These are the largest known birds
and they are native to Africa, but they are growing in domestication
in numerous parts of the United States. The large eggs weighing
three or four pounds are laid in communal nests in the sand; they
are then incubated by the sun. The wings are very rudimentary,
and there are only two toes.

Fig. 307. — Kiwi, or apteryx, a wingless bird from New Zealand which is about
the size of the domestic fowl and loolts lilce an overgrown chick. (From Krecker,
General Zoology, published by Henry Holt and Company, Inc., after Evans.)

Rheiformes (Rheas). — This is another form of running bird. They
are partially feathered on the neck, have three toes, and inhabit
South America.

Casuariiformes (Emus and Cassowaries). — These are flightless,
running birds with very small wings. The former inhabits Australia
and the latter New Guinea. The cassowaries, which are smaller than
ostriches and have shorter necks, have a headgear with bright colors
on the head and neck.

Apterygifornies (Kiwis). — This is the fourth order of running,
flightless birds. The feathers of this form are hairlike. The kiwi is
about the size of a hen, but its wings are much reduced. It is noc-
turnal, and it makes a nest in a hole in the gromid.

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TEXTBOOK OF ZOOLOGY

Crypturiformes (Tinamous). — This is a group of little known
quaillike birds of northern South America, Central America and
southern Mexico.

Sphenisciformes (Penguins). — These are flightless, diving birds.
The feathers are almost scalelike, and the wings are modified as
"flippers" to be used under water. They dive for fish which can be
swallowed under water. They live on barren rocky shores of the
Antarctic. Here they nest in colonies, each female laying one or two
eggs in depressions among the rocks.

Fig. 308. — Penguins. The wings and feet are highly adapted to swimming. (Prom
Hegner, College Zoology, published by The Macmillan Company.)

Gaviiformes (Loons). — Our common loon is checked with black and
white over the back. These birds are also expert divers and swim-
mers under water. They have a very peculiar call that sounds like a
weird or crazy laugh. Their nesting range is between northern
United States and the Arctic circle. The wintering ground is prin-
cipally the Gulf Coastal States.

Colymh (formes (Grebes). — This is a group of small or medium-
sized diving birds with lobed feet. The most common one is the pied-
billed gTebe or hell-diver. The legs are far posterior on the body.
There are about twenty-five species generally distributed over the
world.

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Procellariifonnes (Albatrosses and Petrels). — The former are birds
of the open sea with a wing spread of ten or twelve feet. They will
follow ships for days without landing by gliding on nearly motion-
less wings. They come to land usually on islands in colonies, to lay
their eggs. Laysan Island, far out in the Pacific, is noted for them.
The petrels are small and may be found in midocean. They nest in
crevices on rocky shores and islands. The members of this order
have tubular external nostrils, fully webbed toes, and long narrow
wings.

Fig. 309. — American bittern, Botaurus lentiginosus, stake driver. Sometimes
called shikepoke in common parlance. (From Metcalf, Textbook of Economic
Zoology, published by Lea and Febiger.)

Pelecaniformes (Totipalmate swimmers). — The cormorants are the
widely distributed, well-known representatives of the order. They
have long necks, long, hooked bills, and webbed feet. Their habits
are gregarious, and they dive for fish which can be eaten under
water. The oriental peoples take advantage of this ability to catch
fish, and train them to retrieve for the master.

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TEXTBOOK OF ZOOLOGY

Ciconiiformes (Long-legged Waders). — This order includes two
prominent families : Ardeidae, including bitterns, herons, egrets, and
storks, and Phoenicopteridae, including flamingos; of the latter, one
species, Phoenicopterus ruber, inhabits the Gulf States. It is a tall
bird with long, rosy colored legs and a long, curved neck. The bill
is large and peculiarly shaped, with a curved matching of upper and
lower parts. It is used for seining food in the form of small animals
out of the mud. The tall, conical nests are built of mud on mud
flats. Two eggs are laid in the hollowed top. The birds of the first

<i-V-ii'M<l'rl\.: .

Fig. 310. — Turkey vulture (buzzard), Cathartes aura septentrionales. (From Met-
calf. Textbook of Economic Zoology, published by Lea and Febiger, after Snyder.)

family, particularly bitterns, herons, and egrets, are structurally
similar but their habits are quite different. They are medium-sized
to large birds with long bills, necks, and legs. The bitterns live in
marshes and grasses along shores while herons live in open shore
waters. The great blue and lesser blue herons with some color varia-
tions are quite common all through the middle United States, and in
the South and Southwest. The green heron is another medium-sized
bird. The American bittern, or shikepoke is a long-billed fishing bird
as are also the herons. The Southern States have two species of
egrets, Herodias egretta and Egretta candidissma, with long plumes

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589

or aigrettes on the head which were formerly hunted for millinery-
trade until the birds were almost extinct. Fortunately this has been
prohibited and the birds are reported as on the increase.

Anseriformcs (Ducks, Geese, Swans, and Mergansers). — These are
swimming birds with short legs and completely webbed toes. The
bill is rather broad and flattened. Nearly all of them are strong,
swift fliers, and are excellent game and food birds. There are more
than two hundred species of these ducklike birds, and many of them
frequent the waters of the United States. Geese and swans are some-
what larger than ducks. They migrate to the Southern States for
the winter, flying in V-shaped formation.

Fig. 311. — One of the "blue darters," Cooper's hawk, Acciinter cooperi. (From Met-
calf, Textbook of Economic Zoology, published by Lea and Febiger, after Snyder.)

Falconiformes (Birds of Prey). — In this order there are three
families of these strong birds with powerful wings, stout hooked bills,
and strong claws. It includes vultures, eagles, hawks, falcons, kites,
and ospreys. The turkey vulture (buzzard) and black vulture are
common birds of the Southwest. The California vulture, Pseudo-
gryphus calif ornianus , and the condor of the Andes Mountains of
South America are very large birds. Most of our species of hawks.

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TEXTBOOK OP ZOOLOGY

including the large red-tailed and red-shouldered, are extremely
beneficial because of the rodents and insects they devour. Only the
sharp-shinned hawk, Accipiter velox, and Cooper's hawk, Accipiter
cooperi, are particularly destructive to other birds and poultry. They
are bluish gray, swift fliers, frequently called ' ' blue darters. ' ' There
are two American eagles quite generally distributed over the conti-
nent. They are : the national bird, Aquila chrysaetos, or golden eagle,
and the bald eagle, Haliaeetus leucocephalus.

Fig. 312. — Killdeer, Oxyeclius vociferus. A common shore bird. (From Metcalf,
Textbook of Economic Zoology, published by Lea and Febiger, after Snyder.)

Gallifornies (Fowllike Scratchers). — Turkeys, quails, pheasants,
prairie chickens, and domestic chickens are the most notable repre-
sentatives of this group. Here are some of the most famous game as
well as commercial birds. The wild turkeys and prairie chickens are
becoming scarce, and considerable effort is being made to save and
rebuild the remnant in the Southwest. Pheasants have been very
successfully introduced to some parts of our country from Asia.
They make excellent game birds. The group are all scratchers, living
largely on the ground and feeding on seeds and insects. Some of
them roost in trees. They have stout bodies, hard bills, and short
wings.

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Oruiformes (Cranes and Rails). — The rails, gallinnles, and coots
are rather small birds, the first two live in fresh- or salt-water
marshes. The common coot or mud hen, Fulica americana, lives on
the water like a dnck. It swims about in small flocks, dives, and can
perch in trees. Its feet are not webbed, but the toes are fringed.
The cranes are large heronlike birds with long necks and legs. The
large whooping crane has a wingspread of about eight feet. These
birds like many of our larger ones need more protection to keep them
extant.

Charadriiformes (Plovers or Shore Birds). — There are representa-
tives of nine families of these birds in North America. The plovers,
snipes, sandpipers, killdeers, curlews, gulls, terns, woodcocks, avocets,
phalaropes, and auks are all fairly well known except the last three.
The jacana bird, Jacana splnosa gymnostoma, a tropical form, ranges
into Texas.

Colunibiformes (Pigeons and Doves). — These birds have a distinc-
tive appearance and are seldom confused with anything else. They
are of medium size and have general distribution. They feed on
seeds and fruits. They produce "pigeon milk" which is regurgitated
as food for the young. The passenger pigeon, Ectopistes migratorius
(Fig. 411), which is now extinct, was at one time our most abundant
bird. The flocks were supposed to contain billions of individuals. They
were killed out by the market-hunters and by the encroachments of
civilization. The mourning dove, Zenaidura macroura, is now a much
sought after game bird. It is known for its very plaintive call. They
breed in pairs, then flock and feed in pastures and grain fields.

Psittacifornies (Parrots, Parakeets, Lovebirds, and Macaws). —
The one native species of southeastern United States is now thought
to be extinct or nearly so. The larger parrots are sought after be-
cause of their ability to talk, and all of the others are brilliantly
colored. The lovebirds are small parrots with affectionate disposi-
tions from Africa; they are commonly used as cage birds. Macaws
are the largest of parrots and range from Mexico to Argentina. They
are exceptionally colored.

Cuculiformes (Cuckoos, Chaparral bird, and Ani). — The "rain
crows," as the cuckoos are called because of their peculiar call which
is said to forecast rain, are long, slender birds with long wings, a
long tail, distinctive bill, feet with two toes in front and two behind.
They build their nests in the form of loose platforms of small twigs

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TEXTBOOK OF ZOOLOGY

in bushes and trees. The road runner or chaparral bird is quite com-
mon along- the roads from central Texas on southwest. It is a slender
bird of fair size with a tail as long as the body. It feeds on grass-
hoppers, mice, lizards, and small snakes. Many strange stories are

Fig. 313. — Burrowing or prairie dog owl, Speotyto cunicularia. Common on the

plains around prairie dog towns.

told about its battles with rattlesnakes. The ani is common in Mexico
and gets over into the United States occasionally. They are thought
to lay their eggs in community nests somewhat as the ostrich does.

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593

Strigifonnes (Owls). — These are large birds of prey with large
eyes, soft feathers, strong hooked beaks, and vicious claws. Their
food consists of mice, rats, insects, small reptiles, occasionally small
birds, and snails. The group, as a whole, is beneficial to man. The
great horned owl, Bubo virgmianus, the screech owl, Otus asio, the
burrowing owl, Speotyto cunicularia hypogaea, the barred or hoot owl,
Strix varia, and the barn owl (monkey-faced), Strix pratincola, are
all well known.

Fig. 314. — Kingfisher, Ceryle alcyon. (From Metcalf, Textbook of Economic
Zoology, published by Lea and Febiger.)

Caprimiilgiformes (Goatsuckers). — The nighthawk and the whip-
poorwill are the best known representatives. They are chiefly noc-
turnal, flying during the evening and morning hours and catching
insects as they fly. They are medium-sized birds with a large gape
and weak bill. They excel in their ability to dodge while flying.
No nest is built ; the eggs are laid in almost any convenient place.

Micropodifonnes (Hummingbirds and Swifts). — The former are
small, swiftly flying, long-billed birds which are confined to the
Americas and are more abunda,nt in the tropics. The feathers of the
male are often brightly colored, and the tail in both sexes is long,
slender, and usually forked. Their wings beat the most rapidly of

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TEXTBOOK OF ZOOLOGY

any birds. They feed on nectar from flowers. The chimney swifts
are universally distributed. They have a short bill, broad mouth,
short tail with terminal stiff quills, and long', pointed wings. They
nest in chimneys, caves, hollow trees and on cliffs, in nests made of
twigs and leaves glued together with adhesive saliva. Some forms
make the nest entirely from saliva and in China these nests are eaten
by man.

Fig-. 315. — Mocking-bird, Mivius polyglottos leucopterus. One of the most common
birds of the South and Southwest. (Drawn by Edward O'Malley.)

Coraciiformes (Kingfishers). — This group is represented by the
belted kingfisher, Ceryle alcyon, which breeds from northern Canada
to the Gulf States. It lives along streams and near ponds to dart
down and catch a fish at every opportunity. They nest in holes in a
bank, usually four to six feet deep.

Piciformes (Woodpeckers). — These are arboreal (tree living) birds
with stout beaks for boring holes in tree trunks. They feed largely
on wood-boring insects, their larvae, and ants. The tongue can be
extended some distance beyond the bill in capturing prey. The tail
feathers are stiff and are used to prop the animal against the tree
while it is working. The flicker (yellowhammer), Colaptes auratus,

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the red-headed woodpecker, Melanerpes erythrocephalus, the hairy,
and the downy woodpecker, Dryohates puhescens, are the most com-
mon species. These birds are quite destructive to some orchard trees,
telephone poles, and occasionally roofs of buildings.

Passeriformes (Song or Sparrowlike Birds). — More than half of
the known birds belong to this order. They all have four toes, three
in front and one behind on the same level as the other three. There
are about sixty families described. They are usually small and most
of them have characteristic songs. The arrangement of the toes is an

Pig. 316. — Starling. Sturnus vulgai-is. An introduced European bird. (From Met-
calf, Textbook of Economic Zoology, published by Lea and Febiger, after Snyder.)

adaptation for grasping and perching. There are nineteen families
of common birds which make up this large group, including such birds
as sparrows of several kinds, cardinals, buntings, tanagers, swallows,
kingbirds, larks, phoebes, scissor-tailed flycatchers, crows, jays, star-
lings, magpies, blackbirds, grackles, cowbirds, bobolinks, swallows,
martins, waxwings, shrikes, vireos, warblers, wrens, thrashers, mock-
ingbirds, nuthatches, chickadees, robins, and bluebirds. The starling,
Sturnus vulgaris, was introduced from Europe in about 1850. Since
that time it has spread westward from the Atlantic seaboard until

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TEXTBOOK OF ZOOLOGY

at the present time it reaehes the Rocky Mountains and has gone far
into Texas on the southwest. It became noticeable in central Texas
in the winter of 1935-1936 and plentiful during 1936-1937. It has
increased rapidly and it promises to be a nuisance. It may be recog-
nized by its gregarious habits, its rapid darting flight in flocks, its
short, thick body, and the fact that it walks on the ground instead of
hopping. It is about the size of a cowbird and its color is greenish
or purplish black with lustrous brown flecks or streaks.

KACTS RKt,ATJXG TO TUB ENT.I.ISH SfAimOW

ITS FXX.)0 Is CHtEFf.y.GttAIN.

TT IXlJiS SKUIOirs HAKM TO STANDING

KIELDfs AN 11 GKAIX IN' THE SHOCK.

rr DEFKATS MAX;8 EFFOKTS TO tTCED

AXU lUUnOR NATIVE BirSD.S BV COX-

.S'OMtNG THK VXHilK

rr DJttvKs AWAV oi"u nativk biros
WHOHK riiESF.jCcK 1^ Mi;rti xoheto be

I>ESIHK1).

NCSTl-NG USS THA,V OS! VIIU 1\S

AOULT

Fig. 317. — Facts concerning English sparrow. (Courtesy Conrad Slide and Pro-
jection Company.)

Economic Relations

Birds constitute one of our very valuable groups of animals be-
cause of their several services to man. At the same time there are
some forms which have only a negative importance. The meat of
several birds has long been used as food for man. The chief
groups among the wild birds that are used as food are ducks, geese,
quail, doves, grouse, pheasants, and turkeys. Many other birds are
abundant enough and palatable enough, but they are too small.
The sparrow is such an example. There are others that are abun-
dant and easy to secure, but are not particularly palatable; this is

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true of the flesh-eating, fish-eating:, and particularly carrion-eating
forms, such as vultures, eagles, and hawks. During the last few
years the common crow has been coming into use as an article of
food. Since it lives on an omnivorous diet largely of plant origin,
it has a palatable flesh. It certainly is abundant, and it is great
sport hunting it. Some restaurants serve it occasionally as a
regular part of the menu. The wild ducks and geese have been
hunted so extensively that they are no longer a very important
source of food ; too, they are necessarily strictly protected at present.
The chief domestic birds that are used for food are chickens
(domestic fowls), ducks, geese, guinea fowl, pigeons, and peacocks.
The domestic chicken is the chief one of these. It likely descended
originally from the jungle fowl of India and the East Indian
Islands. The original birds were used all through that part of the
world as gamecocks. The present game chickens are nearest the
jungle fowl of any existing breeds. From this beginning have
arisen dozens of valuable breeds of domestic chickens. In thickly
populated areas poultry raising is on the increase, not only because
the hen is an efficient apparatus for converting grain and garbage
into meat and eggs, but also because a large number of chickens
may be kept in a relatively small area. Most of the commercial egg
production comes from the chicken, which produces more than
2,600,000,000 dozen eggs a year in the United States alone.

The turkey is a domesticated form of the native wild turkey of
North America. It is a very popular and profitable meat-producing
bird. Millions are produced for the markets in the southwestern
part of the United States each year. The guinea fowl is the de-
scendent of a small turkeylike bird of Africa. It has dark flesh
which is prized by some people. The pigeon is used as food prin-
cipally while it is young. The young pigeon is known as a "squab,"
and it can be raised in close quarters, such as back yards, in cities.

Besides the production of meat and eggs there are some other im-
portant products and services which come from birds. Feathers
are used both for ornaments and for pillows. Guano, which is a
mixture of the excreta, soil, feathers, unhatched eggs, and dead
birds, is rich in nitrogen and is one of the best sources for nitrogen
•fertilizer. Hundreds of thousands of tons of it are mined each year
and sold. It is estimated that in the areas of the bird roosts the

598 TEXTBOOK OF ZOOLOGY

guano accumulates at the rate of about four inches a year or 750
tons per acre per year. Large numbers of different kinds of birds
render service of almost inestimable value in destroying insects,
rodent pests, and weed seed. The hawks and owls are very im-
portant as rat and mice killers. Finches, sparrows, and quails are
destroyers of weed seed. Woodpeckers, chickadees, creepers, wrens,
bluebirds, robins, thrushes, flycatchers, kingbirds, meadow larks,
swallows, warblers, tanagers, vireos, and many others are very im-
portant insect feeders. This asset, if computed financially, would
run into millions of dollars a year for the country. At the same
time the value of the birds as game for sportsmen to hunt cannot
be measured wholly in dollars and cents.

DOMESTIC CHICKEN

Gallus domestica Cbankiva), the domestic fowl, is a convenient ani-
mal for study because of its size, its availability, and its universal
distribution with man. As previously stated, it is thought to have
arisen from the Indian jungle fowl. It is of great interest, not only
because of its immense economic value, but because it shows many
of the adaptations of this form of animal to its ij^e of life. Although
the chicken has partially lost its power to fly, it still retains the
features which adapt birds to a life in the air, such as feathers,
wings, air sacs, hollow bones, and a rigid skeleton. The principal
modification of the chicken from its wild ancestors is the relative
increase in weight to give the body a stocky build, failure to de-
velop and exercise the short wings, and a great increase in egg
production in most breeds.

Habits and Behavior

This is a diurnal perching bird which spends the time between
dusk and daylight sleeping in a squatting position on a perch.
During the daylight hours it is an extremely busy animal at hunt-
ing food, dusting feathers, running, walking, scratching, and for the
females, egg-laying. The flocking habit is fairly well developed,
and usually one cock establishes himself as master of a certain flock
and all other cocks are more or less subservient to him. They are
entirely polygamous and the mating is promiscuous.

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External Structure

The body of the gamecock, the Leghorn, and the Bantam is quite
spindle-shaped and graceful, while that of the Plymouth Rock or
Rhode Island Red is stocky and almost awkward. The body is
divided into three general divisions: head, neck, and trunk. The

t-VAtsJE

-SHAFT

-QUI LU

SHAFX
BAR B

Atsl-TERIOR

barbui-e:
- posterior

BARBUU E
--HOOK1_ETS

Fig. 318. — A, flight feather showing general features ; B, details of a portion of a

feather.

head is prolonged at the mouth region by the pointed heak, whose
horny covering is derived from the skin and is true horny material.
At the base of the beak there is a small fleshy structure which has a
naked, waxy appearance. This structure is known as the cere. In
the birds of flight, this is larger and serves as a shield to keep the
air from blowing directly into the slitlike nostrils and interfering
with breathing. One relatively large eye is located on either side

600 TEXTBOOK OF ZOOLOGY

of the head. From the location, the vision is necessarily monofocal.
The eyeball is slightly movable and is provided with an upper lid,
lower lid, and a semitransparent nictitating membrane which ex-
tends across the eye from the inner corner. The ear is lacking in
external conchal cartilage, but appears below and behind the eye
on each side as the opening to the external auditory meatus. This
is usually guarded by a few stiff short feathers. On top of the
head is the comh and below the angles of the jaws are the wattles.
These are highly vascularized, fleshy modifications of the skin in these
regions, and are frequently different in the two sexes.

The mobile neck is quite long and joins the dorsoanterior point of
the trunk. The trunk is more or less basket-shaped, with the pair of
wings folded at the sides. The skin stretching from the body to the
wing is the alar membrane. The body stands almost erect on the
strong legs. The distal part of the hindlimbs are devoid of feathers
but are covered with scales. The toes are provided with nails or
claws which originate in the epidermis. The dorsal surface consists
of a horny plate which is set in a matrix. The forelimbs and girdle
are modified for flight, while the hindlimbs and girdle are modified
for bipedal locomotion. The skin which covers the chicken is very
thin and does not contain sweat glands, neither does it have a gen-
eral distribution of oil glands. The oil is supplied by a tail gland or
uropygial gland. The bird squeezes a quantity of oil from this gland
into its beak and passes the beak over all feathers one at a time until
they are all preened. This treatment renders the feathers practically
impervious to water.

The feathers are skin developments arising in dermal papillae
which are well set in the skin. There are three types of feathers :
contour (flight or quill) feathers with a stiff shaft and firm texture;
down (plume) feathers with a soft shaft and a free fluffy arrangement
of the barbs ; and filoplume feathers with hairlike structure consisting
of a slender shaft and a few or more branches. A typical feather is
composed of a quill which sets in the feather follicle of the papilla of
the skin and continues up into the feather as the shaft. The shaft
bears slender barbs which extend obliquely from it to make up the
principal surface of the feather. The barbs in turn bear still smaller
and more slender projections known as barbules, which hook the barbs
together. The barbs in normal position make up the vane, and give
the feather a smooth surface. At the end of the quill there is a small

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opening called the inferior wnhilicus and at the point where the quill
emerges from the skin there is another small opening, the superior
umbilicus. In many feathers the hyporachis or after-feather arises
from this point. The shaft is often called the rachis. The feathers in
many parts of the body are developed in rows with long intervals be-
tween the rows in which there are no papillae, but these areas are
simply covered over by feathers growing in the adjacent areas. The
definite feather areas over the body are called pterylae while the inter-
vening featherless areas are apteria (apterylae).

CEPHALIC
PTEIRYLA
CERVI CAl_
APTERiUM
VE rslTRAU
PTERYLA
HUME RA1_
PXERYLA

SPINAL. PT
ALAR PT.

L-ATERAl-
AP TE RIUM'

c'.-vjr'W -J-/ VEN-TRAL. APT.
VirtiJ-J.iWT' — FE MORAL.

CAUDAl. P
CRURAl- P

Fig.

319. — Diagram showing feather tracts, or pterylae, and featherless areas, or
apterylae (apteria) of a rooster. (Drawn by Titus Evans.)

There is a complete but gradual shedding of the feathers or molt
in the fall and a partial one in the spring. Of course, new feathers
replace the old as fast as they are shed. There may be some color
change of the plumage accompanying the molt.

Digestive System

The alimentary canal consists of the principal organs in order:
mouth, pharynx, anterior esophagus, crop, posterior esophagus, pro-
ventriculus, gizzard (ventriculus), intestine, large intestine, cloaca,
anus. The several accessory organs are beak, tongue, salivary glands,
liver, and pancreas. The proventriculus and gizzard constitute the

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TEXTBOOK OF ZOOLOGY

stomach. The chicken feeds largely on vegetal food, particularly
grain and plant seeds. This food is ground in the gizzard by the
muscular activity of the wall crushing it against the sand and gravel
which have been swallowed and lodged there. The walls of the

E SOPHAGUS
T RACHEA

1_EFT UUNG
HEART

GALL BLADDER
PROVENTRICULUS
Gl ZZARD
DUODE NUM
PANCREAS

UARGE IMTESTINE

VAS DEFERENS
■URETER

C L O A C A

Fig. 320. — Visceral organs of male chicken, ventrolateral view. (Drawn by Titus

Evans.)

proven triculus are glandular and secrete gastric juice which softens
the food and begins digestion of the protein. The cro'p, which is
a modification of the esophagus, is used as storage space for food as it
is ingested. In the pigeon the crop is glandular and secretes "pigeon

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603

milk" on which the young are nourished. The small intestine is
about 60 inches long and is made up of duodenum first, beyond the
stomach, and then at the end of the first turn, the ileum begins. It
is coiled considerably and leads to the large intestine. The bile
ducts coming from the gall bladder and the liver enter the small
intestine about fourteen inches below the stomach. The pancreas,
which lies beside the duodenum, also pours its contents into the
small intestine. Two blind sacs, each about seven and one-half
inches in length, and called caeca, extend forward from their point
of origin at the juncture of the small and large intestine. They are
usually partly filled with a soft, pasty material. The rectum of the
large intestine opens into the saccular cloaca. One portion of this
space receives the fecal material from the intestine and another
portion receives products from the urinogenital organs. The cloaca
opens to the outside by the anus. The ileum and caeca serve as the
principal organs in which absorption of digested food by the blood
occurs. The digestive functions of the bird are very potent and
rapid. This seems to be a necessary compensation for the waste
caused by their extensive and energetic motions, and their high
state of irritability.

Respiratory System

The chicken and other birds breathe through the nostrils, nasal
chambers, pharynx, superior larynx, trachea, inferior larynx (syrinx),
bronchi, bronchial tubes, lungs, and air sacs.

Air is brought into the nasal chambers through the slitlike nostrils
in the upper jaw. Within the nasal chamber the air is warmed by
passing over three scroll-like folds which are the turhinated lamina
supported by the turlinated hones. Next it passes to the pharynx
through a narrow slit in the hard palate. There is a row of filiform
papillae or fingerlike projections marking the junction of mouth and
pharynx, and another transverse row of horny ones at the juncture
of the roof of the larynx and esophageal margin. There is no epi-
glottis but simply the slitlike glottis through the anterior wall of the
larynx. This is provided with two lips which can be brought tightly
together so that nothing can fall through into the larynx when food
is being swallowed. From the boxlike superior larynx the air passes
through the tubular trachea to the inferior larynx or syrinx where
it bifurcates. The walls of the trachea are supported by cartilaginous

604 TEXTBOOK OF ZOOLOGY

rings which in different kinds of birds will vary from 90 to 120,
depending on the length of the neck. This inferior larynx is the
true voice box in the bird because it is here that the vocal apparatus
is located. The two semi-lunar internal tympanic memhranes, one
as a fold on each side of the cavity, are caused to vibrate by the air
in sound production. In song birds there is a membrane with a
second glottis across the lower end of the trachea. The bony bar
supporting it is called the pessulus.

The air goes next into two hronchi which enter the tissue of the
lungs. Within the lung each primary bronchus divides into secondary
bronchi and beyond this each of these gives off smaller tertiary bron-
chial branches. The further branches are considered a part of the
lung. The lungs occupy only about one-seventh of the thoracic space.
Bronchioles which are small branches of the tubules carry the air into
the air spaces or alveoli of the lungs where most of the respiratory
exchange of gases is made with the blood. Birds have, some bladder-
like extensions of the bronchial tubes in the form of air sacs which
increase the respiratory surface as well as making the body more
buoyant in the air or on water. These sacs are arranged in the
neck, thorax, and abdomen, and extend into the cavities of bones.
The principal ones are : a single anterior thoracic, a pair of cervical
sacs along the neck, a pair of posterior diaphragmatic sacs behind
the diaphragm, a pair of anterior diaphragmatic sacs, and a pair
of abdominal sacs.

Circulatory System

This system, as a whole, includes the blood vascular system and
the lymphatic system. The first consists of heart, arteries, capil-
laries, and veins, making a closed system through the body, while
the latter is composed of spaces, vessels, and capillaries which empty
into the blood vascular veins near the heart. The lymphatic system
is somewhat of an open system.

The heart of the chicken and most other birds is relatively large
and is located near the median line of the thoracic cavity. The
double-walled membranous sac, the pericardium, surrounds and holds
the heart in place. There are two distinct, thin-walled auricles, and
two distinct muscular ventricles. Blood is drawn into the right
auricle from the systemic veins ; right and left precava, and the
single postcava. This blood needs aeration and passes through the

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right auriculoventricitlar valve to the rigJit ventricle. Then it is
pumped, with the contraction of the heart, through a semilunar valve
into the pulmonary artery which divides to supply each lung. The
aerated blood from the lungs returns to the left auricle by way of
the two pulmonary veins. It passes from the left auricle to the left
ventricle through the hicuspid or left auriculoventricular valve. Then

^u.

L. CAROTID
R. PECTORAL
R. BRACHIAL

R.BRACHIO-
CEPHALIC
R. ATRIUM
L. BRACHIO-
CEPHALI C
^PULMONARY
LaoRTIC ARCH
L. ATRIUM

/-R. V

ENTRICLE

COELI AC
LUMBAR

RENAL

MESENTERIC

DORSAL

AORTA

FEMORAL

SCI ATIC

I LIAC

-POSTER! OR

MESENTERIC

CAUDAL

Fig. 321. — Heart and arteries of domestic chicken from ventral view. (Drawn

by Titus Evans.)

it is pumped through the semilunar valve in the base of the aorta
and out the right aortic arch. Just at the anterior level of the heart
this arch passes through the pericardium and gives off the right and
left brachiocephalic arteries to the neck, head, and arms ; then it
curves to the right and extends posteriorly around the heart as the
dorsal aorta. The dorsal aorta supplies esophageal branches to the

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TEXTBOOK OF ZOOLOGY

esophagus, intercostal arteries to the ribs, lunil)ar branches to the
small of the back, ovarian or spermatic arteries to the gonads, the
coeliac axis to the crop, gizzard, duodenum, spleen, and liver; the
anterior mesenteric to the small intestine ; posterior mesenteric to
the large intestine; external iliac or crural arteries branching to the
pelvis and thigh ; and, the ischiadic continues one to each posterior
limb where it branches to all parts.

r. jugular

r.brachial
r. subclavian
ventricle
Ctipped anteriorly)
r, pre cava

l.precava
r. pectoral

■^R. ATRIUM
L. ATRIUM
PULMONARY
HEPATIC

POSTCAVA
HEPATIC PORTAL

GASTRO-

DUODENAL

MESENTERIC

COMMON ILIAC

POSTERIOR -

MESENTERIC

FEMORAL

RENAL

COCCYGEO-

MESENTERIC

INTERNAL

ILIAC

CAUDAL

Fig. 322. — Veins of cliicken from ventral view. (Drawn by Titus Evans.)

Excretory System

The urinary apparatus of the bird consists of two metanephric
kidneys, from each of which a ureter extends and empties into the
cloaca. The kidneys are about 2i/4 inches long, made up of three
irregular lobes, and located above the peritoneum in the lumbosacral
region. Each is made up of Malpighian bodies, uriniferous tubules.

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607

and the surrounding tissue. Urine passes from the collecting tubules
in the kidneys to the pelvis of each of the ureters, and through the
ureters into the cloaca. From here it passes to the exterior through
the anus with the feces. There is no urinary bladder.

R. SUBCLAVIAN ARTERY
R. CAROTID A.
AORTIC ARCH

BRACHIO-CEPHALIC A.

BRACHIO-CEPHALIC A.
LEFT ATRIUM
PULMONARY ARTERIES
ENTRANCE OF L. SUBCLAVIAN V.
ENTR. OF PULMONARIES
SEMI-LUNAR VALVES
L.ATRIO- VENTRICULAR VALVE
RIGHT ATR.-VENTR. VALVE
ENTR. POSTCAVAL VEIN
RIGHT PRECAVAL VEIN
RIGHT ATRIUM
INTER-VENTRICULAR WALL

RIGHT VENTRICLE
LEFT VENTRICLE

Fis:. 323. — Horizontal section of heart of chicken to show chambers and valves
from ventral view. (Drawn by Titus Evans.)

Nervous System

The bird has a relatively well-developed brain and nervous sys-
tem. In an average chicken the brain weighs about 10 grams.
The cerebrum, the anterior division of the brain, shows fair develop-
ment but no convolutions, while the olfactory lobes which are acces-
sory to it are very small. The optic lobes of the midbrain are well
developed and the cerebellum is exceptionallj' developed. The cere-
bellum consists principally of a median lobe called the vermis or
worm. The ventral part of this division of the brain forms the roof
of the fourth ventricle of the medulla oblongata.

The sense organs of sight and hearing show improvement when
compared with the reptiles. The eye is large, rounded laterally, and
rather flattened anteroposteriorly. It has the usual layers of outer
sclerotic coat continuous with the coriiea in front, the pigmental

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TEXTBOOK OF ZOOLOGY

chorioid just beneath, and ttie membranous retina lining the inside
of the posterior portion. The crystalline lens is of a soft texture, is
nearly round, and is enclosed in a capsule. The cornea is of a horny
consistency and is transparent. Light thus passes rapidly through
it to the posterior part of the eye. There is a comb-shaped membrane
stretching from the entrance of the optic nerve at the posterior wall of
the eye to the posterior surface of the lens. This structure is called
the pecten, and it is supposed that it has something to do either with
the nutrition of the eye or with the keen accommodation possessed by
the bird. The chicken has a lacrimal duct and sheds tears.

- — y^

OLFACTORY NERVES
OLFACTORY LOBES

CEREBRAL LOBES

LEFT OPTIC LOBE

PINEAL BODY
CEREBELLUM
TRIGEMINAL N.

-FACIAL N.
-AUDI TORY N.
-MEDULLA OBLONGATA

Fig. 324. — Dorsal view of brain of chicken. (Drawn by Titus Evans.)

The organ of hearing is embedded in the skull and consists of the
external meatus, tympanic membrane, one ossicle membrane, and the
columella which is attached to the tympanic membrane by a cartilage.
The inner ear is enclosed in the bony labyrinth and contains the
vestibule, three semicircular canals, and the membranous cochlea. The
cochlea has arisen from a pocketlike lagena on the vestibule, and is
much better perfected in birds than in simpler vertebrates. The
Eustachian tube provides a connection between the tympanic cavity

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and the 'pharynx. This provision allows for equalization of air pres-
sure on the two sides of the tjTupanic membrane.

The sense of smell and the olfactory apparatus are poorly developed
in the chicken. The sense of iaste is centered in the taste buds
located on the surface of the tongue and on the dorsal palate. This
sense is only fairly developed. Touch is distributed over the skin
and in certain feathers. The edge and point of the beak are sen-
sitive to touch. Herbst's touch corpuscles are found in all parts
of the skin.

Skeletal System

The skeleton of the bird is remarkable for its rigidity with light
weight. Even in the heavy, nonflying chicken, this is noticeable.
The strongly keeled sternum serves as the attachment for the strong
pectoral muscles which are so important in the flight of most birds.
There are short bracing bones between the ribs, called uncinate
processes. Many of the bones, even in a chicken, have air cavities
in them. These are better developed in the better fliers. Teeth are
entirely lacking in the adult.

The skull is quite well developed and has a relatively large cranial
cavity. The orbit is also large. The cranium consists of three single
bones, the occipital, the ethmoid, and the sphenoid. There are also
three pairs including the frontals, the parietal, and the temporal.
The free quadrate connects the lower jaw with the cranium.

The long flexible neck is composed of sixteen cervical vertebrae,
the anterior two of which are named the atlas and the epistrophei's
(axis). The articulation of the vertebrae is such that the head is
well adapted for use in feeding, fighting, and nest building in some
birds. The centra of the vertebrae are heterocoelous or saddle-shaped.
Following the cervical vertebrae there are five thoracic, thirteen
lumbosacral, and seven coccygeal in order. The total number is
forty-one. The cervical and coccygeal alone have free movement.
The pygostyle at the caudal end supports large tail feathers, which
are movable. There are seven pairs of ribs, two of them articulating
with cervical vertebrae and five pairs related to thoracic vertebrae.
The thoracic ribs join the sternum by sternal ribs. The cervical ribs
are floating. The sternum joins the pectoral girdle by a union in
front with the coracoid. The coracoid articulates dorsally with the
scapula, another girdle bone, and with the humerus of the wing.
Besides the scapula or shoulder blade the two clavicles join the upper

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TEXTBOOK OF ZOOLOGY

PRE tv1AXH_l_A

-IM ASA.l_

LACRIMAL

MAXILLA

FRONTAL

PARI ETAL

OCCI PI XALS

ATLAS

AXIS

QUADRATO JUGAL
J MANDIBLE

CERVICAL VERTEBRA

, HUMERUS

RADIUS

ULNA

I .-.-CARPO-

I ] METACARPUS

I r-PHALANlGES^' THORACIC

VERTEBRAE

/ SCAPULA

/ Rl BS

,' SVrslSACRUM

PYGOSTYLE-,

Fig. 325. — L.ateral view of skeleton of chiiclien. (Drawn by Titus Evans.)

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611

ends of the coracoids and fuse with each other ventrally to form the
"wishbone" or "collar bone." The wing is a greatly modified front
limb, but strictly homologous to the arm. There is a humerus in the
upper arm whose head fits in the glenoid cavity. The radius and ulna
support the forearm. The carpus or wrist is composed of two bones,
scapholunar and cuneiform, followed by two metacarpals. The first
digit or pollex has only one phalanx or joint, the second has two
phalanges, and the third is located at the end of the metacarpal bone.

SARTORIUS

GLUTEUS PRIMUS

SEMITENDINOSUS

GASTROCNEMIUS

FLEXOR PERFORATUS
INDICIS SECUNDUS
PEDIS

FLEXOR PERFORATUS
MEDIUS SECUNDUS

PERONEUS LONGUS

Fig. 326. — Lateral view of tlie muscles of the hind leg of the chicken. (Drawn by

Titus Evans.)

The pelvic girdle is fused quite rigidly to the sacrum. The ilium,
ischium, and pulis of each side are closely fused with each other.
The pelvic limb fits into the rounded concavity on each side of the
girdle. This socket is called the acetabulum. The hindlimb supports
the body in nearly an upright position when the bird is walking. The

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TEXTBOOK OF ZOOLOGY

femur is the bone of the thigh which fits close to the body and is
covered by the feathers. The section below the knee (shank) is com-
posed of the long, larger tibiotarsus and the slender fibula which is
more or less free at the proximal end but is bound to the tibiotarsus
along its side. The next portion is a single bone, the tarsometatarsus,
which has resulted from a fusion of several. Distal to it are attached
the four digits, each composed of phalanges. The first toe is directed
backward and the other three forward, as an adaptation for perching.
In the male chicken there is a horny spur projecting backward from
the tarsometatarsus.

BICEPS
DELTOID

EXTENSOR METACARPI
RADIALIS LONGUS
PRONATOR BREVIS
EXTENSOR INDICIS LONGUS
EXTENSOR OSSIS
METACARPI POLLICIS
FLEXOR DIGITORUM

FLEXOR CARPI ULNARIS

EXTENSOR PROPRIUS POLLICIS
FLEXOR BREVIS POLLICIS
INTEROSSEOUS PALMARIS

Fig. 327. — Ventral view of the muscles of the wing of a chicken. (Drawn by

Titus Evans.)

Muscular System

In the chicken there are 162 voluntary, striated muscles, single
or in pairs. These muscles are named in some cases from their loca-
tion, others from their attachments, some for their shape and form,
others from their use or function, and still others from their direc-
tion in the body. They are covered and bound together by the white
fibrous connective tissue sheath, called fascia.

The muscles of the chest and forelimb are quite well developed
as an adaptation to flight. The muscles of the thigh and shank are

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613

well developed for bipedal locomotion and perching. The muscles
of the long flexible neck are quite intricate. Since this animal does
no chewing, the jaw muscles are not strongly developed. The large
pectoral muscles which extend from the sternum to the upper arm
(Fig. 327) constitute from 15 to 20 per cent of the weight of the body
and make up the breast meat of the bird. The muscles of the thigh and
shank or ''drumstick" are quite well developed, and the chicken is a
strong runner (Fig. 326). Certain muscles and ligaments of the legs
are arranged in such a way that the bird is able to cling to the perch
due to the down pull of the weight of the body. This makes it
possible for the chicken to sleep on the perch without danger of
falling off.

LEFT TESTIS

LEFT VAS DEFERENS

CLOACX

PAP;'LLA OF VAS D.

Fig. 328. — Reproductive organs of male chicken. (Drawn by Titus Evans.)

Reproduction and Life History

The sexes are distinct but there is no definite pairing for repro-
duction; one cock may mate with several hens. The eggs are
usually laid in a rounded nest constructed of hay or other soft
material. Some birds build no nest at all, Avhile others build very
elaborate ones, such as the hanging model of the Baltimore oriole.
Copulation is necessary for the transfer of mature spermatozoa from

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TEXTBOOK OF ZOOLOGY

the cloaca of the male to the cloaca of the female. This is accom-
plished by the cock covering the back of the hen and bringing the
cloacal apertures of the two together. The motile spermatozoa
migrate up the uterus and oviduct to meet the mature eggs and
unite with them in fertilization. In the cock there is a pair of oval,
light-colored testes located near the dorsal body wall anterior to the

OVARY

FUNNEL OF
OV IDUCT

GLANDULAR
P ORTION

SHELL GLAND
PO RTl ON

CLOACA

Fig. 329.— Reproductive organs of female chicken. Notice tlie single left ovary
and oviduct. (Drawn by Titus Evans.)

kidneys. From each testis the vas deferens, a duct, leads posteriorly
to open in the cloaca. Just before joining the cloaca this duct is
dilated to form a storage sac, the seminal vesicle.

The adult hen has only the left ovary and oviduct, the right
having atrophied during embryonic development. The mature ova
rupture the wall of the ovary and pass into the oviduct. The

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615

glandular wall of the oviduct secretes much of the albumen or
''white" around the egg. In the lower part of the oviduct, the
shell membranes are added, and the calcareous shell is deposited in
part here and completed in the uterus and vagina. The time re-
quired for the egg to pass from the ovary to the exterior at laying
averages about twenty-two hours. The average-sized hen's egg
weighs about two ounces of which 11 per cent is shell, 32 per cent
yolk, and 57 per cent albumen. A dozen or fifteen eggs are usually
laid in the same nest by the hen and then incubated with her body
temperature by sitting on them almost constantly for twenty-one
days. They are kept at a temperature between 98° F. and 100° F.,
and are turned over each day to avoid internal adhesion of the
embryo to shell membranes. At the end of this period they hatch
by breaking through the shell with the temporary "egg tooth" on
the beak. The chicks are covered with down, have their eyes open,
and can run as soon as they are dry. For this reason they are said
to be precocial, and since they leave the nest immediately, they belong
to a general group called the nidifugae. Another type of bird which
requires parental care, such as feeding, is said to be altricial, and
since it remains in the nest, it belongs to the group nidicolae. The
individual reaches maturity in from six to ten months, depending
on the breed.

(

CHAPTER XXXI

MAMMALIA

The class Mammalia (mam'ma'li a, milk-forming) includes many
of the animals most familiar to man. Nearly one-third of the total
number of them occur in America. Besides our common domestic ani-
mals, such familiar forms as rats, mice, ground hogs, bats, foxes, bears,
deers, seals, whales, man and many others belong in this group.
The skin of them all is provided with sweat glands and sebaceous
(oil) glands and is more or less covered with hair. These animals
are tj^pieally quadrupeds with five digits on each limb. The females
have well-developed mammary glands which secrete milk for the
nourishment of the young. There is usually a distinct division of
the body into head, neck, trunk, and tail regions. All mammals have
a definite temperature regulation and are said to be homoiothermal,
or warm-blooded. The body temperatures of different species vary
from 77° F. to 104° F. In mammals, as in birds, the heart is com-
pletely divided into four principal chambers. The single systemic
arch of the aorta is the left one. Kespiration is carried on by lungs
and the anterior end of the trachea is modified to form a larynx
capable of sound production. In general. Mammalian blood contains
nonnucleated, circular red corpuscles ; however, in the camel they are
oval in shape. The thoracic cavity of the mammal is separated from
the abdominal cavity by a complete diaphragm.

Classification

The entire class includes approximately 4,000 species of living
mammals and 3,500 fossil forms. The class is divided into two sub-
classes, several divisions and a number of orders.

Subclass Prototheria. — The primitive, egg-laying mammals. This
group includes only a single order.

Order Monotremata. — The most primitive mammals and the only
ones that lay eggs are placed here. They are limited in their dis-
tribution to Australia, Tasmania, and New Guinea. The eggs are
similar to turtle eggs and are laid either in a pouch on the female 's
abdomen, as in the spiny anteater, or in a tunnel in the ground near

616

MAMMALIA 617

water, as in the duckbill. The oviducts in these animals do not
unite to form a vagina, but empty directly into the cloaca, which is
present in this subclass of mammals only. After hatching, the
young are nourished for a time on milk from the mammary glands
of the parent. These glands secrete their milk on to the hair of
the abdomen and the young either suck or lick it up from here.
There are two quite representative animals of this group. One is
the duckbill or duck mole, OrnitJiorhynchus anatinus, which is about
a foot and a half long; it is covered with hair, has webbed feet,
and a peculiar duck bill snout. It feeds on worms which it digs
from the mud with its bill. During the daytime it sleeps in its
grass-lined tunnel, the entrance of which is under water. The eggs

Fig. 330. — The duckbill, Ornithorhynchus anatinus. The "duck mole" of Australia.
(From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.)

are laid and the young reared in this underground chamber. On
the heels of the hind feet of the mole are some spurs which are sup-
plied with poison from a gland located in each thigh.

A second representative of the order is the spiny anteater, Tachy-
glossus aculeatus, which is about one foot long and covered with
stiff spines mixed with coarse hair. It has a head and mouth drawn
out into a long proboscis with a long, slender tongue which is pro-
truded for picking up ants and other insects. This animal lives in
a burrow.

Subclass Eutheria. — The group includes the true viviparous mam-
mals and is divided into two divisions, the marsupials or pouched
mammals (Didelphia) and the placental mammals, Monodelphia.

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TEXTBOOK OP ZOOLOGY

photo

Pig. 331. — Kangaroo carrying young in the marsupial pouch.

MAMMALIA

619

Order Marsupialia. — This group of mammals has no well-developed
placenta and is sometimes designated as a separate subclass, called
Metatheria. Their shell-less eggs absorb food from the wall of the
Icterus. The young are born in a very immature state and make their
way to the marsupial pouch where they cling to the teats and are
nourished on milk until they can shift for themselves. The group is
at its best in Australia and nearby islands. Here is found the true
kangaroo with its short, poorly developed forelimbs, powerful hind-
limbs and tail, and peculiar upright posture and leaping locomotion.
The seven families of the order include not only kangaroos, but
also opossums, phalangers, wombats, bandicoots, dasyures, and Caeno-
lestes. There are several species in South America. Some of these
are no larger than mice or rats and are frequently brought into this
country on bunches of bananas. In the United States, the opossum,
Didelphis virginiana, is the only representative of the group. It is
about the size of the common cat, with a long scaly tail and fur of
dirty yellowish white color. Ten or twelve young are born at a time
and are carried in the pouch of the mother. The young remain with
the mother about two months, often riding as a group on the mother's
back during the latter part of this period. The opposum is quite
active at night, but it usually sleeps through the day.

Order Insectivora. — This group includes the common mole, Scalopus
aquaticus, the hairy-tailed mole, Parascalops Ireweri, the star-nosed
mole, Condylura cristata, the shrews and short-tailed shrews. They
are quite well distributed through North America and Europe, but
are absent from Australia and most of South America. The group
is largely burrowing and nocturnal in habit. They feed chiefly on
insects which they seize with their sharp, projecting incisor teeth.
The moles are well adapted to the burrowing habit of life. They
have rudimentary eyes, no external ears, short stout forelimbs with
strong sharp claws for digging. Their tunnels are just under the
surface of the saaidy loam in which they live. They occasionally
throw up molehills along the tunnel.

The shrews are small, mouselike animals with conical, pointed
heads, ratlike feet, small eyes, and external ears. They may live
in burrows or on the surface of the ground under logs, rocks, or
heavy vegetation. The long-tailed shrew, Sorex personatus, of the
North and the East, and the short-tailed shrew, CryptoUs parva, ex-

620 TEXTBOOK OF ZOOLOGY

tending into the South and Southwest, are the common forms. Be-
cause of their small size and retiring habits, these animals are seldom
observed.

Order Chiroptera. — Bats are the mammals which have developed
the power of flight and are not always distinguished from birds by
the layman. The forearm and fingers are extended, and the skin
stretches between them as well as to the hind limbs. Most of them
are small and nocturnal. They usually have the toes of the hind
feet developed for grasping and are able to hang by them, head
downward, when at rest. The brown bat, Eptesicus fuscus, is the
most common form in the United States. The Mexican free-tailed

Fig. 332. — Little brown bat, Myotis lucifugus, in resting position. (From Metcalf,
Textbook of Economic Zoology, published by Lea & Febiger.)

bat, Tadarida mexicana, extends as far north as central Texas. Carls-
bad Caverns near the eastern boundary of New Mexico is famous for
its evening bat flight. The bats come out like a cloud of smoke, make
a definite flight of about sixty miles along two streams and back to
the caverns before daybreak. Here they remain until the next evening.

Because of their ability to fly the bats are widely distributed
mammals, being found on all of the continents and even on isolated
islands. In the East Indies, Australia, Africa, and Asia, there are
several different fruit-eating bats. In tropical America there are true
and false vampire bats. The true vampires live on the blood of
horses, sheep, cattle, and occasionally sleeping human beings. Their

MAMMAXiIA

621

It is

teeth are well adapted for bringing the blood on the victim,
then lapped up from the wound.

Order Edentata. — This is a group including the giant anteaters,
sloths, and armadillos. The giant anteater is the only one in which
the teeth are entirely absent; they are modified in other forms by
lack of enamel. The giant anteater, MijrmecopJiaga juhata, reaches
a length of six or seven feet. Its long claws are used to open the
anthill, and the long prehensile tongue is used for taking up the
ants. The sloths live in trees, clinging to the underside of the limbs
by the long, clawed feet. The animal is ventral side up. They can
even sleep in this position. Their food is principally leaves and buds.
They are very slow moving animals and inhabit South and Central
America.

Fig. 333. — Nine-banded armadillo, Dasypus novemciiictum.

The nine-banded armadillo, Dasypus novemcinctum, is the only one
of this group inhabiting the United States. It ranges from Argentina
to southern New Mexico and northeast to central and east Texas.
The head is small, tail elongate, and the body is covered dorsally by
bony plates. The ventral parts of the head and body are covered
with bristly hair. The nine bands are formed around the trunk by
the arrangement of the scutes in that region. These animals dig
very rapidly in the ground, and when they are frightened they can
roll themselves into a ball as does a pill bug. The normal litter
of young is quadruplets produced from a single fertilized ovum
(Fig. 334).

Order Pholidot a. —This is a group of scaly anteaters found in
Africa and eastern Asia. They are well protected by epidermal scales
and can roll up like armadillos. They are from one to five feet long.

Order Rodentia. — These animals are the gnawers and compose one
of the largest orders of mammals. Such forms as pocket gophers,

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TEXTBOOK OF ZOOLOGY

rats, mice, ^-ound squirrels, squirrels, chipmunks, prairie dogs, kan-
garoo rats, cotton rats, wood rats, muskrats, woodchucks, porcupines,
guinea pigs and beavers are representative. They all have long
chisellike incisors but no canine teeth.

The prairie dogs {Cynomys ludovicianus) are heavy -bodied, bur-
rowing, rodents that live on the plains west of the Mississippi River.

Fig. 334. — Identical armadillo quadruplet embryos attached to the placenta.

These animals are gregarious, living in "towns" or colonies. The
burrows are provided with a mound around the entrance, and they
are usually quite deep and fifteen to thirty feet apart. Forty or
fifty acres of land may be covered by one town.

The ground hog or woodchuck, Marmota monax, is a larger solitary,
burrowing animal which hibernates during the winter. It bears
about six young in the burrow early in the spring. The ground
squirrels are also burrowing animals somewhat similar to the ground
hog, but much smaller. They are found from the Mississippi basin

MAMMALIA

623

to the West Coast iu shallow burrows in the open fields or low brush.
The striped spermophile is a common midwestern form. The chip-
munk, Tamias striatus, a small striped squirrel, with a slender non-
bushy tail, usually lives on the border of the woods. It feeds

:^::\

%- z^-c

'^muo0¥^

^^'^■■^;^' -;^

■:-^m. c^^

Fig. 335. — Prairie dog, Gynomvs ludovicianus. Lived in large colonies or "towns"
in great abundance over the prairie country at one time.

Fig. 336. — Pocket gopher, Geomys bursar ius. A burrowing rodent with destructive

habits.

on seeds and nuts which it usually stores for times of need. The
squirrels are excellent tree climbers and in fact live in trees most of
the time. The red squirrel, Douglass squirrel, gray squirrel, Abert
squirrel, fox squirrel, and flying squirrel are the common species.
The gray squirrel is more common in the South while the Abert is
found in the Rocky Mountain region and the Douglass is the Pacific

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TEXTBOOK OF ZOOLOGY

form of the red squirrel. Most of these squirrels live on nuts, acorns,
and seeds primarily.

The pocket gopher, of the family Geomyidae, is another burrowing
animal which ranges through the South, Southwest, and Midwest.
They are provided with strong incisors, large cheek pouches, and
large digging claws. They are very able diggers and construct ex-
tensive burrows about 12 to 18 inches below the surface. Surplus
earth is pushed to the surface in mounds. The cheek pouches are
used for gathering tubers, such as potatoes, roots, and seeds, for food.
The pocket mice and kangaroo rats are other related forms found in

Fig. 337. — Banner-tail kangaroo rat, a common resident of the plains and desert.

(Courtesy of Nature Magazine.)

the Great Plains region and Southwest. These mice have cheek
pouches and long, bush-tipped tails. The kangaroo rats usually live
in colonies in sandy land. They build extensive tunnels. These
animals are built like small kangaroos with long hind legs and
strong tails.

The beaver, Castor canadensis, is the largest animal of this order,
reaching a length of from three to three and one-half feet. At one
time beavers were distributed over most of North America, but now
they are reduced to a few in out of the way mountain streams. The
beaver has a stout body, strong cutting teeth, webbed hind feet, and
a broad, flat naked tail. It is well adapted to aquatic life and builds

MAMMALIA

625

dams for its home. The dams are built of trees which are cut with
its teeth, floated into position, and chinked with mud.

The muskrat. Fiber zibethicus, belongs in a large family with lem-
mings, rats, meadow mice, and white-footed mice. It lives in slow
streams, ponds, and swamps, and feeds on roots of water plants,
fresh-water mussels, dock, corn, and other grain when it is available.
It builds houses of stalks, leaves, cattail leaves and mud out in the
water of swamps. In ponds it uses a burrow in the bank. The home
is lined with cattail down or grass ; here it rears the young and spends
the winter.

The porcupine, Erethizon dorsatus, is principally a northern ani-
mal but is found in the mountains as far south as Virginia and also

Fig. 338. — Muskrat, Fiber zibethicus, an important fur bearer. (From Metcalf,
Textbook of Economic Zoology, published by Lea and Febiger.)

in the mountains of the Southwest in Texas and Arizona. These ani-
mals have the hairs of the back modified into spines which normally
lie flat, but which can be elevated by muscles when the animal is
frightened or angered. The black rat, Battus rattus, the Norway
rat, Rattus norvegicus, and the common mouse, Miis musculus, have
all been introduced into this country from Europe. The chinchillas,
viscachas, and the cavies (guinea pigs) have all been introduced from
South America.

Order Lagomorpha. — The rabbits and hares constitute a very inter-
esting and important group. The jack rabbit is the most common
hare of the western plains, mountain region, and Southwest. It is

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TEXTBOOK OF ZOOLOGY

•I

Fig. 339, A. — Jack rabbit, Lepus calif oi-nicus. A common prairie siglit. (Courtesy

of Nature Magazine.)

Fig. 339, B. — Mexican ring-tail cat. (Courtesy of Nature Magazine.)

MAMMALIA

627

famous for its long legs, long ears, and speed. All of the hares build
nests in heavy grass and bear the young here. The cotton tail rabbits,
Sylvikvgus of several species, are generally distributed over the coun-
try. They are grizzly gray above and lighter below, with the under-
side of the tail cottony white. They dig their own burrows, or borrow
burrows from prairie dogs, ground hogs, or badgers. Several litters
of blind, helpless young are born in the burrow or in a nest above the
ground during the breeding season. In the Gulf States, extending

Fig. 340. — Adult golden hamster. (Courtesy Biological Survey House, Chicago.)

as far west as central Texas, is a larger semiaquatic relative of the
cottontail, the swamp rabbit. It is brownish gray above and is said to
have the habit of concealing itself in the water with only the tip of the
nose exposed. In the region above the timber line are found the tail-
less rock rabbits or pikas. They live among the rocks and look a little
like guinea pigs.

Order Carnivora. — Most of the mammals of this order are flesh-
eating, although a few are omnivorous and one or two are vegetarians.
They have the canine teeth well developed and conical premolars as

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TEXTBOOK OF ZOOLOGY

an adaptation to tlieir feeding habits. The suborder Fissipedia in-
cludes the terrestrial carnivores with walking feet, while a second
suborder Pinnipedia includes the aquatic carnivores with swimming
feet.

There is a large group of common mammals that belong to the first
suborder. The fishers, martens, minks, weasels, ferrets, otters,
skunks, and badgers constitute a group of very blood-thirsty killers.
The skunks of genus Mephitis are quite common and are well known
because of their conspicuous white stripes on black fur background
and powerful scent glands. Besides the striped forms there are the
smaller, spotted forms. The badger, Taxidea taxus, extends down
the plains, and there meets the Mexican variety coming up from the
south. It is heavy-set and has short legs with long, sharp, strong
claws. It can dig almost as fast as a man with a spade and usually
comes out winner in a fight with any dog. It is strictly nocturnal
and lives in a burrow.

Fig. 341. — The badger, Taxidea taxus.

In the family Canidae there are besides the domestic dog, the fox,
the coyote, and the wolves. The red fox, Vulpes fulva, is a common
form through the North and East, and has been introduced into the
Southwest by sportsmen who enjoy fox-hunting. It is a cunning
animal and quite difficult to catch or trap. It digs burrows or builds
dens in rocky hillsides where the young are raised. The chosen food
of the fox includes mice, insects, wild birds, and occasional poultry.
Besides the red fox there are the gray fox, the kite fox, and the
Arctic fox. The coyote, Cayiis latrans, is somewhat larger than the
foxes and is quite crafty. It has managed to keep up its numbers in
spite of persistent control measures, such as poisoning, trapping, and
hunting. It lives in dens and among the rocks and feeds largely on

MAMMALIA 629

rabbits, mice, poultry, game, and small livestock. The wolves are
larger than coyotes and frequently hunt in packs. Canis gigas is the
timber wolf.

In the family Felidae are classified the domestic cat, bobcat (lynx),
tiger, leopard, mountain lion, jaguar, and ocelot. Most of the wild
representatives of this group have been pushed far back into the more
remote parts of the Southwest and the Rocky Mountain area. Felis
couguar, the mountain lion (puma, panther, or cougar) ; Felis
hernandesii, the jaguar; Felis pardalis, the ocelot; and Lynx rufus,
the bobcat or catamount, can all be found in southwestern Texas
and New Mexico. The common North American coon or raccoon (Fig.
412) is a plantigrade (walks on entire foot with heel touching the
ground) animal about two and one-half feet long and generally dis-
tributed east of the Rockies. Procyon lotor is its name, and it is much
hunted throughout the South for its fur and its flesh. The bears in-
clude the black or brown bear, Euarctos americanus, which is the most
widely distributed and most common; the large, grizzly, TJrsus hor-
ribilis, of the Rocky Mountains, and the white polar bear, Tlmlarctos
maritimus, of the Arctic region. Brown bears and grizzlies are rather
omnivorous, feeding on fruit, insects, flesh, honey, and even tourists'
lunches. The grizzly bear is more partial to meat than is the brown
bear. The polar bear feeds quite largely on fish.

There are several carnivores which are considerably modified as an
adaptation to a life in the ocean. The body has become fishlike in its
shape and specializations. The appendages in particular have be-
come swimming organs. Callorhinus alascamis, the fur seal, occurs
along the Pacific coast and goes to the Pribilof Islands of the Bering
Sea at breeding season. At this time one male or ''bull," depending
on his ability to fight other males, will set himself up in charge of
from four or five to twenty-five or thirty females. Under present
laws, only the unmated males ("bachelors") are allowed to be killed
for the furs.

Besides the fur seal, there are the California sea lion, Zalophus
calif ornianus; the Pacific walrus, Odobenus divergens or 0. obesus;
and the Atlantic walrus, Odobenus rosniarus. The walruses have
very long canine teeth in the upper jaw and use these for digging
mollusks which are used as food.

Order Artiodactyla. — This is one of the four orders of hoofed ani-
mals and includes those with the even toes. The group includes

630 TEXTBOOK OF ZOOLOGY

cattle, camels, llamas, goats, sheep, pigs, alpacas, reindeer, deer, elks,
moose, giraffes, bisons, hippopotamuses and gazelles. The deer family,
Cervidae, peccary family, Tayassuidae, cattle family, Bovidae, and
the pronghorn antelope family, Antilocapridae all have native repre-
sentatives in North America. The family Cervidae is the largest in
the order. Nearly all males in the deer group have horns which are
solid outgrowths of the skull. These are shed each year and a new
but larger set grown the next season. The moose, Alces americana is
the largest of the group, and it may reach a weight of one thousand
pounds. It is found in the mountainous parts of the Northwest. It
has large, broadly flattened antlers. The elk, Cervus canadensis, is
another large representative which is found only in isolated parts
of our western states. In recent years there has been much effort
made to conserve the remaining ones, particularly in Wyoming,
Montana, and Utah. The antlers of the elk are long, with numerous
slender points. The most commonlj^ distributed deer is the white-
tailed or Virginia deer, Odocoileus virginianus. It thrives in semi-
domestication as well as in the wild. The black-tailed deer, Odo-
coileus crooki and the mule deer, Odocoileus hemiomis are both a
little larger than the white-tailed. The mule deer is common
through the Rocky Mountain states. The black-tailed deer does not
range so far north. Both have doubly branching antlers while the
white-tailed does not.

The caribous, which are native, and the imported European rein-
deer are important meat animals of Canada and Alaska. The cari-
bou, Rangifer cariltou, is heavy-bodied with stout legs and heavy,
irregular antlers in both sexes. The pronghorn antelope, Antilocapra
americana (Fig. 410) which once covered the Great Plains and
ranged over most of the western states is now limited to a few scat-
tered, isolated herds. Much attention is now being given to its resto-
ration and fortunately so. It is nearly as large as a small white-
tailed deer, has peculiar short, single-branched horns in both sexes,
long pointed ears, and a striking white rump patch. It sheds the
horns each year.

The family Bovidae, of course, includes the domestic cow which is
not a native of this continent. The horns of this group are hollow,
occur in both sexes, and are permanent. Rocky Mountain sheep, Ovis
canadensis, is found in the higher western mountains. The horns of
the male are greatly developed and curved. They are the basis for

MAMMALIA

631

the common name, Bighorn. It is very sure-footed and can live on
mountain ledges. The Rocky Mountain goat, Oreamnos montanus,
lives in a similar habitat, but farther north. It has short, dark
horns, shaggy wool, and a beard. The musk ox, Ovibos moschatus,
is a heavy bodied, hump-shouldered animal with stout, down-curved
horns. It lives above the timberline, principally in Canada and

Fig. 342. — Bighorn mountain sheep are still to be found in the Rockies. (Courtesy

of Nature Magazine.)

Alaska. The buffalo. Bison americanus, was extremely abundant all
over the Great Plains until the last quarter of the nineteenth century
when they were killed out by hunters and crowded out by civilization.
They are powerful, heavy-headed, hump-shouldered animals. There
are still a few herds, such as the one at Yellowstone National Park,
that are kept under semidomesticated conditions. Our domesticated

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TEXTBOOK OF ZOOLOGY

hogs belong in the family Suidae and have come down to us from the
European wild boar. In southern Texas and on south to South
America is a piglike animal, known as the peccaries, or javelinas,
Pecari angulatus. They are gregarious, nocturnal, and feed on roots
and nuts primarily. Their heads are large, bodies slender, and tails

Fig. 343. — Buffalo cow and calf. A sight which is no longer seen except In special
preserves. (Courtesy of Nature Magazine.)

short. Hippopotamuses, camels, and giraffes are numerous and impor-
tant African members of this order. The llama and alpaca are useful
South American forms.

Animals, such as camels, cattle, sheep, deer, giraffes, and pronghorn
antelopes, chew their cud and are therefore said to be ruminants.

MAMMALIA

633

They swallow their food partially chewed and, because of the struc-
ture of the stomach, they are able to regurgitate it later for further
chewing. For this reason such animals can consume large quantities
of bulky food in a short time, then retire to the shade and chew
while reclining. The stomach of such an animal is greatly modified
by having four divisions. Following the esophagus is the pouchlike
rumen at the left, then the small middle reticulum, at the right, is
another pouchlike part, the omasum or psalterium, which continues
on to the right and posteriorly into the more elongated al) omasum,
which in turn joins the duodenum. On the first trip to the stomach,
the food passes into the rumen, is stored and moistened. It then goes
in small quantities at a time into the reticulum, and this "cud"

Psa/fer/u/n

Esophagus

Aboma^um

Hunjerj

Fig. 344. — A ruminant stomach. The arrows indicate the direction of tlie move-
ment of the food in the formation and regurgitation of a cud. (From Wolcott,
A-nxinal Biology, published by McGraw-Hill Book Company.)

may pass back to the mouth from here. After it is chewed and re-
swallowed it passes through a valve at the entrance to the stomach
into the omasum, and on to the abomasum.

Order Perissodactyla. — This is the group of odd toed mammals in
which the axis of the foot is through the third toe. There are no
modern forms which are natives of this country with the possible ex-
ception of the horse, and this is very indirect. In the horse, ass, and
zebra, the foot is reduced to one hoofed toe. The ass and zebra are
African and Asiatic forms. Tapirs are piglike with four toes in
front and three behind. They are found in southern Asia, and in
Central and South America. The rhinoceros is a large Asiatic and
African form.

634 TEXTBOOK OF ZOOLOGY

Order Prohoscidea. — This small order includes only two genera
of elephants with one species each of the largest terrestrial animals.
One species, Elephas indicus, is Asiatic, and the other, Loxodonta
africana, lives in the tropical forests of Africa. Both have the nose
extended several feet as a muscular trunk or proboscis which is a
very handy and useful appendage. The skull is very thick, with air
spaces, and the molar teeth are very large, with prominent ridges.

Order Sirenia. — This is a very limited order of sea-cows. They are
aquatic mammals with a pair of flexible anterior flippers and a strong,
rounded tail. The dugongs of the Indian Ocean and Australia, and
the Manatees of the Atlantic Ocean represent the group. The Florida
manatee, Trichechus latirostris is only rarely found.

Order Odontoceti (The Toothed Whales). — All of the whales are
mammals which have become adapted to a strictly aquatic life. The
body is modified for swimming by the reduction of appendages, the
horizontal flattening of the tail and its division into two lobes or
"flukes." The head of these animals is large with long jaw bones.
The nostrils open by a single aperture from which the breath is
spouted when the animal comes to the surface. A thick layer of fat
or "blubber' is deposited beneath the skin and this serves to conserve
heat in the body. The porpoise, Phocaena pJwcaena, is very common
in the Gulf of Mexico and elsewhere. It is about six feet long and
rolls around in the water. It catches such fish as mackerel and
squeteague for food. The sperm whale, Physeter catodon, is a large
whale of about seventy-five feet in length. Such other animals as
the beaked whale, dolphin, and narwhal belong in this order. The
killer whale, Orcinus orca, is generally distributed. It is about
twenty feet long, ferocious and predatory on fish, seals, and even
other whales.

Order Mystacoceti. — This is the whalebone whale group. Their
teeth do not develop beyond the embryonic stage, but they are re-
placed by cordlike plates of baleen or whalebone. Whalebone was
once an important item of commerce, being used in making whips,
stays, and other flexible articles. The largest species in the order
and, in fact, the largest of all animals is the sulphur-bottom whale,
Sihhaldus musculus. It reaches a length of one hundred feet and
lives in the Atlantic and in the Pacific off the coasts of Central
America, Mexico, and California. The gray whale, Bhachianectes
glaucus, is another Pacific form. The Greenland right whale,

MAMMALIA

635

Balaena mysticetus, is a polar inhabitant, each of which yields about
3,000 pounds of whalebone and 300 barrels of oil. The finback and
humpback whales also belong to this order. In feeding, all of these
whales take large quantities of water into the mouth, pass it out
through the whalebone which serves as a sieve, and retain all of the
small organisms and particles of organic matter as food.

Order Primates. — This is the order which includes the lemurs,
monkeys, apes, and man. Because of the large number of primitive
characteristics of representatives of this order, some authors place it
near the first or middle of the list of orders of mammals instead of
at the end. Most of them are tropical, arboreal, and live on nuts,
seeds, fruits, insects, and birds. They have a particularly well-
developed brain. The thumb and also the great toe in most forms are
placed in opposition to the other digits as an adaptation for grasping.
Usually only one young is born in a fairly advanced stage, but rather
helpless at first and requiring considerable care. The order is usually
divided into two suborders, the Lemuroidea and Anthropoidea. The
first includes the lemuroids which have the front teeth separated.
These are typically small or medium-sized quadrupeds of Madagas-
car, with long, bushy tails. The aye-aye is a nocturnal arboreal ani-
mal with long ears, large ratlike teeth, bushy tail, and long digits
with sharp claws. Its body is about one foot long. The tarsiers are
about the size of rats with suckerlike discs on the ends of their fingers
and toes. They are also arboreal and nocturnal. The true lemurs
have an elongate face and a small cranium. Their hindlimbs are
longer than the forelimbs. Most of them live on Madagascar and
nearby islands as well as in Africa and Asia.

The second suborder, Anthropoidea, includes several families of
monkeys and apes.

The marmosets make up another family found in South and Cen-
tral America. They have a flat nail on the big toe, while the thumb
is not opposable to the other digits. The brain case is large, and the
space between the nostrils is broad. The Cehidae, or New "World
monkeys, have long prehensile tails, broad flat noses, and all digits
have nails instead of claws. This group includes the common monkey
of hand-organ fame, the spider monkeys, squirrel monkeys, and
howlers, all of which are natives of Central America and the northern
part of South America.

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TEXTBOOK OP ZOOLOGY

The Old "World monkeys, family Cercopithecidae, have narrow
high-ridged noses. Some have 'long tails, others have short tails.
Certain of them are almost bipedal. Most of them have heavy cal-
louses on the hips which are used as cushions while sitting. Some of

-""axuMSO;;^, '^t^mc^imsi^'mti,^^ .

/ -

l...

...-«n«Hnri;j^a&ifcM

Fig. 345. — Orangutan holding a glass. (Courtesy of Nature Magazine.)

them have cheek pouches. With the exception of the Barbaiy ape,
Macaca sylvana, of the Rock of Gibraltar, this entire family is con-
fined to Africa and India. The baboon, of genus Cynocephalus, a
doglike monkey with a short tail, lives in central Africa. The man-
drill, drill, and macaque monkeys are others in this group.

MAMMALIA

637

The manlike, or anthropoid, apes are grouped in the family
Shniidae which includes the gibbons, Hylolates; the orangutan, Simia
satyrus; the chimpanzee, Anthropopitheais troglodytes; and the
gorilla. Gorilla gorilla. The gibbons are tailless apes with long arms.
They are arboreal and omnivorous. They can accomplish bipedal
locomotion. They are only three feet tall and strictly arboreal ; they
live on the Malay Peninsula and in the East Indies. The orangutan
builds nests in the trees and feeds principally on fruits. It is be-
tween four and five feet tall and has an arm spread of seven feet.
The chimpanzee is perhaps the most intelligent ape. It is easily
tamed and in many respects is more manlike than most of the others.
It lives in central and western Africa. The arms are somewhat
shorter and the skull rounder and smoother than are those of the
gorilla. The gorilla, which is the largest of the group and somewhat
more ferocious, is about five and one-half feet tall. It walks on the
soles of the feet and on the knuckles of the hands. It has prominent
canine teeth and feeds mainly on plants and foliage.

Economic Relations

Nearly all of the important beasts of burden, such as horses, asses,
mules, elephants, camels, llamas, reindeer, oxen, and dogs, are
mammals. The history of the origin of the domestication of most
of these has been lost to antiquity. Cattle, sheep, hogs, goats, rein-
deer, alpaca, and rabbits are the chief meat-producing mammals. A
few years ago it was estimated that more than 20,000,000 cattle,
15,000,000 sheep, and 80,000,000 swine are required to supply the
meat demand in the United States each year. Cattle and goats are
the most important commercial milk-producing animals. The milk
of camels, reindeer, and llama is also used in some parts of the world.
The leather produced by tanning the hides of meat-producing do-
mestic animals particularly is worth millions of dollars. It is used
for making shoes, saddles, harness, belts, and for ornamentation.
Wool is a very important animal fiber used in the manufacture of
fabrics which are resistant to dampness and cold. It is produced
principally by sheep and goats in this country. In some parts of the
world the alpaca and camel are important wool-producing animals.
The skins and furs of many wild mammals, the fur-bearers, are
exceedingly important commercially. They were used as clothing
and ornamentation even by primitive people. The modern people are

638 TEXTBOOK OF ZOOLOGY

demanding more and more furs. The most commonly used and abun-
dantly sold furs are in the following order : mole, rabbit, skunk,
muskrat, opossum, squirrel, fox, ermine, wallaby, mink, wolf, civet
eat, and raccoon. There are several vei*y valuable fur-bearing ani-
mals coming very near the point of extinction, and there should be
an earnest effort made to restore them. In this group would be in-
cluded beaver, fur seal, otter, Russian sable, and chinchilla. A single,
choice, silver fox fur may bring a thousand dollars or more, a fisher
is valued at about three hundred dollars, the beaver and otter, each
at about one hundred ; a wolf is worth about forty as also is the
black bear ; the skunk is valued at about six dollars, and the muskrat
at about three dollars. Fur farming is being practiced with some
species, such as silver fox, mink, muskrat, and rabbits. Rabbit and
muskrat farming have the added advantage of producing salable
meat.

Many of the undomesticated mammals become serious pests at
times when the usual balance in nature is disturbed. Rats and mice
are very destructive of stored provisions, such as fabrics, clothing,
grains, and various foodstuffs. These rodents, along with squirrels,
gophers, prairie dogs, and groundhogs which normally live on wild
plant tubers, seeds, acorns or nuts, are frequently destructive to grain
crops. Field mice, rats, and rabbits sometimes damage young fruit
trees by gnawing the tender bark just at the surface of the ground
until the tree is girdled, thus causing subsequent death. The burrow-
ing forms may be killed by fumigating the burrow with carbon di-
sulphide, calcium cyanide, or carbon monoxide from the exhaust of
an automobile.

Many of the larger carnivorous mammals are quite destructive of
young domesticated animals. The wolf, coyote, fox, and, in some
parts of the Southwest, the puma, are quite predatory, killing calves,
lambs, kids, and poultry. Certain wild mammals are carriers of
disease. In the Rocky Mountain region, ground squirrels and other
rodents carry Rocky Mountain spotted fever and transmit it to man
through the bite of the spotted fever tick. Bubonic plague is carried
by rats and other rodents and is transferred to man by the rat flea.

The deer and bear are about the only large mammals left that
are classified as game animals and hunted for sport. Several others,
such as the pronghorn antelope, buffalo, and elk, have been almost

MAMMALIA

639

completely destroyed. With proper protection, however, these
species might still be restored iu many parts of the country. Squir-
rels and rabbits are the most abundant game mammals of present
times.

THE CAT, A REPRESENTATIVE MAM3VLAL

The common house cat, Felis domestica, is a carnivore which is
familiar to everyone. It is so common that it is always available for
study. The cat is a quadruped which is well adapted for walking
and climbing, at which activities it is very adept. The eyes of the cat
are well adapted to sight at night when it is quite active. As a result
of domestication the carnivorous diet of the cat has been somewhat
modified. The cat is clean in its sanitary habits.

External Structure

The entire body is covered with a high quality hair or fur which
may be one of several colors. On the upper lip and around the eyes
are some especially long stout hairs, whiskers or vibrissas. The upper
lip is somewhat cleft in the center. In the hare this cleft is extremely
prominent, and it is from this that the abnormally cleft lip in man
came to be known as ''harelip." The tall, flexible external ear or
pinna partially surrounds an ear opening, the external auditory
meatus, which leads to the interior of the skull and the tympanic
membrane. The large, oblong nostrils are located in the fleshy nose,
the end of which is naked. Their large eyes have both upper and
lower lids, and each has a sheetlike nictitating membrane which may
be drawn over the eye from the medial corner.

The trunk is conveniently divided into a thorax or chest, supported
by the ribs, and posterior to this the abdomen. Extending posteriorly
is the long slender tail. Along the ventral side are four or five pairs
of nipples or teats which are prominent in the female but rudimen-
tary in the male. They contain the openings of the milk or mammary
glands. Ventral to the tail is the perineal region in which is located
the anus and external genital organs. The two pairs of limbs extend
ventrally from the sides of the trunk. The forelimb is divided into
upper arm, forearm, wrist, palm, and five digits with retractile
claws at their tips. The hindlimb extends from the hip and is divided
into thigh, shank, ankle, arch, and four digits with similar claws.

640

TEXTBOOK OF ZOOLOGY

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MAMMALIA

641

The cat walks on the digits with the remainder of the hand or foot
failing to reach the ground. This is known as the digitigrade gait.
Man and bears use the plantigrade or primitive gait in which the
entire sole and heel make contact with the ground.

Skeleton

Exclusive of the teeth, ear bones, chevron bones, and sesamoid
bones a young fully developed cat has 233 bones. In addition to
the bones there is some cartilage. This skeleton is made up of
cartilage-hones, which are preformed in cartilage and replaced by
bone, and memhrane-hones, a bone formation in the dermis of certain
portions of the skin. The sesamoid ho7ies are pieces formed in ten-
dons. The kneecap is the largest of these. Chevron hones are paired
and extend ventrally from the anterior ends of several of the caudal
or tail vertehrae.

NEURAL SPINE
POST ZYGAPOPHYSIS
ACCESSORY PROCESS
ARTICULATING SURFACE
OF PRE-ZYGAPOPHYSIS

NEURAL CANAL

CENTRUM

TRANSVERSE PROCESS

Fig. 347. — Anterodorsal view of lumbar vertebrae of cat. (Drawn by Titus Evans.)

The axial skeleton, as is usual in vertebrates, is the portion extend-
ing in the main axis of the body and is composed of skull, vertebral
column, ribs, and sternum. There are twenty-six bones that are
usually named and described in the cat's skull. These bones are for
the most part immovably fused together at sutures. In the most
anterior or frontal segment of the skull proper are the two frontal
hones, dorsally, the presphenoid and two orhitosphenoid-s. Behind this
is a middle or parietal segment composed of the hasisphenoid, two ali-
sphenoids, and the two dorsal parieiaU. The most posterior or
occipital segment represents a fusion of four bones. It consists of
hasioccipital, two exoccipitals, and the supraoccipital. In addition
to these rings or segments there are three sense capsules, the olfactory

00
CO

MAMMALIA 643

capsule, auditory capsule and optic capsule, as well as the jaws. The
walls of the olfactory capsule consist of the vomer, mesethmoid,
pterygoid, palatine, maxilla, premaxilla, lacrimal, nasal, and nasal
processes of frontal bones. The auditory capsule is enclosed in a por-
tion of the temporal bone. This bone is formed by fusion of the
squamous, petrous, and tympanic bones.

The eat and other mammals have two sets of teeth, the milk or
deciduous set, and the permanent set. In the first set, the dental
formula for one-half of the mouth is incisors 3/3, canines 1/1, and
premolars 3/2. These teeth begin to appear when the kitten is two
weeks old and are complete at eight weeks. At the end of the fourth
month the milk incisors are being replaced by permanent teeth. The
formula for one-half of the mouth with permanent teeth is : incisors
3/3, canines 1/1, premolars 3/2, molars 1/1 or a total of thirty teeth.
Because of the different forms these teeth are said to be heterodont.
They develop like placoid scales from the epithelial lining of the
mouth, and each tooth consists of an outer, hard covering, the enamel,
under which is the dentine and then the pulp. The permanent set
in man includes 32 teeth and the formula for one-half of the set is
incisors 2/2, canines 1/1, premolars 2/2, molars 3/3.

The vertebral column or backbone is composed of five groups of
vertebrae; seven cervical in the neck, thirteen thoracic in the chest,
seven lumbar in the small of the back, three sacral in the hip region,
and from four to twenty-six caudal in the tail. These vertebrae
articulate on each other. They are separated by intervertebral discs
of cartilage, except the fused sacral group. Intervertebral ligaments
serve to connect the vertebrae. The principal parts of the typical
vertebrae are : the body or centrum, neural arch over the central
canal, a spinous process or neutral spine projecting dorsally from the
arch, two transverse processes projecting laterally one on each side,
zygapophyses or articular processes, a pair projecting anteriorly and
a pair posteriorly to articulate with adjacent vertebrae. Articulating
laterally with the thoracic vertebrae are thirteen pairs of rihs which

Fig. 348. — Muscles of the cat. a.s., occipitoscapularis (rhomioideus capitis) ;
cm., cleidomastoid ; ex. lonq. dig., extensor longus digitorum ; F. G. U., flexor carpi
ulnaris ; flex. long, dig., flexor longus digitorum ; flex. long, hal., flexor longus
hallucis ; F. P. D., flexor profundus digitorum; gastroc, gastrocnemius; int. ohliq.,
internal oblique; L. S. V., levator scapulae ventralis ; P.A., pectoralis ; parot^,
parotid gland ; Pt, pronator teres ; spl, splenius ; sub^n., submaxillary gland ; T,
triceps brachii : trans., transverse abdominis. (From Stromsten, Davison's Mam-
malian Anatomy, published by The Blakiston Co.)

644 TEXTBOOK OF ZOOLOGY

extend in an arch, ventrally. The nine anterior ones are called
true rihs because they articulate dorsally with the thoracic vertebrae
and ventrally with the sternum, or breastbone. The other four are
termed false ribs, three pairs of which articulate with other ribs,
while the fourth has no ventral articulation and is called a floating
rih. The sternum is divided into eight pieces, called sternehrae, which
lie in a row in the median line of the ventral side of the thorax. The
anterior sternehra is called the manubrium and the posterior one is
called the xiphoid or ensiform process.

The two girdles and pairs of limbs are quite well developed. The
forelimb and pectoral girdle are composed of a scapula, poorly de-
veloped clavicle, humerus, uhia, radius, seven carpals, five meta-
carpals, and fourteen phalanges. The hindlimb and pelvic girdle are
composed of the iyinominate bone (ilium, ischium, pubis), femur,
patella, tibia, fibula, seven tarsals, five metatarsals, and twelve
phalanges.

Muscular System

The study of the muscular system is known as myology. All mus-
cles moving the bones are voluntary; i.e., they are under control of
the will. There are over five hundred voluntary muscles in the cat,
each of which is attached by one or both ends to the periosteum of
the bone, usually by tendons. The point of attachment on the fixed
bone, which is not moved by the contraction, is the origin. The end
of the muscle attached to the bone which is moved is known as the
insertion. According to function the skeletal muscles are grouped as
flexors, extensors, adductors, abductors, rotators, elevators, depressors,
and sphincters. A flexor muscle is one which bends the part on itself.
An extensor is one which by its action tends to extend or straighten
the part. An adductor muscle is one attached to a long bone in such
a way as to move it toward the ventral midline of the body. An
abductor opposes the action of the adductor. The caudofemoralis is
an abductor of the thigh and flexor of the shank. The gracilis on the
medial surface of the thigh is an adductor of the thigh. A rotator
muscle is one which causes a part to rotate on its own axis. An elevator
raises or elevates a part, as the splenius of the neck may elevate the
head. A depressor is the antagonist of an elevator. A sphincter
muscle is one encircling an aperture which it closes by contraction.
The orbicularis oris in the lips around the mouth is an example. A

MAMMALIA

645

dilator is antagonistic to a sphincter muscle. The relations of most
of the superficial muscles may be observed by reference to Fig. 347.
Over four hundred of the muscles of the cat are found in man, and
they have approximately the same relative location, function, and
nerve supply.

NASAL CAVITY

MOUTH CAVITY

TRACHEA

OESOPHAGUS

EXTERNAL

JUGULAR

RIGHT CAROTID

ARTERY

SUBCLAVIAN A.

AORTIC ARCH

PRECAVAL VEIN

HEART

RIGHT LUNG

POSTCAVAL V.

DIAPHRAGM

DORSAL AORTA

LIVER

STOMACH

DUODENUM

PANCREAS

SPLEEN

SMALL

INTESTINE

RT. ADRENAL

RT. KIDNEY

LARGE

INTESTINE

R. OVARY

OVIDUCT

CAECUM

FOETUS

UTERUS

URINARY BLADDER

RECTUM
URETHRA

VAGINA

Fig. 349. — Lateral view of visceral organs of a female cat. (Drawn by Titus Evans.)

646

TEXTBOOK OF ZOOLOGY

The Digestive System

The most anterior portion of tliis system is the buccal cavity or
mouth. It contains the teeth, which are set in alveoli or sockets in
the jaws, and the tongue on which are located taste huds. The roof

INTERNAL
CAROTID

EXT. CAROTID

COMMON

CAROTID

SUP. THYROID

SUBSCAPULAR

SUBCLAVIAN
LONG THORACIC
INNOMINATE

PULMONARY
DORSAL AORTA
C CELIAC

HEPATIC

GASTRO-
5 RLE NIC

SUPERIOR
MESENTERIC

ADRENO -LUMBAR

RENAL

OVARIAN

INF. MESENTERIC

ILIOLUMBAR

EXT. ILIAC

INT. ILIAC

CAUDAL

Fig. 350. — Heart and arteries of cat (ventral view). (Drawn by Titus Evans.)

MAMMALIA

647

of the mouth is composed of the hard palate at the posterior margin
of which is a transverse muscular flap, the soft palate, which hangs
down to separate the moutli from the pharynx just behind. At the
lateral extremities are found the tonsils, which are masses of lym-
phoid tissue each held in a pit or fossa. The space between the base

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-SUBSCAPULAR
-BRACHIAL
■SUBCLAVIAN
-VERTEBRAL
■ PRECAVA
-INT. MAMMARY
■PULMONARY
-AZ YGOS

■POSTCAVA

HEPATICS
I NTESTINAL
SUP. MESENTERIC

PANCREATO-
DUODENAL
HEPATIC PORTAL
CORONARY
-GASTRO-
EPIPLOIC
GASTRO-
SPLENIC

INFERIOR
MESENTERIC
RENAL
OVARIAN

COMMON ILIAC
EXTERNAL ILIAC
INT, [LIAC

HYPOGASTRIC
CAUDAL

Fig. 351. — Veins of cat (ventral view). (Drawn by Titus Evans.)

648 TEXTBOOK OF ZOOLOGY

of the tongiie and palate, which provides the opening from mouth
to pharynx, is known as the fauces. Ducts from the salivai-y glands
open into the mouth (Fig. 349).

The pharynx receives the two openings of the internal nares from
above the hard palate and the two Eustachian tubes from the middle
ears. In the floor of the pharynx is the glottis which opens to the
respiratory tract and is guarded by the cartilaginous, flaplike epi-
glottis. Posteriorly the pharynx leads to the aperture of the esopha-
gus which is a narrow, muscular tube leading posteriorly to the
stomach, which lies caudal to the diaphragm on the left side. The
esophageal end of the stomach is known as the cardiac portion, and
the intestinal end is the pyloric portion. Inside, the gastric glands
are embedded in the fine foldings of the internal epithelium. At the
posterior end is a circular fold of the epithelium, embraced by a
sphincter muscle which serves as a valve to open and close the pylorus
or gateway to the intestine. Following the stomach is the small
intestine which is seven or eight feet long and is divided into the
anterior curved duodenum, the jejunum and the much-coiled ileum.
The duodenum receives the pancreatic duct from the pancreas and
the tile duct from the liver. The small intestine leads into the
ascending portion of the colon, which turns transversely across the
abdomen and follows posteriorly as the descending colon to the
rectum. At the anterior end of the colon is a short, blind sac extend-
ing beyond the point of entrance of the ileum ; this is the cecum. In
human beings a narrowed extension of this is the vermiform ap-
pendix. In the rabbit the cecum is about twenty inches in length.
Carnivorous animals usually have a reduced cecum.

Circulatory System

This system includes both the blood vascular portion and the
lymphatic system. The blood system is responsible for the transpor-
tation of oxygen, food, and excretions. The lymph of the lymphatic
system can be compared to blood without red corpuscles. It makes
intimate contact with the tissues. The plasma of the blood seeps
through the capillaries and is collected as Ijonph in the lymph spaces
of the tissues over the body. These spaces are joined by vessels and
many of them empty into the thoracic lymph duct which leads into
the large subclavian vein of the thorax.

MAMMALIA 649

The heart has four principal chambers, as it does in the chicken.
It is fairly large and compact. The pericardial sac holds it in posi-
tion in the body. There is only one systemic arch leading from the
heart, and it turns to the left. The anterior part of the body is sup-
plied largely by the branches of the innominate artery which repre-
sents the right aortic arch (Fig. 350). The two preeaval veins with
their branches collect blood from, the anterior, and the single postcaval
vein and its branches from the posterior, parts of the body. They
bring it to the right auricle of the heart (Fig. 351). The hepatic portal
system is present, serving to collect blood from the visceral organs and
deliver it to the liver. There is no renal portal system. The blood is
composed of the fluid plasma, white corpuscles, and red corpuscles.
The latter are small, disc-shaped, biconcave, and without nuclei, as is
common among mammals.

Respiratory System

In mammals, the special organs of respiration are the lungs,
wherein the carbon dioxide is taken from the blood and the oxygen
from the air is supplied to it. The nasal passages lead through the
nasopharynx to the pharynx and from here through the glottis to the
laiynx, thence by trachea to the lungs. The air is warmed as it
passes in through the nasal chambers and pharynx. The air passes
over the true and false vocal cords of the larynx in the production
of sound. The walls of the trachea are prevented from collapsing by
about forty-five C-shaped rings. The trachea divides into right and
left bronchi just before reaching the lungs. Each lung is completely
invested with a sac of delicate, transparent serous membrane, called
pleura, and is divided into lobes by deep clefts. The left one is com-
posed of two large unequal lobes and one small lobe while the right
consists of four unequal lobes.

Breathing is effected by increasing and decreasing the size of the
thoracic cavity. To do this the ribs are moved forward and spread
while the diaphragm which usually arches anteriorly is contracted to
a flat position. As the chest cavity is thus enlarged, a vacuum is
produced, and to balance the pressure, air naturally rushes into the
lungs from the outside. When relaxed the wall of the thorax and
the diaphragm both return to their original position and force the
air out. The intaking of the air is called inspiration and the dis-
charge is expiration.

650 TEXTBOOK OF ZOOLOGY

Nervous System

The nervous elements of the cat, as of most vertebrates, form sys-
tems, known as the central, peripheral, and autonomic (sympathetic).
The first includes the brain and spinal cord and is protected by three
membranes called meninges. The outer one is the dura mater, under
it the arachnoid, and on the surface of the nervous tissue, the pia
mater. The j)eripheral system includes twelve pairs of cranial nerves
emanating from the brain, and about forty pairs from the spinal
cord. The autonomic system is composed of two trunks which bear
ganglia and extend one on each side of the vertebral column through
its length of the trunk. Branches from these trunks reach all vis-
ceral organs.

The hrain presents a much enlarged cerehrum with fairly well-
developed convolutions. It has spread until the diencephalon and
midbrain have been covered by it. The cerebellum, which is also
quite well developed, is divided into two lateral hemispheres and a
convoluted, central vermis (Fig. 352).

The sense organs are relatively well developed. The olfactory
organ is located in the deep posterior portion of the nasal cavity,
which is quite large. The sense of taste is located in the vallate
papillae and in some fungiform papillae on the dorsal surface of the
tongue, as well as sparsely scattered in the mucous membrane of the
mouth and pharynx. The ear, an organ of hearing, is composed of
the pinna, or external ear, which directs sound waves to the tympanic
membrane, from which they are transferred by three bony ossicles to
the vestibule of the spiral-shaped cochlea. The latter contains the
sensory organ of Corti of the inner ear. The eye is of typical ver-
tebrate form and is developed for keen vision.

Excretory System

The kidneys, two ureters leading from the kidneys, the bladder,
and the urethra constitute the principal organs of this system. Each
kidney is composed of an outer cortical layer about one-half centi-
meter thick and an inner medidlary substance. In the cortical sub-
stance are hundreds of renal corpuscles, each composed of a ball of
capillaries, the glomerulus, and an epithelial wall, Bowman's capsule.
Urine is taken from the blood here and carried through the parts of
the uriniferous tubules to the pelvis of the ureter embedded in the

MAMMALIA

651

CEREBRAL LOBES

CEREBELLUM
MEDULLA OBLONGATA

5 VCERVICAL NERVES
SPINAL CORD

> THORACIC NERVES

LUMBAR NERVES

SACRAL NERVES
FILUM TERMINALE

Fig. 352. — Dorsal view of nervous system of cat. (Drawn by Titus Evans.)

652

TEXTBOOK OF ZOOLOGY

hollowed hilus of the kidney. The slender ureter carries the urine
from each kidney to the urinary bladder. From here it is carried to
the exterior by the urethra.

Reproduction and Life History

The ovaries of the female cat are small, yellowish, oval bodies each
about the size of a large navy bean and located against the dorsal

RIGHT KIDNEY

R. URETER

-URINARY BLADDER

R. VAS DEFERENS

INGUINAL CANAL

PROSTATE GLAND
URETHRA
RIGHT BULBO-
URETHRAL GLAND

PENIS

R. TESTIS

Fig. 353. — Urinogenital system of male cat. (Drawn by Titus Evans.)

body wall just posterior to the kidney. On the surfaces of the
ovaries are a number of projections, the graafian follicles, in each of
which is a developing egg or ovum. Each of the two oviducts or
Fallopiayi tubes has a funnel-shaped end or ostium which fits loosely
around the ovary. The oviduct leads into the horn of the uterus. The
two uterine horns join in the body of the uterus which continues pos-

MAMMALIA 653

teriorly as the vagina. In the ventral wall of the vagina, near the
aperture, is a small body of erectile tissue, the clitoris, which corre-
sponds to the penis of the male. The pair of testes of the male are
contained in a pouch of integ'ument called tiie scrotum, which hangs
beneath the anus. Each testis is about one and a half centimeters
long by one centimeter thick. The epididymis is a mass of coiled
tubules coming from the testis and lying at its dorsal side as a part
of the testicle. The vas deferens is the canal leading from this
cranially through the ahdominal ring and inguiyial canal into the
abdominal cavity, where it enters the urethra on its dorsal side near
the prostate gland. During copulation the mature spermatozoa pass
into the urethra and are discharged into the vagina of the female by
the erected penis. The secretions of the prostate glands and of
Cowper's glands just posterior to them, provide a considerable part
of the seminal fluid.

The cat will first breed at the age of one year. The female will
receive the male at certain periods only. Mature ova rupture through
the wall of the follicle of the ovary and are normally received by
the ostium of the oviduct which covers it. Occasionally an ovum
falls into the body cavity. In the oviduct the ova are moved pos-
teriorly where, if copulation has occurred, they will meet spermatozoa
and be fertilized. Cleavage, or divisions of each zygote follows, and
each resulting multicelled embryonic mass moves down to the uterus
where it becomes implanted in the wall. Here a placenta is formed
from a union of certain embryonic membranes with the internal
lining of the uterus. Parental blood carries nourishment and oxygen
to this membrane where it diffuses through to the embryonic blood
in other vessels in the membrane. The time between copulation and
birth of young (gestation period) in the cat is from fifty-five to
sixty-five days. From three to six young are usually born in each
litter. They are fed from the four nipples of the mammary gland
along each side of the ventral surface of the trunk.

CHAPTER XXXII

ANIMAL ANOMALIES

It is rather generally understood among students of biology that
no two animals, even of the same species, are exactly counterparts of
each other. There is a certain normal range of variation in size and
structure as well as in functional efficiency. Any variations beyond
these rather fixed limits are classified as malformations or anomalies.
A study of such abnormalities is know-n as teratology. Abnormalities
may occur at almost any stage in the life of the individual, but a
large majority result from abnormalities in the process of develop-
ment. Many are due to failure of development and some to over-
development. In turn, much of this is due to malregulation resulting
from failure of balance in the functioning of the endocrine system.
The occurrence of striking malformations in newborn human babies is
in approximately the ratio of one to 165. Fortunately, many of the
most grotesque anomalies do not reach full development and are bom
dead.

Anomalies have been classified into several groups on the basis of
the following conditions :

a. Failure of Development. — The embryonic structure fails to ap-
pear or at least fails to develop to a significant degree. Examples
of this account for single kidneys or legs where they are usually
paired.

b. Arrested Development in which the development stops before
adult condition is reached, as in the cleft palate, harelip, or dia-
phragmatic hernia.

c. Overdevelopment.— In such cases growth is exaggerated or the
number of parts increased as in gigantism (macrosomia) or in-
creased number of digits or accessory mammary nipples.

d. Fusion. — The kidneys are sometimes fused together to form
a horseshoe kidney.

e. Splitting, thus forming extra structures, as in the case of acces-
sory spleens or the splitting of the ureter.

654

ANIMAL ANOMALIES

655

f. Displacement of Organs. — Cases are found where organs oc-
cupy abnormal positions, as both kidneys on the same side, a finger
at the wrist, or the appendix on the left side.

Causes of Anomalies. — There are both internal and external
agencies which bring about malformations in the individual. Both
embryology and pathology contribute to the explanation of the
causes of these abnormal conditions. The development may be per-
fectly normal and a subsequent disease may be the cause of striking
abnormality. On the other hand, certain diseases of the parent will
influence the normal development of the fetus. Even twinning by
itself is an abnormal process in most animals. It, however, is not
usually thought of under the title of anomaly, except when they are

Fig. 354. — Grasshoppers at time of diapause, showing some of the abnormahties
which very infrequently' occur in their natural development. 1, Normal embryo :
2, embryo with two extra heads and mouth parts; 3, embryo with a lateral twm
joined at the abdomen ; J,, almost complete twins back to back ; 5, embryo with a
double abdomen. (From Evans, "Effects of Roentgen Radiation on Embryonic
Organization and Regulation in Melanoplus differentiaUs [Orthoptera]," Physio-
logical Zoology, Vol. X.)

physically connected or the individuals are otherwise malformed.
The causes of anomalies may be summarized as follows :

1. Internal Causes.

a. The germ plasm carries hereditary causes for some, and these
characteristics are transmitted as are normal traits. Color blind-
ness and hemophilia (bleeder condition) are examples.

b. Diseases which cause abnormal growths and conditions, as
elephantiasis. (Fig. 389.)

c. Unbalancing the chemical regulators (hormones) which are pro-
duced by the endocrine glands. (Overactivity of the hypophysis
causes gigantism; cretinism, a dwarf condition, results from deficiency
physically connected or the individuals are otherwise malformed,
in thyroid activity.)

656 TEXTBOOK OF ZOOLOGY

d. Fortuitous abnormalities whose causes are not apparent or are
influenced by certain variations in other organs. Such anomalies
show up most frequently in the circulatory and nervous systems.
Venous or nervous supplies to organs are often modified. Another
example is the rearrangement brought about by diaphragmatic hernia

2. External Causes.

a. Environmental agencies may affect almost any individual whose
development occurs outside the body of its parent. Exposure to
radium or x-ray radiation, sharp variations in temperature, exces-
sive salt content, or contact with toxic substances may all be respon-
sible for various degrees of abnormality. The same factors are also
effective, if present, in the uterine environment of the placental
type of animal.

b. Mechanical factors, such as abnormal pressure, blows, and falls,
may cause some abnormalities.

e. Ahnormal implantation in the wall of the uterus resulting in
deficiencies in nourishment and support of the fetus.

d. Such diseases as syphilis, which may be transmitted from mother
to offspring, are responsible for some types of defects, as impaired
vision,

e. Developmental inhihition or arrest brought about by deficiencies
in metabolism at a time when the rate or efficiency should be high.
The work of several embryologists seems to indicate that the pro-
duction of twins either as normal individuals or otherwise is re-
lated to this condition.

Harelip and Cleft Palate

These two defects are related and are sometimes found together.
The lateral palatine processes may fail to complete growth and
unite properly, thus leaving a gap in the roof of the mouth which
opens directly into the nasopharynx above.

Harelip (cheiloschisis) is a very common defect and is due to the
failure of union of the nasal and maxillary processes. There may
be double harelip or single harelip. In connection with double hare-
lip it sometimes happens that the premaxilla projects beyond the
outline of the face to form what is called wolf-snout. Harelips are
frequently remedied by a surgical operation early in life.

ANIMAL. ANOMALIES

657

Fig. 355. — A case of harelip due to arrested development.

Diaphragmatic Hernia (Open Diaphragm)

An extreme case of this was found in a cat which was being used
for dissection purposes. The animal had lived an apparently normal
life and had been killed for laboratory study without showing evi-
dence of its abnormality until dissected. From all appearances the
diaphragm had not completed its development, but had formed a
fringelike projection which reached inward about half an inch from
the thoracic wall and extended the entire circumference of the in-
side of the thorax. The aperture in its center measured one and
three-fourths inches in diameter.

Due to this condition the arrangement of several visceral organs
was greatly affected. The thorax was somewhat elongated, and the
right side of the cavity comprised about two-thirds of the space
of the chest. The mediastinum (supporting median mesentery of
heart and lungs) had its attachment more than half an inch to the
left of the midline.

Almost the entire liver was turned forward to occupy the right
two-thirds of the chest cavity, and this placed the gall bladder at
the level of the junction of the auricle and ventricle of the heart.

658

TEXTBOOK OF ZOOLOGY

Approximately half of the spleen, the pyloric portion of the stom-
ach, and a large portion of the omentum had been drawn through
the aperture in the diaphragm. The right lung was extremely
crowded and small.

...^*!'" ^-

^ Might lung
^^Leffc lunq

M^'"~ Rt. auricle

% Rt.ventricle

^^ Liver

# Left lung

- Diaphragm
cut edge

Colon

Fig. 356. — Dissection of a fully-grown cat with a congenitally open diaphragm
(diaphragmatic hernia), showing many of the abdominal organs everted into the
thorax.

This anomaly occurs occasionally in human beings, but usually
in much less degree. It is not likely that a man would be able to
reach maturity and carry on normal activities with such an exten-
sive hernia.

ANIMAL ANOMALIES 659

Polydactylism (Extra Digits)

There are numerous abnormal variations in the number and ar-
rangement of digits, ranging from a stumplike structure of no digits
through the ''lobster claw" condition of two or three, to as many
as two more than normal. The polydactylous condition is rather
frequently found in cats. In fact, Wilder once reported that pos-
sibly 25 per cent of the cats of the vicinity of Ithaca, N. Y., possessed
an extra digit on each foot. The forefeet shown in Fig. 357 are
from a living cat which came from a litter of four, two of which
showed these complete characteristics on all feet, one of the litter
had only an extra toe on one foot, and the fourth was normal. Both
hind feet of this animal had five well-developed digits instead of the
usual four. This condition is a hereditary one and is brought about
by partial duplication of elementary structures.

-^'^W ^i:r:-'t

^\|#^ '^lili,"^'^

Fig. 357. — Front feet of a half -grown kitten with six toes. The claws are pro-
tracted on the left foot and retracted on the right There were five toes on the
hind feet of this cat.

Conjoined Twins

Instances occur in which individuals of about equal size or of
unequal size are fused together. If this occurs at the hips with
dorsal sides together, the condition is usually known as Siamese
twins. There are a number of different varieties of fusion, includ-
ing the head region, chest region, or complete fusion of trunk into
a single body with two heads and vice versa. A chicken with two
pairs of legs and wings, two backs, and a single head has been
dissected and described by Dr. F. L. Fitzpatrick. The feather tracts
were double on the neck and trunk. Internal dissection showed
the single digestive system to extend between the two necks, follow
through the combined body cavity with some modifications, and
empty into the cloaca of the right back region. No cloaca was pres-

660

TEXTBOOK OF ZOOLOGY

ent in connection with the left back. There was a single heart, two
very unequal lungs, two pulmonary arteries and veins to the larger
lung, while there was only one common carotid artery, that being
the right. Two trachea were present, the right being rather nor-
mally developed while the left was rudimentary. The brain was
normal, but joining the medulla were two separate and complete

Fig. 358. — Conjoined twins of cat showing single head, but double trunk and

appendages.

spinal cords, one passing to each back region. Most of the internal
organs "favored'' the right side, except the lungs, of which the
left was much more developed.

Dr. P, L. Fitzpatrick reports upon the anatomy of a double pig
also. This freak had two tracheae leading to two sets of lungs.
The brain consisted of a three-lobed cerebrum, two cerebellums.

ANIMAL. ANOMALIES

661

and two medullas. There was a spinal cord in each of the two backs.
A third or median eye was present on a ventral stalk beneath the
third or transverse lobe of the cerebrum. Two pairs of kidneys, two
bladders, four ovaries, double sets of oviducts and uteri, as well as

Fig. 359. — Conjoined human twins showing single hips but double trunk and
head. (Redrawn and modifled from Arey, Developmental Anatomy, published by
W. B. Saunders Co.)

a double cloaca, were present. There was a single but modified
heart and two aortas leaving it. Externally the animal had two
sets of limbs, two tails, two sets of mammae, two anal apertures,
and two urinogenital openings.

662

TEXTBOOK OF ZOOLOGY

Cerebrum

Trachea

--Lung of-
cavity B

Bronchi

Lung of
'1 I \ : i w<fv;--.--:v.i:-.^x {.•:■•■ .•••.•:7i cavityA

\.A Median

V \ sy« / i

\ Medulla /
Cerebelium

Fig. 360. — Left, dorsoposterior view of brain from pig embryo with double body
and single head. Right, larynx, tracheae, and lungs of pig embryo with double
trunk. (After Fitzpatrick.)

Trunk from other heart
R. sabdovian-

Panetalj-.

Dorsal
aorta

R. carotid
--L. carotid
■L. subclavian

Ductus
arteriosus

Porietals

AUantoic-
(/ef t back)

Renal
—Utero- ovarian

^-External iliac

^Allantoic / left^

r J , ^bacK>'
Caudal

Fig. 361. — Dorsal aortae and branches in embryo pig which had a single head and

two bodies. (After Fitzpatrick.)

ANIMAL ANOMALIES . 663

It is suggested that such individuals have developed from a single
zygote which underwent an abnormal cleavage. Examples of these
conditions are shown as they occur in cats. Similar fusions occur
in human beings as is illustrated in the accompanying diagrams.

There are also cases of unequal conjoined twins in which a much
smaller and less developed individual is fused to the abdomen, head,
palate, or sacrum of an otherwise normal individual. Such an indi-
vidual is referred to as a parasite on its larger mate.

Hermaphroditism

There are abnormal cases of sexual development in vertebrates
including man, in which the organs of both sexes are combined in
one individual. In true cases there are present both ovaries and
testes in the one individual, but no cases are known in which both
have functional capacity. The external organs are partly male and
partly female, and the secondary sexual characteristics, such as
voice, mammae, stature, and beard, may be of a mixed nature.
Hermaphroditism is the normal condition in the coelenterates, flat-
worms, annelid worms, and molluscs. Here both sets of organs are
capable of function. Birds and mammals are rarely subject to this
condition.

A condition spoken of as false herma'plirodiUsm involves the pres-
ence of gonads of one sex but the secondary sexual characteristics
and external genitalia of the other. In masculine hermaphroditism
testes are present but not usually descended into the scrotum, while
the external genitalia and secondary characteristics are those of
the female. In feminine hermaphroditism ovaries are present, even
descended into a scrotum in rare cases, but the clitoris is enlarged
and labiae are fused to resemble the penis of the male. In some
instances the lips of the slitlike urinogenital aperture on the under
side of the penis fail to fuse.

There is a very close homology in the organs of the two sexes.
The external genitalia are indifferent or sexless until the end of the
seventh week of embryonic development in human beings. Then
the determination comes, causing normally the modifications of de-
velopment to form the organs of one sex or the other. The develop-
ment of the external organs of the two sexes is strictly parallel.

664 TEXTBOOK OF ZOOLOGY

It is likely that this development is controlled by hormone relation-
ships, and it is in cases in which this balance is disturbed that
hermaphrodites occur. There is still much to be learned concerning
the causes of this condition.

Cardiac Anomalies

Transposition of the heart to the right side of the midline of the
body is known as dextrocardia and occurs rarely. It is usually
associated with displacement of other visceral organs.

There has been a case reported recently in which the autopsy of
a 73-year-old farmer disclosed that he had died because of a rup-
tured left ventricle of an extra heart. One of the hearts of this
man was in the normal position, the other (the one that failed) was
located just above the spleen and below the left lung in the lower
part of the chest. The hearts were both of about normal propor-
tions and both functioned actively. They both joined the aorta.

Due to faulty development, an incomplete interventricular septum
is occasionally found in the four-chambered hearts. The failure of
complete development of the septum between the auricles to close
the foramen ovale is a more common anomaly. It has been reported
that this occurs in some degree in one case in four for the human
being. Actual mixing of auricular blood sufficient to interfere with
normal function is much less common, however, because of the over-
lapping of the membranous walls which are pressed together by the
pressure of contraction. In a small number of human cases the
aerated blood and unaerated blood from the two auricles do mix
and produce a purplish colored blood and in turn affects the color
of the skin. Such a case is known as a ''hlue hahy" and often
leads to early death.

Abnormalities of Brain and Sense Organs

Encephalocoele is due to the protrusion of a sac of the meninges
and part of the brain through a defect in the roof of the cranium.
An abnormally large brain, which is usually associated with the
distention of the cranium by superabundance of cerebral fluid, is
known as hydrocephalus, or the size may be macrocephalus. The
opposite extreme in which the head and brain are abnonnally small
due to failure of development is known as micro cephalus.

ANIMAL ANOMALIES 665

There are cases of cleft nose in which the nostrils are in inde-
pendent projections. This condition is usually associated with hare-
lip and cleft palate. With regard to the eyes there are several
possible abnormalities. Cyclopia is a condition in which there is
a single median eye like that of the Cyclops instead of the usual
paired arrangement. In such cases the nose is usually at the base
of the forehead, above the eye, and cylindrical in shape. Failure
of complete development of the iris or chorioid, thus leaving a gap
or open sector in the margin of the pupil, is known as colohoma.

CHAPTER XXXIII

THE ENDOCRINE GLANDS AND
THEIR FUNCTIONS

The great complexity of the structure of organisms, particularly
of those animals in the higher ranks of the animal kingdom, makes
necessary a means of regulation and coordination of the functions
of the organ systems individually and a means of intercommunica-
tion between them. This work is cared for in part by the nervous
system; but another agency of regulation is present in all higher
organisms and many of the lower forms of life, which is of first
importance in this respect, one so complex and interwoven with
other organs in carrying out its functions that it is not thoroughly
understood at the present time. The group of organs doing this
work we designate as the endocrine glands. They manufacture and
furnish the body with chemical compounds called hormones, a term
which refers to compounds formed in the body and capable of
encouraging or exciting activity in another part of the body; they
are chemical messengers. The endocrine glands pour their hormones
directly into the blood stream without the assistance of ducts, and
the blood carries them to other parts of the body ; but they are never
carried to the outside of the body directly, as in the case of secre-
tions from the glands of external secretion, and for this reason the
endocrines are often referred to as organs of internal secretion, or
ductless glands.

The functions of the hormones are numerous, and current research
and investigation in the new field of endocrinology are constantly
presenting new activities and interrelationships of these secretions.
The field of endocrinology is comparatively new, and a complete
understanding of the complexities involved is impossible until fur-
ther research is completed. The mysterious manner in which the
endocrine organs produce their secretions, the hormones, the man-
ner in which they choose the raw materials, combine them, and
give forth products which are vital to the welfare and happiness
and often to the very existence of an organism, is of most intense
interest and of the greatest importance.

666

ENDOCRINE GLANDS AND THEIR FUNCTIONS 667

The hormones may be classified, arbitrarily, according to function
in three main groups. The first of these groups includes those hor-
mones which arouse specific responses in particular organs or in
localized parts of the body. A second group is composed of those
hormones which affect the general metabolism of the body. The
third group is that of the hormones which affect, in particular, the
growth and development of the organism. Another general method
of classifying substances of this nature terms those substances
which serve as excitors or accelerators ''hormones" and those which
i7iJiihit or depress activities as ''chalones." Illustrations of each of
these will be found in the discussion of the individual organs and
their functions.

Hormones are found in plants as growth hormones, in the inverte-
brate animals, and are best known in vertebrate animals. The follow-
ing examples will serve to demonstrate the nature of endocrine func-
tion in invertebrates: Earthworms whose testes are completely de-
stroyed, do not develop the clitellum, the band-like organ which func-
tions during and following copulation. In Bonellia, another annelid
worm, there seems to be a relation between sex determination and cer-
tain hormones, in that the week-old larvae are indifferent sexually,
but those that attach to the proboscis of the female parent become
males due to some agent received there. All others develop to become
normal females. Among the crabs there is a parasite which attacks
and destroys the gonads, and it is found that the parasitized male
crab will take on distinctly female characters due to the lack of some
humoral agent lost with the destruction of these glands. Too, using
radium to destroy the gonads of Asellus, another crustacean, seems
to indicate that the development of the brood pouch is controlled by a
substance produced in the normal ovaries. Molting and metamorpho-
sis both are regulated by hormones in insects. The supraesophageal
ganglion seems to produce a substance which initiates pupation activi-
ties in moths. The endocrine glands of vertebrates which are best
known and most clearly understood at the present time are the thy-
roid gland, the parathyroid glands, the suprarenal bodies, the pitui-
tary body or hypophysis, the thymus, the gonads, and the pancreas.

The Thyroid Gland

The thyroid gland is the most familiar of the endocrine organs
to the layman. It is a body of two lobes of about the size of wal-
nuts, slightly flattened, placed one on each side of the upper part

668

TEXTBOOK OF ZOOLOGY

of the trachea just below the larynx, or voice box, the two lobes
being connected by a saddle-shaped isthmus. The thyroid is well
supplied with blood, receiving in proportion to weight, three and
one-half times as much blood as the brain; this permits an easy
access of the hormone to all parts of the body. The thyroid is
normally not visible externally, but the pathogenic condition of
the organ caused by overgrowth, and known as goiter, is familiar
to everyone.

A. B. C.

Fig. 362. — Illustrating- cretinism. A, a cretin, 23 months old and B, the same
child after having received thyroid treatment for eleven months. C, an untreated
cretin 15 years old. (From Zoethout, Textbook of Physiology, published by The
C. V. Mosby Company, after Osier.)

The functions of the thyroid gland, as recognized today, are two :
that of control, in conjunction with other endocrine organs, of the
growth and development of the body; and, second, a most important
role in the regulation of metabolism. Any upset of the normal
functioning of the gland or the removal of the gland results in
serious physical and mental disorders, if not in death itself. The
hormone produced by this organ, designated thyroxine, has been
prepared from the fresh thyroid glands removed from various ani-
mals and has also been manufactured synthetically. Chemical diag-

ENDOCRINE GLANDS AND THEIR FUNCTIONS 669

nosis of the hormone reveals the presence of iodine in its compo-
sition, and the amount of iodine available in the body seems to be
the determining factor in the degree of control which this hormone
exerts on bodily functions. In regions of the world in which iodine
is scarce in water and food, goiter is a prevalent disorder, and only
in recent years has the understanding of the cause been complete
enough to suggest as a remedy and a preventive the introduction
of additional iodine into the diet by use of iodized salt. Since the
amount of iodine required for normal purposes is almost infinitesimal,
a sufficient quantity is supplied by this means. The correction of
disorders due to a deficient functioning of the thyroid gland by use
of natural thyroxine or by use of the synthetic product is quite com-
mon today among human beings.

The removal of the thyroid gland in animals which have not ob-
tained their complete growth results in delayed or arrested develop-
ment. An interesting example of this is found among certain
amphibians. The proper functioning of the thyroid gland is essen-
tial for the accomplishment of metamorphosis of the frog tadpole
into the adult frog. The tadpole lacking sufficient thyroid extract
may grow to an unusually large size, but metamorphosis never
occurs without the encouragement of this hormone. In the develop-
ment of human beings, a deficient supply of thyroxine in early
years results in a condition known as cretinism. The growth of
the bones does not take place, the entire body is stunted and de-
formed, mental development ceases, the facial features are misshapen,
growth of hair is scant, and the development of the sexual organs is
inco.mplete. Before work on the thyroid gland was undertaken,
cretins were often seen in certain parts of Europe and occasionally in
America; but the present understanding of the hormone and its use
has made possible the prevention and cure of most cases of this nature.

Improper functioning of the gland in adult years results often
in a condition known as myxedema, in which there is a thickening
and drying of the skin, a puffiness of the eyelids and lips, loss of
hair due to the condition of the skin, a slowing down of metabolism
and heartbeat, a depression of body temperature, the deposition of
large quantities of fat, and a final result, in many cases, of im-
becility. The administration of thyroxine, especially in the early
stages, accomplishes a complete, or at least a temporary, remedy.

An overfunctional thyroid gland, in which the condition is known
as hyperthyroidism, results in an increased metabolic rate, a loss of

670 TEXTBOOK OP ZOOLOGY

body fat, and a condition of hyperirritability of the nervous system.
It seems, therefore, that the difference between an overly energetic and
a sluggish person, and a lean and an obese person, may often be traced
directly to the degree of functioning of the thyroid gland. Hyper-
thyroidism is accompanied by increased excretion of calcium. It dif-
fers from the calcium upset due to parathyroid disturbance in that in
hyperthyroidism its concentration in the blood remains normal.

Work on the lower vertebrate groups suggests the probability of
an important function of the thyroid gland in determining the hiber-
nation periods of certain animals. The thyroid performs additional
functions in conjunction with other of the endocrine glands; e.g.,
the control of sexual activity; but these interrelationships will not
be discussed here. Peculiarly, thyroxin, although an accelerator of
oxidation in vertebrates, has a depressor effect on cell division and
differentiation in such invertebrates as Paramecium, sea urchin, and
the hydroid Pennaria. This has not been completely explained.

The Parathyroid Glands

Connected with the thyroid body are four little glands about the
size of small peas, so insignificant in appearance that they were
overlooked for many years. The removal of these small bodies along
with the thyroid gland in certain operations provoked such startling
results, however, as to attract attention to their presence and to
evoke considerable interest in their investigation. A complete re-
moval of the parathyroids results in unbalancing the blood calcium
and in a type of convulsion known as tetany; death is the usual result.
A deficient supply of paratJioriJione or parathyrin, the hormone of
the parathyroid glands, may be responsible for defective growth of the
bones and for deficient formation of enamel and dentine of the teeth.
Calcium is needed for both the teeth and the bones and the introduc-
tion of either calcium or parathormone into the body is made to sup-
plement a deficient supply of calcium due to malfunctioning of the
parathyroids. Removal of the parathyroids also brings about a fall
in the renal excretion of phosphorus, and the in.jection of parathor-
mone causes an immediate rise in the level of renal phosphorus.
Parathyroid activity is particularly useful to laying hens where so
much calcium is needed in shell formation. There is evidence of a
close functional relationship between the pituitary gland and the
parathyroid. In dogs the removal of the pituitary causes atrophy of
the parathyroids, particularly if the pancreas has been removed also.

ENDOCRINE GLANDS AND THEIR FUNCTIONS 671

Conversely it has been noticed that injections of extract from an-
terior pituitary raises blood calcium in some species because of in-
creased activity of the parathyroids.

Too great an activity on the part of the parathyroid glands results
in a decalcification of the bones, and an increased content of calcium
in the blood and in the excretion of the kidneys. The final result of
this softening of the bones may cause serious disfiguration and stunt-
ing of the body. Accompanying these results are flabbiness of the
muscles, decreased irritability of the nervous system, and other un-
favorable conditions. These may be remedied by the removal of a
portion of the parathyroids.

The Suprarenal Bodies

Lying close to and slightly anterior to the kidneys are two small yel-
low or reddish masses of tissue, which play a prominent role in the
regulation of the body and one of such complexity that much is yet
to be learned concerning its method of functioning. These adrenal
glands, or suprarenal bodies, are made up of an inner and an outer
portion, the medulla and cortex respectively. The former secretes a
substance designated as adrenalin (epinephrine or adrenin), which
acts upon various organs and raises the level of their functioning.
Adrenalin, at times of excitement or emergency, may cause con-
striction of blood vessels, increased rate of heartbeat, a greater
discharge of glucose from the liver to provide additional energy,
erection of hairs, stimulation or inhibition of the various visceral
muscles, etc., to provide greater efficiency. The range of control of
the medullary portion of the adrenals is thus wide and complex,
definitely interrelated with the functions of the sympathetic nervous
system, with other endocrine glands, and numerous processes of the
body, so that its true importance is difficult to estimate. Adrenalin
has been obtained from various animals for use in the treatment of
certain disorders, as an anesthesia in minor operations, and to stop
small hemorrhages. It has also been successfully employed in the
relief of asthma and similar troubles.

A hormone kno^vn as cortin has been isolated from the suprarenal
cortex, and, while the removal of this portion of the adrenals results
in death, the exact functions of the hormone produced therein are
not entirely understood. It does relieve the condition known as
Addison's disease. There is, without doubt, a close relationship
between the cortex and sexual development; and some workers

672 TEXTBOOK OF ZOOLOGY

believe that the cortex regulates the normal flow of blood, which
would account for the fatal results of its removal. An extract has
been secured from the adrenal cortex of cattle in particular and is
used for the treatment of conditions resulting from malfunctioning
of the cortical portion of the adrenals in other organisms. Some
have shown cortin to have capacity for delaying the onset of scurvy
in vitamin C deficiency.

After complete bilateral removal of the adrenal cortex, the fol-
lowing were the average survival periods for these several animals:
opossums, six days; guinea pigs, seven days; dogs, ten days; cats,
twelve days; while rabbits and rats may live on quite normally,
because of the presence of accessory inter-renal tissue. Symptoms
of insufficient cortin are loss of appetite with particular distaste for
fats, vomiting, diarrhea, muscular twitching, tetanic convulsions,
lowering of blood pressure and body temperature, and decline in
urine secretion and heart rate. Lack of cortin is said to disturb
the salt relation in the blood (particularly sodium chloride and
potassium), the water metabolism and redistribution in the body,
the metabolism (especially absorption) of intermediate carbohy-
drates and fats, and milk production in females (at least in cats).

The Pituitary Gland

The pituitary gland is the human being is a body weighing about
0.5 Gm., lodged in a depression at the base of the brain. It consists of
two principal parts : the anterior lobe and the posterior lobe. These
two portions have distinct functions. The vital importance of this
body was not realized for many years, but a series of observations
has placed it in a position of such importance that it has been
referred to as the regulator of the glandular system. The pituitary
gland is now known to be the source of a number of hormones, and
their functions are so closely connected with those of other endo-
crine secretions that they apparently have a part in all hormonal
processes of the body.

The secretions of the anterior lobe of the pituitary affect the
growth and development of the organism in general, the general
metabolism of the body, the development of the sex organs, and
work with other hormones in controlling additional processes. In-
efficiency in the anterior lobe, furnishing the body with either too
much or too little of the growth-promoting hormone, phyrone, re-

ENDOCRINE GLANDS AND THEIR FUNCTIONS

673

suits in the production of giants or dwarfs. A decreased supply
of the hormone in an immature individual, if the condition is not
remedied by administration of the hormonal extract, retards the
growth of the body and may cause a complete cessation of growth.
The dwarfs of the circus furnish examples of this unfortunate con-
dition, although not all dwarfism must be thought due to this cause.
For example, the cretin described previously is the result of thyroid
disorder and is usually a mental dwarf as well as a dwarf in body,
while dwarfism resulting from deficient phyrone is accompanied in
most cases by a normal mental development.

Fig. 363. — A pituitary dwarf at the age of nine and one-half years, compared
with a normal child of the same age. (From Zoethout, Textbook of Physiology,
after Engelbach.)

An overfunctional anterior pituitary results is a marked increase
in the growth of bones, although the general development of the
individual thus affected may be symmetrical and the physiological
processes may be normal in every respect. Cases of this type are
commonly seen. One such instance may be cited in which a nine-
year-old boy measured six feet and one inch and weighed 178 pounds.
His mental condition was normal.

Another condition, known as acromegaly, may result from an over-
functional anterior lobe of the pituitary during adult years. The

674 TEXTBOOK OF ZOOLOGY

facial bones enlarge, particularly in the ridges above the eyes, the
nose, and the lower jaw, and the soft tissues of the face undergo an
overgrowth resulting in a coarsening of the features. The hands
and feet may enlarge, also. The disease may prove fatal if it con-
tinues sufficiently long.

A second hormone secreted by the anterior lobe has a direct in-
fluence on the sex organs. This hormone, prolan, stimulates growth
and activity of the gonads, the testes and ovaries, and therefore
controls the production of the gonadal hormones, which will be
discussed later. The absence of prolan results in an atrophy of the
testes and ovaries, and the cessation of the production of the sper-
matozoa and the ova; its injection increases the activity of the sex
organs.

The posterior lobe of the pituitary also produces more than one
hormone, although pituitrin is the one concerning which we have
the most definite knowledge and which is commonly associated with
this portion of the organ. Pituitrin is known to stimulate the mus-
cles of the arterial system, increasing or decreasing the blood pres-
sure according to the amount of the hormone released in the blood.
It is also a stimulant for the musculature of the uterus and the
intestinal muscles.

It is concerned also with the regulation and disposal of carbo-
hydrates in the body. The body is able to use an increased quantity
of sugar when the secretion of pituitrin is reduced; and, on the
other hand, when the quantity of the hormone is more than normal,
the body needs less sugar; carbohydrates not actually needed are
stored as fat, resulting often in abnormally fat people, extreme cases
of which are seen in the circus. The posterior lobe of the pituitary
probably does not affect the development of the bones, but the func-
tion just discussed is quite definite.

The posterior lobe of the pituitary is concerned also with the
regulation of the secretion by the kidneys. A diseased condition,
known as diabetes iTisipidus, in which the patient voids large quan-
tities of urine, is treated by injection of the postpituitary hormone.
It appears that the hormone probably enables tissues to utilize and
store larger amounts of water than is possible in its absence.

Still another effect of the secretion is found in the case of certain
amphibians and reptiles ; that is, its effect on the pigmentation of
the skin of these animals. The removal of the pituitary gland of a

ENDOCRINE GLANDS AND THEIR FUNCTIONS 675

frog results, among other things, in the bleaching of the animal
and the inability of the frog to alter the color scheme of its skin
to agree with the surroundings. When in a strong light or on a
light background the retinas of the frog's eyes are stimulated by
light rays, and some of the impulses reach the pituitary 's posterior
lobe, resulting in a suppression of its secretion, and consequently
a lightening of the frog's skin. When the light is decreased the
pituitary increases its secretion and the frog has a darker pigmenta-
tion. These reactions probably do not occur so directly in the higher
groups of vertebrates. Kemoval of the pituitary gland tends to
cause atrophy of the other endocrine glands.

The Thymus Gland

The thymus, a small glandular structure located in the chest be-
tween the upper part of the sternum and the pericardium, is a tem-
porary organ, which normally atrophies in human beings by the time
of the onset of puberty. When the gland is too active, a condition
is found in children in which an enlargement of the organ results,
and breathing is rendered difficult. No distinct hormone has been
obtained from this gland.

It has been claimed recently that accruing acceleration in the rate
of growth and development occurs when successive generations of
rats are given daily injections of thymus extract. In third and
fourth generations, the rats at twelve days of age compared favor-
ably with controls of twenty days. Introduction of thymus extract
in young tadpoles causes them to grow rapidly to the size of the
adult frog but still retain their tadpole form and appearance. The
disappearance of the thymus at the time of puberty permits the
differentiation of mature animals and particularly the onset of
activity of the sex glands. Some workers claim that the thymus
contributes to the orderly and proper development of the bones of
the skeleton.

The Gonads and Sex Hormones

In addition to the usual function of producing germ cells for re-
production, the gonads produce hormones which influence the devel-
opment of secondary sexual characters and which have a regulatory
effect on the reproductive processes and activities. Sex differences
are caused in part by various hormones which have a selective
action on the male or on the female secondary and accessory sex

676 TEXTBOOK OF ZOOLOGY

characteristics. The earlier concept that the male sex produces
"male" hormones exclusively and the female produces only "fe-
male" hormones is no longer held. For example, extracts have been
prepared from the urine of women as well as from men which on
injection into capons caused growth of the comb which ordinarily
fails to develop as it would in the cock. The sex hormone sub-
stances affecting the male are spoken of as androgenic and those
affecting the female as estrogenic. The important sex hormones are
androsterone, testosterone, tJieelin, and progesterone (progestin). The
first two are male hormones, and the others are female.

Androsterone is found in male urine and can be crystallized from
it. It has a stimulating effect on development of secondary sex char-
acteristics and a definite regenerative effect on accessory organs
(seminal vesicles, prostate glands, and penis) of castrated male ani-
mals. Testosterone is produced in the interstitial tissue of the testis
but is absent from the urine. This hormone is several times as
effective as androsterone in bringing about regeneration of acces-
sory sex organs in castrated males. Recently androgenic hormones,
which are potent enough to affect the growth of comb in the capon,
have been found in the urine and ovaries of female animals.

Theelin (oestrin, estrogen, folliculin, menoform, progynon) has
been isolated from liquor folliculi, pregnancy urine, the placenta,
and amniotic fluid. This substance causes (1) increased growth of
the accessory female organs (uterus, oviducts, etc.), including changes
in the glands of their linings and in vascularization; (2) contraction
of the smooth muscle of the uterus; (3) initial growth of mammary
glands and nipples; and (4) sudden lowering of theelin concentra-
tion in blood (suggested as cause for bleeding during menses). The
secretion of theelin is influenced by the gonadotropic principle of
the anterior pituitarj^

Progesterone (lutein hormone, corporin, luteosterone progestin) is
a female hormone produced by the corpus luteum, the yellow body
of material which forms in the ruptured Graafian follicle after the
escape of the ovum. It produces the following effects: (1) sensitiza-
tion of the lining of the uterus so that implantation or attachment
of the zygote may take place in case of fertilization; (2) develop-
ment of placenta; (3) arrest of rhythmic contractions of the smooth
muscle of the uterus; (4) inhibition of ovum production and uterine
bleeding.

ENDOCRINE GLANDS AND THEIR FUNCTIONS 677

The influence of other hormones of the body is noted in the repro-
ductive and sexual processes. The effect of the pituitary has already
been mentioned. The thyroid secretion probably plays a role, not
clearly understood, in the female reproductive processes, since the
thyroid always enlarges at puberty and during pregnancy. Another
hormone is thought by some investigators to be formed in the pla-
centa during the development of an embryo. The interrelationships
of these hormones are involved, and doubt exists in some cases as to
their exact functions.

The Pancreas

This is one of the organs serving a dual purpose in the body; it
secretes from a group of its cells, called the islands of Langerhans, a
hormone designated as insulin. Experiments have shown the action
of insulin to be concerned with the metabolism of carbohydrates
and fats. Its presence facilitates the combustion of carbohydrates,
regulates the rate of sugar production by the liver, and promotes
the storage of sugar as glycogen in the muscles. It therefore de-
creases the amount of sugar in the blood.

Extracts of insulin are obtained from the pancreas of animals and
are used commercially for the treatment of the condition kno^vn as
diabetes. This disorder is due to a disturbance of the metabolism
of sugars, provoked by a deficiency of insulin. The blood contains
too great a percentage of sugar, but this is not turned into needed
energy, and much water is excreted by the kidneys in order to elimi-
nate the excess sugar. The patient suffers, therefore, from fatigue,
excessive hunger, and thirst. The injection of insulin subcutane-
ously assists in regulating the condition by restoring the power to
transform glucose into glycogen in the muscles. The patient re-
gains strength and weight as a result, but the treatment does not
perfect a complete cure and additional insulin must be injected at
intervals to maintain normal health. Overdoses of insulin result in
very serious disturbances, which may be relieved by ingestion of
glucose.

Among the functions of the hormone, adrenalin, is the accelera-
tion of the production of glucose in the blood; it is apparent, there-
fore, that the hormones, insulin and adrenalin, are antagonistic in
their effect on sugar metabolism, and an upset of the normal pro-
duction of these secretions results in metabolic disturbances.

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Thus we see the complexity of the performance of these secre-
tions in the body. When they are present in correct proportions,
the organism enjoys a smooth functioning of its physiological proc-
esses, regulated by the hormones. The malfunctioning of any one
endocrine organ results, however, in a disturbance of those processes
which it specifically regulates and usually of others which it influ-
ences in conjunction with its sister glands of internal secretions
or with other regulatory organs of the body. Each gland is called
upon, therefore, to function in full cooperation with the other glands
as well as to perform duties peculiar to itself.

CHAPTER XXXIV

REGENERATION

(By T. C. Byerly, Poultry Research Division, United States
Department of Agriculture)

INTRODUCTION

Regeneration may be defined as the formation of new tissue to
replace that which has been lost. The fact of regeneration was
familiar to Aristotle and Pliny but Spallanzani, who published the
results of his researches in 1768, was the first to apply the experi-
mental method extensively to the study of the problem. Examples
of regenerative ability have been given in previous descriptions of
the various animal phyla. In general, the ability to regenerate lost
parts is more extensive in the simpler animal organisms and more
restricted in animals of great complexity. In every case, regenera-
tion involves the loss of tissue followed by reorganization of the
remaining tissues. Reorganization is usually followed by growth
of the regenerating portion until it reaches approximately normal
size in proportion to the total size of the body. Discussion of
regeneration will include consideration of the control by the organ-
ism of its parts and of the integration of the parts.

REGENERATIVE CAPACITY

Protozoa

Regenerative capacity varies inversely with complexity of the or-
ganism but also varies a great deal among animals belonging to the
same phylum. Among the protozoa, nucleated fragments are gen-
erally capable of regeneration while nonnucleated fragments are
not. Thus, if Stentor is cut into three transverse pieces, each con-
taining a portion of the chainlike nucleus, each fragment regen-
erates by rearrangement of material, forming a small Stentor which
subsequently grows to normal size. In forms such as Amoeba, only
one fragment produced by cutting is likely to be nucleated. This
fragment will regenerate while nonnucleated fragments will round
up but fail to regenerate further.

681

682

TEXTBOOK OF ZOOLOGY

Mill LLL

Fig. 364. — Stentor coeruleus, showing regeneration. A, cut into three pieces ;
B, shows progress of development of anterior portion ; C, progress of regeneration
of middle piece; D, progress of regeneration of posterior piece. z (From Morgan,
Regeneration, by permission of The Macmillan Company.)

Porifera

Some of the sponges have very extensive capacity for regenera-
tion. A series of classic experiments by Prof. H. V. Wilson of
North Carolina demonstrated that regeneration is possible after the
dissociation of the cells comprising the bodies of monaxonid sponges
by pressing them through gauze. The individual cells thus sepa-
rated come together in groups. These cell groups organize them-
selves into spheres, then grow and develop into new sponges. Dif-
ferentiated cells, as well as the undifferentiated amoebocytes, take
part in regeneration.

Coelenterata

The coelenterates have been the subject of very extensive experi-
ments on regeneration. In general, members of this group may be
cut into fragments and each fragment will regenerate a complete
individual. Some forms, such as Eudendrium, are capable of regen-
eration after dissociation in the same manner as the Porifera.

REGENERATION

683

Transverse sections of hydroid stems must be of minimum length,
characteristic for the individual and level from which they were
taken. Pieces shorter than this minimum length may regenerate
tentacles and other structures characteristic of the oral region at
each end of the piece or may fail to regenerate such structures
entirely.

It is possible to split hydroids partially and get individuals with
2, 4, 8, or more, complete oral regions with a single foot.

Platyhelminthes

The free-living flatworms have served as experimental material
for a great many researches on regeneration. Planaria, like the
hydroids, may be cut into several transverse segments each of which

B'V-:/

Fig. 365. — Regeneration in Planaria. A, normal animal with transverse line
to show level of cut to divide the animal; B, B^, regeneration of anterior half;
C, C\ regeneration of posterior half; D. piece cut from middle of body; £)', D".
D', D*, regeneration of D; E, anterior piece; E^, E-, E^, regeneration of B; P. P^.
regeneration in head of another species. (From Morgan, Regene)-atio7ij published
by The Macmillan Company.)

684 TEXTBOOK OF ZOOLOGY

undergoes reorganization and growth to regenerate a new indi-
vidual. Like the coelenterates, very short pieces may regenerate a
head at each end or fail to regenerate a head at all.

Experiments with x-rays have shown that it is possible to prevent
regeneration by this means. X-rays prevent cells in a physiological
condition preliminary to cell division from undergoing division,
though they do not prevent completion of division in cells under-
going mitosis at the time of application.

Some of the flatworms exhibit a type of agamic reproduction in-
distinguishable from regeneration. This is especially true of the
tapeworms. The tapeworm forms new segments just distal to the
scolex, growing to very considerable length if undisturbed. The
more distal segments break off as they become exhausted.

Vermifuges cause the death and shedding of many segments but
are often ineffective in destroying or dislodging the scolex and the
growing region next to it, especially if the scolex is barbed. The
growing region subsequently produces new segments and the worm
may be completely "regenerated."

Annelida

Regeneration is a more complicated process in the annelids than
in the simpler animals and regenerative capacity is somewhat more
restricted.

If a Lumhriculus be cut into a dozen transverse pieces, a dozen
tvorms may be formed. The regenerated head consists of but a few
segments but the tail continues to grow by intercalation of new
segments just anterior to the anal segment.

When the anterior end is cut off, the cut surface is closed very
quickly by contraction of the body wall. The cut end of the diges-
tive tract is contracted and closed. No further extensive changes
are visible for a day or two but new ectoderm is soon formed over
the cut surface. Old ectoderm gives rise to the new. As the new
ectoderm increases in amount, the old is pushed back, leaving a
single layer of cells over the cut surface. The new ectoderm pro-
liferates, producing a bulge in the new head. New cells arise from
the ventral side of the bulge to form the brain, commissures, and
ventral nerve cord. Other ectodermal cells form the muscles.

The blind end of the gut proliferates and grows forward toward
the ectoderm. Invagination of the ectoderm takes place so that

REGENERATION

685

gut and ectoderm are flattened against each other. The cells draw
away from the region of contact and a new mouth is formed. Tail
end regeneration is similar to that at the head end, but simpler since
neither brain nor commissures are formed.

Regeneration in the earthworm is similar to that in Lumbriculus
in many respects but is somewhat more complicated. The anterior
end, when cut off, regenerates a new tail. "When a head is cut off,
another is formed.

Fig. 366. — Loss and regeneration of appendages in the hermit crab, Eupagurus
longicarpus. A, third walking leg; B, next to posterior thoracic leg; B^, posterior
thoracic leg ; C, C, C^, abdominal appendages of male ; D, telson and sixth seg-
ment with last pair of abdominal appendages ; E, regeneration of new leg from
cut-end distal to "breaking joint" ; F, new leg from cut made proximal to the
"breaking joint" ; G, leg regenerating from cut made very near the body. (From
Morgan, Regeneration, published by The Macmillan Company.)

686 TEXTBOOK OF ZOOLOGY

Mollusca

Capacity for regeneration is extensive in this phylum. Foot or
head, mantle or shell, may be replaced under favorable environ-
mental conditions.

Arthropoda

Regenerative capacity of organisms in the phylum Arthropoda
is more limited than in the preceding groups. Regeneration takes
place in the walking legs of decapod Crustacea, such as shrimps,
crabs, lobsters, and crayfish. Spiders, which are members of the
order Arachnida, are also able to regenerate appendages, but the
insects generally lack this capacity.

The hermit crab protrudes three pairs of thoracic legs from the
snail shell in which it lives. These three pairs of legs have breaking
joints, clearly demarked levels at which these legs break when
caught. The last two pairs of thoracic legs are not protruded from
the snail shell and have no breaking joint. When a leg of the first
three pairs is injured, it is thrown off at the breaking joint and the
leg regenerates from the breaking joint outward. Such a regen-
erated leg is small and of little use until the crab molts. The re-
generated leg grows to its proper proportions during the molt. If
a leg of the first three thoracic pairs is cut off between the body
and the breaking joint, a new leg forms with a new breaking joint.
Thus, the breaking joint is not essential to regeneration (Pig. 366).

The last pairs of legs, without breaking joints, are not readily
subject to injury. If, however, they are cut off at any level, the
missing parts are regenerated.

The eyes of Crustacea are also capable of regeneration. When
the eyestalk of Palaemon (shrimp) is cut off near the tip, a new
eye regenerates, but when the eyestalk is cut off near the base, an
antenna is formed.

Echinodermata

Regenerative capacity is very great among members of this phylum.
If an arm be torn from a starfish, not only will the major fragment
of the starfish regenerate a new arm but the detached arm will also
regenerate a new starfish provided even a small portion of the disc
remains with it.

REGENERATION

687

Some Holothurians (sea cucumbers) eviscerate themselves when
roughly handled or kept in an unfavorable environment. Such in-
dividuals may regenerate a complete new viscera.

1 Madreporite
Reqeneratincj ray

\^3

Fig. 367. — Regeneration in tlie starfish. Above, starfish regenerating part of
the central disc and three arms ; below, arm of starfish regenerating the remainder
of the body.

Cliordata

Regenerative capacity varies widely in this phylum. Increased
complexity is generally accompanied by reduced regenerative ca-
pacity within the phylum, but the Pisces form a notable exception.
This order is relatively primitive in many respects, but many fishes
have a relatively limited capacity to regenerate lost parts.

688 TEXTBOOK OF ZOOLOGY

Amphibia

Among the Amphibia, the Caudata are able to regenerate append-
ages. Adult Salientia are unable to do so, though they are more
primitive in many skeletal characters than the Caudata. A great
deal of research pertinent to an understanding of regeneration has
been done with amphibian material.

Salamander larvae frequently snap off each other's legs or gills.
Some salamanders will snap off their tails if they are seized. Gills,
legs, and tails are regenerated in both larvae and adults, though
regeneration is slower in the adult than in the larva.

Regeneration may be accomplished through the activity of pre-
viously highly differentiated cells. When the lens of either a frog
or a salamander eye is destroyed, the cells of the iris undergo a loss
of pigment, differentiate, and form a different type of structure
which develops into a new lens. This process is quite unlike lens
formation in the embryo in which the lens is formed from super-
ficial ectoderm cells under the influence of the optic cup.

New legs are formed by the assembly of undifferentiated connec-
tive tissue cells beneath the surface of the wound and by dediffer-
entiation of muscle and other cells. Such a group of cells is termed
a Mastema. The new leg is formed by the proliferation and differ-
entiation of the cells of the blastema in a manner similar to the
development of the leg in the embryo.

After the leg of a salamander is cut off, new leg bones, muscle,
and other tissues are formed from the blastema over the old stump,
not from the bones and muscle of the old stump directly. The cells
that form the blastema are assembled from their resting place in
nearby tissues. They are not brought together from distant points
in the body. Under local influences they are induced to form a
new leg. The strictly local nature of this induction has been demon-
strated by transplantation experiments. A boneless leg transplanted
to the back of a salamander regenerated its proper complement of
bones. Split limbs develop partial limbs complete as to structures
normally distal to the point of cleavage, producing duplicated limbs.

Nerves are not essential to regeneration of a new leg, but if they
are prevented from growing into it, the regenerated leg is much
smaller than is normal.

REGENERATION

689

Regenerative capacity has been limited in the salamanders so that
while the whole will regenerate certain lost parts, the lost part
even under favorable circumstances is incapable of regenerating the
whole body. A tail cut off and transplanted to the back, and proper
circulation established to insure good nutrition, will regenerate an-
other tail if a proper wound is made, but it will not regenerate a
new body.

Fig. 368. — Regeneration of the lens of the eye from the iris in Triton. (After
Wolff.) A, edge of iris with beginning of lens; B, C, later stages; D, still later
stage, showing rounded formation of lens structure ; E, entire eye with regenerat-
ing lens. (From Morgan, Regeneration, published by The Macmillan Company.)

Regeneration of the lens of the eye from the edge of the iris in
Triton is a different feature in regeneration. Here the new structure
comes from an entirely different germ layer.

690 TEXTBOOK OF ZOOLOGY

Regeneration may be prevented by x-rays, or sometimes by simply
sewing the edges of the wound together. After application of
x-rays, epithelium migrates over the wound surface, but no blastema
is formed.

Reptilia

Some of the lizards are capable of regenerating new tails. Indeed,
some of them have special areas developed where breaking occurs
when the tail is seized just as do the legs of the hermit crab. Spheno-
don, the most primitive of the reptiles, is capable of regeneration of
a new tail. The regenerated tail contains a central cartilage cylinder
instead of vertebrae and the regenerated skin and other tissues are
not quite normal. None of the reptiles is capable of regenerating
new legs.

Aves

The most striking regenerative capacity among birds is the abil-
ity to grow new feathers. Each feather is subtended by a feather
germ, a sort of permanent blastema, from which a new feather is
formed if the old one is lost through molt or accident.

Regeneration of the ovary is quite extensive if operative injury is
inflicted in the young chick. If the left ovary of a young female
chick is removed, the right, normally rudimentary, grows to form
an ovary, an ovotestis, or a testis. Experiments indicate that the
medullary portion of the primary gonad gives rise to a testis and
that some medullary substance remains in the rudimentary right
ovary. When the inhibiting influence of the normally functioning
left ovary is removed, this medullary substance may proliferate to
form a testis. Such sex-reversed birds are usually sterile.

Mammalia

No mammal is capable of growing a new appendage if aji old one
is lost nor of completely regenerating any other major organ. Re-
generation of tissues, however, is quite general. Hair, hoof, horn,
and epithelium regularly regenerate from germinal layers or areas.
If these germinal areas are destroyed, regeneration fails. Baldness
is, of course, a familiar example of failure of regeneration.

Regeneration in the group is largely limited to repair rather than
replacement. In this, as in the case of agamic reproduction, it is
difficult to differentiate between processes which may be classified

REGENERATION 691

as normal growth and those which are regenerative in nature. The
ability of various tissues to regenerate will be described briefly.

Epithelial Tissue. — Cuts and other wounds of the body surface
are healed by the migration of cells from the old epithelium over a
substrate formed by the underlying vascular and connective tissues.
Proliferation of the epithelial cells by mitosis follows.

Glands, such as the mammary and salivary glands, are capable
of considerable regeneration by proliferation of the small cells of
their secretory ducts. The liver is capable of regeneration by pro-
liferation of cells of the small bile ducts. The relation of the re-
generated tissue to the old tissue is usually atypical. If a lobe of
the liver is removed, the other lobes increase in mass, but the lost
lobe is not replaced by a new lobe. This process is called compensa-
tory hypertrophy. The kidney has a very limited capacity for regen-
eration, but if one kidney is removed, the remaining one does undergo
considerable compensatory hj^pertrophy.

Connective Tissues. — Blood cells of all sorts are formed contin-
ually in the mammalian body by special groups of cells. White
blood cells are formed largely in patches of lymphoid and myeloid
tissue scattered throughout the body. Red cells are formed largely
in the red bone marrow. When considerable amounts of blood are
lost, regeneration takes place through increased proliferation of
these special groups of cells.

Small blood vessels are regenerated by sprouting from pre-existing
capillaries. These sprourts may unite distally. The sprouts thicken
and become channeled to complete the vascularization of the injured
area.

Cartilage is regenerated largely from cells of the perichondrium.
Bone is not regenerated primarily from pre-existing bone cells. The
cells of the periosteum or marrow fonn a cellular connective tissue or
the perichondrium forms hyaline cartilage to form a mass called a
callus. Osteoblasts then invade this tissue to form new bone and
complete the repair.

The fibrous connective tissues regenerate by the division of undif-
ferentiated cells. A homogeneous ground substance is formed be-
tween the cells. Fibers appear in the ground substance and complete
the regeneration.

692 TEXTBOOK OF ZOOLOGY

Muscle Tissue. — Smooth muscle has slight regenerative capacity.
Healing is effected largely through the formation of fibrous tissue.
In the case of striated muscle, sareolemma cells sometimes divide
after injury and a limited amount of regeneration takes place.

Nervous Tissue.— Nerve fibers do not reunite after they are
divided. The distal portion of the severed fiber degenerates com-
pletely. The neurilemma cells undergo proliferation and unite with
the neurilemma of the proximal portion. The axone may then grow
into the united sheath. The process takes months. Nerve cells of
adult mammals do not proliferate.

Basis for Regeneration

Certain facts must be reviewed to furnish a basis for an explana-
tion of regenerative capacity. Differentiation of body cells during
embryonic development gives rise to an integrated organism. Genetic
experiments indicate that every cell in the body, with sporadic ex-
ceptions, contains the same genes as every other cell. Differentiation
does not consist of segregation of genes for the various organs.
Different expressions of the same genes at different times in devel-
opment or at different places in the organism or the expression of the
effects of particular genes only at a particular time and place in the
organism must be responsible for differentiation of organs and tissues.

The zygote is capable of giving rise to a whole organism. When
the zygote of some organisms divides, each of the two cells produced
is capable of producing a whole organism, if separated from the
other, regardless of the complexity of the adult organism ; e.g., in the
sea urchin and in man. Such a separation is indicated in the case of
identical twins. In other animals, due to differentiation of the cyto-
plasm, the first cleavage reduces the potency of the resulting cells so
that they will not produce a whole organism if they are separated.
Obviously, if the first two cells remain in normal position, each pro-
duces but a half organism in any case, demonstrating the importance
of the relationship between cells in limiting the potency of each. The
first two cells formed by division of the zygote are thus physiologi-
cally different though genetically identical so long as they remain in
their normal relationship to each other. As development and dif-
ferentiation proceed, potencies of the cells tend to be limited more
and more, the final limitation of potencies in the adult being slight
in simple organisms, great in complex organisms.

REGENERATION 693

Reduction of potency of individual cells of the adult organism is
really only another way of stating reduction of regenerative capacity.
As illustration of this parallel, it has been shown that a few isolated
cells from some of the sponges, kept in a proper environment, will
regenerate the whole organism. In contrast to the formation of an
entire organism by each of the first two blastomeres, if a few cells
from an adult mammal are isolated in a favorable environment, as in
tissue culture, the cells will only reproduce cells of their own kind,
not a new organism. Of course, some cells, such as nerve cells, will
not even reproduce their own kind and thus loss of reproductive or
regenerative potency is complete.

Why does the position of a particular cell, in relation to other cells,
determine its subsequent history?

Every animal organism exhibits polarity. The frog's egg is a
familiar example with its animal and vegetal pole; the nerve cell
with its afferent axone and its efferent dendrites is another. Polarity
may be determined in many ways; in the frog's egg by the relation
of the egg to its nutritive supply, in the nerve cell by the orientation
of the cell with respect to nervous center and periphery or to other
parts of the central nervous system. Polarity implies the presence of
a gradient of some sort between the poles — whether a gradient of ma-
terials as in the frog 's egg, or of permeability, of rate of metabolism,
or of some other sort. Usually the region at the more rapidly func-
tioning end of such a gradient dominates at least a portion of the
gradient.

Dominance of one region over another may be exerted in various
ways. Thus the central nervous system dominates a major portion
of man's body by assorting stimuli from the various portions of the
body and sending impulses to the proper effectors. The endoerines
respond to other stimuli and send substances through the blood
stream. In other cases, substances are produced locally and produce
their characteristic effects after diffusion to nearby cells. Bio-electric
currents may also serve as an agency in the exercise of dominance.
Polarity, with its resulting gradients, dominance of the more active
region of the gradient over the less active, integrate the developing
organism. "When a region grows sufficiently large, a formerly sub-
ordinate level may become so far removed that it escapes from the
dominance of the more active region and itself becomes locaUy domi-
nant, resulting in agamic reproduction in the simpler forms, or in

694 TEXTBOOK OF ZOOLOGY

the formation of a characteristic organ for that time and place in
the development of one of the more complex organisms.

Dominant areas sometimes produce substances which diffuse to sur-
rounding cells and determine their subsequent proliferation and dif-
ferentiation. Such dominant areas are called organizers. The dorsal
lip of the blastopore of the amphibian eg^, which determines the
formation of the central nervous system, is a classic example. Cells
from the vegetal pole of the amphibian egg will form nervous tissue
under the influence of the organizer if such cells are placed over
it instead of the cells normally occurring there. Organizers are
formed in the region of every regenerating structure, if not pre-
viously present. Thus, the stump of the leg of a salamander induces
the cells of the blastema to form those structures which are lacking.
In some cases, as in hydroids, if the dominant area itself is injured
organs characteristic of that region are regenerated rather than the
portion removed.

In the complex organisms, there finally results a maze of inter-
dependent and integrated, locally controlled regions under the gen-
eral dominance of the body as a whole. Each organ has a character-
istic size and rate of activity in proportion to the other parts. These
relations may be maintained from a food-supply in the blood stream
kept at a level which is relatively uniform for all portions of the
body, through different rates of absorption and utilization of food by
the different kinds of cells.

As development proceeds, an ever greater proportion of cells be-
comes functionally differentiated, and the rate of proliferation de-
creases correspondingly. Some cells, such as the germ cells and the
cells which replenish the supply of blood cells, maintain their physio-
logical youthfulness and rate of proliferation, while others, such as
nerve cells, become irreversibly differentiated and cease proliferation
altogether. Animals cease to grow when a balance is reached between
normal wearing out of tissue cells, on the one hand, and proliferative
activity on the other.

Experiments with tissue cultures have clearly demonstrated that
animal cells are potentially immortal. Removed from the dominance
of the organ and body of which they form a part and given adequate,
sanitary, nutrient conditions, they multiply indefinitely, forming
other cells like themselves. Tissue cells from vertebrate animals do
not organize new animals in tissue culture. Apparently, vertebrate

REGENERATION

695

cells older than cells of the earliest cleavage stages in development
have either undergone differentiation which is not completely re-
versible or else tissue culture media have not yet been developed
which permit the formation of proper organizers and fields of domi-
nance.

When a group of cells is isolated from its dominating region, those
cells may regenerate a new body, as in the sponges, or only other
cells like themselves, as in tissue culture. Usually they lack a favor-
able environment, in the case of the more complex animals, and die.
The removal of the cell group reduces asymmetrically the remaining
portion of the body.

When an integrated animal body is disturbed by a wound or by
removal of a part, certain conditions are set up under which healing
and sometimes more extensive regeneration take place. These are :
interference with or destruction of nervous control ; stagnation in the
transport system so that food and metabolic products accumulate;
change in diffusion gradients and the formation of new bio-electric
currents. The normal partition of foods is disturbed. Migration of
cells occurs to close the wound. Cells capable of growth assemble and
proliferate to reproduce the lost part under the influence of re-
maining local tissues. These cells are physiologically younger than
the differentiated cells of surrounding parts, utilize food faster, and
grow faster, so that regeneration takes place even when the organism
as a whole is starving.

Dedifferentiation, when it occurs, is a sort of rejuvenation process
resulting in physiologically young cells with potencies comparable to
embryonic cells.

ADAPTABILITY AND REGENERATION

The capacity for minor repairs is exhibited universally by animal
organisms and is essential to their success. There is certainly no
close correlation, however, between regenerative capacity and suc-
cess of species. In general, regenerative capacity is a limiting fac-
tor to the individual, not to the species. In many cases, it seems to
be more economical from the viewpoint of the species to grow new
individuals by means of sexual reproduction than to repair dam-
aged ones by regeneration. Pishes and insects are highly successful

696 TEXTBOOK OF ZOOLOGY

because of profligate rates of increase. Man succeeds because he
avoids physical injury. These three dominant groups all have
limited regenerative capacity. On the other hand, it would be
foolish to maintain that extensive regenerative capacity is of no
value to those species possessing it.

Summary

All animals have some regenerative capacity. Some of the simpler
ones may regenerate a new organism from a small portion of an
older one, while in the most complex animals only relatively small
parts may be regenerated after loss.

Regeneration consists of the assembly of physiologically young
cells at the site of injury, usually through migration of such cells,
but sometimes by dedifferentiation, a kind of physiological re-
juvenation, of local, differentiated cells. These physiologically
young cells proliferate and differentiate under the inductive influ-
ence of surrounding parts to reproduce the lost part in so far as
regenerative capacity is characteristic of the species.

CHAPTER XXXV

BIOLOGICAL EFFECTS OF RADIATIONS

(By Titus C. Evans, Colleges of Physicians, New York)

The study of biological effects of radiations is making such rapid
progress that it is difficult to cover all of the important phases.
It is hoped that this chapter vrill serve as an outline of the subject,
and that the reader will study the references listed in the back of
the book for detailed information. A few points in physics and
chemistry must be reviewed to understand this subject.

The Structure of the Atom

The atom is thought to consist of (1) a central nucleus bearing a
positive charge and possessing nearly all of the mass of the atom, and
(2) one or more electrons (negatively charged particles of small
mass) revolving around the nucleus in definite orbits. The nucleus
contains protons (positively charged particles) and electrons. The
protons are in excess and this gives the nucleus its positive charge.
The number of electrons in the orbits is determined by the number
of protons in the nucleus in excess of the ones neutralized by the
nuclear electrons. Hydrogen, for example, consists of one proton in
the nucleus (no nuclear electrons) and one orbital electron.

Atomic Weight. — Atomic weight is the total weight of the nu-
cleus. Heavy hydrogen has a nucleus twice the weight of hydrogen.
It contains, besides the proton, another particle of approximate
equal mass which is electrically neutral. This particle is termed a
neutron and is thought to be composed of a proton and an electron
in close association.

The atomic number is related to the charge on the nucleus. Helium,
for example, contains two neutrons and two protons in its nucleus.
This gives it an atomic weight of four and an atomic number of
two. The atom is electrically neutral, so two orbital electrons are
present to balance the charge on the nucleus. The chemical prop-
erties of an element depend upon the number of orbital electrons
(atomic number). Those in the outermost shell are valence electrons
by which one atom may be joined to another.

697

698 TEXTBOOK OF ZOOLOGY

Radioactive Atoms. — The nucleus of radium is unstable and in
changing to a more stable condition it emits radiation of two kinds.
These are particle and wave radiation. Particle radiations have mass
and vary in velocity. Alpha particles are positively charged (2 plus)
and have a mass of four. They are produced by nuclear disintegra-
tions. Beta particles are high speed electrons that have been removed
from atomic orbits. They carry a negative charge and have very
little mass.

Neutrons have a mass equal to that of the nucleus of heavy hy-
drogen but carry no charge. These are produced by nuclear dis-
integrations.

The cyclotron is a machine which swings particles, such as the
nuclein of heavy hydrogen, through an electric field increasing the
velocity of the particle at each cycle until it finally emerges with
a charge of several million volts. Such particles are able to remove
neutrons from metals such as berillium and are able to produce
artificial radioactivity in atoms such as phosphorus, sodium, carbon,
etc. Particles may also be accelerated to high velocities by static
generators such as the Van de Graaf type.

Wave radiation or electromagnetic radiation is different from par-
ticle radiation in several respects. The units, photons, have no mass,
no charge, and travel at a constant velocity (equal to that of light).
The wave lengths, and some other characteristics, of the various radi-
ations are shown in Fig. 369. The frequency of the vibration is in-
versely proportional to the wave length. The amount of energy
associated with a photon is termed its quantum. The quanta of short
wave length radiations are large whereas those of long wave length
are relatively small.

Roentgen radiation is produced by bombarding a metal target
with high speed electrons in a vacuum. If the voltage of the elec-
trons is high enough, it will remove electrons from an orbit near
the nucleus of the target atom {tungsten, for example). As an
outer electron drops into the vacated orbit near the nucleus it gives
up its excess energy in the form of a photon of roentgen radiation.
High voltage roentgen radiation will penetrate iron and copper to
a depth of several millimeters but low voltage x-rays will be stopped
by even thin sheets of aluminum.

Enough energy can be given to atoms by heat or electricity to
cause them to emit radiation resulting from rearrangement of outer

BIOLOGICAL EFFECTS OF RADIATIONS

699

electrons. Such radiations are heat, infrared, visible, and ultraviolet.
Heat and infrared radiation penetrate slowly and the quanta are
comparatively small. The quanta of visible light are larger and
the wave lengths are shorter. Such radiations are easily absorbed
by even light elements except that some light is transmitted by

"Wavelength

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Pig. 369. — Chart of wave radiations. Visible light contains different radia-
tions which appear as colors to the human eye. The long wave length radiation is
the red, and in order of decreasing wave length the others are orange, yellow,
green, blue, and violet. Visible light covers a range of one octave, as the wave
length of the red radiation is twice that of the violet. The range of wave length
for the infrared is much greater as it can be seen that the wave length doubles
itself thirteen times. (From Sheard, Lifegiving Light, published by Williams &
Wilkins Company.)

glass. Ultraviolet radiations have wave lengths shorter than that
of visible light and penetrate opaque substances to a greater degree.
Ultraviolet is also more chemically active since the contained quanta

700 TEXTBOOK OF ZOOLOGY

are large. The sun is continually transforming mass into radiation
consisting of cosmic rays visible, ultraviolet, and infrared radiations.

Biological Effects of Sunlig-ht

Organisms living on the surface of the earth are dependent on
solar radiation either directly or indirectly. It would be difficult
to imagine human life, for example, without the benefits of food,
heat, fresh water, and light. Most biochemical processes are ex-
othermic (give off heat) and lose energy. Photosynthesis constitutes
the chief means of counteracting the "running down" in the energy
of our planet. Green plants contain chlorophyll which absorbs en-
ergy from sunlight, decomposes the carbon dioxide of the air and
combines its carbon with the oxygen and hydrogen of water to make
carbohydrates (simple sugars). This process of photosynthesis is a
very complex one and some essential factors are chlorophyll, light,
carbon dioxide, oxygen, water, temperature, certain minerals, and
enzymes (organic catalysts which accelerate chemical reactions).
The energy absorbed from the sunlight is employed to activate
atoms which results in the synthesis of new compounds capable of
oxidation and release of energy. Plants also show other reactions
to light. The intensity and wave length of light affects size, time
of fruiting, amount of fruit produced, and transpiration (evapora-
tion of water). Animals, by eating the plants, liberate the energy
and utilize it in doing work, in producing heat, and in building
more living tissue.

The eye is man's most important receptor of radiation. The light
coming into the eye is focused by the lens and the image (inverted)
is thrown upon the retina. Retinal cells contain photosensitive
substances which are affected by the quantity and quality of the
radiation. The photochemical reactions incite nerve impulses which
travel to the visual centers of the brain. Moderate amounts of
sunlight are not injurious to the eye, but extreme intensities are
harmful and may result in temporary or permanent blindness, eye
fatigue, and cataracts. The glare of carbon arcs and the reflection
of bright sunlight should be avoided.

Infrared Radiation

Both sunlight and artificial light sources have much of the radi-
ant energy output in the infrared. The wave lengths of the radia-
tion are too long for visibility and the energy is absorbed in tissues

BIOLOGICAL EFFECTS OF RADIATIONS 701

by increasing molecular motion. This results in a local rise in tem-
perature. At low temperatures the chemical activities of a living
organism are slowed to such an extent that extremes will produce
death. A physical factor in causing death by freezing is the forma-
tion of ice crystals which disrupt the cellular structure of the tis-
sues. Biological processes are accelerated by increases in tempera-
ture and, within limits, the activity may be doubled by each 10
degrees (centigrade) increase in temperature. If, however, the
temperature becomes too high biological processes are retarded and
even death may result. Physical factors associated with death re-
sulting from high temperature are loss of water, inactivation of
enzymes, oxidation, and coagulation of the protoplasm.

High Frequency Oscillations

Hertzian (radio) waves are longer in wave length than the infra-
red radiation. They apparently are not biologically effective under
usual conditions of radio transmission, but high frequency oscilla-
tions of shorter wave lengths applied across two terminals at a
high intensity will subject the intervening material to an immediate
rise in temperature. This "internal heat" is being used in the
treatment of certain diseases in which fever aids the body in fight-
ing the infection. The physiological action is the result of electrical
stress. It has been found to destroy certain bacteria (in milk, for
example), to increase germination of seeds, and to produce abnor-
mal development of certain embryos.

Effects of Ultraviolet Radiation

The radiation contained in sunlight whose wave length is too
short to be visible is the ultraviolet. It may be produced by mer-
cury arc or mercury vapor lamps. Small amounts of ultraviolet
radiation retard growth of bacteria, protozoa, and fungi. Con-
tinued irradiation may cause the death of the organisms. There is
evidence that ultraviolet radiation increases the permeability of
cells. Proteins are altered chemically by long exposures to the
radiation.

The importance of certain fish oils in the treatment of rickets has
long been recognized. Likewise, the importance of sunlight is not
a new discovery. A great step, however, was taken recently when
it was discovered that certain inert substances could be activated
by ultraviolet radiation so that they would function, as cod-liver

702

TEXTBOOK OF ZOOLOGY

oil in the treatment of rickets. It appears that in the body the
sunlight acts on the sterol-bearing fats of the skin to produce the
necessary vitamin D.

Roentgen Radiation

In 1895, W. C. Roentgen, a professor of physics in the University
of Wiirzburg (Bavaria), discovered that the cathode ray tube with
which he was working threw some sort of invisible radiation on a

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Fig. 370. — Diagram of a roentgen ray machine arranged to irradiate some
biological material. The line current is comparatively low and has to be stepped
up by the transformer. The input is usually alternating current and must be
changed by the rectifier to direct current. The roentgen ray tube is of glass, is
evacuated, and contains a filament at the cathode end and a target at the anode
end. The filament is heated to incandescence by means of a low voltage current
and gives off electrons. A high negative potential is applied to the filament and
this drives the electrons across the tube where they are attracted to the positively
charged target. The energy of the electrons is given up to the target (usually
tungsten) as they are stopped and part of this energy is emitted as roentgen
radiation.

barium platinum cyanide screen, causing it to glow. He found that
when his hand was placed between the tube and the fluorescent
screen that bones would absorb the radiation while the softer tissues

BIOLOGICAL EFFECTS OF RADIATIONS 703

allowed the passage of the rays. It was also found that the radia-
tion would darken a photographic plate entirely protected from
light. These new rays which he called "x-rays" are usually spoken
of as "roentgen rays" in his honor. These x-rays were found to
V penetrate paper, wood, aluminum, etc., but were completely ab-
sorbed by thick sheets of heavier metals, such as iron and lead.

It was not long after the physicists had perfected the roentgen-
ray machine until physicians began to apply it in diagnosis of bone
injuries and in locating bullets and other foreign objects embedded
in the tissues of the body. By having the patient ingest a barium
compound it is now possible to obtain a detailed radiograph of the
digestive tract. Use of the fluorescent screen has allowed imme-
diate diagnosis of gastrointestinal disorders and has been of value
in the setting of broken bones. The roentgen ray machine has also
proved of great value to the dentist in locating cavities and pus
pockets in teeth, and in finding teeth that have never broken through
the gums.

The biological effectiveness of roentgen radiation was discovered
in a tragic manner. Many technicians and physicians, who had been
exposing their hands and other parts of their bodies to the radiation
while making radiographs or using the fluoroscope, began to de-
velop severe burns on the exposed areas. Before the severity of the
affliction could be appreciated and before the knowledge of the
danger could become widespread many pioneers in the field of x-ray
diagnosis had unwittingly become martyrs to the cause of medical
science. It was thus realized that the radiation was biologically
effective, and that even low dosages delivered at frequent intervals
might result in incurable burns. Short exposures, such as are em-
ployed in diagnosis, are not sufficient to be harmful to the patient,
but the technician who is around an x-ray machine day after day
should be protected by lead shields. White blood cells and germinal
tissues are especially susceptible to roentgen radiation, whereas
other tissues such as those of the nervous and skeletal systems are
more resistant. This differential susceptibility of cells has allowed
the application of roentgen radiation to the treatment of cancer.
Malignant growths arising from radiosensitive tissue are susceptible
to roentgen radiation. If a cancer is composed of undifferentiated,
rapidly growing cells, it is usually more sensitive to the injurious
action of the roentgen radiation. There are a few notable excep-
tions to the above generalizations.

704 TEXTBOOK OF ZOOLOGY

One of the first problems to confront the radiologist was whether
nonkilling dosages would have any subsequent effects on the normal
cells. This was of interest especially in cases where the gonads had
been irradiated. Cases of temporary sterility have been reported
following roentgen ray treatment of the gonads and it was won-
dered whether the offspring of such treated individuals would de-
velop abnormally. Evidence has come in abundance from the gen-
eticists to the effect that the mutation rate of Drosophila is in-
creased to an amazing degree by irradiating the germinal cells.
If the gonads of either sex are irradiated, it is found that inbreed-
ing will bring out many recessive and harmful mutations in the
third generation (F-2). It should be pointed out that the danger
of harmful mutations is not of the same consequence in the human
because brother-sister matings are not permitted. As the mutant
genes are usually recessive, they would probably never be expressed
unless the practice were widespread enough to allow the mating of
two people both of whose parents had treated gonads.

A contribution of greater theoretical importance than this warn-
ing to the x-ray therapist was the new method of increasing the
mutation rate. This addition of so many new characters in Drosophila
which would breed true allowed the geneticist to make rapid ad-
vances in supporting and clarifying the chromosome theory of in-
heritance. This has made possible the chromosome maps which
indicate the position of genes on the chromosome. Cytologists have
worked hand-in-hand with the geneticists, so that the approximate
location of the genes can be demonstrated visibly under the micro-
scope.

The question naturally arose as to how the x-rays bring about
such changes in the hereditary material of the germ cells. It has
been found that most of the changes in the chromatin were so
minute as to defy attempts to demonstrate them morphologically.
Occasionally, however, the cytologist has found that the x-radiation
has caused parts of a chromosome to be moved from their normal
position. Moreover, in some irradiated cells a part of one chromo-
some has become attached to another. Other experiments which
have proved of great value to genetics have been concerned with
the action of x-rays in increasing the rate of crossing over and in
producing somatic mutations.

BIOLOGICAL EFFECTS OF RADIATIONS

705

Organisms vary widely in their susceptibility to roentgen radia-
tions. Protozoa and bacteria in vitro are unusually resistant.

There is no doubt but that x-radiation produces changes in the
physiological activity of cells, but these changes are not as evident
as the morphological disturbances occurring in the nucleus and
cytoplasm. Such structural changes as lagging of chromosomes in
mitosis, failure of chromosomes to separate in meiosis, disintegra-
tion of chromatin, and coagulation of the protoplasm are produced.
Some physiological disturbances that have been noted are increased
permeability, increase in acidity, and changes in rate of respiration.

Fig. 371. — Drawings of grasshopper embryos. The first figure in the upper left
is a normal embryo. The others are abnormal due to the x-radiation treatments.
Note that affected empryos have attempted to produce a secondary embryo.

Mitotically active cells are usually more susceptible than resting
cells, and irradiated cells in the quiescent condition may appear
normal until the onset of mitosis and then disintegrate. Cells irra-
diated while in the process of dividing may complete the process
but be unable to negotiate a succeeding division. Destruction of
cells following irradiation may be delayed by holding them inactive
by low temperature or by some natural inhibitor. Some recovery
has been noted in a few instances in which quiescent cells were kept
inactive for a long period following irradiation.

706 TEXTBOOK OF ZOOLOGY

In all biological experiments the amount of radiation applied is
very important. It has been found that a very slight dosage may
be temporarily accelerative, a moderate dosage may have no visible
effect, a heavier dose may have a temporary inhibiting effect, and
a very heavy dose will result in permanent injury. There is usually
a lapse of some time following irradiation before the effects appear ;
and it has been found that, as the dosage is increased, this latent
period becomes shorter. There are several physical factors which
affect the amount of radiation produced ; these are the voltage, the
current, the type of target, the distance from the target to the irra-

Fig. 372. — Effect of temperature during the irradiation upon the amount of
injury produced. These rats are now two weeks of age. They were irradiated
when one day of age. Each rat was given a dosage of 2,010 roentgens to a sman
region of the back, indicated by the arrow. The radiation inhibits hair forma-
tion and it can be seen that this injury is much greater in tlie animals irradiated
at the higher temperatures.

diated object, the filtration, the back scatter, and the exposure time.
Radiological experiments have become much more accurate and
dependable with the advent of the dosimeter. Such a device meas-
ures the amount of ionization in air and records this change on a
scale calibrated to read in roentgens. This mechanical measure
allows the operator to determine the intensity and the total dose
delivered.

Roentgen radiation has proved to be a very valuable tool in the
study of experimental embryology. It has been found that the first
cleavage is retarded by appropriate exposures. It has been demon-
strated that development can be stopped by heavy dosages. It has

BIOLOGICAL EFFECTS OF RADIATIONS 707

also been found that embryos become more resistant to radiation
as they develop. Exceptions to this conclusion that the embryos
become more resistant as they develop are stages of extreme sus-
ceptibility, such as gastrulation which involves many critical proc-
esses not experienced later, and stages of unusually high resistance,
such as occur in some insects during early cleavage when the un-
differentiated embryo has remarkable regulative powers. Roentgen
radiation has been employed in upsetting the normal differentiation
of embryos, and the results have shed some light on inter-relation-
ships of cells at different stages in development. It has been found
that injury to young embryos as a result of irradiation tends to be
widespread, whereas radiation injury in older ones is more localized.
Evidence indicates that differential susceptibility of tissues of the
adult is not the same as in embryos. For example, the adult nerve
and bone tissues are apparently resistant, whereas in growing em-
bryos these tissues are susceptible.

One of the interesting and more practical types of experiment
with roentgen radiation has been that of determining the effects on
regeneration. It has been found that the effects are localized to
regions of a body which have been exposed, and regeneration of
amputated limbs can be inhibited by proper exposures even though
the wound may heal over. This may be considered as evidence,
which is in accord with other embryological experiments, that cellu-
lar differentiation can be effected without necessarily destroying the
ability of cells to increase in number.

The radiologist (x-ray physician) and the x-ray manufacturers,
as well as the biologist, have long recognized the need of more
information concerning the fundamental action of roentgen radia-
tion upon cells. Radiological organizations, some x-ray manufac-
urers, and such scientific groups as the Committee on Radiation of
the National Research Council and the American Association for the
Advancement of Science have aided such investigations.

The Fundamental Action of Roentgen Radiation

The photon of roentgen radiation is capable of removing an extra-
nuclear electron from an atom leaving it temporarily charged as an
ion. The electron removed by the photon is called a pJiotoelectron
and is capable of removing other electrons from lower energy levels.
Thus the path of the photon (until all of the energy is absorbed)
is marked, momentarily, by a trail of ions. In such a manner mole-

708 TEXTBOOK OF ZOOLOGY

cules become dissociated and form new molecules on recombination.
This production of ionization is the basic action of roentgen radia-
tion on protoplasm. It is thought that the production of an ion-pair
in a chromosome produces the alteration or destruction of the gene
at that point. Chromatin, especially in stages of synthesis, is ap-
parently the most radiosensitive of cell structures. Some investi-
gators have theorized a "sensitive volume" for a cell or an or-
ganism. This conclusion may be derived from certain analyses of
dose-effect curves. Such interpretation of quantitative data also
yields ideas that in some instances single "hits" kill a cell whereas
others require many "hits" on the sensitive area before being killed.
This interpretation of destruction indicates that the amount of
radiation absorbed is the only factor involved in the killing action.
The "target" theories of radiation action appear to apply best to
the killing action on bacteria and protozoa, and to the production
of mutations and chromosomal abnormalities.

Two important objections to the "target" theories, from a bio-
logical point of view, are (1) the shape of the dose-effect curves
may be explained on the basis of individual variation in sensitivity,
and (2) the metabolic condition of the cell at the time of the radia-
tion affects the amount of injury produced by the radiation. It
therefore appears that ionization is the fundamental action, but that
the full expression of the radiation injury depencffe upon the number
of vital processes in a susceptible condition (one of molecular inter-
changes of energy) at the time the ionization takes place. The
importance of the environmetal condition at the time of irradiation
is shown, for example, in Fig. 372 where the amount of injury pro-
duced w\as different even though the dosage applied was the same
for all of the animals. The lower temperature decreases the physi-
ological activity of cells and this apparently increases the resistance
to the radiation.

BIOLOGICAL ACTION OF RADIUM

Unfiltered radium emits three types of radiation: (1) alpha par-
ticles, (2) beta particles, and (3) gamma rays. The alpha particles
produce dense ionization paths and are very active biologically.
HowcA^er, they do not penetrate deeply into tissue, and may be
entirely absorbed by a sheet of paper. The beta particles may also
be filtered out relatively easily by enclosing the radium in a small
tube of platinum or gold. The biological action of beta particles

BIOLOGICAL EFFECTS OF RADIATIONS 709

is intense but most of them do not penetrate deeply into tissue. The
gamma rays are similar to high voltage roentgen radiation and
their biological action is very much the same. There is a difference
in their application in that the radium is usually placed close to the
biological material and then effects are more localized than that of
roentgen radiation.

Effects of Other Radiations

The ionization produced by neutrons in tissue is initiated in a
manner different from that of the roentgen and gamma radiations
already mentioned. The neutron ionization is much more localized
and intense. The neutron releases its energy by knocking the hy-
drogen nucleus, the proton, free from its outer electron. This recoil
proton is so large that it comes in contact with many atoms (thus
producing ions) within a short range before it dissipates its energy,
picks up an electron and becomes an ordinary hydrogen atom again.
Neutrons are more readily absorbed in light substances rich in hy-
drogen (as biological tissues and water), than in denser substances
such as iron or lead. In terms of ionization in air, the neutrons
appear to be from five to fifteen times more effective than roentgen
radiation in killing cells. Qualitatively, the therapeutic effects of
the fast neutrons have so far been similar to those of roentgen
radiation.

Radioactive substances have proved to be valuable as tracers in
following atoms through metabolic processes in the body. Eadio-
active iodine, for example, can be fed and within a short time the
region of the thyroid will be giving off radiation that can be de-
tected by the Geiger counter (sensitive ionization chamber). Radio-
active phosphorus shows promise of becoming of value in the treat-
ment of diseases related to excess bone marrow development. It
appears that phosphorus is concentrated in malignant bone marrow
cells and thus the concentration of radioactivity becomes great
enough to destroy them.

Cosmic rays abound in the atmosphere and penetrate to depths
of even the heavier materials. Apparently not enough of the radi-
ation is absorbed by biological material to be effective. However,
from time to time one hears of the theory that natural mutations
may be the result of cosmic rays affecting the chromosomes of the
germ cells.

710 TEXTBOOK OF ZOOLOGY

Summary

Biological effects are related to the type of radiation being ab-
sorbed. High frequency oscillations penetrate body tissues and
produce molecular agitation ("artificial fever"). Infrared also in-
creases body temperature (although more peripheral in location).
More resistance is offered to radiations of the visible spectrum,
although the energy changes involved are not extensive enough to
bring about striking systemic effects. Ultraviolet radiation con-
tains more energy and its absorption causes changes in atomic asso-
ciations which may have profound effects on chemical processes.

Roentgen and gamma radiations are very penetrating and when
absorbed they displace electrons and produce ionization. These
changes are fundamental in character and may result in extensive
injury to cells.

The physiological condition of cells appears to affect the reaction
to radiation. Embryonic and highly active cells are apparently the
most susceptible.

The quantity of radiation absorbed is an important factor in pro-
ducing biological effects.

The knowledge of biological effects of radiation at present offers
possibilities of theorizing about the origin and development of life
itself. The most dependable contributions, however, have been the
production of artificial fever by high frequency oscillations; infra-
red therapy ; the understanding of the importance of sunlight in the
production of food by plants; the role of ultraviolet radiation in
the -conversion of ergosterol to vitamin D ; the use of roentgen radi-
ation and radium emanations in increasing mutation rates which
has been of great value to genetical studies; the use of these radia-
tions in the study of embryological development; and their use in
the treatment of cancer, etc.

CHAPTER XXXVI

ANIMAL DISTRIBUTION

(By Willis Hewatt, Texas Christian University)

Life Regions and Zones of the Earth

Every area of the earth has its animal and plant life. The ap-
parently barren sun-baked desert, the ice-capped polar regions, the
highest mountain tops, and the tropical rain forests are all inhabited
by their faunas and floras. Life in the seas extends from the shore
line to the greatest depths. After extensive taxonomic surveys have
been made in practically all areas of the earth and these data are
studied, it is found that the earth can be divided into fairly definite
horizontal regions and vertical zones. These two phases of the dis-
tribution of organisms on the earth are very closely related to each
other but are usually studied separately as geographic or horizontal
distribution and hathymetric or vertical distribution, respectively.

Geographic Distribution. — Many attempts have been made to
divide the earth into horizontal life regions based upon the distri-
bution of various groups of animals and plants. The exact bound-
aries of the regions are not fully agreed upon since several groups
of animals have been used as criteria for the division into life
regions. P. L. Sclater (1829-1917), the earliest zoogeographer, worked
•with the perching birds. These forms were well adapted to such a
study since they have little power of flight, they are widely dis-
tributed, and they had been very closely studied. The early (1876)
classic work of the Englishman, Alfred Eussell Wallace, in which
the earth was divided into six primary regions, is the one most com-
monly followed by modern biologists. Wallace based his divisions
upon the distribution of the mammals. Other significant studies in
geographic distribution have been based upon the geographic ranges
of mollusks, earthworms, moths, butterflies, spiders, fresh-water fishes,
reptiles, and many others. From works dealing with such varied
groups of animals it is readily understood that any classification of
the regions of the earth will depend greatly upon the group of ani-
mals used as a criterion. On the other hand, there are many corre-
lations found in the studies of the geographical ranges of all of these
groups of animals. The zoogeograpliical regions of Wallace and the

711

712

TEXTBOOK OF ZOOLOGY

general boundaries of these areas are given below. The boundaries
of the regions do not necessarily follow the outlines of continents as
will be seen in Fig. 373.

1. The Palaearctic Region includes all of Europe, Africa north of
the Sahara, and all of Asia north of the Himalaya Mountains.

2. The Nearctic Region embraces the North American continent
as far south as Mexico.

3. The Ethiopian Region consists of Africa south of the Sahara,
southern Arabia, and Madagascar.

Fig. 373. — Map showing the life regions of the earth.

A. J. Nystrom Company.)

(Base by permission ol

4. The Oriental Region includes Asia south of the Himalayas, and
that portion of the Malayan Archipelago which lies northwest of
Wallace's line. This famous line dividing the Oriental and Aus-
tralian Kegions passes east of the Philippines between the islands of
Bali and Lombok, and between Borneo and Celebes. Bali and Lom-
bok are separated by a very narrow but deep strait, and there is a
very remarkable contrast between the faunas of the two islands,

5. The Australian Region is made up of Australia and the neigh-
boring islands as far west as Wallace's line. It includes the great
number of small islands east of Australia.

ANIMAL DISTRIBUTION 713

6. The Neotropical Region consists of Mexico, Central America,
and South America.

The present distribution of animals on the earth gives us many
evidences concerning the past history of the earth. The North Ameri-
can fauna is notably different from the South American fauna. This
fact apparently indicates that North and South America were com-
pletely separated from each other by an extensive water barrier until
a relatively recent period. The great contrast between the faunas of
the islands of Bali and Lombok indicate that the Australian conti-
nent and its neighboring islands have been separated from the Asiatic
mainland for a relatively long period of time. The study of distribu-
tion also gives many clues to the ancient changes of climate on the
earth.

Bathymetric Distribution. — In discussing the vertical or altitudi-
nal distribution of animals two fundamental types of habitats must
be considered; namely, the water habitat or Hydroiios and the land
habitat or Geohios. The inhabitants of these two realms differ
greatly, but some few forms occupy both regions.

The hydrobios includes both fresh- and salt-water realms. The
variation in vertical distribution of fresh-water animals is relatively
insignificant when compared to that of marine animals. The seas
have been variously classified into vertical zones. The divisions most
commonly recognized are :

1. The Littoral which is the area between tide marks. It is charac-
terized on rocky shores by growths of barnacles, mussels, and snails.
Life in sandy littoral regions, such as that along the Texas Gulf
Coast, is relatively scarce and consists chiefly of annelid worms and
other boring forms.

2. The Sublitioral includes the subtidal region to a depth of ap-
proximately 100 meters. This is about the lowest depth at which
plants can grow abundantly. It is the most productive region on
the earth as far as variety and numbers of animals are concerned.

3. The Abyssal zone is marked by the absence of light and extends
to the greatest depths of the oceans. Life is not very abundant in
this zone. Plants cannot live at such depths, therefore, the animals
which live in the abyssal region are dependent upon other animals
as a source of food, or they subsist upon the floating plant life which
is continuously dying and sinking to lower levels.

714

TEXTBOOK OF ZOOLOGY

The vertical or altitudiual distributiou of terrestrial animals de-
pends primarily upon the vertical distribution of the vegetation.
The variation in the kinds of plants found at different altitudes de-
pends to a large extent upon the climatic belts ^vhich extend around
the earth between the equator and the poles. The vertical belts of
plants encountered in traveling from a lowland to a high mountain
top correspond very closely to the climatic zones surrounding the
poles of the earth. The communities of animals and plants found in
each vertical zone are similar throughout the world.

The seven vertical life zones and a few of the characteristic animals
and plants found in each zone, as it is represented in the Southwest,
are here given.

1. The Alpine zone (usually above 10,500 feet) is the area above
the timber line. This highest zone is represented in the Southwest

, LOweil SOMO....

Fig-. 374. — Diagrammatic sections of the Grand Canyon and San Francisco peak
siiowing the vertical life zones. (After N. N. Dodge and Merriam.)

on the top of San Francisco Mountain in Northern Arizona and in
the Sangre de Cristo range of the Rockies in Northern New Mexico.
This zone is characterized by the presence of few plants, such as
saxifrages and dwarf willows. Among the fauna are found the
golden eagle, some weasels, and mountain sheep.

2. The Hudsonian zone (9,000-10,500 feet) consists of a forest of
spruces and some firs, and harbors the dusky horned owl, bears,
shrews, and red squirrels. The Hudsonian zone is found in the same
region as the Alpine zone.

3. The Canadian zone (8,000-9,000 feet) is distinguished by the
presence of the Douglas fir and the aspens. Common ajiimals of this
zone are the three-toed woodpecker, one species of shrew, and two

ANIMAL DISTRIBUTION

715

species of field mice. This zone is found in the Rockies as far south
as southern New Mexico. There is some evidence of Canadian fauna
in the Chisos Mountains of Texas.

4. The Transition zone (7,000-8,000 feet) is found in the same
regions as the above zones. It is also found in the Davis and Chisos
and Guadalupe Mountains of Texas. It is covered with pine trees

■i HUDSON
CD. CAN

TRA

UPPC
LZD LOWEE

Fig. 375. — Map of the vertical life zones of Texas and adjacent areas.

Bailey.)

(After

(mainly Pinus ponderosa) . Abert's squirrel is found in the Transi-
tion zone as is one species of horned lizard.

5. The Upper Sonoran zone (6,000-7,000 feet) is the pinon belt
distinguished by the nut or piiion pine and cedars. The zone extends
from western Texas through New Mexico, Colorado, Arizona, and
into southern California. Characteristic animals are the pinon jay
bird, and the large rock squirrel.

716 TEXTBOOK OF ZOOLOGY

6. The Lower Sonoran zone is less homogeneous in its flora and
fauna than are the other zones. In the Southwest it includes most
of Texas, Oklahoma, southern New Mexico, southern Arizona, and
southern California. The eastern third of Texas is typical of the
lower Sonoran of the southeastern states which are characterized
mainly by long-leafed pines, magnolia, and live oaks. Some animals
of the eastern portion are mockingbirds, painted buntings, and
wood rats. The western lower Sonoran is inhabited by typical
desert and semidesert flora and fauna.

7. The Tropical zone is of very little significance in the south-
western United States, but many tropical plants, such as the Texas
palms and bananas grow in a narrow strip of the lower Rio Grande
Valley.

Migration of Animals

The migrations of many animals are not well understood but in
most cases they involve breeding habits, food, or shelter. Most of
these migrations are seasonal, but many permanent changes of loca-
tion have been made by groups of animals as a result of permanent
changes of climate on the earth.

The seasonal migrations of the North American caribou and the
bison are among the most noted examples of migrations of animals
which move about in search of food. The caribou migrate south-
ward in Canada during the winter and follow the melting snow
northward in the summer.

The remarkable migrations of the eels of the Mediterranean area
and of the eastern coast of the United States are among the best
known examples of breeding migrations. The eels from the rivers
of these two regions migrate into the Caribbean Sea and spawn.
Although the two spawning territories overlap, the young eels of
each species journey hundreds of miles back to the rivers inhabited
by their ancestors. A great many species of birds migrate from the
tropics and semitropics and breed in northern regions. The scarlet
tanager spends the winter in northwestern South America and mi-
grates across the Gulf of Mexico into northeastern United States
and southern Canada and breeds during the summer months. The
upland plover is familiar in the Southwest, since it migrates through
this region twice each year on its journeys between Argentina and
British Columbia.

ANIMAL DISTRIBUTION 717

Means of Dispersal and Barriers

The wide distribution of species of animals on the earth depends
largely upon their means of dispersal or means of being carried from
one place to another and upon the barriers which they encounter.
Among the members of a species as well as among the related and
nonrelated species of animals there is a continuous struggle for
existence. Those forms which, by some means, are able to enter new
environments where competition is less severe will have the better
chance at survival.

In practically all marine organisms there exists a means of loco-
motion during some stage of the life history. In a great number
of the forms, especially those which are sessile in the adult stage,
there is a free-swimming larval stage : planula larva of the coelen-
terates; trochophore larva of annelids; and the various free-swim-
ming larvae of mollusks, echinoderms, and crustaceans. The plank-
tonic larvae and adults, i.e., those which swim or float free of tlie
bottom, depend greatly upon oceanic currents for their wide dispersal.

The dispersal of oceanic animals is also greatly enhanced by the
large numbers of eggs and larvae produced by these forms. For
example, one investigator found that a "sea hare," a marine gas-
tropod, deposited 478 million eggs during one four-month spawning
period. The eggs were laid at the rate of 41,000 per minute.

Marine animals are limited in their distribution by such barriers
as temperature, land masses, and salinity of the water. The marine
animals on each side of the narrow Isthmus of Panama are entirely
different. The extreme changes in temperature prevent most of
these species going around the southern tip of South America. Lit-
toral animals are often limited in their distribution by large river
mouths which empty great quantities of fresh water into the oceans.
Only specially adapted forms can live in these brackish waters.

Fresh-water faunas are restricted in their distribution by land
barriers which usually separate the bodies of water in which they
live. These forms depend mainlj'- upon other animals, such as birds
and insects, for the dispersal of their eggs and dormant stages.
Many fresh-water animals, such as snails and clams, attach them-
selves to the bodies of birds or insects and are carried into new
habitats.

718 TEXTBOOK OF ZOOLOGY

Among land animals tlie birds and flying insects appear to be
least restricted in their ranges. Even these, however, are often
limited by mountains, deserts, and large rivers which act as the
most effective barriers to the wide distribution of land animals.
Many of the birds and mammals found on the south rim of the
Grand Canyon of the Colorado River are not able to reach the north
rim of the Canyon. For example, the Abert squirrel is found in
New Mexico and Arizona, south of the Canyon, while the Kaibab
squirrel occurs only north of the Canyon. Small islands, which are
located great distances from the mainland, are often inhabited by
small mammals and reptiles similar to those found on the mainland.
Such forms have probably reached the islands by way of floating
rafts of vegetation, and hollow trees which are known to be carried
several thousands of miles by oceanic currents.

Wind currents may carry birds across the Atlantic Ocean. It is
not uncommon for American birds to land on the coast of England
after severe storms. These same currents also carry spores of Pro-
tozoa, small insects, and the eggs of many invertebrates.

Effects of Man Upon Distribution

The advent of man upon the earth ai^d the development of his
more efficient means of travel have greatly enhanced the wide dis-
tribution of many species of animals. Marine invertebrates attach
themselves to the bottoms of ships (ship fouling) and are carried
to practically all parts of the earth. Rodents and insects are acci-
dentally distributed from one country to another by ships. During
the past man has purposely transported animals from one country
to another. In some instances the animals multiply more rapidly
in the new environment than they did in the original habitat.
English sparrows were introduced into North America in 1859, and
since that time they have become so numerous as to be a great pest
in this country. The starling, which was introduced at about the
same time is rapidly increasing in numbers and in recent years has
been reported as far south and west as Central Texas. By cultivat-
ing extensive prairie lands, man has created insurmountable bar-
riers to grazing animals which once roamed these areas. Deforesta-
tion of wooded regions has destroyed the shelter necessary for such
animals as deer, foxes, wolves, bears, and many others.

CHAPTER XXXVii
THE ANIMAL AND ITS ENVIRONMENT

(By a. 0. Weese, University of Oklahoma)

Ecology is that division of biology which has to do with the rela-
tions between organisms and their environment. The environment
of an organism, for convenience, may be divided into two parts, the
nonliving, including physical, chemical, and climatic factors, and
the living, including other organisms of the same and of different
kinds. The science of ecology, in a sense, is as old as man, because
from the very beginning of his conscious existence it was necessary
that man take cognizance of the fact that his environment was made
up in part of plants and animals and that these organisms in turn
had relations to their environment. It was not until comparatively
recently, however, that ecology came to be recognized as a separate
department of biology. Modern ecology may be said to have begun
with the recognition of the community. Plants and animals are
distributed as they are over the surface of the earth, not because
of any chance coincidence, but because of a combination of circum-
stances, one of which is the fitness of the physical environment for
the proper completion of their life histories and another of which
is the presence of such other organisms as are necessary to furnish
food and to provide other requisite conditions. We can think of a
community of organisms in much the same way as we think of a
community of human beings. The analogy cannot be followed too
closely, for, after all, human beings are much alike while the organ-
isms in an ecological community are of many different kinds, having
different requirements in detail as to food and environmental con-
ditions. "We shall arrive at a better concept if we think of a human
community as made up, not only of butchers and bakers and candle-
stick makers, each with their particular functions as producers and
as consumers, but also of the domestic animals which furnish mate-
rials for food or clothing or which perform labor, the household
pets and pests, the cultivated plants which are utilized in the manu-
facture of food, clothing or shelter, and the host of wild animals
and plants wliich enter into some relation Avith those previously

719

720 TEXTBOOK OF ZOOLOGY

mentioned. The characteristics of such communities vary from time
to time and from place to place. A difference in climate may be
sufficient to change almost every component of the community. The
domestic animals and plants associated with man in the tropics are
quite different from those in arctic regions. Perhaps one species,
the dog, almost as adaptable as man himself, might be considered
as a member of both communities. The removal of a single species
or the addition of a new one may alter profoundly the aspect of
the community. Consider, for example, a human society from which
all cows were removed, or the changes made in the life of certain
sections of the United States with the introduction of the cotton
boll weevil.

Some idea of the complexity of the relationships involved in a
community of organisms may be gained by citing Charles Darwin's
example of the dependence of clover on cats, or Thomas Huxley's
extension of the chain of cause and effect to the responsibility of
the old maids of England for the supremacy of that nation on the
seas. Obviously, since old maids are fond of cats, the number of
the latter is greater when old maids are numerous. Cats eat field
mice, which in turn prey upon the nests of bumblebees. Thus, a
large cat population is favorable for the development of clover
which is fertilized by the bumblebee. Clover is fed to cattle and
it is well known that Britain's sea power is due to "The roast beef
of old England."

Most of the relationships suggested in this series have to do with
food. It is often possible to gain a better idea of relations within
a community by the use of a diagram indicating the more obvious
influences. The accompanying figure, modified from Shelford (Fig.
376), represents food relationships only in a hypothetical prairie
community and may be called a ''food chain diagram." Many
very obvious food chains are omitted in order to avoid a complexity
too great for easy reading, and, of course, any real community
would include many more kinds of animals. The chart is very in-
complete, also, in that no relations other than those directly con-
cerned with food are indicated.

Life exists on the earth only in a relatively very limited space.
We might represent the relationship of the various parts of the
planet on which we live by a series of concentric circles, the area
within the inner circle corresponding to the "solid" portion of the

THE ANIMAL AND ITS ENVIRONMENT

721

earth, the lithosphere. If the lithosphere were uniform in diameter,
it would be covered by a layer of water, the hydrosphere, which is
in turn surrounded by a gaseous layer, the atmosphere. It is only

Fig. 376. — Diagram to show food relationships in a hypotlietical prairie com-
munity. (Redrawn and modifled from Shelford. Animal Communities in Temperate
America, published by University of Chicago Press.)

--Lithosphere

■-Hydposphere

■-Atmosphere

Fig. 377. — Diagram to show the relationship of the general areas of our planet.

where the components of these three layers interact that life is pos-
sible. Living organisms cannot exist in the lithosphere (Fig. 377)
without the presence also of water, and of oxygen from the atmosphere.
Similarly life in water is possible only where it contains in solution

722 TEXTBOOK OF ZOOLOGY

both solids and oxygen. Air is habitable only temporarily and animals
spending time there must return frequently to water or soil. Free
interaction between the three components of the earth is made pos-
sible by the fact that the lithosphere is not of exactly uniform
diameter so that certain portions project above the hydrosphere,
forming continents (and islands). These elevated areas constitute
a rather small proportion of the whole surface of the earth, but it
is with these areas that we shall be concerned, not because there
is no life in the submerged areas, but because the space available
in this chapter is too small for us to consider the great subject of
marine ecology.

Fig. 378. — Distribution of "precipitation effectivity" on an ideal continent
(Modified after Thornthwaite. Drawn by Edward O'Malley.)

The inclination of the earth's axis of rotation to the plane of the
earth's orbit about the sun and the relative positions of the conti-
nents are factors which influence the distribution of climates. The
two principal climatic factors are, of course, temperature and
moisture. If the effect of altitude is not considered, the former
is distributed rather uniformly, so that the familiar torrid, tem-

THE ANIMAL AND ITS ENVIRONMENT

723

perate, aiid frigid zones express rather well the comparative tem-
perature conditions on a continent. Many factors, however, operate
in the control of the amount of precipitation and its distribution.
In general, the distribution of available rainfall on an "ideal" top-
shaped continent without mountain ranges might be expected to
follow the plan of Fig. 378 in which the darker areas indicate
maximum rainfall and the unshaded areas represent very arid re-
gions. Each climatic type makes possible the occupancy of the
region concerned by a definite type of biotic community which can

(m Peppstual snouj or ice
[m Tundra
HlD Evergreen hrest
IM] Deciduous forest

Em Grassland

Scrub (sagebrush)
M] Deserf

Tropical forest <l savanna

Fig. 379. — Hypothetical distribution of biotic communities on an "ideal continent.

be most easily designated by the character of the conspicuous or
dominant vegetation. Taking into consideration the seasonal dis-
tribution of available moisture and the annual variations in tem-
perature, the "ideal continent" might be expected to present an
aspect something like that represented in Fig. 379. It will be seen
at once that in the regions of deficient temperature heat is the de-
termining factor, and that in warmer regions the amount of moisture
is the major influence. How closely this expectation is realized in

724 TEXTBOOK OF ZOOLOGY

the case of North America may be seen by a comparison with Fig.
380. As will be seen later, the animals of a climatic region are
as distinctive as the plants. The soil, which is the result of inter-
action between the climate, the plants and animals, and the rock
substratum, also is characteristic of each climate. Thus, in the
area influenced by each climatic type, there is a definite biotic com-
munity or formation, characterized by plants and animals whose
relations to their environment are similar or equivalent.

The Frincipal Biotic Formations

A rough classification of the climatic formations of North America
includes the following:

1. The Tundra Formation

2. The Coniferous (Evergreen) Forest Formation

3. The Deciduous Forest Formation

4. The Grassland Formation

5. The Sagebrush Formation

6. The Desert Formation

In addition there are transitional communities diflBcult of repre-
sentation on a map because of their discontinuous distribution or
small area. Some of these may be referred to, provisionally, as
Woodland, Desert Scrub, Chaparral, and Swamp communities.

1. The Tundra Formation (Sedge-Musk Ox Biome). — This is the
community of the arctic "barren grounds," beyond the northern
limit of trees and between it and the polar region of perpetual snow
and ice. Included, also, is the region above timber line on high
mountains, where climatic conditions are similar to those of the
far north. The vegetation of the tundra is composed mainly of
grasses, sedges, and lichens (e.g., reijideer moss), although there
are also patches of dwarf willows and other woody plants from a
fraction of an inch to a few feet in height. Typical animals of the
Arctic tundra are the musk ox and the arctic caribou which feed
on the low vegetation. Here, also, are found great hosts of rodents,
including lemmings, whose enormous increases in numbers and
periodic migrations in Europe have been known for many hundreds
of years. Arctic hares and arctic ptarmigan, with those animals
previously mentioned, furnish food for the arctic fox and the arctic

THE ANIMAL AND ITS ENVIRONMENT

725

wolf. Many of these animals, including the arctic fox, the arctic
hare, the ptarmigan, and the collared lemming, are adapted to the
climatic rhythm of their environment by changing in color from
white in winter, when the entire landscape is snow-covered, to a
darker color during the summer. On the tundra nest vast numbers
of migratory birds, some of which, like the golden plover, fly to
winter homes in South America.

Fig. 380. — Distribution of major bio tic regions in Nortii America.

2. The Coniferous Forest Formation (Spruce-Moose Biome). —

South of the Arctic tundra and below the high mountain tundra
along the Rockies and the Sierras is the great coniferous forest
whose dominant vegetation is evergreen, composed chiefly of spruces,
firs, and pines of many species. The moose and the woodland caribou
are characteristic animals over the greater part of the area. They
feed upon grass and shrubs, chiefly in open areas near streams or
lakes. The principal carnivore is the timber wolf, The Canada

726 TEXTBOOK OF ZOOLOGY

lynx is also a creature of the evergreen forest. Black bears are
numerous but are widespread, also, beyond the borders of this for-
mation. The wolverine and the red fox are lesser carnivores. The
varying hare is similar to the arctic hare but lives in more protected
situations.

3. The Deciduous Forest Formation (Oak-Deer Biome). — Most of
the United States lying east of the Mississippi River is characterized
by deciduous forest. In this area the dominant vegetation consists
of trees whose leaves are shed during the unfavorable winter sea-
son, such as oaks, beech, maple, hickory, and elm. The most wide-
spread large herbivorous animal is the Virginia deer, although the
wapiti (elk) was formerly of frequent occurrence. The wolf and
the cougar were once widely distributed but are almost extinct now.
The bay lynx, a close relative of the Canada lynx, preys upon the
numerous rodents and birds. Other characteristic animals are the
opossum, the raccoon, the flying squirrel, and the woodchuck. Many
of these inhabit open areas in the woods rather than the dense forest.

4. The Grassland Formation (Grass-Bison Biome). — No single
grass is included in the "common" name of this community because
it is probable that the name of no single genus of grass is sufficiently
familiar. Important grasses in the great central grassland area are
the bluestems (or beard grasses), June grass, porcupine grass, grama
grass, and buffalo grass. The grassland region was the habitat of
the bison which once ranged over the prairies from Texas to Sas-
katchewan in enormous numbers. It is estimated that at the com-
ing of the Avhite man there were over 75 million of these animals,
upon which the plains Indians depended for food, clothing, and
shelter. The American pronghorn or antelope was also abundant.
Both were held in check by the gray wolf. The black-tailed jack
rabbit is an animal of the open grassland while the cottontail oc-
cupies wooded or at least bushy areas. In the drier western portion
of the grassland the prairie dog and the badger (Fig. 341) are com-
mon. There are many small rodents, including the pocket gopher,
the ground squirrels, and prairie deer mice.

5. The Sagebrush Formation (Sagebrush- Jack Rabbit Biome). —
This community is centered in the Great Basin of Nevada, Utah,
and neighboring states. It is an area of deficient rainfall, and
sagebrush is a prominent constituent of the vegetation. Most of

THE ANIMAL AND ITS ENVIRONMENT 727

the larger auimals belong to groups which are also found in neigh-
boring formations, although the smaller mammals are mostly of
different subspecies. Rabbits and rodents are especially abundant.

6. The Desert Formation (Creosote Bush-Kangaroo Rat Biome).
— It is difficult to characterize this community by the name of a
dominant plant and an important mammal, as there are many types
of desert associations. They have, however, one characteristic in
common — a great deficiency in available water. Plants must guard
against water loss and conserve water between the infrequent moist
periods. Some of the adaptations which meet such conditions are :
reduction of leaves (thorns), fleshy stems, thick cuticle, extensive
underground organs, etc. Animals conserve water by remaining
underground or in sheltered places during the hotter parts of the
day and coming out only at night or at dusk. Reptiles and birds
are able to reduce water loss by the absence of sweat glands and
by giving off their nitrogenous excretion in the form of uric acid
which does not need to be dissolved in water.

Adaptation

From what has been said above, it appears that each particular
set of climatic and other environmental conditions accommodates
a particular group of plants and animals. The activities and struc-
ture of these animals are such as enable them to survive best under
the conditions in which they are found. The barren ground caribou
is not fitted for life on the central grassland area nor is the bison
able to survive on the tundra. Each is said to be adapted to the
particular conditions under which it exists. Many so-called adap-
tations, however, seem to be worthless. One might think, for ex-
ample, that the shovellike brow tines of the caribou would be a
remarkable adaptation for shovelling snow from the vegetation on
which the animal must feed during the winter — but the antlers are
shed in the late autumn and do not grow out again until the fol-
lowing spring. One must be careful not to assign adaptive func-
tions to animal structures without careful consideration of the
habits and life histories of the animals concerned.

Succession

It must not be thought that, within the areas of the great climatic
formations, there is entire uniformity in the communities of plants

728 TEXTBOOK OF ZOOLOGY

and animals. This is far from true. There exist minor differences
of climate, soil, and physiographic conditions and of biological his-
tory which result in differences in plant and animal population.
There are always areas which, for example, have been denuded by
physiographic or other processes. Fire, flood, and human utilization
are among the more common influences which may cause partial or
total destruction of the biotic communities of a region. "When
denudation occurs, a region does not long remain unpopulated, but
the slow process of redevelopment of a community characteristic
of the existing climate begins as soon as the destructive force has
ceased its action. This process of development is called succession
because it is characterized by the appearance, first, of pioneers,
which give way to other groups of organisms which, in turn, alter
conditions in such a way that still others find conditions favorable
for their existence. The final result, after a long time, is the de-
velopment of a community which is in equilibrium with its en-
vironment and will not change unless the environment changes.
Such a community is called a climax community. An analogy might
be sought, again, in human experience. The human pioneers who
first penetrated into the broad plains of the "old west" conquered
the wilderness to such an extent that some who required more of
the comforts of civilization were able to find suitable homes. Many
of the original pioneers, however, moved on to new frontiers when
the land became too thickly settled. The "climax human commu-
nity" has not yet been attained, as man is forever seeking more
perfect adjustment to his environment.

It is often possible to study the stages in the development of a
community without waiting for the entire process to take place in
one locality. For example, the broad flood plains of many of our
rivers are subject to frequent inundations. Such inundations may
remove all living things from their paths, but since the flood plains
are so broad and the course of the river is never the same from year
to year, the same tracts are not covered each spring, and areas may
be found which have been undisturbed for a few weeks, a few
months, a year, two years, five years, fifty years, etc. In the flood
plain of the Canadian River (in Oklahoma) mud flats from which
the water has receded recently are soon occupied by an assemblage
of organisms including blue-green algae, two small beetles, and a

THE ANIMAL AND ITS ENVIRONMENT 729

Succession on the Canadian River Flood Plain. (Data From Hefley.)

STAGE IN DEVELOPMENT 1 A 6 ^ O o

Paralimna appendiculatum x*

Heteroeerus pallidas x

Beetle (Algae and detritus)

Bembidion laevigatum x x

Beetle (Algae and detritus)

Cicindela hirticollis x x x

Tiger Beetle (small insects)

Cicindela cupraseens x x x

Tiger Beetle (small insects)

Cicindela punctulata x

Tiger Beetle (small insects)

Mutillidae x

Velvet ants (insects and spiders)

Apion pennsylvanicum x

Weevil (cocklebur)

Haltica bimarginata x x x

Beetle (willow)

Phalacrus politus x

Beetle (willow)

Stictocephala lutea x

Tree hopper (willow, cottonwood)

Cicadella gothica x

Leaf hopper (willow)

Dorytomus squamosus x

Weevil (willow)

Strongylocomis stygicus x

Bug (coral berry)

Epitrix brevis x

Flea Beetle (Miscellaneous plants)

Brief descriptions of stages:

1. Mud flat with blue-gi-een algae, later liverworts.

2. Sedges, willow and cottonwood seedlings in addition to 1.

3. Second level. Sand, sedges, willows, cottonwoods, cockleburs.

4. Third level. Grasses, willows, cottonwoods.

5. Third level. Cottonwoods, fewer willows.

6. Fourth level. Elm-oak forest with shrub undergrowth.

*x indicates the presence of the species as an important member of the com-
munity.

fly. Tiger beetle larvae come a little later, and then seedlings of
willow and cottonwood. During the course of a year or so blowing
sand is deposited so that the level of the ground is raised several
feet, willows and cottonwoods increase in size, and other plants

730 TEXTBOOK OF ZOOLOGY

are added. Soil and sand continue to be deposited, the trees in-
crease in size, and the ground becomes more thickly covered by
vegetation. Finally, after a hundred years or so a still higher level
is attained and the willows and cottonwoods give way to oaks and
elms. The accompanying table indicates some of the insects to be
found in each stage of this developmental series. After the name
of each insect is given, in parentheses, its principal food.

Animal Populations

A great deal can be learned about the relations of organisms by
quantitative methods making possible an estimate of the numbers
of various species present in a given community at any one time.
Comparisons may then be made between the populations of com-
munities differing in some observable respect, or between the popu-
lation of the same community at different times of the year. It is
comparatively easy, although somewhat tedious, to determine the
relative numbers of various plants in a given region by blocking
off sample areas and counting the plants. Most animals, however,
will not remain stationary while a census is being taken, and vari-
ous less exact methods of obtaining population data must be em-
ployed. Some of these methods may be suggested. The number of
birds may be estimated by the number seen within a given area
or during a certain time, or a census of nesting pairs may be taken
by counting nests. Larger mammals may be counted by experienced
observers, burrows of rodents may be counted (but it is also neces-
sary to determine the average number occupying a burrow) or the
relative frequency of tracks, fecal pellets, or other evidences of the
presence of the animals may give an idea of their numbers. The
relative number of rodents is often estimated by the frequency with
which they are caught in traps. The most frequently used method
of estimating the number of insects is the use of the insect net.
A net is swept through vegetation a definite number of times and
all insects caught are counted. While all such methods are neces-
sarily inaccurate, a great deal of valuable information may be
obtained.

A list of the ten most abundant groups of insects found in two
adjacent areas, one heavily overgrazed and the other lightly grazed,
is given in the accompanying table. The figures indicate their relative

THE ANIMAL AND ITS ENVIRONMENT

731

:tio-

y*1a

ar-fy Opr-in^ Oprins

•Summer

Ja/y

Aug

Aufumn

A^oy

£>e<:

Jan F^b

Fig. 381. — Numbers of insects collected in average catch with 100 sweeps of an
insect sweep net in a prairie ravine (Oklahoma) arranged according to orders
through the seasons of the year. (Data from Carpenter.)

732

TEXTBOOK OP ZOOLOGY

Fig. 382. — Total numbers of insects collected in average catch with 100 sweeps
of insect net in prairie ravine (Oklahoma). (Data from Carpenter.) The scale
for this graph is one-fourth of that in the preceding figure.

O
■«

Jan. Fih. Alar Apr /^aj/ June

JuU'

Aaq

Sepf-. Oci-

Nov

Dec.

Fig. 383. — Fluctuations in populations of four Protozoa in an artificial lake

(Oklahoma). (Data from Bragg.)

THE ANIMAL AND ITS ENVIRONMENT 733

Relative Numbers of Ten Most Abundant Insects From Overgrazed and
Normal Grassland. (Wichita Mountains Wildlife Refuge.)

GENUS

common name

normal

OVERGRAZED

Melanoplus

(grasshopper)

11.0

220.0

Campylenchia

(tree hopper)

0.5

144.0

Mermiria

(grasshopper)

14.0

138.0

Scolops

(plant hopper)

10.0

120.0

Elleschus

(weevil)

0.9

80.0

Poeciloscytus

(leaf bug)

1.2

54.0

Agallia

(leaf hopper)

8.0

38.0

Harmostes

(plant bug)

8.0

17.0

Deltocephalus

(leaf hopper)

0.5

3.4

Brucliomorpha

(plant hopper)

11.0

0.4

abundance in ten collections of fifty sweeps of the insect net, each.
These collections were taken over a period of a month and indicate
a very real difference in the insect populations of the two areas.

Seasonal Changes

It is a matter of common observation that the animals observed
in any one place vary greatly from season to season during the
year. This is not only true of those animals which, like many birds,
migrate southward on the approach of winter and return to north-
ern climates for nesting. Some animals hibernate or aestivate, and
others spend a part of the year in an inactive stage, such as the egg,
or the pupa. The life cycle of an organism must be adjusted to the
annual climatic cycle of the climate in which it lives. Some idea
of the variations in the number of insects during the year may be
gained from a study of the accompanying charts (Figs. 381 and 382)
giving the average catch with one hundred sweeps of an insect net
at different seasons.

A study of the abundance of Protozoa in an artificial lake shows
a similar difference in the time of abundance of the various species.

Summary

This chapter has considered very briefly the distribution of the
biotic communities of North America in relation to climate. The
phenomena of succession and seasonal fluctuation of populations
have been discussed, with examples. Attention has been directed
toward the community rather than toAvard the individual organism

734 TEXTBOOK OF ZOOLOGY

or the species. Similarly the sum total of physical environment as
expressed in climate has been stressed rather than single factors,
such as moisture, temperature, light, etc. The animal in nature is
subject always to the action of a complex environment and its dis-
tribution and reactions are the result of its response to the whole.
The student is referred to the chapter on Animal Behavior for a
discussion of the response of the individual organism to modifica-
tions in single environmental factors.

CHAPTER XXXVIII

ANIMAL PARASITISM

(By Sewell H. Hopkins, Texas A. and M. College)

SOCIAL RELATIONS OF ANIMALS

It has been explained in the previous chapter that no animal is
ever entirely independent of others, since all plants and animals
are influenced, directly or indirectly, by all the other organisms in
the community. Most animals, however, can and do catch and eat
their own food, and such animals are said to be "free-living."
But there are thousands of species which depend either completely
or partially on others to provide them with a livelihood. The vary-
ing degrees of dependence are called commensalism, mutualism, and
parasitism. The term symbiosis is applied to all cases of two different
kinds of animals living together, and thus includes commensalism,
mutualism and parasitism.

In commensalism, one animal receives all of the benefit from the
association while the other is neither benefited nor harmed. The
jackal which follows the tiger and cleans up the carcass of the prey
when the tiger has eaten his fill, the small fishes which accompany
sharks and feed on the scraps wasted by the shark in feeding, and
the oyster crab Avhich lives inside the oyster's shell and feeds on
the organisms brought in by the oyster's feeding movements, are
examples of commensalism.

Mutualism is the kind of symbiosis in which both animals receive
benefit from their association. One species of hydra (Hydra viridis)
is green in color because a certain species of green alga lives within
its cells ; the alga receives protection and some nourishment from
its host, while the hydra benefits from the food manufactured by
the green plant. A case of mutualism so far developed that the
two animals cannot live separately is the relationship between
wood-eating termites and their intestinal protozoa. The termite
cannot digest the wood which it eats; the protozoa in the termite's
intestine break down the wood into a form in which it can be used

735

736 TEXTBOOK OF ZOOLOGY

by the host; on the other hand, the protozoa are absolutely de-
pendent on the termite for food and the proper environment;
neither termite nor protozoan can live without the other partner.

The word parasitism in its broad sense applies to all cases in which
one animal depends on another to furnish it with food; for instance,
ornithologists call cowbirds parasites because they lay their eggs in
the nests of other birds and leave the foster-parents to feed and
care for the young cowbirds. Most zoologists, however, use the
word parasitism only for cases in which the parasite lives in or on
the body of its host; for exam^Dle lice live on the bodies of many
animals, and tapeworms live in them.

Origin of Parasitism

How did it happen that some animals became dependent on others
to furnish their food, that is, how did parasitism arise? There is
a considerable amount of evidence for the belief that all parasites
are descendants of free-living ancestors, and that these descendants,
in the course of generations, gradually became more and more de-
pendent on certain hosts, until in some cases they are now abso-
lutely unable to make their own living. For example, certain species
of nematodes which are free-living inhabitants of the mud at the
bottom of ponds and streams are able to live in the large intestine
of a frog if they happen to be swallowed by a frog. Other species,
very similar to the mud-dwelling nematodes, have found the intes-
tines of frogs such a good habitat that they live nowhere else; in
other words, they have become parasites. Some intestinal parasites,
in the course of many generations, have lost their locomotor struc-
tures or even their digestive organs and yet continue to thrive
because there is little or no need for locomotion or digestion when
all food is brought to the parasite already digested by the host's
intestine. Since such degenerate parasites are unable to secure food
elsewhere, they are condemned by their peculiar structure to live
as parasites in the intestine of their host.

Degrees of Parasitism

Free-living animals which sometimes become parasites when they
get into another animal (by swallowing, for instance) are called
accidental or occasional parasites, as in the case of the mud-dwelling

ANIMAL PARASITISM 737

nematodes mentioned above. "Vinegar eels," nematodes in vinegar,
sometimes establish themselves as harmless parasites in the human
urinary bladder. Facultative parasites are able to live almost equally
well as free-living animals or as parasites; many leeches are faculta-
tive parasites. Obligate parasites, on the other hand, cannot live
without the host. Parasites which are free-living during part of the
life cycle, as in the case of the horsehair worms and some ticks and
mites, are called temporary parasites, while animals like Acantho-
cephala and tapeworms which are parasitic during the entire life
cycle are called permanent parasites.

The Successful Parasite

Like all other ways of living, successful existence as a parasite
requires certain modifications or adaptations in structure and func-
tion. Parasites which live on the outside of the host's body are
called ectoparasites; they must have special organs for attachment
in order to maintain their hold on the host; for example, lice have
hooklike feet with which they hold on to the skin, hair, or feathers
of the host, and ectoparasitic trematodes have either muscular
suckers or chitinous hooks for attachment to the outside skin or to
the gills of the fishes on which they live. On the other hand, ecto-
parasitic insects have no need for wings, so fleas and bedbugs con-
tinue to thrive without them. Many ectoparasites, such as fleas,
lice, bedbugs, mites, and ticks, also have specially constructed
mouth parts for piercing their host 's skin and sucking blood. Endo-
parasites, which live inside their hosts, also require special adapta-
tions. For maintaining their positions in the intestine or other
organs they must have some sort of attachment organ, such as the
muscular suckers of trematodes and tapeworms and the hooks of
thorny-headed worms. On the other hand, they live in the dark
so eyes may be entirely lacking without inconveniencing the endo-
parasite; usually all sense organs are either absent or very poorly
developed. There is little or no need for rapid locomotion, so most
endoparasites have locomotor structures much reduced or even en-
tirely lacking. Many endoparasites also have less of a digestive
system than their free-living relatives ; parasites in the liver, lungs,
blood vessels, etc., usually have some sort of digestive apparatus,
but many intestinal parasites, such as tapeworms and thorny-headed
worms, have no sign of digestive organs whatever, but depend on

738

TEXTBOOK OF ZOOLOGY

the host to furnish them with food already digested and ready for
absorption. On the other hand, most endoparasites have their re-
productive organs enormously developed, sometimes so much so
that 90 per cent of the body is taken up by the reproductive sj^stem.
This is in keeping with the general rule that animals whose offspring

//,

Fig. 384. — Arthropod parasites. A, human itch mite, female, Sarcoptes scabiei,
ventral surface; B, ventral surface of male itch mite; C, body louse (cootie),
Pediculus humanus corporis; D, head louse, P, humanus capitis; E, crab louse.
Phthirius pubis. (From Sutton, Diseases of the Skin, published by The C. V.
Mosby Company.)

have the least chance to survive usually produce the largest number
of offspring. In the ease of a tapeworm, for instance, the chance
of any one egg being eaten by the right kind of host, so that it can
develop into another tapeworm, is only one in a million, and tape-

ANIMAL PARASITISM

739

worms would long ago have become extinct except for the fact that
each tapeworm produces manj^ millions of eggs. The peculiar
habitat and mode of life of endoparasites also makes necessary
peculiar adaptations in the functions or physiology of the parasite,
A parasite in the intestine, for instance, must be able to carry on
respiration in almost complete absence of oxygen, must secrete
substajices to counteract the digestive juices of the host in order
to prevent its being digested, must be adapted to a high concen-
tration of salts, acids, and other substances in solution in the fluid
around it, and if in a warm-blooded animal must be able to live
at a relatively high constant temperature. The fact that no host
is immortal makes it necessary for a parasite to have some special
provision for its offspring to escape to another host, in order to
maintain the existence of the species ; this necessity is met by various
peculiar adaptations in the life cycle or development, such as the
complex succession of larval stages in the endoparasitic trematodes
and cestodes.

Fig. 385. — Diagram of the tunnel of an itcli mite in human skin. The female
animal is depositing eggs. (Reprinted by permission from Introduction to Human
farasitologii by Chandler, published by John Wiley and Sons, Inc. Adapted from
Riley and Johannsen.)

Some parasites are able to carry on parasitic activities without
injuring their hosts, Avhile others may weaken or destroy the host.
Parasites which injure their hosts are said to be pathogenic (disease-
producing), while those which cause no appreciable injury are said
to be nonpathogc7iic or commensal. Since most parasites cannot live
without their hosts, a parasite which shortens the life of its host
destroys its own home and means of livelihood ; nonpathogenic para-
sites are more likely to be successful in the long run, and are there-

740 TEXTBOOK OF ZOOLOGY

fore more abundant. Some parasitologists consider pathogenic
parasites to be imperfect parasites because they are not quite per-
fectly adapted for successful parasitic life, while nonpathogenic
species are considered perfect parasites. However, no hard and fast
line can be drawn between the two. Many parasites which are so
perfectly adapted to their customary host that they produce no ill-
effects have been found to be strongly pathogenic to other hosts
where the adaptation is less perfect; for example, certain trypano-
somes which are harmless to the antelopes of Africa, their natural
hosts, produce the highly fatal African sleeping sickness when in-
jected into men.

Means of Infection and Transmission

Many different means of transfer from host to host have been
developed by the various kinds of parasites. These may be classified
as below:

A. Passive transmission.

1. In food or in water.

2. By bite of insects.

3. By sexual intercourse.

4. By direct contact.

B. Active invasion under own power.

By ** passive transmission" is meant the transfer of eggs or larvae
from one host to another without any action of their own. For
example, the eggs of cestodes and of some nematodes, such as
Ascaris, pass out of the host's intestine in the feces; if food of
other animals is contaminated by these feces, animals which eat
this food will swallow the eggs, which hatch into larval worms
within the digestive system of the second host and thus establish
a new infection. Sheep may become infected with liver flukes by
eating the encysted larvae on grass or swalloAving encysted larvae
while drinking water. Trichina larvae encysted in hog meat de-
velop into adult worms in the human, intestines if infected pork is
eaten raw or improperly cooked.

The second means of passive transmission is also widely used.
Malaria parasites, the trypanosomes which cause African sleeping sick-
ness, and many parasites of domestic and wild animals are carried

ANIMAL PARASITISM 741

from infected individuals to new hosts by biting insects wliicli suck
up the parasites with the blood and inject it into the new host with
their salivary secretions.

The third means is used by only a few parasites, most of them
Protozoa. The spirochete which causes syphilis, a flagellate pro-
tozoan called TricJiomonas vaginalis, and a trj'panosome parasitic
in the reproductive organs of horses are examples. Some nematode
parasites of insects are also transmitted in this way.

The fourth means includes a few cases, such as the acquiring of
dog tapeworm by people who kiss dogs and the transmission of
pinworm from the hands of infected people who do not have clean
habits.

Hookworms and the human blood flukes, called schistosomes, are
examples of parasites that invade new hosts under their own power.
Their larvae are able to penetrate the skin.

Parasitism and Host Specificity

Since the beginning of the scientific study of parasitism, it has
been recognized that different animals have different parasites ; for
instance, the parasites found in and on goats are nearly all different
from those of man. Some of the early parasitologists leaped to the
conclusion that each species had its own peculiar species of parasites
found nowhere else, and carried this idea of species specificity so
far that they considered presence in different hosts to be sufficient
evidence of specific difference of the parasites.

Modern knowledge reveals that while some parasites are actually
species specific others have a wide range of hosts. Thus the beef
tapeworm. Taenia saginata, is found in the adult stage in man only,
but the fish tapeworm, Diphyllohothrium latum, seems to.be able to
live in nearly all mammals which eat fish.

Three main factors determine whether a parasite will infect any
given host: (1) opportunity for infection of host, determined by
habits or mode of life of parasite and host (malaria parasites may
be injected into any land animal by bite of mosquito, but strictly
aquatic animals, such as fish, would not be bitten) ; (2) the environ-
mental condition of the habitat furnished by the body of the host,
involving such factors as body temperature, nature of outside sur-

742

TEXTBOOK OF ZOOLOGY

face, size, chemical content of internal organs, etc. (intestinal para-
sites of birds are seldom found in mammals, which have lower body
temperatures, and parasites adapted to the oxygen-rich interior of
a frog's lung can find no suitable habitat in a lungless fish) ; and
(3) ability of the parasite to adapt itself to the wide range of en-
vironmental conditions found in different hosts; thus D. latum,
though it finds its optimum conditions in the humaji intestine, is
adaptable enough to survive under the very different chemical con-
ditions found in the dog, while Taenia saginata is usually unable to
survive under these conditions.

PARASITES AND THE GROUPS IN THE ANIMAL KINGDOM

What kinds of animals are infested by parasites? Not only all
phyla and classes, but all known species and probably all individuals
of higher forms serve as hosts for some kind of parasite. Even in

Fig. 386. — Giardia lamblia, an intestinal flag-ellate. A, face view; B, semlprofile
view; G, cyst. (Reprinted by permission from Introduction to Hmnan Parasitology
by Chandler, published by John W^iley and Sons, Inc.)

the microscopic Protozoa many individuals harbor still smaller
protozoans. For example, several species of parasitic Protozoa are
found in Amoeba proteus.

To which of the main groups of animals do parasites belong ? All
animal phyla, except Echinodermata, include some species which

ANIMAL PARASITISM

743

live as parasites, but the great majority of parasites belong to one
of these four phyla : Protozoa, Platyhelminthes, Nemathelminthes,
and Arthropoda.

To cerebrospinal fluid cau&'ing steeping
sickness and dealh.

Transmission by
bite of tsetse fjy.

^ Man, Antelope, etc.

Xrypanoaomes

in human blood causing
Trypanosome jever-

Transmission by bite
of tsetse fly.

Tsetse Fly

n salivary glands
'or re- infection.
-•' 30"- day)

Critbidial ^orms in.
salivary glands
(2, or »3 days later)

Forms in miclgut,(V6/
after infective meal).

(lewly arrived form in
.salivary gland.
(I2«'lto,20"'days.)

Long slender forms In pnoventriculus.
^about IO*''tol5*''dcry3)

Fig. 387. — Life history of Trypanosoma gamhicnse. (Reprinted by permission
from Introduction to Human Parasitology by Chandler, published by John Wiley
and Sons, Inc.)

,/J

;■!

|r.

Protozoa. — Of the four classes in this phylum, one, Sporozoa, is
entirely parasitic; the other three (Sarcodina, Mastigophora, In-
fusoria) also contain a number of parasitic forms. Examples of
parasitic Sarcodina are the three common human amoebae, Endor-

744

TEXTBOOK OF ZOOLOGY

moeha histolytica (Fig. 391), which invades and destroys the intestinal
lining, thns causing amoeljic dysentery; Endamoeba coli, a harmless

Fig. 388. — Tsetse fly, Glossina, the transmitting- agent for trypanosoma, which
causes African sleeping sickness. (Reprinted by permission from Introduction to
Human Parasitology by Chandler, published by John Wiley and Sons, Inc.)

Fig. 389. — Balantidmm coli, an infusorian parasite of the intestine. Active
form from intestine. c.iK, anterior contractile vacuole; cyt., cytostome ; f.v. food
vacuole ; n, nucleus. (Reprinted by permission from Introduction to Human
Parasitology by Chandler, published by John Wiley and Sons, Inc.)

commensal in the intestine ; and Endamoeha gingivalis, a very common
parasite in the human mouth, usually harmless but sometimes ap-
parently injurious to the gums. Examples of parasitic Mastigophora

ANIMAL PARASITISM 745

are the human intestinal flagellate, Giardia lamhlia (Fig. 386), and
the blood-inhabiting trypanosome, Trypanosoma rhodesiense, caus-
ative agent of African sleeping sickness which is carried by the
Tsetse fly, Glossina (Fig. 388). Examples of parasitic Infusoria are
the human intestinal ciliate, Balantidium coli (Fig. 389), the various
species of Opalina, and related genera found in the excretory bladder
or cloaca of frogs and toads. Of the thousands of species of Sporozoa,
all of which are parasitic, probably the best known are the three
species of the genus Plasmodium (Fig. 393), which cause human
malaria, and Babesia higemina, which produces Texas tick fever of
cattle.

Platyhelminthes. — This phylum also contains four classes, two of
which, Trematoda (flukes) and Cestoda (tapeworms), are all para-
sitic, while the other two, Turbellaria and Nemertinea, are mainly
freeliving but contain some species which are parasitic on aquatic
invertebrates. Among the best known examples of Trematodes are
Fasciola hepatica (Figs. 398 and 399), the sheep liver fluke; Clon-
orchis sinensis (Fig. 397), the Chinese human liver fluke; and Schis-
tosoma Jiaematohium, one of the three species of human blood flukes.
Probably the best known tapeworms are Taenia saginata (Fig. 402),
the beef tapeworm. Taenia solium, the pork tapeworm and Diphyllo-
hothrium latum, the broad fish tapeworm, all three common parasites
of the human intestine, and EcJmiococcus granidosus, a dog and wolf
tapeworm whose larval stage is the cause of a horrible human disease.

Nemathelminthes.— The single class Nematoda includes at least
95 per cent of the species in this phylum ; most of them are free-
living, but there are also thousands of parasitic species. Examples
of parasitic species are the human hookworms, Necator americanus,
the American hookworm, and Ancylostoma duodenale (Figs. 394
and 395), the Old World hookworm; Ascaris lumhricoides (Fig. 90),
the large intestinal roundworm of hog and man ; Dracunculus
medinensis, the Guinea worm, often over a yard long, which
crawls around under the human skin (believed by some to be
the "fiery serpent" mentioned in Exodus) ; Trichinella spiralis,
which causes the often fatal human disease, trichinosis, when
its larvae, encysted in pork, are eaten by man ; and Wuchereria han-
crofti, the fiJaria which is injected into the human blood by certain
tropical mosquitos and causes elephantiasis, a disease in which the
infected limbs may become larger than the body of the victim.

746

TEXTBOOK OF ZOOLOGY

Onciiocerca volvulus, transmitted by certain biting flies, is a com-
mon cause of blindness in some parts of Mexico. Besides the
human nematodes there are thousands of others parasitizing lower
animals, both vertebrates and invertebrates. The other two classes
of the phylum Nemathelminthes are entirely parasitic; the Acan-
thocephala, or thorny-headed worms, are common intestinal para-
sites of many vertebrates, including the hog and occasionally man;

Fig. 390. — Elephantiasis, some extreme cases. A, of legs and feet; B, of
scrotum ; C, varicose groin gland ; D, of scrotum and legs ; E, of mammary glands.
(Reprinted by permission from Introduction to Human Parasitology by Chandler,
published by John Wiley and .Sons, Inc. A and B sketched from photograps from
Castellani and Chalmers; C, D, and E from Manson.)

the Gordiacea or horsehair worms (Fig. 88) are parasites of insects
until nearly mature; they crawl out of their insect hosts when the
latter fall into water, become sexually mature, and lay their eggs.

Arthropoda. — All of the classes in this phylum are predominantly
free-living, but several classes also include parasitic species. The

ANIMAL PARASITISM

747

class Hexapoda or Insecta contains, besides several hundreds of thou-
sands of free-living insects, the parasitic fleas, lice, and bedbugs;
the class Arachnida, characteristically free-living, contains the para-
sitic ticks and mites, and the class Crustacea, though mostly free-liv-
ing, includes a number of species parasitic on fishes and other aquatic
animals. While most of the parasitic arthropods are ectoparasites,
there are also a few endoparasitic species. For example the horse
bot, Gastrophilus, which is the larva of a fly, is parasitic in the stom-
ach of horses; long wormlike arachnids known as Linguatulids or
tongue worms are found in the intestines of some reptiles and mam-
mals; and Sacculina (Fig. 404), a crustacean, parasitic on crabs and
lobsters, sends rootlike outgro\vths all through the body of its host,
although the saclike body remains on the outside.

Some Representative Parasites
Protozoa. — The very small amoebalike protozoans of the genus
Endamoeba are examples of parasites only slightly modified for
parasitic life. There are two distinct stages in the life cycle, the

Fig. 391. — Amoeba histolytica, one of the important protozoan parasites. It is
the causal a.erent of amoebic dysentery. A, Stained vegetative amoeba ; B, cyst
with four nuclei ; n, nucleus, showing peripheral chromatin granules and central
karj-osome ; r.b.c, ingested red blood corpuscles; chr.b., chromatoid body. (Re-
printed by permission from Introduction to Human Parasitology by Chandler, pub-
lished by John Wiley and Sons, Inc., after Dobell.)

active form being much like a small amoeba except that the pseudo-
podia are shorter and move more slowlj'"; these active forms finally
round up and become surrounded by a semirigid, resistant cyst wall.
In this encysted condition Endamoeba is passed from the host
with the feces or other body excrements. While in the encysted con-

748

TEXTBOOK OF ZOOLOGY

dition the parasite divides by binary fission into first two, then four,
and finally (in E. coli) eight little amoebae. If the cyst is swallowed
by another host, the cyst wall is dissolved and the four or eight
young amoebae come out to begin the active stage again. The com-
mon Endamoebae of man are E. gingivalis which lives in the mouth
and is usually transmitted by kissing, and the two intestinal species
E. coli (nonpathogenic) and E. histolytica. The latter species breaks
down the cells of the intestinal lining by means of enzymes which it
secretes, and then ingests the broken cells in the same way that the
common free-living amoebae take in their food. The disease caused
by E. histolytica is known as "amoebic dysentery." Infection occurs
as the result of eating food or drinking water which has been con-
taminated by the feces of infected people, as in the case of the

Fig. 392. — Endanioeba coli. A, stained vegetative amoeba; B, cyst witli eight
nuclei, n., nucleus, showing coarse peripheral chromatin granules, chromatin
granules in "clear zone" between periphery and kai-yosome which is eccentric in
position ; chr.b., remnant of chroniatoid body. Numerous food vacuoles in vegeta-
tive form. (Reprinted by permission from Introduction to Human Parasitology by
Chandler, published by John Wiley and Sons, Inc., after Dobell.)

Chicago hotels where contamination of drinking water by water
siphoned up from the toilet drains into the water pipes caused a
serious outbreak in 1933.

The malaria parasites, of which there are three species infecting
man (Plasmodium vivax, P. falciparum, and P. malariae, each caus-
ing a different form of malaria), are Protozoa belonging to the class
Sporozoa, and are very highly modified for parasitic life. The
adaptations for parasitism and for transmission from host to host
involve a very complex life cycle. The two main phases of the life

ANIMAL PARASITISM

749

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750 TEXTBOOK OF ZOOLOGY

cycle are the vegetative or schizont stage (merozoites) and tlie sex-
ually reproductive or sporont stage. The biology of this parasite
has been discussed in the earlier chapter on Protozoa under class
Sporozoa and Economic Relations of Protozoa.

Nematodes. — Of the thousands of species of parasitic nematodes,
space permits mention of only a few which are particularly impor-
tant because of their danger to man.

Hookworms. — In the United States the most important human
nematode parasite, from the public health viewpoint, is the American
hookworm, Necator americanus. Although called the "American
hookworm" this species probably came originally from Africa and
was introduced into America by the negro slaves. The pioneer work
on hookworm in the United States was done by Dr. Charles W. Stiles
in 1901. Hookworms are slender threadlike nematodes about one-
half inch long; the females are tapered to a point at each end, while
the slightly smaller males have on the posterior end a fanlike ex-
pansion, the copulatory bursa, with curved riblike supports. Both
sexes have a large mouth containing hooklike chitinous teetii by means
of which they tear holes in the walls of the intestine and start blood
flowing from the wounds. A muscular esophagus leading back from
the mouth cavity gradually broadens into a large muscular bulb ; by
means of rhythmic contractions and expansions of the bulblike esoph-
agus blood is drawn into the mouth and forced down into the straight
intestine where some of it is digested and the rest passes on through
and out of the anus near the posterior end. Because of the large
number of worms present there is a serious loss of blood resulting in
anemia and lack of energy; in children the growth is stunted or re-
tarded by hookwonns, and often there is also a lack of proper mental
development. Individuals, very heavily infected during childhood
and early youth, may fail to develop sexually. Treatment is fairly
easy, hookworms being easily killed by doses of anthelmintics, such
as carbon tetrachloride and hexylresorcinol (which are poisonous and
should be taken only under doctor's supervision).

Each female hookworm produces 9,000 eggs per day; these eggs
pass out with the feces of the host; if the infected person defecates
on the ground, the eggs hatch and the larvae crawl around in the
soil; there they develop into infective larvae which live for several
months on the surface of the ground. If bare human skin comes in
contact with these microscopic worms they bore through it to the

ANIMAL PARASITISM

751

Fig. 394. — Ancylostoma duode-
nale, female and male, with head of
Necator americanus drawn to same
scale. a7i., anus ; b^ bursa ; b.c, buc-
cal capsule ; cem.gl., cement gland ;
cerv.gl., cervical gland ; cerv.p.,
cervical papilla ; cl., cloaca ; c.sp.,
caudal spine ; ex.d., so-called excre-
tory' duct; int., intestine; n.ceph.gl.,
nucleus of cephalic gland ; n.r., nerve
ring ; oes., esophagus ; ov., ovaiy ;
ovej., ovejector ; sp., spicules ; t.,
testes ; ut., uterus ; vag., vagina ;
V.S.. vesicula seminalis. (After
Looss from Chandler, Hookioorm
Disease. Reprinted by permission
from Introduction to Human Para-
sitology by Chandler, published by
John Wiley and Sons, Inc.)

752

TEXTBOOK OF ZOOLOGY

,- gn

d»l_-

Fig. 395.-^Life history of hookworms from egg to infective larva. 1, &^^ of
Necator aniericanus at time of leaving host; 2, same of Ancylostoma duodenale;
3 to 7, segmentation and development of embryo in egg ; 8, newly hatched embryo ;
9, same of Strongyloides for comparison (note length of oral cavity and size of
genital rudiment, </.?•.) ; 10, second stage larva; 11, fully developed lai'va ; an., anus;
ex. p., excretory pore ; g.r., genital rudiment ; int., intestine ; n.r., nerve ring ; oes.,
esophagus; oes.b.. esophageal bulb; or. cav., oral cavity; sh., sheath. (After Looss
from Chandler, Hookioorm Disease. Reprinted by permission froiu Introduction
to Hunia/n Parasitology by Chandler, published by John Wiley and Sons, Inc.)

ANIMAL PARASITISM 753

blood vessels, are carried by the blood to the lungs, then migrate up to
the trachea and pharynx, into the esophagus, then down through the
esophagus and stomach to the small intestine; meanwhile they in-
crease in size, so that on arrival in the small intestine tliey are ready
to attach themselves to the wall of the intestine, feed on the blood of
the host, and become adults.

Since the larval hookworms must go through part of their develop-
ment in the soil, and a person can become infected only by direct
contact with contaminated soil, the distribution of hookworm in the
United States is determined by the following factors: (1) freezing
of soil in winter (kills the larvae) ; (2) texture of soil (hookworm
larvae live best in light, sandy loams) ; (3) moisture (hookworm
larvae can live only in damp soil) ; (4) customs of the people in dis-
posal of feces ; the bad hookworm districts are sections in which sani-
tation is very primitive and sanitary toilets are not in universal use ;
deposit of feces on the ground is particularly conducive to spread of
these animals.

From the public health standpoint hookworm disease is a social
problem rather than a medical problem. Few if any people are killed
by hookworm, and infected individuals are easily cured if they go
to a physician for treatment. On the other hand, such a large pro-
portion of the population, in hookworm territory, are kept in bad
health and a listless condition that the social welfare of the whole
community is injured. Prevention of hookworm disease is theoreti-
cally easy; hookworms could be killed out of a community in a few
months if everyone would defecate only in sanitary toilets, if every-
one would take treatments for hookworms at tlie same time, or if
everyone would wear good shoes ; but so far, it has been impossible to
get the cooperation of all the people in hookworm districts. The work
of the medical profession, with the help of certain state agencies, has
reduced hookworm disease in the United States, but there are still
considerable districts in which over 20 per cent of the populations are
infected. In parts of East Texas 33 per cent of the people examined
have hookworm, even now.

Trichina. — TricJiinella spiralis, commonly knowai as Trichina, is an
example of a nematode with an alternation of hosts and a passive
means of transmission. The microscopic lar\^ae are encysted in the
muscles of various meat-eating animals, being particularly common
in hogs and rats ; within the cyst, the larva is coiled in a tight spiral,

754

TEXTBOOK OF ZOOLOGY

which g-ives the species its name. If pork containing trichina cysts
is eaten by a man, the cysts are digested off in the stomach, the larvae
become active and penetrate the mucosa of the small intestine to
moult. They soon become full grown, sexually mature adults, some-
times beginning copulation only forty hours after being swallowed.
The adult worms are short-lived and are not harmful to the host, but
each fertile female, about a week or ten days after the infected meat
was swallowed, begins to produce thousands of microscopic larvae
which she deposits in the walls of the intestine, usually directly into

Fig. 396. — Larvae of Trichinella spiralis, encysted in voluntary muscle. The
adults are parasites of the intestine. (Photomicrograph by Albert E. Galigher,
Inc.)

a lymph vessel or blood vessel. The larvae are carried by the blood
or lymph to the heart, and from there they are carried by the blood
to all parts of the body. The larvae which reach voluntary muscle
enter the muscle fibers and coil up into spirals, grow rapidly to a
len^h of about 1 mm., and in a few weeks become enclosed in a cyst
of connective tissue which grows around them. Trichina larvae are
likely to be most abundant in active muscles, such as the diaphragm,
the intercostals, and the muscles of the larynx, tongue, and eye. The

ANIMAL PARASITISM 755

symptoms of trichinosis (the disease caused by trichina worms) vary
according to the stage of the infection. In the first week, wlien the
adult females are burrowing into the walls of the intestine to deposit
their larvae, the symptoms are so much like those of typhoid fever that
many cases are diagnosed as typhoid by good physicians. Nine or
ten days after the beginning of infection, when the larvae are mi-
grating to the muscles, there are severe muscular pains and aches
(sometimes diagnosed as rheumatism), and the inflammation of the
muscles used in breathing, chewing, etc., may interfere with these
functions. As the parasites become encysted, about six weeks after
infection, pains become worse and swelling of the infected parts
occurs, accompanied by anemia and skin eruptions. If the victim
survives this period he usually recovers, as the parasites are now
walled off by cysts of connective tissue formed by the host; later
calcium carbonate is deposited in these cysts, walling off the parasites
so completely that they die, but the calcified cysts remain as hard
grains in the muscles and may cause some rheumatic pains for years.
Until the worms are completely walled off fever is caused by poison-
ous substances produced by the larval worms. Kecent studies of the
cadavers used in medical schools have revealed that about 20 per I

cent of the American population probably have cases of trichino-
sis in some degree at some time during life, since about this propor-
tion of the cadavers had trichina cysts in the muscles.

All danger of trichinosis can be avoided by cooking pork thor-
oughly before eating it, as the larvae are killed by a temperature of
55° C. (131° F.). Investigations of the United States Bureau of
Animal Industry, in which the author assisted, indicate that pre-
pared sausages seldom contain living worms, most of them being
killed by the salts and seasoning or by long-continued cold storage;
the greatest danger is from fresh pork. Contrary to popular im-
pression. Federal inspection does not guard against trichina, as there
is no effective way to inspect meat for trichina on the large scale that
would be necessary.

Trematodes. — Schistosoma haematohium, a Human Blood Fluke. —
The blood flukes (Family Schistosomatidae) are distinguished from all
other trematodes by having separate sexes. The male has a thick body
with the lateral edges bent ventrally, thus forming a long groove on ;|)

the ventral surface, the gynecophoric canal. The female is long and
slender, almost threadlike in some species; when an adult male and

1.4

756 TEXTBOOK OF ZOOLOGY

female happen to come in contact, the male folds his body around
the female so that she is held fast in the gynecophoric canal, and the
pair be^n copulation. After once becoming paired they remain in
copula during the rest of their lives. Three species of the genus
Schistosoma are human parasites : Schistosoma mansoni, found in
Africa and the tropical parts of the New "World, S. japonicum of
Japan and China, and S. haematobium, the Egyptian blood fluke
found in north Africa and southwestern Asia. Schistosoma haema-
tobium is a parasite in the large blood vessels of the rectum and
urinary bladder. The female lays an enormous number of eggs,
which collect in the capillaries of the bladder and intestinal walls
and block the flow of blood, causing the infected parts to become
swollen and ulcerlike. Eventually the eggs are released into the
lumen of the rectum or bladder and pass out of the host's body in
the feces and urine, along with considerable quantities of blood from
the torn tissues. Aside from the pain caused by the egg-filled swell-
ings, the loss of blood is the most serious effect of blood-fluke infec-
tion. Infected individuals are kept in a run-down anemic condition.
The parasite is a serious public health problem in Egypt, where 80
per cent of the population of the Delta region is infected.

The egg must fall into water for the embryo to develop into a
ciliated larva, the miracidium. When the miracidium is fully de-
veloped, the cap or operculum on the end of the egg shell is pushed
open and the liberated miracidium begins to swim around in the
water. Certain species of snails seem to emit a chemical which at-
tracts schistosome miracidia ; if a snail of the right species is in the
vicinity, the miracidium swims to it and enters its body. If no suitable
snail is available, the miracidium dies after swimming a few hours.
After penetration into a snail the miracidium loses its cilia and de-
velops into a long sausage-shaped sac, the sporocyst, without any
recognizable organs. Germ cells within the sporocyst develop into a
number of young sporocysts, which escape from the mother sporocyst
into the tissues of the snail, grow to full size, and then in turn give
birth to a new generation of sporocysts, or, under some conditions,
these sporocysts may give rise to a different kind of larva, the
cercaria, which has a long, forked, muscular tail, a pair of eyespots,
two suckers (one anterior and one midventral), and a rudimentary
digestive system. When fully developed, the cercaria forces its way
out of the snail and begins to swim through the water. If unsuccess-

ANIMAL PARASITISM 757

ful in finding a host, the cercaria dies in a few hours, but if it comes
in contact with the skin of a man, or any other mammal, the cercaria
enters the skin, with the aid of glands in the head region which seem
to be used in digesting or destroying the skin tissues. After pene-
trating the skin the cercaria soon finds its way into a blood vessel
and begins a voyage through the circulatory systen, carried along by
the current of the blood stream, meantime growing into an adult
fluke. The mating of males and females usually occurs in the larger
veins, and the pair moves to the veins in the walls of the rectum and
bladder, where egg-laying begins.

Irrigated districts, such as the Nile Delta, are especially favorable
for the development of blood flukes because the eggs have more
chance of getting into water and because the field workers often get
into the water while working around the irrigation ditches. The
chances of infection are increased by the customs of defecating and

^ i

Fig. 397. — ClonorcMs sinensis. Oriental liuman Uver fluke, showing male and
female reproductive organs. (Photomicrograph by Albert E. Galigher, Inc.)

urinating into the water, and using water from irrigation ditches for
drinking and washing. In Japan the number of human blood fluke
cases has been greatly decreased by improved sanitation and by
killing the host snails. In 'Egy])t, public clinics (by injecting fuadin
into the blood) treat thousands of cases of this disease, but it will
probably remain a public health problem for years because of the
refusal of the Egyptian peasants to change their old customs.

ClonorcMs sinensis, the Chinese Liver Fluke. — This is an impor-
tant human parasite in parts of the Orient. Clonorchis sinensis also
occurs in other fish-eating mammals, including dogs, cats, and pigs.
The adult worm lives in the bile passages of the liver. In man it l|

often causes enlargement of the liver, diarrhea, jaundice, anemia, 11

and extreme weakness, sometimes resulting in death. Hundreds of
worms may be found in a badly infected man. The eggs laid by the

758

TEXTBOOK OF ZOOLOGY

Proboscis (extruded)

Serve-ganglion

Esrg,

YoUcceUs

.^Intestine

Ventral sucker.
Nephridium

Germ-cells

Cercaria

Germ-cellf

Fig. 398. — The liver fluke, Fasciola hepatica. A, egg; B, miracidium ; G, sporo-
cyst ; D and E. rediae : F, cercaria; G. encysted staa;e ; H. adult (showing dip:es-
tive and excretory systems). (From Hegner, College Zoology, published by The
Macmillan Company.)

ANIMAL PARASITISM 759

adults pass from the liver to the intestine of the host by way of the
bile duet, then pass from the body in the feces. Snails probably be-
come infected by swallowing the eggs, while feeding on fecal matter
in the water. After hatching, the miracidia migrate into the lymph
spaces of the snail and develop into elongated sporocysts, each of
which gives birth to a number of redia (differing from sporocysts by
possessing a pharynx and a rudimentary gut). Each redia gives
birth to six or eight cercariae, which emerge from the snail and swim
around in the water by means of a very large, undivided tail. When
a cercaria comes in contact with a fish, it enters the skin and encysts
either in the skin or in the muscles just below the skin. It is now
called a metacercaria or agamodistomum. Man becomes infected by
eating these metacercariae in poorly cooked fish. When swallowed,
the cysts are dissolved by digestive juices of the host, tlie larva escapes
into the duodenum, migrates up the bile duct to the liver, and there
develops into an adult. There is evidence that the adult Clonorchis
may live as long as twenty years in the liver of man.

Treatment of clonorchiasis is not very satisfactory. Prevention
is simple : avoid eating fish which are not thoroughly cooked.

It will be noted that the Clonorchis life cycle involves three hosts:
a mammal as the final host, a snail as the first intermediate host,
and a fish as the second intermediate host. Infection of the fish is
by active invasion of the cercariae, and infection of the final host is
passive.

Other Trematodes. — One of the best known parasites of domestic
animals is the sheep liver fluke, Fasciola hepatica, which occurs in
all sheep-raising countries in which wet pastures are common. It is
also a common parasite of goats and cattle. Like Clonorchis, Fasciola
lives in the bile passages, and its eggs pass out Avith the feces of
the host, but unlike Clonorchis, the eggs hatch in water and the
free-swimming miracidium actively seeks and penetrates the snail
host. Sporocijsts in the snail give rise to rediae which produce
cercariae, but the cercariae encyst on any surface, including grass |

blades and even the surface film of the water. Sheep become infected I

by eating grass bearing encysted larvae or by swallowing floating !,

cysts while drinking water. Fascioloides magna, the large liver fluke >}

of cattle and sheep in Louisiana, Arkansas, and Texas, is very similar i

in structure and life history.

760

TEXTBOOK OF ZOOLOGY

Other important flukes are the human intestinal fluke, Fasciolopsis
huskii, which is common among the Chinese, who become infected by
eating the cysts on various aquatic food plants; the human lung
fluke, Paragonimus westermanii of eastern Asia, where the natives
become infected by eating the encysted larvae in fresh-water crabs

Fig. 399. — Life history of the liver fluke. Fasciola hepatica. A. adult in liver of
sheep ; B, freshly passed egg as it leaves the body of the host ; C, developing em-
bryo, ready to hatch in the water ; D^ ciliated miracidium embryo in the water and
about to enter the pulmonary chamber of snail ; E, F, sporocyst containing rediae ;
G, redia containing daughter rediae ; H, redia of the second generation containing
cercaria ; J, same having emerged from snail into water ; K, cercaria encysted on
blade of grass ; L, cercaria liberated from cyst after ingestion by sheep ; M, young
fluke developing in liver of sheep. (Reprinted by permission from Chandler,
Introduction to Human Parasitology, published by John Wiley & Sons Inc.)

ANIMAL PARASITISM

761

Fig. 400. — Structure of tapeworm to show different stages of maturity. At lower
left. Taenia insiformis, dog- tapeworm, scolex with hooks and sucker discs. At
lower right, mature proglottids. Above, scolex and proglottids of three ages. The
reproductive organs and pore are shown in most proglottids. (Courtesy or Wards
Natural Science Establishment, Inc.)

762

TEXTBOOK OF ZOOLOGY

and crayfishes; Gotylophoron cotylophorum, a stomach parasite of
cattle in Louisiana, and Dicrocoelium lanccahnn, a common liver fluke
of herbivorous mammals in Europe and Asia.

Numerous ectoparasitic trematodes, of the Subclass Monogenea,
occur on the skin and gills of fish and are often of economic im-
portance because they kill goldfish and other aquarium fishes, and
also young fishes in state fish hatcheries.

Fig-. 401. — Development of tapeworm. A, six-hooked embryo ready to become
embedded in muscle ; B, cysticercus, or bladder worm as encysted ; C. section
through developing scolex in cysticercus; D, later stage; E, scolex everting as it
protrudes from bladder; F, extension of scolex from bladder; G, later stage ;^,
formation of proglottids. (From Parker and Haswell, Zoology, published by The
Macmillan Company, after Jijima and Hatschek.)

The Tapeworms (Class Cestoda).— Cestoda differ from Trematoda
in the complete lack of a digestive system. In fact, Cestodes never
have any sign of a digestive organ at any time during life; they
receive their nourishment by absorbing through the surface of their
bodies the food already digested for them by the host. Most cestodes
also differ from trematodes by having the body divided into a series
of segments, one behind the other, each segment having a com-
plete set of reproductive organs. This structure characteristic of

ANIMAL PARASITISM

763

tapeworms is usually referred to as segmentation of the body, but
it is probably more correct to consider a tapeworm as a linear colony,
in which the segments are really individuals in various stages of
maturity.

Taenia solium, the pork tapeworm of man, may be taken as an ex-
ample to illustrate the structure and life history of a cestode. The
adult tapeworm consists of a scolex or head provided with four mus-
cular suckers and a snoutlike rostellum surrounded by a row of

Head

flead

/Mature serine nis

/Aature

Fig. 402. — Common tapeworms, showing different regions of the body. At the
left above, scolex of Taenia saginata, beef tapeworm ; left below, proglottids of
Monie:sia, sheep tapeworm ; middle, scolex and proglottids of Taenia solium, pork
tapeworm ; right, scolex and proglottids of Dipylidium caninum, a dog tapeworm.
(Courtesy of General Biological Supply House.)

chitinous hooks, which serve as means of attachment to the wall of
the human intestine; a narrow unsegmenfed neck behind the scolex,
and then a series of several hundred proglottids (the segments) be-
coming progressively larger as they get farther from the scolex. The
whole chain of proglottids is called the stroMlus. New proglottids
are constantly budded off from the neck; consequently, the young-
est proglottid is the first one back of the neck and the oldest one is

764 TEXTBOOK OF ZOOLOGY

the one at the end of the strobilus farthest from the scolex. The
youngest proglottids contain no recognizable structures, except the
paired longitudinal nerve cords and longitudinal excretory vessels
which run the full length of the strobilus, and a transverse excre-
tory vessel in each proglottid. As the proglottids become older and
are pushed farther away from the scolex, the reproductive organs
begin to develop; each proglottid develops a complete set of both
male and female reproductive organs; when these become mature
and ready to function, the proglottid is a mature proglottid. Each
mature proglottid is capable of copulating with itself, by bending
the cirrus down into its own vagina, or it may copulate with another
mature proglottid of another tapeworm if one is present close by.
After copulation the male organs begin to degenerate, the uterus
becomes filled with fertilized eggs and takes up more and more
space, then the ovaries, vitellaria, and other female organs degen-
erate and leave the uterus to occupy nearly the whole proglottid;
in this condition the proglottid, now hardly more than a sack of
eggs, is called a gravid proglottid. The gravid proglottid at the
extreme end of the strobilus breaks off and passes out of the host's
intestine with the feces. It continues to live and crawl slowly
through the feces like an independent animal for a few hours, then
usually dies, but the embryos within the proglottid remain alive
much longer. If gravid proglottids or separate eggs are eaten by
a hog, the six-hooked hexacanth larva hatches in the hog's intestine,
bores through the intestinal wall, and migrates to other parts of
the body where it changes into a bladder-Avorm or cysticercus, which
is a saclike larva with an inverted scolex. The cysticerci remain
in the flesh of the hog until the pork is eaten by man ; under the
influence of human digestive juices the cysticerci become everted
so that the scolex is on the outside of the saclike part, then the
scolex attaches itself to the wall of the human intestine, proglottids
begin to bud off from the neck, and an adult tapeworm is formed
within a few weeks.

Taenia saginata, the beef tapeworm, has a similar life history, but
uses cattle instead of hogs as intermediate hosts. Taenia serrata,
a common dog tapeworm often used as a laboratory specimen, is
very similar to the two human species in structure and life cycle.

ANIMAL PARASITISM 765

Other important cestodes are Hymenolepis nana of man and mice;
the broad fish tapeworm, Diphyllodothrium latum, which man gets
by eating raw or poorh^ cooked fish ; the peculiar Echinococcus granu-
losus, adult in dogs, whose cysticercus stages are dangerous parasites
of man (man being the intermediate host in this case) ; and many
tapeworms of domestic animals, such as Dipylidium caninum of dogs,
Thysanosoma and Moniezia of sheep, goats, and cattle, and majiy
others.

CHAPTER XXXIX

MARINE ZOOLOGY

It is true that a good many people never have an opportunity to
study the conditions present in the ocean or to observe the animals
found in its waters and on its beaches. However, with the improved
transportation and awakened interest in the subject, there are more
and more students of this fascinating subject. Many of the animal
forms seem peculiar and spectacular to those of us who reside in-
land. With the modern facilities for travel four hundred miles is
not remote from the seashore, and every student majoring in biology
in colleges or universities within that distance should be given
marine experience, first hand.

The life of the ocean is known as lialohios. Marine animals are
affected and limited by many factors which in turn influence their
distribution. They are affected by the temperature of the water,
the height of the tides, the velocity of currents, salinity of the water,
its turbidity, light, pressure, oxygen content, and the nature of the
bottom and the shore. The occurrence of the proper food greatly
affects the range and abundance of any particular species. Rocky
shores harbor the choice food of certain species and muddy lagoons
supply other groups. The adaptation and adjustment of the marine
animal to the salinity of the water is one of the first considerations.
The salt content is effective both quantitatively and qualitatively.
If an oyster is taken from the ocean and placed in a fresh-water
pond or stream, it absorbs water and swells up excessively. On the
other hand, a clam taken from fresh water and placed in the ocean
loses water and shrinks. In either case the effects will finally be-
come lethal. A few animals, such as lamprey, eels, shad, salmon,
and even gar pike and mullet are able to make the transfer from
marine to fresh water and vice versa.

The salinity of the water of the marine habitat is an important
and interesting feature to be studied. Along the Texas coast the
water is either that of the Gulf of Mexico proper or of the various
bays. The analyses* which have been made on the Gulf water out

♦Reported by Mr. J. G. Burr, Texas Game, Fish, and Oyster Commission.

766

MARINE ZOOLOGY

767

from Matagorda and out from Port Aransas show a high degree
of salinity. The readings range from 36.6 to 37.1 parts of salt pei*
one thousand. The average given for the Atlantic Ocean is be-
tween 35 and 36 parts per one thousand. These readings on the
Gulf are being checked by further data, but the cause of this dif-
ference has not been explained.

The salinity of the bays is quite variable, and Galtsoff in 1926
made comparative studies of a number of them. It was found that
in bays with a large fresh-water stream entering, as Nueces Baj^

;T— L/mbrella

-Manubrium

7mm-

y/M oral
~^ tentadei

■ Central moufch

Fig. 403.

-Cabbage-head jellyfish, Stoviolophus meleagris, a very common form in
the Gulf of Mexico.

the range is from 6.06 parts per thousand in June to 33.06 parts
per thousand in September. In Mesquite Bay at Belden Dugout
Beacon the range is from 5.03 parts per thousand in June to 18.44
parts per thousand in October. In Aransas Bay, where there is
relatively small fresh-water intake, the salinity ranges on the aver-
age between 14.79 parts of salt per thousand in June and 25.47 parts
per thousand in September.

These wide ranges of salinity in the bays create problems for the
organisms attempting to live there. It requires a high degree of

768

TEXTBOOK OF ZOOLOGY

adaptation to salinity changes to be able to do it. Not only do
floods coming" in from streams bring serious disturbances, but the
opposite effect may be brought about by storms rolling the waves

Fig-. 404. — Sacculina. A common parasite on the crab. It is recognized as an
arthropod only by its larval stages because the adult looks like a sponge or fungus.
A. and B, larval stages with jointed appendages ; C, crab infested with adult Sac-
culina. The figure shows it on one .side only. (From Lull, Organic Evolution^ pub-
lished by The Macmillan Company.)

in from the sea. There are opportunity and need for much more
study of the effects of these phenomena.

MARINE ZOOLOGY 769

In general, marine animals are adapted to one of three habits of
life : dwelling on the bottom, swimming at various levels, or simply
floating at the surface. There are three or four forms of marine
animals according to the position they take in the water. (1)
Benthos include all nonswimming bottorm-dwelling forms, and they
may be vagrant or sessile (stationary). Hydroid and crinoid types
exemplify the sessile form and urchin or starfish the vagrant. The
benthos may extend from the shore line to the deep sea. (2) Nekton
is the name of the entire group of swimming animals that are able
to maintain themselves in the water and do not rest on the bottom.
The fishes, whales, and porpoises are typical examples. (3) Plankton
is the collective name for plants and animals that float in the water.
Many of the plankton animals are nearly transparent, and the
smaller ones are surprisingly abundant. Because of their trans-
parency and size, the casual observer seldom sees this group of
animals. The nature of the tissue of the bodies of these animals
is largely of gelatinous material and the shape is either that of an
umbrella or of a mass with projecting processes. The composition
of the tissue is such that it is bulky without great weight. In fact,
it is largely water and has a specific gravity slightly greater than
water. These adaptations make it possible to float with an easy
distribution of weight and very little if any effort on the part of
the animal. A large number of Protozoa, jellyfishes, and Entomo-
straca (microscopic Crustacea) are typical plankton. (4) Pelagic
animals are all of the surface-living forms away from the shore in
the open sea.

Another expression of the distribution of marine animals is by the
following zones. (1) Littoral zone, which is the shallow water along
the shore. Here the fluctuations of wave action, temperature, and
depth (because of tides) are extreme, and the light intensity is at
its maximum. Such aaiimals as certain coelenterates, worms, Crus-
tacea, echinoderms, and molluscs are common inhabitants of this
zone. It requires a rather sturdy type of body and hardy proto-
plasm to Avithstand the hardships of the shore and near-shore. Some
clams and oysters, also corals, sea urchins, and starfishes can live
out of the water for several hours. The majority of corals live
where they can depend on wave action to carry floating objects to

770

TEXTBOOK OF ZOOLOGY

them for food. The zone extends from shorelme to a depth of from
50 to 75 feet. (2) Intermediate (sublittoral) zone includes relatively
shallow water of from 50 to 500 feet and supports hydroids, sponges,
some corals, sea cucumbers, sea urchins, and starfish. There is a grad-
ual increase in stability of conditions through this zone. (3) Bathyal
(abyssal) zone is in the depths of the ocean ranging from 5,000 to

10.

11.

12.

Fig. 405. — A group of typical mollusks occurring along- the Texas coast of the
Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Cheatum). 1, Solen
viridis. 2, Dosina discus. S, Seniicassis gibba. !i, Oliva sayana. 5, Donax roenieri.
6, Venus campechiensis texana. 7, Crepiduln fornicata. 8, Polinices duplicnta.
9, Thais haeniostoma floridana. 10, Architectonica granulata. 11, Littorina iiTO-
rata. is. Modiolus tulipus.

25,000 feet. The pressure increases fifteen pounds per square inch with
each thirty feet of increased depth. The conditions become even more
constant in this zone. Below 800 feet there is no light and enormous
pressure. In the great depths the temperature is nearly constant
and is about 2° C, or just above freezing. Most of the animals
found in this zone are relatively small and somewhat modified

MARINE ZOOLOGY

771

structurally as well as physiologically in order to adjust to the con-
ditions. Approximately 150 species are known to exist at 15,000
feet or deeper. There is a tendency toward the development of
phosphorescent organs and telescopic eyes among animals inhabiting
the bathyal regions.

Fig. 406. — Common bivalve and univalve mollusks that occur along the Texas
coast of the Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Chea-
tum.) 1, Dinocnrdiuni robustum. 3, Busucon perversus. 3, Barnea costata.
i, Ostraea virginica. 5, Plagioctenium irradiens. 6, Atrina seminuda.

The methods of studying the conditions of the ocean are largely
by survey methods, use of light disks, pressure gauges, electric
thermocouple temperature recorders, hydrogen ion analysis ap-
paratus, oxygen tension analyses, and chemical analysis for salinity.
Samples of sea water can be obtained from various depths by use
of a sampling tube or cylinder which is lowered while open so that
water passes right through it. When the sample is desired, a lead
messenger is sent down the cable to trip the trigger and allow the
cylinder to snap shut at both ends. The cylinder with the sample

772

TEXTBOOK OF ZOOLOGY

Fig. 407. — Peropliora, a common tunicate which rangres from British Columbia
to San Diego along the Pacific Coast. Notice siphons, digestive tract (black), and
gill slits in wall of pharynx. (Courtesy of Albert E. Galigher, Inc.)

PAGURUS

LIBINIA

Fig. 408. — ^Representative crabs. Vca (fiddler), CalUnectes (blue edible),
Pagurus (hermit), Libinia (spider). Abundant on shores of Gulf of Mexico.
(Courtesy of General Biological Supply House.)

MARINE ZOOLOGY

773

can then be drawn up. Many of the smaller animals will be in-
cluded in such samples. Other methods of collecting the animals
is by use of metal dredges, dredge nets, shrimp nets, cord mops
or tangles, dragnets along shore, townets, and fishing tackle gen-
erally. It is necessary to use a spade and seine in the beach sands
and some sharp instrument for scraping pilings and rocks. The
shore and the drift line offer many opportunities of collecting and
observing a wide range of animals with no special apparatus.
Studies of distribution and migration of marine animals have been
made by tagging large numbers of individuals and compiling rec-
ords of the return of these tags.

Fig. 409. — -The common pompano, TracMnotus carolinus, a valuable food fish
which is taken abundantly along the southern part of our Atlantic Coast and in the
Gulf of Mexico.

In an effort to give an idea of the typical animal life of the sea-
shore and shore waters, two tables of representative animals are
included. The first includes many of the marine and shore animals,
excluding vertebrates, of the Pacific coast, while the second pro-
vides a similar representative group including fish from the shores
and waters of the Texas portion of the Gulf of Mexico. The rocky
shores of our western coast furnish an abundance of life. To the
casual observer standing almost ajiywhere on the Texas beach, it
seems almost like a desert at the seashore. The shore fauna is rela-
tively sparse because the bare sand and shell fragments are almost
sterile of food. Closer observation, however, will reveal much more
life than at first thought, as will be indicated by the representative
lists which follow.

774

TEXTBOOK OF ZOOLOGY

Typical Pacific Shore Animals

NAME

HABITAT ZONE

Porifera

Beneira cmeria, purple sponge
Plocamia karykinos, red sponge
Leucosolenia eleanor, cream colored sponge
Leucon/ia heatM, sharp-spined sponge

Coelenterata

Garveia annulata, hydroid
Eudendrvum calif orniaum, hydroid
Tubularia marina, solitary hydroid
Aglaophenia struthionides, ostrich plume

hydroid
Gonionemus vertens, hydroid medusa
Velella lata, siphonophore
Stylatula elongata, sea pen
Benilla amiethystina, sea pansy
Cribrina xanthogrammica, large green

anemone
Cribrina elegantissima, verrucose anemone
Epiactis prolifera, brown anemone
Metridimm, dianthus, anemone
Balanophyllia elegans, orange-red coral

Ctenophora

Pleurobrachia bachei, "cat's eye" comb

jelly

Bolinopsis microptera, comb jelly

Platyhelminthes

Planocera califomica, large flatworm
Leptoplana acticola, polyclad worm
Polychoerus caudatus, red flatworm
Nemertina

Amphiporus bimaculatus, orange nemertine

worm
Emplectonema gracile, yellowish green

nemertine worm

Bryozoa

Bulgula califormca, purple Bryozoa
Bulgula pacifica, bryozoan

Annelida

Audounia luxuriosus, hairy gilled worm
Nereis vexillosa, mussel worm
Nereis brandti, large mussel worm
Lwrnbrinereis sp., nereid worm

MoUusca

Pelecypoda

Siliqua paUila, razor clam
Tivela stultorum, pismo clam
Mytilis edulis, edible clam
Mytilis californiamts, mussel
Ostraea lurida, western oyster

Middle tide pool rocks

Lower tide pools
Lower tide pools
Lower tide pools

Lower tide pools
Eel grass at water edge
Driven ashore by storm
Mud flat below tide
Sand flats

Rocks, lower tide pools
Eocks, middle tide
Eocks, middle tide pools
Low tide pilings
Rocks, middle tide pools

Cast ashore (Planktonic)
Cast ashore (Planktonic)

Eocky shore, high tide levels
Eocky shore, upper tide pools
Middle tide pools

Middle tide pools
Middle shore

Lower tide pools
Middle tide pools

Between rocks and in mud

Cosmopolitan

Mud and sand flats

Sandy beaches
Sandy beaches
Sandy beaches
Sub-tide and half-tide
Low tide, rocky bay

MARINE ZOOLOGY

775

Typical Pacific Shore Animals — Cont'd

NAME

HABITAT ZONE

Pecten hmdsii, Bcallop
Pecten drcidaris, thick scallop
Cardita subqundrata, bivalve
Schizothaerus Tmttallii, gaper clam

Gastropoda

Anisodoris noiilis, sea lemon (nudibranch)
Dialula sandiegensis, nudibranch
PleurophylUdea californica, nudibranch
Tethys californica, sea hare
Littorina planaxis, gray littorine
Littorina scutulata, checkered littorine
Littorina sitchana, Sitka periwinkle
Littorina rudis, North Pacific white peri-
winkle
Purpura folitita, a conch
Cypraea spadicea, the cowry
CoTvus californicus, cone shell
Teg-ula funehralis, black turban snail
Tegula trunnea, brown turban
Norrisia norrisii, smooth turban
Tegula ligulata, banded turban
Thais lamellosa, wrinkled purple, snail
Polinices lewi9ii, moon snail
OliveUa biplicata, purple olive snail
Urosalpinx cinereus, oyster drill
Crepidula adunca, hooked slipper
Lottea gigantea, owl limpet

Acmnea scaira, scab limpet
Acmaea limatula, file limpet
Acmaea digitalis, limpet
Acmaea scutum patina, plate limpet
Acmaea ca^ssis pelt a, shield limpet
Acmaea mitra, white cap limpet
Acmaea depict a, painted limpet
Haliotis nifescens, red abalone

Amphineura

Mopalia muscosa, moss chiton
KathaHna tunicata, black chiton
Cryptochiton stelleri, giant chiton
Lepidochitona lineata, lined chiton

Cephalopoda
Polypus iimaculatus, octopus
Loligo opalescent, opalescent squid

Echinodermata

Asteroidea
Pisaster ochraceous, five-rayed starfish
Pisaster giganteus capitat\is, five-rayed

starfish
Pisaster hrevispinus, short-spined starfish
Evasterias troschelii, small starfish
Pycnopodia, helianthoides, large starfish
Leptasterias aequalis, six-rayed starfish
Patiria mininta, webbed starfish

Low tide, rocky bay
Low tide, rocky bay
Low tide pools
Substratum burrow

Middle tide pools
Eocks, middle tide pools
Low tide level, open bay
Eocky middle tide pools
Eocky shore, high tide level
Eocky shore, upper tide pools
High tide bay

High tide bay

Eocky shore, lower tide pools

Upper tide pools

Eocks, middle tide pools

Eocky shore, lower tide pools

Eocky shore, bay

Low tide level, open bay

Mud flats, intertidal

Middle tide pools

Eocky shore, high intertidal

zone
Eocky shore, upper tide pools
Eocky shore, upper tide pools
Eocky shore, high tide level
Eocks, middle tide pools
Eocks, middle tide pools
Eocks, middle tide pools
Eocks, lower tide pools
Lower tide pools

Intertidal rocks
Intertidal rocks
Eocks at low tide
Low tide level

Lower tide pools

Washed ashore occasionally

Lower tide pools under rocks

Lower tide pools under rocks
Below tide line
Below tide line
Below tide line
Middle tide pool rocks
Middle tide on rocks

776

TEXTBOOK OF ZOOLOGY

Typical Pacific Shore Animals — Cont'd

NAME

habitat zone

Ophvwroidea

Ophioderma panamensis, brittle starfish
Ophioploous esmarlci, sand-colored brittle

star
Amphipholis pugetana, small brittle star-
fish
Amphiodia ocoidentalis, brittle star
OphiophoUs amleata, brittle starfish
Echivjoidea

Strongylocentrotus franciscamis, large sea

urchin
Strongylocentrotus purpuratiis, purple sea

urchin
Lovenia cordiformis, red heart urchin
Dendraster exoentricus, sand dollar
EchinarachniAis parma, sand dollar
Eolotfuu/roid^a

Cucumaria miniata, sea cucumber
Leptosynapta inhaerens, dirty white sea

cucumber
Stichopus calif omicws, brown sea cucumber

Crustacea

Copepoda

Tigriopus fulvus, copepod

Cirripedia
Balarms cariosus, large barnacle
Balanus glandula, acorn barnacle
Chthamalus fissus, small brown barnacle
Balanus rmbilis, large barnacle
Mitella polymerias, goose barnacle
Balanus tintinnahulum, barnacle

AmpMpoda

Amphithoe sp., amphipod
Melita palmata, amphipod
CapreUa scaura, amphipod
Orchestia traslciana, beach hopper
Orchestoidea calif orniana, large beach hop-
per

Isopoda

Cirolana harfordi, drab isopod
Ligyda ocoidentalis, large isopod

Stomatopoda
Pseudosquilla higelowi, mantis shrimp

Becapoda

Crangon dentipes, snapping shrimp
Betaeus longidactylus, long-fingered shrimp
PavAilirus interruptus, spiny lobster
Pagurus samuelis, hermit crab
Pagurvs hirs^itiiiscuhos, common hermit
Pagurus granosimanus, small hermit
PagviTUS heringanus, large hermit
Pagii/rus hemphilli, hermit crab
PetroUsthes cinctipes, porcelain crab

Lower tide pools under rocks

Middle tide, under rocks

Under rocks
Low tide
Low tide

Lower tide pools

Lower tide pools
Sand flats
Sand flats
Sand flats

Lowest intertidal

Middle tide pools
Lower tide pools

Upper tide pools

Eocky bay shore
High tide rocks
High tide rocks
Pilings and boats
Intertidal rocks
Pilings and boats

Middle tide pool rocks
Upper tide pools
Kelp, sand, or mud
Sand above tide

Sandy beach among rocks

Under rocks, all levels
Eocky shore, above tide level

Lower tide pools

Lower tide pools under rocks
Middle tide pools under rocks
Lower tide pools
Upper tide pools
Middle tide pools of bays
Upper tide pools of bays
Lower tide pools of bays
Middle tide pools of bays
Middle tide pools of bays

MARINE ZOOLOG"?^

777

Typical Pacific Shore Animals — Cont'd

NAME

HABITAT ZONE

Pugettia gracilis, crab

Pugettia prodxhcta, kelp crab

Cancer oregonensis, round Oregon crab

Bandallia omata, round crab

Portv/ims zantusii, sand crab

Pachygrapsus orassipes, striped shore crab

Kemigraspus nudus, shore crab

Cancer antennarkis, edible rock crab

Ca/ncer productus, crab

Lophopanopeus leucomanus, crab

Speocarcvrvas calif orniensis, burrowing

crab
Uca crenulata, common fiddler

Araclmoidea

Pycnogonv/m stea/rnsi, pycnogonid (sea
spider)

Chordata

Hemichordata

DoUchoglossus pusillus, acorn tongue worm

UrocJwrdata

Styela stvtnpsoni, red tunicate
Ealocynthia nmostor, large tunicate
Perophora annectens, greenish tunicate
Glossophorum plarmm, lobed tunicate
Amaroucvwm califomicum, colonial tuni-
cate

CephaZochordnta

Branchiostoma calif omiense, Amphioxus

Lower tide
Lower tide
Lower tide
Lower tide
Kelp, sand
Upper tide
Middle tide
Lower tide
Lower tide
Lower tide
Mud flats,

pools

pools of bays
pools of bays
pools of bays

or mud
pools

pools

pools under rocks
pools under rocks
pools under rocks
burrow

Mud flats, burrow

Lower tide pools

Below tide

Rocky shores, low tide

Lower tide pools

Below tide

778

TEXTBOOK OF ZOOLOGY

Typical Animals From the Gulf of Mexico — Texas Coast*

NAME

HABITAT ZONE

Porifera

Clione celata, yellow-boring sponge
Coelenterata

Obelia hyalina, branched hydroid
Plumularia inermis, branched hydroid
Clytia naliformis, simple hydroid
Tubularia crocea, matted hydroid

Velella mutica, siphonophore (sp. not defi-
nitely determined)

Physalia pelagica, Portuguese-man-of-war

Porpita liwaaecma, (sp. identification not
yet confirmed)

Daatylometra quinqueoirrha, jellyfish

StomolopJms meleagris, cabbage-head jelly-
fish

Benilla remformis, sea pansy (sp. not defi-
nitely determined)

Ptilosarcus quadrcmgularis, sea pen (sp.
not definitely determined)

Stylatula elongata, sea pen (sp. not defi-
nitely determined)

Leptogorgia carolinensis, yellow gorgonid
coral

Astrofngia danae, common coral (sp. not
definitely determined)

Aurellia cmrita, clear jellyfish

Sea anemone (Classification not deter-
mined)

Chiropsalamus sp.

Ctenopliora

Beroe ovata, sea walnut or comb jelly
BoUnopsis vitrea (not definitely deter-
mined)
Mnemiopsis sp., small sea walnut

MoUuscoidea

Bugula turrita

Zobotryon pellucidum, clear bryozoan
(Another bryozoan of family
Valkeriidae encrusting sargassum ; classi-
fication undetermined)

Platyhielinintlies

Eustylochus meridianalis, flatworm

Annelida

Glycera abranchiata
Nereis Ivmbata, sandworm
Nerme mvrmta

Common on oysters

Attached to floating sargassum
Common on Sargassum
Attached to floating sargassum
Washed ashore, and taken in

shrimp nets
Floats, but may be cast up

Floats generally, cast up
Floats, may be cast ashore

Floating in bays and coves
Floating in gulf, bays, and

coves
Sand bottom, shallow water,

gulf
Bottom, deep water

Mud bottom, shallow water

Encrusting shells

Low tide sands, encrusting

small shells
Floating in gulf and bays
Attached

Floating in bays and bayous
Floating in bays and bayous

In gulf and bays on the bot-
tom

Among oysters and barnacle?

Oyster beds and beaches

♦Appreciation is expressed to Mr. Gordon Gunter, Marine Institute of Science,
University of Texas, and to Professor Elmer P. Cheatum, Southern Methodist
University, who have assisted in the revision of this section.

MARINE ZOOLOGY

779

Typical Animals From the Gulf of Mexico — Texas Coast — Cont'd

NAME

HABITAT ZONE

Euphole grubei

Lumhrinereis sp. (possibly young fiori-

dana), red sandworm
Capitellides teres

Paraxiothea (Olymenella) torguata
Polydora ciliata, burrowing worm

Mollusca

Gastrofoda

Tethys protea, sea hare
FleurophyUidm, nudibranch
Crepidula plana, flat slipper shell
Crepidula fo-rnicata, arched slipper shell
Littorina irrorata, periwinkle
Acmaea antiUarum, limpet
Sinum per sped ivmn, ear shell
EupJeura candata, oyster drill
Thnis haemostoma floridana. purple
PoUnices duplicata, duplicid moon shell

Buscycon pyrum, pear conch

Busycon can<iU<ntlatus, channeled whelk

Busycon (Fidgur) perversa, left-handed

whelk
Oliva scA/ana, lettered olive shell
Miirex spinicostata, spine-ribbed murex
Murex pomii/m, apple murex
Semic-assis gihha, helmet shell
Fasciolarm tulipa, tulip band shell
Fasciolarm gigantea, giant band shell
Architectonica granulata (Solarium), gran-
ulated sundial shell
Triton tigrinis, tiger triton

Between tide levels of sandj
beach

Common in ovster shells

Attached to shells and rocks
Attached to shells and rocks
Jetties
Jetties

Common in gulf
Jetties, oyster beds
Jetties, shallow water
Feeds on clams, etc., just un-
der the surface of the sand

Sandy ocean floor
Sandy ocean floor

Under surface on sandy beach

In gulf

Creep and swim on sandy floor

Pelecypoda

Area pexata, ark shell
Area incongrua
Area, ponderosa, ponderous ark
Area transversa, rhomboid ark
Plagiocteniun irradians, calico shell
Pecten ru)dosibs, knobbed scallop
Ostraea virgirdca, oyster
Pinna seminuda (atrinia serrata), half-
naked pen shell
Tellina tenera, small sunset shell
Dinocardhim rohustum, large cockle
Cardmm mii/ricattmi
Dosini-a discus, disk Dosinia
Modiolus di^smissus, horse mussel
Barnea costata, angel's wings
Chione cancellata, cross-barred vemts
Venu^ mercenarin, hard-shelled clam or

round clam
Venus campechiensis texanu

Donax roemeri, wedge shell Beach

Donax tumidn, wedge shell Beach

Scavengers in both shallow and
relatively deep water

Eocky crevices
Rocky crevices

Beach

Beach
Sandy beach

T 1

780

TEXTBOOK OP ZOOLOGY

Typical Animals From the Gulf of Mexico — Texas Coast — Cont'd

NAME

habitat zone

Donax variabilis, variable wedge shell
Solen viridis, green, razor clam
Brachiodontes hamatus (Mytilus), hooked

mussel
Brachiodontes recurvus (Mytilus)
S'lnsula similis, dish shell
Macrocallista nimbosa
Mulina sp.
Echinochama sp.

Martesia cuneivieris, boring clam
Anomia simplex, false oyster or jingle

shell
Teredo navalis, shipworm
Cephalopoda

Loligo brevipermis, squid

Loligo opalescens

Octopus (not identified), devil fish

Arthropoda, Crustacea

Stomatopoda

Chloridella empusa, mantis shrimp

Decapoda

Clibanarius vittatus, hermit crab
Pagurus longioarpus, hermit crab
Pagurv/S floridantis, hermit crab
Peneus setiferus, common shrimp
Penaeus astccus

Palaemonetes vulgaris, common prawn
Macrobrachium acnnthurus, tropical shrimp
Crangon armillatus, snapping shrimp or

pistol crab
Callinectes sapidus, blue crab

Porturms gibhesU, swimming crab
Ovalipes ocellatus, lady crab (probable)
Sesarma cinereum
Ocypode albicam^, sand crab

Panopeus herbstii, mud crab
Eurypanopeus depressus, mud crab
Metoporhaphis calcarata, spider crab
Libinia emarginata, spider crab
Arenaeus cribrarius, calico crab
Menippe mercrnaria, stone crab
Persephonn crinita, purse crab
Persephona punctata aqidlonaris
Calappa flammea
Hepatu^ ephelitious
Pinnixa chaetopterana
TJca minax, fiddler crab
Uca pugnax, fiddler crab
Uca pugilator, fiddler crab

Amphipoda

Orchestia platensis, beach hopper
Orchestia grillus, beach hopper
Caprella acutifrons

Beach

Brackish water

Beach

In oysters

On oyster reefs

"Wooden pilings

Muddy and gravelled shore

Muddy and gravelled shore
Shallow water

On muddy bottom
Brackish to fresh water

Near jetties; shallow water
and bayou

Sandy beaches

mark,

Back of high-water

sandy beaches
In oyster reefs

Muddy bottom and oyster beds
Gulf waters along shore
Gulf waters along shore
Gulf, shallow water
Oyster reefs
Gulf, along shores

Around worm tubes
Salt marshes, burrows
Salt marshes, burrows
Burrows in sandy or muddy
beaches

Under beach debris
Under beach debris
In algae growing on jetties

MARINE ZOOLOGY

781

Typical Animals From the Gulf of Mexico — Texas Coast— Cont'd

NAME

HABITAT ZONE

Grubia compta
By ale brevipes
Allorchestes sp. beach flea
Amphithoe sp. beach flea
Jassa sp.
Cirripedia
Lepas anatifera, goose barnacle

Balarms ebii/rneus, acorn barnacle
Isopoda

Aega psora, parasitic isopod
Ligyda exotica, common isopod

Echinodermata

Astropecten articularis, yellow starfish
Echinaster sp. violet starfish
Ophioderma sp. brittle star

Mellita qiimquiesperforata, keyhole urchin
Moira atropos, heart urchin (probable
identity)

Sea cucumber, sp., has not been iden-
tified

Chordata

Urochordata

Tunicate sp. (Has not been identified)

Elasmobranchii

Galeocerdo arciicnis, tiger shark
Carcharias platyodon, cub shark
Eeniceps (Sphyrna) tihuro, bonnethead or

shovelhead shark
Sphyrna tudes, hammerhead shark

Alopias vulpes, thresher shark
Carcha/rodon carchaHas, man-eater shark
Dasyatis sabina, stingaree
Dasyatis americana, southern sting ray
Pteroplatea miorura, butterfly ray
Manta birostris, great manta
Stoasodon narinari, whip or eagle ray
Narcine brasiliensis, electric ray
Rhinoptera bonasus, cow-nosed ray
Pristis pectinatus, sawfish
Pristis sp.

Scoliodon terrae-novae, sharp-nosed shark
Baja ackleyi, ray

Pisces

Atractosteus spatula, alligator gar
Tarpon atlanticus, tarpon
Elops satirus, ten pounder
Dorosoma cepediaimm, gizzard shad
Signalosa aJchafalayae
Galeichthys felis, sea catfish
Bagre marina, gaff-topsail catfish

Under beach debris (wrack)
Under beach debris (wrack)

On floating wrack, such as logs

and boards
Jetties and piles

On sharks

Jetties and beach debris

Off shore

Tide pools in jetties and off

shore
Sandy beach, low tide

Sandy beach, low tide
Specimen came from ship chan-
nel, Galveston

Off shore

Along shore and harbors

Along shore and harbors in

spring
Along shore and harbors in

spring

Off shore

In bays and off shore

Along shore and bays

In gulf

In oyster beds

Oyster and clam beds
Bays and open gulf
Bays and open gulf

Along shore

Bays

Open gulf

Bottom open water

Along shore

Bays and gulf

Along shore and shallow bays

782

TEXTBOOK OF ZOOLOGY

Typical Animals From the Gulf op Mexico — Texas Coast — Cont'd

NAME

HABITAT ZONE

Ogcocephalus radiatus, short-nosed batfisli
Chilomycfterus schoepfi, spiny boxfish
Trichiurus leptwrus, cutlass fish
Synodus foetens, lizard fish
Ancylopsetta quadrocellata, oceEated fluke
Symphurus plagiusa, tongue fish
Chaetodipterus faber, spadefish
Spheroides marmoratus, southern swellfish
Lagocephalus loevigattis, smooth swellfish
Sphyraena guachanco, barracuda
Trachinottis carolinus, common pompano
Mentici/rrhus Uttoralis, kingfish
Brevoortia patronus, menhaden
Lactophrys tricornis, cowfish
Pogonias crotnis, black drum
Stellifer lanceolatus, star drum
Cynoscion nebulosus, speckled trout
Cynoscion arenarius, sand trout
Cynoscion nothus, silver squeteague
Nautopaedium porosissimus, midshipman
Opsanus beta, toadfish
Astroscopus y-graecum, southern stargazer
Prionotus punctatus, spotted sea robin
Prionotus trihulus, southern sea robin
Anchoviella epsetus, striped anchovy
Hippocampus stylifer, sea horse
Syngnathus louisicmae, pipefish
Gohius hostatus, sharp-tailed goby
Gdbius lyricus, lyre goby
Micropogon undulatus, croaker
Conodon nohilis, barred grunt
Lepophidvwm hreviharbe, short-bearded

cusk eel
Remora remora, pilot fish
Sdaenops ocellata, redfish
LeiostoKmus xantfmrus, spot
Lutiarms campechanu^, red snapper
Lutianus grismis, gray snapper
ScomberomoTus maculatus, Spanish mack-

eral
Mugil cephalus, common mullet
Vomer setapinms, moonfish
Selene vomer, look-down
Bairdiella chrysura, silver perch
Poronotus triacanthus, butterfish
Chloroscombrus chrysurus, bumper
Citharichthys spiloptems, spot-finned whiff
Lobotes surinamensis, triple-tail
Centropomus undedmalis, pike, snook, or

roballo
Lagodon rhomboides, pinfish
Orthopristes chrysopterus, pigfish
Hypsoblennius ianthas, freckled blenny
Chasmodes bosqwianus, striped blenny

Shallow bays, sandy bottoms

Bottom, bays and off shore

Bays

Along shore

Shore waters

Open gulf

Along shore

Open gulf

Shrimp trawls

Deep water gulf

Deep water

Along shore and bays

Along shore

Open gulf

Off shore

Shallow water, among weeds

Sandy bottom

Bottom, along shore

Usually open gulf

Bottom, deep water

Along shore
Along shore

About docks, shallow water
Off shore, deep water

Along shore, bay, and gulf
Along shore, bay, and gulf

Along shores
Off shore

Along shore

Gulf

Gulf in shrimp trawls

Taken in shrimp trawls

In shrimp trawls
Oyster reefs
Oyster reefs

MARINE ZOOLOGY

783

Typical Animals From the Gulf of Mexico — Texas Coast — Cont'd

NAME

HABITAT ZONE

Gobiesox strumosus, cling fish
Menidia beryllina peninsulae

Trinectes maaulatus (Achirus fasciatus),

hog choker
Achirus lincatus, striped sole
Paralichthys lethostigmus, southern fluke

or "commercial flounder"
Efropus crossotua, fringed flounder
Polynemios octonemus, threadfin
Caranx hippos, common jack or Jiguagua
Caranx ruber, ship jack
Fundxilus similis, killifish
Fundulus heteroclitus grandis, killifish
Cyprinodon variegatu,s, broad killifish

Reptilia

Chelonia mydas, green sea turtle

Caretta kempil, bastard loggerhead turtle

Mammalia

Tursiops truncatus, porpoise, bottle-nosed

dolphin
TrichecJius vumatus latirostris, manatee,

very rare

Oyster reefs and pilings
Silversides, along shore and in

bays
Muddy or sandy bottom, shal-
low
Bottom in shallow water
Bottom in shallow water

Taken in shrimp trawls

Sandy shores

Off shore

Along shore

Shore waters and bays

Along shore, Gulf water
Open sea

Bays and along shore
Back bays and estuaries

CHAPTER XL

WILDLIFE CONSERVATION

(By Walter P. Taylor,* Senior Biologist, United States
Fish and Wildlife Service)

The Abundance of Wild Animals

* ' The manor of living nature is so ample that all may be allowed to
sport on it freely. The most jealous proprietor cannot entertain any
apprehension that the game will be diminished or even perceptibly
thinned." So wrote Dr. Richard Harlan, in his book, Fauna Ameri-
cana, published in Philadelphia in 1825, just 122 years ago.

What a panorama of abundant wildlife must have greeted the ex-
plorers and pioneers, east, west, north, and south, to inspire so uni-
versally the blind faith in the inexhaustibility of our natural re-
sources. This was a false faith that even yet has not been wholly
eradicated, although the evidence of pitifully small remnants of some
of our valuable wildlife species, and some of these still diminishing,
should teach us better.

It is true that in some of the unbroken forests, deer may not have
been so abundant as they are today in the same areas ; for deer and
similar wildlife like borders and developmental stages of vegetation
better than they do pure forest stands. Similarly, bobwhite quail may
have been less generously represented in the broom-sedge of the
southeastern United States than in the more varied environment of
the present. On the whole, however, taking account of variations in
numbers dependent on habitat, the generous abundance of the wild-
life resources encountered in those early days is difficult for us to con-
ceive. Let us examine some of the records.

Buffalo were widely distributed on the North American continent,
almost from the Atlantic to the Pacific, and almost from the Arctic
barrens to the Gulf of Mexico. Their numbers were estimated at not
less than sixty or seventy million individuals. The American bison,
as they grazed the plains country or migrated with the seasons, must
have been one of the outstanding wildlife phenomena of all time.

♦In charge, Texas Cooperative Wildlife Research LTnit. (The Agricultural and
Mechanical College of Texas ; the Texas Game, Fish, and Oyster Commission ; the
American Wildlife Institute ; and the Fish and Wildlife Service, U. S. Department
of the Interior, cooperating.)

784

WILDLIFE CONSERVATION

785

It is well known that the pronghorned antelope is found on the
continent of North America and nowhere else in the world. This crea-
ture, combining as it does some of the characters of the cow and the
deer, is unique. It is the only member of its species, genus, and fam-
ily. While North America shares such animals as the bear, deer, moose,

.ts^

Fig. 410. — Prong;horn antelope, Antilocapra aniericana. A strictly American ani-
mal of the open plains and foothills.

elk, and beaver with the Old World, the pronghorn is peculiarly our
own. Like the buffalo, the antelope was found very widely over the
plains, although its range was not so extensive as that of the buffalo,
and by some authorities it is estimated to have been equally abundant.

In order to obtain a hint of the number of passenger pigeons in the
early part of the nineteenth century let us examine a calculation of

786

TEXTBOOK OF ZOOLOGY

pigeon numbers made by John James Audubon, the celebrated early
American artist and ornithologist. He estimated a total of nearly
five billion birds in the three states of Kentucky, Ohio, and Indiana
along the Ohio River in one year's flight. Old-timers still living in
eastern Texas report passenger pigeons as still numerous in the
1870 's in parts of that State. So abundant were these birds over
their original range that when, after a long period of exploitation,

Fig. 411. — Passenger pigeon, Ectopistes migratorius. This bird was once extremely
abundant but is now extinct. (Courtesy of General Biological Supply House.)

the inevitable happened, and the species disappeared, many and
varied were the explanations offered. But there is really no mystery
at all. Carloads of birds were shipped direct to the big city markets
for weeks during the nesting season. If there is any mystery, it is
that the birds persisted as long as they did.

The deer family, including some of the most magnificent animals in
the world — caribou, moose, elk, mule deer, black-tailed deer, and the
Virginia deer — has fortunately, with few exceptions, proved to be

WILDLIFE CONSERVATION 787

tough and tenacious. Caribou may still be observed in large numbers
in the ]\Iount McKinley region and in other suitable parts of Alaska.
Moose, although they have been extirpated from New York, Pennsyl-
vania, and some other states, still persist over much of their former
range, a remarkable fact when the size and character of the animal
are considered. 'Elk, or wapiti, now reduced to a few favorable locali-
ties in the West, were originally of general distribution nearly
throughout the country. True deer of the several varieties mentioned
above have persisted over much of their former ranges, at least wher-
ever habitat conditions coupled wnth an interest in deer conservation
on the part of intelligent landowners have made such persistence pos-
sible. Although in a few places, as in parts of Pennsylvania, deer may
be more abundant now than formerly, as a result primarily of man-
made openings in the forests, in most instances deer have decreased.
Strecker points out that between 1845 and 1853 no fewer than 75,000
deer hides were marketed by a single firm dealing in furs and hides
and operating out of Trading House Creek near Waco, Texas. At
present deer are scarce in this general region. This is a striking ex-
ample of what has happened throughout the country.

The collared peccary is the only North American wild pig. For-
merly found widely throughout southwestern Texas, and still occurring
over a considerable area in that state as well as in southern Arizona,
New Mexico, and old Mexico, the peccary has been forced back into a
relatively restricted range. This has been given legal protection
recently as a game animal in Texas.

The former range of the beaver, one of our most valuable rodents,
was almost coextensive with the outlines of North America, excepting
only in the extreme North, the ultra-arid region, and parts of the low-
lying South and Southeast. Its numbers must have been correspond-
ingly great. It is said that some of Canada's great modern cities are
founded on the sites of former beaver colonies. The experience of Wis-
consin, Michigan, New York, and Pennsylvania, however, indicates the
entire practicability of the restoration of this highly useful animal
over vast stretches of its former domain. The desirability of such
restoration is increasingly appreciated as man recognizes the value of
the beaver as a soil conservationist ; for this most interesting rodent is
the original check-dam engineer. The beaver was in the business of
soil-building, soil conservation, and flood prevention a hundred thou-
sand years ago.

788

TEXTBOOK OP ZOOLOGY

Along with the beaver, certain other fur animals, among them the
mink, raccoon, marten, fisher, otter, wolverine, skunk, opossum, lynx,
muskrat, badger, and fox, have for long been sought for their furs.
Some, as the otter and fisher, were formerly present in far greater
numbers than they are now. All, save possibly the opossum,
muskrat, and fox in certain areas, are much reduced. From a total
of $500,000,000 in 1929, the retail turnover in the fur business of the
United States declined to $200,000,000 in 1935, owing partly to cur-
tailment of business and deflation of values and partly to actual re-
duction in the numbers of wild fur animals. In Texas, where a fur-

Fig-. 412. — Raccoon, Procyon lotor, a valuable fur-bearing animal.

tagging law permitted an accurate check to be made, there was an
actual decline in the number of furs taken from 1,407,884 in 1932-1933
to 912,276 in 1934-1935. This means a 35 per cent decrease— 500,000
pelts, worth $325,000 — in three years. This same decline has been
noticed over the entire country.

Ducks, geese, and swans have been favorite objects of sport from
the earliest days of our Republic, and formerly were present in such
numbers that little concern was felt, although spring shooting was the
rule in many localities and the birds were freely sold in the markets.
During the past few years, however, difficulties in securing good
hunting have been so obvious that extraordinary efforts have had
to be exerted to save these popular game birds. During 1938 and
since, the ruddy, wood duck, canvasback, redhead, and bufflehead are

WILDLIFE CONSERVATION 789

given complete protection under the Federal Migratory Bird Act.
The season, bag limit, and manner of hunting of all the other species
are rigorouslj^ regulated.

We can only guess at the numbers attained by some of the smaller
animals, but it is certain that, in many instances, they were very large
indeed. In some parts of the country the squirrels are important ob-
jects of sport. They function also as tree planters and as possible
balance wheels on the increase of certain other forms of wildlife. Bas-
ing his figures on the known abundance of the gray squirrel in some
localities and on the extent of its geographic range, Seton estimated
that the gray squirrel population may easily have numbered several
billions in eastern North America at the beginning of the nineteenth
century.

Bailey, of the United States Biological Survey, in 1905 made a study
of the prairie dog (Fig. 335) in western Texas. Usually, as he pointed
out, the animals were found in scattered colonies, "... but over an ex-
tensive area lying just east of the 'Staked Plains' they cover the whole
country in an almost continuous and thickly inhabited dog town, ex-
tending from San Angelo north to Clarendon in a strip approximately
100 miles wade by 250 miles long," a 25,000 square mile colony con-
taining 400,000,000 prairie dogs. Their numbers have been extremely
depleted by poisoning.

The whales of the sea were formerly numerous enough to support
an extensive industry maintained by nationals representing various
maritime countries. The American share in this business centered in
the village of New Bedford, Massachusetts, and reached large propor-
tions. Like most other wildlife resources, the whales were subjected
to unreasonable slaughter ; indeed, the history of the whaling indus-
try as a whole has been a rather sad story of declining possibilities
and steadily more restricted opportunities, owing to the remorseless
pursuit and crassly unbusinesslike exploitation to which these greatest
of living creatures have been subjected by man.

The Natural Range of "Wild Animals

As indicated in the foregoing paragraphs, the natural range of
many game animals of great potential value was formerly much more
extensive than it is at present. The buffalo which ranged so widely
over the continent is now virtually extinct in a wild state. The prong-

790 TEXTBOOK OF ZOOLOGY

horned antelope, considered by Seton to have occupied a primitive
range of approximately 2,000,000 square miles, or 1,280,000,000 acres,
is now represented by scattered remnants only.

The wild turkey, a strictly North American product, once occurred
generally throughout the wooded sections of the eastern, southern,
and southwestern United States, as well as in Mexico. Now it is gone
from many of its former homes and its future is decidedly uncertain
unless there is marked change in its treatment by man.

Conservationists are becoming much concerned about the prairie
chickens. Formerly represented in the Northeast by the now
extinct heath hen, and at present in the northern Great Plains
area and the Middle West by the greater prairie chicken, on
the southern plains by the lesser species, and on the Gulf Coast by
the Attwater prairie chicken, these birds are worthy of real attention.
Handsome and grouselike birds, weighing up to one and a half pounds
and exhibiting some of the most interesting courting habits known
among American game birds, the loss of any of them is a real calamity.
The heath hen has become extinct in recent years. The greater and
lesser prairie chickens are much reduced. They have been so crowded
out by field-crop agriculture and cut down by drought, overgrazing,
and too much shooting, that their status is not encouraging in any
part of their range. The Attwater prairie chicken, which formerly
occurred on the Gulf coastal prairie from Louisiana westward into
Texas and south to a point beyond Corpus Christi, has been all but
eliminated. It is gone from more than 90 per cent of its former range,
and is probably in worse condition than any other of the still existing
forms.

The jaguar, el tigre of the Mexicans, really an American leopard
and justly entitled to protection as an object of sport wherever it would
not interfere too much with livestock, formerly ranged north in
Arizona occasionally to the Grand Canyon of the Colorado, and was
found widely throughout Texas. Of recent years there are few au-
thentic records of its occurrence in the United States, though the
species is seemingly quite abundant still in parts of Mexico. This is
likewise true of the ocelot or leopard cat. Now within the United
States it is restricted to a limited locality in the brush country of
southern Texas. Likewise, the puma is becoming greatly reduced
and restricted.

WILDLIFE CONSERVATION 791

One last instance, the grizzly bear, noblest of American flesh-eaters,
was formerly present widely over the western United States, as well
as in Alaska and the extreme northern parts of North America, and
in northern Mexico. Merriam's studies show what a wealth of forms
were represented. Unlike the black bear, which is among the shyest
of animals, the grizzly was courageous and much more inclined than
the black to dispute man 's encroachment on his hereditary range. In
this way poor old Ursus liorrihilis (the name itself has a majestic
sound) proved to be not so well adapted to modern conditions as the
black bear. Now all the grizzlies are gone from the United States, ex-
cept in a few scattered localities in the West, where they maintain a
precarious existence under special protection or other unusual condi-
tions. Fortunately, there are a good many of these splendid animals
still remaining in Canada and Alaska.

Detailed consideration of our valuable wildlife shows a discourag-
ing decrease in almost every instance during the three hundred years
or more of our occupation of North America. However, there are
a few cases in which the natural range of wild animals has been
increased as a result of man's activities. Everyone is familiar with
the English sparrow and the starling in America, the muskrat in
England and on the continent of Europe, the rabbit in Australia, the
house mouse and the Norway rat generally wherever civilized man is
found. In most cases, unfortunately, these increases result from
man's unintelligent meddling with natural conditions, and result in
much damage to his interests. The Norway rat is undoubtedly the
most dangerous and costly mammalian enemy of mankind in the
whole world.

In a few places, favorable ecological conditions combined with pro-
tection by man or a reduction in the number of natural enemies have
resulted in an abnormal increase in numbers of game species ordi-
narily considered valuable, and sometimes developed detrimental sur-
pluses. Protection of deer on certain royal forests in England and on
the continent of Europe has sometimes seriously interfered with the
normal growth of the trees and other vegetation. On the Kaibab
Plateau north of 'the Crrand Canyon of the Colorado River, in the state
of Arizona, the deer, numbering normally perhaps 3,000 or 4,000
individuals, increased to a number estimated at 30,000 to 100,000 ani-
mals. The future of the deer's own food supply, and indeed of the
Kaibab Forest, was threatened. There is no more reason for op-

792 TEXTBOOK OF ZOOLOGY

posing an open season on does or "antlerless" deer in such cases
than for opposmg the marketing of breeding cows on an over-
stocked livestock range.

The Coming of Civilization and a Declaration of Indefensibles

The story of the development of America is full of meaning as it
bears on our natural resources. Man's inventive genius, coupled with
his occupation of the land, has ''modified the earth," to use Marsh's
expressive statement, and has helped, in all too many instances, to
make his surroundings less favorable for his own future.

Below are reviewed some of the things for which civilized man is
responsible. Here is a sort of Declaration of Indefensibles:

Man has cut and burned the trees of the forest, in many instances
leaving the affected area so poor that it cannot recover without costly
plantings.

He has precipitately drained the marshes for his agriculture or for
the alleged protection of his health, sometimes wisely, often waste-
fully. Not seldom have drainage enterprises turned out to be of little
value for agriculture or for mosquito control, but they nearly always
eliminate much valuable wildlife (waterfowl, fishes, fur animals, and
a host of other creatures).

With his roads man has penetrated nearly every remaining fastness.
The wild game has few remaining refuges. Except for a few preserves,
the man with the gun can ride nearly anywhere and enjoy a "success-
ful" hunt mth a minimum of physical exertion or mental stimulus.

Man has polluted the streams and lakes of America, and even the
waters of the sea itself. Salt water from oil wells, chemical wastes
from industry, and raw sewage from towns and great cities are poured
into our great drainage ways. Formerly clear and sparkling, with
an abundance of fish, what are they now 1 All too often merely nause-
ating reminders of what once was a splendid past.

Man has plowed much land that should have been kept for livestock
and wild game. He has grazed a great deal more land that should
have been reserved for wildlife. He has planted his field crops in such
a manner that erosion of the soil, mother resource of all the rest, is
accelerated often to the point of destruction. The results of all this
land mismanagement are becoming annually more obvious, through
the increase in number and volume of floods, the augmented turbidity

WILDLIFE CONSERVATION 793

and even opacity of streamways that should be clear as crystal, the
destruction of valuable fish resources, the silting and choking up of
reservoirs and of irrigation works, the lowering of water tables, the
drying up of springs, the increase in black blizzards of dust, and the
abandonment by settlers of homes and lands.

Man has unnecessarily and unintelligently reduced the food and
cover for wildlife. He seems not to have realized that every time he
cuts a needed tree or a bush, every time he cleans a fence row or
grazes the underbrush out of a woodlot, he thereby removes the life-
giving food and essential cover of many living creatures — insects,
mammals, birds, and game. It is of little use to liberate fish in streams
if no fish food is there.

Man has perfected his mechanical devices so that all creatures are
threatened: guns, stronger, better, more of them; more efficient am-
munition ; automobiles, comfortable, speedy, easily capable of taking
the sportsman five hundred miles in a single day to the place where
good shooting or fishing is reported; airplanes that still further ac-
celerate the process; traps and scents that lessen the chances for
unwary fur animals to escape; huge juggernauts, called tree-dozers,
to remove the jungles which in some cases afford the only remaining
protection to some of our most interesting forms of wildlife !

Man has thoughtlessly removed the dead trees and brush which are
so necessary to such forms as woodpeckers, owls, and raccoons, as well
as to game; he has "cleaned up" the native plants on which the bob-
whites principally feed at certain times of the year; he has cam-
paigned against the marsh hawks and other hawks and owls that must
be relied on to help keep the snakes, rodents, and insects in a proper
balance ; he has at times eliminated rodents that are the necessary food
of fur animals and valuable raptores; he has fought the predators,
sometimes quite unnecessarily, seeming not to realize that in them-
selves the predators hold possibilities, on certain areas, of developing
into as high class objects of sport as can be found anywhere, as well
as being one element in a balanced wildlife program.

Man has devleoped an agricultural system so efficient that a seem-
ing overproduction is one of its acute problems. He has built rail-
roads, bridges, cities, and mechanical devices of all sorts that enable
the well-to-do to live on a scale of luxury scarcely imaginable in other
days. In short, he has developed the mightiest mechanical civiliza-
tion of all time.

794 TEXTBOOK OF ZOOLOGY

But what does this civilization rest on ? The answer is clear : First,
on the renewable natural resources — the soil, the waters, the forage
growing on the open range, the trees of the forest, and the wildlife;
and second, on the nonrenewable natural resources — oil, sulphur,
stone, and minerals. Wildlife conservation is but one aspect of the
larger world problem of conservation, on the solution of which de-
pends the entire future of the human race.

The Problem of Restoration

A rather discouraging outlook, it seems, and so it is, but restoration
is by no means hopeless. Some farmers are encountered who have de-
cided to get their share of game while the getting is good, and sci-
entists, who have almost turned defeatist in the fight for conserva-
tion, have concluded that we are living at the close of the "Age of
Mammals," and that our interesting mammals and birls — and in all
probability we ourselves — will shortly pass out of the picture. Per-
haps so, but, in my opinion, there is no justification for such a view.

It is the considered opinion of the writer that if the American peo-
ple want fish in their streams, bobwhite quail in their coverts, deer
and wild turkey in their forests, song birds and insectivorous species
about their farms and homes, they can have them in generous abun-
dance. This opinion is based on some concrete instances in which actual
results constitute unmistakable demonstrations of the possibilities.
Three of these demonstrations will now be considered.

1. Bobwhite Quail.- — The painstaking scientific research work of
H. L. Stoddard and C. 0. Handley in southern Georgia and northern
Florida has resulted in an encouraging increase in the bobwhite popu-
lation on the great plantations of the region. Here is a positive demon-
stration on a grand scale of the possibility of substantially increased
production of bobwhites throughout the vast southern pine forest area,
extending over parts of at least fifteen southern states.

2. Big Game in the National Forests. — During the past several
years, under the enlightened management policies of the Federal
Forest Service, big game is reported to have increased 140 per cent
in the national forests. Nor has this increase entailed a rigorous pro-
gram of absolute protection, except in unusual cases, but rather, true
conservation through wise use. Here, then, we have a second demon-
stration, on a series of forests and ranges that embrace more than

WILDLIFE CONSERVATION 795

180,000,000 acres of well-handled public land, of the possibility of
saving our big game, whenever we wish to do it.

3. Deer and Wild Turkey. — It is well known that deer and wild
turkey are more abundant in the Edwards Plateau country in Texas
than in many similar places, and that the receipts of many ranch
owners from hunting leases are important ranch assets in the region.
It will be remembered that the county agents of six Edwards Plateau
counties in Texas estimated the income derived from outside hunters
entering these counties to hunt at not less than a million dollars in a
single season (1935-1936).

The Edwards Plateau situation, like the others, convincingly indi-
cates the entire practicability of building up the deer and turkey pop-
ulation over a vast section of the Southwest which at present is char-
acterized by a shortage or even absence of these excellent game ani-
mals.

There are some rather simple measures, however, that wdll have to
be adopted if American wildlife is to be properly maintained and in-
creased. The willingness and ability of the public to do these things
will be a measure of their real desire for game. Here are some of the
essentials :

1. Support a strong educational program, including research, teach-
ing, and extension, so that the present and future generations will
have the facts, and will be inspired to act in the light of them.

2. Take politics out of the state game departments in all instances,
so that merit and good behavior will be the sole criteria for service in
conservation administration.

3. Give the responsible state agency full powers and responsibilities
to set seasons and bag limits, and to make other regulations regarding
the taking of game and the conservation of wildlife, subject to legisla-
tive approval.

4. Give the game department money enough to work with. The
easiest way to do this, at present, is to adopt the so-called universal
hunting license law, which, while it means a slightly increased cost of
hunting, will afford the sportsman a chance to get something when
he goes afield.

5. Provide food and cover for wildlife. For the most part this can
best be done, not by an expensive planting program, but by protect-
ing the native vegetation from overgrazing, and letting Nature her-

796 TEXTBOOK OF ZOOLOGY

self do the job. Fencing off the heads of gulleys €0 that valuable
plants will have a chance. Eliminate overgrazing. If the vegetation
is removed to get rid of the boll weevil or other insects, numerous
song and insectivorous birds will be eliminated at the same time, and
the game will go, too. Earlier ideals of ultra clean farming might
well be replaced by a better balanced program according to the best
information available at present.

6. Know your wildlife. Count your quail, turkeys, and deer, and
so regulate the take that the seed stock will not be drawn upon. If
you are a landowner, join with your neighbors to form a game pre-
serve demonstration association in cooperation with the Extension
Service at your nearest agricultural and mechanical college.

7. "Work out a cooperative agreement between farmers and sports-
men, such that the interests of both will be more adequately safe-
guarded. The farmer wdll have to be equitably reimbursed for his
game, the sportsman assured of better hunting.

8. Keep in touch with your game department, with your agricul-
tural and mechanical college, with the United States Fish and Wild-
life Service, and practice the best modern methods of game manage-
ment. The game wardens, county agents, and teachers of vocational
agriculture will help you if you call on them.

9. Do not forget the fur animals. As rapidly as practicable, work
out a plan whereby trapping can be permitted on a basis that will
avoid depletion of the stock, but will pay an appreciable return to
the landowners as well as the trapper himself.

10. Wildlife restoration, in the opinion of the most enlightened
and advanced wildlife managers, is not so much a matter of expen-
sive plantings as of letting Nature grow her own vegetation ; it is less
a matter of propagation in captivity, and more a matter of encour-
aging maximum natural production of game; it is less a matter of
introducing exotics or stocks of game from somewhere else, and more
a matter of making conditions favorable for wildlife already on the
ground. Restoration is not only possible, but is highly desirable. It
should supplement the existing income of the landowner, and pro-
mote the utilization of lands not now used to best advantage. It
should result in the substantial increase of a resource already esti-
mated to be worth more than a billion dollars to the United States
each year. It should add to the store of those natural treasures that,

WILDLIFE CONSERVATION 797

all money considerations aside, make life more interesting, more
worth living.

11. Make full use of the natural crops on wild land — forest land,
marshland, bottom land, range land. Perhaps such areas will pro-
duce more actual income through the natural crops: fur animals,
fishes, lumber (harvested on a sustained yield basis), stovewood,
wildlife, recreation, water supply, a moderate amount of grazing, and
natural woods' products (nuts, berries, etc.), than they would if
made over into farms. Much land is not well suited to field crops or
grains.

i :

CHAPTER XLI

COMPARATIVE EMBRYOLOGY

(By a. Richards, University of Oklahoma)

Embryological knowledge has been a common property of mankind
since before the period of recorded history, and Aristotle embodied
with surprising accuracy in his extensive treatises on animal life a
great many facts in regard to the development of animals. Because
of the nature of embryos, however, it is obvious that there was a very
definite limit to any except the most superficial knowledge until the
development of the microscope early in the seventeenth century
opened the way for scientific embryology. The first extensive
study of the embryo was made by William Harvey and was pub-
lished in his work, Generation of Animals, in 1651. He was fol-
lowed by Malpighi, who published De formatione pulle in ova, in
1672. These investigators studied especially the chick embryo, but
Malpighi 's account includes also various invertebrates. In the mid-
dle of the eighteenth century came the great question of preforma-
tion versus epigenesis, with which the names of Wolff, Haller, and
Bonnet are connected. Workers later came to see that neither the
egg nor the sperm contains a perfectly formed small embryo, and
that development is a gradual growth from small beginnings. They
realized also that Wolff's claims that the germs develop from homo-
geneous material do not fully represent facts, although his concepts
were better than those of his opponents. To the student of present-
day embryology these difficulties are seen to be concerned with the
nature of organization of the embryo. The earlier investigators were
trying to understand the manner in which the mechanism of life
begins to operate, as well as the steps by which the structure of the
animal is produced. The next step came in 1828 when von Baer pub-
lished his germ-layer doctrine, according to which all the parts of an
embryo develop from three sheets of tissue, and the organisms are
formed by outpocketings, foldings, thickenings, and other mechanical
devices applied to these three sheets of tissue. This theory is said
to have established embryology as a science. It was a very
stimulating generalization and contains much of truth; yet it

798

COMPARATIVE EMBRYOLOGY 799

would probably not have had quite the place in the history of embry-
ology that it does if the cell doctrine had not developed before it.
The cell doctrine, coming in 1838-1839, made it possible to carry
knowledge of the organization of the embryo back beyond the stages
of germ-layer formation and it gave the basis from which an under-
standing of the formation of tissues and organs could proceed. The
ceU theory, the improvements on the microscope, and the develop-
ment of new methods of fixing, sectioning, and staining material made
possible the accumulation of a great deal of information about the
origin and the details of development of the embryos of all kinds of
animals, and this was a special contribution of the last third of the
nineteenth century. The twentieth century has extended this body
of knowledge and interpreted it upon the basis of extensive experi-
mentation.

For convenience, the developmental history of an animal may be
divided into stages, but the student must remember that this is an
arbitrary action on our part, and that the development from the ani-
mal's standpoint is a continuous process. It must be borne in mind
that ontogeny, which means the entire life history of an organism
from the earliest beginnings to old age and death, is a continuous
and ever-changing set of processes, and the sum of the characteristics
of all the stages of development makes up the characteristics of the
animal. The hen's egg is as much a representative of the genus and
species, Gallus domesticus, as is the feathered animal to which most
of us attach that name.

Embryology really begins with the formation of the germ cells in
the bodies of the parents, and it is known in some forms that the
prospective germ cells are set aside very early in the development of
the organism, even in the cleavage stages, and that they may remain
latent until the organism has become independent in its existence.
The development of the germ cells, or gametes, through the active
stages of gametogenesis (Fig. 47) involves the multiplication period
during which they are known as "gonia" {oogonia and spermato-
go7iia), the growth period, the maturation period proper, that is, the
period of the "cytes" (oocytes and spermatocytes), ending with the
fully formed or mature oogonia and spermatogonia. These gametes
are now ready for the process of fertilization, which takes the form of
two steps, the first the initiation of development on the part of the
egg following the entrance of the sperm ; and, second, the union of

800

TEXTBOOK OP ZOOLOGY

the egg and sperm pronuclei, by means of wliich the chromosomal com-
plex of the egg is restored to the diploid condition and the hereditary
mechanism is enabled to function. The last period is that of cleavage,
during which the egg segments into many cells and the egg substances

Fig. 413. — Fertilization in the sea urchin, Toxopneustes. S, mature spermato-
zoon ; m, transformation of spermatozoon into male pronucleus ; /, female pronu-
cleus. (Reprinted by permission from Outline of Comparative Embryology by
Richards, published by John Wiley and Sons, Inc., after Wilson.)

are distributed into the appropriate cells, so that differentiation and
subsequent division of labor can take place. The formation of germ
layers follows the organization of the Uastomeres into the three sheets
of tissue whose further development leads to the formation of the

COMPAKATIVE EMBRYOLOGY 801

organs. The period of organ formation is succeeded by a period of
histological differentiation, the result of which is a fully formed but
small organism. Its subsequent groAvth is accompanied by changes
which are spoken of as metamorphosis, leading to the adult animal.
Summarizing the stages in the embryological history of an organism,
we find : gametogenesis, fertilization, cleavage, germ layer formation,
organ formation or organogenesis (Fig. 110), histological differentia-
tion or histogenesis. The ontogenetic cycle may be completed if we
carry the history of development on through adolescence, adulthood,
and old age.

The first problem to which the developing embryo must be adapted
is that of a store of food to enable it to grow until it can obtain food
for itself. At the same time it must be protected from the adversities
of the environment; these are of chemical and physical nature and
also include other organisms which would prey upon it for food. The
first of these problems is solved by the storage of a sufficient amount
of yolk (Figs. 287 and 288) in the maturing egg to carry the embryo
through its period of helplessness. The duration of this period de-
pends upon the habitats in which the embrj^os develop, and hence there
are various kinds of yolk with respect to form and deposition in the
egg.

To solve the second problem, that of protection, many devices are
to be seen throughout the animal kingdom. Conspicuous among them
are the types of membrane, including shells and horny structures
which protect embryos from physical harm and regulate the chem-
ical exchanges with the surrounding medium. There are also many
delicate adjustments, as for example, the oil droplets which cause some
eggs to have a lighter specific gravity and thus to float at the surface
of the water w^here certain forms of animals cannot so readily destroy
them. Egg membranes are usually classified into three groups ac-
cording to their origin. Those formed from the egg itself are spoken
of as "primary membranes," and of them the vitelline is an exam-
ple. Those formed from the follicular cells are "secondary mem-
branes," and of these the true chorion is an example, and finally
those formed from the wall of the oviduct or uterus are "tertiary
membranes," of Avhich the albumen and shell of the bird egg are
examples.

The amount and kind of distribution of yolk are to no small extent
responsible for the types of cleavage found in the animal kingdom.
Without explaining this relation in detail, we may say that there are

802 TEXTBOOK OF ZOOLOGY

six types of cleavage, as given below. In general, according to the
amount of yolk present, eggs are spoken of as of three kinds. Iso-
lecithal or homolecitJial eggs are those in which only a small amount
of yolk is present and that uniformly distributed throughout the
cytoplasm. Telolecithal eggs are those in which the vegetative half
of the egg contains most of the yolk (the vegetative pole is opposite
the animal pole, where the polar bodies are given off). In some telo-
lecithal eggs the amount of yolk is not very large, and it is in the
form of distinct globules; in others, it is concentrated into a dense
mass in the form of a large yolk spherule, which is not penetrated by
the cytoplasm and therefore cannot be divided by it when the ceU
undergoes cleavage. The third characteristic mode of division is that
in centrolecithal eggs in which the entire egg is well filled with yolk
at the beginning of cleavage, with a nucleus surrounded by cytoplasm
forming an island in the center of the egg. Successive divisions
carry the plasma islands to the surface and leave the yolk as a cen-
trally located mass.

Cleavage is therefore divided first of all into two kinds, total and
partial, depending on whether or not the cytoplasm is able to divide
the yolk completely. Eggs having total cleavage are spoken of as
holoblastic; those having partial cleavage, as merohlastic. In each of
these subdivisions the cleavage patterns take different forms, and
therefore allow us to recognize six distinct modes by which the sepa-
ration of egg substances into definite areas is accomplished; thus the
different plans of embryonic organization are established. These six
types are radial, disynmietrical, bilateral, and spiral, which occur in
holoblastic eggs ; and superficial and discoidal, in meroblastic eggs.

Radial cleavage (Fig. 415) occurs in eggs which are not heavily
yolk-laden, and hence is marked by a high degree of regularity and
symmetry. Eggs of the echinoderms are commonly chosen to illustrate
this type, which also occurs in the Porifera and Coelenterata. The
first cleavage plane cuts the egg into half from the animal to the
vegetal pole, and may pass through any diameter of the egg. The sec-
ond cleavage plane again cuts through the poles at right angles to the
first. The third cuts the egg somewhere near the equatorial region; it is
therefore best to speak of it as latitudinal. The sixteen-cell stage is
reached by a meridional cleavage, and meridional and latitudinal
cleavages alternate until there is developed a well-formed blastula, the
coeloblastula (see below).

COMPARATIVE EMBRYOLOGY

803

FiE- 414 —Superficial cleava&e in the centrolccithal egg of tlie bristle-tail Insect,
Campodik staphylinns. In A the fertilized egg appears as a nucleus surrounded
bv cytoplasm forming an island in the center of a yolk mass. B, C, ii.ii, stages
of cleavage In F the thickening on the ventral side will give rise to the blasto-
derm (Reprinted by permission from Outline of Comparative Embryology by
Richards, published by John Wiley and Sons, Inc., after Uzel.)

804

TEXTBOOK OF ZOOLOGY

Disymmetrical cleavage is found in only one group of animals, the
Ctenophora, and is so specialized that it may be omitted from the
discussion here.

In eggs having bilateral cleavager a degree of organization of the
egg substances is present only in the single-celled condition, a fact
which is discoverable in various ways in different kinds of eggs be-
longing here. The restriction of yolk to certain portions of the egg,-
pigmentation in the outer layers, or granular cytoplasm as distin-
guished from clear areas may be the indications of bilaterality in

Fig. 415. — Comparison of radial and spiral cleavage. A, radial type; B, spiral
type in third cleavage; C and D, radial and spiral types respectively in fourth
cleavage. (Reprinted by perinission from Outline of Comparative Emhryology by
Richards, published by John Wiley and Sons, Inc., after Korschelt and Heider.)

the egg even before the first cleavage plan appears. There can only
be one plane which will divide the egg into tAvo equal halves in exam-
ples of this kind and, as a result, the two blastomeres contain
approximately equal amounts of all the egg su])stances. Again the
second plane is at right angles to the first and the egg is divided into
quarters. In some animals the order of sequence of these two planes
is interchanged, but in the four-cell stage of all bilateral eggs two of
the cells must belong to the right and two to the left half of the

COMPARATIVE EMBRYOLOGY

805

future animal. Bilateral cleavage is fairly widespread throughout
the animal kingdom, occurring in the Nematoda, Tunicata, amphioxus,
and in all the Vertebrata which have holoblastic cleavage ; that is, all

^s-

bid.

GASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK— AMPHIOXUS.

PASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE
AMOUNT OF YOLK— AMPHIBIA.

GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE-
AMOUNT OF YOLK— BIRDS.

Fig. 416. — Gasti-ulation in three types of embryos. Typical discoidal blastula
formation (left) and gastrulatlon (riglit) in the telolecithal egg of the bird, below.
blc, blastocoele ; bid., blastoderm ; blp., blastopore ; ect., ectoderm ; ent., endoderm ;
riiit., cells undergoing mitosis. (From Patten, Embryology of the Chick, published
by permission of The Blakiston Co.)

except the selachians, teleosts, reptiles, and birds, in which the cleav-
age is meroblastic, and the mammals, which are so specialized as not

806 TEXTBOOK OF ZOOLOGY

to permit a clear-cut designation of the types of cleavage. Bilateral
cleavage is greatly influenced by the amount of yolk, and irregu-
larities in the cleavage pattern develop much earlier than in the radial
type. The degree of organization present in the egg at the beginning
indicates earlier differentiation, both physiological and morpholog-
ical, and embryos of this type are correspondingly reduced in their
power of regulatory adjustment.

In spiral (Fig. 415) cleavage the tendency toward earlier organiza-
tion of the egg is carried to a further extent, and consequently these
eggs are among the most extreme of the types spoken of as determina-
tive; that is, they have a very much reduced power of regulation and a
much more precise sequence of stages leading to the formation of the
embryonic body. This type of cleavage occurs in the annelids and
mollusks, and in some smaller groups, for example, the subclass, Poly-
cladida, of the flatworms.

Eggs of the meroblastic type contain so much yolk that their cleav-
age is quite modified from the more simple ones, previously men-
tioned, of the holoblastic type. There are two general kinds, discoidal
and superficial. In the discoidal eggs (Fig. 416) the yolk is in the
form of a single, fairly large and fairly dense body, surrounded l)y a
very thin cytoplasmic film which is totally unable to divide the entire
mass. It is possible to derive this type of cleavage from the extremely
telolecithal eggs of the bilateral sort by considering the amount of yolk
to increase and concentrate and the amount of cytoplasm to decrease
in proportion to the egg. Upon fertilization the cytoplasm accu-
mulates on one side of the egg to form a small disc-shaped or lens-
shaped mass. It is only this discoidal portion of the cytoplasm that
undergoes cleavage; hence the term applied to it. It divides suc-
cessively into two, four, eight cells, all lying in one plane, but at the
fourth or fifth cleavage some cells are cut off interiorly and there
arises a disc-shaped mass, several cells in thickness. With continued
cell division the discohlastula is produced. After gastrulation and
the formation of the germ layers, this cap of cells grows down, so
that it entirely encloses the yolk mass ; but at no time does the yolk
undergo any sort of division. Discoidal cleavage occurs chiefly in
the vertebrate classes, Elasmobranchii, Teleostei, Reptilia, and Aves.
There is also a type of discoidal cleavage to be found in the Cephalo-
poda, which is an extreme case of the spiral type, and a further

COMPARATIVE EMBRYOLOGY 807

illustration is seen in the scorpions, which might be considered as an
extreme case of superficial cleavage.

Superficial cleavage (Fig. 414) predominates in the arthropods, al-
though not all arthropods have this type of cleavage. It occurs also in
some coelenterates, and it has been described for one sea cucumber. In
superficial cleavage the yolk is in the form of a great many spheroids
which are separated from each other by very thin cytoplasmic films.
In the center of the uncleaved egg the nucleus lies in an island of
cytoplasm. Cleavage is accomplished by the repeated divisions of
the nucleus and the surrounding cytoplasm. These small nuclear
cytoplasmic islands gradually move about, and as their division con-
tinues, they are arranged into the form of a layer of cells which finally
reaches the surface of the egg and forms the blastoderm there. This
is a superficial layer with the yolk mass left in the center of the egg
and constitutes the so-called superficial blastula. Some cells remain
behind to help digest the yolk and are known as vitellophages.

The cleavage period, regardless of the type, terminates with the
formation of the blastula, a one-celled embryo. Of the blastulae there
are seven kinds. Most students of embryology think of cleavage in
terms of the holoblastic eggs, and of blastulae as hollow spheres of
cells of the type of the coelohlastula. Actually this is only one of the
varied kinds of structures, all of which answer the definition of a
blastula, "a one-layered embryo."

Eggs having radial cleavage usually develop into coeloblastulae,
as do those other holoblastic types in which the amount of yolk is
smaU. Coeloblastulae, that is, blastulae containing cleavage cavities,
may be either equal or unequal, depending upon the amount of yolk
present. If the size of the cavity is greatly reduced, we approach
the second type, the stereohlastula, which is a solid embryo, but here
again the dimensions of the cells may be uniform or quite variable,
and we likewise have equal and unequal stereoblastulae. In the
stereohlastula, however, all of the cells reach the surface of the entire
embryo, none of them being cut oK at the interior as in the case of the
type of blastula known as the morula. Formerly, it was common to
speak of the morula as a stage in development of any embryo, but this
usage has been abandoned and the term is now given to a particular
type of solid blastula in which certain cells are entirely cut off from
the surface of the embryo. The placula is a type of blastula in which
the cavity is reduced by a shortening of the egg axis which runs from

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TEXTBOOK OF ZOOLOGY

the animal to the vegetal pole, so that instead of being spherical it is
a mere flattened slit. The circumference of the placula corresponds
to the equator of the coeloblastula much as if two saucers were placed
with their cavities together. The amphihlastula of a sponge (Fig. 56)
is a free-swimming larval stage, the cells of the animal half being
sharply distinguished from those of the vegetal half by their smaller
size and ciliated covering. The superficial blastula, as already de-
scribed, is formed when the dividing plasma islands reach the sur-
face of the egg and form a blastoderm covering it. The discoblastula
results from the repeated divisions of the disc on the surface of these
eggs, and is in the form of a disc several cells in thickness.

Fig. 417. — Unequal stereoblastula of the moilusk Crepidula (boat shell). (Re-
printed by permls.sion from Outline of Comparative Embryology by Richards, pub-
lished by John Wiley and Son.s, Inc., after Conklin.)

Examples of these various types of blastulae occur as follows : equal
eoeloblastulae are found among the echinoderms and in amphioxus;
unequal eoeloblastulae (Fig. 288), in the frog, annelids (Fig. 110) and
molluscs; stereoblastulae, in those forms having spiral cleavage in
which there is a large amount of yolk; morulae, chiefly in the
coelenterates ; placulae, in ascidians and in some nematodes; amphi-
blastulae, limited to the sponges; superficial blastulae, chiefly among
the arthropods, and discoidal blastulae, of the same disposition as
discoidal cleavage.

The formation of the blastula marl^s the end of the first great period
of embryological development. It is followed by germ-layer forma-
tion which takes place in two steps; the first, endoderm formation
or gastridation (Fig. 416), and the second, the formation of primary
germ layers (primary ectoderm and primary endoderm), from which
are derived the definitive germ layers, ectoderm, mesoderm, and endo-

COMPARATIVE EMBRYOLOGY

809

derm. The processes by which the formation of these definitive germ
layers is brought about we speak of as rnesoderm formation. There
are really four methods by which gastrulation is accomplished in the
animal kingdom : embolic or invaginate gastrulation, epibolic gastrula-
tion, gastrulation by polar ingression, and gastrulation by delamina-
tion. As in the case of the coeloblastula (Fig. 48), the invaginate
gastrulation is the kind people usually think of in following the story
of development, but it again is only one of the several types. It is
the most simple type, however, and occurs in those embryos which
have a cleavage cavity of fair size. The cells at the vegetal pole
flatten and gradually push in, so that they come to form a lining layer
enclosed by cells of the other half of the egg. The new cavity formed

Fig. 418.

Fig. 419.

Fig. 418. — Morula of the coelenterate, Clava squamata. Notice the "mulberry
appearance. (Reprinted by permission from Outline of Comparative Embryology
by Richards, published by John Wiley and Sons. Inc., after Harm.)

Fig-. 419. — Placula of the tunicate, Cynthia partita. (Reprinted by permission
from Outline of Comparative Embryology by Richards, published by John Wiley
and Sons, Inc., after Conklin. )

by their inward movement becomes the gastrocoele or primitive gut
cavity, and will develop into the archenteron of the embryo. The best
illustration of this type of gastrulation is found in echinoderms, but
many others are scattered throughout the animal kingdom.

Epibolic gastrulation (Fig. 289), on the other hand, is a process of
overgrowth occurring in those embryos with very large yolk-contain-
ing cells, small ectodermal cells with no cavity between, and spiral
cleavage. The ectodermal cells begin a gradual process of overgrowth,
which when completed converts the stereogastrula into an embryo con-
taining a relatively few primary endodermal cells surrounded by a
layer of numerous small primary ectodermal cells.

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TEXTBOOK OF ZOOLOGY

Polar ingression resembles invagination except in one particular.
In invagination the cells, whether few or many, move inward as a
definite layer. In ingression the cells move inwards, but not as a defi-
nite layer. They do not retain their mass formation, but move as
individual cells or as very small groups of cells and rearrange them-
selves into the form of a layer with an interior cavity.

The fourth means of gastrulation is delamination, which means the
formation of a second layer, divided off from the primary ectoderm
because the spindles of the cells are directed at right angles to the
surface of the embryo. The second layer is split off, so to speak, from
the first. The net result of gastrulation is a two-layered embryo of
primary ectoderm and primary endoderm.

Fig. 420. — Formation of the endoderm in the gastropod, Patella, by unipolar
ingression. (Reprinted by permission from Outline of Comparative Embryology
by Richards, published by John Wiley and Sons, Inc., after Patten.)

The formation of mesoderm changes the two-layered embryo into
a three-layered or triplohlastic one. Obviously there are only two
sources from which the mesoderm can arise, primary ectoderm or pri-
mary endoderm, and when it has been produced, definite endoderm,
ectoderm, and mesoderm result. If the mesoderm is derived from
ectoderm, it is known as eeto-mesodenn. In general, ecto-mesoderm
may be looked upon as of temporary importance, often being lost
when larval metamorphosis is accomplished. Important embryolog-
ical questions are connected with the formation of this kind of meso-
derm, and it has a bearing also upon the phylogeny of different ani-
mal groups. If the mesoderm is formed from the endoderm, it is
known as endo-mesoderm, and is the type with which we are most
familiar in the animals usually studied in courses in embryology.

COMPARATIVE EMBRYOLOGY 811

There are several processes leading to the formation of endo-meso-
derm, but it always arises from a group of cells lying at the sides of
the gut and gradually extending themselves to make a mass which
later hollows out to form the mesoblastic somites.

Enterocoele formation is the evagination of a series of pockets from
the cavity of the primitive gut and occurs, for example, in amphioxus.
The third type of coelome formation is by a solid outgrowth of cells
arising from the enteron, a common method among the vertebrates.
Lastly, is mesenchymous coelome formation, the formation of meso-
blastic somites by the fusion of scattered mesenchymous cells. This
is rather rare and of little importance in general embryology.

Mesoderm formation results in a larva consisting of three germ
layers and completes the second great period of embryological devel-
opment. From the three germ layers the different organs of the body
are developed by invagination, evagination, foldings, and the reten-
tion of embryonic thinness in certain regions while neighboring areas,
because of differential growth, thicken and become elaborated into
the organs as we know them; this latter process is especially well
illustrated in the development of the brain.

In many animals development is direct, the embryo passing directly
from a stage of organ formation into a small, but adultlike creature.
Others, especially those which develop in the sea and the terrestrial
insects, undergo a process of metamorphosis, their earlier adaptations
to one type of habitat being insufficient to fit them to live in the dif-
ferent habitats of the adult. A small organism essentially like the
adult is the result, and only the processes of histological differentia-
tion and of growth are necessary to produce the mature organism.
Organ formation and histological differentiation complete the proc-
esses which we commonly speak of under the heading of embryology.

CHAPTER XLII

MAMMALIAN DEVELOPMENT

In mammals the reproduction is entirely sexual and in all of the
higher forms the development is intrauterine (within the uterus).
With the exception of those of the lowest group, Prototheria, the eggs
of mammals bear only a meager amount of yolk.

The ovaries are small, ovoid, paired organs which are attached one
on either side of the wall of the coelom by the supporting mesenteries,
mesovaria. The histological structure of the ovary includes the outer
{germiTwl) epithelium, the stroma (connnective tissue and few smooth
muscle cells), ovarian follicles with ova (eggs) in different stages of
development. Fig. 421 shows the origin of a follicle from the germinal
epithelium in the formation of an ovigerous tube. Certain cells of the
epithelium are the primordial germ cells, which become ova and the
adjacent cells form the lining of the primary follicle. The wall of the
follicle cells thickens by multiplication of these cells. Soon a cavity
forms in this wall of cells as the beginning of antrum or space of the
follicle. The space does not entirely surround the developing ovum.
The layer of the cells immediately surrrounding the ovum {corona
radiata) are joined at one side to the wall of the follicle. The band
of cells extending to the wall is known as the cumulus. The outer-
most layer of a mature follicle consists of two thin bands of con-
nective tissue, the internal theca and external tJieca. The human ovum
measures only 0.22 millimeter in diameter and that of a dog is still
smaller, measuring only 0.15 millimeter. The follicle becomes dis-
tended with fluid (follicular liquor) and at maturity projects beyond
the general surface of the ovary.

The ovum develops in an ovarian (Graafian) follicle. It passes
through the stages of oogenesis to the secondary oocyte here (Fig.
421). At this point of development the follicle reaches its maturity
and its outer wall ruptures to free the maturing ovum. The ruptured
follicle from which the ovum has been discharged, immediately fills
with a blood clot in which are many of the follicle cells. It later
takes on a yellowish color and appears as a mass of epithelial cells.
If pregnancy ensues, this body becomes a functional corpus luteum
and produces essential hormones, if not, it degenerates to become a

812

MAMMALIAN DEVELOPMENT

813

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814 TEXTBOOK OF ZOOLOGY

scarlike corpus albicans. In human beings the usual procedure is for
only one ovum to mature in one of the ovaries each four weeks
(approximately). At the end of the succeeding period an ovum ma-
tures in the other ovary. This alternation proceeds from month to
month in the female from puberty to menopause, except during preg-
nancy. When the ovum leaves the ruptured follicle of the ovary it
is technically in the body cavity, but in practice the funnellike
ostium of the oviduct receives it immediately and starts it down the
oviduct. It is here in the upper part of the oviduct that maturation
of the ovum is completed and fertilization occurs. Spermatozoa, car-
ried in semen, are introduced into the vagina of the female genital
tract in the act of copulation or coitus. These motile spermatozoa
swim up the oviduct and meet the ovum shortly after it enters. Fer-
tilization of the ovum by union with a spermatozoon occurs and the
zygote continues to move slowly down the oviduct. Cleavage, the next
step in development, takes place while the embryo proceeds along
the oviduct. The later stages are comi^leted normally in the uterus.
This process of successive cell divisoin is modified somewhat when
compared with that described for starfish on page 116 and frog on
page 533, but the same ultimate purpose of rapidly increasing the
cells is accomplished. Cleavage in mammals is complete and nearly
equal. The stages of the earlier divisions have been obtained and
observed in such mammals as Macacus monkey, rabbit, guinea pig,
rat, pig, sheep, and horse. It is thought that cleavage in the human
being is similar to these. One of the four cells resulting from the
second cleavage division is different from the others. It is the fore-
runner of a differentiated group of cells which soon becomes sur-
rounded by the other cells (Fig. 422). This enclosed group is then
known as the inner cells. The outer layer which surrounds these inner
cells is known as the trophohlast. Shortly, small pouches or vesicles
filled with watery fluid which is secreted by the newly formed tropho-
hlast, appear beneath it. These join to form a common cavity between
the trophoblast and the inner cells, except at one pole, where the two
groups of cells remain in contact. This cavity goes under the name
of blastocyst cavity and the whole structure, which is comparable to
a specialized blastula, is called a blastocyst or blastodermic vesicle.
The embryo's body will develop from the inner cell mass. The tropho-
blast becomes closely associated with the inner lining of the uterus
of the mother and soon plays a part in nutrition, respiration, and
excretion of the embryo. It sinks into the uterine lining carrying

MAMMALIAN DEVELOPMENT 815

the inner cell mass with it. This is known as implantation. The en-
tire blastocyst makes a rapid growth and the cavity becomes distended
with the lymphlike fluid.

Next, a simplified process of gastrulation takes place, in that two
successive layers of cells are shed into the blastocyst cavity from the
free margin of the inner cell mass. These are layers of endoderm
cells. The ends of the layers join to form an elongated enclosure.
The inner cells which are joined to the trophoblast become the
ectoderm of the body and the trophoblast remains as extraembryonic
ectoderm. The cavity enclosed by the endoderm is the archenteron
or primitive gut. In coelenterates and sponges development stops
with this two-layered or gastrula condition.

In higher forms, the mesoderm or third germ layer is formed
immediately following gastrulation. At this stage in mammals and
most other vertebrates a primitive streak appears along the dorsal
midline of the posterior portion of the blastoderm (embryo). When
sectioned, the primitive streak appears as a thickened band which is
continuous with the ectoderm. The ventral side of this streak pro-
duces many cells which organize as a sheet of mesoderm at each side
and spread both laterally and caudally between the other two germ
layers. The mesoderm continues to spread until it finally encircles
the endoderm. The mass of cells thus formed, is soon divided into two
layers by the development of a small cavity in it at each side of the
archenteron. These cavities grow ventrally in the mesoderm until
they meet each other at the ventral side of the archenteron. This
cavity is the coelom or body cavity. The layer of mesoderm forming
the outer or lateral wall (next to the ectoderm) of the coelom is
known as somatic or parietal mesoderm and that on the side next to
the endoderm is the splanchnic or visceral mesoderm. Later in de-
velopment, when the somatic layer unites with the ectoderm, it forms
the somatopleure (body wall). Similarly the splanchnic layer unites
with the endoderm to form the splanchnopleure or wall of the ali-
mentary canal.

Almost coincidental with the early development of the mesoderm, but
independent of it, is the formation of the neural plate as a thicken-
ing and depression in the ectoderm along its dorsal midline. This
begins just above the primitive streak mentioned previously and
grows anteriorly. This is the portion from which the nervous system
develops. This plate sinks, neural folds develop along its sides form-

816

TEXTBOOK OF ZOOLOGY

MAMMALIAN DEVELOPMENT 817

ing a neural groove. These folds meet each other over the groove to
form the neural tube (Fig. 423).

The mesoderm along each side of the neural plate becomes or-
ganized into blocklike thickenings. These are somites, and they are
paired opposite each other, marking segmentation in the body. In
a chick embryo of thirty-six hours of incubation there are fourteen
pairs of somites and in a pig embryo 6 millimeters long there are
thirty-two pairs.

The younger embryos of different groups of vertebrates are so simi-
lar that it is impossible to distinguish them from each other. This is
illustrated in Fig. 444 in a later chapter. At a sufficiently early
stage, the embryos of the human being, the pig, the rat, the alligator,
the salamander, and the fish all appear very similar. The gill slits
and segments are conspicuous in all of them. IMuch later the limbs
develop from lateral pairs of limb buds in the mesoderm. The hind
limbs develop first and the front ones follow. The human body can
barely be discerned in embryos of one month, and the embryo must
be nearly two months of age before it can be identified definitely as
human on the basis of morphological features.

Organs and Systems

After the three germ layers (ectoderm, endoderm, and mesoderm)
have been established in the embryo, the next step is differentiation
of these layers each in various ways for the formation of particular
organs and systems of organs. The fate of the germ layers has been
concisely summarized in the last paragraph in the chapter on meta-
zoan organization.

Embryonic Membranes. — All three of the terrestrial groups of
vertebrates (reptiles, birds, and mammals; i.e., amniota) produce
extensive embryonic membranes as a feature of their development
which is not found in the aquatic forms. These membranes serve
to give the developing embryo added protection, as well as increased
facilities for the functions of nutrition, respiration, and excretion.
In birds and reptiles the early embryo is flat and the somatopleure
extends over the surface of the yolk far beyond the limits of the
embryo proper. A fold of this sheet of somatopleure (ectoderm
and parietal mesoderm) grows dorsally along each side of the embryo
(Fig. 424). These folds finally meet each other above the embryo and
fuse together, thus forming an enclosed cavity between the dorsal sur-

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TEXTBOOK OF ZOOLOGY

face of the body and this new membrane. The name of this membrane
is amnion and the cavity formed is the amniotic cavity. The super-
ficial limb or layer of the original fold of somatopleure over the body
is the serosa. In mammals the serosa unites with another embryonic
membrane, the allantois, to form the real chorion which is highly vas-
cularized and serves as the embryonic portion of the placenta. The
allantois is a ventral evagination (outgrowth) from the ventral side

H tad Fold

■/^eso6/ashc Somife

Cloied fifevra/ 7v6e

Open /l/eural Qrooi^e

Tail Fold

Fig. 423 — Dorsal view of an embryo in the neural tube stage of development.
Both ends of the tube are still open. The brain forms at the anterior end
later.

of the cloaca of the embryo. It extends out through the open, ventral
side of the body and spreads between the amnion and serosa until
it covers most of the body like a double-walled sac. After develop-
ment is complete, its stalk within the body cavity is retained as the
urinary bladder. In embryonic birds and reptiles the allantois thus
lies quite close to the shell membrane and since it is richly vascu-
larized, serves as the respiratory organ.

MAMMALIAN DEVELOPMENT

819

In mammals, the allantois spreads between the amnion and serosa
in about the same way, and becomes fused with the inner side of the
serosa to form the chorion. Many branched processes, chorionic villi,
extend from the outer or serosa surface of this membrane and come
to fit into corresponding pits in the internal uterine wall of the
mother.

The yolk sac is another membrane which extends ventrally from
the mid-gut and out through the ventral side of the body wall just
anterior to the allantois. This structure is present in the fish and
amphibia as well as in the amniotes. It is large in the shark for

Embryo
Ectoderm

Ectoderm
Parietal
Mesoderm

Splanchnic
Mesoderm

Endoderm

Yolk Sac

Parietal
Mesoderm

Fig. 424. — ^Diagram showing development of tlie embryonic membranes of a
vertebrate. The amnion and serosa (chorion) each consists of a fusion of
ectoderm and somatic mesoderm ; while the yolli sac and allantois each consist
of a fusion of endoderm and splanchnic mesoderm. The serosa is composed of
the two outer layers of the diagram. Notice that the allantois is a double-walled
sac from the ventral stalk, and it extends well around the embryo.

Amnion

■Amniotic Cai^ity

■Allantois
'Parietal

Mesoderm

Splanchnic
Mesoderm
Yolk Sralk

Endoderm

Vitelline
Membrane

example. In reptiles and birds it carries the large mass of yolk used
for nutriment by the embryo. In mammals it is much reduced be-
cause the embryo soon develops a means of nourishment through the
placenta.

Placenta. — This membranous sac in which the embryo and later
the fetus (designation after distinct body form is apparent) is
formed from two sources, one embryonic and the other maternal.
As suggested in the paragraph above, the chorion of the embryo and
the mucous membrane (internal lining) of the uterus unite (actually

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TEXTBOOK OF ZOOLOGY

fuse in some forms) to form this saclike organ, the placenta. The
chorion is richly supplied by blood from the fetus while the uterine
layer is similarly supplied by maternal blood. Although the em-
bryonic and maternal parts of the placenta may be almost interwoven
in mammals like the human, there is no actual circulation of maternal
blood through the vessels of the fetus. The exchange of materials
(nutrition, respiration, excretion) is by diffusion through the tissues
here in this intimate relation. As in the case of circulatory connec-

Yofk Sac

Muscu/ar Layers
of Uterus //

Basal Plate-
of Placenta

Umbilical Cord-

Amnion

Chorion & Decidua-
Capsularis

Fig. 425. — Sectional view of a fetus in normal position in the uterus, showing
also the intimate relations of embryonic membranes and uterine wall. The chorion
is the outer embryonic membrane. (Modified after Ahlfeld. )

tions, there are no nerves passing from parent to fetus. The stalklike
extension from the abdomen of the fetus to the placenta is the um-
hilical cord. It carries the umbilical arteries and vein, extending from
the chorion to the fetus; the allantoic stalk; and, a vestigial yolk
stalk from the intestine. The Tiavcl of the adult is the scar where the
body wall has closed in about the umbilical cord at the point of its
severance at birth. The fetus is suspended within the amniotic cavity
which is filled with the watery, lymphlike amniotic fluid.

CHAPTER XLIII

GENETICS AND EUGENICS
(By Frank G. Brooks, Cornell College, Iowa)

The History of a Great Discovery

"Like father like son" is an ancient adage. Since man has
been able to think, he has pondered the problem of heredity. Al-
though very early he had observed evidence of the inheriting of
parental characters by offspring of the various forms of life with
which he was familiar and was convinced by these observations that
heredity did take place, he has not known the "how" or "why" of
it until recently. The fundamental law on which heredity is based
was announced by Gregor Johann ]\Iendel in 1866. However, Mendel,
an Austrian monk, published his discovery in an obscure journal
and it did not receive general recognition until its rediscovery in
1900. Therefore, genetics is really a twentieth century science.

Mendel's success in finding the underlying principle of heredity
was due, in part, to his choice of an experimental unit. Instead of
following the usual trend by considering how a parent conveys his
various traits to his offspring, thus making the individual the unit of
observation, he chose a definite inheritable character and considered
how it was transferred from many parents to all their offspring. In
addition to this wise choice of an investigational unit, the patience,
mathematical ability, skill as a gardener, and analytical insight of the
investigator contributed also to the success of the research.

Mendel chose the garden pea as the material for his work. This
was a fortunate choice for, although the law that he was to discover
underlies practically all inheritance, it is not always as free from
complications as it is in the case of those traits of the pea which he
investigated.

Mendel's Law

For one of his projects he planted seeds from stock that had been
known to produce nothing but tall plants for many generations. He
planted also seeds from stock that had produced nothing but dwarf
plants. When the two types of plants were in blossom, he transferred
the pollen from the stamens of the one to the stigmas of the other.

821

822

TEXTBOOK OF ZOOLOGY

The seeds that resulted from the cross were collected and planted.
To the amazement of the investigator they produced nothing but tall
peas; peas as tall as the original tall parents. Dwarf ness seemed to
be lost and tallness was certainly dominant. A less thorough investi-
gator might have called his experiment finished and have proclaimed
as a rule, the fact that a trait can disappear when it is crossed with
its opposite. But not so with the patient monk. He pollinated these
plants with their own pollen and planted another generation. This

Fig. 426. — Diagrram to show the result of crossing tall and dwarf peas. The 1:2:1

ratio appears in the F2 generation.

time three-fourths of the plants were tall and one-fourth of them
were as short as the original dwarf parents. Not being ready even
yet to formulate a law, he self -pollinated his plants for several more
generations and got results that required his best mathematical skill
to interpret. The dwarf peas produced only dwarf peas. Of the tall
peas, one-third of them (constituting one-fourth of the whole num-
ber) produced tall peas without deviation, but the other two-thirds
(half of the whole number) produced stock that repeated the produc-

GENETICS AND EUGENICS S23

tion of tall and dwarf peas in the same proportion as did the preced-
ing generation. Here, then, was a definite ratio of 1:2:1. Mendel
tested his findings with six other traits of the pea and with more than
a dozen other kinds of plants, and after verifying his results he was
able to announce the following law : When members of a species hav-
ing contrasting characters are crossed, all the immediate offspring
will show the trait that is dominant, hut if the members of this gen-
eration are bred to themselves, one-fourth of the offspring ivill show
the dominant trait and breed true for it; one-fourth will show the
recessive trait and breed true for it; the remaining two-fourths of
the offspring will show the dominant trait, but will reproduce the
contrasting characters in the same proportion as did their immediate
parents.

Derivatives of Mendel's Law

From the fundamental law which Mendel discovered, several corol-
laries can be drawn, based on factors responsible for the distribu-
tional behavior of inheritable characters. These corollaries are :

Principle of Dominance. — The determiner for one member of a
pair of contrasting characters (pairs of contrasting characters are
called allelomorphs) may take precedence over the other member
when the two have been brought together in a cross between unlike
parents. Which trait is dominant and which is recessive can be de-
termined only after a cross has been made, and the determination
holds only for the species observed. Thus, taUness may be dominant
over dwarfness in one species of plant and recessive in another. In
many cases, dominance is not complete, and in some instances it is
lacking altogether. These cases will be discussed later in this
chapter.

Independence of Unit Characters. — The determiner for any given
character acts as an independent hereditary unit as it is passed along
from generation to generation. In each generation it may have a
different set of associates, but its associations in one generation do not
affect the company it may keep in the next. Thus tallness in peas may
be associated with yellow pod and with wrinkled seed coat in one
generation, but it may form a combination with green pod and smooth
seed coat in the next. This important principle is responsible for the
great variation we find in plants and animals and makes it possible
for a breeder to bring about any desired combinations of the traits

824 TEXTBOOK OF ZOOLOGY

possessed by the species of plant or animal with which he is working.
Thus the modern Shasta daisy has a combination of the three impor-
tant characters: size of flower, gloss of petal, and prolific growth,
each of which was procured from an original variety having one, but
not the other two, of these traits.

Principle of Segregation. — Since unit characters are independ-
ent, it follows that they can go into various combinations and
are free to segregate out again. This was illustrated by dwarf-
ness in Mendel's peas which went into a cross with tallness,
but segregated out again to form peas that were as consistent
for dwarfness as was the original dwarf stock. To illustrate
both the principles of unit characters and the related phenome-
non of segregation we might use an analogy. If a gallon of white
marbles were mixed with a gallon of black marbles, they would make
a mass of marbles that would look speckled-gray. But the mixture
is not irrevocably gray as would be the case if a gallon each of black
and white paint had been mixed. Inheritable traits, like marbles,
can be placed in various combinations. These may last for one genera-
tion only, after which they can make new combinations with other
traits.

The Physical Basis

With the simple facts in mind of how heredity works, it is desirable
to consider the germinal background of the laws governing it.

The tiny sperm cells of both plants and animals and the egg cells
which, deprived of their food-containing yolk, are about equally
small, contain within them something that determines all the traits
which the individual resulting from their combination will develop.
In fact, the body of the sperm cell and the nucleus of the egg eon-
tain little else than a material that carries the combined inheritable
traits of one generation over to the next. This important material is
called chromatin and during mitosis it becomes arranged in series of
hereditary units called genes. Each gene has a definite causal rela-
tionship to an inheritable trait. Both the sperm cell and the egg cell
contain a complete gene complex, and each, under favorable condi-
tions, could produce a complete individual.

When the two totipotential gametes are brought together at fertili-
zation, the resulting zygote contains two genes for each allelomorph.
When the two genes are for the same trait, the resulting individual

GENETICS AND EUGENICS

825

is said to be homozygous for that trait and, of course, shows it. When
each of the two genes is for a different member of the allelomorph,
in which case we use the term heterozygous, one gene takes precedence
over the other, but the recessive gene is in no way destroyed ; it sim-
ply lies dormant and bides its time. How these genes are arranged
into a limited number of "packages" called chromosomes and how
genes and chromosomes are distributed in the process of sperm and
egg formation was explained in Chapter VIII.

Plotting Crosses

The Monohybrid Cross. — When the genetic constitution of paren-
tal stocks is known, it is possible to plot out the results of various
kinds of crosses. The checkerboard is the simplest device for such

•Kl

T£

Fig. 427.

Fig. 428.

Fig. 427. — The outcome of a monohybrid cross between two heterozygous in-
dividuals is according to the ratio 1:2:1, i.e., (TT). (Td), (Td), (dd).

Fig. 428. — The checkerboard may be used to determine the possible gene com-
binations in the ova. and sperm of a dihybrid cross.

computations. The genetic constitution of the various kinds of male
gametes are set down along the ordinate and that of the various kinds
of female gametes along the abscissa. These values are then copied
into the squares horizontally and vertically and their sums give the
values of the various kinds of gametes that will result from the cross.
In the Fa cross of Mendel's experiment with tall and dwarf peas,
half of the male gametes and half of the female gametes contained a
gene for taUness (T) and half in each case contained a gene for
dwarf ness (d). The outcome, therefore, is shown in Fig. 427.

The Polyhybrid Cross. — It is often desirable to know the outcome
of a cross in which two or more allelomorphs are considered together
as in the case of crossing a taU, green-podded pea (Tg)* with a dwarf,

•It is customary to indicate the dominant trait by using a capital letter and
the recessive trait by using a lower-case letter.

826

TEXTBOOK OF ZOOLOGY

yellow-podded pea (dY). The determination of the possible kinds
of gametes may be simplified by first making a small checkerboard
for them (Fig. 428). Thus we find that in a dihybrid cross there
are four kinds of male gametes and four kinds of female gametes
which may be listed as TY, Tg, dY, dg. Now we make a checkerboard
of sixteen squares and proceed as we did before. (Fig. 429.)

The computation of the dihybrid cross indicates that nine-sixteenths
of the progeny will show the two dominant traits because (Fig. 429)
we find at least one T and one Y together in that many squares. Three-
sixteenths will sJiow the dominant trait of the first allelomorph and the
recessive trait of the other. Another three-sixteenths will show the re-

Tg
TY

dg
TY

TST
Tg

Tg
Tg

dg
Tg

Tg
dY

dY
dY

dg
dY

TY
dg

Tg
dg

dY
dg

dg
dg

Fig.

429. — The outcome of a dihybrid cross between two heterozygous individuals
is according to the ratio 9:3:3:1.

cessive trait of the first allelomorph and the dominant trait of the sec-
ond. One-sixteenth of the offspring will show both recessive traits. It
will be noticed, however, that the nine squares showing individuals
that will be tall and yellow vary with each other in regard to their
entire content. Further examination of the squares will indicate that
there are nine different combinations of letters and that in only one
case as many as four are exactly similar. Individuals whose genes
cause them to look alike are called iihenotypes; those whose genes are
exactly alike are called genotypes.

A trihybrid cross, such as one between a tall yellow-podded pea with
a wrinkled seed-coat (TYw) and a dwarf green-podded pea with a

GENETICS AND EUGENICS

827

smooth seed-coat (dgS) can be plotted by using a checkerboard of
sixty-four squares. The ratio of phenotypes of a trihibrid cross is
27:9:9:9:3:3:3:1.

The Back Cross. — The checkerboard is also useful in computing
the results of a cross between a heterozygous and a homozygous in-
dividual as would be the case in which the offspring of a cross between
a homozygous dominant and a homozygous recessive individual is
bred back to either of the parental stocks ; a procedure often followed
in practical animals and plant breeding. When such a cross is made
to the homozygous dominant stock the results, in terms of pea traits,
are TT, TT, Td, Td. (Fig. 430.) All four will look alike, i.e., they
are phenotypes, but there are two pairs of genotypes.

T T d d

Fig. 430. Fig. 431.

Fig. 430. — A cross between heterozygous and homozygous tall peas produces a
2:2 ratio.

Pig. 431. — A cross between heterozygous tall and homozygous dwarf peas also
produces a 2:2 ratio.

When the heterozygous stock is bred back to the homozygous reces-
sive parental stock, a 2 :2 ratio is also produced. This time the two
kinds are not only genetically unlike, but they also appear unlike
(Fig. 431).

Complications of Mendelian Inheritance

If all inheritable characters followed Mendel's law in as simple a
fashion as do the pea traits that Mendel first investigated, the science
of genetics would be much more easily mastered. Unfortunately this is
not the case. Although Mendel's law is found so consistently as the
underlying principle of heredity that it can be said to be practically
universal in its application, it is often modified and complicated in
various ways.

Multiple Allelomorphs. — Instead of the allelomorph always consist-
ing of the usual two factors, a larger number of alternatives sometimes

828 TEXTBOOK OF ZOOLOGY

appears. Dominance in these cases occurs in a graded series, each
member, between the extremes, being dominant to the lower members
and recessive to the higher members of the series. A simple case of
multiple allelomorphs is found in the inheritance of color in rats where
there are three factors; namely, ordinary pigmentation, ruby-eyed
dilution, and albinism. In a cross between heterozygous parents any
two of the three factors may be brought together in the offspring. If
ordinary pigmentation is present with either of the other two, it will
determine the color, since it is first in the series of dominance. If the
other two are the ones present, the color will be ruby-eyed dilution,
the second member of the series.

A larger series is presented by eye color in the common fruit fly,
Drosophila melanog aster, in which case there are eleven members
which are, in the order of their dominance : red, apricot, coral, ivory,
ecru, buff, tinged, blood, cherry, eosin, and white. Each parent may
have any two of these in its germ plasm and transmit either of them
to the offspring.

Multiple Genes. — Several cases formerly interpreted to be simple
blending inheritance not conforming to Mendel's law have been
explained by the discovery that there can be more than one allelo-
morphic pair concerned with the inheritance of the trait. Thus, in-
stead of there being one gene located at some definite place on a
chromosome, there are two or more genes variously located. Cases of
multiple genes fall into two categories. In the one, each gene inherited
as a dominant, produces part of the result, and the effects are cumula-
tive. In the other type of case, the inheritance of one dominant gene
produces the entire effect,

A common example of the cumulative type is the inheritance of skin
color in man. It has long been known that a cross between a white
person and a pure-blooded negro produces offspring of a medium
shade called mulatto, and that a cross between two mulattoes will pro-
duce offspring with a range of color varying from intense black to
a shade that may allow the person to pass for white. This is ex-
plained by the fact that there are two allelomorphic pairs concerned
with the inheritance of color, the dominant of either of which will
produce a certain amount of pigment per square millimeter of
skin surface. The negro of pure skin inheritance is homozygous,
having the dominant genes of each pair. Using P and P' to repre-
sent the dominant factors of these two allelomorphs and p and p' the

GENETICS AND EUGENICS

829

recessives, the homozygous negro would have PPP'P' while the ho-
mozygous white person would have ppp'p' and the mulatto resulting
from the first cross would have PpP'p'. The cross between two mulat-
toes would be a dihybrid cross which could be plotted by a checker-
board essentially similar to that of Fig. 432. The accompanying chart
(Fig. 432) shows that there can be four, three, two, one, or no dom-
inant factors present, thus accounting for the series known under
the following technical terms: ''negro," "chocolate," ''mulatto,"
"quadroon" and "pass for white."

ppt

Pp*

pP'

PP*

pp»
p?»

negro

Pp*

ppt

choc.

pP*
ppi

choc.

PP*
ppt

mulat

ppt
I''

choc.

Pp'
mulat

pP»
mulat

PP'

quadr

ppt

pP»
choc.

Pp»

pP*
mulat

pP'

pP*
mulat

PP*
quadr

ppt

pr>»

mulat

Pp*

K
Quadr

pP»
Quadr

PP*

PP*
white

ppl

Pp*

pp«

PP*

Fig. 432. — The inheritance of color by children of mulatto parents can be shown by

the checkerboard for a dihybrid cross.

The other type of case in which multiple genes occur is exemplified
by a certain brown-seeded variety of oats in which there are two pairs
of genes concerned with the determination of color. Here the pres-
ence of the dominant gene of either pair produces the entire effect.
This case, too, can be plotted on the dihybrid checkerboard, but the
result is a 15 :1 ratio, for fifteen of the sixteen squares would show at
least one dominant gene. Crosses between a red and a white variety of
wheat in which there are three pairs of genes for color, produce a 63 :1
ratio. This is a trihybrid cross in which each dominant gene can pro-
duce the entire effect.

Complementary Genes. — In a number of cases color is produced
by two allelomorphic pairs of genes, the dominants of which must
react on each other to produce the color effect. In the sweet pea, if
the dominant factor of the red allelomorph (Rr) is acted on by the

830 TEXTBOOK OF ZOOLOGY

dominant factor of the color allelomorph (Cc), then red is produced.
If either of these dominant genes is present without the other, white
flowers are produced, and if two white-flowered plants of the genetic
constitution Rrcc and rrCc are crossed that part of the progeny will
be red in which both R and C are inherited. A student familiar
with elementary chemistry may understand this better if he thinks
of the red factor as being represented by a colorless solution of
phenolphthalein and the color factor as a colorless alkaline solution.
When both solutions come together, red is produced, but neither solu-
tion can produce the color without the other.

Supplementary Genes. — But sometimes purple sweet peas appear
when two white varieties are crossed or when a red variety is crossed
with a white variety. Purple is produced by a third gene which
intensifies red if it is already present, but has no effect unless the
RC genes are likewise inherited. Thus we find that its effect is to
supplement that of the two genes R and C which are complements to
each other in the production of red. The accompanying list shows
some of the surprising crosses that may be made with sweet peas. Of
course the combination of genes shown in the last column is not the
only one that will come about in each of the various cases, but it is
the one that emphasizes the point desired. Bb represents the in-
tensifying allelomorph.

White + White = Purple RcB + rCb = RGB

Red + White = Purple RCb + RcB = RCB

Purple + Red = Red RCB + RCb = RCb

Purple + White = Purple RCB + rcb = RCB

Purple + White = WJiite RCB + rcb = rcB

Purple + White = Red RCB + rcb = RCb I

Lack of Dominance.— In several classic cases, some of which are
among plants and others among animals, neither factor of an allelo-
morph is dominant over the other. In the F^ generation, these cases
seem to produce perfect examples of blending inheritance, but the Fa
generation exemplifies the 1 :2 :1 ratio so beautifully that these excep-
tional cases are often used to explain Mendel's law to beginning stu-
dents.

The Andalusian breed of chickens includes both black and white
individuals. When black fowl are crossed with white fowl, all the off-
spring are of a slate color technically known as ' ' blue. ' ' When a blue

GENETICS AND EUGENICS 831

chicken is crossed with another blue chicken, one-fourth the progeny-
are black, one-fourth are white and the remaining two-fourths are
blue. The blacks and the whites are homozygous for their respective
colors while the blues are heterozygous. The inheritance of color in
short-horn cattle, and of the color in the four-o'clock flower are also
examples of this. In still other cases in which there is not neutrality
of dominance, the dominant effect may not be complete. Very often
the heterozygous individuals can be picked out from the homozygous
dominant ones by casual inspection.

Inheritance of Sex

In certain lower phyla of animals and in the early embryos of
higher forms, including mammals, both male and female repro-
ductive systems are present in each individual. Typically in

Fig-. 433. — Half of all possible fertilization combinations will be (XY) and half
will be (XX). The XY combination will be males and the XX will be females.

higher forms, one of these systems is repressed, and we speak
of the individual as being either male or female. The deter-
mination of whether the male or the female system will develop is an
inherited trait. The chromosome that carries the gene of sex deter-
mination has come to be designated as the X-chromosome. In typical
cases, the female has in each of her germ cells, two of these X-chromo-
somes and the male has only one. Therefore, when, in the process of
spermatogenesis, the chromosomes match with their homologous chro-
mosomes and separate again to give each of the resultant cells one
complete set of chromosomes, the X-chromosome cannot pair with an-
other like itself, and when the chromosomes are distributed in sets,
half the sets will lack an X-chromosome. The sperm cells that do not
have an X-chromosome will produce males while those that have such
a chromosome will produce females.

832 TEXTBOOK OF ZOOLOGY

In other cases that include man and Drosophila, the X-chromosome
will pair with a Y-chromosome which has been described as " an empty-
case " or a " ghost chromosome. ' ' In cases in which a Y-chromosome is
present, we cannot say that the male has one less chromosome than
the female, but it practically amounts to that.

Linkage

Since the number of inherited traits of an animal or plant is very
great while the number of chromosomes is comparatively small, it is
apparent that a single chromosome must contain many genes. This be-
ing so, there is not absolutely free assortment of traits as they are
passed from parent to offspring, but rather the offspring must inherit
his traits in groups, that are not, ordinarily, broken up. Therefore,
when two genes, A and B, are on the same chromosome of one of the
parents, if the offspring inherits gene A, he must inherit gene B. In
Drosophila we Imow that the gene which determines whether or not
the wing will be fringed occurs on the same chromosome as the one
that is responsible for the body being or not being black. Now if one
of the parents is fringed-winged and black-bodied, the offspring will
have to inherit both these traits from that parent if he receives either
one of them. This phenomenon is called linkage.

Sex Linkage

Since there are other genes than those concerned with sex on the
X-chromosome, it is to be expected that their inheritance will differ
somewhat in the two sexes. This is strikingly illustrated by certain
abnormalities of man in cases where the abnormal condition is inher-
ited as a recessive; for example, color blindness, blindness resulting
from atrophy of the optic nerves (Leber's atrophy), and hemophilia or
bleeder's disease, in which great loss of blood occurs even from slight
wounds because clotting will not take place. Since the abnormality is
a recessive trait produced by a gene on the X-chromosome, its effect
can be offset by a dominant for normal in the female, but not in the
male, because in the latter case there is not another X-chromosome
present. Therefore, the daughter of a color-blind father will not show
the defective trait but may pass it to her son.

Fig. 434 shows a cross between a color-blind father and a normal
mother. Two kinds of male gametes are produced, half of which carry
the father's gene for color blindness on the X-chromosome, and half of

GENETICS AND EUGENICS

833

which have no X-chromosome and, therefore, have no such gene. All
female gametes have one X-chromosome bearing a gene for normality.
The various possible combinations of the two kinds of sperm with the
single type of egg will produce two types of offspring; namely, sons
who will be strictly normal in regard to the trait in question and
daughters who will inherit the gene, but who will not show the trait
because its effect will be overcome by the presence of a dominant gene

Fig. 434. — In the cross between a color-blind male and a normal female, tlio
results (N, cb), (N, cb), (N), (N) indicate that all daughters, being heterozygous,
will not show the defect though they might transmit it to their offspring, while
the sons are entirely free from it.

Fig. 435. — Results (N, N), (N, cb), (N), (cb). In a cross between a normal
male and a female who is heterozygous for color blindness, half of the sons will
be normal and half will be color blind. Half the daughters will be homozygous
for normal and half will be heterozygous.

for normal. Therefore, the daughters will be heterozygous for the
trait and might be spoken of as carriers. Fig, 435 shows a cross be-
tween such a daughter and a normal male.

Inspection of this figure will show that such a cross will produce
four types of offspring; namely, normal daughters, "carrier" daugh-
ters, normal sons, and color-blind sons. Thus, we see that half the
grandsons of a color-blind man will receive the trait through their
mothers, and that all daughters and half the granddaughters of the

834

TEXTBOOK OF ZOOLOGY

color-blind male will be carriers of the trait, but will have normal
color vision. Of course, from the mating of such a person with a color-
blind man, all the sons and half the daughters will be color blind, but
a young woman who is used to having her father buy pink neckties
thinking they are blue, is not likely to marry a man similarly defective.

Crossing Over

Genetics might be defined as an exact science which has exceptions
to many of its rules. A previous paragraph describes the principle of
linkage which provided for the inheriting as groups of those traits
whose genes occur together on a chromosome. Certain gametogenetic

D D

B D

Fig. 436. — Crossing over occurs when chromosomes break apart after synapsis.

accidents bring about occasional exceptions to this principle. It will
be remembered (page 114) that at synapsis, the chromosomes pair
and the homologous chromosomes become loosely attached to each
other. At times, the pairing chromosomes will even twist around each
other. "When they come to separate in the maturation division, instead
of making a complete separation, they sometimes break off at some
point so that a part of each will be joined to a reciprocal part of the
other, as is shown in Fig. 436. Thus, as a result of crossing over, in-
stead of traits A and B, and C and D being associated in the offspring
as they were in the respective parents, A will be associated with D,
and C with B.

GENETICS AND EUGENICS

835

too

Q.i

,0.+
\ Q5

, ', 1.5

\'P-
- 13.t

'4.5

. \ 5,3

-■'. '.A9

',13.1

M6.+

M.±

20.

21.

27.5

27.7

yellowCB)
Hairy winaC'V)
Scu+eCHf
lefhoil-7
brooiolCw;
prune CE)
white CE)
face+CE)
No+ch CB) ^
AbnormalCB;
echinus(E)
bifid (W)
ruby(E)
crossveinless (W>
club(W) ,
del+ex(W;
cuf(W),
singed (.H)
tanCB)
lozenge CE)

{

0.
2.
3.±

6.±

12.±
IJ.
14 .±
16.

+elegroiph(W)
Star (E)
aris+cp<less(B)

expanded (W)

Guil(W)
Trunca+efW)
dachsous (B)
S+reak(B)

roughoid CE)

bentfW)
shaven CB)
eyeless CE)
rotated CB)
Minute-IY(H)

■■31.

-■35.

33. Vermillion CE)

36.1 miniatuneCW)

36.2 dusky CW)

38.1 furrowed CE)

43. sable (B)
444 goirnetCE)

54.2 small wing
54.5 rudimen+aryiJ'V)
56.5 forked CH)
57. BarCE)
58.5 small eye
59. fused CW)
59.5 BeadexCW)
62. Minute-nCH)
65. cleft CW)

-170. bobbed CH)

doichs CB)
Ski-n (W)

-■41. Jammed CW)

46.± Minute-e(H)

48.5 black CB)

487 Jaunty CW)

54.5 purple CE)

575 cinnabar(E)

60.t safraninCE)

-64.± pink-wingCEW)

67. vestigial CW)

68.± telescope CW)

72. Lobe(E)

74.t gapCw)

.- /4.t gap i.vv; i

■■75.5 curved CW) ..

■■83.5 fringed CW)

--90. humpy CB)

■ 20. divergent CW)

■ • 26. sepia CE)
26.5 hairy (B)

- ■ 35. rose CE)

■ ■ 56.2 cream-m CE)

40.1 Mi'nute-h CH)
:: 40.2 tiltCW)

40.4 Dichoiete CH)
- 42.2 thread CB)

44. scarlet CE)

-. 48. pink CE)
49.7 maroon CE)
f50.± dwarf (B) ,
150. curled.(W)
-- 54.8 Hairy wing supr

58.2 Stubble (H\
: : 58.5 spineless (Hj
* 587 bithorax(B)
..'~3Q.5 bi thorax- b
•f.62. stripe CB)
i"'63.I glass (E)
T 66.2 Delta CW)
69.5 hairless Ch)
70.7 ebony (B)
72. bandCB)

75.7 card ina 1(E)
76.2 white ocelliCE)

male fertility

Long bristled

- - 99.5 arc CW)
■ - 100.5 plexus (W)
-- 10?.± lethal -Ha
(105. brown (E) ,
-ll05.t blistered CW)
106. purpleoidCE)
II07.± morulaCE)
1107 speck CB) ,
107.5 balloon (W)

91.1 rough CE)
93. crumpledCW)
93.8 Beaded CW) ^
94.1 Painted CW)

100.7 claret CE)

101. Minute (H)

106.2 Minute-gCH)

male fertility

Fig. 437. — Chromosome map of drosophila. (From Sharp, Introduction to Cytology,
published by McGraw-Hill Book Company.)

836 TEXTBOOK OF ZOOLOGY

Crossing' Over, a Useful Tool. — The process of crossing over has
proved to be a valuable means of determining something of the nature
of genes and chromosomes and, especially in Drosophila, it has enabled
us to locate the positions of many of the genes on their respective
chromosomes. If two linked traits are separated every time crossing
over occurs, it is obvious that their genes lie at opposite ends of the
chromosome, but if they are separated only half the times, it is ap-
parent that half the length of the chromosome lies between them.
Therefore, by computing the percentages of such separations and
noting which traits are affected, it has been possible to make chromo-
some maps showing the positions of the various genes and the distances
separating them. The chromosome maps for Drosophila are quite com-
plete (Fig. 437).

Mutations

When chromosomes separate from each other at synapsis, aside from
breaking so that a piece of one will adhere to the correlative piece of
the other, various additional "chromosomal accidents" will occur
which change the organization of the chromosome. A piece of chro-
mosome, for example, might break off from the end of one chromosome
and adhere to the end of its homologue. Any disarrangement of the
genes of a chromosome results in structural or physiological changes
in the organism into which it goes. Such changes are inheritable and
are called mutations. Since there are places on the chromosomes where
these accidents occur more frequently than at other places, there is a
tendency for certain mutations to occur with fairly determinable reg-
ularity, say once in every hundred thousand births. Some mutations
are useful and are preserved to the species to such an extent that De
Vries believed they were the principal factor in bringing about evolu-
tion. Other mutations are a disadvantage to the species, and the or-
ganisms possessing them are eliminated in Nature's fierce struggle
for existence unless they are saved from that cruel fate by man's
interference.

Human Heredity

Man, very naturally, is interested most in the heredity of man. In
spite of this supreme interest, his knowledge of his own heredity is
much more limited than his knowledge of inheritance in any of a num-
ber of other organisms. There are two reasons for this : namely, he is
not free to experiment with his own kind; and, as would be expected.

GENETICS AND EUGENICS

837

the application of Mendel's law to inheritable traits in this, the most
complex of all living forms, is correspondingly complicated. Although
there are eases in which the 1 :2 :1 ratio occurs in as simple a form as
in Mendel's peas, there are many more cases in which the Mendelian
principle is manifested as multiple allelomorphs, multiple genes, modi-
fying factors, complementary factors, incomplete dominance, change-
able dominance, irregular dominance, etc. There is a tendency
among some to depreciate our knowledge of human heredity on the
ground that there is so much that we do not know. It is right to
admit the extent of our lack of knowledge, but it is wise to give
proper credit to our present store of information and to take cog-
nizance of the rapidity with which the gaps in our knowledge are
being eliminated. One by one the complicated problems of human
heredity are being solved by patient investigators, and by putting
what we already know to use, we stimulate the accession of more data.
Some Cases of Human Heredity. — To give a complete summary
of our knowledge of human heredity would be beyond the scope of
this chapter. The accompanying table gives some of the data that
have been accumulated:

Data on Human Heredity

TRAIT

General Physique and Skeleton

Stature

Body build

Polydactyly (extra digits)
Brachydactyly (short digits and

limbs)
Symphalangism (fused fingers and

toes)
Zygodactyly (webbed fingers and

toes)
Club foot

Lobster claw (split hands and feet)
Exostoses (outgrowths of long bones)
Abnormal fragility of bones
Amputation (entire absence of hands

and feet)
Shortened arms

Skin, Hair and Countenance

Ichthyosis (scaly skin)

HOW INHERITED

DOMINANCE

A composite

character
Multiple genes,

two or more

pairs
Simple
Simple

Simple

Simple
Simple
Simple
Simple
Simple

Irregular dom-
inance

Simple. Lethal
when homozy
gous,

Many genes for shortness

are dominant
Factors for stoutness are

dominant

Abnormality dominant
Abnormality dominant

Abnormality dominant

Abnormality recessive
Abnormality dominant
Abnormality dominant
Abnormality dominant
Abnormality dominant

Abnormality dominant
Abnormality dominant

838

TEXTBOOK OF ZOOLOGY

Data on Human Heredity — Cont'd

TRAIT

HOW INHERITED

dominance

Keratosis (thickened skin on palms

Sex-linked

Abnormality dominant

and soles, suggestive of hoofs)

Cutis laxa (rubberlike skin)

Simple

Abnormality dominant

Thick lower lip ("Habsburg")

Simple

Abnormality dominant

Prominent chin

Simple

Abnormality dominant

Wide nostrils (negroid)

Simple

Dominant to Caucasian
type

High nasal bridge

Multiple genes.

Dominant to low or

four pairs

"snub"

Normal pigmentation

Plural genes

Cumulative

Piebald (spotted skin)

Simple

Piebald dominant

Premature grayness

Simple

Abnormality dominant

Blaze (a white forelock)

Simple

Blaze dominant

Epidermolysis (excessive formation of

Simple

Abnormality dominant

blisters)

Albinism (lack of pigment in skin,

Simple

Recessive to all types of

eyes, and hair)

pigmentation

Beaded hair

Simple

Abnormality dominant

Wavy hair

Probably simple

Partial dominance

Kinky hair (negroid)

Simple

Dominant

Alopecia (baldness)

Sex-linked

Dominant in men
Recessive in women

Hair color (brunette or blonde)

Multiple genes

Dark shades dominant

Red hair

Probably simple

Red pigment dominant

Eyes

Eye color

Modifying fac-

Dark shades tend to be

tors and varia-

dominant over blue

tion in domi-

nance

Hereditary cataract

Simple

Abnormality dominant

Color blindness

Sex-linked

Abnormality usually re-
cessive

Night blindness

Sex-linked

Abnormality usually re-
cessive

Atrophy of optie nerve

Sex-linked

Abnormality recessive

Large, irregular pupils

Simple

Abnormality, an irregu-
lar dominant

Almond eyes (Mongolian)

Simple

Recessive

Ear Structure and Hearing

Complete absence of external ears

Simple

Abnormality dominant

Cup ears (small, deformed, inverted

Simple

Abnormality dominant

pinnae)

Otosclerosis (progressive hardening of

Simple

Abnormality dominant

ear drums)

Deaf mutism

Simple

Abnormality recessive

Nervous System

Feeblemindedness

Can be simple

Abnormality recessive

Amaurotic family idiocy

Simple

Abnormality recessive

Huntington's chorea (St. Vitus'

Simple

Abnormality dominant

dance)

Manic depressive insanity

Plural genes

Abnormality chiefly dom-
inant

GENETICS AND EUGENICS

839

Data on Human Heredity — Cont'd

TRAIT

HOW inherited
Plural genes, 32

dominance

Dementia praecox (schizophrenia)

Abnormality seems to oc-

pair

cur as an incomplete
recessive

Dipsomania

Probably sim

pie

Abnormality recessive

Average intelligence

Dominant to very high or
very low

Diseases and Diatheses

Hemophilia

Sex-linlced

Abnormality recessive

Allergy (predisposition to hay fever,

Simple

Abnormality dominant

asthma, eczema, migraine, etc.)

Gower's muscular atrophy

Simple

Abnormality recessive

Diabetes insipidus (excessive produc-

Simple

Abnormality dominant

tion of urine)

Mating-s Among Defectives

It is obvious that persons who have inherited scaly skin, lobster
claws, amputated hands and feet, exostotic bones, or who might have
any of many other inheritable defects that incite pity or repulsion
will find difficulty in securing mates. When these abnormalities occur
as mutations, the afflicted persons are not likely to marry on their
own social level, but will probably mate with others who are of lower
grade mentally or who have other abnormalities that make them ob-
jectionable to normal people. Thus there is a tendency for defective-
ness to be precipitated to a social group that can clearly be called
dysgenic. By her process of eliminating the unfit who could not sur-
vive the fierce struggle for existence. Nature formerly kept this group
at minimum size. Today its numbers are being added to, not only by
recruits from higher groups who have a poor heredity either by un-
fortunate segregation of undesirable genes or by the occurrence of
such mutations as have been mentioned, but also by increased repro-
duction by the members of the dysgenic group itself.

The Differential Birth Rate

Nature keeps her creatures fit by giving reproductive advantage to
the best members of each species. Various dioecious animal forms
produce from dozens to millions of young per pair from which, on an
average, two individuals are selected to replace the parents. As a rule,
the two selected are the ones that are strongest and the most free of
defects — these are usually the ones that are best adapted to their en-
vironment. Man has, in the case of his own kind, preserved the weak
and defective individuals that Nature would have eliminated. This has

840

TEXTBOOK OF ZOOLOGY

been done through the application of medical science, together with
public health and other measures that have come with the development
of a humanitarian consciousness. Nothing but praise should be given
to an altruism that saves lives and relieves suffering, but the effect on
our race of man's present practice of preserving individuals that
Nature would have destroyed, without safeguarding the reproductive
advantage of the fitter group, is worthy of consideration.

Fig. 438. — Family from Brazil showing- hereditary absence of hands and feet.
The man in the picture is the uncle of the children shown. The father, who is dead,
had the same deformity. Of the twelve children six were normal and six w^ere
deformed. (From Holmes, Human Genetics and Its Social Import, published by
McGraw-Hill Book Company.)

It has been shown by Lorimer and Osborn* that certain large groups
are increasing so rapidly while others are so diminishing, that the
surviving children of a million women of reproductive age of the first
category will be twice as numerous as the surviving children of a
similar group of women of the second classification. Carried on at the
same rate for three generations (which is only a long lifetime) the
descendants of the first groups will be sixteen times as numerous as
those of the second groups.

♦Lorimer, F., and Osborn, F. : Dynamics of Population, Macmillan.

GENETICS AND EUGENICS 841

Casual observation will make evident that such grouping is likely
to be on a basis of eugenicity and dysgenieity. A number of studies
have been made of the reproductive rate of groups classified by voca-
tion. These studies reveal that passing from the professional and suc-
cessful business classes through the various occupations to that of the
unskilled, transient, agricultural laborer, the number of children per
family rises steadily.

Family Size in Eugenic Groups

The vocational group made up of college teachers might be taken as
an example of a profession whose members have a low reproductive
rate. A recent study made by Kunkel* shows that 4,567 college teach-
ers have 5,932 living children, an average of 1.3 children per teacher.
Dividing the teachers surveyed into three groups according to age, he
found that in the oldest group there is an average of 1.6 children, the
middle group averages 1.42, and in the youngest group, which consists
of those of less than forty-three years of age, there are 0.86 children
per teacher. Since the families of this last group are not complete, the
average of the other two groups, or about 1.5 children per family,
might be taken to indicate the reproductive rate of faculty families.

A correspondingly low birthrate is found among other groups whose
members would be expected to possess traits that should be preserved
for our race. Cattell reports that the average number of children in
the families of the persons listed in American 31 en of Science is 1.88.
But small families are not limited to college professors and scientists,
for those distinguished persons whose names are recorded in Who's
Who in America have families averaging only slightly more than two.

Since the families from which college students come can reasonably
be taken as a eugenic group, several studies have been made of the
sizes of the families represented on the campuses of various American
colleges and universities. The writer kept a record for a ten-year
period of the sizes of the families represented by the students of a city
university of the Southwest. The average number of children m those
families was found to vary from year to year from slightly under
three to slightly over three. Since there were no childless families
represented, these figures are high for the social stratum concerned.

*Kunkel, B. W. : A Survey of College Faculties, Bulletin of the Association of
American Colleges 23: No. 4, Dec, 1937.

842 TEXTBOOK OF ZOOLOGY

What effect college education may have on family size may be inferred
from other studies. Harvard graduates whose year of graduation
would give us reason to suppose that their families are complete, have
produced 1.9 children per married alumnus; allowing for the mem-
bers of the group who did not marry, the average falls to 1.6 chil-
dren. Corresponding averages for Yale are 1.9 and 1.5, for Swarth-
more 2.15 and 1.9, and for Vassar 2.15 and 1.25.

A false sense of eugenic security might be prompted by the belief
that the figures exceeding two in the foregoing citations indicate that
the parents are being replaced and that any residual value represents
a gain. But in the cases of two-child families, what assurance have we
that those children will live to reproductive age, that they will marry,
and if they marry that they in turn will have children ? Considering
these possibilities, it is evident that fertile families must provide for
more than replacement if the group to which they belong is to be per-
petuated. Various computations have been made of the average num-
ber of children per fertile family necessary to maintain the numerical
strength of a group. These estimates range from 3.1 to 4. Consider-
ing that the current incidence of childless families in America is
20 per cent, probably the higher number is more nearly correct, and
it is not safe to place the figure at less than 3.5, or stated another

., . , , Grandmothers t ^t ^

way, It IS only when ^^^^^^^ ^^ daughters -■ ^ *■>"' " «'™° «™"P
is maintaining itself.

Faanily Size in Dysgenic Groups

Various studies have shown that larger families occur among peo-
ple who have but a poor store of those qualities of intelligence, sta-
bility, and physical traits that go to make up racial excellence. Lori-
mer and Osborn found in their study of the school children of selected
eastern cities that children with the lowest intelligence scores (I.Q.
below 60) came from families that averaged nearly six children; that
those with medium scores (I.Q.'s 90 to 110) came from families aver-
aging less than four; and that the children of superior intelligence
(I.Q.'s 140 and over) came from families that averaged less than two
and a half children.

The writer's study of nearly a thousand improvident families of a
type well known to the social workers of southwestern cities revealed

GENETICS AND EUGENICS

843

that in this group the number of births occurring in completed families
averaged 7.9 and the average number of children born to mothers of all
ages was 5.7. The average number of surviving children of the two
groups was 6.1 and 4.6 respectively. The significance of these data in
comparison with those concerned with the size of the families from

III

• i i

I£CSND

Bale without trait

Female without trait

klale and female with trait

Sax unknown; 7 inheritance unknown

Fig'. 439. — Standard pedigree chart. Charts such as this can be used in tracing
a trait tlirough several generations of a family. This particular chart shows the
inheritance of a dominant trait starting with a cross between a male who was
heterozygous for the trait and a female in whom it was homozygously recessive.

which college students come can be realized from the fact that if the
reproductive rates in these two groups continue for ten generations,
the descendants of one hundred families of the dysgenic group will
number more than twenty-eight thousand, while in the same genera-
tion, one hundred families of the present-day college-student group
will be represented by eleven persons.

844 TEXTBOOK OF ZOOLOGY

What Can Be Done?

It must be admitted that the present racial situation has been
brought about through the advance of humanitarianism and science.
The cure for the situation must come through the application of
greater humanitarian and scientific measures. The many corrective
panaceas that have been suggested must be tested carefully and ap-
plied cautiously. Some reforms can be applied at once and are being
applied today to some degree ; others must wait until the time for their
application is more opportune.

Some Eugenic Measures

The step that would be most fruitful of racial betterment is general
education along eugenic lines. If a sufficient number of thoughtful
citizens were informed about racial trends, a eugenic consciousness
would be developed that would cause every proposed social or eco-
nomic change to be considered from the standpoint of its genetic
significance.

The marriage and divorce laws of the various states should be uni-
fied and rewritten along eugenic lines. They should provide for ad-
vance notification of applications for marriage licences as is provided
for by the California laws, and sliould contain provision for health
examinations as already enforced in a number of states. The latter
practice should be extended to include the examination of family
histories.

Many positive measures have been proposed for granting aid of
various kinds to large families of good eugenic stock. At the present
time most of these proposals are impractical, but we might look for-
ward to wage adjustment to family size starting with government
employees, and rental rates of government-owned houses based on a
fixed percentage of the family income regardless of the size of the
house required.

It has been proposed that those who are clearly unworthy of parent-
hood should be segregated in colonies of their own sex. The expense of
this as well as the probability of many social and other problems that
would arise in such a situation challenges the wisdom of such a meas-
ure beyond the degree it is now being practiced in our eleemosynary
and punitive institutions.

Twenty-nine of our states have adopted laws providing for the
eugenic sterilization of such persons as those wlio have been committed

GENETICS AND EUGENICS 845

to sanitaria for mental cases because of an inheritable type of insanity
and who are to be returned to their families. A few states provide also
for such sterilization of habitual criminals and those who are clearly
feebleminded. Eugenic sterilization consists of vasectomy and sal-
pingectomy— operations that bring about sterility without interfering
with endocrine function or normal sexual reactions. Over twenty-five
thousand such sterilizations have been performed under the present
laws.

It is evident from the data previously discussed that family limita-
tion is being practiced by the eugenic group. It is suggested by many
who are facing squarely the problems of racial welfare that those who
are mentally and in other ways far below the majority of our citizens
should have made available to them the means of similarly limiting
the sizes of their families. The number of clinics where such measures
are made available to persons who will make proper use of them al-
ready number in the hundreds.

Some of these eugenic measures are questioned on the grounds that
they violate human rights. AVe should also be considerate of the rights
of the unborn. It is reasonable to say that every child that is to be
born into this world has :

The right to he horn with a sound mind.

The right to he horn with a strong and normal hody.

The right to he horn into an environment in which

his inherited potentialities will have a fair

chance to develop.

CHAPTER XLIV

ANIMAL BEHAVIOR
(By Ina Cox Gardner)*

Introduction

To most people the study of either human behavior or the behavior
of other animals means a study of the mind. This conception came
from the early definition of psychology, which meant the science of
soul or mind. The term mind is an abstract one, and whether or not
any animals including human beings have what we call mind, is a
matter of inference. In the past we have sought to explain behavior
by the use of such terms as instinct, consciousriess, mind, intelligence,
memory, and many others. These abstract terms became supposedly
concrete entities and much valuable time was spent in a search for
them. Psychologists in the modern period reject these nouns and make
use of adjectives such as conscious or mental, which have more precise
meaning in the description of behavior.

The movement which led to the development of animal psychology
into a distinct scientific discipline began with the work of Darwin.
This does not mean that the field had been altogether neglected up to
that time. Even prehistoric man must have been interested in the
behavior of animals that supplied him his daily food. The ancient
Greeks observed animal activities intensively with the definite aim of
attempting to understand and explain them. From this time onward
the problem of the nature of infrahuman behavior has attracted the
serious attention of scientists. In fact, this problem has led to almost
endless controversy from age to age.

Two extreme views have grown out of this controversy. They are
antipodal — at one end of the pole animal behavior is interpreted in
the best possible light. The higher, nonhuman animals are often re-
garded as almost human in intelligence. This view arises from the
use, more or less uncritically, of anthropomorphic analogy. The basic
inference in such analogy seems to be that if an animal acts like a
man it must also feel and reason as a man does. While at the other

♦Acknowledgment is made to Warden, Jenkins, and "Warner, and the Ronald
Press of New York for permission to use certain of the materials included in
this chapter.

846

ANIMAL BEHAVIOR 847

end of the pole, other animals are rated very low in intelligence.
Simple organisms are usually regarded as mere living machines with-
out mental life of any sort. The behavior of even the higher, non-
human animals is explained as due, in the main, to the operation of
blind instincts, although a certain degree of intelligence may be ad-
mitted. Both of these views have been rejected by present-day com-
parative psychologists.

The history of the study of animal behavior may be divided into two
periods. The first, known as the anecdotal period, had its origin about
the time of Darwin, and the collection of Romanes anecdotes may be
taken as a representation of the better classes of anecdotes. Scores of
anecdotal collections appeared in which the tendency to humanize the
mental powers of the other higher animals was carried to the ridicu-
lous. The anecdotes were often taken from unreliable sources and
accepted without critical comment. The anecdotal collections were
widely read and the popular imagination was deeply stirred. This
movement which lasted from about 1859 to 1890 was not altogether
without value, in spite of its absurdities and vagaries, since the inter-
est aroused was later turned into more scientific channels.

A reaction against the humanizing tendency began about 1890 and
led into the experimental period. This period was characterized by a
more critical use of anthropomorphic analogy which led in time to a
rejection of it altogether in favor of a strictly objective position, and
the use of more precise observation and the gradual development of
carefully controlled experimental methods.

The new movement arose from the work of Lubbock, Loeb, Newman,
and Lloyd Morgan. Lubbock, the English naturalist, became inter-
ested in the behavior of insects through the personal influence of Dar-
win. Loeb and Newman, both physiologists of the German school,
sought to analyze the behavior of lower organisms along rigidly scien-
tific lines. The work of Morgan, an English biologist and philosopher,
was restricted mainly to the higher vertebrates. Lubbock was the
first of the group to make use of experimental methods, his studies on
insect behavior appearing in collection form in 1882. Each of these
men, however, was an original and independent worker : Lubbock and
Morgan were most directly influential in the spread of the animal
laboratory among psychologists after the turn of the century.

Morgan was the first to extend experimental methods to the higher
vertebrates. As early as 1891, he spoke of the "trial and practice"

848 TEXTBOOK OF ZOOLOGY

factor in learning, and in 1893 announced the famous Canon of Mor-
gan. This runs as follows : "In no case may we interpret an action
as the outcome of the exercise of a higher psychical faculty if it can
be interpreted as the outcome of the exercise of one which stands lower
in the psychological scale. ' ' This is merely the law of parsimony ap-
plied to animal psychology. While it does not rule out the use of
anthropomorphic analogy entirely, it did serve to introduce a measure
of restraint into such speculation. This canon was very influential
among those, who, while objecting to the extreme view of Loeb, were
ready to reject the absurd anthropomorphism of the anecdotalists.

Two observation methods are used in the study of animal behavior :
(1) The genetic method, wherein observations are made upon an ani-
mal's behavior as it naturally occurs from the beginning to the end of
the life of the animal; (2) The training method, wherein the observa-
tions are made upon an animal's behavior during training and after
it has been trained in certain habits of action or habits of discrimina-
tion among stimuli of different sorts. Every act in an animal's be-
havior is a response to one or more stimuli, produced by conditions
internal or external to the organism. In the whole phylogenetic series,
the internal stimulus is frequently, if not usually, due to the metabolic
processes going on constantly within the protoplasm. The restlessness
of all organisms is to be ascribed to impulses generated by the meta-
bolic processes going on within that organism. An external stimulus
is any change in the environment which affects the organism generally,
or some special organ or part of it in particular, so that the animal's
behavior is modified to some extent at least. No organisms, as a rule,
live under conditions that are always the same, i.e., all organisms are
subject to the effects of stimuli, and their responses are termed reac-
tions. One-celled organisms react to all classes of stimuli that call
forth reactions in the many celled organisms. The nervous system and
sense organs constitute merely a more delicate, complex, and effective
mechanism for the reception of the stimuli to which even the undiffer-
entiated protoplasm of Amoeba responds.

Such stimuli may be classed as chemical, electrical, thermal (heat),
mechanical, or photic (light). The reactions of animals to any and
aU stimuli are classed as tropisms, reflexes, instincts, and habits. The
first three of these are used to connote the many types of native be-
havior, whereas acquired activities are spoken of as habits. In so far
as these terms imply a sharp distinction between heredity and en-

ANIMAL BEHAVIOR 849

vironment, they are objectionable. They are seldom used in the same
sense by different writers in the classification of types of behavior.

Tropistic Behavior

The concept of tropism has its origin in plant physiology where it
was applied to the positive or negative orientation of plant to light,
gravity, or other external stimuli. The term was later used by Loeb
to cover similar growth responses in lower organisms and simple re-
sponses of the reflex type as well. In time, Loeb extended the concept
to include practically all reflex and instinctive actions. Loeb used the
tropistic theory as a definite explanatory principle. He assumed that
symmetrical parts of the body surface of an organism possess identical
irritability value. Orientation behavior is due to a forced turning of
the organism so that the corresponding sides are equally stimulated.
This simple explanation ignores the factor of internal regulation. This
has been pointed out by Jennings, Mast, Buddenbrock, and others.
Tlie tropistic theory is useful, however, in describing the behavior of
Protozoa. Since these forms are composed of but a single cell,
any response is likely to involve the whole body. But even in
Protozoa it has been asserted that the metabolic or internal activities
influence the tropisms. Thus the chemotropic response might be en-
tirely different toward a substance that was food to Protozoa, if
the animal was hungry rather than satiated. Also it has been found
that the reactions of an animal to a strong stimulus may be the op-
posite of its reaction to a weak stimulus. Some insects are negatively
phototropic to daylight while they are positively phototropic to weak
artificial light. And again it has been demonstrated that tropistic re-
actions may be modified through experience. Paramecium is a free-
swimming organism, and when it finds its progress in one direction
hindered by some obstacle, such as a strand of thread, it reverses its
course and heads in a new direction. If in the next trial, the obstacle
is not avoided, the little animal continues its trials until it does
succeed in avoiding the obstacle. The analysis of the behavior
of many organisms shows the trial and error method to be a very im-
portant factor in their behavior. Many revisions have been made in
our concepts of the tropisms during the past few years, but it has not
altered the significance of the theory as a means of describing the be-
havior of Protozoa and lower vertebrates. Various tropisms are
described by the response animals make to these stimuli. Thus we

850 TEXTBOOK OF ZOOLOGY

have phototropism, response to light; thermotropism, response to
heat ; chemotropism, response to chemicals ; thigmotropism, response
to contact; geotropism, response to gravity.

Reflex Behavior

Tropistic behavior is the dominant type of behavior in the organism
without a well-differentiated nervous system. The protozoa are with-
out a nervous system, and though a few of them have threadlike struc-
tures running through their bodies in such a way as to suggest a cor-
relating mechanism of some sort, it seems that, in general, this sensi-
tivity and conductivity are properties of the protoplasm itself. In
such animals as Hydra and the jellyfish, as well as in all higher ani-
mals, we have to do with activities promoted by nervous tissue. In
such animals, we no longer call their behavior tropistic but speak of
their simplest reactions as reflexes. In principle there is little dif-
ference between a tropism and a reflex except in the relative com-
plexity of the mechanism involved. The reflex is the dominant type of
behavior in the segmented animals which have a simple ganglionic
type of nervous system. Also according to Coghill reflexes represent
secondary patternings of behavior in the developing vertebrates. The
sensitivity and conductivity involved in the production of a reflex are
the same qualities of protoplasm as are concerned in tropisms, but
they are enhanced by the special differentiation of nervous tissue in
these particular directions.

The human being, as weU as other animals, exhibits reflexes. They
are sometimes called physiological reflexes and are reactions, such as
the wink of the eyelids which follows the entrance of a grain of sand
into the eye or which follows the sudden approach of an object to the
eye. Other reflexes are the knee jerk, contraction of the pupil of the
eye (pupillary reflex) when a strong light is flashed into the eye, and
the general start of the whole body when a sudden and unexpected
noise occurs.

These reflexes have two characteristics which are important. First,
they are brief in duration, consisting, in general, of a single contrac-
tion or relaxation pattern of a group of muscles. Second, they are
predictable ; a particular stimulus always, in the normal subject, pro-
duces the reaction. If the reaction (such as the knee jerk or the pupil-
lary reflex) does not follow the appropriate stimulus (the blow on the

ANIMAL BEHAVIOR 851

tendon or the flash of light) this failure is evidence of an abnormal
condition of the reaction.

Reflexes terminating in action patterns of smooth muscles and
glands also occur. The saliva starts to flow when the odor of food
assails the nostrils. It has been conventional to describe reflexes as
unconscious reactions, but as a matter of fact there are no such reflexes
in the normal human being, and apparently none in other vertebrate
animals. Striking a blow on the patellar tendon, for example, does
not merely produce a reaction terminating in contraction of the quad-
riceps muscle which propels the lower leg forward. Effects are demon-
strable in other parts of the body and included in the total response
is the perceptional awareness of the blow. Expectant attention of the
blow and attention to other stimuli affect the leg movement. The pre-
dictability and brevity of the reflex are only relative. The important
difference between a reflex and a perceptional reaction is that the re-
flex would occur and be still more predictable, even if the animal were
so mutilated that the neural patterns, aside from the parts most di-
rectly affected, were eliminated. For example, if by accident the spinal
cord of a person is broken above the lumbar region, and several
months are allowed for the effects of shock to disappear, the knee jerk
may be elicited and is more pronounced and more uniform for a given
force of blow than before. The person in such a condition does not
perceive the blow unless he is allowed to see it or hear it, and he can-
not perceive the jerk of the leg except by sight. The discrimination of
the lower part of the spinal cord, through which the essential part of
the reflex transit occurs from the upper part of the cord and brain,
prevents the stimulus from affecting more than the leg muscle. Similar
results can be obtained from dogs and cats by experimental severing
of the cord at proper levels. In these cases the reflex becomes a real
unconscious reaction, which it is not in the normal animal, and it is
unconscious because it is restricted to a part of the organism.

Chain Reflex Behavior

Chain reflex behavior in animals is also often called instinctive be-
havior. Watson defines instinct as a series of concatenated reflexes
which unfold serially upon proper stimulation. In insects and other
animals, we find that reflexes often occur in series ; one directly called
out by some appropriate stimulus, itself becomes the cause of another
which follows it, and so on through a longer or shorter series. These

852 TEXTBOOK OF ZOOLOGY

are often called chain reflexes or instincts. Psychologists are inclined
to drop the word instinct, because of the disagreement in definition.
The term chain reflex seems to describe better the reaction of animals.
A chain reflex, like a simple reflex, is an inherited type of reaction
and may appear so complicated and purposeful in character that the
inexperienced observer is often led to describe it as intelligent experi-
ence or reason. It differs from a simple reflex, furthermore, in that
it involves generally the reaction of the animal as a whole rather than
that of single parts of organs. In its most complex forms it often de-
fies analysis, as in the case of migration of birds, or can be only in-
completely analyzed, as in nest-building, care of young, and many
others.

Habitual Behavior

The terms instinct and habit are usually applied to relatively com-
plex types of behavior — the one native and the other acquired. Ber-
nard has shown that psychologists find it difficult to separate complex
behavior into the two categories. This is true because most instinctive
acts involve a considerable amount of practice in becoming estab-
lished. Practice is the earmark of habit. The capacity to form habits
is Dunlap's definition of instinct. Habits are also defined as the
modification of response tendencies which show probabilities of cer-
tain high degree, that particular responses will occur in particular
circiunstances.

As we have seen, the animal has a set of native reactions to external
stimuli which we caU reflexes. A habit is an acquired response that
somehow came to be associated with a stimulus to which it is not ordi-
narily attached. Formerly the animal, learning this new association,
was said to possess associative memory and this was thought to be indic-
ative of consciousness in such animals. Associative memory was ac-
credited only to higher animals. Today in many of the lower animals
experimentalists find a reaction at first directly called forth by a cer-
tain stimulus may later be provoked by another, even an unrelated
stimulus. Ahnost every modern student of psychology or physiology
knows of Pavlov's experiment with the dog whose mouth watered
when a bell was rung as a signal of feeding time. This Pavlov called
a conditioned reflex. It has been produced experimentally in a great
many kinds of animals. Conditioned reflexes years ago were called
associations and were taken to indicate the presence of associative
memory. The term associative memory implies a certain subjective

ANIMAL BEHAVIOR 853

interpretation, and psychologists today are seeking to leave out all
such terms in the study of animal behavior.

Most of the behavior of animals can be explained on the basis of
tropisms, reflexes, chain reflexes, and conditioned reflexes used in the
formation of habits. In most cases the other animals' capacity to
form conditioned reflexes and handle themselves in a complex environ-
ment is much more limited than in the average man. Whether the
higher animals, other than man, can reason is a question that has been
much debated. Reasoning is a term upon which there is not unanimous
agreement as to meaning. If it merely means problem solving, one
would say the individual is able to reason because most higher ani-
mals can solve a problem if the problem is suited to the animal. If
reasoning involves the use of symbolic processes or abstract ideas,
then we are much more in doubt.

The cases of so-called intelligent animals such as "Clever Hans"
and "The Horses of Elberfield," when examined by comparative
psychologists were found to be making use of motor cues, and these
had been conditioned by trainers. These so-called intelligent horses
could add, subtract, multiply, divide, and extract square root and
cube root. They could solve difficult problems, as well as easy ones —
but they could not give correct responses when answers were unknown
to experimenters and audiences. Reasoning could not be accredited to
them.

In Hunter's experiments on delayed reaction, the rats and dogs suc-
ceeded in going immediately to the lighted box after a period of de-
lay only when the body or a part of the body was kept oriented to-
ward the lighted box. The monkeys and children were able to succeed
when the body was not oriented, thus giving evidence of the use of
some other process than bodily orientation.

If we hold to the definition that reasoning involves the use of sym-
bolic processes, there is little evidence of reasoning in animals, other
than human.

CHAPTER XLV

PALEONTOLOGY
(By W. M. Winton, Texas Christian University)

Paleontology deals with fossils. One of the most difficult biological
concepts to define satisfactorily is the term fossil. Broadly speaking,
the word should refer to records left by organisms and not by non-
vital forces. Expressions such as "fossil ripple marks," "fossil sand
dunes" and the like should never be used.

On the other hand, any recognizable trace left by an ancient or-
ganism is a fossil. Bones and shells, of course ; but also scales, plates,
impressions, burrows, leaf prints, intestinal droppings or coprolites,
all are fossils.

Among the most interesting fossils are tracks. The poet's "foot-
prints on the sands of time" in Triassic valleys of New England,
the "thunderbird dance floors" of the Navajo reservation, and the
huge tracks around Glenrose, Hamilton, and Hondo, Texas, are all
dinosaur tracks. Abundant as are skeletons of Mesozoic dinosaurs,
many are known only by their footprints hardened in the shores of
ancient seas.

Other indirect records may be of considerable biological importance.
In the deposits of the Lower Cretaceous division of Texas a few ex-
amples of the extinct oyster, Exogyra, have been found which exhibit
pearls. These pearls (the best specimen is in the Museum of the Bu-
reau of Economic Geology) are of the kind known as body pearls. A
quarter of a century of research by the Marquesan commissions of the
French Government has shown us that pearls of this kind are
caused only by parasitic flatworms. With the Gallic romantic touch
evident even in a scientific report, one writer puts it, "our most
beautiful pearl, then, is but the sarcophagus of a miserable worm."

Pearls in Lower Cretaceous oysters tell us that parasitic flat-
worms appeared on the earth at least as early as mid-Mesozoic

854

PALEONTOLOGY 855

times. With this great span of time (150,000,000 years)* one can
partly understand how each species of mammal today has one or
more species of tapeworms adapted to life in the body of its host
only.

Some pathogenic bacteria when attacking an animal body leave
unmistakable lesions in various parts of the skeleton. The late
Dr. Roy L. Moodie, working first at the Baylor Medical School and
later at the University of Illinois, was able to date the appearance
on the earth of many important disease bacteria. Dr. Moodie
studied large collections of skeletons of ancient reptiles and mam-
mals and found lesions which the modem pathologist associates
with the various bacteria in question.

Before leaving the matter of indirect records, one more instance
will be mentioned. Some predatory carnivorous dinosaurs left on
the vertebrae and other bones of their victims, deep tooth prints
and scoriations as the only record which keeps them from oblivion.
Are these tooth scratches fossils!

The last fifteen years have seen an unprecedented rise in the in-
terest in paleontology and in the number of full-time workers in
this field. Shortly after World War I, the pressing search for
more petroleum brought out strongly the value of fossils in estab-
lishing "horizons*' or levels which the oil geologist could use in
determining favorable domes and other structures into which to
drill. Immediately, workers in what had, hitherto, been one of the
most academic subjects were lured from classrooms and museum
benches into one of the most competitive and commercial industries.

At first attention was given only to the usual larger fossils found
at the surface, but as the science of petroleum geology advanced,
it became more and more important to know the levels or horizons

*How we have come to change such expressions of the earlier geologists as "an-
cient," "very ancient" and "extremely ancient" to round figures is a long and fasci-
nating story which cannot be taken up here. The students of geochronology or age
of the earth after many unsuccessful attempts based on rate of increase of salinity
of the sea, change of the ellipticity of the earth's orbit, addition of the elements of
the known geological column and others which produced figures which made us
human beings, earth's most recent inhabitants, gasp, have now found a dependable
check on absolute time.

The earlier estimates, and that is all they were, are characterized by only one fea-
ture common to all and that is a large figure for the age of the earth. As large as
these figures were, they have been replaced by others which are truly staggering, and
the new figures are believed accurate. At least different workers get closely parallel
results.

The unvarying time clock of a disintegrating radio-active mineral and its use in
determining the absolute length of time since the beginning of the various geologic
periods are discussed fully in any new book on historical geology or perhaps more
entertainingly in Wells' Science of Life or Jeans' Through Space and Time

856

TEXTBOOK OF ZOOLOGY

which were successively penetrated by the drill. Only microscopic
specimens persisted, as the drill crushed the others to unrecogniz-
able bits. Then came the remarkable rise of the young science of
micropaleontology, a field in which women have distinguished them-
selves and proved themselves equal to or better than men.

A D O A

a""A'n'"A"0

"A'0'0"'

first M titled.

S econi MetKoA

Fig. 440. — Diagram to show the vertical distribution of fossils. AjD.OjA.H.O
are characters, each indicating a species which has a thousand feet or more definite
vertical range. The middle of the figure illustrates the first method described In
the text, and the right portion represents the second method.

Some changes have been made in the classic methods of the
science of paleontology, although in the main the newer methods
are identifiable as merely refinements of the older.

Men have long known that some species lived yn the earth much
longer than others. In the case of the ordinary marine species, the

PALEONTOLOGY 857

relative length of the life of the race in question is shown by its
vertical extent in the sediments. A short-lived species left its fos-
sils in a very few feet of sediments ; others are recorded in hundreds
of vertical feet ; others in literally thousands ; and a few species
(of Globigerina) have lived uninterruptedly from the Cretaceous
period to the present, representing many miles of vertical extent in
the sediments.

"Whether dealing with larger fossils or with microscopic ones,
certain features are desirable when doing geologic work. An ideal
fossil species: (1) had a short species life; (2) should have left
abundant fossils, since a species which sharply marks off a narrow
part of the column is valueless if it is very difficult to find; (3)
should have had wide distribution, leaving its record over a large
area ; (4) should be readily recognizable, requiring a minimum of
time-consuming measures, such as the counting of plates and spines
or measurements of different features.

The only fossils which completely satisfy the above-mentioned
requirements are the ammonites (Fig. 148). The various species
were short-lived; in some cases the fossil specimens are actually
thicker than the sediments which were laid down during the life
span of the species. Most ammonite species occurred in large num-
bers, literally dominating the seas. Most ammonites had a wide
distribution, sometimes world-wide. Often the chambered shells,
comparable to those of nautili, acted as airtight bulkheads after the
death of the animals, and the shells floated to great distances. Thus
they dated sediments of the same time, whether the animals lived
in the waters above any particular locality or not. Finally within
a region, most of the ammonite species are characteristic and easily
identified.

But ammonites are valuable only when working with rocks ex-
posed at the surface and only in a limited part of the geologic
column.

Efforts have been made, naturally, to use less perfect fossils, and
methods have been developed to overcome difficulties when the
species concerned lack one or more aspects of the ideal set forth
above.

Since too great a vertical range is the most serious difficulty en-
countered in establishing precise levels in the strata, a brief sum-

858 TEXTBOOK OF ZOOLOGY

mary will be given of the particular methods which apply to this
problem. Two of the methods are here described diagrammatically.
First, is the device of bracketing or use of associations, the term
being used here with much less exactness than in ecology. Several
long-lived species are represented by the symbols (triangles, squares,
circles) , Each is a species which has a thousand feet or more of ver-
tical range, and the ranges are known. In a well sample the appear-
ance of no single one of these species would be of much use to the
paleontologist. Certain associations might indicate limited horizons
upon which the petroleum geologist could carry his structures from
well to well.

Using this method of bracketing on the larger fossils in the Lower
Cretaceous period of North and West Texas during the past twenty
years the writer has been able to locate for engineers, where highways
should run to have the greatest volume of the cuts in soft marls. He
has also, on many occasions, been able to foretell for builders the
exact depth to which foundations would have to be carried for large
buildings, dams, and other masonry structures.

The greatest development of this method has been in the work
with microfossils. As long ago as 1922, three young women,* work-
ing in the laboratories of oil companies in Houston, astounded a
large audience of petroleum geologists by demonstrating beyond
question that they could locate horizons as narrow as twenty feet.
This was done in the soft Tertiary sediments of Southeast Texas,
where geologists, up to that time, had been satisfied with divisions
running from five hundred to a thousand feet.

The second methodf is based on the fact that many marine or-
ganisms suddenly disappear from the seas which they have long
occupied. This extinction is caused by a change in food supply, a
change in temperature or other factor, or by the appearance of new
enemies. A rapid extinction of a species, in terms of the record in
the sediments, may be said to be almost instantaneous. In other
words, the end of a species is more likely to be startlingly abrupt
than is its beginning. This is true, notwithstanding the fact that
with favorable conditions a species can populate an area of land
or a portion of the sea in a very short time. An everyday example

•Misses Hedwlg Knicker and Alva Ellisor and Mrs. P. L. Applin.
tFlrst published by Mr. N. L. Thomas, paleontologist for the Pure Oil Company,
ftltliough not claiming- the method as original.

PALEONTOLOGY 859

of the rapid disappearance of a species from the face of the earth
is the tragic case of the passenger pigeon.

Working on this basis, the paleontologist often will use the
''top" of an extended horizon as a fairly precise marker. This
method is especially favorable for the micropaleontologist working
with well samples, as the drill encountei-s the top first.

Biologists working exclusively with living organisms or at least
living species sometimes are puzzled by the conventions which paleon-
tologists have developed. Show a new flower to a botanist and his
first effort is to place the specimen in the family to which it belongs.
Show an insect to a zoologist and his first thought is to refer it to its
proper order. Just as the botanist recognizes families and the zoologist
recognizes orders the paleontologist recognizes (or attempts to do
so) both of these divisions in his fossils. But the working unit in
paleontology is the genus. Undoubtedly, many of the so-called genera
of fossils, if we could see the animals restored to life, might actually
be families or in some cases might be species.

The old question of the biologist of "what is a species?" is even
more difficult to answer in the ease of fossils. A convention which
is fairly well adopted is to consider the different examples of a num-
ber of varying fossils as the same species if the varying characters
exhibit gradations between the extremes. Thus, sometimes, quite
variable fossils are put in the same species. On the other hand,
examples which do not appear superficially to differ very greatly
may be placed in separate species because certain differing charac-
ters do not intergrade.

Some paleontologists consider, also, that vertical range should be
taken into consideration. The time factor is an important one in
all branches of geology. No one has ever seen a part of any living
species of plant or animal change into a new species. Such changes
are common in fossils. No observant person denies the customary
assumption that the Kaibab squirrel of the north rim of the Grand
Canyon descended from an isolated colony of the more familiar
Abert squirrels. This change required many thousands of years,
yet in a vertical cliff face of forty or fifty feet, exposing ancient
sea bottoms, we may see a record of a quarter of a million years of
the history of the sea and its life.

The paleontologic picture, however, is a badly distorted one, and
the imagination must be exercised vigorously to reconstruct the

860 TEXTBOOK OF ZOOLOGY

ancient seas and their life. If we examine the life in a modern
tropical sea as shown in a Williamson motion picture, it is difficult
to realize how few of the objects we are viewing are apt to leave a
permanent record.

The corals, of course, will survive eternally. The sea anemones
will leave no trace unless some individual may die and leave the
impression of its body in the mud before the tissues are disinte-
grated by bacteria. Most of the gorgeous fishes will leave no rec-
ord. As one grows ill and weak it is attacked by its fellows and
the leftover scraps if they reach bottom will be snatched up by
Crustacea. Only an occasional tooth or vertebra Avill survive.

Sometimes in geologic history a great underwater volcanic deto-
nation would kill all of the fish in a small area of the sea, laying
them down to be buried in the mud. Thus we get aji occasional
glimpse of masses of tangled fish skeletons furnishing us a clue to
the great numbers of fish which have lived and of which we have
no record. Volcanic detonations of this sort were more common in
Devonian time than in ajiy period before or since. For a long time
geologists incorrectly called the Devonian "The Age of Fishes,"
although we know now that since this time, there have been many
more kinds of fishes and more of a kind.

Returning to our under water movie, we may note that the
holothurioids will leave only a few microscopic denticles and dermal
plates. The marine worms, if they are tube builders, will leave their
tubes. As for the clams and pectens, some of them will leave their
shells, although most of the shells will be crushed by octopuses and
other scavengers and the small bits will be passed repeatedly through
the bodies of marine worms in an effort to squeeze the last milli-
gram of nutrient material from them. Some of the bivalves will
settle in the mud, and even if the shells disintegrate, neat mud casts
will be left. In the Southwest mud casts are the most common
fossils.

The foraminifera, ostracods, diatoms and other minute organisms,
which the picture does not show us but which we know are there,
will likely leave a record, provided they have hard tests. Many
thick deposits of shales exhibit no fossils which may be seen by
the unaided eye, but are teeming with microfossils, principally
foraminifera.

PALEONTOLOGY 861

The chances of the ordinar}- mariiie animal persisting as a fossil
are decidedly slim but are infinitely better than those of a land
animal. Land animals are most likely to leave a fossil record in
some natural trap in which large numbers of individuals became
concentrated. "Well-known examples of this phenomenon are the
La Brea asphalt pits of California and the sandstones of the Dino-
saur National Monument in Utah. In one, asphalt underlay shallow
pools of water where Pleistocene mammals came to drink. The
larger herbivores often became mired and their cries would attract
their fellows and also the predatory carnivores. Many would be-
come trapped. The concentration of some kinds of skeletons in these
pits is astonishing. The hard Mesozoic sandstone from which so
many reptilian remains have been chiseled in the Dinosaur Monu-
ment was originally a quicksand which served as a trap.

Since most finds of vertebrate fossils are fragmentary, the art of
reconstruction has been highly developed. An exaggerated idea has
become widespread that a paleontologist needs only a single bone
or tooth to reconstruct the entire animal. It is true that some fos-
sil species have been named, based on a single tooth or bone. In
some cases, more of the skeleton has been found or remains of other
individuals of the species have been turned up.*

When the famous fossil Archaeopteryx was found in the litho-
graphic limestone of Solenhoffen, comparative anatomists and em-
bryologists already were agreed that the birds sprang from a branch
of the reptiles. The finding of this specimen, exhibiting a true half-
way stage and showing both reptilian and avian structures, started
another incorrect idea which has persisted to this day. This is the
notion commonly held by unscientific persons that paleontologists
are constantly searching for ''missing links."

Another incorrect idea, spreading unfortunately to scientific circles,
concerns the gaps in the geological record. Admittedly, gaps still
exist, but they are far fewer and far less significant than they were
in the days when Darwin and Wallace commented so vigorously up-
on this matter. In fact, the major developments in paleontology
have been made since the daj's of Darwin and Wallace, and the
findings have been incorporated in the general disciplines of bio-

•The methods used in reconstructinsr vertebrate fossils are describecl in Scott's
History of Land Mammals of the Western Hemisphere.

862 TEXTBOOK OF ZOOLOGY

logical science. Courageous, indeed, is the modern biologist who
will utter generalizations which contradict the ever-growing story
told by fossils. ■

Some points, of course, may never be settled. Paleontologists,
for example, without exception are neo-Darwinists and believe im-
plicitly in natural selection as the major cause of the new species
constantly appearing on the earth. In this respect, they are con- \
sidered by some biologists as being old fashioned. Certainly paleon- /
tologists could profitably inform themselves on some of the newer
aspects of genetics, and it is eciually certain that biologists should
know more about fossils

CHAPTER XL VI

PHYLOGENETIC RELATIONS OF ANIMAL
GROUPS AND THE THEORY OF EVOLUTION

It is likely that no student of modern biology has gone far before
he has realized something of a progressive relationship between the
various groups of animals in the animal kingdom. Phylogeny refers
to the background of what has gone before in producing a race or
phylum and incorporates the thought that different species have
arisen from common ancestors. The general idea of it, as now ac-
cepted by most biologists, is that all living organisms have been
derived through normal reproduction and variation by adaptation
from simpler, more generalized ancestors. Phylogenetic relation-
ships of organisms and the origin of species have long been topics
of exceptional interest to biologists. Two questions which have
intrigued the thought of many are : first, Avhence came living ma-
terial? and second, what has been its course of events since the
time of origin?

The observations and thought of Charles Darwin gave the first
substajitial support to the idea of all modern species originating
from preexisting organisms. Results of his careful studies were
presented in 1859 in his book The Origin of Species, which is now
a classic in the field. There is still lack of agreement among scien-
tists concerning the details of origin of life and species, but there
seems to be little doubt that species, genera, and even larger groups
have progressively developed since life originated on earth. The
ideas of continuity, development, and differentiation of living organ-
isms are quite generally accepted by those who have studied these
problems. The lines of kinship of animal groups are traced only
through common ancestry, and it is not ordinarily assumed that
direct relationship exists between representatives of modern groups.

Sources of evidences of relationships are quite widespread, coming
from such studies as zoogeography, comparative anatomy, embry-
ology'', physiology, paleontology, and even others. By combination
of information gained from such sources much has been learned of
the past history of many groups of organisms. Even these available
sources give incomplete and sometimes inconclusive evidence of the
history of any particular organism. The individuals are recognized

863

864 TEXTBOOK OF ZOOLOGY

as being only points on a long line of modifications. The two ends
of this progressive line are seldom recognized with any finality.
Darwin, and many others since his time, recognized three funda-
mental facts that bring about a progressive continuity among living
things: heredity, with the tendency for organisms to resemble their
parents ; variation, in that no two organisms are exactly alike, hence
the resemblance between parent and offspring is not absolute ; and
constant 'production of more offspring than can survive. The phylo-
genetic relations of animals show all indications of having been
brought about by the operation of the above-mentioned factors in
an orderly, evolutionary progress.

Variation is one of the most obvious facts and consistent phe-
nomena in the animal kingdom. As long as differences among in-
dividuals of a group are minor or irreguarly distributed the group
is apt to be considered a species. However, the species is not rigidly
bounded and it must be remembered that continuous variation is
in progress within this group as one generation follows another. It
is becoming apparent to man that new groups have arisen and are
arising within old groups of animals, also that old groups have broken
up, some to become new ones and others to become extinct. These new
groups were separated by small differences at first but gradually they
attained greater and greater divergence from the common form. This
is particularly likely where the different groups have become widely
separated geographically or exist under distinctly different conditions
of life. Frequently, it is only the widely separated extremes of the
group which carry on the posterity, the intermediate forms having
died out. In this way rather widely separated species may have
arisen from common stock, and the lack of intermediate forms may
accentuate the wide divergence between them. During the millions
of years which life has existed on earth it seems evident that this
process of divergence between groups has been in operation until
there now exist many definable groups with distinct lines of rela-
tionship. Our natural system of classification is based on the rela-
tions and differences established for the different animal groups.
Comparative studies of numerous animal groups will help to show
some of these relationships.

Colony Formation in Certain Protozoa

Due to incomplete separation of cells following division in Pro-
tozoa, colonies are begun. In some instances groups of cells in some

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 865

such colonies, e.g., volvox, become specialized as gametes for repro-
ductive purposes and the other cells remain as somatic cells. Be-
cause of the similarity between various mastigophoran colonies and
blastula stages in metazoan development, this group has frequently
been cited as the predecessors from which Metazoa at some time
originated.

Development of the Gastrula

The primitive gastrula is thought to have had its origin from a
spherical colony of Protozoa by a more rapid growth of the cells
at one pole which brought about an infolding or invagination to
produce a new cavity from the exterior. This development estab-
lishes a body composed of two general layers. Modern adult coe-
lenterates, like hydra, and others demonstrate precisely this condi-
tion today, with the outer ectoderm and the inner endoderm, each
composed of cells serving vital general functions. This allows all
cells surface exposure either directly on the exterior or to the cavity.
By this means larger and more complex organisms were developed
and have lived.

Trochophore Larva

In Platyhelminthes, Annelida, Molluscoidea, Trochelminthes and
Mollusca there occur larval forms of the trochophore type. Al-
though the surface ciliation and some other superficial features
differ somewhat, a direct relationship among these forms is trace-
able through the larva. The fundamental morphology of the cteno-
phore is similar to that of the trochophore larva, and the Platy-
helminthes are generally thought to have been derived from Cteno-
phores. The regular arrangement of gonads and the even, orderly
distribution of the diverticula from the intestines of the flatworms
are interpreted by many to indicate preparation for segmentation
as it appears in annelids.

Peripatus and the Wormlike Ancestry of Arthropoda

Peripatus, the only representative genus of class Onychophora, was
considered a segmented worm for a long time because of its shape, its
even, conspicuous segmentation, and possession of nephridia for ex-
cretion. But later it was discovered that the respiration is carried
on by tracheae, and the body cavity serves as a blood space, both
of which are typical arthropod features. In addition, the paired

866 TEXTBOOK OF ZOOLOGY

legs are jointed, although similar in appearance to paropodia of
Anjielida. There are two jointed antennae on the head and some
jawlike plates in the mouth. Situated as it is, midway between
annelids and arthropods, this form seems to show an immediate
transition from the one group to the other. Upon this basis it is
usually held that the arthropods have a wormlike ancestry.

Interestingly enough, there exists another idea of arthropod an-
cestry from the fact that all of the lower forms of Crustacea pass
through a characteristic stage knoAvn as the Nauplius. This larva
does not correspond very closely to any strictly annelid stage, but
with its short body and three pairs of appendages it resembles a
modified trochophore larva. The nauplius larva has some features
in common with the rotifers which authorities feel may have arthropod
tendencies.

Echinoderms and Their Larval Relations

Although the adult echinoderms possess radial symmetry, that
seems not to have been in the immediate phylogenetical background.
The larvae of all echinoderms possess bilateral symmetry. These
larvae all have definitely arranged bands of cilia over the body.
Embryologists have pointed out the fact that these larvae are not
directly related to the trochophore. They show more resemblance
to the Tornaria larva of Hemichordates than to any of the non-
chordate forms.

Ancestry of the Vertebrates

The establishment of relationships between the chordate and non-
chordate animals has been one of the perplexing problems in the
study of phylogeny. Different students of this problem have in-
vestigated the possible relationships of such nonehordate groups
as the flatworms, nemertine worms, annelids, arachnids, insects, and
echinoderms. Their investigations have resulted in the formulation
of a number of theories putting forth the various ones of the above-
mentioned groups as the progenitors of, or claiming common an-
cestry with, chordates.

Some authors insist that the chordates have arisen from some
segmented form ; others conclude from their evidence that this is
not necessarily true or essential. All of the theories establish their
relationships to the vertebrates through the protochordates, which
are represented by Amphioxus, the tunicates, and hemichordates.

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 867

Eacli of these has been considered as ancestral stock, which has
contributed to the origin of vertebrates. It is usually conceded by
authorities in this field of study that Amphioxus is a modified an-
cestor of the vertebrates, due to the clear-cut and well-defined con-
dition of the distinctive characteristics of the chordates and the
presence of a midventral endostyle. Next in the line of thought
would come the possibilities of ancestors of Amphioxus. The tuni-
cates have been given this distinction by some. The adult has lost
most of its typical primitive characteristics, but the larva possesses
the distinctive characteristics of chordates and also the ventrally
located endostyle, very similar to these structures in adult Am-
phioxus. It has been suggested that probably the adult tunicate
once existed as an animal similar to its larva of today, and that
its forebear was not only the ancestor of modern tunicates but also
the form from which the Amphioxus group has descended.

Hemichordata, which is usually considered the most primitive of
chordates, is regarded as a jjossible ancestor of, or as possessing
common ancestral stock with, tunicates and Amphioxus. As will
be remembered from the previous study of Hemichordata, its repre-
sentatives possess gill slits, a support in the base of the proboscis
which may be homologous to the notochord, and four longitudinal
nerve cords of which the dorsal is the most highly developed. The
above protoehordate relations are rather generally conceded, but there
is much less agreement concerning their origin, and several theories
have arisen of which the following are important.

Annelid Theory.— The segmental condition of this group, the re-
lationship of the digestive sj^-stem to other circulatory and nervous
systems, and presence of the coelom with related nephridia, present
a close comparison to what is found in the embryonic development
of the vertebrates. It has been suggested by some scholars that by
inverting the body of the nonchordate annelid, the fundamental
systems are brought to resemble their relative locations in verte-
brates. A fibrous cord has been found in some groups of the an-
nelids, and this structure is held to take the place of a notochord in
function and position. These fibers are found just dorsal to the
chain of ganglia in the annelids.

Arachnid Theory. — Such forms as scorpions, Limulus, and other
arachnids have been favorably compared to vertebrates. By com-
parison of these arachnids with the extinct, fishlike ostraeoderms,

868 TEXTBOOK OF ZOOLOGY

an elaborate theory of the possible origin of the vertebrates from
this ancestry is derived.

Echinoderm Theory. — This theory of vertebrate descent goes again
to Hemichordata. The developing egg of this animal becomes
a larva known as Tornaria, which floats in marine waters, has bi-
lateral symmetry, is almost transparent, and possesses bands of cilia
used in locomotion. This larva is almost exactly like that of the
starfish and other echinoderms which live in the same habitat. The
close correspondence of features of these two groups of larvae has
suggested the conclusion that these two types of animals have de-
scended from a common ancestor which was similar to these larvae.
The line of descent of one branch of this stock has presumably passed
through Hemichordata, Tunicata, Ampliioxus, and Vertebrata. The
nonchordate ancestors are not j^et conclusively determined, but the
foregoing theories suggest the thinking and evidence along that line.

Within the subphylum Vertebrata the relations are somewhat more
evident, but the phylogenetic sequence is rather obscure at some
points. Cyclostomes, the simplest vertebrates, are most closely related
to Amphioxus, which has been suggested as the protochordate most
similar to vertebrates. These very primitive fish have an eel-like
body without paired fins and without jaws. They have from seven
to fourteen branchial (gill) apertures in different species, and all
of them possess skeletons composed of cartilage. The cartilaginous
skull is not entirely closed dorsally but resembles a trough with
bars over the roof. The anterior end of the nerve cord has expanded
to become a brain (Chapter XXVI). Next in order of complexity are
the Elasmobranchii, which possess well-developed, paired appendages
(fins) and jaws (Chapter XXVI). They also have a cartilaginous
skeleton, but the skull is much more complete dorsally. The number of
gill arches is reduced to five, but the apertures are uncovered as in
cyclostomes. The number of aortic arches has been reduced
from the sixty to ninety pairs of Amphioxus or seven pairs of
lamprej^ to five pairs. The group of ganoid fishes, which was the
dominant Devonian animal, is generally conceded to have Elasmo-
branch ancestry. Most ganoids have more or less cartilage along
with the bony structure of the skeleton. Their gills are covered
with an operculum, and there are only four aortic arches. The bony
ganoids are usually thought to be the ancestors of true bony fish.

PHYLOGENETIC RELATIONS OF ANIMAL, GROUPS 869

There is an extinct form of Amphibia, Stegocephalia, which shows
relations to the ganoid fishes and for this reason the ganoids are
usually named as ancestors of Amphibia. Some authors hold that
the lungfishes, which represent an independent branch of the Elas-
mobrajich group, are the ancestors of Amphibia because of their
ability to breathe air and live out of water. However, the former
view of the phjdogenetic relation between fish and amphibia is most
commonly held. Amphibia have well-developed bony skeleton with
paired appendages for locomotion on land. Lungs have appeared
as a means of aerial respiration, and the aortic arches have been
reduced to three.

Reptiles are supposed to have descended from Stegocephalia also,
with most modern reptiles coming by way of Bhynchocephalia which
is represented by one living species, Splienodon punctatum. The
snakes, lizards, crocodiles, and the extinct dinosaurs have probably
branched from this group, while the turtles are thought to have de-
scended through Theromorpha, another extinct branch of Stego-
cephalia. The dinosaurs are credited with the ancestry of birds by
way of a toothed, feathered, extinct form known as Archaeopteryx.
It was essentially a flying reptile. The mammals probably descended
from the reptilian group Theromorpha by way of our modern mono-
tremes which lay eggs, hatch them out, and then suckle the young
with milk from mammary glands. The marsupials, such as kan-
garoos and opossums, are next in order, and from these it is thought
the Placentalia have arisen. "Within this group some authorities
hold the view that the Primates, the order including man, have
arisen from Insectivora. The apes and monkeys belong to the Pri-
mate group, and there has been some misunderstanding among lay-
men generally in regard to the possible relationship of man and the
apes. Most people have the misconception that this is a linear
descent in which the most advanced member of the lower group
represents the immediate ancestor of the next higher group. As
a matter of fact, the theory is not that the higher monke3''s are in
the process of becoming apes and the higher apes becoming men,
but that all three of these groups have had origin as different lines
from a common primitive form.

Recapitulation Theory. — In the early part of the nineteenth cen-
tury von Baer observed that the early stages of vertebrate embryos
of different classes had a very close resemblance to each other. He

870 TEXTBOOK OP ZOOLOGY

did not subscribe, however, to the recapitulation theory when it
was formulated later. Haeckel, coming a little later, became con-
vinced that the developing embryo lives over again the stages
through which its whole race has passed, and he formulated the
recapitulation theory or biogenetic law from this idea. In other
words, the organism in its individual life tends to recapitulate the
different stages through which its ancestors have passed in their
racial history. Briefly this same statement is, ontogeny recapitulates
phytogeny. The rehearsal of the phylogeny is in rather slurred form
in some details, but the basis for the idea is readily seen. In brief,
the theory is applied by comparison. Nearly all metazoan organ-
isms begin life by the union of the two germ cells to form a single-
celled zygote which is the new organism. At this time it is com-
parable to the Protozoa. During the ensuing cleavage divisions a
colonial form is represented. Following this, when one side infolds
to form a gastrula with two germ layers, the embryo is almost
identical to the diploblastic coelenterates as represented by Hydra.
Following this, the third germ layer forms between the others and
results in the triploblastic metazoan.

Basis for the Theory of Evolution

One who has thoughtfully studied the field of zoology soon realizes
many relationships or homologies in structural make-up of certain
different groups of animals ; much similarity of embryonic develop-
mental processes, and fundamental coincidences in physiological
activities in all living material. Too, it is recognized that fauna
and flora are not the same in all parts of the earth at the present,
and have not been the same in the past as now. These realizations
and other similar ones have been based on scattered sources of evi-
dence, much of which is indirect. The evidences which have been
discovered, however, have led to the formulation of the theory of
evolution and its rather general acceptance as a working hypothesis
among biologists. There has been confusion regarding this subject
through failure to distinguish between the existence of progressive
evolutionary changes, the course of these changes, and the cause of
such a progressive series of events. The latter point has been the
basis of most of the questions concerning the whole idea, and it is
the most speculative of the three. These three are separate though
related factors.

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 871

In general, scientists are convinced, according to the evidence
they have examined, that organic evolution exists in the form of a
progressive change, which has generally proceeded from simpler to
more complex organization through a long period of time. Although
some sections of the course of this series of developmental events
cannot be charted as completely as a ship's course at sea, numerous
landmarks serve to indicate what has taken place. Biologists are
free to admit that the causes of this process are not understood, and
treat it strictly as a scientific problem.

Most estimates of the time when living organisms first came into
existence range between 60,000,000 and 1,200,000,000 years ago, co-
incident with Pre-Cambrian deposits. The manner of origin of
protoplasm is purely hypothetical. It is suggested that when con-
ditions became suitable, as to chemical elements present, tempera-
ture, pressure, etc., a relatively simple colloidal, protoplasmic mix-
ture arose, having properties of life though perhaps in a very ele-
mentary way. It is assumed that all organisms which have lived
are descendants of this simple origin. Cellular organization, with
division of labor between nucleus and cytoplasm, is taken to be a
first step in the development of protoplasm. It is the simplest vital
unit now recognized. If this speculation is followed further, it may
be supposed that the unicellular organisms of modern times have
descended from such early cells without changing from the simple-
celled state, but have developed many specializations as individual
types, while metazoans have advanced from certain of these simple
forms to a more complexly organized cellular condition.

From the nature of protoplasm and from the evidence in the geo-
logical records, it is usually assumed that orgajiisms first appeared
in the water near the shores of the primitive oceans. Presumably
the first oceans were boiling hot, and the land at that time was a
molten volcanic mass. If this is true, there must have been a long
period after the formation of the atmosphere, seas, and land before
protoplasm, as we know it, could have existed. After these cooled
and became favorable for life the seas are thought to have sup-
ported an abundant life before the land became suitable for its
existence. Along with these several speculative aspects of the sub-
ject there have been offered several forms of evidence to support the
existence of an evolutionary progress of development in organisms.

872

TEXTBOOK OF ZOOLOGY

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Geological Evidence.— Paleontology is the study of fossil remains
of organisms deposited in the strata of the earth's crust. Shells
and other hard parts are mineralized or petrified; in other cases
mud impressions, or tracks, or pitch-preserved individuals, such as
insects, and a few frozen forms constitute the majority of fossils.
It must be kept clearly in mind that the geologist is able to deter-
mine quite accurately the sequence of time or chronological suc-
cession of the layers of the earth's crust. The geological time scale
shows a long period before any life existed, then the appearance of
unicellular plants, then unicellular animals, then colonial forms,
simple many-celled forms, and then the more complex ones. Such
a timetable estimates the relative period of time during each era
and shows some fusion and overlapping of certain types of life.
Certain types of unicellular forms are continuous through the en-
tire scale.

The principal facts shown by the fossil record may be summa-
rized: (a) The fossil forms are not strictly identical with any living
species, and the remains of plants and animals of each geologic
stratum are at least specifically different from the forms in any
other stratum, but they may belong to the same genus; (b) the
oldest strata containing fossils have represented in them most of
the simpler forms of nonchordate animals, while the upper strata
contain fossils of all groups more nearly like modern forms, includ-
ing chordates; (c) in studying these in sequence, there may be
observed a gradual progression from simpler and generalized types
toward more specialized and complex forms as one proceeds from
the older toward the upper or newer strata; (d) only the more gen-
eralized varieties have persisted within the groups that, as a whole,
have become specialized ; many of the others have long since reached
their climax of specialization and have become extinct; (e) many
of the dominant groups of organisms have arisen near the close of
a period during which great climatic changes were taking place and
have enjoyed dominance during the following period because such
a group probably arose in response to the conditions; (f) although
many nonchordate phyla had reached an advanced stage of devel-
opment in the early Cambrian period, where early fossil records occur,
many ancestral sequences have been observed, and these have sup-
plied information making possible the detailed description of the
course of events that has led to the surviving animals of modern

874

TEXTBOOK OF ZOOLOGY

times; (g) the developmental clianges of the chordate groups are
more completely read in the fossil record, with the history of the
mammals in fullest outline, partly because they are relatively recent
and partly because the mammalian skeleton is readily fossilized. The
most complete pedigree in fossils has probably been worked out for
the horse, and a great deal of its material has been located in the
southwestern part of the United States.

The Rise of the Horse. — Primitive horselike animals are thought
probably to have arisen from an extinct group called Condylarthra,
which had five toes on each foot and a large part of the sole resting

Wrist--

Fig. 442. — Positions of the human hand to show the comparative stages of
elevation of tlie horse's foot to the tip of the middle toe. (Courtesy of American
Museum of Natural History.)

on the ground. The first unquestionable horselike form found in
America is the small Eohippus which was about one foot tall and
the fossils of which came from the rocks of Eocene times. It had
the outer four digits complete on the forefoot, but no trace of the
thumb, while the hind foot had three complete digits with vestiges
or splints of the first and fifth. Following the foxlike Eohippus,
later in the Eocene period came the Orohippus with an enlarged
central digit in the forefoot and the loss of the splints in the hind
foot. Mesohippus, about the size of a large dog with a three-toed

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876 TEXTBOOK OF ZOOLOGY

foot both in front and behind but with the side toes much reduced
and a trace of the fifth digit in the forefoot, appeared during the
Oligocene epoch. Merychippus of Miocene times and Pliohippus of
the Pliocene epoch show a continuation of this reduction to a one-
toed type which leads to Equus cdballus of modern times. The
modern horse walks on the tip of the middle toe of each foot with
the vestiges of digits 2 and 4 persisting as splints.

Arthur Dendy, in 1911, wrote that the horse is an example of the
adaptation of a lowland type to become a plains type, as the ex-
tensive, dry, grass-covered, plains developed. The adaptation has
proceeded along two lines. The limbs have become elongated by
the elevation of the heel, thus putting the animal on tiptoe ajid
fitting it for rapid flight from its enemies on the grass-covered open
prairie. At the same time the neck and head became elongated to
enable the animal to graze the ground without bending the legs.
Along with this the teeth changed from a carnivorous form to a
complex, broad grinding type for feeding on grass. In addition
to these changes the brain advanced.

These changes took place gradually through millions of years and
the intermediate forms give the paleontologist a graphic picture of
the history of this modern species. The sequence of these stages
seems to fit in exactly with the theory that each has been derived
from the preceding by a continued adaptation to the changing con-
ditions of life. The horse's pedigree is essentially similar to that
of numerous other forms, such as the elephant, camel, and certain
birds, which have been worked out.

The molluscan, echinoderm, and arthropod fossils give similar
stories among invertebrates. The numerous fossils of fish, Amphibia,
Reptilia, particularly dinosaurs, and birds with the famous Arcliae-
opteryx, all have their testimony to contribute.

Distributional Evidence.— The paleontological distribution of ani-
mals is considered vertically while geographic distribution is in
horizontal plane. It is impossible to separate these two completely.
The study of geographical distribution gives essentially a cross
section of the vertical distribution, thus giving a sort of "still pic-
ture" of the complex developmental relations of animal groups at
one moment in geologic time. In studying this subject, it is neces-
sary to have in mind two fundamental conceptions. The first is that
the ancestors of related genera first appeared or originated in a

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 877

locality which is designated as the common center of origin. A sec-
ond conception is that as the ancestral form became established and
multiplied, mig-ration in search of food and more suitable conditions
occurred. Barriers, many of which were geographic, determined
the direction and extent of this migration. Large bodies of water
blocked the passage of terrestrial animals, as of course land was a
barrier to aquatic animals. High mountains or deserts were barriers
to all animals unable to withstand low temperatures and altitudes
on the one hand, or high temperature and drj^ness on the other.
These forces, and others, are believed to account for the natural
distribution of animals. The eases of the camel group, originating
in North America, migrating to South America and Asia by the
land connections of the Eocene to Pliocene epoch, and the tapirs,
which are represented by distinct species in two widely separated
regions, South Asia territory, and the Central America-South Amer-
ican territory are examples. Here again paleontologieal data show
that in the Pliocene epoch tapirs were distributed over nearly all of
North America, Northern Asia, and Europe. Following that time
they were graduallj^ decreased due to discontinuous distribution until
the one-time world-wide distribution is now isolated in two widely
separated regions. Long isolation of genera in different environ-
ments will bring about definite specific differences. As an example,
a litter of foreign rabbits was introduced to the island of Porto
Santo during the fifteenth century and b}^ the middle of the nine-
teenth century the descendants had become so distinct from the
original ancestors that they were described as a new species. There
are numerous instances of this eifect, demonstrated by isolation on
ocean islands. The islands are either continental, with fauna similar
to those of the nearby continent from which the animals have come,
as the British Isles, or they are oceanic, with a very bizarre assem-
blage of animals which have either drifted in or have been carried
there, as the Hawaiian Islands. Many of the animals on these
oceanic islands are peculiar and are found nowhere else on earth.
Australia has a group of animals, examples of which are very dif-
ferent from those of Asia because the two have been so long separated.
Europe, Asia, Africa, and North America have been connected with
each other hy land bridges in recent enough times that the mam-
mals show similarity. The distribution of the species of a genus
often radiates from the more generalized species which occupy the

878 TEXTBOOK OF ZOOLOGY

center of the range of the genus, and the more specialized species
are found in the scattered outskirts of the range.

From the preceding statements concerning distribution it seems
that any given species originates in a definite locality, that it multi-
plies there and migrates in all possible directions. It modifies as
it goes in response to the various new conditions prevailing and
becomes divided into local varieties which in the course of time
become species. Thus the working method of animal distribution, as
it has been presented, is the principle of descent from preceding
generations with modification.

Morphological Evidence. — Classification of the animals shows in
fact something of the morphological evidence, since current classi-
fication is based chiefly on anatomical features and comparative
anatomy. The groups of the classification are established largely
on anatomical similarities. The differences existing among the rep-
resentatives of all the classes of vertebrates are relatively slight
when set over against the fundamental similarities. Closely related
groups show numerous similarities in the form of homologies. The
flipper of the whale, the wing of the bat or the bird, the foreleg of
the cat and the arm of man all show the same general type of
structure in spite of certain specific differences. There is seldom
any question of their phylogenetic relationship.

The presence of series of similar parts on different segments of
the same animal and the various specializations of these parts show
a progressive development. The highly specialized walking legs,
uropod, claws, and antennae of crayfish, for example, have all devel-
oped from the simple swimmeret type of appendage. They form a
serial homology and are also homologous to the appendages of all
other Crustacea as well.

In higher forms of animals, such as man, there are numerous struc-
tures which seem to be useless and are even harmful in some in-
stances. These are spoken of as vestigial structures. Such parts
correspond in structure and plan to functional parts in other re-
lated forms, but are reduced morphologically and without the origi-
nal function. In man, one probably thinks first of the vermiform
appendix of the colon as such a structure. This is apparently func-
tionless in human beings and can be removed with no loss, but this
same organ in rabbits, some birds, and other animals is an extensive
and functional digestive organ. Man has a coccyx or vestigial tail,

PHYLOGENETIC RELATIONS OP ANIMAL GROUPS 879

and the frog has the urostyle. Pythons and porpoises, neither of
which has the least use for them, have vestigial hind limbs similar
to the functional ones of their relatives. The salivary glands of
certain snakes have become adapted as poison glands, certain sweat
glands have become milk glands, and gill arches have become sup-
porting structures of the tongue, larynx, and throat of adult ter-
restrial animals. The blood supply and nerves both follow the
phylogenetic changes of these organs.

Most animals seem not to have originated in these present forms,
and they do seem to have changed through the long periods of
geologic history. The explanation offered by modern biologists for
these anatomical relations and resemblances between animals is that
the individuals in any group have inherited a similar plan of struc-
ture from the ancestors which was common to all members of the
group. In a group, such as the vertebrates, there have been numer-
ous modifications of various fundamental structures in different
subdivisions in relation to the particular habits of life ; still they
remain fundamentally alike because they have developed from the
underlying plan of organization found in the ancestors. The seal
and the bird, although quite different, show similarities in habits
and otherwise because of common ancestry long ago. The conclu-
sion of biologists of today is that all of the animals in a group, such
as the vertebrates, have arisen by descent with change from a primi-
tive organism which possessed the fundamental organization as
shown from cyclostomes to man.

Embryological Evidence. — Evidences from this field really con-
tinue directly from the previous discussion. The animal is to be
thought of as an individual from the single-celled zygote stage to
the mature stage of old age, no matter what its complexity. In-
timately related types of animals parallel through a large portion
of their development to diverge somewhat in adult condition, more
remotely related forms take separate developmental courses rather
early in life, and unrelated forms may be different almost from the
beginning. In numerous instances the developing stages of more
advanced forms resemble very closely the mature stages of the less
advanced types in a serial fashion. The history of the individual
animal often corresponds in a general way to the history of the
advances of the animal kingdom, up to its state of development.
This apparent repetition of the ancestral development in individuals

880

TEXTBOOK OF ZOOLOGY

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(From Romanes, Darwin and After Darwin, published by the Open Court Publish-

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS

881

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vertebrate animals. Notice the close similarity of the first two stages in all.
ing Company.)

882

TEXTBOOK OF ZOOLOGY

was what led Haeckel to formulate tlie recapitulation theory, ex-
pressed briefly: ontogeny repeats or recapitulates phytogeny, as has
been discussed earlier in the chapter.

In the vertebrate group these apparent relations are much shown.
Most of the embryos of this group are so similar that it is nearly
impossible to distinguish them. They pass through the identical

Fig. 445.- — Diagram to show the modifications of aortic or branchial arches in
different vertebrates types. A. primitive scheme; B. hingflsh ; C, primitive am-
phibian (urodele) ; D, frog; E, snake; F, lizard; G, bird; H, mammal; ec, ic,
external and internal carotids ; va, ventral aorta ; da, dorsal aorta ; db, ductus
Botalli ; p, pulmonary artery ; s, subclavian ; c, coeliac. Vessels carrying venous
blood are black; tliose with mixed blood are shaded; those which disappear are
dotted outlines. (After Boas. Reprinted by permission from Kingsly, Comparative
Anatomy of Vertebrates, published by The Blakiston Co.)

stages of development. Systems, such as the circulatory, nervous,
digestive, and respiratory, follow the same course of development
in all of the vertebrates, no matter how simple or complex. In
earlier stages the similarity is strikingly close.

PHYLOGENETIC RELATIONS OF ANIM.VL GROUPS 883

The course of development and the modifications shown in the
aortic arches or main arteries leaving the heart and passing through
the gill regions is a specific example of the manner in which a spe-
cific set of structures follows out a repetition of ancestral stages in
the development of the individual. Fig. 445 shows a comparison
of the arrangement of the branchial (aortic) arches from the primi-
tive set of six through the fishes, Amphibia, reptiles, to the birds
and mammals, with a modified condition of three arches.

Along with the aortic arch situation are other examples of similar
stages of development in all forms of the group. Gills are present
in all chordates at some time. In the primitive ones the gills are
functional throughout life ; in the more advanced types they are
only transitory gill structures. In frogs and toads, the tadpole stage
is essentially fishlike, and they become amphibian at metamorphosis.
The notochord is a definite structure throughout the life of the
primitive chordates, but is present as such in only the embryo stage
of all others. The development of the heart from the tubular con-
dition through the two-chambered, three-chambered, and finally four-
chambered condition, illustrates the same progressive development.

Physiological Evidence. — The fact that all protoplasm possesses
the same set of fundamental properties or capacities as contractility,
irritability, metabolism, etc., is in itself a definite indication of rela-
tionship of all organisms, since they are all composed of protoplasm.
Too, all protoplasm acts under similar laws and conditions.

Such natural substances of animal bodies as the hormones, or
antibodies or even enzymes are almost universal in their reactions
among chordates and even among nonchordates. Most of them are
interchangeable from one animal group to another. A deficiency
of pepsin or adrenalin in man may be supplied from a cow, a hog,
a cat, a rat, or a dog. The blood of all vertebrates has certain
physiological similarities and some specific differences. The blood
of large groups of human beings will mix without precipitation and
is said to "match." Other individuals' blood may not "match"
in this type but will mix with blood from another group. The ag-
glutination (clumping of red corpuscles) which occurs when samples
of blood from two such individuals are put together, is due to the
reaction of two substances produced by the corpuscles in one or both
of the blood samples. The designation of the blood group depends
upon the presence or absence of one or both of these substances. One

884 TEXTBOOK OF ZOOLOGY

blood group contains neither, another contains one substance, a third
the second substance only, while a fourth group contains l)oth sub-
stances. Serum of the first group will agglutinate corpuscles when
mixed with any of the other three. The groups with the single sub-
stance will agglutinate corpuscles in blood having only the other
substance or blood with both substances. The serum from blood
with l)oth substances Avill not cause agglutination in any of the
others.

In lower mammals some similar blood groups have been found,
but it is only in the apes that the groups correspond to those in the
human. This is an indication of the rather close relationship of
these animal groups. Whether in human or other animals this blood
characteristic is permanent in the individual and it is hereditary.

Serum (blood minus cor^Duscles and fibrinogen) studies also estab-
lish certain relationships among animals. If small quantities of
human blood serum are introduced at intervals into the blood
stream of a rabbit, in time there will be developed in the blood of
the rabbit a substance (antibodies) which, when mixed with normal
human blood will cause precipitation of the proteins here. Serum
from such a rabbit is called antihuman serum. When this serum is
mixed with serum in a certain dilution taken from human, chimpanzee,
gorilla, or monkeys, it will cause precipitation. If the dilution is
increased there is no precipitation when mixed with monkey serum;
at still higher dilution there is no precipitation when mixed with
gorilla blood ; and higher none for chimpanzee blood, until finally none
for human blood. On the basis of these sera precipitation tests the
chimpanzee is closest in its relation to man, then gorilla, and then
the monkeys.

In making similar tests on other vertebrate groups it is found that
crocodiles are more closely related to birds than are the other
reptiles; also among reptiles, that snakes and lizards are more closely
related to each other than to turtles. Too, crocodiles show a closer
relation to turtles than to the other groups of reptiles. Blood
studies of the various groups of vertebrates indicate that there is
more similarity in blood of closely related forms than of others.
At the same time, it is seen that a chemical relationship persists
in the blood throughout the chordate phylum.

The hreeding of plants and animals through long series of genera-
tions of domestication and laljoratory experiments has yielded much

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 885

information concerning the ways of adaptation and phylogenetic
development. A significant result is the demonstration of changes
occurring in animals and plants. From such studies it seems quite
obvious that organisms now living have come to be what they are
by gradual change from generation to generation through a course
of descent from preexisting and varied ancestors, rather than by a
sudden and completely new development. Most of the various
breeds of cattle, chickens, dogs, horses, sheep, crops, etc., have been
developed in each case from a preexisting common ancestor.

Darwin and Studies of Evolution

Most discussions of organic evolution usually begin with mention
of Darwin's monumental work on this suljject and difficult it is to
get away from his fundamental basic thinking on the subject. He
was the first to thoroughly survey the fields of distribution, morphol-
ogy, embryology, and paleontology, and logically relate the data found
there to the theory of evolution. From his studies, many of which were
done along the east and west shores of South America while he was
naturalist of a British Naval expedition on the ship "Beagle," Darwin
formulated a clear-cut and definite argument for evolution on the
basis of Natural Selection. Beginning with Malthus' Law of Popu-
lation, published in 1838, Avhich stated that since man reproduces
in a geometric ratio, the earth would be overpopulated in a few
generations except for such checks as the arithmetric ratio of in-
crease in food production, disease, war, flood, earthquake, fire and
other natural catastrophes reducing population, Darwin formulated
the Natural Selection Theory. This theoiy includes among other
things the application of Malthus' law to all living organisms. The
four basic points on which this theory is developed, may be named in
order as follows: (1) overproduction, (2) struggle for existence, (3)
variation and heredity, and (4) survival of the fittest (natural selec-
tion).

Overproduction is in operation in all thriving normal species.
A single codfish will produce several million eggs in one season.
If every codfish egg were to be fertilized, reach maturity, and repro-
duce with no loss from one generation to the next, it would not be
more than a dozen years until the entire face of the earth would be
covered with codfish and all other animals would be crowded out of
existence completely. Even a form like the elephant, which lives to

886 TEXTBOOK OF ZOOLOGY

be ninety or a hundred years of age and averages only six progeny,
could soon occupy all of the standing room on the face of the earth.
Beginning with one pair of elephants and providing every individual
lived and reproduced even at the average slow rate mentioned,
19,000,000 individuals would be produced in 750 years. If every ele-
phant alive today were to enter into a program like that, both food
and space would become quite scarce before many generations. How-
ever, this doesn't happen on a large scale. All plants and animals
tend to produce more offspring than can ever reach maturity and re-
produce.

The Struggle for Existence is ever present because there are more
individuals produced than the habitat will support. The two most
fundamental needs for which organisms struggle are: (1) food and
(2) opportunity to reproduce and rear young. Of these two, the
struggle for food is very immediate and the food supply is an im-
portant limiting factor on population from season to season. Since
the food supply, on the average, remains quite constant, it is evident
that only a limited number of the increase in individuals can be sup-
ported in a particular habitat. A struggle ensues with each in-
dividual attempting to secure the necessities of life. Not only is there
a struggle for food but also with many factors in the environment
like climate, geographic changes, etc.

Survival of the Fittest was the outcome which Darwin saw result-
ing from such a struggle. Those individuals which were best adapted
to the environment into which they were born have been the ones to
win out in the struggle and leave offspring for a future generation.
The inheritance of favorable or unfavorable characters influences very
strongly the success of the individual in maintaining itself. The
survivors in any generation are those which inherit the most favorable
combination of variations. Many variations, both favorable and
unfavorable to the success of the individual, are hereditary.

When changes in environment come and bring about new living
conditions the animals in the particular habitat must meet these
changes, be able to migrate, or perish. The standard for fitness has
changed under such circumstances and animals with somewhat
different characters and adaptations may now be the "fittest." The
individuals whose variations have brought them to most nearly fit
the requirements for life in their particular habitat will be the ones
most likely to obtain sufficient food supply and adequate provision
for reproduction to rapidly increase their population. As one group

PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 887

is able to do this, it invariably reduces or perhaps entirely eliminates
other species in the locality. Evolutionary changes result from
survivial of the fittest which has come about by accumulation of
favorable variations in each of successive generations. Those in-
dividuals or races which have not been as well adapted to conditions
of the habitat have become inconsequential or extinct. The appear-
ance of characters in an animal is a matter of chance as far as the
individual is concerned. Natural selection may act as an eliminating
agent and determine w^hether the character or trait shall survive after
it appears.

Mutation Theory. — ^Hugo de Vries is the name most prominently
connected with the origin of this theory. He was a Dutch botanist and
in 1886 found some evening primrose plants (Oenothera lamarkiana)
which exhibited discontinuous variation or sudden appearance of
new characters. These sudden sharp variations came to be known as
mutations. There are two types of variations recognized: (1) con-
tinuous or fluctuating variation, such as height of individuals of a
species where they are expected to fall within a normal range thus
allowing a degree of variation among individuals of the species, and ;
(2) discontinuous or sport variation (mutation) where the variation
falls outside the normal range of variation and not connected with
it by intermediate changes.

In the course of several generations of this primrose and involving
approximately 50,000 individuals, six different mutations were found.
The new characters which appeared, were quite different from those
of the typical species as w^ell as being inherited. Since this was the
case, de Vries concluded that he was observing the origin of new
species. He was sufficiently convinced of this to discount Darwin's
conception of the development of new species by the gradual accumu-
lation of continuous variations through natural selection. He pointed
out that mutations are due to changes occurring in the germ plasm
while the continuous variations, individually, are due to changes in
somatic cells.

Following de Vries' work there has been much study of mutations
and numerous ones have been found in nature. Too, it has been
found that spontaneous mutations could be produced in Drosophila
(fruit fly) by x-ray radiation. It is thought that mutations come
as the result of physiological changes in the chromosomes or genes.

888 TEXTBOOK OF ZOOLOGY

While this mutation theory of origin of new species has prompted
much study and thought on evolution it seems not to have displaced
Darwin's general conception of the origin of new species. So far as
Darwin's theory is concerned, the occurrence of mutations only
hastens the process of evolution since they produce quick abrupt
variations instead of the slower smaller continuous variations.
Natural selection will operate with either. Biologists now consider
both small and large variations as mutations, and have turned back
to Darwin's idea of natural selection as the most likely explanation
of the development of new kinds of animals.

REFERENCES

A limited number of selected works wliich are suitable for reference
and collateral reading are presented in the following pages.

Potter, G. E.: Laboratory Outlines for General Zoology, St. Louis, 1938, The
C. V. Mosby Go. A very useful guide to the laboratory portion of the

study.

Biology in General

American Men of Science. Biographical sketches of outstanding scientists, ed. 4,

1927, Science Press.
Biological Abstracts. Eeview of tlie current biological literature. 1926-1947,

Philadelphia.
Carpenter, K. E.: Life in Inland Waters, New York, 1928, The Macmillan Co.
Child, C. M.: Individuality in Organisms, Chicago, 1915, The University of

Chicago Press.
— : Senescence and Eejuvenescence, Chicago, 1915, The University of Chicago

Press.
Conklin, E. G. : Heredity and Environment in the Development of Men,

Princeton, 1922, Princeton University Press.
Garrison, F. H.: History of Medicine, Philadelphia, 1923, W. B. Saunders Co.
Geiser, S. W.: Naturalists of the Frontier, Dallas, 1937, Southern Methodist

University Press.
Hegner, E. W.: The Parade of the Animal Kingdom, New York, 1935, The

Macmillan Co.
Locy, W. A.: Biology and Its Makers, New York, 1915, Henry Holt and Co.
— : The Growth of Biology, New York, 1925, Henry Holt and Co.
Loeb, Jacques: The Organism as a Whole, New York, 1918.
Morgan, Ann: Fieldbook of Ponds and Streams, New York, 1930, G. P.

Putnam's Sons.
Needham, J. G., and Needham, Paul E.: Guide to Freshwater Biology, American

Viewpoint Society, 1927.
— , and Lloyd, J. T.: The Life of Inland Waters, Ithaca, N. Y., 1930, Comstock

Publishing Co.
Nordenshiod, E.: History of Biology, New York, 1928, Alfred A. Knopf.
Ward and "Wliipple: Fresh-Water Biology, New York, 1918, John Wiley & Sons.
\Vhite, E. Grace: General Biology, St. Louis, 1937, The C. V. Mosby Co.
Williams, S. H.: The Living World, New York, 1937, The Macmillan Co.
Woodruff, L. L.r The Development of the Sciences, New Haven, 1923, Yale

University Press.
Young, E. T. : Biology in America, Boston, 1922, Gorham Press.

General Zoology

Atwood, Henry W. : Introduction to Vertebrate Zoology, St. Louis, 1940, The

C. V. Mosby Co.
Borradaille, L. A., and Potts, F. A.: The Invertebrata, New York, 1932, The

Macmillan Co.
Buchsbaum, Ealph: Animals Without Backbones, Chicago, 1938, The University

of Chicago Press.
Curtis, W. C, and Guthrie, Mary J.: Textbook of General Zoology, New York,

1947, John Wiley and Sons.
Guyer, M. F. : Animal Biology, New York, 1941, Harper and Brothers.
Haldane, J. B. S., and Huxley, Julian: Animal Biology, 1927, Oxford at

Clarendon Press.

889

890 TEXTBOOK OF ZOOLOGY

Hegner, R. W. : College Zoology, New York, 1942, The Macmillaii Co.

— : Invertebrate Zoology, New York, 1933, The Macmillan Co.

Jordan, D. S.: A Manual of the Vertebrates, New York, 1929, World Book Co.

Kingsley, J. R. : Comparative Anatomy of Vertebrates, Philadelphia, 1926, P.

Blakiston's Son and Co.
Krecker, Frederick H. : General Zoology, New York, 1934, Henry Holt and Co.
Lane, Henry H. : Animal Biology, Philadelphia, 1929, P. Blakiston's Son and

Co.
Metcalf, Z. P.: Economic Zoology, Philadelphia, 1930, Lea and Febiger.
Newman, H. H. : Vertebrate Zoology, New York, 1920, The Macmillan Co.
— : Outlines of General Zoology, New York, 1936, The Macmillan Co.
Parker, T. J., and Haswell, William A.: Textbook of Zoology (two volumes).

New York, 1930, The Macmillan Co.
Petrunkevitch, Alexander: Morphology of Invertebrate Types, New York, 1916,

The Macmillan Co.
Pratt, H. S. : A Manual of the Common Land and Fresh-Water Vertebrates of

the United States, Philadelphia, 1935, The Blakiston Co.
— : A Manual of Common Invertebrates, Philadelphia, 1935, The Blakiston

Co.
Eeese, Albert M.: Economic Zoology, Philadelphia, 1924, P. Blakiston's Son

and Co.
Shull, A. F. : Principles of Animal Biology, New York, 1934, McGraw-Hill Book

Co.
Storer, Tracy I.: General Zoology, New York, 1943, McGraw-Hill Book Co.
Van Cleave, H. J.: Invertebrate Zoology, New York, 1931, McGraw-Hill Book

Co.
Walter, H. E.: Biology of the Vertebrates, New York, 1928, The Macmillan Co.
Wolcott, Robert H.: Animal Biology, New York, 1946, McGraw-Hill Book Co.
Woodruff, L. L. : Foundations of Biology, New York, 1932, The Macmillan Co.

Protoplasm and the Cell

Cowdry, E. V. (editor) : General Cytology, Chicago, 1924, University of Chicago

Press.
Doncaster, L. : An Introduction to the Study of Cytology, London, 1920,

Cambridge University Press.
Heilbrunn, L. V.: Colloidal Chemistry of Protoplasm, Berlin, 1928, Borntraeger.
Huxley, T. H.: On the Physical Basis of Life, 1868, Collected Essays, Vol.

Method and Results, D. Appleton & Co.
Loeb, Jacques: The Dynamics of Living Matter, New York, 1906, Columbia

University Press.
Painter, Theophilua S., and Griffen, Allen B. : The Structure and the Develop-
ment of the Salivary Gland Chromosomes of Simulium, Genetics 22: 612-

633, November, 1937.
Sharp, L. W.: Introduction to Cytology, ed. 2, New York, 1926, McGraw-Hill

Book Co.
Wilson, E. B.: The Cell in Development and Inheritance, ed. 3, New York,

1925, The Macmillan Co.
— : The Physical Basis of Life, New Haven, 1923, Yale University Press.

Protozoology

Calkins, Gary N.: Biology of Protozoa, Philadelphia, 1926, Lea and Febiger.
Hegner, R. W., and Taliaferro, W. H.: Human Protozoology, New York, 1924,

The Macmillan Co.
Kudo, Richard R.: Handbook of Protozoology, Springfield, 111., 1931, Charles C

Thomas, Publisher.

REFERENCES 891

Marshall, C. E. : Microbiology, ed. 3, Philadelphia, 1926, P. Blakiston 's Son and

Co.
Woodruff, L. L. : The Origin and Sequence of the Protozoan Fauna of Hay

Infusions, J. Exper, Zool. 12: 205-264, 1912.

Porifora, Coelenterata, Platyhelminthes, and Nemathelminthes

Geiser, S. W.: The Distribution of Pectinella magnifica Leidy in the United

States, Field and Laboratory 2: 1934.
Mayer, A. G.: The Medusae of the World, Washington, 1910, Carnegie In-
stitute.
Moore, H. F.: The Commercial Sponges and Sponge Fisheries, Bulletin of U. S.

Bureau of Fisheries 28: 1910.
Nutting, C. C: American Hydroids, I, II, III, Bulletin of U. S. Nat. Museum,

1900, 1904, 1905.
Old, M. C: Taxonomy and Distribution of Freshwater Sponges of Michigan,

Michigan Academy of Sciences, Arts, and Letters, No. 15, 1932.
— : A Contribution to the Biology of the Freshwater Sponges, Michigan

Academy of Sciences, Arts, and Letters, No. 17, 1933.
Potts, E.: Freshwater Sponges, Proc. Acad. Nat. Sci., Philadelphia, 1887.
Smith, F. : Distribution of the Fresh-Water Sponges of North America, HI.

State Lab. Nat. Hist. Surv. No. 14, 1921.

Annelida

Beddard, F. E.: Earthworms and Their Allies, London, 1901, Cambridge Univer-
sity Press.

Harmer, S. F., and Shipley, A. E.: Worms, Cambridge Natural History Series,
Vol. II, New York, 1909, The Macmillan Co.

Mollusca

Eodgers, Julia: The Shell Book, Garden City, N. Y., 1909, Doubleday Page.
Tryon, G. W., and Pilsbry, H. A.: A Manual of Conchology, Phil. Acad. Nat.
Sci., 1878.

Crustacea

Greaser, E. P.: Decapod Crustaceans of Wisconsin, Trans. Wisconsin Academy

Sciences, Arts, and Letters, No. 27, 1932.
Harris, J. A.: An Ecological Catalogue of the Crayfishes Belonging to the Genus

Camharus, Kansas Univ. Sci. Bull. 2: 51-187, 1913.
Hay, W. P.: Crustacea of Indiana, Proc. Indiana Acad. Sci., 1896.
— : Life History of the Blue Crab, U. S. Bur. Fish. Piept., 1905.
Herrick, F. H.: ' Natural History of the American Lobster, Bull. Bureau Fish,

29: 1909.
Herrick, C. L., and Turner, C. H. : Synopsis of the Entomostraca of Minnesota,

Minnesota Geol. and Nat. Hist. Surv., 2d. Eept. State Zoologist, 1895.
Eathbun, Mary J.: The Spider Crabs of America, Smithsonian Inst. N. S. Nat.

Mus. Bull. 129.
— : The Grapsoid Crabs of America, Bull. 97 U. S. Nat. Mus., 1917.

Myrlapoda

BoUman, C. H.: The Myriapoda of North America, U. S. Nat. Mus. Bull. 46,

1893.
Chamberlin, E. V.: The Chilopoda of California, Pomona Jour, of Ent. 4:

1912.
— : The North American Chilopoda and Diplopoda, Ann. Ent. Soc. Am. 5:

1912.

892 TEXTBOOK OF ZOOLOGY

Araclinida

Baerg, W. J.: The Effects of the Bite of Latrodectus mactans, J. Parasitol.,

Ilrbana, 1923, pp. 101-169.
Banks, N. A.: A Treatise on the Mites, Proc. U. S. Nat. Mus., 1904.
— : The Acarina or Mites, Bur. Ent. Eeport 108, U. S. Dept. Agric, 1915.
Chamberlin, E. V. : Kevision of the North American Spiders of the Family

Lycosidae, Proc. Acad. Nat. Sci., Philadelphia, 1908, pp. 157-318.
Comstock, J. H. : The Spider Book, Garden City, N. Y., 1912, Doubleday Page.
Fabre, J. H.: Life of the Spider, New York, 1913, Dodd, Mead & Co.
Petrunkevitch, A.: A Synoptic Index — Catalogue of Spiders of North, Central,

and South America, New York Bull. Amer. Mus. Nat. Hist., 1911, pp. 1-791.
Ean, P.: Some Life Notes on the Black Widow Spider, Latrodectus mactans,

Psyche 31: 162-166, 1924.
Eeese, A. M.: Venomous Spiders, Science 54: 382-385, 1921.
Watson, J. E. : Bite of Latrodectus mactans, Science 55: 539, 1922.

Insecta

Blatchley, W, S. : Coleoptera of Indiana, Indianapolis, 1910, Nat. Pub. Co.
Boyce, E. W.: Mosquitoes or Man? The Conquest of the Tropical World,

London, 1910, John Murray.
Carpenter, G. H.: Insects, Their Structure and Life, Dent, 1928.
— : The Biology of Insects, New York, 1928, The Macmillan Co.
Comstock, J. H. : Introduction to Entomology, Ithaca, N. Y., 1933, Comstock

Pub. Co.
— , and Comstock, A. B.: How to Know Butterflies, Ithaca, N. Y., 1920,

Comstock Pub. Co.
— , and — : A Manual for the Study of Insects, Ithaca, N. Y., 1930, Comstock

Pub. Co.
Dyar, H. G. : The Mosquitoes of the United States, Proc. U. S. Nat. Mus. 62:

1922.
Essig, E. O.: The Insects of Western North America, New York, 1926, The

Macmillan Co.
Fernald, H. G.r Applied Entomology, New York, 1926, McGraw-Hill Book Co.
Folsom, J. W., and Wardle, E. A. : Entomology With Eeference to Its Ecological

Aspects, Philadelphia, 1934, P. Blakiston's Son & Co.
French, G. H. : Butterflies of Eastern North America, Philadelphia, 1886, J. B.

Lippincott Co.
Herms, W. B. : Medical and Veterinary Entomology, New York, 1923, The

Macmillan Co.
Herrick, Glenn W. : Manual of Injurious Insects, New York, 1925, Henry Holt

& Co.
Holland, W. J.: The Moth Book, Garden City, N. Y., 1922, Doubleday Page.
— : The Butterfly Book, Garden City, N. Y., 1931, Doubleday, Doran & Co.
Howard, L. O.: The Insect Book, Garden City, N. Y., 1922, Doubleday Page

& Co.
Imms, A. D. : A General Textbook of Entomology, New York, 1925, E. P.

Dutton & Co.
Kellogg, V. L., and Doane, E. W. : Economic Zoology and Entomology, New York,

1915, Henry Holt & Co.
Kofoid, C. A.: Termites, Berkeley, Univ. California Press.

Lefroy, H. M. : Manual of Entomology, London, 1923, Edward Arnold and Co.
Lubbock, Sir John: Ants, Bees, and Wasps, New York, 1929, E. P. Dutton & Co.
Lutz, F. E.: A Fieldbook of Insects, New York, 1921, G. P. Putnam's Sons.

REFERENCES 893

Metcalf, C. L., and Flint, W. P.: Destructive and Useful Insects, New York,
1928, McGraw-Hill Book Co.

— , and — : A Textbook of Practical Entomology, New York, 1932, McGraw-
Hill Book Co.

Packard, A. S. : Textbook of Entomology, New York, 1898, The Macmillan Co.

Plath, O. E.: Bumblebees and Their Ways, New York, 1934, The Macmillan Co.

Reamur, E. A. F. : A Natural History of Ants (translated by W. M. \Mieeler),
New York, 1926, Alfred A. Knopf.

Snodgrass, E. E.: The Anatomy and Physiology of the Honeybee, New York,
1925, McGraw-Hill Book Co.

— : The Principles of Insect Morphology, New York, 1935, McGraw-HiU
Book Co.

Wardle, E. A.: The Problems of Applied Entomology, New York, 1929, McGraw-
Hill Book Co.

Weed, C. M.: Butterflies (Little Nature Library), Garden City, 1922, Doubleday
Page & Co.

Wheeler, W. M. : Ants, Their Structure, Development, and Behavior, New York,
1910, Columbia IJniv. Press.

Entomological News, published in Philadelphia, the Journal of Economic
Entomology, and the Annals of the Entomological Society of America
contain current articles of importance. Biological Abstracts summarizes
all of the current literature of the world and is an excellent reference for
scattered publications. The Zoological Eecord is also an invaluable source
for bibliographical studies.

Fishes and Amphibia

Adams, L. A.: Necturus — A Laboratory Study, New York, 1926, The Macmillan

Co.
Campbell, Nelle: Organography of IG mm. Ameiurus, Thesis, 1934, Baylor

University Library.
Creaser, C. W. : The Structure and Growth of the Scales of Fishes, etc., Univ. of

Michigan ZooL Pub. 17, 1926.
Daniel, J. F.: The Elasmobranch Fishes, Berkeley, 1928, Univ. California

Press.
Dickerson, Mary C. : The Frog Book, Garden City, 1906, Doubleday Page & Co.
Forbes, S. A., and Eichardson, E. E.: The Fishes of Illinois, Bull. HI. State

Lab. Nat. Hist. Ill, 1908.
Gage, Simon Henry: The Lampreys of New York State, Life History and

Economics, A Biological Survey of the Oswego Eiver System, Supple-
mentary to 17th annual report, 1927, State of New York Conservation

Department, 1928.
Goode, A. M. : American Fishes, Boston, 1926, C. Page and Co.
Holmes, S. J.: The Biology of the Frog, New York, 1927, The Macmillan Co.
Hubbs, C. L. : A Check List of the Fishes of the Great Lakes and Tributary

Waters, With Keys, Univ. Michigan Mus. Zool. Misc. Pub. 15, 1926.
Jordan, D. S.: A Guide to the Study of Fishes, 2 vols, New York, 1905, Henry

Holt & Co.
— , and Everman, W. E.: The Fishes of Middle and North America, U. S. Nat.

Mus. Bulls., 1896.
Kellogg, Eemington: Mexican Tailless Amphibians in the United States,

National Museum Bull. 160, U. S. Nat'l. Museum.
— : The Toad, U. S. Dept. Agr. Bur. Biol. Survey, M. S., 1922.
Kyle, H. M.: The Biology of Fishes, New York, 1926, The Macmillan Co.
Marshall, A. M. : The Frog, New York, 1923, The MacmiUan Co.

894 TEXTBOOK OF ZOOLOGY

Noble, G. K.: The Biology of the Amphibia, New York, 1931, McGraw-Hill

Book Co.
Potter, G. E.: Eespiratory Function of the Swim Bladder in Lepidosteus, J.

Exper. Zool. 49: No. 1, 1927.
— , and Medlen, A. B.: Organography of Gambusia patruelis, J. Morphol. 57:

No. 1, 1935.
— , and Self, J. Teague: Development of Posterior Cardinal Veins in Eelation

to the Swim Bladder in Lepidosteus, J. Morphol. 56: No. 1, 1934.
Kuthven, A. G., Thompson, C, and Gaige, H. T.: The Herpetology of Michigan,

Michigan Handb. Ser. 3, 1928.
Stejneger, L., and Barbour, T.: A Check List of North American Amphibians

and Eeptiles, ed. 3, Cambridge, Mass., 1933, Harvard University Press.
Strecker, J. K. : Eeptiles and Amphibians of Texas, Baylor Bull. 17: No. 4,

1915.
Stuart, Eichard E.: Anatomy of the Bullfrog, Chicago, 1940, Denoyer-Geppert

Co.
— : Anatomy of Necturus, Chicago, 1940, Denoyer-Geppert Co.
Wright, A. A., and Wright, A. H. : Handbook of Frogs and Toads, Ithaca, N. Y.,

1933, Comstock Pub. Co.
Wright, A. H. : Life-Histories of the Frogs of Okefinokee Swamp, Georgia,

North American Salientia, No. 2, New York, 1932.

Reptiles, Birds, and Mammals

Adams, Charles C. : The Conservation of Predatory Mammals, J. Mammalogy 6:

No. 2, May, 1925.
Agassiz, Louis: Turtles, Contrib. to Nat. Hist, of U. S., No. 1, 1857.
Anthony, H. E. : Field Book of North American Mammals, New York, 1928,

G. P. Putnam's Sons.
Apgar, A. C. : Birds of the United States East of the Eocky Mountains, New

York, 1898, American Book Co.
Bailey, Vernon: Beaver Habits, Beaver Control, and Possibilities in Beaver

Farming, Bull. 1078, U. S. Dept. Agric, 1922.
— : Mammals of New Mexico, U. S. Dept. Agric. Bur. Biol. Survey, North

Amer. Fauna 53 (1931).
Barrows, W. B. : Michigan Bird Life, Bull. Michigan Agriculture, 1912.
Beddard, F. E.: The Mammalia, Cambridge Nat. Hist. Series, New York, 1923,

The Macmillan Co.
Beebe, C. W.: The Bird, Its Form and Function, New York, 1906, Henry

Holt & Co.
Bent, A. C: North American Diving Birds, U. S, Nat. Mus., Bull. 107 (1919).
Blanchard, F. N. : A Key to the Snakes of the United States, Canada, and

Lower California, Michigan Acad. Sci. Arts and Letters 4: Part 2, 1925.
Bradley, O. Charnoch: Structure of the Fowl, 1915, published by A. & C.

Black, Ltd., 4 Soho Sq., London.
Bready, M. B. : The European Starling on His Westward Way (Sturnus vulgaris

vulgaris), 1929, Knickerbocker Press.
Burroughs, John: Squirrels and Other Fur Bearers, Boston, 1909, Houghton

Mifflin Co.
Butler, A. W.: Birds of Indiana, Indiana Geol. and Nat. Ees. Eep. 20: 1897.
Camp, C. L. : Classification of the Lizards, Bull. Amer. Mus. Nat. Hist. 48:

1923.
Chapman, F. M. : Color Key to North American Birds, Garden City, 1903,

Doubleday Page & Co.
— : Handbook of Birds of Eastern North America, New York, 1932, D.

Appleton & Co.

REFERENCES 895

Cope, E. D.: A Critical Eeview of the Characters and Variations of the Snakes

of North America, Proc. U. S. Nat. Mus. 14: 1892.
— : The Crocodilians, Lizards, and Snakes of North America, Ann. Eep. U. S.

Nat. Mus., 1898.
Ditmars, E. L.: Snakes of the World, New York, 1931, The Macmillan Co.
— : The Eeptile Book, Garden City, 1908, Doubleday, Doran & Co. Well

illustrated and interesting accounts of the turtles, tortoises, crocodilians,

lizards, and snakes which inhabit the United States and northern Mexico.
Gadow, Hans: Amphibia and Eeptiles, Cambridge Nat. Hist., VIII, London,

1901.
Greene, Eunice C. : Anatomy of the Eat, Philadelphia, 1935, Tr. Am. Philo. Soc,

New Series, Vol. XXVII.
Headley, P. W.: The Flight of Birds, London, 1912, Witherby and Co. (Charles

Scribner's Sons, New York, U. S. agents.)
Heaps, W. : Emigration, Migration, and Nomadism, Cambridge, England, 1931,

W. Heffer and Sons.
Herrick, C. L. : The Mammals of Minnesota, Minnesota Geol. and Nat. Hist.

Surv. Bull. 7, 1892.
HoUister, N.: A Systematic Account of the Prairie Dogs, Bur. Biol. Survey,

North Amer. Fauna 40 (1916).
Hornaday, T. W. : The Destruction of Our Birds and Mammals, 2d Ann. Eep.

of New York Zool. Soc, 1901.
Howell, A. B.: Aquatic Mammals, Springfield, 111., 1930, C. C. Thomas.
Hurter, Julius, Sr.: Herpetology of Missouri, Trans. Missouri Acad. Sc. Vol. 20,

St. Louis, 1911.
Jackson, H. H. T.: A Eeview of the American Moles, U. S. Biol. Survey, North

Amer. Fauna 38 (1916).
Kalmbaeh, E. E. : The Crow in Its Eelation to Agriculture, U. S. Dept. Agric,

Farmers' Bull. 1102 (1920).
Kaupp, B. P.: Anatomy of the Domestic Fowl, Philadelphia, 1918, W. B.

Saunders Co.
Lantz, D. E.: An Economic Study of Field Mice, U. S. Dept. Agric. Biol.

Survey Bull. 31, 1907.
Lewis, John B. : Sight and Scent in the Turkey Vulture, The Auk 45: pp. 467-

470, 1928.
Louisiana Conservation Dept. Birds of Louisiana, 1931.
McAtee, W. L. : How to Attract Birds, U. S. Dept. Agric. Farmers' Bulls. 621

(1914), 760 (1918), 844 (1917), 912 (1918).
Merriam, P. A.: Handbook of Birds of Western United States, Boston, 1927,

Houghton Mifiain Co.
National Geographic Society, The Book of Birds, 1927.
Neff, J. A.: A Study of the Economic Status of the Common Woodpeckers in

Eelation to Oregon Horticulture, Free Press Print, 1928.
Nelson, E. W. : Wild Animals of North America, Nat. Geog. Soc, 1918.
Pearson, T. G. : The Bird Study Books, Garden City, 1923, Doubleday Page

& Co.
Phillips, J. C: American Waterfowl, Boston, 1930, Houghton Mifilin Co.
Eeese, Albert M. : The Alligator and Its Allies, New York, 1915, G. P. Putnam's

Sons.
Eeighard, Jacob and Jennings, H. S., Anatomy of the Cat, New York, 1901,

Henry Holt & Co., Inc.
Eidgway, Eobert: The Birds of North and Middle America, Parts I to VIII,

U. S. Nat. Mus., 1901 to 1919.
Euthven, A. G., Thompson, C, and Gaige, H. T. : The Herpetology of Michigan,

Michigan Handb., Ser. 3, 1928.
Seton, E. T.: Life Histories of North American Animals, New York, 1909,

Charles Scribner's Sons.

896 TEXTBOOK OF ZOOLOGY

— : Lives of Game Animals, Vols. 1, 4, Garden City, 1925 to 1928, Doubleday

Page & Co.
Stejneger, Leonard: The Poisonous Snakes of North America, Eep. U. S. Nat.

Mus. for 1893.
Stoddard, H. L. : The Bobwhite Quail: Its Habits, Preservation, and Increase,

New York, 1931, Charles Scribner's Sons.
Stoner, D.: The Eodents of Iowa, Iowa Geol. Surv., 1918.
Stromsten, F. A.: Davison's Mammalian Anatomy, Philadelphia, 1937, P.

Blakiston's Son & Co.
Thomson, J. A.: The Biology of Birds, London, 1923, Sedg-wick and Jackson.
Viosca, Percy, Jr.: A Contribution to Our Knowledge of the Water Snakes,

Contr. No. 2, Eesearch Dept., Southern Biol. Sup. Co., Inc., 1924.
Vorhies, C, T., and Taylor, W. P.: Life History of the Kangaroo Eat, U. S.

Dept. Agric. Bull. No. 1091 (1922),
Warren, E. E.: The Mammals of Colorado, New York, 1910, G. P. Putnam's

Sons.
— : The Beaver: Its Work and Its Ways, Baltimore, 1926, Williams and

Wilkins Co.
Wetmore, Alexander: The Migration of Birds, Cambridge, Mass., 1926, Harvard

Univ. Press.

Anomalies

Arey, L. B. : Developmental Anatomy, Pliiladelphia, 1947, W. B. Saunders Co.
Evans, Titus C. : Effects of Eoentgen Eadiation on Embryonic Organization

and Eegulation in Melanoplus Differentialis (Orthoptera), Physiol. Zool.

10: No. 3, July, 1937.
Fitzpatriek, F. L. : Abnormal Foetal Pig (Double Body), Proc. Iowa Acad.

Sci. 35: 1928.
— : A Case of Abnormal Development in Gallus domesUca, Proc. Iowa Acad.

Sci. 32: 1925.

Internal Regulation

Allen, Danforth, Daisy, et al. : Sex and Internal Secretions, Baltimore, 1939,

Williams and Wilkins Co.
Banting, F. G., Best, C. H., and Macleod, J. J. E. : The Internal Secretion of

the Pancreas, Am. Jour. Physiol. 59: 1922.
Guyer, M. F. : Internal Secretions and Human Weil-Being, Science 74: 159,

1931.
Schafer, E. A.: The Endocrine Organs, ed. 2, London, 1916, Longmans, Green

& Co.

Kegeneration

Child, C. M. : Physiological Foundations of Behavior, New York, 1924, Henry

Holt & Co.
Morgan, T. H. : Eegeneration, New York, 1901, The Macmillan Co.

Radiation

Baitsell, G. A.: Science in Progress, New Haven, 1939, Yale University Press,

Ch. I and 11.
Butler, E. F. : The Effect of X-rays on Tissue Eegeneration, Scientific Monthly,

December, 1934, pp. 511-518.
Clark, George C. : A General Survey of Eadiobiology, Cold Spring Harbor

Symposia on Quantitative Biology 2: 249-263, 1934.
Clark, G. L.: Applied X-rays, New York, 1940, McGraw-Hill Co.
Duggar, B. M.: Biological Effects of Eadiation, New York, 1936, McGraw-Hill

Book Co.
Evans, T. C: Effects of Eoentgen Eadiation on Embryonic Organization and

Eegulation in Melanoplus Differentialis (Orthoptera), Physiol. Zool. 10:

258-268, 1937.

REFERENCES 897

Fricke, Hugo: The Chemical-Physical Foundation for the Biological Effects of

X-rays, Cold Spring Harbor Symposia on Quantitative Biology 2: 241-248,

1934.
Glasser, Otto: The Science of Radiology, Springfield, 111., 1933, C. C. Thomas.
Henshaw, P. S. : I. Studies of the Effect of Roentgen Rays on the Time of First

Cleavage in Some Marine Invertebrate Eggs. II. Recovery From Roentgen

Ray Effects in Arbacia Eggs, Am. J. Roentgenol. 27: 890-898, 1932.
Henshaw, P. S.: Radiation and the Cell, J. National Cancer Inst. Vol. 1, pp.

277-290.
Laurens, H. K. : Physiological Effects of Radiant Energy, Chemical Catalogue

Company, New York, 1933.
Patterson, J. T.: X-rays and Somatic Mutations, J. Hered. 20: 261-267, 1929.
Pohle, E. A.: Theoretical Principles of Roentgen Therapy, Philadelphia, 1938,

Lea and Febiger.
Quimby, E. H. : The Physical Basis of Radiation Therapy, Ann Arbor, Mich.,

1939, Edwards Bros., Inc.
Sheard, Charles: Life-Giving Light, Baltimore, 1933, Williams and Wilkins

Co.

Animal Distribution

Dice, L. R. : Life Zones and Mammalian Distribution, J. Mammalogy 4: No. 1,

1923.
Merriam, C. Hart: The Geographic Distribution of Animals and Plants in

North America, U. S. Dept. of Agric. Yearbook, 1894-1895, pp. 203-214.
Newbingin, Marion: Animal Geography, London, 1913, Oxford Univ. Press.
Parker, T. J., and Haswell, Wm. A.: Textbook of Zoology, New York, 1928,

The Macmillan Co. (Introduction).
Scharf, R. F. : Origin and Distribution of Life in America, London, 1911,

Constable & Co.
Wallace, A. R. : The Geographical Distribution of Animals, 2 vols. New York,

1876, The Macmillan Co.

Ecology-
Adams, C. C. : Guide to the Study of Animal Ecology, New York, 1913, The

Macmillan Co.
Birge, E. A., and Juday, C. A.: A Limnological Study of the Finger Lakes of

New York, Bull. 32, U. S. Bureau of Fisheries.
— , and — : The Inland Lakes of Wisconsin, Wisconsin Geol. and Nat. Hist.

Survey Bull. 27, 1914.
Chapman, R. N. : Animal Ecology, New York, 1931, McGraw-Hill Book Co.
Elton, Charles S.: Animal Ecology, London, 1927, Sedgwick and Jackson.
Henderson, L. J.: The Fitness of the Environment, New York, 1913, The

Macmillan Co.
Hesse, Richard, et al. : Ecological Animal Geography, New York, 1937, John

Wiley & Sons.
Mast, S. O. : Light and the Behavior of Organisms, New York, 1911, John Wiley

and Sons.
Pearse, A. S.: Animal Ecology, New York, 1926, McGraw-Hill Book Co.
Shelf ord, V. E. : Animal Communities in Temperate America, Chicago, revised

1927, Univ. Chicago Press.
— : Laboratory and Field Ecology, Baltimore, 1929, Williams and Wilkins Co.
— : Naturalist's Guide to the Americas, Baltimore, 1926, Williams and Wilkins

Co.
Stromsten, F. A.: Lake Okoboji as a Type of Aquatic Environment, Univ. of

Iowa Studies in Nat, Hist., No. 12, 1927.

$98 TEXTBOOK OF ZOOLOGY

Viosca, Percy, Jr.: An Ecological Study of the Cold-Blooded Vertebrates of

Southeastern Louisiana, Copeia, No. 115, 1923.
— : Distributional Problems of the Cold-Blooded Vertebrates of the Gulf Coastal

Plain, Ecology 7: No. 3, 1926.
Welch, P. S. : Limnology, New York, 1935, McGraw-Hill Book Co.
"Williams, S. H. : A Systematic Guide to Field Zoology, Author's mimeographed

edition. Univ. of Pittsburgh, 1927.

Parasitology

Chandler, A. C. : Introduction to Parasitology, New York, 1940, John Wiley

and Sons.
Doane, E. W.: Insects and Disease, New York, 1910, Henry Holt & Co.
Ewing, H. E.: Manual of External Parasites, Springfield, 111., 1929, C. C.

Thomas.
Faust, E. C: Human Helminthology, Philadelphia, 1929, Lea and Febiger.
Johansen, O. A., and Riley, W. A.: Handbook of Medical Entomology, Ithaca,

N. Y., 1915, Comstock Pub. Co.
Linton, E., and Stiles, C. W. : Information Concerning Parasitic Worms in

Fish, U. S. Bur. Fisheries, Econ. Circular No. 21, 1916.
Matheson, Eobert: Medical Entomology, Springfield, 111., 1932, C. C, Thomas.
Eiley, W. A., and Christensen, E. B. : Guide to the Study of Animal Parasites,

New York, 1930, McGraw-Hill Book Co.
Shipley, A. E. : Nemathelminthes, Vol. II. Cambridge Nat. Hist., London,

1922.
Stunkard, H. W. : Parasitism: A Biological Phenomenon, Scientific Monthly,

1929, pp. 349-362.

Marine Zoology

Arnold, A. F.: The Sea Beach at Ebb Tide, New York, 1901, Century Co.
Bassler, E. S. : The Bryozoa, or Moss Animals, Smithsonian Eept. for 1920,

pp. 339-380. (Publication 2633.)
Berry, S. Stillman: A Eeview of the Cephalopods of Western North America,
Bull. Bur. Fish. 30: 267-336, 1910.
Notes on Some Cephalopods in the Collection of the University of California,

Univ. California Pub. Zool. 8: 301-310, 1911.
Notes on West American Chitons I. Proc. California Acad. Sci., 4th Series,

Zool. 7: 229-248, 1917.
A Distributional Note on Haliotis, California Fish and Game 7: 254-255,
1921.
Breder, Charles M. : Field Book of Marine Fishes of the Atlantic Coast, New

York, 1921, G. P. Putnam's Sons.
Bush, Mildred : Eevised Key to the Echinoderms of Friday Harbor, Pub. Puget

Sd. Biol. Sta. 3: 65-77, 1921.
Clark, A. H. : Animals of Land and Sea, New York, 1927, D. Van Nostrand Co.
The Holothurians of the Pacific Coast of North America, Zool. Anz. 24:

162-171, 1901.
North Pacific Ophiurans in the Collection of the United States National

Museum, Bull. U. S. Nat. Mus., No. 75, 1911.
Hawaiian and Other Pacific Echini, Mem. Mus. Comp. Zool. Harvard 34:
Nos. 2 and 3, 1909.
Coe, Wesley E.: The Echinoderms of Connecticut, Connecticut Geol. and Nat.

Hist. Surv., 1912.
— : Nemerteans, Harriman Alaska Expedition 11: 1-220, 1904.
— : Nemerteans of the West and Northwest Coasts of America, Bull. Mus.

Comp. Zool. Harvard 47: 1905.
Crowder, AVilliam: Dwellers of the Sea and Shore, New York, 1923, The
MacmiUan Co.

REFERENCES 899

— : Naturalist at the Seashore, New York, 1928, Century Co.

— : Between the Tides, New York, 1930, Dodd, Mead & Co.

Davis, B. M. : The Earlj-- Life-History of Dolichoglossus pusillus Bitter, Univ.

California Pub. Zool. 4: 187-226, 'l908.
Flattely, F. W.: The Biology of the Seashore, London, 1922, Sedgvsdck and

Jackson.
Fraser, C. McLean: The Hydroids of the West Coast of North America, Bull,
from Laboratories of Natural History of State University of Iowa 6:
No. 1, 3-91, 1911.
— : Some H^'droids of the Vancouver Island Eegion, Trans. Eoy. Soc. Canada,

Series 3 8: 99-216, 1914.
— : On the Development of Aequorea forshalea, Trans. Eoy. Soc. Canada,

Series 3 10: 97-104, 1916.
George, W. C, and Wilson, H. V.: Sponges of Beaufort and Vicinity, Bull. U.

S. Bur. Fisheries 37: 1917-1918.
Gunter, Gordon: A List of Fishes of the Mainland of North and Middle
America. Eecorded From Both Fresh Water and Sea Water, American
Midland Naturalist 28: 305-326, 1942.
Harmer, S. F., and Shysley, A. E. : Flatworms, Metazoa, etc. Cambridge Nat.

Hist. Series, Vol."' 2, New York, 1909, The Macmillan Co.
Heath, Harold: California Clams, California Fish and Game 2: 175-178, 1916.
— : Devilfish and Squid, California Fish and Game 3: 103-108, 1917.
Heilprin, Angelo: Animals of Our Seashore, Philadelphia, 1888, J. B. Lippincott

Co.
Johnson, M. E., and Snook, H. J.: Seashore Animals of the Pacific Coast, New

York, 1935, The Macmillan Co.
Mayer, A. G.: Seashore Life, New York Zool. Soc, 1911.
Miner, Eoy W. : Fieldbook of Seashore Life, New York, 1933, G. P. Putnam's

Sons.
— : Animals of the Wharf Piles, Am. Mus. Nat. Hist. Jour. 13: Feb., 1913.
^: Outposts of the Sea: Animals of the Tidal Zone, Am. Mus. Nat. Hist.

Guide Leaflet, Ser. 74, 1929.
Nutting, C. C: American Hydroids. I. Plumularidae, U. S. Nat. Mus. Bull.,

1900.
— : The Hydroids, Papers from Harriman Alaska Expedition, Proc. Wash.

Acad. Sci. 3: 157-216, 1901.
— : American Hydroids. II. Sertularidae, U. S. Nat. Mus. Bull., 1904.
— : American Hydroids. HI. Campanularidae and Bonnevieilidae, U. S. Nat.

Mus. Bull., 1915.
Eankin, E. P.: The Mussels of the Pacific Coast, California Fish and Game

4: 113-117, 1918.
Eitter W. E.: The Movements of the Enteropneusta and the Mechanism by

'which They Are Accomplished, Biol. Bull. 3: 255-201, 1902.
— : The Pelagic Tunicata of the San Diego Eegion, Excepting the Larvacea,
Univ. California Pub. Zool. 2: 51-112, 1905.

and Forsyth, Euth A. : Ascidians of the Littoral Zone of Southern California,

' Univ. California Publ. Zool. 16: 439-512, 1915.
Eobertson Alice: Studies in Pacific Coast Entoprocta, Proc. California Acad.

Sci'., Zoology 2: 323-348, 1900. , ^ ,.. . tt •

Schmitt Waldo L.: The Marine Decapod Crustacea of California, Univ.

California Publ. Zool. 1: 1-104, 1902.
Torrey H B : The Hydroida of the Pacific Coast of North America, Univ.
'California Publ. Zool. 23: 1-470, 1921.

— • Anemones, With Discussion of Variation m Metridium, Proc. Washington

Acad. Sci. 4: 373-410, 1902. ^ t. . * ,

— • Biological Studies on Corymorpha. 1. C. pahna and Environment, J.

Exper. Zool. 1: 395-422, 1904.

900 TEXTBOOK OF ZOOLOGY

On the Habits and Eeactions of Sagartia davisi, Biol. Bull. 6: 203-216,
1904.
The Hydroids of the San Diego Eegion, Univ. California Publ, Zool. 2:

1-43, 1904.
Ctenophores of the San Diego Eegion, Univ. California Publ. Zool. 2:

45-50, 1904.
The Behavior of Corymorpha, Univ. California Publ. Zool. 2: 333-340,
1905.

The California Shore Anemone, Bunodactis xanthogrammica, Univ. Cali-
fornia Publ. Zool. 3: 41-45, 1906.
Biological Studies on Corymorpha. II. The Development of C. palma

from the Egg, Univ. California Publ. Zool. 3: 253-298, 1907,
The Lpeomedusae of the San Diego Eegion, Univ. California Publ. Zool.

6: 11-31, 1909.
Biological Studies on Corymorpha. III. Eegeneration of Hydranth and

Holdfast, Univ. California Publ. Zool. 6: 205-221, 1910.

Biological Studies on Corymorpha. IV. Biol. Bull. 19: 280-301, 1910.

Treadwell, A, L. : Polychaetous Annelids of the Pacific Coast in the Collections

of the Zoological Museum of the University of California, New Syllidae

from San Francisco Bay, Univ. California Publ. Zool. 13: 175-238, 1914.

Ulrey A B.: The Starfishes of Southern California, Bull. So. California

'Acad. Sci., July, 1918, pp. 39-52.

Verrill, A. E.: Monograph of the Shallow-Water Starfishes of the North

Pacific Coast From the Arctic Ocean to California, Harriman Alaska

Series, Smithsonian Institution 14: Parts 1 and 2 (Part 2 consists of

plates), 1914,

Weese A O and Townsend, M. T.: Some Eeactions of the Jellyfish, Aequo rea,

Publ. Puget Sd. Biol. Sta. 3: 117-128, 1921.
Weymouth, Frank W.: The Edible Clams, Mussels and Scallops of California,

Fish Bulletin, No. 4, California Fish and Game Commission, 1920.
— : The Life-History and Growth of the Pismo Clam, Fish Bulletin, No. 7,

California Fish and Game Commission, 1923.
Wilson, C. B.: North American Parasitic Copepods: A List of Those Found
Upon the Fishes of the Pacific Coast, Proc. U. S. Nat. Mus. 35: 431-481,
1909.
Wilson, H. v.: The Sponges, Mem. Mus. Comp. Zool. Harvard 30: No. 1, 1904.
(Eeport of an exploration off the west coasts of Mexico and South
America.)

Wildlife Conservation

Gabrielson, Ira N.: Wildlife Conservation, New York, 1941, The Macmillan

Co. ^ .,

Hornaday, W, T.: Our Vanishing Wildlife, New York, 1913, Charles Scnb-

ner's Sons.
Lydekker, E.: Wildlife of the World, New York, 1915, Fred Warne & Co.

Embryology

Arey, Leslie B.: Developmental Anatomy, ed. 3, Philadelphia, 1947, W. B.

Saunders Co.
Hartman, Carl G.: Studies in the Development of the Opossum, J. Morphol. 27:

1-85, 1916.
— : Breeding, Habits, Development and Birth of the Opossum, Smithsonian

Institution Pub. 2689, 1923.
Lillie, Frank: Development of the Chick, 2nd revised edition. New York, 1927,

Henry Holt & Co.

REFERENCES 901

McEwen, Robert S.: Textbook of Vertebrate Embryology, New York, 1931,

Henry Holt & Co.
Patten, Bradley M.: Embryology of the Chick, ed. 3, Philadelphia, 1929, P.

Blakiston's Son and Co.
— : Embryology of the Pig, ed. 2, Philadelphia, 1931, P. Blakiston's Son and

Co.
Richards, A.: Comparative Embryologj', New York, 1931, John Wiley & Sons.
Wieman, H. L.: An Introduction to Vertebrate Embryology, New York, 1930,

McGraw-Hill Book Co.

Heredity

Altenberg, E.: How We Inherit, New York, 1928, Henry Holt & Co.
Baber, R. A., and Ross, E. A.: Changes in the Size of the American Family

in One Generation, Univ. Wisconsin Studies, 1924, 99 pp.
Baker, J. R.: Sex in Man and Animals, London, 1926, George Routledge &

Sons.
Barnes, I.: The Inheritance of Pigmentation in the American Negro, Human

Biol. 1: 321-381, 1929.
Bauer, E., Fischer, E., and Lenz, F.: Human Heredity, New York, 1931, The

Macmillan Co.
Blacker, C. P.: The Chances of Morbid Inheritance, London, 1934, H. K.

Lewis & Co.
Bonar, J.: Malthus and His Work, New York, 1924, The Macmillan Co.
Bowen, E. : An Hypothesis of Population Growth, New York, 1931, Columbia

Univ. Press.
Carr-Saunders, A. M.: The Population Problem, Oxford, 1932, Clarendom

Press.
— : Eugenics, New York, 1926, Henry Holt & Co.
Castle, W. E. : Genetics and Eugenics, ed. 4, Cambridge, Mass., 1930, Harvard

Univ. Press.
Cattell, J. McK., Families of American Men of Science, Popular Science

Monthly 86: 504-515, 1915; Scient. Monthly 4: 248-262; 5: 368-377, 1917.
Conklin, E. G.: The Direction of Human Evolution, New York, 1921, Charles

Scribner's Sons.
Cowdry, E. V., and Embree, E. R.: Human Biology and Racial Welfare, Vol. 2,

New York, 1930, Paul B. Hoeber.
Darwin, L. : The Need for Eugenic Reform, New York, 1926, D. Appleton & Co.
Davenport, C. B. : Heredity of Skin Color in Negro-White Crosses, Pub.

Carnegie Inst., No. 188, 1913fe.
— , and Muncey, E. B. : Huntington's Chorea in Relation to Heredity and

Eugenics,"^ Am. J. Insanity 73: 195-222, 1916. Also in Bull. Eug. Rec. Off.
17: 1916.
East, E. M.: Mankind at the Crossroads, New York, 1923, Charles Scribner's

Sons.
Fasten, N. : Principles of Genetics and Eugenics, Boston, 1935, Ginu & Co.
Gates, R. R. : Heredity in Man, New York, 1929, The Macmillan Co.
Gosney, E. S., and Popenoe, P.: Sterilization for Human Betterment, New York,

1929, The Macmillan Co.
Gun, W. T. J.: Studies in Hereditary Ability, London, 1928.
Guyer, M. F.: Being Weil-Born, Indinapolis, 1927, Bobbs-Merrill Co.
Herskovits, M. J.: The American Negro, A Study in Race Crossing, New York,

1928, Alfred A. Knopf.
Hodson, C. B. S. : Human Sterilization Today, London, 1934, Watts.
Hogben, L. : Genetic Principles in Medicine and Social Science, London, 1931,

Williams and Norgate.
— : Nature and Nurture, London, 1933, Williams and Norgate.
Holmes, S. J.: The Trend of the Race: A Study of Present Tendencies in the

Biological Development of Civilized Mankind, New York, 1921, Harcourt,

Brace & Co.

902 TEXTBOOK OF ZOOLOGY

— : Studies in Evolution and Eugenics, New York, 1923, Harcourt, Brace

& Co.
— : The Size of College Families, J. Hered. 15: 407-415, 1924.
— : Human Genetics and Its Social Import, New York, 1933, Harcourt, Brace

& Co.
Hunt, H. E. : Some Biological Aspects of War, New York, 1930, Galton Publ. Co.
Huntington, E.: Tomorrow's Children, New York, 1935, John Wiley & Sons.
Jennings, H. S. : The Biological Basis of Human Nature, New York, 1930, W. W.

Norton & Co.
— : Genetics, New York, 1935, W. W. Norton & Co.
Jordan, D. S.: The Human Harvest, Boston, 1907, Beacon Press.
Kuczynski, E. E.: The Balance of Births and Deaths. I. Western and Northern

Europe, New York, 1928, The Macmillan Co.
Landman, J. H.: Human Sterilization, New York, 1932, The Macmillan Co.
Laughlin, H. H. : Eugenical Sterilization, Am. Eugenics Soc, 1926.
Lidbetter, E. J.: Heredity and the Social Problem Group, Vol. I, London, 1934,

Edward Arnold & Co.
Lindsey, A. W.: A Textbook of Genetics, New York, 1932, The Macmillan Co.
Lorimer, F., and Osborn, F. : Dynamics of Population, New York, 1934, The

Macmillan Co.
Ludovici, A. M. : The Choice of a Mate, London, 1934, John Lane.
Morgan, T. H. : The Physical Basis of Heredity, Philadelphia, 1919, J. B.

Lippincott Co.
Myreson, A. : The Inheritance of Mental Diseases, Baltimore, 1925, Williams and

Wilkins Co.
Newman, H. H. : Evolution, Genetics, and Eugenics, ed. 3, Chicago, 1932, Univ.

Chicago Press.
Pearl, E. : Studies in Human Biology, Baltimore, 1924, Williams and Wilkins

Co.
— , and Pearl, E. DeW\: The Ancestry of the Long-Lived, Baltimore, 1934,

Johns Hopkins Press.
Popenoe, P.: The Child's Heredity, Baltimore, 1929, Williams and Wilkins Co.
— , and Johnson, E. S. : Applied Eugenics, ed. 2, New York, 1933, The Macmillan

Co.
Eollins, W. A.: Fertility of College Graduates, J. Hered. 20: 425-427, 1929.
ShuU, A. F. : Heredity, New York, 1931, McGraw-Hill Book Co.
Sinnott, E. W., and Dunn, L. C. : Principles of Genetics, New York, 1932,

McGraw-Hill Book Co.
Stockard, C. E.: The Physical Basis of Personality, New York, 1931, W. W.

Norton Co.
Terman, L. M. : Genetic Studies of Genius. Vol. I, Mental and Physical Traits

of a Thousand Gifted Children, 1925, Stanford Univ. Press.
Thompson, W. S.: Population Problems, ed. 2, New York, 1935, McGraw-Hill

Book Co.
Walter, H. E.: Genetics, New York, 1930, The Macmillan Co.
Whitney, L. F.: The Case for Sterilization, New York, 1934, Frederick A.

Stokes Co.
Wiggam, A. E. : The Fruit of the Family Tree, Indianapolis, 1924, Bobbs-Merrill

Co.

Animal Behavior

Bernard, L. L.r Instinct, a Study in Social Psychology, New York, 1924,

Henry Holt & Co.
Buddenbrock, W. T.: A Criticism of the Tropism Theory of Jacque Loeb,

J. Animal Behavior 6: 341-306, 1916.
Coghill, G. E.: Anatomy and the Problems of Behavior, New York, 1929, The

Macmillan Co.

REFERENCES 903

Dunlap, Knight: Elements of Psychology, St. Louis, 1936, The C. V. Mosby Co.
Holmes, S. J.: Studies in Animal Behavior, Boston, 1916, E. G. Badger.
Howes, Paul G. : Insect Behavior, Boston, 1919, R. G. Badger.
Hunter, VV. S. : The Delayed Reaction in Animals and Children, Behavior

Monographs 2: No. 1, 1913.
Jennings, H. S.: Behavior of Lower Organisms, Part III, New York, 1906,

Columbia University Press.
Loeb, J.: Forced Movements, Tropisms, and Animal Conduct, Philadelphia,

1918, J. B. Lippincott Co.
Mast, S. C. : Light and the Behavior of Organisms, Part I, New York, 1910,

John Wiley & Sons.
Morgan, C. L.: Introduction to Comparative Psychology, ed. 2, Loudon, 1906,

W. Scott.
Romanes, G. J.: Animal Intelligence, London, 1881, Keagan Paul.
Washburn, M. F. : The Animal Mind, Nevy York, 1926, The Macmillan Co.
Watson, J. B. : Introduction to Animal Psychology, New York, Henry Holt

& Co.

Physiology

Bayliss, Wm.: Principles of General Physiology, New York, 1924, Longmans,

Green & Co.
Burton-Opitz, R. : An Elementary Manual of Physiology, Philadelphia, 1936,

W. B. Saunders Co.
Heilbrunn, L. V.: An Outline of General Physiology, Philadelphia, 1937,

W. B. Saunders Co.
Howell, W. H. : Textbook of Physiology, Philadelphia, 1933, W. B. Saunders Co.
Macleod, J. J. R. : Physiology in Modern Medicine, St. Louis, 1938, The C. V.

Mosby Co.
Mitchell, P. H.: General Physiology, New York, 1932, McGraw-Hill Book Co.
Rogers, C. G.: Textbook of Comparative Physiology, New York, 1927, AlcGraw-

Hill Book Co.
Stiles, P. G. : Human Physiology, Philadelphia, 1939, W. B. Saunders Co.
Zoethout, AVm. D.: Textbook of I'liysiology, St. Loui;^, 1938, The C. V. Mosby Co.

Paleontology

Bassler, R. S. : Shelled Invertebrates of the Past and Present, Smithsonian

Scientific Series 10: 1932.
Berry, E. W.: Paleontology, New York. 1920, McGraw-Hill Book Co.
Crabtree, J. H.: Rocks and Fossils and How to Identify Them, London, 1921,

Epworth Press.
Davies, Arthur M. : Introduction to Paleontology, London, 1920, Thomas Murby

Co.
Hawkins, Herbert L. : Invertebrate Paleontology; An Introduction to the Study

of Fossils, London, 1920, Methuen & Co.
Lucas, H. A.: Animals of the Past, Am. Mus. Nat. Hist. Handbook No. 4,

1929.
Lull, Richard S. : Organic Evolution, New York, 1929, The Macmillan Co.
Lydekker, L.: The Horse and Its Relatives, New York, 1912, The Macmillan Co.
Moddie, Roy L. : A Popular Guide to the Nature and Environment of the Fossil

Vertebrates of New York, New York State Mus. Handbook No. 12, 1933.
Osborn, H. F.: The Age of Mammals, New York, 1910, The Macmillan Co.
Scott, W. B. : History of the Land Mammals of the Western Hemisphere, New

York, 1929, The Macmillan Co.
Scudder, Samuel H. : Some Insects of Special Interest From Florissant Colorado

and Other Points in the Tertiaries of Colorado and Utah, U. S. Geol. Surv.

Bull. No. 93, 1892.
Shimer, Hervey AV. : Introduction to a Study of Fossils, New York, 1933, The

Macmillan Co.

904 TEXTBOOK OF ZOOLOGY

Phylogeny

Darwin, Charles: Origin of Species, ed. 6, London, 1880.

— : Variation in Animals and Plants Under Domestication, London, 1868,

— : The Descent of Man, London, 1871.

Dendy, Arthur: Outlines of Evolutionary Biology, New York, 1911, D. Appleton

& Co.
Gaskell, W, H. : The Origin of the Vertebrates, London, 1908, Longmans, Green

& Co.
Jordan, D. S., and Kellogg, V. L. : Evolution and Animal Life, New York,

1908, D. Appleton & Co.
Kellogg, V. L.: Darwinism Today, New York, 1908, Henry Holt & Co.
Lindsey, A, W.: Problems of Evolution, New York, 1931, The Macmillan Co.
Lull, E. S. : Organic Evolution, New York, 1940, The Macmillan Co.
Newman, H. H., et al.: The Nature of the World and of Men, Chicago, 1926,

Univ. Chicago Press.
Newman, H. H.: Evolution Yesterday and Today, Baltimore, 1932, Williams

and Wilkins Co.
Osborn, H. F. : From the Greeks to Darwin, New York, 1894, The Macmillan Co.
— : The Origin and Evolution of Life, New York, 1917, Charles Scribner's

Sons.
Patten, W. : The Evolution of the Vertebrates and Their Kin, Philadelphia, 1912,

P. Blakiston's Son & Co.
Romer, A. S. : Man and the Vertebrates, Chicago, 1933, Univ. Chicago Press.
Thomson, J. A.: Darwinism and Human Life, New York, 1908, Henry Holt

& Co.
Wilder, H. H. : History of the Human Body, New York, 1909, Henry Holt & Co.
— : The Pedigree of the Human Eace, New York, 1926, Henry Holt & Co.

GLOSSAKY*

Abdomen (ab do'men), the portion of the trunk posterior to the thorax of an
animal.

Aboral (abo'ral), opposite the mouth.

Absorption (ab sorp'shun), the process of taking in soluble foods by the circula-
tory medium or by the protoplasm directly.

Accommodation (akom 6 da'shiin), the power of adjustment of the eye to near
and far objects.

Acetabulum (as e tab'u liim), the socket in each side of the innominate bone
of the pelvic girdle into which the femur fits.

Achromatic figure (ak ro mat'ik). the mitotic figure without chromosomes.

Aestivation (es ti va'shiin), state of torpidity induced by heat and dryness.

Allelomorphs (ale'16 morfs), a pair of corresponding genes in homologous
chromosomes, but each produces a different character.

Allergy (al'er ji), acute sensitiveness to a foreign substance, as foreign protein
in the body.

Alternation of generation (al ter na'shun of jen er a'shiin). (See Metagenesis.)

Altrlcial (al trish'al), in reference to birds which are hatched without feathers
and in a helpless condition.

Alveolus (alve'olus), a small cavity or pit, such as the air sacs in the lung
of a higher vertebrate.

Ambulacral (am bu la'kral), area of echinoderm related to tube feet.

Amino acids (am'ino), organic acids with a (NH,) radical, and derived from
complex proteins.

Amitosis (amit5'sis), direct cell division, occurring without chromosomal
activity.

Amnion (am'nion), inner embryonic membrane of terrestrial vertebrates.

Amoeboid movement (ame'boid), the streaming of protoplasm in a cell to ex-
tend the cell in some direction with the formation of pseudopodia.

AmpMaster (am'fi as ter), the complete mitotic figure of a dividing cell.

Amphiblastula (am fi blas'tu la), the free-swimming larval form in sponges.

Amphimixus (am fi mik'sis), union of nuclear material from two different
cells, as in fertilization.

Ampulla (ampul'a), a bulblike dilatation.

Amylopsin (am i lop'sin), a pancreatic enzyme capable of converting starch
into sugar.

Anabolism (an ab'6 liz'm), the building up of living protoplasm.

Analogous (anal'ogus), differing in structure and origin, but similar in func-
tion.

Anaphylaxis (an afl lak'sis), acute reaction of the body to foreign protein
materials which have a toxic effect; this may be an increased sensitivity
to the material because of previous contact with it.

Anatomy (anat'omi), the science that treats of the structure of organic bodies.

Anus (fi'nus), the posterior opening of the alimentary canal.

Appendicular skeleton (ap en dik'iilar), skeleton of the paired fins of Pisces.

Archenteron (ar ken'ter on), the cavity of the gastrula which is the primitive
digestive cavity.

Artery (iir'teri), the larger blood vessels leading away from the heart.

Asexual reproduction (a sek'shu al), reproduction without sex cells.

Assimilation (as sim i la'shiin), the transforming of digested food into proto-
plasm.

♦Phonetics according to "Webster's New International Dictionary.

905

906 TEXTBOOK OF ZOOLOGY

Asymmetry (a sim'et ri), a condition in which the two sides of an animal are

dissimilar.
Autonomic nervous system (6 to nom'ik), that portion of the nervous system

controlling the involuntary muscles.
Autotomy (otot'omi), self -mutilation.
Axial gradient (ak'si al gra' di ent), the graduation of the rate of metabolism

along the principal axis of an axiate animal.
Axone (ak'son), a nerve fiber serving to conduct impulses away from a nerve

cell body.

Barrier (bar'ier), any physical, chemical, or biological obstruction that pre-
vents migration of animals.

Benthos (ben'thos), life of the deep sea bottom.

Binary fission (bi'nari fish'iin), division of a cell into two daughter cells.

Biramous (bira'miis), a two-branched condition.

Bisexual (bi sek'sh 6b al), a condition in which both male and female organs
are present in one individual.

Bivium (biv'ium), one side of an echinoderm including a pair of ambulacra.

Blastocoele (blas'to sel), the cavity present in the blastula stage of develop-
ment; also cleavage of segmentation cavity.

Blastomere (blas'to mer), one of the segments first formed by the division of
the ovum.

Blastula (blas'tula), a sphere of cells with a hollow cavity resulting from
cleavage of the zygote.

Blepharoplast (blef'a ro plast), the bodj^ in a cell from which a flagellum
arises.

Brachium (bra'kium), arm.

Branchial (brang' ki al), pertaining to gills or branchiae.

Buccal (buk'al), pertaining to the mouth cavity.

Budding (bud'ing), reproduction involving the branching of new individuals
from the external surface of the old one.

Byssus (bis'iis), a tuft of fiberlike threads which attach certain mussels to the
substratum.

Caecum (se'kum), a blind pouchlike pocket of the intestine; usually at the

junction of the small and large intestines.
Calcareous (kal kar'e us), composed of lime or calcium salts.
Calciferous (krd sif'er us), glands which are thought to secrete an alkaline

secretion into the esophagus of the earthworm.
Canaliculus (kan'alik' u lus), one of the tiny canals extending from lacuna

to lacuna to distribute nutriment in bone.
Capillary (kap'ileri), a microscopic branch of an artery which extends into

a tissue and finally joins a small vein.
Carapace (kar'apas), shell-like external covering.
Carbohydrate (kar bo hi'drat), organic compound of carbon, hydrogen, and

oxygen, such as starch or sugar.
Cardiac (kar'diak), pertaining to the heart.
Carnivorous (kjir niv'6 rus), flesh eating.
Caste (kast), any group of distinct forms within a species, as found in some

insects.
Cataholism (ka tab' 6 lizm), process of oxidation or break-down of protoplasm;

destructive phase of metabolism; dissimilation; the oxidation of organic

substances of the body to release kinetic energy and heat.
Catalysis (ka tal'i sis), the initiation or acceleration of a chemical reaction

by the presence of a substance which itself does not enter into the

reaction, as an enzyme.
Caudal (ko'dol), pertaining to the tail.

GLOSSARY 907

Cell theory (sel the'ori), the theory that all living things are composed of

cells.
Centimeter (sen'ti nie'ter), one-hundredth of a meter and the equivalent of

0.393 inches; or 1 inch equals 2..5-i centimeters.
Central nervous system (sen'tral), that portion of the nervous system composed

of the brain and the spinal cord.
Centriole (sen'triol), a small granule within the central part of the aster in

the mitotic figure; also known as centrosome.
Centrolecithal (sen'trO les i thai), refers to the tj-pe of egg with the yolk mass

in the center, as the egg in insects.
Centrosome (sen'tro som), usually considered synonymous to centriole.
Cephalic (sefal'ik), pertaining to the head.
Cephalothorax (sef'alo tho'raks), a fusion of the head and thorax or chest,

as in crayfish.
Cerebellum (ser ebel'liim), the large lobe of the hind brain, in front of and

above the medulla.
Cerebrum (ser'ebrum), the anterior division of the brain.
Cercaria (surka'ria), a tailed larval stage of the liver fluke.
Ctenoid scale (te'noid), a type of fish scale with spines at the free margin.
Cervical (sur'vikal), has reference to the neck region.
Chaeta (ke'ta), one of the bristlelike structures in the body wall of many

annelids, used as organs of locomotion.
Chelicera (kelis'era), an anterior pair of appendages in arachnids.
Cheliped (ke'liped), most anterior thoracic leg of cra^-fish; large pincher.
Chemotropism (kcmot'ropiz'm), response of an organism to chemical changes.
Chlorophyll (klo'rofil), the green coloring matter in plants and a few ani-
mals which is active in photosj'nthesis.
Chromosomes (kro'mo somz), bodies formed in the nucleus during mitosis which

constitute the physical basis of inheritance.
Chitin (ki'tin), the hard material composing the 'exoskeleton or shell of

Crustacea, insects, and others.
Chloragogue cells (klo'ragog), compose the outer layer of the intestine of

the earthworm.
Chondi-in (kon'drin), the material of which cartilage is composed.
Chorion (ko'rion), the outer embryonic membrane of mammals.
Chorioid (ko'rioid), middle or vascular coat of vertebrate eyeball.
Chromatin (kro'matin), dark-staining substance of the nucleus of the cell.
Chromatophore (krd'ma to for), a colored pigment cell.
Chromidia (kromid'ia), scattered chromatin granules through the cytoplasm

of some cells. .

Chromonemata (kro mo nem' a ta), threads of chromatin distinguishable within

chromosomes during mitosis; seen in the resting phase of some._
Chyme (kim), partially digested food material which is in semiliquid con-
dition.
Cilia (sil'ia), hairlike cytoplasmic processes, used by certain protozoans for

locomotion.
Cirrus (sir'iis), a bristlelike appendage.
Cleavage (klev'ij), the cell divisions changing the zygote into an organism

of many cells.
Clitellum (klitel'um), a broadened area in the earthworm about one-third of

the length of the body back from the head. It is glandular and serves

in producing the cocoon.
Cloaca (kloa'ka), the common chamber into which the intestine, and urinary

and genital canals discharge in some forms.
Cnidoblast (ni'do blast), the type of cell of the coelenterate in which the sting

cell or nematocyst develops.

908 TEXTBOOK OF ZOOLOGY

Cochlea (kok'lea), a coiled structure of the inner ear in which is located the
sensory ending of the auditory nerve.

Cocoon (kokobn'), a covering which protects a larva, pupa, or even the adult
stage of certain animals.

CoeloWastula (selo blas'tula), blastula having a hollow center.

Coelom (se'lom) or coelome (se'lom), the space between the walls of the body
and the inclosed viscera.

Commensalism (ko men'sal izm), an association of different species of an-
imals in which at least one benefits without injury to the other.

Commissure (kom'i shoor), a strand of nerve fibers or nerves joining two cen-
ters or ganglia.

Conditioned reflex (kon dish'und), a reflex action which is modified or estab-
lished by previous experience.

Conductivity (kon'duk tiv i ti), the power of conducting or of receiving and
transmitting.

Congenital (kon jen'i tal), conditions existing at birth.

Conjugation (kon job ga'shun), a temporary union of two individuals with
exchange of nuclear material.

Copulation (kop u la'shun), union of genital regions of two individuals during
which spermatozoa are transferred from one to the other.

Corium (ko'riiim), the deeper layer of the skin or dermis.

Cornea (kor'nea), transparent coat of modified epithelial tissue over the front
of the eye.

Cortex (kor'teks), superficial portion or outer layer, as of the brain or kid-
ney.

Cranial (cra'nial), pertaining to the portion of the skull enclosing the brain.

Cretin (kre'tin), a defective individual due to abnormality of the thyroid
gland.

Ctenidia (tenid'ia), gill structure.

Cutaneous (kutaneus), pertaining to the skin.

Cuticle (ku'tikl), the outer surface of the skin of many animals.

Cycloid scale (si'kloid), a scale which is thin and shows concentric lines of
growth without serrated margin.

Cyclosis (siklo'sis), the rotation of the endoplasm of protozoan forms.

Cyst (sist), an organism enclosed by a resistant wall.

Cysticercus (sis ti sur'kus), the bladder worm or encysted stage in the life
history of the tapeworm.

Cytology (sitol'oji), the science that treats of the minute structure of cells.

Cytopharynx (si to far'inks), channel from surface to endoplasm in Euglena.

Cytoplasm (si'to plaz'm), the protoplasm of the cell not including the nucleus.

Dactyl (dak'til), refers to finger.

Daughter cells (do'ter selz), the two cells resulting from a division of one celh

Delamination (de lam i na'shiin), the formation of a new layer of cells parallel

to the old by the division and migration of cells of the primary germ

layers.
Dendrite (den'drit), a nerve fiber which carries impulses toward the nerve

cell body.
Dermis (dur'mis), same as corium.
Dialysis (dial'isis), separation of dissolved materials in crj^stalloids and

colloids by means of semipermeable membrane.
Diaphragm (di'afram), a muscular partition between the abdominal and

thoracic cavities in mammals.
Diastase (di'astas), the class of enzymes capable of bringing about conversion

of starches to sugars.
Diencephalon (di en sef'a Ion), a region of the brain just posterior to the

cerebrum.

GLOSSARY 909

Diflferentiation (dif eren shi a'shun), the formation of special parts, tissues, or
cells from the primitive unspecialized layers.

Diffuse (difus'), to mix with or to spread through completely and thoroughly
another substance.

Digestion (di jes'chun), the conversion of complex unabsorbable food materials
into a form capable of bodily absorption.

Dihybrid (dihi'brid), progeny or offspring of parents differing in two charac-
ters.

Dominance (dom'inans), a condition in which one of two characters present
in the individual appears to the exclusion of the other.

Dimorphism (di mor'fizm), difference of form between members of the same
species.

Dioecious (die'shus), the male and female germ cells being produced by dif-
ferent individuals.

Diploblastic (dip 16 blas'tik), composed of two germ layers.

Diploid (dip'loid), having the base number or double number of chromosomes,
as in somatic cells.

Dissimilation (di sim i la'shun). (See Catabolism.)

Diurnal (diur'nal), active by day.

Diverticulum (di ver tik'u liim), a blind tube branching out of a larger one.

Duodenum (du 6 de'niim), the part of the small intestine between the stomach
and the jejunum.

Ecdysis (ek'disis). (See Molt.)

Ecology (ekol'oji), the science of the relation of an organism to its environ-
ment.

Ectoderm (ek'todurm), the outer cell layer of the wall of a gastrula and its
later derivatives.

Ectoplasm (ek'to plaz'm), substance of the outer layer of cytoplasm or
ectosarc of a protozoan animal.

Ectosarc (ek'tS sark), the superiicial layer of cytoplasm of a single-celled
animal.

Egestion (e jes'chun), the casting out by the body of indigestible food ma-
terial.

Electrolyte (5 lek'tro lit), a substance whose molecules dissociate into ions.

Electrotropism (e lek'trot'ro piz'm), response of an organism to electric cur-
rents.

Embryology (em bry ol'o ji), the science of the origin and development of
the individual.

Endocrine system (en'd5krin), a sy.stem including those ductless glands which
secrete hormones.

Endoderm (en'do durm), the inner cell layer of the wall of the gastrula and its
later derivatives.

Endomixis (en do mik' sis), nuclear reorganization within a protozoan which
does not involve conjugation.

Endcplasm (en'do plaz'm). (See Endosarc.)

Endopodite (en dop'6 dit), the internal or principal branch of a biramous ap-
pendage of Crustacea.

Endosarc (en'do sark), the area of c^-toplasm within a cell which is surrounded
by ectoplasm; substance of this is endoplasm.

Endoskeleton (en do skel'e tiin), the bony, cartilaginous, or other internal
frame work of an animal.

Endothelium (en do the'liiim), the mesodermie lining laj'er of such closed
spaces as blood vessels and lymph spaces.

Enteric (en'terik), adjective form of enteron.

Enterocoele (en'ter 6 sol), a portion of the coelomic cavity that arises by out-
growth from the enteric cavity.

Enteron (en'ter on), a digestive cavity or tube.

910 TEXTBOOK OF ZOOLOGY

Entomology (en'to mol 6 jl), the branch of zoology that deals with the study

of insects.
Entozoic (en'to zo'ik), forms which live within the bodies of other animals.
Enzymes (en'zimz), substances that bring about chemical transformation.
Ephyra (ef'i ra), the free-swimming larval form of the Scyphozoa.
Epiboly (epib'61i), posterior growth of a fold of the blastoderm over the

surface of an embryo in the formation of the enterou during gastrula-

tion.
Eplgenesis (ep i jen'e sis), the conception that the parts of the organism arise

from an undifPerentiated germ cell.
Epithelium (ep'i the li um), a sheet of cells covering an internal or external

surface of the body.
Equatorial (e kwa to'ri al) plate, the platelike arrangement of chromosomes

lying in the plane of the equator of the mitotic spindle during cell

division.
Erepsin (erep'sin), an intestinal enzyme which splits peptones into amino

acids.
Estivation (es ti va'shun), a dormant condition adopted by certain animals

during summer.
Eugenics (ujen'iks), the science of genetics applied to human kind, usually

for the purpose of improvement.
Euglenoid (u gle'noid), resembling a Euglena, as euglenoid movement.
Eustachian (usta'kian) tube, the tube extending from the middle ear to

the pharynx.
Evagination (e vaj'i na'shun), the unequal growth outward of a surface layer,

one of the processes by which differentiation of organs is produced.
Eviscerate (evis'erat), to remove or cast out the internal organs.
Exopodite (eks op'o dit), the external branch of the appendages.
Exoskeleton (ek so skel'e tun), the hardened parts of the external integu-
ment of an animal. ,
Expire (ekspir'), to expel water or air in the process of respiration. j

Factor (fak'ter), one of several interacting elements in a complex process.
Agency influencing the development of an individual, as those carried
in the genes of chromosomes.

Fauna (fo'na), a term referring to animal life.

Feces (fe'sez), the indigestible portion of the food which passes through
the alimentary canal and is discharged by way of the anus.

Fertilization (fur ti li za'shun), the union of a mature ovum and a mature
spermatozoon to produce a zygote.

Fetus (fe'tus), an advanced stage of the embrj-o of a mammal before birth.

Fibrin (fi'brin), the fibrous material in a blood clot; formed when fibrinogen
of the blood is exposed to air.

Filtrahle virus (fil'trab'l vi'rus), an organism too small to be seen with the
microscope and usually within cells of other organisms.

First filial (f first fil'ial), generation, the individuals arising from a particu-
lar mating.

Fission (fish'un), division of an organism into approximately equal parts.

Flagellum (flajel'iim), a whiplike locomotor structure of a cell or single-
celled animal.

Follicle (fol'ik'l), a cellular sac or pocket.

Fragmentation (frag men ta'shun), a process by which individuals of certain
Protozoa and simple Metazoa may divide internally to form several new
individuals.

Freemartin (fre'martin), a modified female member of a pair of cattle twins
which shows certain male features.

GLOSSARY 911

Gametes (gam'ets). (See Germ cells.)

Gametogenesis (gam e t6 jen'e sis), the series of cell divisions in the develop-
ment of germ cells.

Ganglion (gang'lion), a group of nerve cell bodies outside the central nervous
system.

Ganoid scale (gan'oid), rhombic in shape, composed of an inner layer of bone
and outer layer of enamel.

Gastrula (gas'trdo la), the two-layered stage in the development of an embryo.

Genes (jenz), the units of material which function in the transmission of
characters in heredity.

Genetics (jenet'iks), the science that treats of variation, resemblances, and
their inheritance from parent to offspring.

Genotype (jen'otip), the genetic constitution of genetically identical or-
ganisms.

Genus (je'nus), pi. Genera (jen'era), a division of the classification, a sub-
division of a family, and is divided into species.

Geotropism (je ot'ro piz'm), response of an organism to gravity.

Germ cells (jerm) (gametes), cells specialized for reproduction.

Germ layer (jerm), one of the primary cell layers in an embryo.

Germ plasm (jerm plazm), the hereditary material of an organism, the
chromatin.

Gills (gils), (pharyngeal clefts), a series of paired slits in the wall of the
pharynx and liody.

Gonads (gon'ads), reproductive organs.

Gonophore (gon'ofor), a reproductive individual which bears gonads, as in
Hydroids.

GlocMdiiim (gl6 kid'i iim'i, the larva of a fresh-water clam.

Glomerulus (gl6 mer'ulus), a body of capillaries enclosed at the end of each
kidney tubule of the mesonephric and metanephric types of kidneys.

Glycerol (glis'erol), one of the alcohols which enters into the composition
of fats; glycerin.

Glycogen (gli'ko jen), a form of carbohydrate food material as formed and
stored by the liver.

Habitat (hab'itat), the place or area in which an animal or species lives.

Haploid (hap'loid), the reduced or half number of chromosomes of the mature
germ cells.

Heliotroplsm (he li ot'ro piz'm), a response of an organism to light.

Hemoglobin (he mo glo'bin), a protein pigment substance of the blood which is
capable of absorbing oxygen and is red when coml)ined with it.

Hemolysis (he mol'i sis), disintegration of red blood corpuscles.

Hepatic (hepat'ik), pertaining to the liver.

Herbivorous (hur biv'6 riis), herb or plant-eating animals.

Hermaphoditlc (hur maf ro dit'ik) (monoecious), having both male and fe-
male germ cells produced in one individual.

Heterozygote (het er 6 zi'got), an organism which is carrying sets of unlike
characters in its genetical constitution.

Hibernation (hi ber na'shiin), the cessation of activity or dormancy of an an-
imal during the winter season.

Histogenesis (his to jen'e sis), the development and differentiation of tissue
cells.

Histology (histol'oji), the science that treats of the microscopic structure
of the various parts of the animal body.

Holoblastic (hoi u blas'tik), having the tj-pe of egg structure in which cleavage
divides the entire egg.

Holozoic (holozo'ik), the animal nutrition, the ingestion and digestion of
organic material.

Homoiothermal (ho moi'6 thur mal), having a temperature regulation.

912 TEXTBOOK OF ZOOLOGY

Homolecithal (ho mo les'i thai) (isolecithal), eggs having a uniform distribu-
tion of the yolk.

Homologous (ho mol'ogus), similar in structure and origin, but different in
function.

Homonomous (ho mon'o mus), slight or no differentiation of body segments.

Homozygote (h5m6 zi'got), a zygote or resulting organism in which the cor-
responding genes are alike.

Hormone (hor'mon), the essential substance of an internal secretion which
serves to help in metabolism regulation. Produced by endocrine glands
and carried by the blood.

Hyaline (hi'alin), semitransparent or glassy.

Hybrid (hi'brid), a cross, or offspring of parents differing in genetical con-
stitution.

Hydrolysis (hi drol'i sis), chemical rearrangement of a substance by combining
with water.

Hydrostatic (hi dr6 stat'ik), a type of organ which regulates the specific
gravity of an aquatic animal.

Hypertonic (hi per ton'ik), possessing greater osmotic pressure than some re-
lated substance.

Hypostome (hi'postom), a conical projection around and below the mouth in
coelenterates.

Hsrpothesis (hipoth'e sis), an idea as it first develops as the result of pre-
liminary observation and experiment.

Ileum (il'eum), the posterior and longest part of the small intestine.

Ilium (il'ium), dorsal bone of pelvic girdle of terrestrial vertebrates.

Immunity (imii'mti), freedom of susceptibility to disease.

Ingestion (in jes'chun), the taking in of food material by an organism.

Insectivorous (in sek tiv'6 rus), insect-eating animals.

Inspire (inspir'), the drawing in of water or air in the respiration.

Instar (in'star), the period between molts in insect development.

Insulin (in'sulin), a hormone produced by the pancreas and essential to the

proper metabolism of carbohydrates.
Integration (in te gra'shiin), development and correlation to give unity in an

organism.
Integument (in teg'u ment), the outer covering of the body.
Intracellular (in tra sel'u ler), within the cell.
Intracellular differentiation (in tra sel'u ler dif er en shi a'shun), the presence

of a variety of cells within one body.
Invagination (in vaj i na'shun), the unfolding process by which the primary

endoderm is withdrawn into the blastular cavity and becomes enclosed

by the primary ectoderm.
Irritability (ir i tabil'i ti), the capacity of protoplasm for _ responding to

changes in environmental conditions or to external stimuli.
Isolecithal (i so les'i thai). (See Homolecithal.)

Isotonic (i so ton'ik), possessing the same osmotic pressure in related sub-
stances.

Jejunum (jejob'num), the middle division of the small intestine, between the
duodenum and the ileum.

Karyokinesis (kar i 6 ki ne'sis), mitotic cell division.
Karyolymph (kar'i 6 limf ), the more fluid material of the nucleus.
Karyoplasm (kar'i 6 plaz'm), the protoplasm which constitutes the nucleus.
Karyosome (kar'iosom), a "net knot" or a part of the chromatin which

forms a distinct body in the nucleus.
Katabolism (ka tab'6 liz'm). (See Catabolism.)

GLOSSARY 913

Keratin (ker'atin), a nitrogenous substance forming the chemical foundation

of hair, horn, feathers, nails, claws, etc.
Kinetic energy (kinet'ik), energy inherent in motion of a body.

Labium (la'bium), posterior boundary or lower lip of an insect's mouth.
Labrum (la'brum), the exoskeletal anterior boundary or upper lip of the in-
sect's mouth.
Lacrimal (lak'rimal), pertaining to tears.
Lacteal (lak'teal), pertains to milk; refers to lymphatics of the intestinal

region because of their light color following absorption of fat.
Lacuna (laku'na), a cavity or space, particularly that of bone, which contains

the bone cells.
Lamella (lamel'a), a thin layer or plate.

Larva (lar'va), the young stage of an animal, which changes form during life.
Larynx (lar'inks), the expanded upper end of the windpipe or trachea; voice

box.
I Lethal (le'thal), capable of producing death.
Leucocyte (lii'kSsit), a white blood corpuscle.
Ligament (lig'ament), a band of white fibrous connective tissue connecting

structures other than muscles; particularly joining bones at the joints.
Limpet (lim'pet), a small tj-pe of gastropod (Mollusca) with a simple uncoiled

shell.
Linin (li'nin), the delicate threadlike structure which supports the chromatin

granules in the nucleus.
Linkage (lingk'ij), the constant association of particular genes in certain

chromosomes.
Lipase (li'pas), a fat-splitting enzj-me.
Lipin (li'pin), fatty substance.
Lipoid (lip'oid), fatlike substance.
Lophophore (lo'fofor), a disc which surrounds the mouth and bears the

tentacles of the Bryozoa.
Lumbar (lum'ber), pertaining to the region usually known as the small of the

back.
Lumen (lu'men), internal cavity of a tubular duct, gland, vessel, etc.
Luminescence (lu mi nes'ens), the emission of light from the body.
Lymph (limf), the blood plasma and white corpuscles in the lymph spaces

about the tissues.
Lymphatic (limfat'ik), a vessel which carries lymph. In general, pertaining

to lymph.

Macronucleus (mak ro nu'kle us), the large nucleus of certain protozoans sup-
posed to control vegetative functions.

Madreporite (mad're p6 rlt), the strainerlike external aperture of the water-
vascular system of echinoderms.

MalpigMan (mal pig'i an) body, a structure in the cortex of the kidney, com-
posed of a glomerulus and Bowman's capsule which serves to take urine
from the blood.

Mantle (man't'l), a fold of the body wall which partially envelopes the body;
present in most mollusks and here secretes a shell.

Marsupial (mar su'pi al), having a pouch for carrying the young.

Marsupium (mar su'pi um), an external pouch used in carrying the young, as
in the kangaroo or opossum.

Matrix (ma'triks), the mother substance, such as that which encloses anything;
the intercellular material of cartilage or other sustentative tissue.

Maturation (mat ti ra'shiin), the series of changes occurring in the development
of germ cells before fertilization, including a reduction in the number
of chromosomes in the cells.

914 TEXTBOOK OF ZOOLOGY

Maxilla (maksira), the major bone of the upper jaw of vertebrates or the
accessory mouth part just back of the mandibles in many invertebrates.

Medulla (me dul'a), posterior portion of the vertebrate brain; also the median
area of many organs.

Medullary (med'uleri), pertaining to the medulla.

Medullated (med'u lat ed), term used in reference to a nerve fiber which pos-
sesses a fatty or myelin sheath.

Medusa (medu'sa), a free-swimming individual coelenterate, such as a jelly-
fish.

Meiosis (mio'sis), the reduction division in maturation of germ cells.

Menopause (men'opoz), natural cessation of menstruation in women between,
forty-five and fifty years of age.

Meridional (me rid'i 6 nal), a condition in which planes extend from pole to
pole of a spherical body.

Meroblastic (mer 6 blas'tik), having the type of egg structure in which cleavage
is only partial, owing to the accumulation of yolk in the egg.

Mesencephalon (mesen sef'alon), the third region of the vertebrate brain, com-
monly called midbrain.

Mesenchyme (mes'engkim) (Parenchj-ma), undifferentiated mesoderm com-
posed of large cells.

Mesoderm (mes'odurm), the middle germ layer and its later derivatives.

Mesoglea (mesogle'a), a jellylike substance found in Coelenterata between
the ectoderm and endoderm.

Mesonephros (mes 6 nef'ros), the vertebrate kidney of forms from lamprey
to amphibians inclusive.

MesorcMum (me sor'kium), the mesentery or membrane supporting a testis.

Mesothelium (mes 6 the'lium), the mesodermic, membranous lining of the peri-
toneal cavity.

Mesovarium (mes 6 va'rium), the mesentery in which the ovary is suspended.

Metabolism (me tab'6 liz'm), the building up of living protoplasm and its con-
current oxidation.

Metagenesis (met a jen'e sis), an alternation of sexual and asexual generation
in the life cycle of an organism.

Metameres (met'amers), one of a series of similar parts that follow one another
in a vertebrate or articulate animal.

Metamerism (me tam'er iz'm), serial symmetry or succession of segments.

Metamorphosis (met a mor'fo sis), the transformation of one developmental
stage into another without intermediate steps.

Metaphase (met'afaz), the phase of mitosis involving the longitudinal split-
ting of the chromosomes on the equatorial plate.

Metazoa (met'azoa), animals whose bodies consist of few or many cells func-
tioning as a unit.

Micronucleus (mi kro nu'kle us), the small nucleus of certain protozoans sup-
posed to control reproduction.

Micropyle (mi'krdpil), the small opening in the egg where sperm enter in
certain forms of animals.

Milt (milt), the light-colored spermatic fluid of male fish.

Miracidium (ml ra sid'i iim), the early larval stages in the flukes.

Mitochondria (mit 6 kon'dri a), small structures in the cytoplasm of animal

cells; their significance is not entirely understood.
Mitosis (mito'sis), indirect cell division, involving the formation and splitting
of chromosomes and their equal distribution to daughter cells.

Molt (molt), a complete or gradual shedding of the outer covering.
Monodelphia (mon 6 del'fi a), having a placenta.
Monoecious (mo ne'shiis). (See Hermaphroditic.)

Monohybrid (mon 6 hi'brid), an offspring of parents which differ by only one
character.

GLOSSARY 915

Morphology (morfol'o ji), the science that treats of the form and structure

of the lindies of animals.
Morula (mor'ula), a type of blastula characterized by the absence of a seg-
mentation cavity.
Mucosa (muko'sa), a cellular membrane lining such cavities as those of the

digestive tract.
Mucus (mu'kus), a viscous secretion which contains mucin (mu'sin). Mucous

IS the adjective form.
Mutation (mtita'shun), a heritable change in an organism due to changes in

one or more genes of germ cells.
Mutualism (mii'tualiz'm), animals of different species associating together for

the mutual advantage of each.
Myelencephalon (mi e len sef'alon), the fifth or most posterior division of the

vertebrate brain; the medulla oblongata of the adult.
Myelin (mi'elin), fatty substance surrounding the axone in medullated nerve.
Myoneme (mi'6nem), contractile fiber or strand in the cytoplasm of certain

protozoans.
Myotomes (mi'6 toms), segmental divisions of the muscles.
Myxnoidea (mik si noi' dea), subclass of Cyclostomes including hags. The

name means slime and form.

Nares (na'rez), the openings into the nasal chambers in vertebrate animals.

Nauplius (no'pliiis), a larval stage of certain Crustacea.

Nekton (nek'ton), the pelagic aquatic animals which are independent of the

effect of wind and waves.
Nematocysts (nem'a to sists), stinging bodies found in the tentacles of certain

coelenterates.
Nematode (nem'a tod), a roundworm belonging to class Nematoda of phylum

Nemathelminthes.
Neoteny (neot'eni), the indefinite persistence of the immature condition of

an animal.
Nephridium (nefrid'iiim), a form of excretory organ, as found in the earth-
worm.
Neplirostome (nef'ro stom), the funnel-shaped aperture at the medial end of a

nephridium.
Neural (nii'ral), pertaining to the nervous system or to a nerve.
Neurilemma (nurilem'a), the membranous outer coat of a nerve fiber.
Neuroid transmission (nu'roid), primitive transmission of impulses from cell

to cell.
Neuron (nu'r5n), a nerve cell together with its processes.
Nidamental gland (nid'ament al), one of the reproductive organs of the female

squid.
Notochord (no'tokord), a flexible rod extending anterior to posterior in the

longitudinal axis of the body dorsal to the digestive tube and ventral

to the nerve cord in chordates.
Nocturnal (nok tur'nal), reference to night. Contrasted to diurnal which per-
tains to daytime.
Nodes of Ranvier (ranvya'), constrictions in medullated nerve where the

myelin sheath is interrupted.
Nomenclature (no'men kla tur), a system of naming objects or ideas.
Nondisjunction (non dis jungk'shiin), the failure of homologous chromosomes

to separate after synapsis and both go to one daughter cell with none

to the other.
Nucleolus (nu kle'6 liis) (Plasmosome), a body within the nucleus containing

material that is not chromatin.

916 TEXTBOOK OF ZOOLOGY

Nucleus (nti'kleus), a typically spherical body within the cell that contains the

chromatin.
Nymph (nimf), the larval stage of an insect which undergoes incomplete

metamorphosis; also the larval stage of a few vertebrates.

Ocellus (osel'us), a simple type of eye, as in some insects.

Ommatidium (6m a tid'i um), one of the numerous rodlike units of the com-
pound eye.
Ontogeny (on toj'eni), the entire development and life history of an individual

organism.
Oocyte (6'6 sit), the female germ cell before maturation is completed.
Oogenesis (6 6 jen'e sis), the maturation of the female germ cell.
Oogonium (6 6 go'ni lim), the female germ cell during the multiplication and

growth stages of maturation.
Operculum (6 pur'kii lum), a fold of skin, bone, and scales, which covers the

gills of fishes and certain Amphibia; also the bony structure closing the

aperture of certain snail shells.
Organ (or'gan), an arrangement of two or more tissues as a part of the body

which performs some specific function or functions.
Organism (or'gan iz'm), any independent living being.
Orthogenesis (or tho jen'e sis), the theory which holds that animals tend to

develop along lines leading constantly in the same direction because

they are determined by internal factors.
Osmosis (osmo'sis), diffusion of substances dissolved in fluid, through a semi-
permeable membrane.
Ossicle (os'ik'l), a small bony structure.
Ostiuna (os'tiiim), a mouthlike opening or entrance.
Otocyst (o'tosist), the primitive organ of hearing.
Ova (6'va), mature female germ cells. Sing., ovum (6'vum).
Ovary (6'vari), the female gonad.
Oviduct (6'vidukt), the duct for the passage of ova from the ovary to the

exterior of the animal.
Oviparous (ovip'arus), pertaining to those animals which lay eggs that hatch

after exclusion from the body.
Ovipositor (6 vi poz'i ter), an organ of female insects and others which serves

in helping to deposit the egg.
Ovoviviparous (6 v6 vi vip'a rus), a condition of retention of the egg in the

mother 's body where it is nourished by the yolk of the egg.
Ovulation (6 vu la'shun), the process of discharging mature eggs from the

ovary.
Oxidation (6k si da'shun), a chemical combination of oxygen with another

element.

Paleozoology (pale 6 z6 6r6 ji), the science that treat." of the animals of the
past as represented by fossil remains.

Parasite (par'asit), an organism that lives on or within and at the expense of
another organism.

Parenchyma (pa reng'ki ma) . (See Mesenchj-me.)

Parietal (pari'etal), pertaining to the walls of the coelom.

Parthenogenesis (par the no jen'e sis), the development of an egg without
fertilization.

Pathology (path6r6ji), the study of abnormal structures and abnormal func-
tioning of life processes.

Pedal (ped'al), pertaining to the feet.

Pedicellaria (ped i sela'ri a), pincherlike structures found over the surfaces of
sea urchins and starfishes.

Peduncle (pedung'kl), the stemlike attachment of certain shells and barnacles
to other objects.

GLOSSARY 917

Pelagic (pelaj'ik), floating near the surface of water.
Pericardial (per I kar'di al), situated around the heart.
Periosteum (per i os'te um), the membranous covering of bone.
Peripheral nervous system (pe rif'er al), that part of the nervous system ex-
clusive of the brain and spinal cord.
Peristaltic (per i stal'tik), forcing the food along the intestine by rhythmical

contractions of the intestinal wall.
Peritoneum (peri tone'um), the membrane that lines the coelom of vertebrates.
Petromyzontia (pet ro mi zon'ti a), subclass of Cyclostomes including the lamp-
reys. Tlie name means stone and suck.
Phagocyte (fag'6 sit), a white corpuscle which engulfs and destroys bacteria

and other foreign material.
Pharynx (far'ingks), the region between the mouth and the esophagus.
Pharyngeal (fa rin'je al), pertaining to the pharynx.
Phenotype (fe'notip), a tj-pe of organism possessing a complex of characters

in its external features.
Phenotypic (fe n6 tip'ik), pertaining to phenot\T3e.

Photosynthesis (fo to sin'the sis), the process by which green plants manufac-
ture starch from raw materials.
Phototropism (fo tot'rfi piz'm), response of an organism to light.
Phylogeny (fi loj'eni), the study of the origin and relationships of the different

groups and races of organisms.
Physiology (fiziol'o ji), the study of the function of the parts of an organism

as well as its living processes as a whole.
Pia mater (pi'ama'ter), the membrane which is the immediate covering of

the brain and spinal cord.
Piliditmi (pT lid'ium), helmet-shaped larva of certain forms.
Pineal (pin'e al) body, a dorsal projection from the diencephalon and thought

to be the vestige of a third or median eye in vertebrates.
Pituitary (pitu'iteri) body, a glandular structure attached to the neutral
side of the diencephalon of the vertebrate brain. It is an endocrine
organ.
Placenta (plasen'ta), the vascular membrane which connects the embryo with

the parent.
Placula (plak'ula), a t\-pe of blastula in which the animal and vegetative

halves are somewhat compressed toward each other.
Plankton (plangk'ton), the small pelagic organisms which are at the mercy of

the waves.
Plasma (plaz'ma), the fluid portion of the blood.

Plasmagel (plaz'ma jel), the viscous or semisolid portion of protoplasm.
Plasmasol (plaz'ma sol), the more fluid phase of protoplasm.
Plasmosome (plaz'mo som). (See Nucleolus.)

Pleural (pldbr'al), pertaining to the cavity which contains the lungs.
Plexus (plek'sus), a network.
Polar (po'ler) body, a small nonfunctional cell or oocyte produced during the

maturation divisions of the female germ cell.
Polarity (p61ar'iti), referring to a condition in which points or poles of con-
centration or dominance are established in a body.
Polocyte (po'16 sit), a technical name for a polar body.

Polyandry (pol'I an dri), the practice of one female mating with several males.
Polygamy (pSlig'ami), having more than one mate at the same time.
Polygyny (p6 1ij'ini), the practice of one male mating with several females.
Polymorphism (poli mor'fiz'm), the occurrence of two or more forms of in-
dividuals within a species.
Polyp (pol'ip), the attached phase of the life history of a coelenterate animal.
Precocial (pre ko'shal), tjT)e of bird which leaves the nest and has downy cover-
ing at time of hatching.

918 TEXTBOOK OF ZOOLOGY

Predaceous (pre da'shus) animal, one which preys on others.

Predatism (pred'a tiz'm), the practice of one animal preying on another.

Primordial (pri mor'di al), the first or primitive form.

Proboscis (probos'is), an extension of the head or mouth parts. May be nose,

as in elephant; mouth parts, as of moth; or pharynx, as of planaria.
Proctodeum (prok to de'um), the pocket in the ectoderm ventral to the posterior

part of the enteron of the embryo; primordium of the anus.
Proglottid (pro glot'id), one of the sections or individuals of the chain making

up the bod.y of a cestode, such as the tapeworm.
Pronepliros (pr6 nef'ros), the^ first kidney structure to form in the developing

vertebrate.
Pronucleus (pro nu'kleus), one of the two nuclei within a fertilized egg before

cleavage occurs.
Propagation (prop a ga'shun), the production of new individuals.
Prophase (pro'faz), the preparatory stages of mitosis during which the forma-
tion occurs of spindle, spireme, and chromosomes.
Proprioceptor (pro pri 6 sep'ter), the receptor or end organ of the nervous

system located within a certain tissue receiving stimulations in reference

to bodily position or orientation.
Prostate (pros'tat) gland, one of the male reproductive organs producing part

of the semen.
Prostomium (pro sto'mi iim), portion of the anterior segment of annelids which

overhangs the mouth.
Protein (pro'tein), one of the organic compounds found in protoplasm. It

contains the elements carbon, oxygen, hydrogen, and nitrogen.
Protoplasm (pro'to plaz'm), the living matter of which all organisms are com-
posed.
Protopodite (pro top'o dit), the proximal section of the crustacean appendage.

It includes coxopodite and basipodite.
Prototroch (pro'to trok), the band of cilia extending around the equatorial

region of trochophore larva.
Protrusible (pro trob'sib'l), the ability to be put out or extended from the

body.
Proventriculus (pro ven trik'ulus), the anterior, secretory portion of the

stomach in certain animals, as the bird.
Pseudopodia (su do po'dia), protoplasmic processes (false feet) formed by

certain protozoans and used for locomotion.
Ptyalin (ti'alin), the starch-digesting enzyme of saliva; a diastase.
Pupa (piVpa), the encased, inactive stage between the larva and adult condition

in many insects and other animals.
Pyloric (pilor'ik), pertaining to the pylorus.
Pylorus (pilo'riis), the junction of the posterior portion of the stomach with

the small intestine.

Radial symmetry (ra'dial sim'etri), applied to a body that can be equally

divided by several radial planes.
Radiant energy (ra'diant enerji), inherent power or energy transmitted

through space, as that from the sun, radium, or x-ray.
Radula (rad'ula), the sheetlike rasping structure of the mouth of gastropods;

used in mastication of food.
Recessive (reses'iv), in reference to a gene which is carried in the cell without

expressing its character unless there is absence of its dominant mate.
Recapitulation (re kapit u la'shiin), repetition in development of an individual

organism of its phylogenetic history.
Redla (re'dia), second phase of the life history of the fluke.
Reflex (re'fleks) action, automatic reaction to a stimulus from a receptor

neuron and passed on to an adjustor neuron; performs an involuntary,

appropriate act.

GLOSSARY 919

Regeneration (re jen era'shuii), the replacement of mutilated parts or au
entire animal from a portion of one.

Renal (re'nal), pertaining to the kidney.

Rennin (ren'in), an enzyme constituent of gastric juice of mammals and
capable of coagulating the protein portion of milk.

Reproduction (re pr6 duk'shun), the production by an organism of others of its
kind.

Respiration (res pi ra'shun), the exchange within an organism of oxygen enter-
ing the protoplasm and carbon dioxide leaving it.

Response (respons'), the reaction of an organism to a stimulus.

Rete (re'te), a limited meshlike arrangement or network.

Reticulum (re tik'ti liim), a fibrous or tubular network.

Retractile (re trak'til), that which can be withdrawn.

Retrogression (ret ro gresh'vin), going behind or moving backward.

Rhabdites (rab'dits), special structures found interspersed among the epidermal
cells of flatworms.

Rheotropism (re ot'ropiz'm), response of an organism to mechanical currents.

Roe (ro), ovary and eggs of fish.

Rudiment (roo'di ment), partially developed or embryonic structure; usually
without function.

Rugose (rob'gds), possessing many ridges and folds.

Ruminants (roo'mi nants), animals which chew the cud.

Saprophyte (sap'rofit), an organism which absorbs nonliving organic matter

in solution directl}' through the surface of the bod}-.
Sarcolemma (stir ko lem'a), the delicate membrane immediately enclosing the

striated voluntary muscle cell.
Sarcoplasm (sar'ko plaz'm), the cytoplasm of muscle cells exclusive of the

sarcostj'les or fibrils.
Sarcostyles (sar'ko stils), cytoplasm fibrils in the structure of cytoplasm of

voluntary muscle cells.
Schizogony (ski zog'o n50> asexual reproduction.

Sclerotic (skle rot'ik), the dense fibrous outer coat of the vertebrate eye.
Scolex (sko'leks), knoblike "head" on anterior proglottid of cestode.
Scute (skut), a plate on the ventral side of the body of the snake.
Sebaceous glands (seba'shus), small subcutaneous glands, usually connected

with hair follicles.
Sedentary (sed'en ter i), unattached forms which remain in one place.
Semen (se'men), fluid which carries the spermatozoa in the males of most

animals.
Seminal (sem'inal), pertains to spermatozoa.
Semipermeable membrane (semipur'meabl), one which permits the passage

of solvents through it but not solutes, unless they dissolve in the

membrane.
Senescence (senes'ens), period of old age and its effects.
Septum (sep'tiim), a wall dividing two cavities.
Serial homology (serial homol'ftji), presence of structures of similar origin

and form in different segments of the same animal.
Serosa (sero'sa), membrane covering peritoneal surface of internal organs;

one which secretes a watery fluid.
Sertoli cells (ser to'le sels), modified, supporting or nurse cells for forming

spermatozoa in the testes.
Serum (se'rum), the fluid part of the blood which remains after clotting.
Sessile (ses'il), attached directly and incapable of locomotion.
Seta (se'ta), hairlike spine or bristle, found in animals.
Sinus (si'nus), a cavity in a bone or other part, or a dilated vessel or canal.

920 TEXTBOOK OF ZOOLOGY

Siphon (si'fon), a canal or passageway, as the waterways in clams or tunicates.
Somatic cells (somat'ik), the cells of the body exclusive of the germ cells;

body cells.
Somites (sS'mits), segments of the body of a segmented animal.
Spermatogonium (spur ma to go'nium), a male germ cell during the period

of multiplication and growth in maturation process.
Spermatocyte (spur'ma to sit), the male germ cell before its maturation is com-
pleted.
Spermatogenesis (spur ma to jen'e sis), the maturation of the male germ cells.
Spermatozoa (spur ma t6 zo'a), mature male germ cells.
Sphincter (sfingk'ter), a muscular band surrounding a tube or aperture which,

by its contraction, closes the lumen.
Spicule (spik'ul), one of numerous needlelike, solid structures found supporting

the tissues in the body wall of sponges.
Spiracle (spir'ak'l), openings of air tubes in insects, or modified opening of

first gill slit of certain fish.
Spireme (spi'rem) thread, the coiled bead-like string of chromatin material

that appears during the prophase of mitosis.
Splanchnic (splangk'nik), has reference to the visceral organs.
Spongtn (spun'jin), the skeletal material of a sponge.
Sporulation (spor u la'shun), production of spores by division of a protozoan

while encysted.
Statoblast (stat'6 blast), an encased, asexual winter bud of a bryozoan.
Statocysts (stat'6 sists), sense structures assisting in maintaining equilibrium

in certain forms.
Steapsin (steap'sin), one of the pancreatic enzymes which is capable of chang-
ing fats to fatty acids and glycerin.
Stereoblastula (ster e 6 blas'tula), a blastula in which all the cells are in close

contact and no blastocoele is formed.
Stigma (stig'ma), a sensitive pigment spot of Protozoa or the opening of a

spiracle of insects.
Stomatodeum (sto ma to de'um), the opening of the developing alimentary tract

in an embryo.
Stratum (stra'tum), a layer of a series.

Striated (stri'at ed), type of muscle with more dense areas across the fibers.
Strohila (strobi'la), a series of individuals produced by linear budding, as

certain Scj-phozoa and tapeworms.
Succession (suk sesh'un), the successive occupation of a given area by several

species, either hourly, daily, or seasonally.
Sustentative tissue (sus'ten ta tiv), binding together or supporting the various

parts of the body.
Suture (su'tur), to sew together; a line of junction.
Symbiosis (simbi 6'sis), the living together of two organisms.
Synapsis (sinap'sis), the pairing of the chromosomes in the germ cells at one

stage of maturation.
Syncjrtium (sin sish'i um), a mass or layer of protoplasm with numerous nuclei

but without distinct cell boundaries.
Syngamy (sin'gami), union of mature gametes to form a zygote.
System (sis'tem), an aggregation of organs to perform some general function

of life.

Taxis (tak'sis), a tropismal response involving movement of an organism as a

whole.
Taxonomy (taks on'o mi) (systematic zoology), the classification or orderly

arrangement of organisms according to their natural surroundings.
Tegimientary (tegumen'tari), referring to the skin.

GLOSSARY 921

Telencephaloii (tel en sef a Ion), the anterior division of the vertebrate brain.

Telolecithal (tel 6 les'i thai), type of egg with abundant yolk unequally dis-
tributed.

Telophase (tel'ofaz), the final stage in mitotic divisions.

Tentacle (ten'tak'l), flexible, armlike extension of the body of many non-
chordates.

Terrestrial (ter res'tri al), a land form; living on or in the ground.

Testis (tes'tis), male gonad.

Thermotropism (ther mot'rfi pTz'm), response of an organism to temperature.

Thlgmotropism (thigm6t'r6 piz'm), response of an organism to contact.

Thoracic (tho ras'ik), of or pertaining to the thorax or chest.

Thorax (tho'raks\ the middle region of the body.

Threshold (thresh'old), the minimum strength of stimulus necessary to get a
response.

Thrombin (throm'bm), the substance of the blood which plays an important
part in clotting.

Thyroxin (thi rok'sen or -sin), the hormone which is produced by the thyroid
body.

Tissue (tish'u), an organization of similar cells into a layer or group for the
performance of a specific function.

Toxin (tok'sin), any poisonous substance.

Trachea (tra'ke a),'the windpipe or a tube for conveying air to the lungs; air
tubes in insects.

Trichocyst (trik'6 sist), saclike structure in the ectosarc of Paramecium.

Triploblastic (trip 16 blas'tik), composed of three germ laj-ers.

Trivium (triv'ium), the three anterior ambulacra of Echinodermata, collec-
tively.

Trochophore (trok'ofor), a semispherical type of larva with cilia; found among
flatworms, annelids, mollusks, etc.

Tropism (tro'piz'm), the movements of an organism in response to a stimulus.

Trypanosome (trip'a n6 som), genus of parasitic Protozoa (Mastigophora) in-
cluding the causal agent of African sleeping sickness.

Trypsin (trip'sin), a pancreatic enzyme which converts proteins to amino
acids.

Tsetse (tset'se) fly, a species of fly which serves to transmit the causal agent
of African sleeping sickness.

Tundra (tobn'dra), level plains region of the arctic region.

Tympanum (tim'panum), cavity of the middle ear or more generally any organ
serving to receive sound waves.

Umhilical (um bil'ikal) cord, the cordlike connection between the fetus and
the placenta.

Umbilicus (um bil'i kus), the navel or the point of attachment of the umbilical
cord to the abdomen.

Uncinate (iin'sinat), in the shape of a hook.

Ungulate (ung'gulat\ hoofed.

Unguiculate (unggwik'ulat), having claws.

Urea (ure'a), a nitrogenous compound which is produced as a protein by-
product in metabolism.

Ureter (ure'ter), the duct which conveys urine from the metanephric kidney
to the cloaca or bladder.

Urethra (ure'thra), the duct which leads from the urinary bladder to the
exterior of the bodv.

Uropods (u'ropodz), the sixth pair of abdominal appendages of a crustacean.

Vacuoles (vak'uolz), small cavities in a cell filled with water, gasses, or oils.
Vagina (vaji'na), the cavity between the uterus and the external genital
aperture of the female in many animals.

922 TEXTBOOK OF ZOOLOGY

Vascular system (vas'kuler), the circulatory system.

Vascular tissue (vas'kuler), fluid tissue consisting of cells known as corpuscles

in a fluid medium, plasma.
Vein (van), the larger blood vessels leading to the heart.

Ventral (ven'tral), side away from the back; literally belly; opposite to dorsal.
Villus (vil'us), a fingerlike, vascular process of the internal lining of the

small intestine.
Vitamins (vi'tamins), substances which occur in small amounts in numerous

foods and are essential regulatory substances for the animal body.
Vitelline (vitel'lin), the outer membrane of an egg.
Vitreous (vit'reus), glassy in appearance.
Viviparous (vi vip'a rus), the retention and development of the egg in the

mother's body and nourishment of the embryo from the blood of the

mother.
Volant (vo'lant), able to fly.

Zoogeography (z6 6 je og'rafi), the study of the geographical distribution of

animals.
Zoology (zoo'loji) (animal biologj^), the study of the science which treats

of animals.
Zygote (zi'got), a fertilized egg, or embryo, after fertilization.
Zymogen (zi'm6jen), a pre-enzyme; a substance which is produced in a gland

cell and becomes an enzyme when it is discharged and activated by some

other substance, perhaps another enzyme.

INDEX

Abdomen, 348, 639
Abducens nerve, 527
Abductor muscle, 524
Abomasum, 633
Abyssal zone, 713, 770
Acanthocephala, class, 175, 178, 179
Acarina, order, 266, 295. 297
Accipiter, 590
cooper i, 589
Accommodation, 402
Acetabulum, 495, 522, 611
Achiridae, family, 452
Aciculum, 19 6
Acidian larva, 367
Acipenser fulvescens, 447
Acipenseridae, family, 446
Acnidosporidia, 73
Acoela, 160
Acontia, 141
Acris gryllus, 485
Acromegaly, 6 73
Actinaria, order, 140
Actinophrys, 67, 69
Acusticolateral areas, 435
Adambulacral plates, 229
Adaptability and regeneration, 695
Adaptation, 727
Addison's disease, 671
Adductor mandibulare, 468

muscles, 250, 254
Adhesive papillae, 367
Adipose fin, 442, 457
Adrenal glands, 671
Adrenalin, 671
Adrenin, 671
Aedes, 80

Aegeria exitiosa, 328
Aegeriidae, family, 328
Aeolis, 241

Aeolothrips fasciatus, 320
Agamodistomum, 759
Agassiz, 43
Agassiz' tortois! , 550
Aglypha, 556
Agnathostomata, 412
Agriculture, 34
Albatrosses, 587
Albulidae, family, 448
Albumin gland, 246
of hen's egg, 615
Aloes americana, moose, 630
Alcyonacea, order, 143
Alcyonaria, subclass, 143
Alcyonium, 143
Alfalfa beetle, 324
Alimentary canal, 109
AUantois, 818
Allelomorph, 823, 824
Allelomorphs, multiple, 827
Allergy, 839

Alligator mississippiensis, 561
Alligators, 560
AUocosa parva, 295
Alopecia, 838
Alpacas, 630
Alpine zone, 714
Alteration of generation, 112
Altricial birds, 615
Alveoli, 566
of lungs, 514

Alytes obstetricans, 480
Amblema costata, 248
Amblyopsidae, family, 451
Ambulacral groove, 218, 224, 227, 228
ossicle, 229, 230
plate, 228
Ambystoma, 484
axolotl larva, 481
texanum, 480
tigrinum, 478, 480
Ambystomoidea, suborder. 484
Ameiuridae, family, 450
Ameiurus, arteries, 461
cranial nerves, 469
internal structure, 458
life history, 470
natalis, 457
skeleton, 466
veins, 462
Ametabolous, 3 08
Amiidae, family, 4 47
Amino acids, 57
Amitosis, 61
Ammocoetes, 420
Ammonite, 259
Amnicola, 245

comalensis, 247
Amnion, 818
Amoeba, 85

assimilation, 88
behavior, 91
digestion, 87
excretion, 88
histolytica, 744
locomotion, 91, 92
metabolism, 86
proteus, 67
reproduction, 89
Amoebic dysentery, 78
Amoebina, 67
Amoebocytes, 233
Amoeboid acivity, 152
Amphiaster, 63

Amphibia, breeding habits. 479
class, 411. 472
classification, 482
coloration, 474
economic importance, 486
enemies of, 477
families of, 484
hibernation, 480
means of defense, 478
regeneration, 477
secondary sexual characters, 480
Amphibian voice, 478
Amphibians, tailless, 485
Amphiblastula, 128, 808
Amphicoela, suborder, 485
Amphicoelous centrum, 444
Amphids, 175
Amphineura, 256
Amphioxus, 361, 368
circulatory, 371. 372
respiratory system, 373
Amphiplexus, 409
Amphipoda, order, 265
Amphipods. 283
Amphisbaenidae, family, 555
Amphitrite, 195
Amphiuina, 473
Amphiumidae, family. 4 84
Ampulla. 222, 229, 230, 231

923

924

INDEX

Ampullae of ]L,orenzini, 437
Amyda, 550

Amylopsin, 385, 386, 502
Anabolism, 86, 377
Anabrus simplex, 315
Anacanthini, suborder, 451
Analogy, 271
Anaphase, 62, 63
Anapsida, subclass, 547
Anasa tristis, 318
Anastomosis, 107
Anatomy, gross, 20
Ancestry of Arthropoda, 865
Anchoviella mitchilli, 448
Anchovies, 448
Ancylis comptana, 326
Ancylostoma, hookworm, 745, 751
Andalusian chickens, 830
Androgenic hormone, 676
Androsterone, 676
Anecdotal period, 847
Anglers, 455
Ansuidae, family. 552
Anguillidae, family, 449
Angulosplenial bone, 520
Ani, 591

Animal and its environment, 719
anomalies, 654
behavior, 846
cell, 58

distribution, 711
kingdom, 25
parasitism, 735
Animals of Gulf of Mexico, 778
Anisoptera, suborder, 321
Annelida, 26, 194

phylogenetic advances, 216
theory of evolution, 867
Anniellidae, family, 553
Annuli, 212
Annulus, 268
Anodonta, 249

stewartiana, 248
Anolis, 552

Anomocoela, suborder, 485
Anopheles mosquito, 74. 79
Anoplura, order, 266, 333
Anseriformes, order, 589
Ant lions, 266, 322
Anteater, giant, 621
scaly, 621
spiny, 617
Antebrachium, 495
Antedon. 225
Antelope, 630, 785
Antennae, 269, 271
Antennata, division, 265
Antennules, 269, 271
Anterior cardinal veins, 434

fontanelle, 428
Anthelmentics, 750
Anthonomus grandis, 324
Anthozoa, class, 140
Anthropoid apes, 637
Anthropoidea, suborder, 635
Anthropopithecus troglodytes, 637
Antibodies, 395, 884
Antigen, 395
Antilocarpa americana, antelope,

785
Antipathes, 143
Antipathidea, order, 143
Antithrombin, 395
Antrum, 812
Ants, 266, 331
Anura, order, 485
Aortic arches of vertebrates, 882
Apes, 635, 637

630,

Aphelodactyla, 224
Aphid, beet, 317

black cherry, 318

rosy apple, 316
Aphididae, family, 318
Aphids, 318, 340
Aphis lions, 266

maidi-radicis, 340

roseus, 318
Aphrodita, 195
Apical organ, 237
Ap.on proclive, 324
Aplodinotus, 454
Apodes, suborder, 449
Apopyles, 124

Ai-pendicular skeleton of frog, 520
Appendicularia, 361
Appendix, vermiform, 878
Apteria, 601
Apterygiformes, 585
Ap^erygoi-a, subclass, 309
Apteryx, 585
Aqueous humor, 529
Aquila chrysaetos, 590
Arachnid theory of evolution, 867
Arachnida. class, 266, 292

classification, 295
Arachnoid, 650
Arachnoidea, division, 266
Aranea gemma, 296
Arane.da, order, 266, 295
Arbacia, 220, 222
Arcella, 67

Archaeopteryx, 861, 869
Archenteron, 117, 210. 535
Archianneiida, 194, 212
ArchiteuLhis princeps, 260
Arciferal pectoral girdle, toad, 542
Ardeidae, family, 588
Arenicola, 195
Argentines, 449
Argen.inidae, family, 449
Argiopldae, family, 296
Argulus, 264, 265, 284
Ariidae, family, 450
Aristotle, 3 6
Aristotle's lantern, 221
Armadillidium, 282
Armadillo, nine-banded, 621

embryos, 622
Arrowworms, 193
Artemia, 283

Arteries, efferent branchial, 463
of frog, 503
of horned lizard, 568
of toad, 542
Arthrobranchiae, 271
Arthroleptella lighlfooti, 479
Arthropuda, phyiogeneiic advances.

286

phylum.

263, 746

wormlike ancestry, 865
Arthropodial membrane, 268
Arthrorhabdinus, 291
Articular bone, 467
Artiodactyla, order, 629
Ascaphus, 479

truei, 485
Ascaridia lineata, 176
Ascaris, 175, 176, 179
in.einal anatomy, 181
intestinal worm, 745
life cycle, 182, 183
relations to man, 183
Ascaroidea, order, 176
Ascidiacea, 361
Ascon, 124, 125
Ascorbic acid, 390
Asellus, 281, 282, 283

INDEX

925

AsUidae, family, 330
Asphalt pits of La Brea, 861
Aspidiotus perniciosus, 316. Sis
Ass, 633
Assimilation, 278
in Euglena, 81
in Hydra, 152
Astacus, 266
Aster, 62, 63
Aster ias, 228

cleavage, 116
Asteroidea, class, 217, 226
Asteromeyenia, 121
AsLhma, 839
Astrangia, 142, 143
Astropecten, 218
Athermidae, family, 452
Atlas, 494
Atom, 697
Atomic weight, 697
Atrial cavity, 365, 370
Atrina seminuda, pinna shell, Mi
Atriopore, 370
Auidae, family. 296
Auditory meatus, 639

nerve, 527
Audubon. John J.. 786
Auks, 591
Aurellia, 138

lite cycle, 139
Australian region, 712
Autonomic function, 528
Autotomy, 219, 234, 281
Aves, class, 411, 582
classification, 584
economic relations, 596
Avicularia, 184. 185
Aviculariidae, family, 296
Avocets, 591
Avoiding reaction, 101
Axial gradient, 173

skeleton, 378
Axoloil larva of Ambystoma tigrinum,

481
Axone, 107
Aye-aye, 635

Babesia bigemina, 80
Baboon, 636
Back cross, 827
swimmers, 318
Badgers, 627, 628
Bagre marina, 450
Bailer, 270

Baker's mealy bug, 318
Balaena mysiicetus, right whale, 635
Balance in life, 32

in nature, 31
Balanoglossida, order, 361
Balantidium coll. 70, 71, 744
Balanus, 265, 285
Baleen, 634
Balistidae family, 4o5
Barb, 599, 600
Barbary ape, 636
Barbels, 457
Barbule. 599. 600
Barnacle, 264

acorn, 264

goose, 264
Barnacles, 284
Barnea costata, 771
Barracudas, 453
Barriers to distribution, 717
Basal disc, 140. 146, 147
Basiliscus, 552

Basipodite, 270
Basisphenoid bone, 465
Basket star, 219
Basommatophora, 258
Bass, freshwater, 454
Basses, eea, 454
Bat, brown, 620

Mexican free-tailed, 620
Baihyal zone, 770
Bathymetric distribution, 713
Batoidei, order, 424
Batrachoididae, family, 455
Batrisodes, 340
Bais, vampire, 620
Bdellostoma, 412
Bears, 629, 791
Beaver, 624, 787
Beavers, 622
Beche-de-mer, 235
Bedbugs, 318
Bee hies, 330
Bees, 266, 331

BeeJe, Colorado potato, 3J3
Beetles, 265, 322
ambrosia, 33 5
passalus, 335
Behavior, chain reflex, 851
habitual, 852
reflex, 850
tropistic, 849
Belfrage, G. W., 48
Belonidae, family, 451
Belostomatidae, family, 318
Benthos, 769
Beroe ovata, 157, 159
Bidder's organ, 542
Bighorn, 631
Billbugs, 324
Binary fission, 90
Biogenetic law, 870
Biological effects of radiations, 697

point of view, 17
Biology, 17, 20
Biomes, 724

Bios, 17 . „„„

Biotic communities, 723

formations, 724
Biotin, 390
Bipes biporus, 555
Bipinnaria larva, 23 2, 234
Biramous appendages, 2(0
Bird migrations, 582
B.rds, 411, 582

economic relations, 596
Birth rate, 839

Bison amencanus, buffalo, bii
Bisons, 630
Bittern, 587
Bivium, 227, 228
Black bass, 453

widow spider, 293
Blackbirds, 595
'Blasendarm," 190
Blastema, 688
Blastocoel, 210

Blastocoele, 117, 534, 53o, 80o
Blastocyst, 814
Blastoderm, 805
Blastodermic vesicle, 814
Blastomeres, 171. 533, 800
Blastopore, 534. 805
Blastostyles, 132
Blastula. 117, 134, 155
earthworm, 210
of frog, 533 .
Blatella germanica, 315
Blattidae, family, 315
Blind cave salamander, 482

926

INDEX

Blindworm. 553

Blister beetle, 322
Blood corpuscles, 107
matching, S83
white cells, 513
Blubber, 634
Blue baby, 664
crab, 282
darter, 589
Bluebirds, 595
Boar, wild, 632
Bobcat, 629
Bobolinks, 595
Body louse, 738
Boidae, 556, 557
Boll, Jacob, 48
Bombyliidae, family, 330
Bonellia, 214, 215
Bonitos, 454

Bonnethead shark, 422, 439
circulatory system, 433
Book lice, 333
scorpion, 266
Bot fly, 747
Botalius, duct of, 491
Botaurus lentiginosus, 587
Botflies, 330

Bovidae, family of cattle, 630
Bowfln, 447

Bowman's capsule, 398, 650
Box-elder bug, 318
Brachial artery, 506
Brachiolaria larva, 232, 234
Brachiopoda, 27, 187
Brachium, 495, 563
Brachypephus magnus, 302
Brain of frog, 526

ventricles, 526
Branchial arches, 465
Branchiata, 264
Branchiopoda, order, 264, 283
Branchiostegal membrane, 463
Branchiostegites, 268
Branchiostoma, 361, 368
Branchipus, 264

Breeding habits of amphibia, 479
Bremidae, family, 338
Breviciptidae, family, 486
•'Bristle jaws," 193
Brittle star, 218
Bronchi, 566, 604, 649
Bronchioles, 566, 604
Brontosaurus, 546
Brown, Robert, 49
Brownian movement, 55
Brucophagus funebris, 33 4
Bryobia praetiosa, 298
Bryozoa, 27, 184, 185
Bubo virginianus, 593
Buccal cavity of frog, 500
Buccopharyngeal respiration, 514
Buffalo, 631, 632
Buffon, 44
Bufo americanus, 538

marinus, marine toad, 473. 478

punctatus, 474

toad, 485

valliceps, 485, 487

woodhousii, 485, 538
Bufonidae, family, 485
Bugs, 266
Bulgula, 184, 185

Bulimulus dealbatus liquabilis, 239
Bullfrog. 473, 497
Bullhead, internal structure. 458

life history and reproduction. 470

nervous system, 469

skeleton, 466

yellow, 457

Bumblebees, 338
Bunchgill, 321
Buprestidae, family, 324
Bursa, copulatory, 179
Bursaria, 71
Busycon, 248

egg capsules, 247

shell, 771
Butterflies, 266, 325
Butterfly ray, 424, 425
"Buzzard," 588
Byssus, 253

Cabbage-head jellyfish, 138
Caddis flies, 200, 321
Caeca, 603

pyloric. 459
Caecilianella. 237
Caecilians. 472
Calciferous gland. 204
Calcispongie, class, 120
Calendra mormon, 324
Calliuectes, blue crab, 282. 772
Callorhinus alascanus, fur seal. 029
Callus, 691
Calorie, 388
Calyx, 224
Cambarus. 266

clarkii, 207
Camels, 630
Campanella. 130
Campanularia, 131
Campeloma decisum. 247
Campodoidae, family. 311
Canadian zone. 714
Canidae, family. 028
Canis gigas. timber wolf. 029

latrans. coyote. 028
Cauon of Morgan, 848
Cantharidin. 341
Canthon simplex. 335
Capillaries, 392
Capitophorus potentillae. 320
Capitulum. 495
Caprella. 204

Caprimulgiformes, order. ,593
Carabidae, 324
Carangidae. family. 454
Carapace. 208. 578
Carbohydrates, 50
Carbon cycle, 28
Carcharhinus. 439
Carcharias, platydon. 422
Carcharodon carcharias, 422
Carchesium. 72. 73
Cardiac chamber. 272

muscle. 381

plexus, 528
Cardinals, 595
Cardo, 305. 345
Caiibous, 630
Carinatae, 584
Carnivora, order, 027
Carnivores, aquatic, 629
Carotid arch. 505

gland. 505
Carp, 450

leather. 449

lice. 284

scale. 449
Carriers. 833
Carrion beetles. 324
Carunculina texasensis, 248
Case flies. 321
Cassowaries. 585
Castor canadensis, beaver. 024
Casuariformes. order, 585

INDEX

927

Cat, 411. 639

circulatory system, 64S
digestive system. 646
excretory system, 650
Mexican ringtail, 626
muscles, 642
nervous system. 650
reproduction. 652
respiratorv svstem, 649
Catabolism, S6. SS, 153, 278, 377
Catamount. 629
Cataphracti, suborder. 454
Catfish, 411, 450
freshwater. 450
sea, 450
Cathartes aura septentrionales, 588
Catostomidae. family, 450
Cats, 629
Cattle, 630
Caudata. order, 484
Caudina. 224
Cave fishes. 451
Caviar, 447, 456
Cavies, 625

Cebidae. family of monkeys. 635
Cecum, 648
Cell division. 61
principle, 49
theory, its influence. 52
Cellular differentiation. 104

organization. 105
Centipedes, 265
Central disc, 217
Centrarchidae, family. 454
Centrolecithal egg. 803
Centrosome, 59, 60
Centrum, amphicoelous. 465

procoelous. 520
Cephalic ganglion, 16S
Cephalin, 395
Cephalization. 375

Cephalochorda. subphylum. 361, 368
Cephalodiscida, order. 361
Cephalodiscus, 361, 364
Cephalopoda, 2.58
Cephalopods, evolution of. 259
Cerambycidae. 324
Ceratite. 259
Ceratium. 75

Ceratobranchial cartilage. 428
Ceratodontidae. family. 455
Ceratohyoid bone, 467
Ceratopharys. 472
Ceratosa, order. 122
Cercaria, 756, 758, 759
Cerci, 348
Cercomonas, 66

Cercopithecidae, family of monkeys. 636
Cere 599
CereiDellum, 406, 469. 607

frog. 526
Cerebral commissure. 252
ganglia. 245
ganglion of Helix, 243
hemispheres. 469
vesicle, 372
Cerebratulus, 163
Cerebropleural ganglion, 252
Cerebrum, 406. 607
Cerianthus. 140
Cervical groove, 268

vertebrae, frog, 520
Cervidae, family of deers. 630
Cervus canadensis, elk. 630
Ceryle alcyon. 593
Cestoda, class, Kil. 762
Cestodes, 160. 161. 174
Cestus veneris. 157
Chaetae. 197

Chaetognatha. class. 27. 184. 193
Chaetopoda. 194
Chagas disease. 79
Chain reflex, 851
Chalcis-fly, 334
Chalones. 667
Chambers. Dr. Robert, 61
Chaos diffluens. 85
Chaparral bird, 591
Characters, unit. 823
Charadriifonnes, order, 591
Charybdea. 137
Cheek pouches. 624
Cheiloschisis, 656
Chela, 270
Chelicerae. 292
Chelonia, order, 548
Cheloniidae, family, 550
Chelydridae, family. 549
Chemotropism, 77
Chenopodium, 183
Cherniidae, family. 318
Chevron bones. 641
Chicken, 590, 597. 598
circulatory systein, 604
digestive sjstem. 601
muscles of hind limb, 611
muscular system, 612
reproduction and life history. iil3
respiratory system, 603
skeletal system, 609, 610
Chilodon, 70
Chilomastix, 67
Chilomonas, 66
Chilomycterus schoepfli, 443
Chilopoda, order, 265, 288, 290
Chimaera affinis. 425

monstrosa. 425
Chimaeras, 411
Chimpanzee. 637
Chinchillas, 625
Chipmunk. 622
Chiroptera, order, 620
Chitin, 106, 343
Chitinous exoskeleton. 286

Chloragogen cells, 204

Chlorella vulgaris, 144

Chlorochroa sayi, 318

Chlorohydra. 144

Chloropisca glabra. 331

Choanocytes, 119. 120, 12G

Chondrostei. order. 446

Chordata, 27, 360
classification. 361
phvlogenetic advances. 3(12

Chorioid, 401

layer of eye, 529
plexus. 527

Chorion. 818

Chorionic villi. 819

Chromatin. 59. 824

Chromatophore. 82

Chromomeres, 60, 63

Chromonema. 63

Chromonemata. 60. 61

Chromosome map. 835

Chromosomes. 61. 114

Chrysaora. 139

Chrysomelidae. 324

Chrysopa californica. 322

Chrvsopidae, family. 322

Chvme. 385. 501

Cicadas, 266, 318

Cicadellidae, family. 320

Cicindelidae. family. 324

Ciconiiformes, order, 588

Cilia. 94. 102
I Ciliata. 70

928

INDEX

Cimex lectularlus. 318
Cimicidae, family, 318
Cinclides, 142

Circulatory or vascular system, 110
Circulatory system, cat. 648
chicken, 604
open type, 2 63. 273
phrynosoma, 566
pisces. 459
toad. 542
turtle. 580
Circumesophageal commissures. 274
Circumoral canal. 221. 231
Circumpliaryngeal connectives. 208

ring. 176
Cirri, 197. 224
Cirripathes, 143
Cirripedia. 284

order, 265
Cirrus. 179

Clam, circulation, 252
digestive system, 250
excretion. 253
fresh water, 248

nervous system and sense organs. 252
reproduction. 253
respiration. 251
Class Infusoria. 93

Sarcodina. 85
Classification of man. 22
Clavicles. 609
Cleavage, 280, 374, 802
in asterias, 116
in cat. 653
disymmetrical. 804
divisions, 374
earthworm. 211
in frog, 533
holoblastic. 802
in mammal, 816. 814
merotalastic. 802
spiral, 804
superficial. 803
Cleft palate, 656
Click beetles. 324
Cliona. 121

Climax community, 728
ClUeUuiii. 199, 200. 209. 214
Clitoris of cat. 653
Cloaca, 613
Clonorchis. 161. 759

sinensis, Chinese liver fluke. 745. 757
Clupea harengus, 448
Clupeidae. family. 448
Clypeaster. 222
Clypeus, 345. 357
Cnemidophorus. 554
Cnidoblast. 148. 149. 150
Cnidocil. 148, 149
Cnidosporidia, 73
Cobras, 558
Coccidia, 73

Coccinellidae, family, 324
Coccygeal vertebrae. 609
Coccyx. 878
Cochineal. 341
Cochiopa texana. 247
Cochlea. 403. 60S. 650
Cockroaches. 313
Cocoon. 171. 214
earthworm. 209
Codfishes. 451
Cod-liver oil. 456
Codonosiga, 60, 75
Coelenterata, classification. 131
economic relations of. 156
phylum. 26. 130
Coelenterates, phylogenetic advances of,
1.56

Coeliac axis. 606
Coeliacomesenteric artery. 506
Coeloblastula. 807
Coelom. 194. 211

development, 536
Coitus. 409. 814
Colaptes auratus, 594
Coleoptera. order. 265. 322
Coleps, 70
Collar cells, 119
Collared lizard, 553
Collembola, order, 265. 311
Colloid. 53
Colloidal state. 53
CoUum. 288
Colotaoma. 665
Colony formation. 864
Color blind inheritance, 833
Coloration in amphibia. 474
Colubridae. 557
Columbiformes. order. 591
Columella, 574, 608
Colymbiformes, order, 586
Comb jellies, 157
Commensalism. 735
Commissure. 199
Conditioned reflex. 852
Conductivity. 54
Condvlarthra. 874

Condylura cristata, star-nosed mole. 619
Conger eels. 449. 473
Congridae. family. 449
Coniferous forest formation. 725
Conjugation. 96. 98
Couocephalus vicinus, 310
Consciousness. 54
Conservation of wildlife. 784
Continuity of germ plasm. 48
Contractile vacuole. 88, 94
Contractility. 54. 381
Conus arteriosus. 490. 510

Coon. 629

Cooties, 206

Coots. 591

Cope, E. D.. 48

Copepod. 284

Copepoda. order, 265

Copperheads, 556. 559

Copulation, 613. 6.i3. 814
earthworm. 209

Copulatory bursa, 179

Coraciiformes, order. 594

Coracoid bar. 430
bone, 465. 495. 521. 609

Coral, 140. 143
snake. 558

Corallium rubrum. 143

Coregonidae, family, 448

Coreidae, family, 318

Corizldae, family. 318

Cormorants. 587

Coin-ear worm. 320

Cornea. 270. 301. 401. 528. 607

Corneageu cells, 276, 277

Corona radiata, 812

Corporin, 076

Corpus albicans, 813
luteum, 812

Corrodentia. order. 266. 333

Corti. organ of. 6.50

Cortin. 671

Corythucha distincta. 318

Cosmic rays, 700

Cotton boll weevil. 324

Cotvlophoron. 702

Cougar, 629

Cow. 630

Cowper's glands. 653

INDEX

929

Coxa. 293. 306. 347
Coxopodite. 270
Coyote. 628
Crabs. 264. 265. 772
Cranial cre.sts. toad. 540

nerves. .527

of bullhead. 469
Cranium, 380

of fish. 465

of frop. 518
Craspedacusta. 131
Crawfishes. 267
Crayfish. 264. 265. 266

appendages. 269. 270

development. 279. 280

di^jestive system. 272

excretory system. 274

habitat and behavior. 267

metabolism. 27S

regeneration. 281

reproduction. 278

respiratory system. 271

vascular system. 273
Creatinin. 381. 464
Cremastocheilus annularis. 340
Creosote bu=-h-kangaroo rat biome. 727
Crepidula. 808
Cretinism. 655, 668
Cricket frog acris, 485

Mormon. 312
Crickets. 265
Crinoid. 225
Crinoidea. class, 224
Cristatella. 186
Croaker. 453
Crocodiles. 411. 548. 560
Crocodilia, order. 548. 560
Crocodilus acutus, 561
Crop, 601. 602
Crossing over. 834
Crossopterygii, order. 446
Crotalidae. family, 559
Crotalus, 5.59
Crotaphvtus collaris, 552
Crows, 595
Crura cerebri. 526
Crustacea, class. 264. 266
Cryptobranchidae. family. 484
Cryptobranclioidea. suborder, 484
Cryptobranchus, 473, 484
Cryptodira, suborder. 547. 548
Cryptotis parva, short-tailed shrew. 619
r'—i-ntn'-ifores. order. .586
Crystalline cone, 276

style, 251
Cteni. 444

Ctenoid scale. 444. 464
Ctenophora, phylum, 26, 1.57
Cub shark. 422
Cubomedusae, 137
Cuckoos, .591
Cuculiformes order. 591
Cucumaria, 224
Culicidae, family, .330
Cumulus. 812
Cuneiform, 611
Curculionidae, 324
Curlews, 591

Cutaneous artery of frog. 508
Cuticle. 81. 201

nonchitinous, 194
Cuticula. 308
Cybiidae. family. 453
Cycloid scale. 444
Cvclophylliriea, order, 161
Cyclopia, 665
Cyclops. 264. 265. 281. 283
Cyclosis. 95
Cyclospondyll, order, 422

Cyclostomata, class. 410, 412

economic relations, 413
Cynocephalus, a baboon, 636
Cynoglossidae, family, 452
Cynomys ludovicianus, prairie dog, 622,

623
Cynthia, 361
Cyprinidae, family, 450
Cyprinodontes, 451
Cyprinus carpio, 449
Cypris, 264
Cyrtoceracone, 2.59
Cysticerous, 762. 764
Cytology. 21
Cytopharynx. 81
Cytoplasm, 60
Cytosome. 86

Dace. 450

Dactylozooids, 135, 137
Daddy longlegs, 266, 299
Damsel flies, 265, 321
Danaidae, family, 327
Danaiis menippe, 327
Daphnia, 264, 281, 283
Dart sac, 246
Darters, 454
Darwin and Evolution, 885

Charles, 44
Dasyatis, 456

americanus, 423
Dasypus novemcinctum, armadillo, 621
Daughter cells, 62, 64
Decapoda, order, 265, 282
Deciduous forest formation, 726
Declaration of indefensibles, 792
Dedifferentiation, 695
Deduction, 18
Deer, 630, 786
Delamination, 810
Dementia praecox, 839
Demospongiae, class, 121
Dendrobates. 480
Dendrocoelum lacteum. 165
Dengue fever. 80
Dental formula. 643
Dentalium. 257. 2.58
Dentarv bone, 495. 520
Dentine, 643

Dermal branchiae, 226. 229, 233
r)ermaptera. ordei". 265. 320
Dermochelidae. family, .550
Desert formation, 727
Desmognathus. 484
Desor's larva, 163
Development, arrested, 654

of nervous system, .536

of sexual reproduction. Ill

of starfish. 2.32. 2.34
Developmental stages of several em-

brj-os. 880. 881
de Vries. Hugo, 48, 8,87
Dextral, 240
Dextrocardia, 664
Diabetes. 677

insipidus, 674
Diaphragm, 109
Diaphragmatic hernia. 6.57
Diapophysis. .543
Diapsida. subclass. 548
Diaptomus. 283
Diastase. .384
Diastole. .396
Dibranchiata. 260
Dicamptodon en=atus. 484
Dicrocoelium. 762
Didelphia. 617
Didelphis vlrglniana, 61Q

930

INDEX

Didinium. 70
Diencephalon, 419, 4fa9

of frog, 526.
Differential birth rate, 8^9
Differentiation, 801

intercellular, 104
Difflugia, 67
Digenea, 161
Digestion in frog, 501

intracellular. 130 , . . +-„„,

Digestive enzymes and their functions,

387
system, 109
cat. 646
chicken, 601
toad, 541
tract, solitary wasp, .50»
Digitigrade gait, 641
Dihybrid cross, 820
Dinocardium, shell. 7 a
Dinosaurs, 869
DioctophjTne. 176
Dioctophymoidea, order, 17b
Diodontidae, 455
Dioecious, 112
Diphyllobothrium, 161
latum, fish tape, 741, 742
tape, 765
Diplasiocoela, suborder 486
Diploblastic condition, 130

form, 120
Diploid condition, 800

number. 114
Diplopoda, 265, 288, 290
Dipnoi, subclass, 455
Dipsomania, 839

Diptera, order, 266, 329 ^

Dipylidium caninuin, dog tape, 763, .bo
Discoblastula, 806
Discocephali, suborder. 4o4
Discomedusae, 137 . ^
Dispersal in distribution a<
Dissosteira spurcata, 344^
Distribution of animals, ai
Diverticula, 167
Diverticulae. 213
Dobson flies, 322
Dog, 628
Dogfish, freshwater. 447

.shark. 422 .,

Dolichoglo'sus, nervous system, obJ

kowalevskii, 362
Doliolum. 361

Dolphin. 634 . . ,^ „- 090
Dominance, principle ot, iiz6

Dominant, 45
Doodlebugs, 322
Dorosomidae, family, 448
Dorsal aorta. 605
Dosimeter, 706
Dourine. 80
Doves, 591

mourning, 591 ^

Dracunculus, Guinea worm. (4.j
Dragonflies, 265, 319. 321
Drassidae. family, 295
Drassids, 295
Drassus neglectus. 296
Prill. 636 , ^ „„„

Drosophila melanogaster. 82S
Drumfishes. 4.54
Dryobates pubescens, 59o
Duck mole, 617
Duckbill. 617
Ducks. 589, 788
Duct of Botallus. 491

of Cuvier. 445
Dujardin. 49
Duodenum. 382

Dura mater, 527, 650
Dussumuriidae, family. 448
Dwarfism, 673
Dysgenic. 839

E
Eagles, 589
bald, 590
golden. 590
Ear structure. 402
Eardrum, 498
Earthworm. 199

circulatory system. 20o
development, 210
internal anatomy, 201
Earwigs, 265
Ecdysis. 263
Echeneidae, family. 4.54
Echinarachnius, 222
Echinaster, 218
Echinococcus, 756

dog tape, 745
Echinoderm theory of evolution. SbS
Echinodermata. 26
Echinoderms, larval relations. 8bb
Echinoidea, class, 219
Echiuroidea, order, 194, 214
Ecology, 24, 719 ^ ^ , ,. o,a

Economic importance of entomology, 6w
relations of birds, 596
of mammals, 637
Ectethmoid, 406
Ectoderm, 117, 130

of frog, 535
Ectoparasite. 737
Ectopistes, 786

migratorius. 591
Ectoprocta, 184
Ectopterygoid, 467
Ectosarc, 81. 86
Edentata, order, 621
Eel, 443
Eels, 449
conger, 449
morey. 4.50
Eelworm, 175, 180
Egg. centrolecithal, 803
guides. 349
of hen, 615
"Egg tooth," 615 -
Egret, 588

Egretta candidissma, 08*5
Elapidae, family, 558
Elasmobranchii. class, 411, 4_V

economic relations. 425
Elateridae. family, 324
Electrons, 697
Electrotropism, 77
Elephantiasis. 745. 746
Elephants. 634 ^ roj.

Elephas indicus, elephant. 6u4
Eleutherodactylus latrans. 479

ricordii, 472
Elks, 630
Elops saurus. 447
Elytra. 322. 354
Embiidma. order. 2b.\ 666
Embids. 205
Embryology, 23
comparative, 798
of frog, 532

later stages, 537
of toad, 544
Embryonic membranes 81 1
Empoasca fllamenta, 317. 3^u
Emulsoid. 53

Emus. .585 _,_ ^__

Emvdidae, family of turtles. :i49, on
Emys blandingii, 549

INDEX

931

Enamel. G43
Encephalocoele. 664
Endamoeba, 747
coli, 78
g-ingivalis, 78
histolytica, 67. 78
Endocrine glands. 666
summary, 678
system, 110
Endodei-m, 117. 130, 155

of frog, 535
Endolimax nana, 78
Endolympii, 403, 503
Endomixis. 97
Endoparasite. 737
Endoplasm, 81
Endopodite, 2G9. 270
Endoprocta. 184
Endosarc. 81, 86
Endoskeleton, 380. 517
Endostyle, 366
Engranlidae, family, 448
Ensiform process. 644
Enterobius vernicularis, 176
Enterocoele formation. 811
Enterokinase. 385
Enteronomas. 67
Entomobrvidae, family, 312
Entomostraca. subclass, 32, 204. 283
Entosphenus. 413
tridentatus. 415
Entozoic animals. 66
Enzvmes, 56, 383
Eohippus, 874
Ephemerella grandis. 321
Ephemerida. order, 26.'5. 321
Ephvdatia, 121
Ephyra, 139. 140
Epibolic gastrulation. 809
Epibranchial cartilage, 428
Epicauta puncticollis, 323
Epicoracoid cartilage, 521
Epicranium, 343
Epidermis. 499
Epididvnii.'--. 438. .572. 653
Epigyiium. 293
Epinephrine. 671
Epiotic. 465
Epipharynx, 357
Epiphragma, 238
Epiphysis, 419. 469
Epipodite. 269. 270
Episternum of frog. 521
Epistropheus. 609
Epistvh"?. 72. 75
Epitheliomuscular cell, 148, 150
Eptesicus fuscus, brown bat. 620
Equatorial plate. 62, 63
Eavius caballus. horse, 876
T^romobates. 298

Erethizon dorsatus, porcupine, 625
Eriophyes pyri, 298
Ervthrocvtes. 108. 395

of fi-og. 503
Erythroneura comes. 320

ziczac. 317
Escocidae. 450
Esophagus. 203
Esox. 451

Estrogenic hormone. 676
Etheostomidae. family, 454
Ethiopian region. 712
Ethmoid bone. 465

Euarctos americanus, brown bear, 629
Eubranchipus, 283
Euconulus chersinus trochulus. 240
Eucosmidae. family. 328
Eudorina, 73
Eugenic grouDS. 841
measures, 844

Eugenics, 821

Euglandina singleyana, 240
Euglena, 67, 81
behavior. 84
reproduction, 83
respiration, 83
Euglenoid movement, 84
Eulamellibranchiata. 256
Euophrys inonadnock, 296
Eupagurus, regeneration, 685
Euplectella. 120
Euplotes. 71
Eupomatus larva. 237
Eurypelma steindachneri. 297
Euselachii, order, 422
Euspongia, 122
I'jusuchia. suborder. 548, 560
Eustachian tube. 608

frog. .530
Eutheria, subcla.ss of mammalia, 617
Eutherodactj'lus. 485
Eventognathi. suborder, 450
Involution, annelid theory. 867
arachnid theory, 867
distributional evidence. 876
echinoderm theory. 868
embryological evidence. 879
geological evidence. 873
morphological evidence. 878
physiological evidence, 883
theory, 863
Excitor hormones. 667
Excretion, starfish. 233
Excretory organs, cat, 650
cliicken. 606
fish. 464
frog, ,515
or urinarj' system. 110
Bxoccipital bones. 463. 641
Exocoetidae. family. 451
Exogyra. 854
Exopodite. 269. 270
Exumbrella. 132
Eye of chicken. 607
of frog. 528
worm. 176
Eyeball, 401
Eyespot, 233

Facets, 345
Facial nerve. 527
Fairy shrimps. 283
Falconiformes. order, .589
Falcons, 589
Fallopian tubes, 652
Family elopidae. 447

size, 841

in dysgenic groups, 842
Fances, 648
Fascia. 612
Fasciola. 101 ^ _^„

hepatica. liver fluke. 743. 758, (u9. 760
Fascioloides. 7.39
Fasciolopsis. 760
Fat bodies of frog, 532
Feathers. 582. 600
Feeblemindedness. 838
Felidae. family. 629
Felis cougar, mountain lion. 629

domestica. house cat. 639

hernandesii, jaguar, 629

pardalis. ocelot. 629
Femoral vein, frog, 509
Femur, 306. 347

of frog. 522
Fenestra ovalis. 574
Ferrisia. 240

excentrica. 238

932

INDEX

Fertilization. 614

in bullfrog-. 531

membrane. 533
Fiber zibethicus. muskrat. 625
Fibrogen. 393, 395
Filaria, 175. 176. 745
Filaroidea, order, 176
Fileflshes, 455
Filibranchiata. 256
Filiform papillae, 603
Filoplume. 600
Filum terminale. 651
Firmisternal girdle, 543
Fish, bony. 442

excretory org^ans, 464

flying, 442

lice. 284

motii. 310

respiratory system. 463

skeletal system, 464

typical bony, 457
Fisher. 638

Fisheries and zoology, 34
Fission, 111

Fissipedia, suborder, 62S
Flagellum, 81, 82, 126
Flame cells. 160, 167, 189. 190, 397
Flat-fishes. 451
Flatworms, 26
Fleas, 266. 329
Fleming. 62
Flicker. 594
Flies, 266. 329
Flounders. 451
Fluke, human blood. 755
Flukes. 160
Fly, big-headed. 332
Flying fishes. 451
Folliculin. 676

Folsomides decemoculatus. 312
Foot-gland. 190
Foramen magnum, 519

ovale. 664
Foramina. 527
Foraminifera. 68. 78
Formicidae. family, 339
Fossil, 854

reptiles. 546
Fossils, distribution. 856
Fourth ventricle, 435. 469
Fox. 628
Frog, appendicular skeleton, 520

blastula nnd gastrula, 533, 534

blood, .503

brain, 526

circulatory system. 502

cleavage, 533

common tree. 474

digestive system. 499

embryology, 532

excretory .system. 515

feeriing habits. 477

fertilization. 531

hairy. 476

heart. .510

heart beat. 512

internal ear. 529

later development. 537

muscular system. 523

nervous svstem. 523

pelvic girdle. .521

reproductive organs. .531

respiratory system. 514

sen'se organs. 528

skeletfi system. 517

skin. 475

skull. 517

Bplnal cord. .')27

Strecker's. 482

swamp tree. 473

Texan cUff. 478

Frog — Cont'd

tic-e. 4SC1

vascular system, 503

veins. 508

vertebral column. 520

visceral skeleton. 519
Frogs. 411. 472
Frons. 343
Frontal bones. 465
Frontonia. 70. 71
Frontoparietal bone, 517
Fruit flies. 330
Fulica americana. 591
Fungiform papillae. 650
Funiculus. 185. 186
Furs, 637

Gadidae, family, 451
Galea, 305. 345
Galeichthys felis. 450
Galen. 21. 37
Galeocerdo arcticus. 422
Galliformes. order, 590
Gallinules. 591
Gallus domestica. 598
Galvanotropism, 77
Gambusia. 451
Game, big. 794
Gametes. Ill

of hydra, 1.54
Gametocytes, 74. 749
Gametogenesis. 113. 799
Gammarus. 32, 264, 283
Ganglia, 399
Ganoid scale. 444
Ganoin, 444
Gapes in chickens. 176
Garfishes. 4.51
Garpike, 447

Gasterosteidae, family. 451
Gasterostomata. order. 161
Gastric digestion. 385

mill. 272
Gastrocnemius. 523
Gastrocoele. 805
Gastrocopta armifera. 240
Gastrophilus, horse hot, 747
Gastropoda, 257
Gastrotheca. 480
Gastrotrioha. ISS
Gastrovascular cavity. 130. 14S, 150, 1-58.

167
Gastrozoids. 1,S5. 137
Gastrula. 117. 1.55

development. 865

enrthworm, 210

of frog, .'^.33. .534

starfl=;h, 234
Gastrulation. 805. .808

earthworm. 211

in Tnammal. 815
Gaviiformes. order, .586
Gazelles. 630
G'^bia stpllpta. 239
"Geoko." 551
Geese. .589
Gci«er. S. W.. 48
Gpkknnidae. family, 551
Hel Pt5>tp, .53
G<^mmulp='. 121
Genae. .^43
Gene"?. 61. 824

comn'<^'"nen'^ary. 829

mult''nle. 828

siinniementary. 830
Genetics. 23

qnd eugenics. 821
Ge^iohvo'd mu^'^ie. 468
Geni*f<l nl-^te. 221

rachls. 2?l

INDEX

933

Genotype. 826

Geobios, 713

Geographical distribution. 711

Geologic time scale, 872

Geomyidae, family of gophers, 624

Geomys bursarius, pocket gopher, 623

Geotropism. 77

Gephyrea. 194. 214

Germ cells. 104

layers, 117
fate of. 118

plasm. 104

continuity of. 48
Gerrhonotus. 552
Gerridae, family, 318
Gestation, 653
Giant water bugs, 318
Giardia. 67

lamblia. 742
Gibbons. 637
Gigantism, 655
Gill plate, 197

rakers. 465
Giraffes. 630
Gizzard, 203. 204, 601

shads. 448
Glands, ductless, 666

of internal secretion, 666
"Glass snake," 552
Glenoid fossa. 495, 521
Globigerina, 69. 78
Glochidium, 2.i3. 2.54
Glomerulus. 398. 650
Glossa. 357
Glossarj', 905
Glossina, 744

palpalis. 79
Glossiphonia, 214
Glossopharyngeal nerve, 527
Glottis. 492. 564, 603
Glucose. 56

Glycogen, 56, .383. 387
Glycogenase. 388
Gnawers, 621
Goats. 630

Rocky Mountain. 631
Goatsuckers, 593
Gobies. 454
Gobiidae. family. 454
Gobioidea. suborder, 454
Goiter, 668
Golden eagle, 590
Gonadotropic hormone, 670
Gonads, 104
Gonangium. 134
Goniatite, 2.59
Goniobasis. 245. 258

comalensis. 247
Gonionemus. 131, 132
Gonotheca, 133
Gonozooids. 337
Gooseberry fruitworm, 327
Gophers, pocket. 621, 623
Gordiacea. clas«:. 175, 177
Gordius. 177. 178
Gorgonacea. order, 143
Gorgonia. 143
Gorr'onocpnhalus, 219
Gorilla. 637

gorrilla. 637
Graafian follicles, 652, 812
Gracilis. 644

muscle of frog, 523
Crackles, 595
Gmnt'a. 121. 122
Granulocytes, 513
Grass-Bison Biome. 726
Grasshopijcr, circulatory system, 349
Grasshoppers, 265, 343
Grassland formatlop. 72Q
Graylings, 448

Grebes. 586

Green glands, 110. 286

Gregaloid colony, 76

Gregarina, 72

Gregarinida, 73

Grew. 50

Ground beetles, 324

hog, 622

squirrels, 622
Growtli. 54

Gruiformes, order, 591
Grynophilus porphyriticus. 4S4
Guanin. 474
Guano. 597
Guanophores. 474
Guinea pigs, 622. 625

worm, 175, 176
Gular fold, 562
Gullet. 140
Gulls, 591
Gurnards, 454
Gynecophoric canal, 755
Gyroceracoue. 2.59

Habit. 852

Habitual behavior. 852

Hadurus hirsutus. 298

Haeckel, 43. 284

Haemal arches. 405

Haemoglobin. 196

Haemosporidia. 73

Hagfishes. 412

Hags. 410

Haliaeetus leucocephalus. 590

Halibut. 451

Haliclona. 121

Haliclystus. 137

Haliver oil. 456

Halobios. 766

Hamster, golden, 027

Haploid number. 115

Haplomai. suborder, 450

Haplosporidia. 73

Harelip, 6.39, 656

Hares. 625

Harvestmen. 266. 299

Harvey. William. 40, 798

Hawk. Cooper's. .589

Hawks, 589, 590

Heart anomalies. 664

Heart beat, branchial. 261

of chicken. 607

of frog, 510, 512

systemic. 261
Hectocotylized arm, 261
Helicina orbiculata, 258

tropica, 240
Heliothis obsoleta. 326
Holiozoa, 69
Holisoma. 244

trivolvis lentum. 238
Helix. 2.37

anatomy of. 243

circulation. 244

excretory organs, 245

genitalia. 246

nervous system. 245

reproduction and life history. 245

respiration, 244
Hellbender. 484
Hell-diver, 5S6
Heloderma suspectum. 553
Helodermatidae. family. ,553
Homichordata. subphylum. 361, 362
Hemidactylium scutatum. 4S4
Hemimetabolous. 308
Hemi penes, 572
Hemiptera, order, 266, 318
Hemocyanin, 244, 394

934

INDEX

Hemoglobin, 57, 205, 375, 513
Hemol>-mph, 394
Hemophilia, 839
Hemophiliacs, 395
Hepatic caeca, 229

portal vein, 434, 509
Heredity, 821

physical basis, 824
Hermaphrodite, 154
Hermaphroditic condition. 112
Hermaphroditism, 40S, 063
Hermit crab, regeneration. 685
Herodias egretta, 588
Heron, 588
Herring, 447, 448
Herrings, round, 448
Hertzian waves, 701
Hetaerina americana, 321
Hetaerius tristriatus, 340
Heterocera, suborder, 325
Heterocercal tail. 426
Heterocoela, order, 121
Heterocoelous vertebrae. 609
Heterodont condition, 643
Heteronomous condition, 203
Heterophyllidea, order. 161
Heterosomata. suborder. 451
Heterotrichida, 71
Heterozygous individual, 825
Hexylresorcinol, 183
Hibernation, 476

in amphibia, 480
Hiodon tergisus. 448
Hiodontidae. 447
Hippiscus, 315

corallipes, 310
Hippoboscidae, 330
Hippocrates, 36
Hippoglossidae, family, 452
Hippopotamuses. 630
Hirudinea. 194. 212
Hirudo. 195, 212
Histogenesis. 801
Histology, 21
History of zoology, 36
Hogs, 632

Holoblastic cleavage, 802
Holocephali, subclass, 425
Holometabolous, 308
Holostei, order, 447
Holothuria. 224
Holothuroidea. class, 222
Holotrichida, 70
Holozoic animals, 66
Homo sapiens, 22
Homocercal tail. 443
Homocoela, order, 120
Homoiothermal condition. 616
Homology. 271. 663

serial. 271
Homonomous condition, 263
Homoptera, order, 266, 316. 318
Homozygous individual. 825
Honey bee, 357
Honeydew, 340
Hoofed animals, 629
Hooke. Robt, 49
Hormones, 666

of non-chordates, 667
plant growth, 667
sex, 675
Horned lizard, 561
arteries. 568
internal structure, 566
reproduction, 571
urinogenital system, .571
veins, 570
Horned "toad," 561
Horntails, pigeon, 331

Horse, 633

Equus caballus, 876
its rise, 874

phylogenetic development, 875
Horsefly, green-headed, 331
Horsehair snake, 175, 177
Horseshoe crab, 266
Host specificity, 741
Housefly, life history, 330
Hudsonian zone, 714
Human blood cells. 396

heredity, 836
Humboldtiana. 242

ciiisosensis. 239
Hummingbirds. 593
Huxley, Thomas H.. IS, 47
Hyaline cartilage, 107
Hyaloplasm, 00
Hyalospongiae, class, 121
Hydatina, life cycle. 191
Hydra, 131, 144
life cycle, 153
locomotion, 145
metabolism, 151
regeneration. 156
viridissima, 144, 146
Hydranths, 132
Hydrobios, 713
Hydrocaulus, 132
Hydrocephalus, 664
Hydrocorallina. 136
Hydrolysis, 384
Hydromedusae, 135
Hydrophidae, family, 559
Hydropsyche, partita, 322
Hydrorhiza, 132
Hydrostatic organ, 464
Hydrotheca, 133
Hydrozoa, class, 131
Hyla, 486
cinerea, 474
crucifer. tree frog, 483
Hylobates, gibbons, 637
Hyman, Libbie H., 144
Hymenolepis, 161
nana, 765
Hymenoptera, order, 266, 331, 357
Hyoid arch, 465
Hypaxial portion of body. 496
Hyperbranchial groove. 373
Hyperoartii. 412
Hyperotreti, 412
Hyperthyroidism, 669
Hypertrophy, 691
Hypnotoxin, 149
Hypobranchial groove, 372
Hypodermis, 308
Hypopharyux. 305
Hypophysis. 469

frog, 526
Hyporachis
Hypostome,
Hypothesis,
Hypotrichida

601
134,
18
71

147

I
if

Ichthyomyzon, 413

Ichthyophis glutinosus, the caecilian, 473
Icius similis. 2!)(i
Ideal continent, 722
Idotea, 282

Iguanidae, family. 551
Ileocaecal valve, 565
Ileum, 499, 564
Iliac arteries of frog, 507
Ilium. 611
of frog. 521

INDEX

935

Imago, 309
Implantation. 815
Incus, 403
Inclefensibles, 792
Infrared, 699
Infundibulum. 158
Infusoria, 71

class, 70
Ingestion in amoeba, 87

in hydra, 151
Ingression, polar, 810
Inguinal canal, 652. 653
Iniomi, suborder, 450
Inner cell mass, 816
Innominate vein of frog, 508
Insect characteristics, 301
Insecta, class, 265, 300
Insectivora, order, 619
Insects, economic relations. 340

hemimetabolous, 320

holometabolus, 321

nerve winged, 322

representative. 343

shellac scale, 341

social life, 334
Inspiration, 648
Instinct, 846. 852
Insulin, 677
Integument, 377
Integumentary system, lOS
Interambulacral plates, 219. 220
Interauricular septum, 510
Intercalary disks, 107
Intercellular digestion. 151
Intercostal muscles. 581
Intermedin. 678
Internal carotid artery, 433
Interradial pouches. 222
Interstitial cell, 149, 150
Intem'ertebral discs. 643
Interzonal fibers. 62
Intracellular digestion, 119
Invertase, 386
Iris. 401

of frog, 529
Irritability, 54
Ischiopubis, 430
Ischium of frog, 521
Ischnochiton, 257

conspicuus. 256
Islands of Langerhans, 110, 677
Isopoda, order. 265
Isoptera, order, 265. 320
Isospondyli, subordei-. 447
Istiophoridae. family, 453
Itch mite, 739
Ixodidae, family, 298

.Tacana spinosa gymnostoma, 591
Jaguar, 629
.lapanese beetle. 31
Japyx hubbardi. 311
.Tavelina. 032
Jays. 595
Jellyfishes. 26
.loint snake. .")52
Jugular veins of frog, 508
Jugulares, suborder. 4.55
Julidae. family. 2S9
Julus. 289

Jumping spiders, 290
June beetle. 324
bug. 354

life history. 350
Jungle fowl, 597

Kangaroo. 618

rat, 624
Karyolymph, 61
Karyoplasm. 61
Karyosomes, 61
Katydids, 315
Keratosis, 838
Kerona, 71
Kidneys, 398

of cat, 650
Killdeers, 590, 591
Killifishes, 441
King crab, 266
Kingbirds, 595
Kingfisher, 593. 594
Kinosternidae, family, 548
Kissing bug, 79
Kiwi, 582, 585

Labia minor, 320

Labial palps, 357

Labium. 345

La Brea asphalt pits, 861

Labrum, 345, 357

Lacertilia, suborder, 547, 551

Lacewings, 322

Lacinia, 345. 305

Lacrimal bone. 407. 643

Lacrymaria. 71

Lactase, 3S6

Lacteals. 513

Lactogenic hormone, 678

Ladybird beetles, 324

Ladyfishes. 448

Lagena, 437, 60S

Lagomorpha, order, 625

Lamarck, 43

Lamprey, 413

larva. 420

life history, 421
Lampreys. 410
Lampsilis. 248

anodontoides. 249. 254
Langerhans. islands of, 677
Larva, hexacanth. 764
Larvacea, 361
Laryngeal chamber. 564
Lasius. 340

Lateral line system. 470
Latrodectus. 293

mactans, 295
Laveran and Manson. 79
Leaf hoppers. 320

roller, strawberry, 328
Leber's atrophy, 832
Leeuwenhoek, 49
Lemuroidea, suborder. 035
Lemurs. 635
Lens. 401
Leopard. 629
I^epas. 265

Lepidoptera, order, 266, 325
Lepidosaphes becki, 320
Lepidosiren paradoxa. 455
Lepidosirenidae, family, 455
Lepidurus. 204
Lepisma saccharina. 310
Lepismidae. family. 310
Lepisosteidae. family. 447
Leplodactylidae, family. 485
Leptinotarsa decimlineata. 323
Leptocoris trivittatus. 318
Leptodea fragilis. 248
T>eptodora. 264
Lepton squamosum, 239
I^eptosynapta. 224
Leptotyphlopldae, family, 557

936

Lepus, 626

Lestes uncatus, 321

Leucocytes. 393

of frog. 503, 513
Leucosolenia. 120
Levator muscle, 524
Leydig, duct of, 493
Libinia, spider crab, 772
Lice, 266. 333
Life, attributes of. 30

history, ameiurus, 470
housefly. 330
lampiey. 421
monarch butterfly. 328
mosquito, 329

zones, 713. 714
Ligula. 305
Ligulae. 345
Limacina australis. 241
Limulus Polyphemus, 299
Lineus socialis, 162
Liuin, 59

net, 61
Linkage, 832
Linneas, 42
Lion, 629
Lionotus, 71

Liopelmidae, family, 485
Lipase. 384
Lipophores. 474
Liriope, 136

Litanseutria obscura, 318
Littoral zone. 713, 769
Liver, function, 386
Lizard, alligator, 552

collared. 552

fishes, 450

horned, 552
Lizards, 411, 551

night, .'554

sand, 554

worm, 555
Llamas. 630
"Lobster claw," 659. 837
Lobsters, 265
Locust. 343

Locustidae. family, 315
Loligo, 2.58

brevipennis, 2.57. 260
Long-horned wood beetles. 324
Loons. 586

Lophiidae, family, 455
Lophophore, 185, 186, 188
Lovebirds, 591
Lower jaw of frog, 520
Loxodonta africana, elephant, 634
Lubber grasshopper, western. 302
Lucernaria. 137
Lucunae. 107
Lumbricus. 195

terrestris. 199
Lungflshes. 455
Lutein, 676

Lntinnidae. family. 454
Lutianus campechanus. 454
Lycosa. 295
Lycosidae, family, 295
Lygaeidae, family, 318
Lygus pratensis, 318
Lymnaea, 242

bulimoides, 256
techella, 238

palustris. 237

stagnalis, 238
Lymph, 108

Lymphatic system, 502, 513
Lymphocytes, 513
Lynx, 629
^L-yre snake, 556

INDEX

Macaca sylvana, 636
Macaque monkey, 636
Macaws. 591
Machilidae, family, 310
Machilis orbitalis, 310
Mackerels. 453

Spanish. 452
Macroccphalus. 664
Macrogametes. 74. 112
Macromeres. 534
Macroperipatis, 288
Madrepora. 142
Madreporaria. order. 142
Madrepoiic plate. 218
Madreporite, 217, 220, 221. 223. 226, 2L's,

230. 231
Magellania. 188
Maggots, 330

Malacostraca, subclass, 265
Malaria, 74, 79, 748
Malformations, 654
Malleus, 403

Mallopliaga, order, 266, 333
Malpighi, 40, 49
Malpighian bodies, 606

corpuscles, 398, 492

tubules, 110. 286, 352
Malthus' law, 885
Mammalia, 411

class, 616

classification, 616

economic relations, 637
Mammalian development, 812
Mammary gland, 653

glands, 639
Mammogenic hormone, 678
Man, 6^5
Manatee, 634
Mandrill, 636
Man-eater shark, 422
Maniliin, human, 109
Mantidae, family, 318
Mantispids, 322
Mantle, 365

cavity, 250
Manubrium, 131, 138
Manus, 522
Marine Biological Laboratory, 43

zoology, 766
Marmosets, 635

Marmota monax, woodchuck, 622
Marsupia. 253
Marsupial frogs, 480

pouch, 61S, 619
Marsupialia, order, 619
Marsupials, 617
Martins, 595
Mastax, 189
Mastigamoeba, 66
Mastigophora, 66

class, 81
Mastodonsaurus, 473
Mating among defectives, 839
Maturation of germ cells. 113
Maxillae, 270. 357
Maxillipeds, 269, 270
May beetles. 354
Mayflies, 265. 319, 321
Meantes, suborder, 485
Meckel's cartilage, 520
Mecoptera, order, 266, 333
Mediastinum, 657
Medulla oblongata, 435, 607
of flsh. 469
of frog, 526
Medusae, 113, 132. 133. 134
Megalopidae, family, 447

I
I

INDEX

937

Megalops, 285

Melanerpes erythrocephalus, 395
Melanophores, 474
Melanoplus atlanis, 343

femur-rubrum, 310
Melophagus ovinus, 330
Membranous labyrinth, 403. 530, 574
Memory, associative, 852
Mendel, Gregor J., 45, 821
Mendel's law, 821
Menetus dilatatus, 238
Meninges, 650
Menofoini, 676
Mentomeckelian bone, 520
Mentum, 305, 345
Mergansers, 589
Mermiria bivittata, 310
Meroblastic eggs, 546
Merozoites, 74
Merychippus, 870
Merzoites, 749
Mesenchyme. 165
Mesethmoid, 456, 643
Mesoblast cell. 211). 211
Jlesoderm, 117, 809
of frog, 536
parietal, 819
splanclinic, 815
Mesoglea. 132, 150. 151
Mesohippus, 874
Mesonephric tube. 464
Mesopterygium. 430
Mesorchium. 492, .572
Mesothelium, 201
Mesothorax, 305. 347, 3.57
Mesotubarium. 492
Mesovaria, 812
Mesovarium. 438, 573
Mesozoa, 120
Metabolism. 28. 54. 377
Metacarpals. 522
Metacercaria. 759
Metacrinus. 225
Metagenesis. 113, 135
Metamere, 211
Metameres, 194. 361
Metamerism. 375

Metamorphosis. 308, 801
in acidia, 367
starfish, 232

Metanephros, 397, 545

Metaphase, 62, 63

Metapleural folds. 370

Metapterj'gium, 430

Metargiope trifasciata, 290

Metatarsal bones of frog, 522
tubeicles, toad, 540

Metatheria, 619

Metatliorax, 305. 347. 357

Metazoa characteristics. 103

Metazoan organization, 103

Metridium, 140, 141

Microcephalus, 664

Microgametes. 74

Microhyla, 486

Micromeres. 534

Micronuclel. 98

Micropodiformes, order. 593

Mocropogon undulatus. 453

Microptci'us salmoides, 453

Microsporidia, 73

Midbrain of fish, 469

Miescher's tube, 73

Migration of animals. 716
airwavs. birds. 583

Millepedes. 265

Millepora, 136

Millers, 327

Millipede, 289

Mlmus polyglottos, 594
Mind, 846
Mink, 638
Minnows, 450

mud, 450
Miracidium, 756, 758, 759
Miridae, family, 318
Mirids, 318
Misumena vatia. 295
Mite, human itch. 738
Mites, 266
Mitochondria, 60
Mitosis, 62
Mnemiopsis, 157
Mockingbird, 594, 595
Mole, 619
Molgula, 361

manhattensis, 365
reproduction, 366
Moilusca, 27

classification, 256
economic relations, 255
internal morphology, 243
phylum. 236
Molluscoida. 27. 184
Mollusk shells. 771
Molt. 280. 601

Monacanthidae. family, 455
Monarch butterfly, 326

life histoiy, 328
Monaxon, 124
Monaxonida, order, 121
Moniezia, sheep tape, 763. 765
Monkeys, 635
Monocytes, 513
Monodelphia, 617
Monoecious condition. 112
Monogenea. 161

subclass. 762
Monohybrid cross, 825
Monos.ga, 66

Monotremata, suborder, 616
Mooneyes, 447
Moose, 630
Moray eels. 450
Momion cricket. 312
Morphology. 20
Morula, 807, 809
Mosaic vision, 277, 278
Mosquito fish, 80, 451

life history. 329
Mosquitoes. 266
Mother-of-Pearl. 255
Moths. 266. 325
Motor cells. 400
end plates, 400
nerve root, 527
Mouse, 625

Mouth parts of insects, 301
Mud hen, 591

puppy. 487
Mugil curema, 452
Mugilidae. family, 452
Mulatta, 828
Mliller, Johannes. 43
Miillerian tube, 493
Mullets, 452

Muraenidae, family. 450
Murex tenuispina, 241
Murgantia histrionica, 318
Mus. 625

Musca dome?tica.330
Muscidae. family. 330
Muscle, actions of. 524
attachments, 524
involuntary, 524
Muscles of cat. 642. 644
of fish. 467
of hind limb, chicken, 611

938

INDEX

Muscular or contractile tissue, lOG
system, 1C9

chicken. 612

frog-, .523

phrynosoma, 576
Musculium ferrissi, 248
Musculocutaneous vein. .508
Musli ox, 631

turtle, 549
Muskrats, 622, 625
Mustelus canis, 425
Mutation theory, 887
Mutations, 836
Mutualism, 735
Mycetozoa, 69
Mylohyoid muscle, 408
Myocomma, 370
Myocytes, 125
Myoflbrils, 106
Myology, 644
Myosepta, 496
Myoseptum, 370

Myotomes, 370, 372, 427, 496, 537
Myriapoda, class, 265, 287. 289
Myrmecophaga jubata, giant anteater,

621
Myrmecophiles, 339
Myrmeleonidae, family, 322
Mysis, 285

Mystacoceti, order of whales, 634
Myxedema, 669
Myxidium, 73
Myxine, 412
Myxinidae, family, 412
Myxinoidea, subclass, 412
Myxobolus, 73
Myxosporidia, 73
Myzus cerasi, 318

Nabidae, family, 318

Nabis ferus, 318

Nacreous layer, 250

Nagana, 80

Narwhal, 634

Nasal bones, 465

Natural resources, restoration, 794

selection. 885
Naturalists of the frontier. 48
Nauplius larva, 285
Nautilus. 258
Navel, 820
Nearctic region, 712
Necator, 750

hookworm, 745
Nectocalyces, 137
Necturus, 485

circulatory system, 490

digestive system, 489

internal structure, 488

maculosus, 487
muscular system, 496
respiratory system, 491
skeleton, 494
urinogenital system, 492
Neelidae, family, 312
Nekton. 769
Nemathelminthes. 26, 745

phylum, 175
Nematocyst, 134

Nematocysts, 136, 1.3S. 142, 147, 148
Nematoda, class, 175
Nematode, 750

Nematognathi, suborder, 4.50
Nemertina, class, 161
Nemertinea, 27, 192
Neoceratodus fosteri, 455

Neocomatella, 225
Neoechinorhynchus emydis. i7S
Neoscona benjamina, 296
Neotropical region, 713
Nephridia, 110, 188. 195, 206. 207. 214, 39S
Nephridial ducts, 189
Nephridiopore, 195, 206, 207
Nephridiopores, 201
Nephridium, 203
Nephrostome, 195, 199, 206. 207
Nephrostomes, on frog kidney, 514
Nereis virens, 195, 196
Nerves, cranial, of frog, 529
Nervous function, 404
system, 110

of bullhead (ameiurus), 469

of cat, 650

chicken, 607

development, 536

of frog, 525

phrynosoma. 573

starfish. 233

sympathetic, 528

toad, 543

vertebrates. 399
tissue. 107
Nettling cells, 135
Neural arch, 465
folds, 536
plate, 536, 815
tube, 537, 817, 818
Neurocoele, 372, 527
Neuroepitlielial, 404

cells, 153
Neuron, 107
Neurones, 405
Neuropodiuni, 196. 197
Neuroptera, order, 266, 322
Neutrons, 698

Newt, common (triturus). 484
Nicotinic acid. 389
Nictitating membrane, 562, 600, 639
Nidamental gland, 262
Nidicolae, 615
Nidifugae, 615
Night lizards. 554
Nipples, 653
Nitrogen cycle. 29
Noctiluca, 67
Noctuidae, family, 327
Nodosaria hispida. 69
Nomenclature, binomial system, 22, 43
Notochord, 360, 372
Notonectidae, fnmily, 318
Notopodium, 197
Nuclear membrane, 59, 60
Nuda. 157

Nutritive-muscular cell, 151
Nymph. 319
Nysius ericae, 318

Oak-deer biome. 726
Obelia, 113, 131. 132
Occipital condyle. 494
Ocean, conditions, 771
Ocelli, 301, 345
Ocelot, 629
Ocular plate, 221
Oculina, 142
Oculomotor nerve, 527
Odobenus, walrus, 629
Odocoileus, deer, 630
Odonata, order. 265. 320
Odontoceti, order of whaler, 634
Odvnerus dorsalis, 335, 359
Oestridae, family, 330

INDEX

939

Oestrin. 676

Oligochaeta, 194, 196. 199
Omasum, 633
Ommatidia. 277, 302
Ommatidium. 276
Omosternum, 521
Onchocerca volvulus. 745. 746
Oniscus. 282
Ontogeny. 113. 285

recapitulates phylogeny. 870
Onychophora, class, 265. 287
Oocyst, 74

Oocyte, secondary, 115
Ooecium, 185, 187
Oogenesis, 113. 114
Oogonia, 114, 799
Ookinete, 74
Opalina, 70, 745
Operculum, 537
Opliichthyidae, family, 449
Ophidia, suborder, 548, 556
Ophioderma, 219
Ophiothrix, 219
Ophisaurus, 552
Ophiura, 219
Opliiuroidea, class, 219
Opisthobranchiata, 258
Opisthoglypha, 556
Opisthotic bone, 494
Opossum. 619
Optic chiasma. 526

ganglion. 262

lobes. 469
Oral groove. 93

hood, 370

lobes, 138

ossicles, 229
Orangutan. 636. 637
Orcinus orca, killer whale. 634
Oreamnos montanus. 631
Oreaster, 218
Organ. 108

of corti. 403, 650
Organ pipe coral, 143
Organic catalysts, 57
Organizers. 694
Organogenesis. 801
Organs and system. 817
OrioTital region. 712
Origin of species, 43, 863
Ornithorhynchus anatinus. 617
Orohippus. 874
Orthoceracone, 259
Orthoptera, order, 265. 313
Orytricha. 71
Osculum. 122
Osmeridae. family, 448
Osphradium, 253
Ospreys, 589
Ossicles. 227. 228
Osteoblasts. 691
Ostia, 122. 124. 273
Ostium. 531, 573. 652
Ostracoda. order, 204, 284
Ostraea virginica, oyster. 771
Ostrich. African. 584. 585
Otocyst. 237, 367
Otter. 638
Otus asio. 593
Ova, 104

Ovarian follicle. 812, 813
Ovaries of cat, 6i>2
Ovary of hen, 614
Ovibas mo.schatus. musk ox. 631
Oviducts. 407

of frog. 531
Oviparous. 407
Ovipositor. 348

Ovis canadensis, mountain sheep, 630

Ovotestis, 243

Ovoviviparous condition, 248. 487

Ovum. 652

Owls, 593

barred or hoot, 593

great horned, 593

prairie dog. 592

screech. 593
Oxyechus vociferus. 590
Oyster drill. 241

extinct. 854

shell, 771
Oysters, 255 '

Pacific shore animals, 774
Paddlefishes, 446
Pagurus, 282

hermit crab. 772
Palaearctic region. 712
Palatine bones. 517
Paleontology, 854
Paleozoology, 24
Pallial line, 249, 250
Palpigradi, order, 266
Palps, 197

labial, 250
Palpus, 305
Pancreas, 490, 677
Panaorma. To
Panther. 629
Pantothenic acid, 390
Papillae, filiform, 603

fungiform, 650

vallate, 650
Papula, 226

Parabacillus coloradus, 318
Paragastric canals. 158
Paragonimus, 161, 760
Paragordius, 178
Parakeets, 591
Paramecium, 70,93, 100

locomotion. 102

metabolism. 95

reaction to temperature. 100

reproduction, 96. 98

structure. 93
Paramylum. 83
Paramj-xine. 412
Parapodia. 196
Parapophyses. 465
Parapsida, subclass. .547
Parascalops breweri, hairy-tailed mole,

619
Parasite, accidental, 736

occasional, 736
Parasites, facultative, 737

their transmission, 740
Parasitism, 735

Parasphenoid bone, 465, 495, 518
Parathormone, 670
Parathyrin. 670
Parathyroid glands, 670
Parenchyma, 105
Parietal bone, 465, 494
Parotoid gland, toad, 540
Parrots, 591

Parthenogenesis, 112, 191. 410
Parthenogonidium. 76
Passalidae. family. 335
Passalus cornutus. 335
Passenger pigeon, 591. 786
Passeriformes. order, 595
Pasteur, 47
Patella, 240. 293. 644

larva. 237

940

INDEX

Pathology, 23

Paurometabolous, 308

Pavlov. S52

jr'tjaris, ai tifical, 255

Peas, dwarf. 822

Pecari angulatus, javelina. 632

Peccary, 630, 632, 787

Pecten, 357, 608

Pectinatella, 186, 187

Pectine, 298

Pectoral girdle, toad. 542

muscles, 524
Pedal ganglion, 245
Pedicellariae, 218. 226. 229
Pediculati, suborder, 455
Pediculus, louse, 738
Pedigree chart. 843
Pedipalpi, 292

order, 266
Peduncle, 187
Pelagic animals, 769
Pelecaniformes, order. 587
Pelecypoda, 256
Pellicle, 94

Pelmatohydra oligactis, 144
Pelobatidae, family, 485
Pelvic girdle, frog, 521
Pemphigus betae. 318
Penguins. 586
Penial setae. 181
Penis, 246, 261
Pennatula, 144
Pennatulacea, order, 143
Pentacrinus. 225
Pentastomoids. 292
Pentatomidae. family, 318

Pepsinogen, 383
Peranema. 66

Perca flavescens, 454. 457

Perch. 411. 454

internal structure, 460
skull, 467
yellow, 457

Percidae, family, 454

Percomorphi, suborder, 452

Pereiopods. 270

Pericardial cavity, 376
sinus, 272, 274

Pericardium. 567, 604, 649
of frog. 510

Perichondrium, 691

Pericolpa, 137

Periganglionic gland, 528

Perihemal space, 229

Perilymph. 530

Perioral membrane. 227

Periosteum, 378, 644

Periostracum. 249

Peripatus. 265. 287, 865

Periphylla, 137

Periplaneta americana, 315

Periproct. 221

Perisarc, 132

Perissodactvla, order, 633

Peristalsis, 382

Peristaltic contractions. 151, 196

Peristome. 227

Peristomium. 197

Peritoneum. 376

Peritrichida, 71

Perivisceral coelom. 229
space. 274

Perla modesta. 321

Peromedusae, order, 137

Perophora, a tunicate. 772

Pessulus. 604

Petrels. 5S7

Petromvzon. 413
marinus, 415

Petromyzontla, subclass, 412

Petropedetes newtoni, 480

Phacus. 66

Phagocytosis, 395

Phalanges, 611

Phalangida, order, 266, 295. 299

Phalanx. 611

Phalaropes, 591

Pharyngeal clefts, 360

Pharyngobranchial, 428

Phasmidae, family, 318

Pheasants. 590

Phenotype, 826

Phidippus workmanii, 296

Philodina, 190

Philomycus carolinensis, 240

Pliocaena phocaen i, porpoise, 634

Phoenicopteridae, family, 588

Phoenicopturus ruber. 588

Pholcidae, laniily, 2^7

Pholcids. 297

Pholcus phalangoides, 297

Pholidota, order, 621

Phoronidea, 27

Phosphorescent organs, 771

Photoelectron. 707

Photons, 698

Photosynthesis, 28, 29, 700

Phototropism. 77

Phrynosoma, 552

circulatory system. 566
I digestive system, .563
ear, 574

muscular system, 576
nervous system. 573
respiratory system. 566
skeleton. 574
skull. 575

Phthirius pubius. crab louse, 738

Phyllophaga, 354

anxia. 355
Phylogenetic advances, arthropoda. 286
relations of animals, 863

Phylogeny. 23. 285

Physa anatina. 238
eggs, 247
gyrina. 237
humerosa. 238

Physalia, 136, 137

Physeter catodon, sperm whale, 634

Physiology, 22

Physoclisti. 447

Physocyclus, 297

Physostomi. 447

Phytonomus posticus. 324

Pia mater, 526, 650

Piciformes. order. 594

Pickerels. 450

"Pigeon milk," 591, 602

Pigeons, 591

Pigment cells, 474

Pigmentation of skin, 674

Pigs, 630

Pikas, 627

Pikes 450

Pilidium larva. 162. 163

Pill bug, 264. 265

Pine scale. 320

Pineal body of fish, 469

Pinna. 639

Pinnipedia, 628

Pinnules. 224

Pinworm, 176

Pipa. Amazonian frog, 480

Pipefish, 451

Pipunculus subvirescens, 332

Plsaster, 218
ochraceus, 227

INDEX

941

Pisces, 442

circulatory system, 459
class, 411, 445
economic relations, 455
muscles, 467
Pitocin, 678
Pitressin, 678
Pituitary body of frog, 526

gland, 672
Pituitrin. 674
Placenta, 653, 819
of armadillo, 622
Placoid scales, 422, 427
Placula. 807, 809 .
Plagioctenium irradiens, pecten, 7/1
Planaria, 160, 163
metabolism, 170
regeneration, 173
reproductive system, lbs
Plankton, 769
Plant lice, 318
Plantigrade gait, 641
Planula, 128, 134

larva, 139, 140
Plasma, 108, 205, 392
of blood, 503
membrane, 59, 60
Plasmagel, 86
Plasmalemma, 86
Plasmasol, 86
Plasmodium, 79, 745
falciparum, 74
life cycle, 749
malaria parasite, 748
malariae, 74
vivax, 74
Plasmosome, 61
Plastron, 578
Platyhelminthes, 26, 745
economic relations, 174
pliylogenetic advances, 174
phylum, 160
Platyphylax designata, d22
Platypodidae, family, 335
Plecoptera, order, 265, 321
Plectognatlii, suborder, 455
Pleopods, 270
Plethodon cinereus, 479

glutinosus, 472
Plethodontidae, family, 484
Plethodontids, 472
Plethopsis wrighti, 485
Pleura, 649
Pleural ganglion, 245
Pleurobrachia bachei, 15 <
Pleurobranciiiae, 271
Pleurocera, 245, 258
Pleurodira, suborder, 547
Pleuron, 268. 347
Pleuronectidae, family, 452
Pleuroperitoneal cavity, 430
Pleurum, 306
Pliny, 36
Pliohippus, 876
Plovers, 591
Plumatella, 186, 187
Pneumatic duct, 463
Pneumatophore, 135, 136
Podical plates, 348
Podobranchiae, 271
Podophrya, 72
Poduridae, family, 312
Poeciliidae, family. 451
Poikilothermous condition, 545
Polar body, 115
ingression. 810
"Polian vesicles, m, ^^^
Polistes, 338

Pollen basket. 357
brush, 357
combs, 357
PoUex, 563, 611
Polvaxon spicule, 125
Polybia, 338
Polychaeta, 194, 196
Polycladida, 160
Polydactylism, 659
Polydactyly, 837
Polydesmidae, family, 290
Polydon spathula, 448
Polygordius, 212, 213
Polygyra dorfeuilliana, 240
roemeri, 239
texasiana, 240
Polyhybrid cross, 825
Polynemidae, family, 453
Polyp, 130. 131
Polypide. 184
Polypteridae, family. 44fa
Polypterus. 446
Polypus. 258

bimaculatus, 257
Polystomella crispa. 68
Polyzoa. 184
Pompano. 454. 7i3
Populations. 730
Porcellio. 282
Porcupine. 622. 625

fishes. 455
Porifera, 26

phylum, 119
Porocyte, 126
Porospora gigantea, 59
Porpoise, 634

Portuguese man-of-war, idb
Postcaval vein of frog, 508
Postorbital crest, toad, o4U
Potamobius, 266
Potato psyllid, 318
Prairie chickens, 590

dog, 623 . ^ ^^^
Precocial bird, 615
Prehallux, 522
Prepartoid crest, toad, 540
Prepollex. 522
Prezygapophyses, 641
Prickleback, 321
Primates, order, 635
Primordial germ cell, lid
Pristis pectinatus. 424
Proboscidea order 634
Proboscis, 161, 164. 179, 197, 362
Procellariiformes, order. 587
Procyon lotor. coon, hlv, /o5
Progesterone. 676
Progestin. 676
Proglottids, 161. 761. 763
Progynon, 676
Pronotum. 345
Pronuclei, 800
Pronucleus, 116
Pro-otic bone, 494, 517
Prophase, 62, 63
Proptera purpurata, 248
Propterygium, 430
Prorodon, 71
Prosobranchiata. ^o8
Prosopyles. 124
Prosostomata, order, Ibl
Prostate gland, 653
Prostoma rubrum, 162
Prostomium. 197 198, 200 212, 21o
Protandrous condition, 245
Protease, 384
Proteida, suborder. 48o
Proteins, 56

942

INDEX

Proteoses, 385
Proteroglypha, 556
Proterospongia, 119
Prothorax, 305, 345, 357
Prothrombin, 395
Protobranchiata, 256
Protogynous condition, 245
Protons, 697

Protopipa, Amazonian frog, 480
Protoplasm, 49

and the cell, 49

characteristics, 53

chemical nature, 56

fundamental activities, 54

physical nature. 55

structure, 53
Protopodite, 270
Protopterus, 455
Prototheria, subclass, 616
Prototracheata, division, 265
Protozoa, 25

classification, 65

colonial, 75

economic relations, 77

phylum of, 65
Protractor muscle. 249
Proventriculus, 601
Prozoea, 285
Psalterium, 633
Pseudacris, 486

ocularis, 473

streckeri, 482
Pseudemys, 549

troostii elegans, 577
Pseudobranchus stria tus, 485
Pseudococcus maritimus, 316, 318
Pseudocoel, 181

Pseudogryphus calif ornianus, 589
Pseudolamellibranchiata, 257
Pseudophyllidea, order, 161
Pseudopodia, 67, 70, 86
Pseudopodiospore, 90
Pseudoscorpionida, order, 266
Psittaciformes, order, 591
Psocids, 333

Psychichthys afflnis, 425
Psyllid, 316

Pteronarcella bodia, 321
Pteroplatea micrura, 425
Pteropods, 258
Pterosauria, 546
Pterotic bones, 466
Pterygoids, 465, 517, 643
Pterygoquadrate, 465
Pterygota, subclass, 309, 313
Pterylae, 601
Pubis, 521

Pulmocutaneous arch, 507
Pulmonary veins of frog, 510
Pulmonata, 258
Pulmonate snails, 238
Pulvillus, 347
Pulvinaria vitis, 318
Puma, 629
Pupa, 330

Pupil of frog eye, 529
Pupipara, 331
Pupoides marginatus, 240
Purkinje, 49
Pycnogonids, 292
Pygidium, 316
Pygostyle, 582, 609
Pylangium, 511 /
Pyloric caeca, 228, 229, 459

chamber, 272
Pyralididae, family, 327
Pyrenoid bodies, 81
Pyridoxine, 389

Quadrate bone, 465, 467
Quadratojugal bone, 517
Quadratopterygoid, 428
Quadroon, 829
Quadrula forsheyi, 248
Quadrupeds, 616
Quadruplets, 621, 622
Quail, 590, 794
Quantum, 698

Rabbits, 625

cotton tail, 627

jack, 625, 626

swamp, 627
Raccoon, 629, 788
Race runners, 554
Racemose bodies, 230
Rachis, 601
Radial canals, 124

cleavage, 802

nerve, 229
Radiata, 217
Radiations, 697
Radio waves, 701
Radioactive atom, 698
Radiolaria, 69, 78
Radium radiation, 708
Radula of snail, 243
Rails, 591
"Rain crow," 591
Raja erinacea, 424
Rana catesbeiana, 497

grylio, 497

pipiens, 486
Range of wild animals, 789
Rangifer caribou, 630
Ranidae, family, 486
Rapliidiidae, family, 322
Rat, Banner-tail kangaroo. 624
Ratitae, subclass, 584
Rats, 622, 625
Rattlesnakes, 556, 559
Rattus, 625
Ray, butterfly, 424
Rays, 424
Reason, 853

Recapitulation tlieory, 284, 869
Recessive, 45
Rectal caeca, 229, 233
Rectus abdominis, 524
Red corpuscles, 395
Rediae, 758, 759
Red-legged locust, 310
Reduction division, 114
References, bibliographic, 889
Reflex arc, 405

behavior, 850

circuit, 400
Reflexes, 852
Regeneration, 681

earthworm, 211

in amphibia, 477

in chordata, 688

in hydra, 156

in planaria, 173

in platyhelminthes, 683

of starfish, 234
Regenerative capacity, 681
Reichenbachia, 340
Reindeer, 630

Relations of animals and plants, 27
Remoras, 454
Renal corpuscles, 650

portal system, 509
vein, 434, 491

INDEX

943

Reniceps, internal anatomy, 440

tiburo, 422, 439
Renilla, 144
Rennin, 385
Reproduction, 54 ^ , oo .

and life cycle, starfish, 234 ^

and life history, chicken, bio

asexual. 111, 153

in cat, 652
Reproductive function, 4U(

system, frog, 531
Reptiles, classification, 54*

flying, 546

fossil, 546
Reptilia, 411

class, 545
Resources, nonrenewable, 7 9 4

renewable, 794
Respiration, aerial, 392
Respiratorj' system, 110
chicken, 603
frog, 514
phrynosoma, obb
vertebrates, 390
tree, 223

Rlltomtion^'of^ natural resources, 794

Rete testis, 572

Reticulitermes hesperus, 3^0

Reticulum, 633

Retina, 401, 529 -

Retinella indentata paucilirata, 24U

Retinula, 276

Retortamonas, 67

Retractor muscles, 224, 2^0

Rhabdamina, 69

Rhabdocoelida, 160

Rhabdom, 276, 277

Rhabdopleura, 361, 364 „,,,^i„ ro.

Rhachianectes glaucus, gray whale, 634

Rhagon, 124, 125

Rheas, 585

Rheiformes, order, 585

Rheotropism, 77

Rhincocephalia, order, 560

Rhinoderma, frog of Chile, 480

Rhizocrinus, 225

Rhizopoda, 67 ,. ^. .„ ,

Rhomaelia microptera. digestive system

350
reproductive system, 353
Rhopalocera, suborder, 325
Rhyacotriton olympicus, 484
Rhynchites bicolor, 324
Rhynchocephalia, 869

order, 548
Riboflavin, 389
Ricord's frog, 472
Ring canal, 223, 230
Roaches, 313
Robber flies, 330
Robins. 595
Robler frogs, 40
Rock rollers, 321 ,^ , . „„

Rocky Mountain spotted fever, SO

Rodentia, order, 621

Roe of starfish, 235

Roentgen, 698
radiation, 702

Roman scholars, 36

Ross, Major Ronald, 79

Rostellum, 763

Rostrum, 268, 427

Rotifers, 27, 188

Roundworms, 26, 175

Rugae, 564

Rumina decoUata, 238

Ruminants, 632

Sable, Russian, 638

Sacculina, 265, 284, 74 (, 768

Sacculus, 403, 530

Sacrum, 380

Sagartia, 140

Sagebrush formation, t ^t

jack rabbit biome, 726
Sagitta hexaptera, 192
Sailfishes, 453 ,^ , , „,„„^

Salamander, blind cave (Typhlomolge),

482
Salamanders, 411, 472
Salamandra salamandra, 4i9
Salamandroidea, suborder, 484
Salientia, order, 485
Salinity of ocean, 767
Salmon, 447, 448
Salmonidae, family, 448
Salpa, 361 .
Salticus senicus, 296
Salts, inorganic, 58
Sand cricket, 315
dollar, 222
fleas, 265
Sandpipers, 591
Santonin, 183
Saprophytic, 66

nutrition, 82
Sarcode, 49
Sarcodina, 67

Sarcolemma, 106 „

Sarcoptes scabiei, itch mite, i6b
Sarcosporidia, 73
Sartorius, 611
Sauria, 551
Saw-fish, 424
Scale, ctenoid, 444, 464
cycloid, 444
ganoid, 444
insects, 266
San Jose, 316
Scalopus aquaticus, mole, 619
Scaphiopus, 485

couchi, 475
Scaphognathite, 270
Scaphopoda, 258
Scapula, 465, 495
Scarabaeidae, family, 335
Sceloporus, 552
Sceroblasts, 125

Schistocerca, shoshone, 310 ^
Schistosoma, blood fluke, 745, <•>&
Schizogony, 73
Schizont, 750
Schizopod, 285
Schleiden, 43. 50
Schneider, 62
Schwann, 43, 50, 51
Sciaenidae, family, 454
Sciatic vein of frog. 509
Scientific method, 18
Scincidae, family, 555
Sclera, 401

of frog eye, 529
Sclerites, 301, 305, 345
Scolex, 161, 761, 762, 763
Scolopendra, 291
Scolytidae, family, 335
Scomberomorus maculatus, 4i)^
Scorpion flies, 266, 333
Scorpionida, order. 266, 295
Scorpions, 266, 298
Screech owl, 593
Scrombridae, family, 453
Scrotum of cat, 653
Scudderia, 315

944

INDEX

Scutigera, 290
Scypha, 121, 122

anatomy of, 123
Scyphomedusa, 137
Scyphozoa, class, 137
Sea butterflies, 258
cows, 634
cucumber, 223
eg-gs, 235
horse, 443, 451
lion, 629
robins, 454
slug, 241
walnuts, 26, 157
Seal, 629

Seasonal changes, 733
Sebaceous glands, 616
Secondary sexual characters, amphibia,

480
Secretin, 385, 501
Sedge-musk ox biome, 724
Segregation, 824
Selachii, subclass, 422
Self, J. Teague, 194
Semicircular canals, 437, 529, 608
Semilunar valves, 459, 510
Seminal groove, 200, 209
receptacles, 202
vesicle, 437, 614

frog, 531
vesicles, 202
Sense organs of frog, 528
Septae, 201
Serial homology, 271
Serosa, 818

Serous membrane, 649
Serpentes, 556

suborder, 548
Serranidae, family, 454
Serum, 393

antihuman, 884
Sesamoid bones, 641
Setae, 195, 196, 200, 201
Sex hormones, 675

inheritance of, 831
Sexual reproduction, 408

development of. 111
Shagreen, 425

Shark, nervous system, 436
pilots, 454
skeleton, 427
urinogenital system, 437
visceral skeleton, 428
Shark-liver oil, 456
Sharks, 411, 422
Sheep, 630

Rocky Mountain, 630
Shikepoke, 587, 588
Ship worm, 241
Shore animals of Pacific, 774
Shovelnose shark, 422
Shrews, 619
Shrimps, 265
Siamese twins, 659
Sibbaldus musculus, whale, 634
Silphidae, 324
Silver moth, 265, 310
Silverfish, 310
Silversides, 452

Simia satyrus, orangutan, 637
Simiidae, family, 637
Simulium vittatum, 332
Sinistral, 240
Sinu-atrial valve, 490
Sinus venosus, 459
Siphlurus occidentalis, 321
Siphonaptera, order, 266, 329

Siphonoglyphe, 141, 142. 143
Siphonophora, order, 136
Siphonophore colony, 135
Siphons, 222, 365
excurrent, 250
exhalant, 252
Siphuncle, 259
Sipunculoidea, 27, 194, 215
Siren lacertina, 473, 485
Sirenia, order, 634
Sirenidae, family, 485
Sistrurus, 559
Skates, 424
Skeletal system, 108
chicken, 609
fish, 464
frog, 517
phrynosoma, 574
Skeleton, appendicular, of fish. 465
of Ameiurus, 466
of cat, 640
of chicken, 610
of shark, 427
of toad. 543
of turile, 580, 581
Skin, frog, 475

human, 376
Skinks, 555
Skull, frog, 517
perch, 467
phrynosoma, 575
Skunk, 638

Sleeping sickness, African, 78
"Sliders," 549
Slimy salamander, 479
Sloths, 621
Slug, 323
Smelts, 448

Sminthuridae, family, 312
Snail, 237

egg masses, 247
Snails, 257

pulmonate, 238
terrestrial, 240
Snake eels, 449
Snake, harlequin, 558

pit vipers, 559
Snakes, 411, 548, 556
coral color bands, 558
lyre, 556
worm, 557
Snappers, 454
Snipes, 591
Social life, insects, 334

relations of animals. 735
Sol state, 54
Solaster, 218

Solenogiypha, 556 ^

Soles, 451

Solitary wasp, digestive tract, 358
Solpugida, order, 298
Somatic cells, 104

mesoderm, 53 6
Somatoplasm, 105
Somatopleure, 815, 817
Sonoran zone, lower, 716
Sorex personatus. long-tailed shrew 619
Sound production in mammals. 616
Sow bugs, 265
Spadefoot toad, 475
"Spanish fly." 341
Sparrows, 595
English, 596
Speotyto cunicularia, 592

hypogaea, 593
Spermatid, 114, 115
Spermatogenesis, 113
Spermatogonia, 799

INDEX

945

Spermatophores, 171, 493
Spermatophoric sac, 2 62
Spermatotheca, 246, 353
Spermatozoa, 104, 115, 407
Sphaeiella, 66
Sphaeriidae, 256
Sphenethmoid bone, 518
Sphenisciformes, order, 586
Sphenodon, 546, 548, 869

punctatum, 560
Sphenodontia, suborder, 548
Sphincter, 644
Sphyraenidae, family, 453
Spicules, 121. 125

types of, 125
Spider, digestive system, 293
Spiders, 266, 292
crab, 295
comb-footed, 295
orb- web, 296
Spinal cord, frog, 527
Spiny dogfish, circulatory system, 4d2
Spiracles, 349, 422, 427
Spiral valve, 422
Splanchnic mesoderm, 536
Splanchnopleure, 815
Splenial bones, 495
Sphnts, 874

Sponge, histology of, 126
metabolism, 127
reproduction of, 127
Sponges, 26, 119

diagrams of different types, 124
economic relations, 129
fresh \Yater, 121
phylogeneuc aavances, 129
Spongilla, 121
Spongin, 125
Spongioblasts, 125
Spongioplasm, 60
Spongovostox apicedentatus, 320
Spoonbills, 446
Sporoblasts, 74
Sporocyst, 756, 758, 759
Sporogony, 73
Sporont stage, 750
Sporosac, 135
Sporozoa, 111

class, 72
Sporozoites, 74
Sporulation, 74, 90, 749
Springtails, 265, 312
Spruce-moose biome, 725
Squalus acanthias, 422

internal anatomy, 429
Squamata, order, 547, 551
Squamosal bone, 494, 518
Squamous epithelium, 107
Squash bug, 318
Squid, dissection of, 261

eye of, 262
Squirrels, 623
Stapes, 403

Stapliylinidae, family, 340
Starfish, 226

development, 232, 234
metamorphosis, 232
regeneration, 234
Starling. 595

Statoblasts, 187 „„ „„„

Statocysts, 121, 133, 159. 253, 262, 276
Statolith, 159, 253
Stauromedusae, order, 137
Steapsin. 385, 502
Stegocephalia. 869
Stegomyia. 80
Stem-mother, 112

Stenopelmatus fasciatus, 312
Stenostomvim, 160
Stentor. 7 0. 71

regeneration, 681
Stereoblastula, 807
Sternal artery, 272
Sternebrae, 644
Sternum. 268, 306. 345
Sticklebacks, 451
Stigma, 81
Stigmata, 366

Stimuli, classes, 848 '

Sting, 357

ray, 423, 456
"Stingaree," 425
Stipes, 345

Stomach, ruminant, 633
Stomodeum, 140, 158
Stomolophus, 138
Stone canal, 221, 230, 231

flies, 265, 321
Strawberry leaf roller, 326
Strecker's frog, 482
Strepsiptera, order, 265. 333
Streptostylic condition. 547
Strigiformes, order, 593
Strix varia, 593
Strobilization, 140

Strobilops labyrinthica texasiana, 240
Strobilus, 763
Strobula, 139. 140
Strongylocentrotus. 222
Strongyloidea, order, 176
S.rongyloides stercoralis. 176
Struggle for existence. 885, 886
Struthio camelus, 584
Struihioniformes, order, 585
Sturgeons, 446
Sturnus vulgaris, 595
Stylommatophora, 258
Stylonychia. 71
Stylopids, 265

Subclavian veins of frog, 508
Subesophageal ganglia, 275
Subgenital plates. 348
Sublittoral zone. 713, 770
Submentum, 345

Subpharyngeal ganglion, 199, 208
Subumbrelia, 132
Succession. 727
Succinea avara, 240
Suckers, 450
Suctoria, 70, 72
Suidae, family, 632
Sunfishes, 454

Sunlight, biological effects, 700
Superciliary crest, toad, 540
Supraclavicle, 465
Supracondyloid fossa, 640
Supraoccipital bone, 465
Suprapharyngeal ganglia, 199

ganglion. 208
Suprarenal glands, 671
Suprascapula of frog, 521
Surra, 80 „„. „„„

Survival of the fittest, 88o, 886
Sustentative tissue, 106
Suture, 343, 641
Swans, 589
Sweat glands, 616
Swifts, 593
Swimmerets. 270
Swimming clam, 284
Sycon, 124, 125

Sylvilagus. cotton tail rabbit, bZ7
Symbiont, 340
Symbiosis, 144, 735

946

INDEX

Sympathetic nerves of frog, 526

nervous system, 528

system, 399
Sympetrum rubicundulum, 321
Symphysis, pubic, 576
Symplectic bone, 467
Synangium, 511
Synapse, 406
Synapsis, 1U7, 114
Synch^onlidae, family, 450
Synentognathi, suboi'der, 451
Syngamus trachea, 176
Syrinx, 603

Syrphidae, family, 330
Syrphids, 330
Syrrhophus, 485
System. 108
Systemic arch, frog, 506

veins, 508
Systole, 396

Tabanus phaenops, 331

Tachardia lacca, 341

Tachinidae, family, 330

Tachinids, 330

Tachyglossus aculeatus, 617

Tadarida mexicana, 620

Taenia, 161
pisiformis, dog tapeworm, 761
saginata, beef tapeworm, 741, 742
serrata, dog tape, 764
solium, pork tape, 745, 763

Taeniolae, 137

Tailed amphibia, 484

Tantilla, 556

Tapeworm development, 762 764

Tapeworms, 762

Tapirs, 633

Tarantula, 266, 296

Tarpon, 447

Tarsals, 495

Tarsiers, 635

Tarsometa tarsus, 612

Tarsus, 293, 347

Taste buds of flsh, 470

Taxidea taxus, badger, 628

Taxis, 77

Taxonomy, 21

Tayassuidae, family, 630

Tegmina, 347

Teiidae, family, 554

Teleostei, order, 447

Teleostomi, subclass, 446

Telophase, 62, 64

Telosporidia, 73

Telson, 268

Tendons, 106, 524. 644

Ten-pounders, 447

Tentacles, 131, 133, 137, 147, 197

Tentaculata, 157

Tentaculifera, 72

Tentaculocysts, 137

Teratology. 654

Teredo navalis, 241, 255

Tergum, 268, 306, 347

Termites. 265, 320

Termitophiles, 339

Termopsis nevadensis, 320

Terns, 591
Test, 219

Testes. 572

of cat, 653
Testicle, 653
Testosterone, 676
Testudinata, order, 547, 548
Testudinidae, family. 550

Tetany. 670

Tetrabranchiata, 258

Tetragnatha laborlosa, 296

Tetranychus telarius, 298

Tetraphyllidae, order, 161

Tetrastemma, 163

Tetraxonida, order, 121

Tettigonidae, family, 315

Texas fever, 80

Thalarctos maritimus, polar bear, 629

Thaliacea, 361

Theca, 812

Theelin, 676

Thenea. 121

Theory, 18
of evolution, 863
of recapitulation, 397
Theridiidae, family, 295
Thermotropism, 77
Theromorjiha, 869
Thiamin, 389
Thigmotropism, 77
Thomisidae, family, 295
Thoracostel, suborder, 451
Threadflns, 453
Threadworms, 26, 175
Threshold stimulus, 77
Thrips, 265, 320

tabaci. 320
Thrombocytes of frog, 503
Thunnidae, family, 454
Thymallidae, family, 448
Thymallus tricolor, 448
Thymus gland, 675
Thyone, 222, 224
Thyroid gland, 110, 667
Thyrotropic hormone, 678
Tliyroxine, 668

Thysanoptera, order, 265, 320
Thysanosoma, 765
Thysanura, order, 265, 309
Tibia, 293, 306, 347
Tibioflbula of frog 52"'
Tibiotarsus, 612
Tick, sheep, 330
Ticks, 266, 298

Tiedemann's bodies, 230, 231, 233
Tiger, 629

beetles, 324

salamander, 484

ambystoma tigrinum, 480

shark, 422
Time scale, geologic, 872
Tmamous, 586
Tingitidae, family, 318
Tissue, 105

epithelial, 105

muscular, 106

nervous, 107

sustentative. 106

vascular, 108
Toad, arteries, 542

Common, 487

digestive system, 541

embryology, 544

marine, 473

muscles, 543

narrow-mouthed, 486

nervous system, 543

obstetrical, 480

spadefoot, 475

skeleton, 543

urinogenital system, 541

vascular system, 542

veins, 542
Toadflshes, 455
Toads, 411, 472, 538
Tocopherol, 390

INDEX

947

Tomato fruitworm, 326

Tongue flsh, 451

"Tornaria" larva, 364

Totipalmate swimmers, 587

Toxocara canis, 176

Toxopneustes, 800

Tracheae, 110, 603

Tracheata, section, 265

Trachelmonas, 66

Trachinotus carolinus, pompano, 773

Trachvlina, order, 136

Tramatodes, 160, 161, 174, 755

Transitional zone, 715

Transmission of parasites, 740

Trapezius muscle, 427

Trematoda, 161

Trepang, 235

Triatoma, 79

Triaxon, 125

Trichechus latirostris, manatee, 634

Tricliina, 176, 745, 753

Trichinella, 175, 176, 745, 753

Trichinelloidea, order, 176

Trichocysts, 94, 102

Trichomonas, 67

Trichoptera, order, 266, 321

Tricladida, 160

Trigeminus nei-ve, 527

Triggerflshes, 455

Triglidae, family, 454

Trihybrid cross, 826

Trimorphodon, 556

Trionychidae, family, 550

Trionychoidea, suborder, 547, 550

Triploblastic embryo, 810

Tripneustes, 222

Triradiate spicule, 125

Triturus, common newt, 484

Trivium, 227, 228

Trochanter, 293, 306, 347

Trochelminthes, 27, 184

Trochlea, 576

Trochlearis nerve, 527

Trochophore larva, 188, 192, 199, 213,

237, 865
Trochospongilla, 121
Trophoblast, 814
Tropical zone, 716
Tropicorbis liebmanni, 238
Tropism, 77, 849
Tropisms in hydra, 146

in planaria, 164
Tropistic behavior, 849
Trout, 448

eggs and embryos, 470
Truncus arteriosus, 504
Trypanorhyncha, order, 161
Trypanosoma, 79, 743
Trypanosomes, 80
Trypetidae. family, 330
Trypsin, 502
Trypsinogen, 385
Tsetse fly, 744
Tuatara, 560
Tube foot, 221
Tuberculum, 495
Tubularia, 131
Tundra formation, 724
Tunic, 365
Tunicata, 361
Tunnies, 454
Turbellaria, class, 160
Turbinated bones, 603
Turkev vulture. 588
Turkeys, 590

wild, 795

Turtle, circulatory system, 580

digestive system, 578

leatherback, 550

loggerhead, 550

mud, 548

painted, 549

pond, 577

respiratory system, 579

skeleton, 580. 581

snapping, 549

Troost's, 577

urinogenital system, 581
Turtles, 411, 545, 548
Twins, conjoined, 659
Tympanic membrane, 403. 562, 604
Tympanum, 498, 530, 574
Typhlomolge rathbuni. 4 82
l^phlosole. 204
Tyrannosaurus, 546

Uca, 282

fiddler crab, 772
Ultraviolet, 699
Umbilical cord, 820
Umbilicus, inferior, 601
Umbo, 249

Umbridae, family, 450
Uncinate processes, 609
Undulating membrane, 94
Unionidae, 256
Unit characters, 823
Upper Sonoran zone. 715
Ureter. 398, 572, 606
Ui-eters of cat, 650
Urethra, 653

of cat, 650
Urinary bladder, 490. 650
Urine. 650

Uriniferous tubules, 398. 464, 515
Urinogenital system, 541
horned lizard, 571
turtle, 581
Urochoi'da, subphylum, 361, 365
Urodela, order, 484
Uroglena, 67
Uropods, 270
Uropygial gland. 600
Urosalpinx. 241

cinerea. 255
Urostyle, 520

Ursus horribilus. grizzly bear. 629
Uterus of cat, 652
Utriculus, 403, 437

frog ear, 530

Vagina of cat, 653

Vagus nerve, 527

Vallate papillae, 650

Valve, semilunar. 511

van Leeuwenhoek. 41

Vane, 599

Variation, 864

Vas deferens, 572, 613, 653

Vasa efferentia, 531

Vascular system, functions, 110

tissue, 108
Veins of frog, 508

of horned lizard, 570

of toad, 542
Vejovidae, family, 298
Vejovis mexicanus, 298
Ventral abdominal vein, 491, 509
Ventricles of brain, 526
Ventriculus, 601

948

INDEX

Venus's comb, 241

flower basket, 120, 121

girdle, 157
Vermetus spiratus, 240, 241
Vermiform appendix, 878
Vermis, 650

Vertebra of frog, 519, 520
Vertebral column, 378
cat, 643
frog, 520
Vertebrata, classification, 410

digestive system, 382

subphylum, 362, 375
Vertebrate circulatory system, 392

excretory system, 396

skeleton, 378
Vesalius, 21, 37
Vespidae, family, 338
Vespula, 338

pennsylvanica, 333
Vestibule, 608
Vestigial structures, 878
Vibracula, 185
Vibracularia, 185
Vibrissae, 639
Villosities, 476
Vinegarroon, 266
Virchow, 64
Viscachas, 625
Viscera, 109
Visceral ganglia, 245

skeleton, fish, 465
frog, 519
Vitamin D, 390

K, 390
Vitamins and their functions, 388
Vitellarium, 190, 191
Vitelline cells, 171

membrane, 115, 532, 819
Vitellophages, 807
Vitrella, 276
Vitrina glacialis, 237
Vocal cords, 649
Voice of amphibia, 478

box of frog, 514
Voluntary muscle, 524
Volvox, a colony, 76, 77
Vomerine teeth, 500
von Baer, K. E., 23, 43, 284
von Mohl, 49
Vorticella, 70, 71
Vulpes fulva, red fox, 628
VuKure, 589

California, 589

Walking legs, 270
Wallace, Alfred R., 711
Walrus, 629
Wasps, 266, 331

solitary, 335
Water, 58

dog, 487

fiea, 264

moccasins, 556, 559

striders, 318
Water-vascular system, 230, 231
Wattles, 600

Wave radiations chart, 699
Waxwings, 595

Weevils, 265, 324, 325
Welsmann, 47
Whale, killer, 634

sperm, 634

sulphur-bottom, 634

toothed, 634
Whalebone, 634
Wheel animalcules, 188
Whipworm, 176
White corpuscles, 513
Whiteflshes, 448
Wild turkey, 795
Wildlife conservation, 784
Wolff, 798
Wolf-snout, 656
Wolves, 629
Woodchuck, 622
Woodcocks, 591
Woodpeckers, 594

downy, 595

red-headed, 595
Worm lizard, 553

shell, 241
Wrens, 595
Wuchereria bancrofti, fllaria, 745, 746

Xantusiidae, family, 554
X-chromosome, 831
Xenodusa, 340
Xerophthalmia, 389
Xiphisternum of frog, 521
Xiphoid process, 644
Xiphosura, order, 266, 295, 299
X-rays, 703
Xysticus nervosus, 295

Yellow-jacket, 333
Yolk glands, 169, 191

plug, 535

sac, 819

Zalophus californlanus, sea lion, 629

Zebra, 633

Zenaidura macroura, 591

Zoantharia, subclass, 140

Zoea. 285

Zona pellucida, 816

Zonitoides arboreus, 240

Zooecium, 184, 187

Zoogeographical regions of Wallace, 712

Zoogeography, 20, 24

Zooid, 135

Zooids, 184

Zoology, 19

history of, 36

subdivisions of, 19
Zophodia grossulariae, 327
Zoraptera, order, 333
Zorotypus, 333
Zygapophyses, 495, 520. 643
Zygodactyly, 837
Zygonectes notatus, 451
Zygoptera, suborder, 321
Zygote, 111, 115, 134, 139, 493, 824
Zymogen, 383

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