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103 



INSTRUCTOR'S COPY 



Compliments 
of 

THE C. V. MDSBY COMPANY 

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



SENT AT THE REQUEST 
OF 

MR. FRANK A. VOLK 



Your opinion of this book will be 

appreciated when your review of tt 

has been completed. 



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TEXTBOOK ^ 

OF 



ZOOLOGY 



BY 



GEORGE EDWIN POTTER, Ph.D. 

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



SECOND EDITION 



With 445 Text Illustrations 



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ST. LOUIS 

THE C. V. MOSBY COMPANY 

1947 



Copyright, 1938, 1947, By The C. V. Mosby Company 
(All rights reserved) 



Printed in U. S. A. 



Press of 

The C. V. Mosby Company 

St. Loviis 




DEDICATED TO 
PROFESSOR FRANK A. 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. 

9 

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. 

I 



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. 

17 



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 



19 



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- 



20 



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 



21 



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 



23 



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 



25 



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

Classification of the Animal Kingdom 

Very few people realize how many different kinds of animals 
there are and how greatly they vary in size, structure, ajid habits 
of life. The estimated number of kinds is all the way from 1,000,000 
to 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 



27 



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 



28 



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/ 




Sl l /~^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 



29 



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, 



32 



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. 



I 



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 

36 



HISTORY OF ZOOLOGY 



37 



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

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 



38 



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 



39 



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 



40 



TEXTBOOK OF ZOOLOGY 



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



41 



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 



42 



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. 



44 



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 



45 



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. 



46 



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 



47 



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 

49 



50 TEXTBOOK OF ZOOLOGY 

the nature and appearance microscopically of several organs of the 
body; Grew had made rather extensive microscopic studies of plants, 
and in 1831 Kobert Brown had discovered the nucleus of the cell, 
but not until the work of Schleiden in 1838 and Schwann in 1839 
was the cell theory formally enunciated. The former a botanist and 



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

the latter a zoologist, each working independently, came to the same 
conclusion and in 1839 collaborated their ideas. This theory, as 
they gave it, was in substance, All living things (plants and animals) 
are composed of cells. 



PROTOPLASM AND THE CELL 



51 



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 



m 



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 



53 



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 



59 



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 



62 



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. 

65 



66 



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 



67 



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 



68 



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 



69 



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- 



70 



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 



71 



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 



75 



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- 



76 



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 









B 



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

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 

81 



82 



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. 

S5 



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 



87 



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 



89 



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 

93 



94 



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 



95 



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 

if 

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 



98 



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 



99 



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. 





11 








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


■ - 









I » . . • 



B. 

* ^^^■^^- — 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 



a 



——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. 





i 



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 





B 



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 



126 



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 



138 



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. 



142 



TEXTBOOK OF ZOOLOGY 



These protrude through pores (cinclides) in the body wall to the 
outside, and they are supplied with nematocysts and secretory cells. 
They serve as defensive as well as offensive structures. 

Asexual reproduction by budding from the margin of the basal 
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^.- ^»* 






A 



"^^^ 




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- 



150 



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- 



152 



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- 



154 



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 



158 



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 



51 



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) .i T . ' 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 



162 



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 




CX 







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 



I 



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- 



172 



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 



i 



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 



178 



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 



180 



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/: 



3 




'/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 



182 



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. 



186 



TEXTBOOK OF ZOOLOGY 



The mouth of the larger, regular zooid is located at the free end 
and is surrounded by a tuft of ciliated tentacles. This 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- 



3^ 



liaublt 




K rXUAVO'ELLA, 







f- 











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 



188 



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 

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 



198 



TEXTBOOK OF ZOOLOGY 



which extends through about six segments and which has a digestive 
gland opening into it from both sides. The remainder of the 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 



200 



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 



zm 



, 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). 



202 



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 



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':- 









9^ 



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



Anybu/cfcraf 
p/afes 




Fig. 117. — Dried test of the sea urchin, Arhac'xa. A, shows arrangement of the 
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. 



po 




Fig. 128. — Development and metamorphosis of the starfish. A, Dorsal view of 
early ciliated larva showing ciliated bands, and left and right coelomic pouches. 
B, Ventral view of bipinnaria larva showing the extension of the left and right 
coelomic pouches. C, Dorsal view of the same larva showing the left madreponc 
pore and water tube, and the fusion of the left and right coelomic pouches to form 
an anterior coelom. D, Dorsal view of an older larva showing the budding of the 
five water tubes from the left coelom. E, Left side view of a still older larva 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.) 



I 



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, 



238 



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. 



f 



t 



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, 



240 



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, 



i 



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 



242 



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 




A 



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 



I 



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 




i 



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 




D 




D 






3. 






'© 




©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 



I 




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 

I 



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 



I 



( 



i 



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. 



252 



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 



I 



I 



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 



I 



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 



I 



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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 



A2 

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.., 



M. 

^.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 ■ 



a 




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. 





D. 

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. 

th 




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- 



290 



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- 



302 



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 



A. 




B. 

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 



I 



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, 



304 



TEXTBOOK OF ZOOLOGY 



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 



306 



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- 



310 



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. 




i 




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- 



312 



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 





c. 




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.) 



324 



TEXTBOOK OF ZOOLOGY 



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. 



326 



TEXTBOOK OF ZOOLOGY 





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- 



328 



TEXTBOOK OF ZOOLOGY 



wing moths, Aegeriidae, a rather distinctive family, are represented 
by the peach-tree worm, Aegeria exitiosa Say, a serious enemy of the 
peach in most parts of the United States (Fig. 191). The strawberry 




Fig, 192. — Life 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. / 



330 



TEXTBOOK OF ZOOLOGY 



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. 



332 



TEXTBOOK OF ZOOLOGY 



! 



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- 



334 



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 



V 



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 



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■A 




^' 










r.-cj. L 



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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 



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5t. 



-A\.X 



H'lnt. 



n 



-to. 



-■R;. 



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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 

S. 

Fig. 215. — Cross section of Amphioxus thirough the level of the posterior portion of 

the pharynx. 



two-thirds of the ventral surface of the body are thought to be their 
forerunners. The ventral and dorsal fins are supported by small 
vertical rodlike fin rays. On the ventral side, just posterior to the 
metapleural folds, is an opening, the atriopore, and beside the ventral 
margin of the caudal fin is the anus. The segmental divisions of the 
muscles are apparent on the body wall. There are from fifty-eight 
to sixty-four of them on each side in B. lanceolatus but sixty-nine 
in B. calif orniense and they are known as myotomes. The myotomes 
on the two sides are not paired, but alternate with each other. 
Adjacent ones are separated by a myocomma or myoseptum. 



PHYLUM CHORDATA 



371 



Buccal cirri 



QUI slit inwall 
of phorynx 



Afferent branch- 
Jal arteries 



Ventral aorta 

Dorsal aorta 

. —Notochord 
_ -Spinal cord 



■^ 



-Distribution throuqh 
liver 



..HepaticV. 

. Subintestinal vein 
. -Atriopore 



-Vcntro- intestinal V. 
.Dorso-intestinai A. 



Anus 

Caudal vein 

1 Caudal artery 



Fig. 216. — Diagram of the circulatory system of Amphioxus. 



if 

372 TEXTBOOK OF ZOOLOGY 

Internal Structure and Metabolic Activities. — In small cleared 
specimens the internal organs are easily observed. The notochord 
extends the length of the body as a slender rod of vacuolated cells 
which are 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) 

(c) Thoracic basket 

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 

(c) Glycogenases — liver and muscles 
Converts glycogen to glucose 

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 

(c) Sucrase (invertase) 

4. Proteases or proteolytic enzymes — split complex proteins 

(a) Pepsin in gastric juice 

(b) Trypsin in pancreatic juice — functions in small intestine 

(c) Erepsin in intestinal juice 

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, 

(c) Effects of deficiency: Pellagra in primates (man and monkeys). 
Black-tongue in dogs. Swine pellagra. 

4. Be or pyridoxine (CgHuOaN). 

(a) Sources: Milk, liver, cereals, yeast. 

(b) Functions: Necessary for growth. May influence oxidation of food. 

(c) Effects of deficiency: Paralysis in chickens. 



♦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. 

(c) Effects of deficiency: Graying in black rats. Dermatitis in rats and 
chickens. 

6. Biotin (doHieOsNjS). 

(a) Sources: Egg yolk, yeast, cereal grains, molasses. 

(b) Functions: Essential for growth. 

(c) Effects of deficiency: Thickening of skin and dermatitis in clucks 
and rats. 

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. 

(c) Effects of deficiency: Scurvy in human and guinea pig (bleeding in 
mucous membranes, beneath skin and into joints). 

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. 

(c) Effects of deficiency: Sterility in male fowls and rats. Failure of 
spermatogenesis. Death of rat embryos in uterus. 

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). 

(c) Effects of deficiency: Blood fails to clot. 

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 



I 



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 



V 



•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 



m 



m. 



-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 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 

M 



,. 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 









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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*. <«»',•' 



T 



"^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. 
Com m on 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 / 



h 






Irtternal care ^^ ' 

■\uricL:.iari '--■ 



\ 



I .5»> ,-j1 



_ ■-; ■ 


/ < 


J 


\nl3 "iC- r 


■r,3SinT.:.r:c^:. 


r 


PosX L : 


— *> 


m^ 


/:. ■ , 






-' ■ 


:,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 



i 



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 



540 



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 



542 



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. 

545 



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 

Styl