Marine Biological Laboratory

p . , July 15, 1948

Keceived *^ '

. . , 62131
Accession No.

^, ^ The C. V. Hlosby Co.

^ St. Louis, IIo. (/

Place. \

ESSENTIALS OF ZOOLOGY

ESSENTIALS ^

OF

ZOOLOGY

EMPHASIZING PEINCIPLES OF ANI3IAL

BIOLOGY

BY

GEORGE EDWIN POTTER, Ph.D.

Professor of Zoology, Agricultural and Mechanical College of Texas

College Station, Texas

SECOND EDITION

With 224 Text Illustrations

ST. LOUIS
THE C. V. MOSBY COMPANY

1948

Copyright, 1940, 1948, By The C. V. Mosby Company

(All rights reserved)

Printed in the
United States of America

Press of

The C. V. Moshy Company

St. Louis

DEDICATED TO
the late

PROFESSOR \V. B. WILSON

Ottawa University
my former teacher and inspiration in biology

PREFACE TO SECOND EDITION

The present edition represents a complete editing of the previous
edition with some revision and addition of material at certain
points. The general form and content of the book have been re-
tained. A number of condensed and pertinent tables have been
added to the chapter on Physiology, and a new section dealing
with the snail has been included in the chapter on mollusca. The
author makes acknowledgment here for many valuable suggestions
which came from Dr. C. J. Goodnight of Purdue University. Also,
appreciation is expressed to Betty Ann Potter for her valuable
assistance with the index of this book.

George E. Potter.

College Station, Texas.

PEEFACE TO FIRST EDITION

There has come into being a considerable number of relatively
short but fundamental zoology courses. Often they are given in a
single semester. Such courses usually involve rather intensive lab-
oratory study of the systems of one or two vertebrate forms, a few
selected invertebrates, and a number of special relationships as para-
sitism, reproduction, development, etc. The recitation or theory
portion of such a course is ordinarily concerned with some coordina-
tion and summary of the selected materials treated in the laboratory,
but perhaps more particularly with the lessons in principles which
may be drawn from them.

It is the presence of this type of course in a number of different
zoology or biology departments Avhich has prompted the organiza-
tion of the present book. It is the purpose of the author to bring
into the book the essential fundamentals which every zoology stu-
dent should have, regardless of the length of the course he happens
to take.

It is likely true that the majority of the students taking elemen-
tary zoology will have only that one course in the field and for that
reason some emphasis is placed on the economic aspect, on the human
relation, and on the principles involved. It is assumed that the
teacher will elaborate upon the details of particular topics of interest
to the class beyond the limitations of detailed information contained
in this volume.

The arrangement of the chapters has been placed in an order
which seems logical. However, the chapters are written in such a
way that the teacher may change the order with no difficulty. Since
this is true, the teacher has some option in the possibility of striking
a workable combination of 'Hypes" and ''principles" material as
well as balance in laboratory -lecture program.

The author is indebted to, and extremely grateful for the coopera-
tion of, the following who have contributed to the manuscript:

J. Teague Self, University of Oklahoma, Earthworm

Elmer P. Cheatum, Southern Methodist University, Fresh-Water
Clam and Snail

Vasco M. Tanner, Brigham Young University, Locust

8

PREFACE 9

Ottys Sanders, Southwestern Biological Supply Company, The Frog
Sewell H. Hopkins, Agricultural and Mechanical College of Texas,

Animal Parasitisyn
Willis Hewatt, Texas Christian University, Animal Distribution
A. 0. Weese, University of Oklalioma, The Animal and Its Environ-
ment
Frank G. Brooks, Cornell College, Genetics and Eugenics

Acknowledgment is made to Ivan Summers for the excellent art
work he has put into the book. Appreciation is likewise expressed
to Dr. Titus Evans, Mrs. Ruth Sanders, Miss Joanne Moore, and Mr.
Edward O'Malley, ^vho have prepared certain of the illustrations.
Finally, the author wishes to express appreciation to the Agricultural
and Mechanical College of Texas, and the staff of its Biology Depart-
ment for the cooperation which has made the organization of this
book possible.

George E. Potter.

College Station, Texas.

CONTENTS

CHAPTER I PAGE

Introduction ___-_____---------- 17

Zoology, a Biological Science, 19; The Subdivisions of Zoology, 19;
Classification of the Animal Kingdom, 22; Balance in Nature, 24;
Vital Relations of Animals and Plants, 26; Zoology as Related to
Man, 29; Agriculture and Zoology, 29; Fisheries and the Application
of Zoology 30,

CHAPTER II

History of Zoology _________________ 31

CHAPTER III

Protoplasm and the Cell _______________ 39

Living Matter, or Protoplasm, 39; The Cell Principle, 39; General
Characteristics of Protoplasm and the Material of the Cell, 41 ; Funda-
mental Properties or Activities of Protoplasm, 42; Physical Nature of
Protoplasm, 43; Chemical Nature of Protoplasm, 44; Structure of a
Typical Animal Cell, 47; Cell Division, 50.

CHAPTER IV

Metazoan Organization _______________ 54

Cellular Differentiation, 54 ; Cellular Organization, 55 ; Development of
Sexual Reproduction, 61.

CHAPTER V

The Bullfrog as a Typical Vertebrate Animal (Class Amphibia)

(By Ottys Sanders) _________----- 64

Habitat of the Bullfrog, 64; External Structure, 65; Digestive System
and Digestion, 67 ; Circulatory System, 70 ; The Veins, 73 ; Respiratory
Organs and Respiration, 80; Excretory System and Excretion, 81;
Skeletal System, 83; Muscular System, 88; Nervous System, 90; The
Sense Organs, 93; Reproductive Organs, 95; Embryology, 96; Eco-
nomic Importance of Amphibia, 102; Classification of Amphibia, 103;
A List of Families of the Amphibia in the United States, 104; Order
Caudata (Tailed Amphibians), 104; Order Salientia (Tailless Am-
phibians), 105.

11

12 CONTENTS

CHAPTER VI

PAGE

The. Rat, a Representative Mammal ____________ 107

Habitat and Habits, 107; External Structure of the Rat, 108; Skeleton,
110; The Muscular System, 115; Digestive System, 121; Glands Asso-
ciated With the Digestive System, 124; Circulatory System, 126; Re-
spiratory System, 131; The Nervous System, 132; Excretory System,
140; Reproductive System, 142.

CHAPTER VII

Chordates in General ________________ 146

Characteristics, 146; Classification, 147; Phylogenetic Advances of
Cliordata, 148; Subphylum Hemichorda, 148; Subphylum Urochorda,
Molgula, 150 ; Subphylum Cephalochorda, Amphioxus, 152 ; The Verte-
brate Animal: Subphylum ^'ertebrata, 157.

CHAPTER VIII

Physiology ____________________ 166

Support and Protection, 166; Movement and Locomotion, 168; Diges-
tion, 168; Functions of the Liver, 173; Absorption and Utilization of
Food Materials, 173; Vitamins and Their Functions, 174; Respiration,
176; Circulation, 178; Nervous Function — Reception and Conduction,
180; Functions of the Spinal Cord, 183; Functions of the Divisions
of the Brain, 183; Sense Organs and Their Function, 183; Exci'etiou,
188; Kidneys, 189; Reproduction, 191.

CHAPTER IX

The Endocrine Glands and Their Functions ________ 195

The Thyroid Gland, 196; The Parathyroid Glands, 198; The Supra-
renal Bodies, 199; The Pituitary Gland, 201; The Thymus Gland,
203; The Gonads and Sex Hormones, 204; The Pancreas, 205.

CHAPTER X

Sexual Reproduction and Development of the Individual _ _ _ _ 209
Body Form, 216; Organs and Systems, 217; Nervous System and Sense
Organs, 218; Intrauterine Development, 223; Biogenetic Law, or
Theory of Recapitulation, 224.

CHAPTER XI
Animal Anomalies _________________ 226

Causes of Anomalies, 226; Harelip and Cleft Palate, 228; Diaphrag-
matic Hernia (Open Diaphragm), 228; Polydactyly (Extra Digits),
229; Conjoined Twins, 230; Hermapliroditism, 233; Cardiac Anom-
alies, 234; Abnormalities of Brain and Sense Organs, 234.

CONTENTS 13

CHAPTER XII

PAGE

Genetics and Eugenics (By Frank G. Brooks) _____i___ 235
The History of a Great Discovery, 235; Mendel's Law, 235; Deriva-
tives of Mendel's Law, 237; The Physical Basis, 238; Plotting Crosses,
239 ; Complications of Mendelian Inheritance, 241 ; Inheritance of Sex,
245; Linkage, 246; Sex Linkage, 246; Crossing Over, 248; Mutations,
249; Human Heredity, 249; Matings Among Defectives, 252; The
Differential Birth Rate, 253; Family Size in Eugenic Groups, 253;
Family Size in Dysgenic Groups, 255; What Can Be Done? 256;
Some Eugenic Measures, 256.

CHAPTER XIII

Classification of Animals ______________ 259

Rules of Nomenclature, 260 ; Summary of the Classification of Animals,
261; Phylum I. Protozoa, 261; Phylum II. Porifera, 262; Phylum
III. Coelenterata, 262; Phylum IV. Ctenophora, 263; Phylum V.
Platvhelminthes, 263; Phvlum VI. Nemathelminthes, 263; Phvlum
VII. Annelida, 263; Phylum VIII. Echinodermata, 264; Phylum
IX. Mollusca, 264; Phylum X. Arthropoda, 264; Groups of Non-
chordates Whose Systematic Position Is Uncertain, 265; Phylum
XI. Chordata, 266.

CHAPTER XIV

Protozoa in General ________________ 268

Characteristics, 268; Classification, 268; Colonial Protozoa, 277;
Tropisms and Animal Reaction, 278; Economic Relations of Protozoa,
279.

CHAPTER XV

Representative Protozoa — Euglena, Amoeba and Paramecium _ _ _ 282
Euglena, of Class Mastigophora, 282; Habitat and Characteristics,
282; Structure, 282; Food and Assimilation, 283; Respiration and
Excretion, 284; Reproduction and "Life Cycle, 284; Behavior, 285;
Locomotion and Flagellar Movement, 285; Amoeba, of Class Sar-
codina, 285; Characteristics and Habitat, 286; Structure, 286; Metab-
olism, 287; Reproduction and Life Cycle, 289; Behavior, 291;
Amoeboid Movement and Locomotion, 292; Paramecium, of Class In-
fusoria, 293 ; Characteristics and Habitat, 293 ; Structure, 293 ; Metab-
olism, 295; Reproduction and Life History, 296; Behavior, 299;
Locomotion, 302.

CHAPTER XVI

Hydra, of Phylum Coelenterata ____________ 303

Classification of the Phylum, 304; Habitat and Behavior of Hydra,

14 CONTENTS

PAGE

311; External Anatomy, 313; Internal Anatomy, 314; Metabolism,
318; The Nervous System and Nervous Conduction, 319; Reproduction
and Life Cycle, 319; Regeneration, 322; Economic Relations of the
Phylum, 322; Phylogenetic Advances of Coelenterates, 322.

CHAPTER XVII

The Flatworm, Planaria, of Phylum Platyhelminthes _____ 323
Classification, 323; Habitat and Behavior of Planaria, 324; External
Anatomy, 326; Internal Anatomy, 326; Metabolism, 331; Reproduc-
tion and Life History, 331; Regeneration, 333; Economic Relations
of the Phylum, 334; Phylogenetic Advances of Platyhelminthes, 334.

CHAPTER XVIII

The Roundworm, Ascaris, of Phylum Nemathelminthes _____ 335
Classification, 335; Habitat and Behavior of Ascaris, 337; External
Anatomy, 338; Internal Anatomy, 339; Reproduction and the Life
Cycle, 340; Relations to Man, 341.

CHAPTER XIX

Earthworm, of Phylum Annelida (By J. Teague Self) _____ 342
Class Chaetopoda, 342; External Anatomy of the Earthworm, 345;
Internal Anatomy, 346; Reproductive Organs, 347; Digestive System,
348; Circulatory System, 350; Respiratory System, 351; Excretory
System, 351; The Nervous System, 352; Reproduction, 352; Regenera-
tion, 355; Importance of Annelids to Man and Other Animals, 356;
Phylogenetic Advances of Annelida, 357.

CHAPTER XX

Starfish, of Phylum Echinodermata, and Other Echinoderms _ _ _ 358
Classification, 358; Habitat and Behavior of Starfish, 361; External
Anatomy, 363; Internal Anatomy, 363; Reproduction and Life Cycle,
368; Regeneration and Autotomy, 370; Economic Relations, 371.

CHAPTER XXI

Fresh-Water Mussel and the Snail, of Phylum Mollusca (By Elmer

P. Cheatum) _________________ 372

General Characters, 372; Habitat and Behavior of the Clam, 373;
External Features, 374; Internal Anatomy, 375; Digestion, 376;
Respiration, 376; Circulation, 377; Nervous System and Sense Organs,
377; Excretion, 378; Reproduction and Life Cycle, 378; The SnaU,
380; Habitat and Behavior, 380; External Anatomy, 383; Internal
Morphology, 386; Respiration, 386; Circulation, 387; Nervous System,
388; Excretory, 388; Reproduction and Life Cycle, 388; Economic
Relations of the Phylum, 391.

CONTENTS 15

PAGE
CHAPTER XXII

The Crayfish, a Crustacean Arthopod __________ 393

Classification, 393; Habitat and Behavior of Cra\iish, 396; Ex-
ternal Structure, 396; Internal Structure, 400; Metabolism, 406;
Eeproduction, 407; Regeneration and Autotomy, 409; Economic Re-
lations, 410; Characterization of Other Crustacea, 410; Recapitula-
tion Theory or Biogenetic Law, 413; Phylogenetic Advances of
Arthropoda, 414.

CHAPTER XXIII

The Locust, a Representative of Insects (By Vasco M. Tanner) _ 415

CHAPTER XXIV

Animal Parasitism (By Sewell H. Hopkins; ________ 428

Social Relations of Animals, 428; Origin of Parasitism, 429; De-
grees of Parasitism, 429; The Successful Parasite, 430; Means of
Infection and Transmission, 433; Parasitism and Host Specificity,
434; Parasites and the Groups in the Animal Kingdom, 435; Some
Representative Parasites, 440.

CHAPTER XXV

The Animal and Its Environment (By A. O. Weese) ______ .457

The Principal Biotic Formations, 462; Adaptation, 465; Succes-
sion, 465; Animal Populations, 468; Seasonal Changes, 470; Sum-
mary, 471.

CHAPTER XXVI

Animal Distribution (By Willis Hewatt) _________ 472

Life Regions and Zones of the Earth, 472; Migration of Animals,
477; Means of Dispersal and Barriers, 478; Effects of Man Upon
Distribution, 479.

CHAPTER XXVII

The Theory of Evolution ______________ 480

Colony Formation in Certain Protozoa, 482; Development of the
Gastrula, 482; Trochophore Larva, 482; Peripatus and the Worm-
like Ancestry of Arthropoda, 483; Echinoderms and Their Larval
Relations, 483; Ancestry of the Vertebrates, 484; Basis for the
Theory of Evolution, 488.

GLOSSARY
Glossary ____________________ 510

ESSENTIALS OF ZOOLOGY

CHAPTER I
INTEODUCTION

The question, What is Life? is the greatest riddle in the biological
world. The term life is an abstraction with no objective reality ex-
cept 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 mate-
rial is able to sustain and reproduce itself under favorable conditions.

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

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.

'^Trained and organized common sense" was the definition of
science given by Thomas H. Huxley, an eminent English biologist

17

18

ESSENTIALS OF ZOOLOGY

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 iden-
tical results with frequent repetition and by numerous observers.

Fig:. 1. — Divisions of study in the field of biology.

Science lays its foundation on accurate observations and depends
on the ability of the senses to reveal the truth. Established facts
represent truth, and the scientist respects truth while to him tradi-
tion or mere opinion counts for little as such. Science is, therefore,
a changing, increasing body of knowledge which is ever becoming
more thoroughly established.

INTRODUCTION' 19

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 biology 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

Zoology as one of the divisions of the general field of biological
science is such a broad field that it is necessary to subdivide it into
several subdivisions for convenience in study. It has been a rela-
tively short time since all of the know^n biology, geology, and re-
lated subjects were studied under the head of natural history. But
now the subject matter of zoology alone has grown to such mag-
nitude 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. It is one of the older fields, and is 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 dissection, observation, and study
of the parts, form, and relationship of parts of the digestive system
of the cat would be a good example of anatomical study.

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.

20 ESSENTIALS OF ZOOLOGY

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. 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 natural
relationships. This field is often spoken of as systematic zoology.
The number of described species of animals as given by different
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 impossible to do
business if a company were to provide a large floor space, go out
and buy the thousands of different kinds of articles that are handled
by a department store, and just throw all of them on its floors at
random.

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: Chordata

Subphylum : Vertebrata
Class: Mammalia

Order : Primates

Family: Hominidae
Genus: Homo

Species: sapiens

INTRODUCTION 21

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 binomial
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 Dutch 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 func-
tions, such as digestion, circulation, respiration, excretion, glandular
secretion, nervous activity, muscular contraction, and others. Many
of the processes which occur in the developing embryo are also in-
cluded here. Much of the present study referred to as cytology is
physiological. Physiology, like morphology, is an old branch of zoology.
Physiology depends upon an understanding of physics and chemistry
on one hand, and anatomy on the other.

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.

5. Embryology is a study of thfe origin and development of the
individual and may be spoken of as ontogeny. It usually involves
the changes occurring in the organism from the time of fertilization
by the union of two cells, one derived from each parent, through
the numerous cell divisions, growth, organization, and differentiation
leading to the adult condition.

6. Genetics is the division which deals with the study of varia-
tions, resemblances, and their inheritance from parent to offspring.
Fairly definite laws governing this inheritance of qualities have been
established by the geneticists.

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.

22 ESSENTIALS 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 organism
to the medium in which it lives to temperature, to light, to food, to
competition, to enemies, to mating, and many other factors, all be-
come a part of an ecological study.

9. Zoogeography or geographical distribution of animals is con-
cerned with the extent of the regions over which species are dis-
tributed and the association of species in individual regions. It is
concerned 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 fa-
vorable 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 unsur-
mountable obstacles. Conditions which prevent dispersal of animals
from one area to another are known as harriers. Oceans, mountains,
forests, deserts and land are all barriers to different types of ani-
mals. The Starling, which originated in Europe, was not found in
America until after it was introduced by man, and in recent years
it is becoming 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 ani-
mals 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 de-
scent 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.

Classification of the Animal Kingdom

Few people realize the number of different kinds of existing ani-
mals and their variation in size, structure, and habits of life. The
estimated number of kinds is all the way from 1,000,000 to 10,000,-
000. To date, approximate!}^ 840,000 species have been named and
described.

The entire kingdom is divided into two subkingdoms: Protozoa,
or all single-celled animals, and Metazoa, the many-celled animals.

INTRODUCTION 23

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. Later in the book there is a chapter
devoted to the classification of animals but the principal phyla are
listed and briefly described here :

Phylum Protozoa. — Individuals consist either of a single cell or of
aggregates of cells, by each of which are performed all the essential
functions of life. They are mostly microscopic in size and largely
aquatic in habit. Some live in the ocean, some in fresh water, and
still others as parasites in man and other animals. About 15,000 are
known.

Phylum Porifera (Sponges). — Mostly marine aquatic metazoans
which live attached! The body is supported by fibrous, calcareous,
or siliceous spicules, and the body wall is perforated 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 cavity, and tentacles provided with stinging bodies, nemato-
cysts. The described species number at least 4,500.

Phylimi 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, enlongated worms which possess
both a mouth and an anus. Some are free-living, others are parasitic.
The hookworm, ascaris, and the ''horsehair worm" are common rep-
resentatives. 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 locomotion.
The common representatives are starfishes, sea urchins, sea cucum-
bers, and sea lilies. There are about 4,500 known living species.

24 ESSENTIALS OF ZOOLOGY

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.

Phylum Arthropoda. — The group includes crayfishes, lobsters,
crabs, centipedes, scorpions, and all insects. Their bodies are seg-
mented, and they have segmented appendages. This is by far the
largest single phylum. Some authors believe as many as 675,000
species belong to it.

Phylum MoUusca. — Unsegmented 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. Approx-
imately 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 ; Tro-
chelminthes — 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 ; Chaetog-
natha — 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 Molluscoida.

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

INTRODUCTION

25

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26 ESSENTIALS OF ZOOLOGY

be balanced, all groups are held in bounds 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.

The story of the rabbit in Australia is likewise an interesting
example of the effect of balance or lack of it. Not many years ago
Australia had no rabbits. It was hoped and intended by English
immigrants there, that a few imported pairs of rabbits would in-
crease sufficiently so that the old English sport of riding to the
hounds might be developed in Australia. To the surprise and dis-
may of these people, the rabbits flourished until now they are jeop-
ardizing the enterprises of man.

Again, we have an example of the effect of the natural agents of
repression. The Japanese beetle which was recently introduced in
the United States by accident has ravaged the vegetation in several
eastern states and threatens other areas. When our investigators
went to Japan to study the enemies of the beetle in an effort to find
a means of control, they had to search for weeks to find a seriously
infested area. So impressed are some biologists becoming with the
potential danger of interfering with the natural balance, that even
when some irritating pest is under discussion, 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 check on others that are still more obnoxious.

Vital Relations of Animals and Plants ,

There are certain single-celled organisms that are claimed as ani-
mals by zoologists and as plants by botanists. It is difficult to draw
an absolutely clear-cut line of distinction. Of course, it is easy to
recognize the extremes. Anyone 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

INTRODUCTION

27

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

28

ESSENTIALS OF ZOOLOGY

depend on green plants for their organic food. It 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)

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in sunlight 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 photosynthesis is in progress, oxygen is discharged into the

INTRODUCTION 29

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

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 humarn 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 loss of their
lives is constantly saving millions of human lives. One of the most
obvious uses of other animals is as a source of food supply. We
have only to think of such forms as 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. 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.

Agriculture and Zoology

A recent instance of the economic importance of zoological knowl-
edge is found in the saving of the entire citrus industry in Florida

30 ESSENTIALS OF ZOOLOGY

from the Mediterranean frnit fly. Injurious insects alone cause an
annual loss in the United States of more than one and one-half bil-
lion 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 important are caused
each year by the parasitism of our domestic animals by bacteria,
protozoans, worms, and arthropods.

Agriculture has benefited greatly from the application of the prin-
ciples of heredity to plant and animal breeding. Much fundamental
knowledge has come from the extensive studies on the genetics and
breeding of the common fruit fly, Drosophila. It produces a new
generation about once every nine days. More improvement of strains
of animals and plants, too, can be made in one man's lifetime than
was previously possible through ages. The United States Department
of Agriculture, the Division of Entomology and the United States
Fish and Wildlife Service 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 Wildlife Service does an extensive and re-
markable work in the study, propagation, and care of this natural
zoological resource. Even with this work and that of all the State
Fisheries Departments, the natural fish life does not flourish as it
might, had 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 free-
dom from chemical, soil, 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.

CHAPTER 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 considered
throughout the text rather than in a given chapter.

There were individual persons interested in and studying natural
history long before there was any organized 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 assistance of
the armies of Alexander the Great in collecting materials. Alexander
had been one of Aristotle's pupils and had become interested in the
development of scientific endeavor. He made a grant of 800 talents
($200,000 or more) for use by Aristotle in his investigations. Thus
even in those times endowments were being set up for the support
of research. The other Greeks who followed Aristotle 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 'Mark
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

31

32 ESSENTIALS OF ZOOLOGY

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

Galen (a.d. 131-201), coming in the midst of the ''dark ages" as
he does, should be particularly credited for the contributions he
made. He was of Greek ancestry but moved to Rome early and
became a successful physician. His anatomical studies were made
principally from direct observations on elephants, Barbary apes, and
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, Galen 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 an active
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
then allowed. He later became professor of surgery there. He was
the first, since the time of Aristotle and Galen, to prove that direct
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
observations 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
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

HISTORY OF ZOOLOGY

33

in capillaries, studies on glands, and his work on the structure and
metamorphosis of the silkworm take rank with outstanding con-
tributions to zoological knowledge. Numerous organs of the human

Fig. 6. — Vesalius (1514-1564), pioneer anatomist. (From Locy, Growth of Biology,

published by Henry Holt and Company, Inc.)

body are named for this renowned scientist of his time. Like other
early microscopists, he had to build his own microscope.

34

ESSENTIALS OF ZOOLOGY

Antonj van Leeuwenhoek (1632-1723) lived almost contemporane-
ously with Malpighi and like him made many contributions to the
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
valuable contributions to the development of the microscope are the
enviable accomplishments of this man.

Fig 7 — Linnaeus (1707-1778), first great student of taxonomy. (Reprinted by per-
mission from Locy, Growth of Biology, Henry Holt and Company, Inc.)

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
of a species and introduced the use of anatomical features in dis-

HISTORY OF ZOOLOGY 35

tingmshing 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
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 system, but it is specifically known by the
genus name and species 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 anatomy. He was of French ancestry and largely
self-educated by his studies at the seashore. A number of anatomical
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, which 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

36

ESSENTIALS OF ZOOLOGY

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.

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

\

Pig. 8. — Charles Darwin (1809-1882), the author of Origin of Species. (From Garri-
son, History of Medicine, W. B. Saunders Company.)

persist and produce progeny. His conception of the factors and
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
ideas before him. It was the vast accumulation of facts covering

HISTORY OF ZOOLOGY

37

a period of twenty years which commanded the attention of scien-
tists as well as of the public generally. In 1858 he read a joint paper
with Alfred Russel Wallace, 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.

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

Fig. 9. — Louis Pasteur (1822-1895) contributed much to the knowledge of bacteria
and disease. (From Garrison, History of Medicine, W. B. Saunders Company.)

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 offspring 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.

Louis Pasteiir (1822-1895) was a French scientist who had been
trained in chemistry but became one of the outstanding pioneers

38 ESSENTIALS OF ZOOLOGY

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
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.

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 Nat-
uralists of the Frontier. This book is extremely interesting to read.

References

Locy, W. A.: Biology and Its Makers, New York, 1915, Henry Holt and Com-
pany, Inc.
— : The Growth of Biology, New York, 1925, Henry Holt and Company, Inc.
Nordenshiod, E.: History of Biology, New York, 1928, Alfred A. Knopf, Inc.

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
always 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 which 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 living
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 made 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.
Leeuwenhoek saw spermatozoa and bacteria and included them with
single-celled animals as ''little beasties"; Malpighi had described
the nature and appearance microscopically of several organs of the

39

40 ESSENTIALS OF ZOOLOGY

body ; Grew had made rather extensive microscopic studies of plants,
and in 1831 Robert 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
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.

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.

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
material produced by the cells, the cell principle 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

PROTOPLASM AND CELL 41

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 and 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 theory.

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 translucency 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.

42

ESSENTIALS OF ZOOLOGY

It is in a colloidal state of the emulsoid type. 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 Avatery 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 dispersed 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 become 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

Fig 10 The change of a colloidal emulsion from sol to gel state. In A the drop-
lets of the disperse phase (not stippled) are shown scattered through the disper-
sion medium (stippled), and the emulsion is a sol; in B the droplets are shown
taking up liquid and swelling; in C this is continued until they press upon one
another- in D the droplets are so crowded as to become contmuous and to have be-
come in' fact the dispersion medium, while that which was the dispersion medium
is now in droplets and has become the disperse phase. The emulsion has become a
gel. (From Wolcott, Animal Biology, McGraw-Hill Book Company, Inc.)

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. Irritahility, which refers to the capacity present in all proto-
plasm for responding to changes in environmental conditions, or ex-
ternal stimuli.

PROTOPLASM AND CELL 43

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. Metabolism, 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.

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 millimeter in diameter. These par-
ticles 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 Broivnian
movement (characteristic of colloidal substances). Protoplasm dif-

44 ESSENTIALS OF ZOOLOGY

fuses sloAvly 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 known 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
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

PROTOPLASM AND CELL 45

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
making up the remaining 1 per cent. These elements are found
combined to form compounds. The organic compounds include car-
bohydrates, fats, proteins, and also enzymes. The inorganic com-
pounds 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 CpHigOe-
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 water (HgO) 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
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 weight 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 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. Pre-

46 ESSENTIALS OF ZOOLOGY

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

Enzymes are substances whose exact chemical nature is not yet
known, but whose importance to protoplasm is probably unequaled.
Chemically and physically they seem to be more like proteins than
anything else. These substances are not only found 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
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.

PROTOPLASM AND CELL 47

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,
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. Cer-
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.

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.

48

ESSENTIALS 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. It
is living and semipermeable.

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 kno^vn as kino-
plasm and is made up of two parts, the larger centrosphere, enclosing

.CELL MEMBRANE
PLASMA MEMBRANE
CENTROSOME
CENTRIOLE

NUCLEAR MEMBRANE
NUCLEOLUS

CHROMATIN RETICULUM

-NUCLEAR SAP

PLASTIO

VACUOLE
ENDOPLASM

ECTOPLASM

CHONDRIOSOME

) NUCLEUS

Fig. 11. — Diagram, showing a typical celL (From Parker and Clarke, Introduction
to Animal Biology, The C. V. Mosby Company.)

a (two if divided) cent Hole. Vacuoles are often present as small
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 granules 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

PROTOPLASM AND CELL 49

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
portion of this is karyolympJi, 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 kno^vn
as chromatin, which is thought to be the center of functional activities
of the nucleus. In instances where the chromatin is fibrillar, the
threads of it are called chromonemata. During division of the cell
this granular material becomes arranged into definite bodies, the
chromosomes. It is generally thought that in these bodies are lo-
cated the units of material (genes) which function in the trans-
mission of hereditary characteristics from one generation to the

^■c^;■^^l^,

t^--; ir^:.:-fv.; /l'M

tJi-,'*'-" i'^-f ■•.■■.'•■ •■ *^Vi«

A

d e £ g

Fig. 12. — The upper surface of a fully developed salivary gland chromosome
(large vesicle type) from Simulium fly larva. The longitudinal, threadlike bands
are called chromonemata, and these consist of a linear series of granules, the
chromomeres, which have a specific arrangement of grouping. A is a semidiagram-
matic 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 b is composed of 15 or 16
vesiculated chromomeres closely pressed together, c to h are other groupings of
chromomeres along the chromonemata of the chromosome. (From Painter and
Griffen: Chromosomes of Simulium, Genetics 22: 616. 1937.)

next. There are usually one or two knots of more dense chromatin
in the nucleus which are called karyosomes. Then besides these,
most nuclei have another 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

50 ESSENTIALS OF ZOOLOGY

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
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 study, these changes will be set out as six phases.
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
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
amphiaster. The nucleolus disappears and the chromatin becomes
organized into a long, tangled fiber, which is called the spireme
thread.

In the middle prophase stage the centrioles have migrated still
farther from each other, and the spindle fibers between the centrioles
as well as the astral rays around them have become well established.
The spireme thread has thickened and divided into segments in the

PROTOPLASM AND CELL

51

formation of chromosomes. During these changes the nuclear mem-
brane begins to degenerate. In the late prophase stage the centrioles
have reached the polar positions on opposite sides of the nucleus. The
spindle extends between the two asters, and the chromosomes become

CEI_1_

MEfs/IBRANE
—CENTROSPHERE
-MUCL.EAR

MEMBRANE

CHROMATIN
NUCt_EOl_US
CVTOPL.ASM

— CENTRlOUe

RE ME

RESTING CELU

SPINDt_E
ASTRAU

RAys

— CHROMOSOME

EARLY P ROPHASE

-EQUATORJ AU

LATE PROPHASE

METAPHASe

77 ADJACENT
CELL.

INTERZONAL,

FI esRs

EARLY ANAPHASE

l-ATE ANAPHASE

CL.EAVAGE
FURROW

TELOPHASE

DAUGHTER CEl_LS

arranged on the spindle fibers, end to end in a row at the equatorial
position. This plate of chromosomes constitutes what is called the
equatorial plate. The nuclear membrane has almost completely de-

52 ESSENTIALS OF ZOOLOGY

generated. The chromosomes lying in the equatorial plane of the
spindle now undergo an equatorial division, each being divided into
two corresponding halves. The actual division pf the chromosomes
here constitutes the metaphase stage. Following this stage each of
the chromosomes resulting from the division migrates along the
spindle fibers toward its respective centriole, or pole. The stage dur-
ing which occurs the migration of the chromosomes from the equa-
torial position to the poles is called the anaphase.

As the chromosomes near the poles of the spindle they crowd very
close to each other. At this time a constriction of the cytoplasm be-
gins 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 group, 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 significance. 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

PROTOPLASM AND CELL 53

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.

References

Cowdry, E. V. (editor) : General Cytology, Chicago, 1924, University of Chicago

Press.
Heilbrunn, L. V.: Colloidal Chemistry of Protoplasm, Berlin, 1928, Borntraeger.
Sharp, L. W. : Introduction to Cytology, ed. 2, New York, 1926, McGraw-Hill

Book Company.
Wilson, E. B.: The Cell in Development and Inheritance, ed. 3, New York, 1925.

The Macmillan Company.

CHAPTER IV

METAZOAN ORGANIZATION

All animals whose bodies consist of few or many cells functioning
as a unit are called metazoans.

The cells of their bodies are definitely organized and classified mor-
phologically as well as physiologically. There is a well-regulated
division of labor. Among the single-celled animals each cell is largely
independent of its fellows, doing for itself all that is necessary to
carry on living processes. In the many-celled animal, as in a highly
developed society of men, certain individual cells become more pro-
ficient 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 civilization
advances; so it is with development of complexity in metazoans.
Another characteristic of Metazoa is the presence of a definite
center of control localized in a particular group of cells which be-
comes the nervous system in higher forms.

Cellular Differentiation

In Protozoa (single-celled animals) there is seen fair development
of intracellular differentiation, 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 differ-
ences between them. The presence of a variety of cells within one
body is spoken of as intercellular differentiation. The modification^
of metabolic activity is the basic factor in the development of all
differentiation. Certain groups of cells become specialists in a par-
ticular phase of the metabolic activity. Some become protective sur-
face cells, others secrete special enzymes, still others specialize in ex-
cretion, and so on.

The entire metazoan body is usually divided into gerjn cells, which
are specialized for reproduction, and somatic cells or body cells,
which compose the remainder of the body and are grouped in layers.

54

0

METAZOAN ORGANIZATION 55

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.
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; siis-
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-

56 ESSENTIALS OF ZOOLOGY

velops various protective structures in the different groups of ani-
mals : the hard, horny chitin of insects ; scales of fish ; horny plates
and scales of reptiles ; feathers of birds ; hair and nails of mammals.
The glands of the body are developed from epithelium. Secretions
from these various glands lubricate the surfaces, contain enzymes
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

ft

A

Fig. 14. Germ cells. A, Ovum of female; B, spermatozoa of the male.

' ' 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.

METAZOAN ORGANIZATION 57

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, giving the general ap-
pearance 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
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 axoncy usually a single unbranched fiber except for
infrequent collateral branches; and (b) dendrites, frequently much
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
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

58

ESSENTIALS OF ZOOLOGY

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

Fig. 15. Typical somatic cells from vertebrate animals. 1, Squamous epithelial

cells- 2 section through a portion of bone showing Haversian canal (in center),
bone'cells, lacunae, canaliculi, and matrix; S, section of hyaline cartilage showing
cartilage cells in lacunae, and matrix between lacunae ; .',, section of tendon which
is composed of white fibrous connective tissue; 5, longitudinal view of smooth (m-
voluntary) muscle cells; 6, striated (voluntary) muscle; 7, motor nerve cell, show-
ing processes; 8, human red blood corpuscles (nonnucleated) and human white
corpuscles (nucleated). (Drawn by Titus C. Evans.)

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.

METAZOAN ORGANIZATION 59

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 skeleton, and
they serve for the support of the body as a whole and for the pro-
tection of the internal, vital organs.

c. The Muscular System consists of muscles, the voluntary, stri-
ated group moves skeletal parts and accomplishes locomotion; the
nonstriated, involuntary group is concerned with the movements of
the internal organs (viscera), and the cardiac muscle produces the
heart action.

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-
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) to collect and transport to the
point of exit carbon dioxide, liquid wastes, bacteria, and other foreign
matter.

60

ESSENTIALS OF ZOOLOGY

g. The Excretory or Urinary Syste7n 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,
urinary bladder, and urethra are accessory to the kidneys. The kid-

.duqudarVm
.Carotid ArUry

.JTrachca

...SiibclcLvianV.
PrecavalV.

Oorsoil Aorta

Pulmonary A.

Left Aun'cfe

LeftVcntr/d2

Luna

Diaphracfm

1 Livcp

Uiiodaniim

.. .Stomach

Jtunivcrse C.Q\on

Dcicendlnc] Co/on
Aiccndm^ Colon

Ileum

Fig. 16. — Human manikin showing parts of the principal systems from ventral

view. (Drawn by Edward O'Malley.)

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.

METAZOAX ORGANIZATION 61

li. The Reproductive System is a closely related arrangement of
glands, ducts, and accessory structures which function to produce
new individuals of the species. Additional features of sexual repro-
duction are given in a separate section on that topic below.

i. 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.

j. 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.

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 systems 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

Reproduction 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 begins
life as a single cell resulting from the fusion of two sex cells, one
produced by each parent. In son;e 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

62 ESSENTIALS OF ZOOLOGY

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 budding 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
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 egg 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

METAZOAN- ORGANIZATION 63

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
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. The medusae produce
eggs and sperms which unite in the water and develop into asexual
colonies. Metagenesis really involves two methods of reproduction
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 sexua;lly reproducing plants have a
similar alternation of sexual and asexual generations.

References

Maximow, Alexander A., and Bloom, William: A Textbook of Histology, Phil-
adelphia, 1938, W. B. Saunders Company.

Hewer, Evelyn E.: Textbook of Histology for Medical Students, St. Louis, 1938,
The C. V. Mosby Company.

CHAPTER V

THE BULLFROG AS A TYPICAL VERTEBRATE

ANIMAL

(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. 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. With the exception of
caecilians, they do not have scales as do the reptiles. The caecilians,
none of which has been reported from the United States, are worm-
like 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. 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.

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
through their skin, which is richly supplied with blood vessels.

Habitat of the Bullfrog

The bullfrog is a solitary animal except during the breeding season.
It is strictly aquatic and does not leave the pools as does the leopard

64

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

65

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. Crayfish, insect larvae, water beetles,
snails, and other aquatic organisms 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 Rockies from
Canada to Mexico. They have also been introduced into the western
portion of the United States and into various foreign countries.

Fig. 17. — The bullfrog, Rana catesbeiana. (Courtesy of Southern Biological Supply

Company. )

External Structure

Bullfrogs obtained in the South and Southwest are usually of tAvo
species, Rana catesbeiana Shaw, the common bullfrog, or Rana grylio
Stejneger, the southern bullfrog. Individuals of the former species
attain larger sizes, and the giant "bullfrogs of the southern swamps
usually are Rana catesbeiana. The two species differ not only in size
but also in external appearance, particularly when alive. However,
they are essentially the same anatomically, and this chapter is based
on a study of Rana catesbeiana.

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.

66

ESSENTIALS OF ZOOLOGY

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 t7'unk. Attached
to the trunk on either side anteriorly are the forelegs and posteriorly
the liindlegs.

The head has two prominent eyes which protrude above its sur-
face. These can be drawn back into their orbits and forced some-
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

i.i"^'

h . ■•^': ■

A

B

Fig, 18. — Melanophore from Rana temporia. A, Pigment distributed in response
to light; B, pigment contracted. (Redrawn and modified from Noble, Amphihia of
North America, McGraw-Hill Book Company, Inc.)

of each eye is a circular oval area, the tyinpamim 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
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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 67

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 femur, which
joins the trunk; the tibio-fibula; and the ankle, or tarsus. Follow-
ing the tarsus is the foot with five digits (toes), which are connected
by a web, producing a very efficient swimming organ. The point at
which the tarsus joins the tibio-fibula is known as the heel.

Digestive System and Digestion

The mouth cavity, or buccal cavity, is large. It narrows toward
the esophagus, which is a short gullet leading directly from the
mouth 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
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 which are cemented to two bones in
the roof of the mouth, known as vomers. These teeth consequently
are 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 crayfish, 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 bicornate. Its anterior half is attached

68

ESSENTIALS OF ZOOLOGY

to the floor of the mouth just back of the tip of the lower jaw, and
its posterior end is free. 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 sw^allowed by
the frog, in which case the stomach turns inside out and protrudes
into the mouth cavity.

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

Vomerine teeth

Floor of orbit —

Esophagus -||v*

Vocal sac

Maxillary t^cth
_ Internal na'-es

_ SjIojs marqinaiis

_ E.U5tachian tuoe
Glottis

c Toncjue

Figr. 19. — Mouth, or buccal cavitj^ of the bullfrog.

is the gall bladder, 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 probably contains no digestive enzymes. Al-
though its function in altering fatty substances is important, of
prime importance is its ability to store glycogen and the fat upon
which a hibernating 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

BULLFROG AS TYPICAL VERTEBKATE ANIMAL 69

digestive enzymes. This secretion is taken from the pancreas by
pancreatic ducts which empty into the bile duct that runs 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 stom.ach. Here
the gastric glands secrete hydrochloric acid and an enzyme, pepsin,
which convert the proteins to peptones. The muscles of the stomach
cause a thorough mixing of the gastric juice with the food and then
pass the partly digested food (chyme) posteriorly into the small in-
testine. 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 duodenum its
highly alkaline secretion. In addition, this pancreatic juice contains
three digestive enzymes : a protease, trypsin, which continues the di-
gestion begun by pepsin in the stomach, converting proteins to amino
acids ; an amylase, amylopsin, 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.

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 sugai* 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, lymphoid organ, the functions of which are but
incompletely known. The destroying of red blood corpuscles is an

70 ESSENTIALS OF 2O0L0GY

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 Endocrine Glands and Their Functions.

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 Endocrine Glands
and Their Fu7ictions.

*

Circulatory System

The circulatory system comprises the blood 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-
sary for metabolism and 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 contractions,
forces the blood to flow to the tissues.

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 and destruction, hormones being trans-
ported from organs of one part of the body to another, or foreign
substances accidentally introduced.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 71

The capillaries are very small vessels, the walls of which are made
up of endothelium continued from the linings of arteries and veins.
They 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 cell
walls, and thus become free to engulf bacteria or other harmful
objects.

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
which divides just above the auricles into a right and left truncus
arteriosus. Each of these trunks splits into three arches going to
each side of the body, the anterior carotid arch, the middle systemic
arch, and the posterior pulmocutaneous arch.

The Carotid Arch. — Each carotid arch divides into two branches.
The more ventral, lingual arteiy, or external carotid, passes forward,
giving branches to the thjrroid, pseudothyroid, muscles of the hyoid
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 goes to the base 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 esoph-
agus 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 occipitovertehral artery the large subclavian

72

ESSENTIALS OF ZOOLOGY

artery rises 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

External carotid

Pa I at in e

Auricularis

Cutaneous
Carotid oland

Conus arteriosus
Pulmonary.

Systemic arch

Lateralis
Dorsa lis

Ophihalmic
Cerebral

Internal carotid

Brachial

Vertebral

Hepatic

Coetiaco.rn-esenteric

.Left qastric

PANCREAS

IRiqht (gastric
C celiac
Anterior
mesenteric

Splenic

Recto.vesical
Sciitic

/^ffl^^ZL-Posterior mesenteric
.''r^'-*\^Femora I

Fig. 20. — Arteries of the bullfrog from ventral view. (Drawn by Ruth M. Sanders.)

the large coeliaco-mes enteric 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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 73

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
vessels which send numerous smaller arteries to the small and large
intestines.

The urinogenital arteries consist of about four to six small mucli-
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.

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 supplying 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 rectovesical artery is sent
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 pulmocutaneous arch takes blood to the respiratory organs:
the lungs, skin, and buccopharyngeal cavity. The pulmocutaneous
arch on each side divides into a pulmonary artery to the lungs and
a large cutaneous artery, which passes outward to the skin.

The Veins

The veins 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

74

ESSENTIALS OF ZOOLOGY

or hepatic filters to eliminate urea and other waste products from the
blood or chemically to alter it. Pulmonary veins from the lungs
carry oxygenated blood.

The venous circulation, therefore, may be divided into four main
systems: the systemic, hepatic portal, renal portal, and pulmonary
systems.

Linqual
Mandibular

Brachial.

Sinus

venosus

^Internal jaqular

External ju.julnr

. Suipscapalar

Innominate

Cardiac

Hepatic

Cutaneous —

Posterior
vena caval

Pre cava I

' LIVER
LOWER urr ;-5^0;V/,^\

LOBc .kV ^"^^X Hepatic
\ portal

XOasfric

Spermatic.

Dor so. lumbar.

Renal

t iSplcnic
\_1A^_/ Mesenteric

.SPLEEN

Abdominal —
Renal portal.
Vesical

External iliac

Femoral

Fig, 21. — Veins of the bullfrog from ventral view. (Drawn by Ruth M. Sanders.)

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.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 75

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.

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 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 network 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 over the pancreas to join it. It then contin-
ues 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
aMominal vein. As the abdominal vein proceeds toward the heart
along the median portion of the ventral body wall, it receives vesical
veins from the bladder, parietal veins from the body Avail and, at its
anterior end, a cardiac vein from the heart. In the region of the liver

76 ESSENTIALS OF ZOOLOGY

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, like the hepatic portal system, diverts
blood to a purifying organ instead of carrying it directly to the heart.
In this case, 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, passes anteriorly and joins the sciatic, to make the renal portal
vein. Near the kidney this vein receives the dorsolumbar veins from
the body wall and, in the female, several vessels from the ovisacs.
The renal portal vein follows the dorsolateral margin of the kidney,
sending numerous transverse branches into the organ, where they
break up into capillaries. Blood which passes through these capil-
laries is purified of some of its waste products and then leaves the
kidney through the renal veins which empty into and form the pos-
terior vena cava of the systemic system.

Pulmonary 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 com-
ing from the skin and bueco-pharyngeal 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 conus 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 walls tends to close the small oblique aper-
ture when the auricle contracts,

BULLFROG A!S TYPICAL VERTEBRATE ANIMAL

77

Both auricles pass blood into the ventricle through a common open-
ing, the auriculo-ventricular 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 poeketlike semilunar valves which open inwardly into the conus

Carotid A.

System.A.

Pulmocata-
neous f\.

R. auricle -
ipirril yaVje.

Conus Qrt(i-_
rio5Uj

SzvnWuDQ'c mqWz

-Jruncus at-tenosas

-Pulmonary aperture
-T-Sinu-auriculor oper-

—Left auricle

— Irttzraurlcular sep-
tum

--Briitle inpulmo-

cutaneoas A.

- -/lariculo-ventricular
valve

- -s^^:^/ Ventricle

Fig. 22. — Heart of the frog with the ventral wall removed and bristles shown In
the arteries branching from the truncus arteriosus.

when blood is passing out but are tightly closed at other times. The
proximal portion of the conus is known as the pylangium, and the
distal portion as the synangium. 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.

78 ' ESSENTIALS OF ZOOLOGY

Near the anterior free end of the spiral valve where it is the
widest, there is a pair of small synangial valves which, together
with the end of the spiral valve, separate the pylangium from the
synangiunL 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
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 resist-
ance to blood flow, the blood tends to enter the larger systemic
arteries first. As the systemic arteries fill, they offer more resistance

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 79

to the blood, while resistance in the carotid arteries decreases 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.

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 Physiology). After the frog has been injured, the
formation of a clot prevents indefinite bleeding and makes it pos-
sible for the tissues to begin repair.

The white blood corpuscles or leucocytes are of three kinds : lym-
phocytes, monocytes, and granuloctjtes. Their outlines are irregular,
due to their amoeboid movement, and the shape of their nucleus
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 chiefly by the spleen. The spleen
seems to be the primary organ concerned in supplying new leuco-
cjrtes, while the bone marrow furnishes most of the new erythrocytes.
Leucocytes may also increase by fission.

Lymphatic 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 fluid
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,

80 ESSENTIALS 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 Ijmiphatic 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 wall of the larynx is 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 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 hucco-pharyngeal respiration. With the glottis
closed, air is drawn into the mouth cavity and forced out by rhythmi-
cal movements of the throat. Oxygen is taken up by blood vessels in
the lining of the mouth 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 is known as cutaneous respiration. During

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 81

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 respiration are discussed in the
chapter on Physiology.

Excretory System and Excretion

The two kidneys lie betv^een the parietal peritoneum and dorsal
body wall in the posterior region of the body cavity. The space in
which they are enclosed is called the cisterna magna. They are
dark red in color, flattened and elongated. They are made up of
a very great number of uriniferous tubules. 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 Endocrine Glands and Their Functions.

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. It has been estimated that, while man ex-
cretes about one-fiftieth of his weight per day, the frog excretes
about one-third of its weight. During the winter, how^ever, in com-
mon with the slowing down of its other body functions, the kidney
function of the frog is practically stopped.

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

82

ESSENTIALS OF ZOOLOGY

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.

Ostium —

Postcaval V-/F

Ovary —

— Mdrenal cjland
- - Renol vein

Oviduct — -Sn

*^.'':.'---i

L. intestine — V---^

W /^

L/terui -%

OoacQ —

»•«*-.

Adus - -

^ ^

■':y^- W\dr)2\j

-nesonephric duct

Urinary 31

Fig. 23. — Urogenital system of the frog from ventral view. Male organs shown
on right side of figure and female on the left. The single urinary bladder is present
in both sexes.

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,

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 83

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 pomposed of cartilage,
cartilage bones, and membrane bones, forms a case for the brain and
capsules for the sense organs. Cartilage bones are the sphenethmoids,
pro-otics, exoccipitals, pterygoids, palatines, and cartilaginous quad-
rates. 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 are the
premaxillaries, maxillaries, nasals, frontoparietals, quadratojugals,
squamosals, parasphenoids, and vomers. The bone* 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
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 cra-
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

84

ESSENTIALS OF ZOOLOGY

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.

"^ Pr^maxHlary

Moxi/lflKy
— Nasal
^ Sphcnethnoid

— Fronbo -parietal

^j y^ Pterygoid
-VI- Squamosal

Pro otic

'4^ - 1 - Squamosal

\<mf—Quadratqjuqal
--xoccipitxil I *■ ' •""' "^ '

Columella
Fig. 24. — Dorsal view of the skull and upper jaw of the bullfrog.

^ — Premaxil/ary

— Palatine

Fiff. 25.

—fronbo-panetal
Pterygoid

Parasphenoid

|- -Squamosal

- Quadratojugat

Quadrate car-
' $. tilaqe

-Ventral view of the skull and upper jaw of the bullfrog.

The visceral skeleton is that part of the axial skeleton which is
composed of the jaws and hyoid arch in the adult. The gill arches
of tadpoles are included in this portion.

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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

85

arches supporting the gills, and evidence 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 forming the sides, and a pair of short
quadratojugals as the posterior portions. The premaxillae and
maxillae each bear a row of small conical teeth. 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 subsequently cov-
ered anteriorly by a dentary bone and posteriorly by an angulo-
splenial bone. The jaws are attached to the cranium by a combina-
tion of three bones on each side, the squamosal, pterygoid, and
palatine, to form a suspensory mechanism.

Dorsal fissure

GraywatieK
Zygapophysis _

White mattec

Central canal
Dorsal root-

Ventmal fissure. ^/

___Neural spine

Neural arch
— Transverse process

Duta mater

_ Qanglion
Pia mater

^. /verve fcrunh
Ventral root

Centrum

i:'i&. 2b.— structure of a single vertebra and cross section of the spinal cord of
J^^\ (Redrawn and modified from Holmes, Biology of the Frog, by permission
of The Macmillan Company, after Howe, Atlas of Zootomy.)

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 from the tadpole tail with the tenth.

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 tj^pical vertebra is kno\vn as the centrum.
The centrum is concave in front and convex posteriorly, and there-
fore is procoelous. Attached to the centrum is a bony ring, the
neural arch, which extends dorsally from the centrum around a por-
tion of the spinal cord. The neural arch has extending from its sides

86

ESSENTIALS OF ZOOLOGY

a pair of riblike transverse processes to which muscles are attached,
and a dorsal projection is the neural spine. In addition, the neural
arch has at each end a pair of processes known as zygapopJiyses by
which the vertebrae are coupled together, the posterior zygapophyses
of one vertebra overlapping the anterior zygapophyses of the succeed-
ing one. 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 an intervertebral
foramina protected by the cartilaginous pads between the vertebrae.

js- -^^^ Episternum

-fc-^i'=s=^^=^ — Omosternum

Epicoracoid
Corb'ilaqe

Mesosternum

T#-^^ Xiph'isternum

Fig. 27. — Ventral view of the pectoral girdle of bullfrog in natural position.

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 sternuni 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.
Each side of the dorsal part of the girdle is composed of a large flat
bone, the suprascapula, which curves ventrally and joins the scapula.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

87

From the ventro-aiiterior end of the scapula two bones extend to the
mid-ventral line of the body and would meet their fellows from the op-
posite side except that the narrow epicoracoid cartilage intervenes.
The anterior of these two bars is the clavicle and the posterior one
the coracoid. At the junction of coracoid and scapula a depression is
formed, known as the glenoid fossa, into which the forelimb articu-
lates. The ventral sternum is separated into two portions by the
pectoral girdle. The anterior portion is composed of a bone, the
omosternum, to which is attached anteriorly a rounded plate of car-
tilage, the episternum. The posterior portion is composed of a bone,
the sternum proper, and a rounded cartilage, the xiphisternum, which
has a notch at its posterior margin through which the abdominal vein
runs as it leaves the body wall.

8^ vertebra

Sacral

diapophyses
cyfmarical

Ilium

Urostyle

Ac€t<abul(^/D

- -hchium
Fig. 28. — Pelvic girdle of bullfrog, dorsal view.

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 pubis. The more slender ilium is at-
tached anteriorly to the transverse process of the ninth vertebra, and
posteriorly it fuses with the pubis and ischium, forming a dishlike
concavity, the acetabulum, which receives the hindlimb. The pubis
is ventral to this point of fusion, and the ischium is posterior.

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

88 ESSENTIALS OF ZOOLOGY

rudimentary fifth near the thumb, the prepollex, consisting of only
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 essentiallj^ 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
tihio-fibula, 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 (astragulus) and fihulare (calcaneum),
and two small tarsals. There are also two extremely small bones form-
ing the prehallux, or rudimentary sixth toe. Distal to the tarsals are
five long metatarsals. 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.

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.

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 kno\vn 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 insertion. 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.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

89

Adductor— one that draws the limb toward the median ventral line.
Abductor — one that draws the limb away from the median ventral
line.

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90 ESSENTIALS OF ZOOLOGY

The pectoral muscles cover the chest and ventral portion of the
upper body region; the rectus abdominis 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.

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) autonomic or sympathetic nervous system.

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
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, and
organs of respiration.

Central Nervous System. — The brain is covered with a pigmented
membrane known as the pia mater. The brain has three main divi-
sions, the forebradn, midbrain, and Jiindbrain. 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 lobes. 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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 91

chiasma is the infundibulum, and somewhat behind this is the vitu
itary body, or hypophysis. The pitiiitaiy is of dual origin, developing
m 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.

Olfactory tract
Olfactory lobe

Cerebrum

Optic neh-ve
Pineal body

^ -Diencephalon

Trlqem'inus^

Tacial

Auditory

Thoracic
enlargement

'Spinal cord

Lumbar

^nlanjement

n/umterminafe,

Optic lobe

Cerebellum

Medulla oblongata

^^ ventricle

J?/O55opbaryngea/

Vaqus nerve

J^ Spinal nerve

2.^ spinal nerve

Brachial
plexus

3^ spinal nerve

.+ - spinal nerve

XalcamoQs body
6p\na\ nerve

Spinal nerve

~ Spinal nerve

.^^^ spinal nerve

3^ spinal nerve

JOtb spinal nerve

Sciatic plexus

Sciatic nerve

Fig. 30. — Dorsal view of nervous system of a frog.

The hmdbrain 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 triangular

92 ESSENTIALS OF ZOOLOGY

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
the brain the cavities, known as ventricles, form a continuous chan-
nel for the flow of cerebrospinal fluid. The ventricles are connected
one with another 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 tri-
angular fourth ventricle in the medulla oblongata. Vascular nets of
blood vessels called chorioid plexuses are found in each of these four
regions, and most of the cerebrospinal fluid is derived 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 within 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 com-
prising 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 neur'ocoele, 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 or autonomic
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, oculomotor, troclilearis, trigem-
inus, dbducens, 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 in 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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 93

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
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 brachial 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 or Autonomic Nervous System. — From the first sym-
pathetic ganglion, nerves are given off Avhich 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 throughout the tissues of the body, all being connected by
sympathetic nerve fibers and finally communicating with the sympa-
thetic trunks.

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-

94 ESSENTIALS OF ZOOLOGY

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 iris 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.

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 cells of the eye which pass
the stimuli received on to the optic nerve. 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.

The ear of the bullfrog is covered externally by a membrane, the
tympanum. A Eustachian tuhe 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.

The inner ear lies in a cavity of the skull known as the auditory
capsule. The structures of the inner ear compose a memhranous
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 saccidus 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.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

95

Sound progresses in the following fasliion. 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 saceulus 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.

Fig. 31. — The right internal ear of the frog, lateral view.

Reproductive Organs

The ovoid testes of the male bullfrog are attached to each kidney
by a fold of peritoneum, the mesorchium. In this fold, running be-
tween the testes and kidneys, are several small ducts, the vasa ef-
ferentia. These ducts connect with the mesonephric duct through
Bidder's canal and 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, mesovarium,
ventral to the kidneys.

96 ESSENTIALS OF ZOOLOGY

The two oviducts are greatly convoliited 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
ostia, 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 ovarj^ and its peritoneal covering and are
free in the body cavity. They make their way to the 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 enormously when it contacts water.

Fertilization in the bullfrog is external, and the spermatozoa of
the male enter the eggs after they have been laid in the water.

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 laying. Eecent
experiments have also shown that these fat bodies are essential for
allowing the normal development of the sex organs and for main-
taining their health. When they are removed, there is a deteriora-
tion 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 2i/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 tha?i 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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

97

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

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(Yolk P/u^)

Thi'^'f; hni^^itni^^'^^.^^ ^f^P cleavage stages, blastula and gastrula of the frog.
;;f ili « * «^^^^i?' unequal type of cleavage. The upper, shaded portion of earl
ot ^^o^^t.^l%^^f^^^^:^^ y^r^^^"^^^ the animal /tem^/ie.'erand the low^r porttSn
toe yolk pmg ^^ ^^ hemisphere. The circular plug seen in the gastrula stage i?

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.

98

ESSENTIALS OF ZOOLOGY

The eggs are fertilized externally by the male, who is clasping
the female as the eggs are laid and discharges spermatozoa into the
water. The first spermatozoon to swim to the egg and enter it by
piercing the vitelline membrane initiates fertilization. After the
sperm has entered, a fertilization memhrane 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 spermatozoan nuclens, fuses with
the nucleus of the Qgg to complete fertilization and start development.

micromcrcs

m&cromerc5

TROaEGGL, CK065 6ECT10M Or BLA'STULA

(Courtesy of General

Fig. 33. — Section through blastula stage of developing frog

Biological Supply House.)

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 blast omeres. The third cleavage is parallel to
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 (micr omeres)
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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL

99

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 hlastula, 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.

Ectodernn ,

Endoderm--

Archenteron -

or

primitive
Intestine

Dorsa] lip of blastopore

Yolk pluqi -Ventral l\p of blastopore

Fig. 34. — Section through gastrula stage of developing frog.

Gastrulation. — Gastrulation begins with the appearance of a small
groove slightly below the equatof of the egg. The upper edge of
this groove is known as the dorsal lip of the blastopore. 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 the others;

100

ESSENTIALS OF ZOOLOGY

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 gastrulation
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 plug.

While the animal cells have been advancing and covering the
vegetal cells, changes have been going on within the egg. The ap-
pearance of the groove or dorsal lip of the blastopore was caused
by an infolding or invagination of outside cells. This invagination
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.

otic pit

ectoderm

rKurftl fube

mesockrm

notochord
hypochord
^omaiic mesoderm
splanchnic mesoderm

mic^ut

forejjut

^^oik

Action through oiic pit

Section through mld-^ut

Fig. 35. — Sections through two levels of the neural tube stage of the developing
frog. (Courtesy of General Biological Supply House.)

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
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 known 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

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 101

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
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.
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
forms a groove known as the neural groove. The neural folds or
lateral edges of the plate curl in and meet in the dorsal midline, where
they fuse to form the neural tube. The neural plate is wider at its
anterior end than it is nearer the blastopore, and at this expanded
anterior end the brain arises, while the remainder becomes 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 sucker 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 may pass out after coming into the mouth over

102 ESSENTIALS OF ZOOLOGY

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 heak somewhat like that of a
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 horny 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 dis-
cussed in the chapter on Meiazoan Organization.

Economic Importance of Amphibia

The entire group of Amphibia is 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.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 103

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
leather. Dried salamanders have been used as a vermifuge. Adult
frogs and salamanders, as well as larval stages, are widely used as
laboratory animals.

Classificatioii of Amphibia

There are estimated to be about 1,900 known species of living
frogs, toads, and salamanders in the world, and about 60 species
of caecilians. None of the caecilians have been reported from the
United States. In the United States there occur about 70 species

^^^^^^^^^^^^^^^^^^^^H

^■1

BBHi

HH

■i

HH

■

1

■

iH

'\.tgmmM

■

1

Fig. 36. — Amhystotna texanum, a very common salamander in Texas. (Photograph

by Ottys Sanders.)

of salamanders and about 67 species of frogs and toads. Many of
these species are subdivided into several subspecies. The Southwest
contains a large proportion of all of these, and Texas has within
its boundaries more species of toads than does any other state in
the Union.

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;

104 ESSENTIALS OF ZOOLOGY

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; and other factors.

Some characters used in classifying adult frogs and toads are:
color markings; length of body and of hind limb; shape of head;
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 ; and other characters.

The student interested in classification and identification of species
should consult appropriate keys for the various groups of Amphibia.

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 (Tailed Amphibians)

Suborder Cryptohranchoidea

Family Cryptobranchidae

Cryptobranchus alleganiensis (1 species). This so-called "hellbender*'
ranges from the eastern states west to Iowa, south to Louisiana.

Suborder Ambystomoidea

Family Ambystomidae

Amby stoma (about 12 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). Washington to Southern California.
Bhyacotriton ■ olympicus (1 species). Olympic Mountains, Wash.

Suborder Salamandroidea

Family Salamandridae

Triturus (about 3 species in the United States). The common newt of the
Southwest is Triturus viridescens louisianensis. The other species repre-
sented is T. meridionalis.

Family Amphiumidae

Amphiuma (2 species). A. tridactylum, the three-toed congo eel, ranges
from northern Florida to eastern Texas.

Family Plothodontidae

Gyrinophilus porphyriticus (1 species). Eastern states west to Kentucky,
south 10 Georgia.

BULLFROG AS TYPICAL VERTEBRATE ANIMAL 105

Pseudotriton (2 species). Pennsylvania to Louisiana.

Eurycea (7 species). Range from New England to Texas. E. quadridigitata,
the 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 rathluni (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.
hrimleyorum, Brimley's triton.

Plethodon (11 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.

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.

Hydromantes platycephala (1 species). Yosemite salamander.
Sul order Proteida

Family Proteidae (with external gills and 2 pairs of limbs)

Nectwus. According to a recent revision of the genus by Mr. Percy Viosca,
of New Orleans, describing two new species from Alabama and one new
species from Louisiana, the number of species in the U. S. is increased
from three to six. 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. beyeri Viosca.

Suborder Meant es

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 (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 (3 species). One species in the East; all three species in the
Southwest. These are the spadefoot toads, the pupils of whose eyes are
vertical when in daylight.

106 ESSENTIALS OF ZOOLOGY

Suborder Procoela

Family Bufonidae (Toads)

Bufo (16 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. fowleri, B. punctatus, B. valUceps, and B. wood-
housii.

Family Leptodactylidae (Robber Frogs)

Leptodactylus alhilabris (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.

Pseudacris (5 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
Texas and Oklahoma to Mississippi, is* a very colorful species, and its
high-pitched staccato chirp is one of the earliest to be heard at breeding
pools in Texas.

Kyla (14 species). Various species in all of the United States. They are
the most colorful of all the frogs. Common species in the Southwest
include : H. arenicolor, H. cinerea, H. crucifer, H. squirella, H. versicolor.

Suborder Diplasiocoela

Family Ranidae (True Frogs)

Bana (18 species). Various species occur in all parts of the United States.
Common species in the Southwest are: E. sphenocephala, R. pipiens, B.
catesbeiana, B. clamitans.

Family Brevicipitidae (Narrow-mouthed Toads)

Hypopachus cuneus (1 species). In southern Texas.

Microhyla (2 species). Ranges from Virginia to Texas, northward to Mis-
souri and Indiana.

References

Dickerson, Mary C: The Frog Book, Garden City, N. Y., 1906, Doubleday Page

& Co.
Holmes, S. J.: The Biology of the Frog, New York, 1927, The Macmillan

Company.
Kellogg, R. : Mexican Tailless Amphibians in the United States, National Mus.

Bull. 160, U. S. National Museum, Washington, D. C.
Noble, G. K.: The Biology of the Amphibia, New York, 1931, McGraw-Hill Book

Company.
Stuart, Richard R.: Anatomy of the Bullfrog, Chicago, 1940, Denoyer-Geppert

Co.

CHAPTER VI

THE RAT, A REPRESENTATIVE MAMMAL

This animal is regarded with disdain usually because of the dam-
age and destruction it does, but it is a very valuable mammal for
study because of its availability and convenient size. Its structures
and metabolic activities are similar to those of the larger mammals
including the human being. The most common example in this
country is the brown or Norway rat, of which the white or albino
is ordinarily a variety.

The classification of the brown rat in the animal kingdom may
well be summarized at this point. It of course belongs to the phylum
Chordata, subphylum Vertehrata, class Mmyimalia, subclass Eutheria,
order Rodentia, family Muridae, genus Rattus, and species norvegicus.
The scientific name therefore is Rattus norvegicus. Its albino variety
is Rattus norvegicus alhinus. The black rat, which is less abundant,
is named Rattus rattus. Another rat which is very common in some
localities of the southern United States is the long-tailed Rattus
alexandrinus. The home of the brown rat is thought to have been
in Northern Asia originally. In the wild form the color is brown
above and gray beneath, but many color varieties have developed
to include gray, yellow, and albino. The most conspicuous charac-
teristic distinguishing the brown (Norway) rat from the black rat
or the roof rat is the shorter tail of the former. Its tail is shorter
than the body while in the other two the tail is as long as the body
or longer.

Habitat and Habits

The rat is secretive in its abode, living most commonly in wood-
piles, trash, stacks of hay or straw^; under the floors, and in the walls
of buildings, attics, and basements. Nests are built of available
material such as cloth, paper, shavings, twine, grass, and fibers. A
warm temperate climate where there is an abundance of available
food and nest-building material furnishes the ideal habitat. Rats
are largely nocturnal in their habits and quite shy in their activities.
They are capable of gnawing their way through wooden partitions
and even lead sewer pipes. Rats are capable of an independent life,
but they flourish when associated with the haunts of man because

107

108 ESSENTIALS OF ZOOLOGY

of the abundance of available food and shelter. Natural enemies of
the rat are dogs, cats, coyotes, foxes, minks, weasels, skunks, owls,
hawks, king snakes, black snakes, and bull snakes.

External Structure of the Rat

The rat is a quadruped and is quite well adapted for climbing as
well as for walking. The body, with the exception of the tail, is
covered with thick fine fur mixed with stiff hairs. The heavy hairs
or '^feelers" about the face are called vibrissae. Those around the
mouth are known as buccal vibrissae and are divided into two
groups: the mystacial in several rows above the upper lip in front
of the eye, and the submental group on the chin. Above the eye is
the superciliary group and the single bristle on each side of the face
below the outer corner of the eye is a genal vibrissa. The interramal
group is a tuft in the ventral midline between the angles of the
jaws. All of these bristles are very sensitive to touch and are of
great use to an animal whose habits are primarily nocturnal.

The body of the rat may be divided into four regions : head, neck,
trunk, and tail. In addition, there are the two pairs of appendages.

The head is rather pointed at the anterior end, and located termi-
nally are the two nostrils, each shaped like an inverted comma and
capable of being closed in case the animal goes under water.

The naked skin around the nostrils is very sensitive to touch. The
mouth is located in a subterminal position. There is a cleft in the
upper lip which exposes the incisor teeth, even when the mouth is
closed. The two sides of the cleft extend inward toward the mid-
line behind the upper incisors almost to meet each other and prac-
tically exclude these teeth from the buccal cavity proper. In the hare
this cleft in the upper lip is very prominent, and it is from this that
the abnormally cleft lip in man has come to be known as ^'harelip."
The eyes of the rat are small with relatively large pupils. They are
well set in the lateral orbits and protected by upper and lower eye-
lids. There is a nictitating membrane, supported by a semilunar
cartilage, in the medial corner of the eye. The eyelashes are fine and
short. The eyeballs are kept moist by secretions from Plarderian
and lacrimal glands, spread over their surfaces by winking. The
ears are located well posterior on the head. The pi^ma which is the
conspicuous part of the external ear is large, erect, and supported

RAT, A REPRESENTATIVE MAMMAL 109

in part by cartilage. It is nearly devoid of hair. At the base of
this is an ear opening, the external auditory meatus, which leads to
the interior of the skull and to the tympanic membrane. The head
region is well supplied with the endings of all the five principal
senses : sight, hearing, smell, touch, and taste.

The trunk is the large portion of the body posterior to the head.
The relatively short cervical region, or neck, joins the head and
trunk. Behind the cervical region is found the chest or thoracic
region and following this is the abdominal region. The latter may
be divided into the lumbar region (small of the back) and sacral or
hip region. Continuing posteriori}^ from the trunk is the tail or
caudal region. Attached to the sides of the trunk are the two pairs
of limbs or appendages. The fore limbs or arms are composed of
the following parts in order: upper arm or brachium, forearm or
antebrachium, wrist or carpus, and hand or manus with its fingers
or digits. There are four well-developed digits and a vestigial thumb.
Beginning at the body each hind limb consists of thigh, shank, ankle
(tarsus), and foot (pes) with its plantar region (sole) and five digits
(toes). There are claws on the digits of both fore and hind limbs.
Since the rat walks with the soles and palms of the feet in contact
with the ground, it is said to be plantigrade in its gait. Animals
like cats and dogs which walk with only the lower sides of the
digits on the ground are digitigrade while animals like horses and
cattle which walk on the tips of the digits are unguligrade.

Along the ventral side of the abdomen are five pairs of nipples or
teats in the albino and six pairs in the Norway rat. In the perineal
region, just beneath the base of the tail is located the anus or opening
through which the fecal matter is discharged from the digestive
tract. Situated also in the perineal region are the external genital
organs. In the male the scrotum is the conspicuous swelling ventral
to the anus. The two testes (sex glands) are usually held inside the
scrotum, but sometimes they lie up in the body cavity instead, in
which case they are poorly developed and much reduced. The ex-
ternal opening of the male urogenital system is in the midventral
line near the anterior end of the scrotum. The prepuce (foreskin),
which is hairy externally, forms the aperture and surrounds the
distal end or glans of the penis. The latter is withdrawn. In the
perineal region of the female there are three openings: the anus,
ventral to the base of the tail; the vaginal orifice (exit from female

110 ESSENTIALS OF ZOOLOGY

reproductive organs) just ventral to the anus, and below this is the
smaller opening from the excretory system at the distal end of a
prominent elevation or papilla.

Skeleton

Characteristic of verebrates the skeleton of the rat is principally
an endoskeleton which is completely enclosed by the soft parts. It is
composed of 223 bones (exclusive of the teeth, chevron bones and
sesamoid bones) and a number of cartilages. There are present
both cartilage hones, which are preformed in cartilage and then re-
placed by bone, and memhrane hones, which are formations of bone
in the dermis in certain portions of the skin. Chevron hones are
paired and extend ventrally from the anterior ends of several of
the caudal or tail vertebrae. The sesamoid bones are pieces of bone
formed in the tendons. The kneecap is 'a good example of these.

The principal functions of the skeleton are to support the body,
give it form, furnish a sturdy attachment for voluntary muscles,
and to protect internal organs. The bones performing the latter
function are usually flattened while those which serve for attach-
ment primarily are long, somewhat cylindrical, and are joined by
movable joints.

It is customary to divide the skeleton of higher vertebrates into
axial skeleton and appendicular skeleton for convenience in study.
The axial portion extends in the main axis of the body and consists
of the skull, vertebral column, ribs, and sternum. The appendicular
portion includes the bones of the fore and hind limbs as well as the
pectoral and pelvic girdles which support them.

The skull is rather conical in shape and somewhat elongated. It
is composed of the cranium, which encases the brain, the jaw struc-
tures, and the hyoid. For the most part these bones are joined to
each other by immovable joints or sutures which are usually marked
by a line of fusion. There are forty-two bones included in the skull.
The dorsal portion or roof of the cranium includes the paired frontal
bones, paired parietals, and the interparietal. The lateral walls are
composed of the tympanic and squamosal portions of the temporal,
alisphenoid, orhito sphenoid, and the lateral portions of the frontals
and parietals. The floor is formed by the hasioccipital, hasisphenoid,
and presphenoid. in order from posterior to anterior. The posterior

RAT, A REPRESENTATIVE MAMMAL,

111

inCraorbitqf ri'ssurQ

L

acnma

(

-\--malar brocess
Molars

-ZuQOTTiatlC

1 suTl

coronal suture

•TumlDaniC'
— saqittal suluTQ

3osT-turnbanlc hooK

ateral tiaTt

baramasloid brocess

— occibital con^ule

Occibital

external occfJDital crest

'?if;.e?.lv~^^"^i PJ ^^^ ^^^' dorsal aspect. (From Greene, Anatomy of the Rat,
iransactions of the American Philosophical Society, New Series, Vol. XXVII, 1935.)

112 ESSENTIALS OF ZOOLOGY

end of the skull is composed of the exoccipitals lateral to the foramen
magnum (opening), supraoccipital dorsal to it, and the basioccipital
ventrally.

The face is composed of the nasals, located just anterior to the
frontals, the bones of the jaws, and the zygomatic arches or cheek-
bones. The upper jaw is formed by the two anterior premaxillae,
and the two longer maxillae. The palatine processes of the maxillae
and premaxillae fuse with the palatine bone to form the hard palate
or roof of the mouth. The skeleton of the nasal chamber is formed
by the vomers and palatines as a floor, ethmoids as walls, and nasals
as the roof. The zygomatic arches are composed of the zygomatic
processes of the maxillae anteriorly, the malar or jugal bone, and the
zygomatic processes of the temporal (squamosal) bones. The lower
jaw is composed of the two dentary bones which meet at the anterior
midline in the mandibular symphysis. The hyoid bone is U-shaped
and lies at the base of the tongue just anterior to the larynx or
voice box.

In addition to the nasal capsule (chamber) mentioned above, there
are the optic capsule or orbit and the auditory capsule. The orbit is
formed by the frontal bone, the lacrimal, orbitosphenoid, alisphe-
noid, and the zygomatic arch. The auditory capsule is enclosed in
the tympanic bulla, a part of the temporal bone.

The rat has only the per^manent set of teeth, and does not have
these preceded by the milk or deciduous teeth found in many other
mammals. There are sixteen teeth in all, including a pair of in-
cisors and three pairs of molars in the upper jaw and the same
number in the lower. The incisors, located at the front, are long
and chisel-like. The lower pair protrudes farther into the mouth
cavity than the upper pair. The upper incisors incline slightly
posteriorly. On the jaws just behind the incisors is a toothless
space, which is known as the diastema. The molars are broad and
cusped for grinding the food gnawed free by the incisors. The
number and location of teeth in an animal's mouth are often given
in a dental formula which shows the teeth of either the right or left
half of the mouth. The formula for the adult rat would be : incisors
1/1, molars 3/3 (1/1, 0/0, 0/0, 3/3). This is a marked modifica-
tion for the mammals. The dental formula for the cat is: incisors
3/3, canines 1/1, premolars 3/2, molars 1/1. For the hog it is:
incisors 3/3, canines 1/1, premolars 4/4, molars 3/3 which is the

RAT, A REPRESENTATIVE MAMMAL 113

typical complete dentition for mammals. This formula for the sheep
is 0/3, 0/1, 3/3, 3/3; for the horse: 3/3, 1/1, 3 or 4/3, 3/3; for
rabbit: 2/1, 0/0, 3/2, 3/3, and for man 2/2, 1/1, 2/2, 3/3. The
teeth of mammals are set in sockets or alveoli in the jaws, a condition
known as thecodont, and since there are different kinds in the same
mouth they are also heterodont.

The skull serves primarily as a protection for the brain and the
special sense organs of the head and furnishes a means of securing
and masticating food material. Numerous muscles are attached to
the external surfaces of its bones.

The vertebral coliiynn or backbone is a series of bones, the vertebrae,
extending from the skull through the axis of the trunk and tail.
This column is divided into five regions or groups of vertebrae :
seven cervical in the neck, thirteen thoracic in the chest ; six lumbar
in the small of the back or loin region ; four in the sacral or pelvic
region ; and twenty-seven to thirty in the caudal or tail region. Each
vertebra articulates with the adjacent one but is separated from it
by intervertebral discs of cartilage. Intervertebral ligaments serve
to connect the adjacent vertebrae. The vertebral column supports
the ribs and pelvic girdle as well as serves for attachment of numer-
ous muscles.

The typical parts of the typical vertebra are the solid, rather
cylindrical, ventral body or centrum; the neural arch at the dorsum
over the central canal; the neural spine or spinous process projecting
dorsally from the arch ; the transverse processes, one extending later-
ally on each side ; zygapophyses or articular processes, a pair project-
ing anteriorly and a pair posteriorly from the arch to articulate
with adjacent vertebrae. The two most anterior cervical vertebrae,
the atlas and axis, are highly modified. The atlas is a heavy ring
of bone with a distinct ventral process; broad, flat transverse proc-
esses; a large spinous process; and anterior articular processes, artic-
ulating with the occipital condyles of the skull, and posterior
zygapophyses articulating with the axis just behind. The axis has
a broad, flat centrum, reduced transverse and ventral processes, and
a large neural spine. The caudal vertebrae are modified by presence
of ventral chevron bones on the anterior ones and almost complete
loss of processes as well as reduction of size in the posterior ones.

There are thirteen pairs of ribs which articulate with the thirteen
thoracic vertebrae. The anterior seven pairs are the true ribs, for

114 ESSENTIALS OF ZOOLOGY

they are attached to the sternum by costal cartilages. The other
six pairs are false ribs because their costal cartilages do not join
the sternum. The eighth, ninth, and tenth pairs attach to the poste-
rior border of the seventh rib and by cartilage to the sternum. The
three posterior are free at the ventral ends and are known as floating
ribs. The sternum or breastbone consists of seven segments called ster-
nebrae, which join each other end to end. The most anterior one is
known as the manubrium and the seventh one, which terminates in
a rounded plate of cartilage, is the xiphoid process.

The appendicular skeleton, consisting of the two girdles and pairs
of limbs, is quite well developed. The pectoral girdle is composed of
the shoulder blade or scapula which is a dorsally located, broad, flat,
more or less triangular bone, and the collar bone or clavicle, which
is a slender, double-curved rod of bone extending from the acromion
process on the apex of the scapula to the manubrium of the sternum.
At the level of the junction of the clavicle and scapula a socket is
formed for articulation with the bone of the upper arm. This is
the glenoid fossa. The bone of the upper arm is the humerus. On
its proximal head portion are the greater and lesser tuberosities which
are muscle attachments. The shaft is nearly cylindrical except in
the proximal portion where it is flattened to form the prominent
deltoid ridge on its anterior surface. The distal end is formed by a
medial head, the trochlea, and a lateral head, the capitulum. The
two bones of the forearm are the radius on the thumb side and the
ulna, parallel to it, on the opposite side. The radius articulates
with the capitulum of the humerus, and the ulna articulates with
the trochlea. The ulna is the larger and longer of the two bones.
Its projection beyond the articulation with the humerus forms the
elbow or olecranon process, and its projection at the articulation
with the wrist forms the styloid process. The radius is slender and
shorter. Its articulation with the capitulum forms a pivot and
allows some rotation of the forearm at the elbow. The hand is
composed of the nine car^pals of the wrist, arranged in two series;
the five metacarpals of the palm (the first is small) ; and the pha-
langes with three in each digit except the thumb which has only two.
The claw or nail is borne on the terminal phalanx.

The pelvic girdle supports the hind limbs. This girdle is com-
posed of three pairs of bones. The three bones of each side are very

RAT, A REPRESENTATIVE MAMMAL 115

closely fused together to form the innominate bone. The three bones
here are the long, anterior ilium, the ventral pubis, and the posterior
ischium. The socket into which the bone of the thigh fits is located
on the side of the innominate at the position where the three bones
unite. This socket is the acetabulum. Ventrally the two pubic bones
meet and fuse in the pubic symphysis. The bone of the thigh is the
femur, and it extends from the acetabulum, into which its head
articulates, to the knee. Besides the head at the proximal end there
are the greater and lesser trochanters. At the distal end the femur
is divided into medial and lateral condyles which articulate with the
bones of the shank. The small patella is the kneecap. The tibia, the
large bone of the shank, extends to the ankle. The slender arched bone
is the fibula, which articulates proximally with the lateral tuberosity
of the tibia and returns distally to fuse into the side of the tibia.
The ankle is composed of eight tarsals arranged in two rows. The
large bone of the proximal row is the calcaneum or heel. The arch
of the foot is supported by the five metatarsals, and the bones of
the digits (toes) are also phalanges. There are three in each except
the first toe, which has only two.

The Muscular System

Myology is the term applied to the study of muscles. The funda-
mental property of contractility in protoplasm is highly specialized
in muscular tissue. The function of muscular tissue, to produce
movement, is accomplished by shortening the muscle cells and later
relaxing them. Those muscles which are contracted under control
of the will are voluntary, and the others are involuntary. Most of
the skeletal (attached to the skeleton) muscles are voluntary, and the
visceral muscles of the internal organs are involuntary in their action.
The voluntary, skeletal muscles are the ones considered in this
chapter. This group functions primarily in locomotion. For the
most part each skeletal muscle is attached by one or both ends to
the periosteum (membranous covering) of bone, usually by tendons.
The origin of a skeletal muscle is the point of attachment on a fixed
bone or other part, which does not move as a result of the contrac-
tion of the muscle. The end of the muscle attached to the bone that
moves when the muscle contracts is the insertion. According to
function, muscles are classified as flexors, extensors, protractors,

116

ESSENTIALS OF ZOOLOGY

supinators, levators, depressors, sphincters, dilators. A fiexor mus-
cle is one wliich bends the part on itself. An extensor is one which
by its action tends to extend or straighten the part. An adductor
muscle is one attached to a long bone in such a way as to move it
toward the ventral midline of the body. An ahductor opposes the
action of the adductor. Pronator muscles rotate the part forward
toward the ventral side while supinators rotate the part back toward
the dorsal side. A levator (elevator) raises or elevates, and a de-
pressor lowers the part concerned. A sphincter muscle is one encir-
cling an aperture which it closes by contraction. The orhicuUrts ons
in the lips around the mouth is an example. A dilator is antagonistic

sTeTnomasToIdeus----'

clavotTafsezius--'
levaToTclaviculse
acTomiodelToideus-
sfiinodelToldeus-

K.. 38.-Superflcia. -f - »' ^ Tmn^'cTionfortte ImericTn' Pwt^sipS
(From Greene, Anatomy of the Rat, iiansacuons oi uie u^

Society, New Series, Vol. XXVII, 1935.)

to a sphincter muscle. Protractor muscles cause the part concerned
to be thrust out as in the case of the tongue while the action of retrac-
tors is to withdraw the part.

Muscles are usually attached to bones in such a way that the
bones serve as levers with the articulations serving as fulcra. Ad-
vantage of power is usually gained in machines by causing a small
force to act through a great distance by use of a long lever thereby
moving a heavy load a short distance. It has been observed by
students of myology that most skeletal muscles operate m an almost
opposite manner, in that usually a powerful muscle acts on a shorter
lever to produce a rapid movement of a small load at the end of a
longer lever. This arrangement produces more speed but it is less

RAT, A REPRESENTATIVE MAMMAL

117

efficient. The two common classes of levers used in the body are
(1) those in which the fulcrum lies between the load and the mus-
cular attachment, and (2) those where the fulcrum is at one end of
the lever, with the muscle attached in the middle, and the load at
the other end.

'^^

3~

k.

en

\

'v^

^t>/l

^

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)

m cd o

Pi

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fc.+J o
0) •" K

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m —

C> C3 S

O c! C

d 0) oJ

^-

03 %se

S"

Ha Ch-T

I Sh -'03

a, o o

.^^-^^

0) P

fe

The following table has been prepared to give a summary of
pertinent information concerning the conspicuous superficial mus-
cles of the rat:

118

ESSENTIALS OF ZOOLOGY

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RAT, A REPRESENTATIVE MAMMAL

119

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(-1
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120

ESSENTIALS OF ZOOLOGY

be

cn
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I

RAT, A REPRESENTATIVE MAMMAL 121

The structure of muscle is discussed in more detail elsewhere in
the book but a brief statement concerning it may be appropriate
here. Each skeletal muscle is composed of a large number of fibers
or cells as they are called. These are bound together by connective
tissue (fascia). An individual fiber is slender and enclosed in an
elastic membrane called the sarcolemma. Within, is the general mass
of protoplasm, which is composed quite largely of the longitudinally
disposed myofibrils. These are surrounded by the more fluid portion
of the sarcoplasm. Each fibril in certain states of contraction shows
alternate dense and light areas along its length. Adjacent fibrils
across the entire fiber have these areas at approximately the same
level, thus giving the appearance of cross-striation through the en-
tire fiber. This is the basis for speaking of this tissue as striated
muscle. Each fiber possesses numerous nuclei, making it multi-
nucleate. Smooth muscle of the visceral organs does not possess the
dense and light areas on the fibrils of the cells.

In detailed study of the muscles they are usually taken by
regions or groups according to location. It is assumed that the
student will have more detailed descriptions of the muscles in labor-
atory directions if they are studied there. Reference should be
made to Greene, ''Anatomy of the Rat," Tr. Am. Philosophical Soc,
Vol. XXVII, 1935, for more detailed descriptions and illustrations.

Dig^estive System

This system is in the form of a tube with various outgrowths and
extends from the mouth to the anus of the animal. It serves pri-
marily in digestion of complex food material, converting it into a
soluble form which may be absorbed by the blood and lymph. The
entire system may be divided into a number of parts as follows :
buccal cavity (mouth), pharynx, esophagus, stomach, small intestine,
large intestine, salivary glands, liver, and pancreas.

The most anterior portion, the buccal cavity, is bounded dorsally
and ventrally by the upper and lower lips. The space between the
teeth and the inner surface of the lips and cheeks is called the
vestibule. The teeth have been described previously with the skull.
The tongue is a well-developed organ lying in the floor of the mouth,
but it does not have the usual frenulum, or band of connective tissue,
extending from its ventral side to the floor of the mouth. There are
papillae (horny processes) over the anterior and lateral portions of
the dorsal surface. There are taste buds in the walls of some of

122 ESSENTIALS OF ZOOLOGY

these papillae. The roof of the mouth is composed of the hard
palate at the posterior margin of which is the soft palate, a muscular
flap hanging down to separate the buccal cavity from the pharynx
just behind. The space between the base of the tongue and the
palate, which provides the opening between mouth and pharynx, is
known as the fauces.

The pharynx also receives the naixs just dorsal to the palate and
this portion of it is called the nasopharynx. It contributes then to
both digestive and respiratory systems. The two Eustachian tubes
from the middle ears enter the pharynx. There are two passages
continuing posteriorly from the pharynx. The aperture leading to
the esophagus which is usually constricted except when food is pass-
ing through it, and the glottis, which is located just posterior to the
tongue, leads to the larynx. The glottis is guarded by the flaplike
epiglottis to prevent food material and water entering it. The epi-
glottis is quite important in swallowing.

The esophagus is a distensible tube extending from the pharynx
to the stomach. It passes through the dorsal portion of the thoracic
cavity, and immediately posterior to the diaphragm it enlarges to
become the stomach. The esophageal end of the stomach is known
as the cardiac portion, and the more inflated, left-hand portion
which follows the cardiac portion is the fundus. The tapering pos-
terior portion which comprises the right-hand side of the organ is
the pyloric portion. In the epithelial lining of the internal wall are
many fine foldings in which are imbedded the gastric glands. They
secrete gastric juice into the cavity. This is an acid solution con-
taining the enzymes (ferments), pepsin and rennin. The former
converts complex proteins to peptones, an intermediate stage of
digestion of this class of food, and the latter coagulates (clots) the
protein portion of milk to concentrate the casein. The pyloric por-
tion of the stomach leads directly into the small intestine through a
sphincter valve located at the junction of these two. This is the
pyloric valve, and it is usually closed. When the chyme (food in
process of digestion) reaches a certain level of acidity (0.4 to 0.5
per cent), the nerve endings controlling the valve are stimulated
and cause it to open, allowing the chyme to rush into the intestine.

The small intestine is the longest division of the alimentary canal,
being approximately six times the length of the body from snout to
anus. Coiled in a manner which almost defies description, it occu-
pies a large portion of the abdominal cavity between the liver and

RAT, A REPRESENTATIVE MAMMAL

23

■ J(?ffi^ffThoTacicand Abdominal Vrsceia

Fig. 40.— Thoracic and abdominal viscera of a male rat, ventral view rFrom
Nrw''s%rferCl!'xUn,l9'35T"'''^''''''' "' '''" American S^llotiphiclf S^Se??:

124 ESSENTIALS OF ZOOLOGY

pelvis and ends by joining the colon of the large intestine near the
right hip bone. Its first turn behind the stomach is the duodenum
and takes the shape of an inverted J turned backwards, to give a
short ascending limb from the stomach and a long descending limb
proceeding posteriorly. The pancreas, which is a branched (often
divided), thin gland, is held in the gastroduodenal mesentery. It fol-
lows the curve of the duodenum extending posteriorly and to the left
into the gastrosplenic mesentery. The pancreatic juice, which it pro-
duces, is collected by numerous, small, paired pancreatic ducts. The
ducts join the distal portion of the bile duct which enters the duo-
denum. Posteriorly the duodenum continues into the jejunum whose
boundaries are not very clearly defined anatomically but more readily
distinguished in histological studies. The ileum is the rather tightly
coiled portion which extends to the large intestine. The contents of
the ileum at this point pass through the ileocolic valve which com-
municates with both the colon (large intestine) and cecum. The
latter is a broad, curved sac on the right side of the abdominal cavity
and is about an inch to an inch and one-half in length. It occupies
the position of the vermiform appendix of man. This blind sac serves
as a cavity for the storage of partially digested food and fecal matter
in the rat. The cecum and ileum join the colon or large intestine
in its ascending portion. This part extends anteriorly to the region
of the stomach where it crosses to the middle of the cavity or beyond
as the transverse colon, turns posteriorly, and continues as the de-
scending colon. The rectum is the continuation of this portion of the
colon through the pelvis, and it terminates externally at the anus.
The fecal matter forms pellets and the masses of these pellets in the
colon infiuence the shape of certain portions of it.

Glands Associated With the Digestive System

Salivary Glands. — The parotid gland is a rather loosely organized
but conspicuous structure extending from the ventrolateral surface of
the neck to a position around the base and behind the ear. Its pos-
terior extremity reaches the shoulder, covering the outer half of the
clavicle. The parotid duct (Stenson's duct) is formed by the union
of three branches and proceeds across the masseter muscle to enter
the mouth through the cheek just opposite the upper molar teeth.
The submaxillary glands are large, elongated, and located on the ven-
tral surface of the neck. They are in contact with each other along

RAT, A REPRESENTATIVE MAMMAL 125

r

the midventral line from the hyoid almost to the manubrium. A sub-
maxillary duct (Wharton's) leaves the anterior end of each gland
and leads anteriorly along the medial border of each mandible, enter-
ing the floor of the mouth just posterior to the incisor teeth. Joining
the anterior ends of the submaxillary glands and covering their ducts,
the sublingual glands are found, the ducts of which enter the floor of
the mouth also. The salivary glands secrete saliva, which contains
mucus and the starch-digesting enzyme ptyalin. This enzyme con-
verts starches in the food to the intermediate sugar, maltose.

The Liver. — This large gland is located ventrally in the cavity,
just posterior to the diaphragm at about the level of the junction of
esophagus and stomach. It is divided into four parts : the two lateral
lohes; the median (cystic) lobe; and the caudate lobe, which is smaller
and partially encircles the esophagus. No gall bladder is present.
The bile duct (ductus choledochus) is formed by tributaries (hepatic
ducts) from various lobes of the liver. This duct in the rat seems not
to substitute for a gall bladder as it does in some other animals where
the bladder is absent. The bile duct enters the duodenum a little more
than an inch from the pylorus. The liver not only secretes the bile
which is important in alkalizing the chyme and emulsifying the fats
in the intestine as well as possibly producing some fat and carbohy-
drate digesting enzymes, but it is also an organ for the storage of
digested carbohydrates and fats. Furthermore, it is an important
organ in the formation of urea which is then carried by the blood to
the kidneys for excretion.

The gastric glands located in the fine folds (crypts) of the mucosa
of the stomach have been mentioned already. The mucosa or inner
lining of the small intestine also has numerous simple glands which
secrete intestinal juice. This juice protects the walls of the intes-
tine, lubricates them to assist in the movement of food, contains the
protease enzyme (protein-splitting) erepsin for digestion of peptones,
and probably produces maltase for the conversion of maltose to glu-
cose. It may be that there are even other enzymes produced here.

Of course, the digestive action of the three pancreatic enzymes oc-
curs in the small intestine. These are as follows: trypsin, a pro-
tease, converting complex proteins and peptones to amino acids; amy-
lopsin, a diastase, converting starches into maltose; and steapsin, a
lipase, converting complex fats into fatty acids (oleic, stearic, butyric,
palmitic) and glycerin.

126 ESSENTIALS OF ZOOLOGY

In general, the primary function of the digestive system is to con-
vert the complex food material into an absorbable solution which may
readily be taken up by the blood and in turn by the protoplasm of
the tissues. The digestive enzymes are indispensable agents in this
function.

Circulatory System

This system includes the blood vascular and lymphatic vascular
portions. The former consists of the heart, arteries, capillaries, and
veins. This system transports dissolved food materials, oxygen, car-
bon dioxide, excretions, hormones, and antibodies to and from the
cells over the body. In addition, the distribution of the blood helps
in equalizing the temperature throughout the body. Blood and
lymph are the two vascular fluids.

The Heart.^ — This muscular organ is located in a division of the
thoracic cavity, the pericardial cavity, which is separated from the
remainder of the chest by the transparent pericardium. The heart
of the rat is quite representative of the mammalian heart with its two
auricles (atria) and two ventricles. The right and left sides of the
adult heart are quite distinct. The right atrium (auricle) receives
systemic blood from the precaval (superior vena cava) and postcaval
(inferior vena cava) veins which collect the venous blood from the
various organs of the body. Contraction of the right atrium forces
the blood through the right auriculoventricular orifice which is
guarded by the tricuspid valve and into the right ventricle. At the
same time the contraction of the left atrium, which has received
aerated blood from the lungs, forces that blood through the left atrio-
ventricular orifice which is guarded by the bicuspid or mitral valve
and into the' left ventricle. Immediately following the simultaneous
contraction (systole) of the two atria, the ventricles contract. The
blood from the right ventricle is forced into the pulmonary arteries
and carried to the lungs while the blood from the left ventricle is
entering the aortic arch to be distributed to all parts of the body.
There is no direct connection between the cavities of the two sides of
the heart. The principal valves of the heart are the tricuspid and
bicuspid mentioned above, and the two semilunar valves. There is
one of the latter in the proximal portion of the pulmonary artery
and the other in. the aorta. The function of these valves is to prevent
backflow of blood, in order that the blood may ])e kept in circulation

RAT, A REPRESENTATIVE MAMMAL

127

in a given direction. The heart is a muscular pump which forces the
blood through the body. Its complete failure causes almost immediate
death.

L. Common Caroiid A.
R. Common Caro^jci A.

R. Subclavian A,

= L. Subclavian f\.

Innom* na-^ e A.
Ao »-^ »C A re Va

—rj From L. Ven-^ricle

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Co«\iac A.

Renal A.

Superior HesertVericA.

K idney
Mio\u«v»V>a»- A.

p«rma^ic. A.
ll-.ac A.

Fig-. 41. — Diagram of the arteries of the rat from ventral view.

Blood Vessels.— The large artery leaving the left ventricle is the
aorta, and it soon branches to supply the principal organs of the body.
From its arch arise the innominate, left common carotid, and then

128

ESSENTIALS OF ZOOLOGY

the left subclavian arteries. The innominate artery then divides quite
shortly into the right commo7i carotid and the right subclavian arter-
ies. The carotids supply the head and neck; the siibclavians supply

External Ja^ulat V.

-L . Aa\ \\ar>^ V.
-L Subclavian V.
Vcvtebral V.

L. S up^r io r

Vena Cava
-R. Atrium

-J- \/en1:ric\e

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-Suora»-©nal V.
-I. H^nal V.

-K\<Jnev
Iliolumbar V

LJntemal . .
Dp«rma"tCc V.

c

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Wiac V.

Fig. 42. — Diagram of the veins of the rat from ventral view.

t

Ihe shoulders and arms ; the aorta takes a mid-dorsal position and ex-
tends the length of the trunk and tail. It is known as the dorsal aorta
and from anterior to posterior it gives rise to the intercostal arteries
to the ribs; phrenic artery to the diaphragm; celiac artery to the

RAT, A REPRESENTATIVE MAMMAL 129

esophagus, stomach, duodenum, liver, pancreas and spleen; superior
mesenteric artery to the pancreas, small intestine, cecum, and colon;
infer'ior mesenteric artery to the colon and rectum; lumbar arteries,
paired to the back; renal arteries, paired to the kidneys; and finally
at the posterior, the division into the two common iliac arteries in the
pelvic region. These each branch into external and internal iliac ar-
teries and supply the pelvis and hind limbs. The aorta proceeds pos-
teriorly as the caudal (coccygeal) artery to supply the tail.

In the tissues of the organs over the body the arteries continue to
branch and rebranch until their diameter becomes small enough to
allow them to pass between individual cells. These minute vessels
are called capillaries and while in them the blood exchanges oxygen
and food material for carbon dioxide and excretory wastes from the
cells of the tissues. After passing among the cells of an organ the
capillaries converge to form small veins (venules), which join each
other in forming the larger ones.

The large veins joining the heart have already been mentioned.
They are the postcava from the posterior and precavae from the an-
terior. The single postcava is formed by the union of the caudal and
the paired common iliacs in the pelvis. This vessel lies in the mid-
dorsal line, receiving paired renal veins from the kidneys, genital
veins from the gonads, and the suprarenal veins from suprarenal
glands. At the level of the liver, hepatic veins join the postcava.
Anterior to this it enters the right atrium. The blood in the stomach,
intestines, pancreas, spleen is collected by the branches of the hepatic
portal vein, which delivers the blood to the liver where it distributes
through the special capillaries (sinusoids) and is collected by the
hepatic veins. The precaval veins are formed one on each side by
the subclavian veins from the forelimbs, the internal and external
jugular veins from the head and neck, and the vertebral from the
brain. The precavae, like the postcava, enter the right atrium of the
heart bringing in the venous blood.

Blood. — Generally speaking, thirty rats are approximately equal
to one human metabolically. If this holds true in regard to the
quantity of blood in the body of a rat, it has approximately 170 c.c.
of blood since it is held that the human body carries between 5 and
6 liters. Blood is a scarlet substance composed of a fluid in which
cells float. Its specific gravity is about 1.055. The liquid basis is
called plasma, and the cells are known as corpuscles. Plasma con-

130 ESSENTIALS OF ZOOLOGY

stitutes approximately 60 per cent of the volume of blood. It is
itself about 90 per cent water and 10 per cent dissolved solids, chiefly
proteins and salts. Sodium chloride is the most abundant salt here.
Calcium and phosphorus salts are present in small proportions but,
nevertheless, are very important. An interesting suggestion has even
been made: that salts of the plasma are the same in kind and pro-
portion as those which were present in the prehistoric sea. The
primitive marine animals are thought to have had body fluids based
on sea water. Their descendants seem to have carried this composition
down through time. However, modern sea water has become about 3
times as concentrated. The plasma carries also carbon dioxide col-
lected from tissues, hormones, and antibodies. The hormone materials
serve to regulate the metabolism of different tissues over the body,
and the antibodies bring about immunity to certain diseases.

The corpuscles, or formed elements, as they are often called, are
of two classes, the red corpuscles, or erythrocytes, and the white cor-
puscles, or leucocytes. The red corpuscles are small, nonnucleated
cells measuring about %2oo i^^ch across the face of each in man, and
approximately the same in the rat. These cells contain hemoglobin,
a red pigment, which is apparent in the mass but not evident in a
single corpuscle. Single corpuscles have a greenish amber cast, and
are in the shape of biconcave discs. There are about 5,000,000 red
corpuscles in each cubic millimeter of blood in the male human
(4,500,000 in female). This furnishes an idea of the number which
may be present in the rat. Hemoglobin is capable of combining
readily with oxygen in such a way that the oxygen may be given up
easily to cells with lower oxygen content. Red cells, then, are the
oxygen carriers of the blood. In adult animals these corpuscles are
formed primarily in the red bone marrow. In earlier life the liver
and spleen supplement this. The total surface area of all the red
corpuscles in the body of a rat has been estimated as the equivalent
of about 3 square rods.

The leucocytes are nearly colorless in their natural condition. They
are quite variable in size and shape. The majority of the white cells
are of the amoeboid type and many have different types of granules
in the cytoplasm. These cells are classified according to nuclear
condition or shape, size of cell, and staining reaction of these cyto-
plasmic granules. These serve the body in devouring foreign mate-
rial, including bacteria. In this way they protect the animal's body
against toxins and disease.

RAT, A REPRESENTATIVE MAMMAL 131

Because of the presence of the substances fibrinogen (a protein),
throiJihin, and calcium salts, in the blood, it clots upon exposure to
air. The solid fibrous material formed in clotting is fibrin. The
thin, watery straw-colored liquid which separates from the clot is
sei^um.

Lymphatic System. — As the blood passes through the tissues of
the various organs, it loses a portion of its fluid base. This liquid
which seeps from the capillaries is lymph and is collected in the
Ijmiph spaces surrounding the cells in most tissues. It is quite simi-
lar to plasma, and it distributes food and other necessities to the in-
dividual cells and collects carbon dioxide and other waste products to
be returned to the blood when it finally returns to it by way of the
thoracic lymphatic duct and the subclavian veins. The thoracic duct,
the largest in the body, lies in the dorsal part of the trunk between
the dorsal aorta and the vertebral column. It receives the various
Ijonphatic vessels. These vessels have ultimately been formed by the
union of the lymph spaces between the cells in the tissues. Along the
lymphatic vessels at certain points are glandular masses, the lymph
glands or nodes. These bodies produce Ij^mphocytes, a type of white
cell carried in the IjTnph and blood. In certain portions of the body
these glands are abundant, as the inguinal (groin), axillary (armpit),
popliteal (knee), cubital (elbow), tracheal, and submaxillary regions.

Respiratory System

In mammals the special organs of respiration are the lungs,
wherein the carbon dioxide is taken from the blood and oxygen is
taken from the air by the blood. The nasal passages lead through
the nasopharynx to the pharjmx and from here through the glottis
to the larynx (voice box), thence by trachea (windpipe) to the
bronchi, and into the lungs. The air is warmed as it is passed
through the nasal chambers and pharynx. The turbinated bones
form extensive convolutions in the lining of the inner surface of
the nasal chamber. The tear ducts lead from the eyes into the nasal
chamber while the Eustachian (auditory) tubes lead from the
middle ear into the naso-pharynx. Air passes from the pharynx,
around the fingerlike epiglottis which guards the slitlike glottis lead-
ing into the larynx or voice box. The epiglottis closes the entrance
of the glottis only during swallowing in order that food or other
material may not enter. The vocal cords are located on the inner wall

132 ESSENTIALS OF ZOOLOGY

of the larynx. They appear as folds of the epithelial lining. The
larynx which is similar to a small box continues into the tubular
trachea or windpipe posteriorly. The presence of a number of
C-shaped cartilages embedded in its wall prevents it from collapsing
as air is drawn through. The trachea extends parallel to, and is in
contact with, the ventral side of the esophagus. At the posterior end,
it bifurcates to form two primary h7'onchi, each of which leads to a
lung in which it is partially embedded. The left lung has but one lobe
while the right has four, three of which may be seen from the ventral
surface.

Each lung is completely invested externally by a delicate, though
tough, transparent serous membrane called the pleura. This mem-
brane is continuous with the lining of the wall of the pleural cavity.
The portion adherent to the surface of the lung is the visceral pleura,
and the portion lining the inner surface of the body wall is the
parietal pleura. Within the lungs the primary bronchi branch and
continue to branch as secondary and tertiary bronchi, finally becom-
ing small hronchioles which lead into the alveoli, or tiny terminal
air sacs.

Breathing is affected by increasing and decreasing the volume of
the thoracic cavity. To do this the ribs are moved forward and
spread by intercostal and other muscles, while the diaphragm which
usually arches anteriorly is contracted to a flat position. As the chest
cavity is thus enlarged, the internal pressure is reduced, and to bal-
ance the pressure, air naturally rushes into the lungs from the out-
side. When relaxed, the walls of the thorax and the diaphragm both
return to their original positions and expel the air. Drawing air into
the lungs is called inspiration, and discharging it is expiration. While
the air is in the alveoli of the lungs, oxygen is absorbed from it by
hemoglobin in the blood distributed in the capillaries of the pulmo-
nary veins embedded just beneath the epithelium here. At the same
time carbon dioxide is given up by the blood to the air. The pulmo-
nary veins return the oxygenated blood to the heart for redistribution
to all parts of the body.

The Nervous System

In the simple forms of life and in individual cells of the higher
forms, the cell membrane is the area where vital reaction takes
place. The two fundamental properties, irritability and conduc-
tivity, operate to receive the stimuli and conduct the impulses in the

KAT, A KKPRESENTATIVE MAMMAL 133

protoplasm. As nervous tissue develops, these properties are spe-
cialized, and the efficiency of the organism increases as the awareness
of the environment increases. The development of sense organs in-
creases and enhances this awareness. Although animals without nerv-
ous systems exhibit excitation, conduction, and to some extent cor-
relation, these functions expand in range and power when the sense
organs and nerve pathways become available. As complexity and
range in the mechanism of the nervous system increase, centers of
correlation arise for a more precise analysis of the excitation in order
that an appropriate reaction will result. Thus, means is provided
for the organism to relate itself better to the environment.

The fundamental units of the nervous system consist of a sensory
neuron and a motor neuron. The neuron (nerve cell with all of its
processes) is the unit of structure in nervous systems, from the sim-
plest to the most complex, even to include those of rats and men. Two
or more neurons may be associated to function as a reflex arc, and this
arc rather than the single neuron becomes the functional unit. In
higher forms of vertebrate animals there is more and more organiza-
tion and concentration of neurons which brings about increasing
prominence of the brain portion of the system. In man, cerebral ac-
tivity takes a dominant part in the functions of the entire system;
however, there has been no loss of significance or extent of develop-
ment and activities of the other parts. The entire system controls
and coordinates the activities of the other parts of the body.

The entire system of the rat and other higher vertebrates is usually
divided into four general divisions: (1) central, including the brain
and spinal cord; (2) peripheral nerves, including cranial and spinal
nerves; (3) sense organs, as eyes, ears, tactile corpuscles, taste buds,
and olfactory structures; and (4) autonomic (sympathetic), includ-
ing the two trunks of ganglia.

Central Nervous System. — The central portion of the system is
tubular, having developed as a simple neural tube. This portion of
the system is covered by three membranes called meninges. The
outer tough membrane is the dura mater; beneath this is the delicate
arachnoid layer ; and adhering to the surface of the nervous tissue is
the vascular pia mater. The anterior region of this central portion
is the brain. It is considerably broadened and thickened, presenting
five divisions. From anterior to posterior these divisions are: tel-
encephalon, diencephalon, mesencephalon, metencephalon and mye-
lencephalon.

134 ESSENTIALS OF ZOOLOGY

The anterior division is the telencephalon, which includes the
olfactory lobes and cerebrum. The former are paired lobes at the
most anterior end. They are well developed and serve as centers of
the sense of smell which is quite specialized in the rat. The cerebrum
is divided longitudinally into two hemispheres one on each side of
the dorsal median fissure. There are no deep furrows or convolutions
in the surface of the cerebrum as is the case in the cat, man, or a
number of other mammals. The cerebrum is relatively large and has
spread until the two middle divisions of the brain have been covered
by it. Between the two hemispheres is a broad band of fibers, the
corpus callosum, which connects them. The fornix, anterior commis-
sure, and posterior commissure are other bundles of nerve fibers con-
necting the two hemispheres. Thus there is coordination of function
in the two halves of the cerebrum. This division of the brain serves
as the center of voluntary control, correlations, and many associations.

The diencephalon or thalamus which is in the anterior portion of
the brain stem and is covered by the cerebrum forms the principal
connections between the cerebrum and other parts of the brain. In
particular there is a relationship between the olfactory lobes and this
part. The diencephalon is another important center of the sense of
smell.

The mesencephalon, or midbrain, is located just posterior to the
preceding division and is also covered by the posterior portion of the
cerebral hemispheres. On its dorsal side are located the corpora
quadrigemina. There are four of these prominences, and they are
homologous to the paired optic lobes of the frog and other simpler
vertebrates. The ventral portion of the midbrain consists primarily
of two large bundles of nerve fibers called the cerebral peduncles.
The fibers of these make the numerous connections between the cere-
brum and medulla, which in turn leads to the spinal cord. There
are visual and auditory centers located in the midbrain.

The cerebellum is the convoluted portion of the brain which ap-
pears externally, just posterior to the posterior margins of the cere-
bral hemispheres. Its position is just posterior to the midbrain also.
Its median lobe is the vermis, and at the sides of this are the right
and left hemispheres. The large bundles of fibers extending to the
ventral side of the brain at this level constitute the pons and repre-
sent the only portion of the cerebellum to appear on the ventral side.
This division of the brain serves as a center of coordinated movement
and is particularly associated with balance.

RAT, A REPRESENTATIVE MAMMAL

135

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136 ESSENTIALS OF ZOOLOGY

The medulla oblongata is the most posterior division of the brain.
Its anterior portion is somewhat covered by the cerebellum, and ven-
trally it has only the pons between it and the midbrain. The cavity
of the medulla (fourth ventricle) is ventral to the posterior portion
of the cerebellum and is covered by a vascular membrane which is a
part of the chorioid plexus. The respiratory center and certain of
the other reflex centers are located in this division. It serves also as
the important pathway for the bundles of nerve fibers extending from
other parts of the brain to the spinal cord.

The brain developed as a specialized anterior portion of the em-
bryonic neural tube and has retained the hollow form. This original
cavity is modified into spaces within the principal divisions and lobes
in the form of ventricles. The first two, the lateral ventricles, are lo-
cated one in each of the cerebral hemispheres, and extend anteriorly
into the olfactory lobes. Posteriorly they join the third ventricle of
the diencephalon by way of the foramen- of Monro. This third ven-
tricle extends ventrally into the infundibulum, which is a downpush-
ing in the floor of the diencephalon. A narrow channel, the cerebral
aqueduct (aqueduct of Sylvius), leads posteriorly from the third ven-
tricle through the midbrain and ventrally to the cerebellum to join
the broad fourth ventricle of the medulla oblongata. The fourth ven-
tricle continues posteriorly into the narrow cavity (neurocoele) of
the spinal cord.

Spinal Cord. — This is the division of the central portion of the
nervous system which extends through the length of the trunk and
into the tail. This elongated cord is in the shape of a somewhat
flattened cylinder which tapers to a fine-pointed filum terminale at
the posterior end within the anterior portion of the tail. There are
two enlargements along the length of the cord. They are the cervical
enlargement in the neck-shoulder region, and the sciatic enlargement
in the lumbar region. The large nerves supplying the front and hind
limbs arise at these levels, respectively. Spinal nerves are distributed
in pairs and leave the cord segmentally. The neurocoele is the small
continuous cavity through the length of the cord, joining the fourth
ventricle at the anterior. The gray matter containing nerve cell
bodies surrounds the neurocoele in the shape of the letter H or, per-
haps more accurately, in the shape of a butterfly with wings spread.
The white matter, which is largely nerve fibers, surrounds the gray
matter. These fibers extend lengthwise throuerh the cord and connect

RAT, A REPRESENTATIVE MAMMAL 137

different levels of the cord with each other and the brain. Each of
the spinal nerves, which were mentioned above, joins the gray matter
of the cord by two roots, a ventral root which enters the ventral
column and a dorsal ix)ot which enters the dorsal column of gray
matter. The dorsal root carries the afferent nerve fibers to the cord,
while the ventral root carries the efferent fibers. A spinal ganglion
containing cell bodies of afferent fibers is associated with each dorsal
root.

The trunk and limbs receive man}^ impulses produced by environ-
mental stimuli, as well as entertaining most of the activities of locomo-
tion, digestion, circulation, excretion, respiration, reproduction, etc.
All of these must be kept properly related and coordinated in order
that the organism be kept alive and carrying on normal activities. This
is done largely by the spinal cord and associated autonomic portion of
the system. Certain impulses are transmitted by the cord to the brain
for association and reaction there. In general, it may be said that
the cord functions as a center with some power of integration, par-
ticularly for reflexes of the trunk and limbs, as well as a pathway
to the brain.

Peripheral Nerves. — Twelve pairs of cranial nerves leave the brain
of the rat and thirty-four pairs of spinal nerves are given off by the
spinal cord. These make connections with the sense organs (recep-
tors), motor end plates in muscles (effectors), as well as the auto-
nomic portion of the system. The nerves are composed of bundles of
nerve fibers, and each fiber is the extended process of a neuron. These
processes are axons (neurites) and dendrites. All portions of the
head, trunk, and limbs are supplied with nerve fibers, which are in
communication with the central nervous system.

The paired cranial nerves all pass from the skull through foramina,
and all except one are distributed to the head and neck. They are
divided according to function into motor, sensory, or mixed (having
both motor and sensory fibers). The olfactory (I), optic (II), and
auditory (VIII) are the only cranial nerves which are wholly sensory.
The oculomotor (III), trochlear (IV), abducent (VI), spinal acces-
sory (XI), and hypoglossal (XII) are motor. The trigemiyial (V),
facial (VII), glossopharyngeal (IX), vagus (X) are mixed. The last
two named are mainly motor, having only a small number of sensory
fibers. Some of the sensory roots of these nerves bear ganglia (aggre-
gations of nerve cell bodies). The more important of these is the
Gasserian (semilunar) ganglion of the trigeminus.

138

ESSENTIALS OF ZOOLOGY

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RAT, A REPRESENTATIVE MAMMAL 139

A summary of pertinent information concerning this group of
nerves is organized into the accompanying table.

The paired spinal nerves are connected to the spinal cord by a
dorsal (sensory) root and a ventral (motor) root. The former trans-
mits impulses to the cord, and the latter consists of fibers carrying
motor impulses away from the cord. A small ganglion is located in
each dorsal root. The two roots forming each nerve join each other
while still within the neural canal of the vertebral column, and the
nerve then emerges through the intervertebral foramen of that level.
Immediately outside the intervertebral foramen each nerve gives off
a dorsal branch (ramus) to the muscles of the back, the main ventral
branch, and a small connecting bundle (ramus communicans) to the
autonomic system. In general, a pair of spinal nerves leaves the
spinal cord at each meeting of adjacent vertebrae for most of the
length of the vertebral column. In certain regions these nerves anas-
tomose (join in a network), forming plexuses. In the neck region
is the cervical plexus; in the region of the shoulder and forelimb, the
brachial plexus; in the region of the loins, the lumbar plexus; and in
the region of the sacrum, the sacral plexus.

Sense Orguns. — The organs of sense, or receptors, and specialized
peripheral endings of the sensory nerves, are constructed to be par-
ticularly efficient in receiving a certain type of stimuli. The eyes,
ears, olfactory membrane of the nose, taste buds, pressure and
tactile corpuscles are all endings of this kind. The stimuli for the
eye are in the form of light waves ; those of the ear are ether vibra-
tions ; those of taste and smell are chemical ; and those of touch are
pressure and contact. These organs and their function will be dis-
cussed in the chapter on Physiology.

Autonomic Nervous System. — The mere sight of food to a hungry
person causes the flow of both saliva and gastric juice. This activ-
ity occurs without conscious control and is one of the multitude of
examples of the functioning of the autonomic portion of the nervous
system. The principal parts composing this system include two
trunks or chains of ganglia, one at each side of the vertebral
column on the dorsal wall of the coelome. The anterior extremity
of each trunk is the superior cervical ganglion near the angle of the
jaw and covered by the submaxillary gland. In each trunk there are
cervical (or cranial) ganglia with fibers extending to the ciliary
body in the eye, salivary glands, heart, bronchi, stomach, intestines,

140 ESSENTIALS OF ZOOLOGY

liver, pancreas, kidneys, and spleen; thoracicolumhar ganglia with
fibers to muscles of hairs, skin arterioles, eye, viscera, blood vessels,
and certain urogenital structures; sacral ganglia with fibers to ex-
ternal genital organs, urinary bladder and large intestine. Auto-
nomic impulses either stimulate or inhibit action in the organs sup-
plied. Each autonomic ganglion is connected to the nerve of its
level by a band of fibers called the ramus communicans.

Excretory System

The principal organs constituting this system are a pair of kidneys,
a pair of ureters, a single bladder and a single urethra. These
organs relieve the blood of urea, various salts, excess water, etc.,
and discharge these substances from the body as urine.

The kidneys are bean-shaped structures located dorsal to the peri-
toneum in the lumbar region of the trunk. The two kidneys are not
exactly opposite each other on the two sides of the midline, the left
one being about a half a length more posterior. At the medial side
of each kidney is a depression called the liilum where the renal
blood vessels and ureter join it. The ureters are tubules leading,
one from each kidney, to the urinary bladder. The internal struc-
ture of the kidney consists of a more peripheral or outer layer, the
cortex, which lies just internal to the capsule, and the inner medullary
portion. The cortex contains the Malpighian corpuscles which are
essential structures in receiving urine from the blood. The medul-
lary portion is composed largely of tubules. These tubules converge
in the papillae which lead to the sinus in the hilum. Here the
papillae empty into the widened end of the ureter, which fills the
renal sinus and is called the renal pelvis. The Malpighian (renal)
corpuscle is composed of the inverted, swollen end of a uriniferous
tubule and a mass of blood capillaries enclosed by it. The former
is known as Bowman's capsule and the knot of capillaries enfolded
by it is the glomei^ulus.

The urinary bladder is a thin-walled oblong sac in the posterior
portion of the abdominal cavity. It receives the urine from the
ureters. The narrowed neck which continues from it to the exterior
is named the urethra. The urethra becomes tubular, and in the
male extends through the penis to its tip, serving both excretory
and reproductive functions through most of its length. In the

RAT, A REPRESENTATIVE MAMMAL

141

female it is strictly excretory and its aperture is near the clitoris.
In passing from the blood to the exterior, urine would follow about
the following course: glomerulus, Bowman's capsule, uriniferous
tubule, collecting tubule, papilla, renal pelvis, ureter, urinary blad-
der, urethra to the external orifice. The urine which is excreted

uTeT'ine
uTerus

uTeteT

bladdcT"

uTethra--'

vaaina---

TecTum-

anus— -

diabhiagm
infeTioT vena cava
su[DTaTena!

kidneu

ureteT

ovaTLj with follicles;
oviduct

ovaiian aiteTu-
TecTum

Fig. 44.— Female urinogenital system of the rat, ventral view. (From Greene.
^natomy of the Rat, Transactions of the American Philosophical Society, New
Series, Vol. XXVII, 1935.)

142 ESSENTIALS OP ZOOLOGY

carries excess nitrogenous materials and must be eliminated. These
wastes are principally in the form of urea, uric acid, and creatinin
in water solution. The passage of urine from the blood to the
uriniferous tubules is a complex process combining secretory activ-
ity and osmosis.

The kidneys of the rat, all other mammals, birds, and reptiles be-
long to the type known as metanephros or true kidney. Fishes and
amphibians in adult form have a different kidney structure, called
mesonephros. The higher vertebrates possess this as an embryonic
structure also.

Reproductive System

In the early stages of the development of the individual the organs
of sex show no indication as to its future sex. The gonads and ex-
ternal genitalia of both sexes are similar up to a certain point, and
both sets of accessory tubules develop in each individual. At a
certain stage in the development of the individual the sex becomes
established. The gonads take on characteristics of either ovaries or
testes, one set of accessory ducts is accelerated in development while
the other is inhibited, and there is a modification in the developing
external genitalia in keeping with the sex to be. The manner in
which this differentiation occurs is shown in the following table :

INDIFFERENT EMBRYONIC
CONDITION

MATURE MALE MATURE FEMALE

Primitive genital gland Testis Ovary

Mesonephros Epididymis Epoophoron

Paradidymis Paroophoron

Mesonephric duct Vas Deferens Disappears

Miillerian duct Vestigial appendix of Oviduct, uterus, and

testis vagina

The principal function of this system is the production of germ
cells (sex cells or gametes). Female germ cells are produced in
the ovaries and are called ova. The male germ cells are produced
in the testes and are called spermatozoa.

The Female Reproductive Organs. — Included under this title are
external genitalia, two ovaries, pair of oviducts (Fallopian tubes),
uterus, and vagina. The external structures are in the perineal
region just ventral to the anus. The diminutive clitoris is a fold of
tissue here which is terminated by the glans clitoris. The clitoris

RAT, A REPRESENTATIVE MAMMAL 143

is homologous to the penis of the male and is similarly erectile
tissue which becomes engorged with blood during sexual excitement.
The vaginal orifice is immediately ventral to the anus and the urinary
aper^ture just ventral to this, being at the base of the clitoris.

Internally, the ovaries are located in the pelvic portion of the ab-
domen. They are small and flattened. The mesentery which sup-
ports each ovary is the mesovarium. The oviducts are rather slender
and less than an inch in length. The mouth of each oviduct is near
the ovary of that side and is called the ostium. It is funnel-shaped
and has a row of fingerlike fimbria around its margin. From this,
the tube extends to the horn of the uterus. There is no direct con-
nection between the ovary and oviduct, but the ostium usually
covers the side of the ovary through which a mature ovum is to
rupture in order that the germ cell will be received by the oviduct.
In this animal there are two horns to the uterus^ each receiving an
oviduct. These horns converge and almost immediately join the
vagina which leads to the exterior. Embryos develop in the horns
of the uterus.

In summary, the germ cells are produced in the ovary, are freed
by rupture through its wall, are received by the ostium of the
oviduct, are moved through the oviduct (fertilized here, if fertilized)
to the horn of the uterus where the fertilized ovum (zygote) becomes
implanted and a placenta is formed. If not fertilized, the ovum
passes from the uterus to the vagina and out by way of vaginal
orifice. The vagina receives the penis during copulation (sexual
act), and the spermatozoa are discharged from the penis here.

Male Reproductive Organs. — Included here are the penis, scrotum,
testes, epididymis, vasa deferentia, seminal vesicle, urethra, prostate
glands, and Cowper's glands. The penis is composed of erectile
tissue and covered by the integument. The erectile tissue is en-
gorged with blood during sexual excitement and causes the organ
to become enlarged and rigid. The parts of the penis are the hase
or bulb at the proximal end, the body, and the glans or head at the
distal end. The prepuce is the loose skin which covers the glans.

The scrotum is a prominent sac in the perineal region of the male.
Internally it is divided into two pouches which are strictly out-
pushings of the coelomic cavity in that the peritoneum continues
through each inguinal canal to line each of these pouches. The
testes descend from the abdomen to the scrotum during breeding

144

ESSENTIALS OF ZOOLOGY

season and are again withdrawn following it. The testes are oval
in shape, being about twice as long as broad. In large fully matured
rats the testis may be two centimeters in length. Associated with
the testes, in the composition of what is often called the testicle,

supraienal

mbaT

eTer
Inteinal^l^eTmatic
common iliac

[)T06Tat
fjTosTale

buiDocaveinosus

bulbouT

TecTun

JDididnmal
-TesTiGuIar

Ljrn[)hL|5i5 bubi6

cabuT ebidi'dumis

Testis

ductus dejeiens

COT bus ebididumis

ij^ididy

mis

Fig 45. — Lateral view of a dissection showing male urinogenital system of the
rat (From Greene, Anatomy of the Rat, Transactions of the American Philosoph-
ical Society, New Series, Vol. XXVII, 1935.)

is the tubular epididymis. The testis is the reproductive gland. It is
composed of seminiferous tubules and interstitial tissue. Sperma-
tozoa are shed from the inner surface of these tubules where they

RAT, A REPRESENTATIVE MAMMAL 145

mature. With a microscope it is possible to study the different
stages of spermatogenesis in the epithelial walls of these tubules.
The spermatozoa of the rat are somewhat the shape of diminutive
harpoons with a head, middle piece, and tail.

The vas deferens leaves the scrotum along with the nerve, artery,
and vein as the spermatic cord. It passes through the inguinal canal,
then looping anteriorly and dorsally, to pass dorsal to the ureter
and return posteriorly to join the urethra (tube from bladder to
exterior). The seminal vesicle (vesicular gland) is a conspicuous
sac which joins the vas deferens near its point of entrance into the
urethra. The seminal vesicle probably does not store spermatozoa
in the rat but is thought to produce an alkaline solution which is
mixed with the sperm. Partially surrounding the urethra at this
point and also on the ventral side of the urinary bladder is the
prostate gland. Both parts of this gland empty into the urethra.
It has been suggested that their secretion assists in the locomotion
of spermatozoa as well as their nourishment in rodents. The urethra
leads on through the length of the penis as a common excretory and
reproductive canal and ends in a slit in the end of the glans.

Rats are sexually mature at two or three months of age. Ovulation
(discharge of mature ova from the ovary) occurs at intervals of
about three weeks after maturity in the female, except during preg-
nancy. This continues until the age of 15-18 months, when ovulation
ceases. The stoppage of sexual activity at this time is known as
menopause.

References

Adams, L. A.: An Introduction to the Vertebrates (Chap, on Mammals), New-
York, 1938, John Wiley & Sons, Inc.

Greene, Eunice C. : Anatomy of the Eat, Transactions of the American Philo-
sophical Society, New Series, Vol. XXVII, 1935, Philadelphia.

Hunt, H. R. : Laboratory Manual of the x\natomy of the Eat, New York, 1924,
The Macmillan Company.

Metcalf, Z. P.: An Introduction to Zoology, Springfield, 111., 1932, Charles C.
Thomas.

Stuart, Eichard E.: Anatomv of White Eat, Chicago, 1947, Denoyer-Geppert
Co.

CHAPTER VII

CHOBDATES IN GENERAL

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. 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.

Characteristics

All individuals classified in the phylum possess three distinctive
characteristics that are most conspicuous in certain primitive forms.
The three features clearly distinguish the phylum from all others
and bind together individuals which are widely separated in appear-
ance but characterized by certain traits peculiar to this group alone.
These three characteristics are: (1) notochord, a flexible rod ex-
tending 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 tuhulai^ 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 chordate 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
cord and brain. In higher forms the anterior end of the tube be-

146

CHORDATES IN GENERAL 147

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 to be studied does not give
as great a leverage for the muscles, but it greatly increases the mechan-
ical 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 (hem i kor' 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
Cephalodiscida with its two genera C ephalodiscus and Rhahdopleura.
These are all small wormlike animals.

TJrochorda (u ro kor' da, tail cord), or Tunicata (tu ni ka' t^)
includes 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. Molgida,
Cynthia, Appendicularia, and Salpa are examples.

Cephalochorda (sef a 16 kor' da, head cord) includes approximately
twenty-eight different species of marine, shore-loving, fishlike forms
of which Amphioxus (Branchiostonia lanceolatus) is the most common
representative.

Vertebrata (ver te bra' ta, jointed) animals with backbone — frog,
man. These are the larger, more conspicuous animals. 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.

148 ESSENTIALS OF ZOOLOGY

Elasmobranchii (e las mo bran' ki I, metal plate and gills). Fish
with jaws, cartilaginous skeleton, persistent notochord, and placoid
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 feath-
ers. The forelimbs have become wings. All birds.

Mammalia (ma ma' li a, mammary or breast). Warm-blooded ver-
tebrates with hair and with mammary glands for suckling the young.
Cats, Men, Monkeys, W^hales, Seals, Bats, etc.

Phylogenetic Advances of Chordata

(1) Notochord and endoskeleton, (2) pectoral and pelvic girdles
with limbs, (3) development of dor sally 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 Annelida and the latter was inde-
pendent. 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 Hemichorda

Dolichoglossus, kowalevskii is the common example studied. It is a
wormlike animal which burrows into the mud and sand along the
seashore. They range from 6 to 10 inches in length. Others of the
genus 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 chamber. The cavity of the proboscis is filled

CHORDATES IN GENERAL

149

with water which is drawn in and expelled through a proboscis pore
or vent located on its dorsal side and just anterior to the collar. Sup-
porting the base of the proboscis is a short skeletal process which is
stiff and extends anteriorly 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 rela-
tionship to the digestive tube which is characteristic in the embry-
onic development of the notochord for certain higher chordates.

Fig. 46. — External features of Dolichoglossus kowalevskii.

Geppert Company.)

(Courtesy of Denoyer-

Apical plate

Mouth

[V Proboscis
ix :'/ coelum

Anus

Fig. 47. — Tornaria larva of Hemichorda. (From Hegner, Collerje Zoology, The

Macinillan Company, after Metchnikoff. )

The mouth opens on the ventral side just anterior to the collar and
leads into the straight alimentary canal which extends to the pos-
terior end of the body 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 numer-
ous paired gill slits, located in the lateral walls of the anterior (sup-
posedly 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 present, water is passed through them for

150 ESSENTIALS OF ZOOLOGY

respiratory purposes, oxygen being absorbed and carbon dioxide be-
ing discharged from the blood here. There is no differentiation of a
distinct pharynx.

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
ventral cord running 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.

These animals are dioecious, with gonads in the form of a genital
ridge extending lengthwise 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 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 re-
semble 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 animal and
went under the genus name of Tornaria.

Balanoglossus and other representatives of its subphylum, though
lacking in complete conformity to chordate characteristics, are classi-
fied here because of the diverticulum supposedly representing a imdi-
mentary notochord, the gill clefts in the alimentary canal, and the
dominance and grooved structure of the dorsal nerve cord.

Subphylum Urochorda, Molg^la

Subphylum Urochorda includes a number of common represen-
tative marine forms, such as Salpa, Cynthia, Ciona, Clavelina, As-
cidia, and Molgula. The latter genus represented by M. manhattensis
will be given particular consideration here. This animal is commonly
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. How-
ever, it does present pharyngeal gill slits. M

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

CHORDATES IN GENERAL

151

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
branchial siphon (oral funnel or 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
the internal organs through the transparent mantle.

These animals are hermaphroditic or monoecious. Each has two
compound sets of gonads, one on the left side in the loop of the
intestine and the other on the right side of the body. Some of the

Incurrent siphon

Excurrent siphon

II Mantle

Tunic

r¥\ Qonqlion

-Anus
-^^ Genital duct

^'%--- Ovary
T^\~~ 0\qestive glands
" — tsophaqus

— -\ntest\m

- -^stomach
— branchial fold

- - Endostyfe

- /Atrium

- - Phar^r)K

Fig. 48. — Diagram of Molgula manhattensis from the left side to show the struc-
ture with the courses of water and food indicated by arrows.

sessile tunicates, as Molgula, reproduce by budding. 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 amphibian tadpoles which are free-swim-
ming. The larva possesses the typical notochord, gills, and nerve
cord of Chordata. For some reason 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

152 ESSENTIALS OF ZOOLOGY

*'chin. " It now undergoes regressive changes involving loss of tail,
notochord, and posterior portion of nerve cord. The anterior por-
tion of the cord becomes a simple ganglion. The paired eyes and
otocysts (ear structures) also disappear. This process of meta-
morphosis has caused an active respectable chordate to become a
lazy, stationary form which is not much more than a water-bag
whose level of development has degenerated almost to that of a
sponge. In a few instances tunicates reproduce one generation
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 : Branchiostoma virginiae, B. floridae, B. hermu-
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 aver-
age 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 usually referred to as a close ancestral rela-
tive of the Vertebrata.

Habitat. — It is found in shore water and on the sandy beaches of
the subtropical and tropical portions of the world. These animals
are found along our Atlantic Coast as far north as Chesapeake Bay,
at certain points in the Gulf of Mexico, and on the southern Pacific
Coast. They may be found along the shores of the Mediterranean
Sea, the Indian Ocean, and along the southern coasts of China.

Habits and Behavior. — It burrows rapidly, head first, in the sand
by means of a vibratory action of the entire body, but comes to
rest with the anterior end exposed to the water. At' times, partic-
ularly 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
a small fish, but it does not have a distinct head. The mouth opens

CHORDATES IN GENERAL

153

Cerebral

vesicle

i^j Oral cirri

■ Velum

Velar tenia-
' cles

Zndostyle

\ i

r

GlHslit

Pharynx

^^. Atrial
." cavity

>«

."'7 '

r

1'

w

Liver

Gonad

■- — Notochord

-"-Meta pleural
fold ^

Spinal cord

Atriopore

Intestine

Ventral fin

m

Anus

Caudal fin

«.. «. Diagram of ^"Phloxu^s^^Branc.io.toj^^^^^^ ,„m the right side.

154

ESSENTIALS OF ZOOLOGY

on the ventral surface of the anterior portion of the body. There are
no clearly defined lateral fins, but a pair of skin structures, the
metapleural folds, extending along the anterior two-thirds of the ven-
tral 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, and they are known as myotomes. The myotomes
on the two sides are not paired, but alternate with each other. Adja-
cent ones are separated by a myocomma or myoseptum.

Dorsal fin

Fin ray

Epidermis

5pinal nerve

Nerve cord

- -Nobocliord

Myofco.'ne musde

tiyocomma

Dorsal Aortxj

-Epibranchial qroove
— Nephridium
— AtriaKavity
^ -.QUI slit
Liver

Fig. 50.

Neurocoele .^^^^s?:.

Notocbordol-
sheath

Coelom j

Atrial cavity-
Pharynx

Gill rod

Gill bars

Qonad

Ventral aorta .

':l;^.Jiypohrorich\al qroove

0i E ndostyle

U4 Coelom

_ ^ _ Metapleural fold

-Cross section of Amphioxus through the level of the posterior portion of

the pharynx.

Internal Structure and Metabolic Activities. — The notochord ex-
tends 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

CHORDATES IN GENERAL

155

Buccalcim

QUI slit in wall
J)f pharynx

Afferent branch-
Jal arteries

I Ventral aorta

Dorsal aorta

Notochord

Spinal cord

-. Distribution throuqh

iver

r

Hepatic V.

^ubintestinal vein
.Atriopore

:vi"

Ventro-'intestinal V.

Dorso-ints5tinal A.

</

Anus

Caudal vein

; Caudal artery

Fig. 51. — Circulatory system of Amphioxus.

156 ESSENTIALS OF ZOOLOGY

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 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 hrancliial 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 those from the opposite side in forming the dorsal aorta.
The dorsal aorta extends posteriorly to the tip of the body giving
off numerous branches to myotom.es 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 suhintestinal
vein receives the blood from the intestine 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
suhintestinal and hepatic portal veins is laden with dissolved nutri-
ment. The blood in these ventral veins flows from posterior toward
the anterior.

Digestive System. — A current of water is carried into the mouth
by the ciliated bands on the inner surface of the oral hood. Sur-
rounding 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. 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
hyperhranchial groove, which is ciliated. In the floor of the pharynx
is the hypohranchial groove in whose Avails is located the glandular
endostyle, capable of secreting mucus. The endostyle 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 peri-
hranchial grooves are carried dorsally to the hyperhranchial groove.
The cilia here move the mass back to the intestine. A blind, finger-
like diverticulum of the intestine, the liver or hepatic caecum, ex-
tends 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 enzymes into the

I

CHORD ATES IN GENERAL 157

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 and support the
gills, contain the blood vessels, and are supported by chitinous rods.

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 narroAv space surrounding the intestine. Be-
hind 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,
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. Following fertilization comes a series of cleavage
divisions which are total and equal. This is followed by the infold-
ing of one side of the spherical body to form the gastmla and this
in turn becomes a free-swimming larva which reaches adult condition
without metamorphosis.

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. Metamerism and bilateral symmetry are universal charac-
teristics among vertebrates. The segmented vertebral columyi and
other supporting structures form an endoskeleton (internal skeleton)
which is the basic support of the body. Paired appendages are usu-
ally present at some time in the life of the individual. The majority
have two pairs of fins or limbs in adult condition. There is a ven-

158 ESSENTIALS OF ZOOLOGY

trally located hem^t which is divided into chambers. The blood con-
tains hemoglobin-hearing red corpuscles and amoeboid white corpus-
cles. In the vertebrate body is a well-developed coelom, which encloses
advanced systems of organs for digestion, excretion, circulation,
reproduction, and, in terrestrial forms, respiration. C ephalization 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 pro-
longation of the body behind the anal opening and is found in some
degree in all vertebrates. The neck 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 ante-
rior, 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 ap-
pendages as arms, wings, pectoral fins, forelegs, and flippers. The
same is generally true for the pelvic limbs.

The vertebrate animal is covered by an integument or shin which
serves as a protective and sensory organ. It also helps in excretion
through the sweat glojids, 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 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. The exo-
skeleton is a rather minor part in vertebrates and consists of nails,
claws, scales, hair, feathers, and other outgrowths. The endoskele-
ton includes the axial and appendicular portions. The first is com-
posed 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. The vertebral column is composed
of segmental divisions, the vertebrae, and is divided into five regions
as follows : cervical vertebrae of the neck, thoracic vertebrae of the
chest, lumbar vertebrae of the small of the back, sacral vertebrae of
the hip region, and the caudal vertebrae of the tail region.

I

CHORDATES IN GENERAL 159

Divisions of Skeleton of Terrestrial Vertebrate

I. Axial Skeleton

(a) Skull

1. Cranium

2. Sense capsules

3. Jaw apparatus
(Visceral arches)

(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)

The muscular system represents a system of cells highly special-
ized in contractility. Voluntary muscles are usually connected with
the skeleton; those of the visceral organs, e.g., intestine, are invol-
untary. Cardiac muscle is the highly specialized involuntary muscle
which makes up the wall of the heart.

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 outgroAvths, 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 re-
ceives saliva from salivary glands. Following the mouth is the
pharynx or throat region. Next comes the esophagus which is usu-
ally tubular and propels the ''swallows" of food posteriorly by
consecutive waves of contraction, 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

160

ESSENTIALS OF ZOOLOGY

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 the convoluted intestine which is
divided into the anterior duodenum, middle jejunum, and posterior
ileum. Its walls produce digestive enzymes from glands and it re-
ceives digestive juices from two other glands: the liver and the
pancreas.

■Craniom
Skull ^ ^^^tm^mr^ Orbit

Cervical
y/erfebrae

Scapula

Mandible
Clavicle

Sternum

Thoracic
vertebrae

Lumbar
vertebrae

Sacrum

Metacarpals

Tibia

Metatarsals

Humerus
Rib

Pelvis

Radius
Ulna
Carpals

Hcvnct

Fig. 52. — Human skeleton.

(From Wolcott, Animal Biology, McGraw-Hill Book
Company, Inc.)

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

CHORDATES IN GENERAL 1()1

most vertebrates. It secretes the bile and also serves to convert
carbohydrates to glycogen (animal starch) to be stored for future
energy 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. 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 respiratory system is at least in part an outgrowth of the
digestive canal. In most aquatic vertebrates respiration is accom-
plished by drawing water through gill slits in the wall of the pharynx.
Air-breathing, terrestrial forms have developed the trachea (wind-
pipe) and lungs as another outgrowth of the pharynx. A certain
amount of respiration takes place through the skin. The respiratory
process is composed of two phases: external respiration which in-
cludes the exchange of the gases, oxygen and carbon dioxide, be-
tween the external medium and the blood; and internal respiration
which is the exchange of the gases between the blood and the proto-
plasm of the cells over the body. Hemoglobin in the red corpuscles
of the blood has a loose affinity for oxygen, is capable of carrying
it, and then gives it up at points where there is demand for it.
Much of the carbon dioxide given up by the cells becomes carbonic
acid and carbonates which may be transported by the plasma (fluid)
of the blood.

The circulatory system is a closed system of vessels supplying all
parts of the body with blood and a system of spaces, sinuses, and
vessels collecting lymph from the various organs to return it to
the blood vessels. The blood circulatory system centers in a con-
tractile heart from which tubular arieries lead out to various organs
of the body where they branch into minute vessels or capillaries. The
capillaries converge as they carry the blood away from the organs
to form the veins which carry the Wood back to the heart. This is
a closed system of vessels. The blood is composed of the clear fluid,
plasma, and the hlood corpuscles. The red corpuscles contain the red
pigment matter, hemoglobin with oxygen-carrying power. The white
corpuscles or leucocytes are of several varieties and they are amoeboid
in character. These cells may make their way among cells of , other

162

ESSENTIALS OF ZOOLOGY

tissues where they engulf bacteria and foreign matter. Upon ex-
posure 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 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.

Veins from uppGP_
part of Body

Lymphatics

Thoracic duct -

Oupcnop vena cava
Pulmonary/ artery

lii^ht aupicla —
Infepiop vena cava-

/

ISi'^hl' vcntpiclc

tactcab —

Hepatic vcirL

Vcini from Iowcp
part of Body

Artcploa to uppcp*

papt of Body

Pulmonapy vein

— Left auricle

- Left vcntplclc

ArtGPJGS to lov/ep
part of Body

. ^.;/mpBati«--A^•■r^^;^\V^HopatlcaptGl^

Fig, 53. — Diagram of circulation of a mammal. The oxygenated blood is shown
in black • the venous blood in white. The lymphatics are black irregular hnes.
(From P'ettibone, Physiological Chemistry, The C V. Mosby Company.)

The excretory system of vertebrates consists of kidneys, excretory
ducts, and often a urinary bladder. The kidneys serve to remove

CHORDATES IN GENERAL

163

from the blood, waste nitrogen products and excess salts in solution
as well as to dispose of excess water. The life history of higher
vertebrate individuals includes successive stages as follows : the
pronephros, the sole kidney for a time ; followed by the mesonephros
which is the dominant functional excretory organ ; and, finally, the
development 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, since these tran-
sitional stages represent adult kidneys for more primitive vertebrates.

Fig. 54. — Cross section of spinal cord and roots of the spinal nerves, showing a
simple reflex circuit. 1, Sensory surface of skin ; 2, afferent nerve fiber with S, its
cell of origin, located in the spinal ganglion ; If, 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, The C. V. Mosby Company, after Morat. )

The nervous system in this type of animal is composed of a train
and spinal cord forming the central nervous sy stern; nerves extending
to all parts of the body, ganglia which are groups of nerve cell bodies
outside the central nervous system, and the sense organs which serve
for receiving stimuli are usually grouped together under the name
peripheral nervous system. A portion of this latter division, con-
sisting of two longitudinal trunks with ganglia distributed along

164 ESSENTIALS OF ZOOLOGY

them, lies parallel to the spinal cord, and constitutes the autonomic
system. The peripheral 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 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 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 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
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 vibrations
carried in the water and is quite important to aquatic animals of
this type.

The vertebrate reproductive system shows a fairly high degree
of development. The sexes are ahiiost 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

CHORDATES IN GENERAL 165

deferentia. The female gonads are ovaries, and they produce ova
or eggs. They are carried from the body by oviducts. The males
of some classes possess for use in copulation certain accessory organs,
which tend to insure fertilization.

References

Atwood, Henry W.: Introduction to Vertebrate Zoology, St. Louis, 1940, The
C. V. Mosby Company.

Walter, H. E.: Biology of the Vertebrates, New York, 1939, The Macmillan
Company.

CHAPTER VIII

PHYSIOLOGY

The maintenance of any living body requires the cooperation of
several functions which will attain similar fundamental results
wherever they occur in living material. 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.

The collective term metabolism is employed when referring to all
of the interactions involved in the living process of protoplasm. It
includes the processes concerned with conversion of food into proto-
plasm, release of energy through oxidation, production of heat,
movement, elimination of wastes, or in other words these processes
are chiefly : ingestion, digestion, egestion, absorption, transportation,
assiyyiilation, respiration, oxidation, and elimination. The processes
concerned with the conversion of food material into protoplasm (build-
ing up) constitute the phase of metabolism known as anabolism. In-
cluded here are ingestion, digestion, absorption, transportation, and
assimilation. The oxidation of materials of the protoplasm to liberate
energy, and the elimination of wastes incidental to it, is known as
catabolism 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 living
organisms, must be concerned with metabolism. It includes all of
the chemical changes and transformations by which energy is sup-
plied for the activities of the protoplasm. For convenience, the
several functions will be discussed separately with the thought of
bringing out their expression in different types of animal life.

Support and Protection

In Protozoa there is no very elaborate adaptation in this direc-
tion. The presence of a cuticle in some and the secretion of a hard

16C

PHYSIOLOGY 167

shell in others seem to be the particular developments related to
these special functions in this group.

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 arjound 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 exoskeleton except that they are principally
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
protection and temperature regulation in most of the groups of
vertebrates. Such structures as scales, shells, feathers, hair, nails,
horns, and even enamel of teeth are of this type.

Primitively the notochord is the original endoskeletal structure of
the chordate group. Around it are developed the basic structures of
the vertebral column which functions as the principal axial support
of all vertebrates. The sternum, girdles, and paired limbs have de-
veloped with the terrestrial life of vertebrates and the necessity for
locomotion on land.

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 the bone if allowed sufficient time, in which ease the bone will
lose its rigidity. Caustic solutions will destroy the animal matter and
make the bone brittle.

168 ESSENTIALS OF ZOOLOGY

Movement and Locomotion

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 certain 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 are seen in Protozoa which move from
place to place by means of pseudopodia, cilia, or flagella. In ciliary
movement the numerous small strands of protoplasm beat rhythmically
with a stroke in one direction, so timed that the beat passes in a
wavelike progression from one end of the ciliated area to the other.

The development of a high degree of contractility in muscle cells
makes possible muscular movement. A muscular locomotor system
consists 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) jn the reaction. During
the shortening of the muscle there is a hydrolysis or absorption of
water by the protein product, creatine-phosphoric acid. By-products
of the process include carbon dioxide, lactic acid, urea, creatinine,
and phosphoric acid.

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.

Digestion

The general function of digestion, which is to convert complex
raw food material into a soluble, absorbable form, has already been
pointed out. The materials commonly used for foods have large
molecules, and are usually colloidal in nature. Digestion then must
serve to break up these large molecules into smaller ones, thus
forming solutions of substances which will readily diffuse through
membranes. Digestive enzymes are responsible for placing the food

PHYSIOLOGY 169

materials in solution. By enzyme action, 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 presence
under certain conditions will cause or hasten chemical reaction be-
tween other substances without itself being consumed. The enzymes
are formed in the protoplasm of cells, and their action is similar to
that of a catalyst, since they accelerate chemical action. There are
different types of enzymes each capable of producing specific kinds of
reactions. There are oxidizing enzymes (oxidases) capable of bring-
ing about oxidation; reducing enzymes (reductases), which produce
reduction in tissues; coagulating enzymes (coagulases), which cause
clotting or coagulation; and hydrolyzing enzymes (hydrolases), which
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 activating agent
or co-enzyme. As an example, the precursor of pepsin is pepsinogen
which becomes activated in the presence of dilute acid.

Classes of Digestive Enzymes

1. Diastases or di-astatic enzymes — split carbohydrates

(a) Ptyaliii 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 disaccharids to the less complex monosaccharids
(simple sugars) — intestinal juice

(a) Maltase

(b) Lactase

(c) Sucrase (invertase)

4. Proteases w 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)

170 ESSENTIALS OF ZOOLOGY

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 significant.
In carnivorous animals it is about one-half the area of the skin while
in herbivorous animals it is about twice the area of the 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
splitting. The splitting of the disaccJiaride, maltose, will serve as
an example of this process :

Ci2 H22 Oil + H2 O =: 2 Ce H12 Og
(Malt sugar) (Water) (Glucose)

The two molecules of glucose formed are in a form for ready
absorption.

Gastric Digestion. — The tubular gasttnc 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. Rennin 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-

PHYSIOLOGY 171

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 amylopsin, 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
efficiently than does pepsin. There are nineteen amino acids that
are regarded as building stones of the protein molecule. In a com-
plex protein like casein, as many as sixteen of these amino acids will
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 is the pancreatic diastase, and it is able to bring about
hydrolysis of carbohydrates in the alkaline niedium of the intestine
without activation. It produces dextrin and maltose (malt sugar).
The pancreatic lipase, steapsin, brings about the splitting 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 in-
troduced by the bile, combine with these fatty acids to form soaps
which help in emulsifying the remaining fats, thus making them more
readily split.

Intestinal secretions or succus entericus which are produced by
glands in the mucous membrane of the small intestine include five
enzymes. Enterokinase, which activates trypsinogen to form trypsin,
has been mentioned already. Erepsin, the intestinal protease, supple-
ments the activity of trypsin by converting proteoses and peptones
into amino acids. Maltase converts maltose and dextrin into dextrose
(glucose). Invertase changes sucrose (cane sugar) into dextrose and
levulose. Lactase converts milk sugar (lactose) into galactose and
dextrose, both simple sugars.

172

ESSENTIALS OF ZOOLOGY

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

GastriG
lipase

Stomach

Emulsifies
fats

Glycerol and
fatty acids

None

Liver

Bile

Emulsifies
fats

None

Amylase or
a my lop sin

Produced in
pancreas

Pancreatic
juice pro-
duced in
pancreas
but acting
in small
intestine

Carbohy-
drates

1

Maltose

Steapsin

Produced in
pancreas

Lipins

(fats)

Glycerol and
fatty acids

Trypsin

Produced in
pancreas

Proteoses
and pep-
tones in
alkaline
medium
with enter-
okinases

Polypeptids

Erepsin

Small
intestine

Intestinal

Polypeptids

Amino acids

Maltase

Small
intestine

Maltose

Glucose
(dextrose)

Lactase in
mammals

Small
intestine

Lactose

Glucose and
galactose

Invertase or
sucrase

Small
intestine

Sucrose

Glucose and
levulose

*The esophagus and colon do not secrete any enzymes.

The undigested residue passes into the large intestine where prob-
•ah\y no enzyme digestion occurs. Certain bacteria {Escherichia coli
and others) attack any undigested foods (especially protein) and
bring about putrefactive fermentation. Products of this action may
be absorbed; some of them are frequently toxic and must be elimi-
nated 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.

PHYSIOLOGY 173

Functions of the Liver

The secretion of the liver is bile and is discharged into the duo-
denum of the small intestine by way of the common hile 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 mentioned above. Cho-
lesterin and two pigment materials are excreted in the bile. Bile is
secreted continuously, but between meals it is stored in the gall hlad-
de7' 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 wea (and uric acid in some forms) is formed in the liver
and carried by the blood to the kidneys for excretion. The removal of
toxic protein substances and toxic heavy metals is another function
of the liver.

Absorption and Utilization of Food Materials

The soluble products of digestion are absorbed through the semi-
permeable epithelial lining of the intestine into the blood of the
adjacent capillaries, or in the case of fats into the lacteal lymphatics
and from here into the subclavian vein through the thoracic lymph
duct. 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. Carbohydrates and then fats are more economical and
efficient as sources of fuel for production of heat and energy. Oxi-
dation of protein requires the disposal of much more waste products.
The comparative heat production values of the three are as follows:

One gram of protein z= 4100 small calories*

One gram of carbohydrate = 4100 small calories
One gram of fat = 9305 small 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

*A small calorie equals the amount of heat necessary to raise one gram of water
one degree centigrade under standard conditions, while a large calorie equals the
amount of heat necessary to raise one liter of water one degree centigrade under
standard conditions.

174 ESSENTIALS OF ZOOLOGY

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 related to the
regulation of certain phases of metabolism. They are recognized usu-
ally through the abnormal condition brought on by their deficiency.
There is little danger of vitamin deficiency for adults living on a bal-
anced and mixed diet. Much of our knowledge concerning the symp-
toms brought on by lack of different substances has been gained by
feeding experiments on different kinds of laboratory animals. This
information is then applied to human beings. The following outline
summarizes the essential points concerning vitamins.

The Vitamins and Their Characteristics

I. Vitamiji A (C20H30O) — antixerophthalmic — fat soluble.

(a) Sources: carotene (C40H56) 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. Bi or Thiamin (C12HJ7ON4S) — Antineuritic.

(a) Sources: Germ of wheat and other cereal grains, peanuts, liver, and
egg yolk.

♦There are still other recently discovered fractions of Vitamin B, whose func-
tions are specific.

PHYSIOLOGY 175

(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. Eiboflavin (Cj.HjoOeN^).

(a) Sources: Eggs, liver, milk, green leaves, yeast.

(b) Functions: Necessary for growth, active relation to several enzymes
with 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 (CsHgNOz) — 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. Bg or pyridoxine (CgHijOsN).

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

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

(c) Effects of deficiency: Paralysis in chickens.

5. Pantothenic acid (C9H17O5N).

(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 (C10H16O3N2S).

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

(b) Functions: Essential for growth.

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

III. Vitamin C or Ascorbic Acid (CeHgOe) — 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 (C28H44O) — antirachitic.

(a) Sources: Tuna and cod-fish liver oils. Exposure of skin to ultra-
violet radiation.

(b) Functions: Eegulation of calcium and phosphorus metabolism. Ee-
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.

176 ESSENTIALS OF ZOOLOGY

V. Vitami/n E or Tocopherol (C29H50O2) — 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 (CsiH^gOz) — 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.

Respiration

R-espiration has been defined as the process involving the ex-
change of gases between the protoplasm of an organism and its en-
vironment. 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
depends on the partial pressure of the particular gas on the two
sides of the membrane. Gas will flow in the direction toward the
least pressure.

In the larger and more complex animals where the volume of
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.

♦ PHYSIOLOGY 177

In these larger organisms, the respiration process is divided into
external respiration, the gas exchange occurring between the circula-
tory medium and the environmental medium (air, water, etc.), and
internal respiration, the exchange of gases occurring between the cir-
culatory medium (blood, etc.) and the protoplasm of the organism.

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.

Aerial respiration is accomplished in terrestrial animals through
special internal surfaces which must be kept moist. In insects a
system of branched tubes called trachea, 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 muscles which control the breathing actions in vertebrates
are automatically stimulated through the nervous system to contract
when the carbon dioxide level of the blood reaches a certain point.
A respiratory center, located in the medulla oblongata, is affected
by the carbon dioxide and determines the rate of respiratory move-
ments. 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 respiratory action. In addition to exchanging gases the lungs
also discharge moisture and give off a certain amount of heat.

178 ESSENTIALS OF ZOOLOGY ■ 9

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 substances, 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 will be 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, i.e., the vessels carry the blood through its entire
circulation without being interrupted by extensive sinuses, 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 mol-
lusks and crustaceans there is a similar respiratory pigment carried
in the plasma, which is called hemocyanin. Instead of iron, copper
is the principal constituent of this pigment. Vertebrate blood is
largely water carrying dissolved materials and suspended corpuscles.
The fluid part is known as plasma. The amount of blood in a mammal
is approximately one-tAventieth of the body weight, or in the average
man a little more than a gallon. The plasma contains enough inor-
ganic 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 un-
equally 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 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 cor-

PHYSIOLOGY

179

puscles (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 approximately 20,000,000,000,000 (20
trillion) of them. Each erythrocyte is essentially a little capsule filled
with hemoglobin which is a compound peculiarly fitted to unite with
atmospheric oxygen. When united with oxygen it is known as oxy-
hemoglobin, 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.

Average Characteristics of Human Blood Cells

KINDS OF CELLS AND

AVERAGE NUMBER

STRUCTURE

PER CUBIC MILLI-

COLOR WITH

METER OF 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

Reticulo-

Amoeboid; can

6,000 to 10,000

joined by thread;

endothelial

leave blood ves-

stains dark lilac,

cells outside

sels and enter

cj-toplasm pale;

capillaries of

tissues

blue with gran-

bone marrow

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 immunitv

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, refractile, no

Bone marrow

Provide substance

200,000 to 400,000

nucleus; dark
blue to lilac;
2 to 3

needed in clotting

(Reproduced by permission from Textbook of Zoology by Storer, copyrighted,
1943, by McGraw-Hill Book Co., Inc.)

180 ESSENTIALS OF ZOOLOGY

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 plasma of the blood contains a group of substances called
antibodies (immune bodies). These have been produced by various
tissues of the body upon contact with certain foreign proteins. Since
bacteria and pathogenic Protozoa react as foreign protein, they stimu-
late the body tissues to the production of specific protective antibodies
and physicians have come to make use of these antibodies in sterile
serum lor prevention and treatment of several diseases. Some of
these antigen substances bring about the clumping or agglutination
of foreign bacteria, 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 liemopJiiliacs 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 protliroyiibin 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
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.

Nervous Function — Reception and Conduction

Irritability and conductivity are fundamental functions of all
protoplasm, whether it be in the body of Amoeba or 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

PHYSIOLOGY 181

effects of previous stimuli. In the final analysis, the perceptions
and reactions of man are but expressions of these primitive func-
tions in a more specialized organism.

The protozoan organism has no special nervous system but de-
pends on the primitive properties of irritability and conductivity
to guide its activities. In the simpler Metazoa, such as the coelenter-
ates, there are scattered nerve cells connected with each other by
fibers to form a nerve net. The neuroepithelial or neuromuscular
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 will be studied in Planaria. In Annelida and Arthro-
poda the nervous system is a modified ladder type in which the two
longitudinal cords of ganglia have fused along most of the midventral
line. Toward the anterior end, the cords separate at a paired gan-
glionic enlargement, the sub esophageal ganglion, arid encircle the ali-
mentary canal to join on the dorsal side as the pair of supraesophngeal
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 circumoral nerve
ring around the mouth, and radial branches extend into each arm.
Branches from these communicate with the sensory structures of the
skin and tube feet.

Concentration of the tissue of the nervous system into definite
organs 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 types of nervous system, from worms to

182 ESSENTIALS OF ZOOLOGY

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-
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 nerve
which brought in the sensory impulse. The motor axone carrying
the impulse from the motor nerve cell in the gray matter usually
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. Keflex 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,

PHYSIOLOGY 183

Functions of the Spinal Cord

This organ serves as a system of reflex centers which control the
actions of glands of the trunk, visceral organs, and skeletal muscles.
The spinal cord is also a nervous pathway between the brain and
numerous organs of the body. It is said that more than half a million
neurones join the cord through the dorsal roots of the spinal nerves.

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 spontaneous actions. The midbrain is one of the cen-
ters of coordinated movement which has to do with posture and eye
muscles. The cerebellum 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 co-
ordinaiod 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 breath-
ing and may be an inhibitor on heart action. It also regulates
digestive secretions, movements of digestive organs, and vasomotor
activity of the blood vessels. As a whole, the brain serves as the
organ of communication between the sense organs and the body
and is the coordinator of the bodily activities.

Sense Organs and Their Function

There are five special senses which have rather particularly
defined function, and, in at least four of them, quite specialized or-
gans for perception. These five are sight, hearing, smell, taste, and
touch. They are all in contact with, and referred to, the external
world. The organs in which these senses are stimulated are, respec-
tively, the retina of the eye, the org^n of Corti of the ear, the olfac-
tory membrane of the nose, taste buds in the epithelium of the
tongue and related parts, and the tactile corpuscles, as well as free
nerve endings in the epithelium.

In addition to the senses centered in these specialized sense or-
gans, there are several general senses, such as hunger, nausea, re-
spiratory sensations, and sexual sensations.

Cutaneous Sense Organs (Touch, Pressure, Temperature). — There
are endings of sensory nerves in all parts of the skin and the mucous

184 ESSENTIALS OF ZOOLOGY

membranes lining the mouth, nose, vagina, and urethra. The senses
of pain and temperature are probably accommodated by unmodified
free nerve endings. Sensations of pain may be produced in almost
every organ of the animal's body. Meissners tactile corpuscles are
located in the dermis generally over the body, but more particularly
on the soles of the feet, at the bases of the vibrissae, at the tip of the
snout, and tip of the tongue. In human beings the sense of touch is
twice as discriminating on the tongue as it is on the finger tip ; almost
thirty times as great as that of the back of the hand; and sixty-five
times as great as that of the middle of the back. Pacinian corpuscles
are organs of pressure sense located in the deeper dermis, mesenteries,
certain visceral organs, and at the joints.

Sense of Smell. — The organ of the sense of smell lies in that part
of the mucous membrane lining the posterior part of the nasal
cavity. The portion of the mucous membrane containing the sen-
sory olfactory cells is known as the Schneiderian membrane and is
grayish in color in a fresh specimen. The remainder, the respira-
tory membrane, is red in color. The olfactory nerves conduct the
olfactory impulses to the brain. This sense is a chemical sense, and
stimulation is produced by volatile substances which go into solution
in mucus on the membrane.

Sense of Taste (Gustatory Sense). — The mucous membrane of the
tongue contains most of the taste buds of the manmial. Fewer taste
buds are found on the soft palate and epiglottis. Some animals have
taste buds on the lips and snout, and in the catfish the feelerlike bar-
bels around the mouth, and even on the tail, possess taste buds. This
sense too is a chemical sense and materials must be in solution be-
fore they stimulate the gustatory cells. Many taste sensations are
dependent on olfactory cooperation. Taste sensations are grouped
into four classes : hitter , sweet, salty, and sour. In the human being
taste buds receiving sweet sensations are located in the tip of the
tongue, bitter at the base, salty along the sides, and sour (acid)
rather generally in all of these areas. The mid-dorsal portion of the
tongue is not very sensitive.

Sense of Sight. — The eye is the special organ of sight, and within
it the retina contains the sensory cells. Each eye is held in an orbit
which is located in the skull. The conjunctiva is a thin membrane
which lines the eyelids and passes over the front of the eye between
the lids. This latter portion of it is transparent. Tear (lacrimal)
glands, which are located in the orbit under the base of the upper

PHYSIOLOGY - 185

lid, secrete tears, This is a salty liquid which lubricates and washes
the surface of the eyeball and inner surface of the lids. Seven
muscles control the movements of the eyeball. These muscles take
their origins on the inner surface of the orbit and have their inser-
tions on the surface of the eyeball. These seven include four straight
ones (recti), two oblique ones, and one retractor: superior rectus,
mesial (internal) rectus, inferior rectus, lateral (external) rectus,
superior oblique, inferior oblique, and retractor oculi. The four recti
muscles each pull the eyeball in the direction indicated by the
name ; for example, the superior rectus moves the eyeball upward,
and the inferior one moves it do^vnward. The action of the superior
oblique muscle is modified by the fact that it pulls through a pulley
which results in a twisting or rotating movement for the eyeball.
The inferior oblique opposes it by direct attachment at the opposite
position. The retractor oculi originates on the boundary of optic
foramen and divides into four heads which are inserted in the
sclerotic coat (sclera) near the entrance of the optic nerve. This
muscle is almost covered by the recti muscles.

The eyeball is composed of three tissue layers, a lens, and two
humors. The outer, firm sclera is almost white in color and continues
forward into the transparent cornea which covers this surface. In-
side the sclera is the choroid coat which is generally black in color.
On the inside surface is an irregular area which has a metallic luster
and is called the tapetum. The choroid extends forward as the iris
or colored portion in human beings, but in albino rats the pigment is
lacking and it is pink, due to the color of the blood in its tissues. The
iris is .incomplete at its forward side, and the aperture in its center is
the pupil. Light is admitted through this circular opening. Its size
may be adjusted by contraction or expansion of the iris, and in this
way the amount of light and divergence of rays are regulated. The
inner layer, lining the inner surface of the posterior three-fourths of
the eye, is the retina. It is a thin gray membrane and contains the
sensory cells (rods and cones), which are connected to nerve fibers
coming in by way of the optic nerve which enters the back side of the
eye. The convex lens, which is held in position just behind the iris
by the ciliary ligament, is crystalline and focuses the light rays on
the retina. The area of the retina directly behind the lens is called
the fovea. This is the most sensitive part of the retina. Slightly
medial and below this is the entrance of the optic nerve, at which
point the retina is devoid of rods and cones. The very descriptive

186

ESSENTIALS OP ZOOLOGY

name, blind spot, has been applied to this area. The eyeball is divided
into two compartments, the aqueous chamber in front of the lens and
the vitreous chamber behind it. The aqueous humor filling the outer
chamber is of a watery consistency while the clear vitreous humor
filling the inner chamber is jellylike. The light, in reaching the retina
of the rat (or human being), would enter the eye through the cornea
and pass through the aqueous humor, pupil, lens, and vitreous humor
in that order.

The eye is capable of perceiving several features of light such as
intensity, distance, color, etc. There are several theories of visual
perception, but those most commonly accepted are based upon chemi-
cal changes in the rods and cones of the retina. Such an explanation

Con/uncf/Vcr

Poster/or.
chamber

■ScJerofic layer

Choroid layer
Retinal layer

Pigmerrh
layer

Fovea
centralis

Optic
nen^e

Anterior
chamber

Eyelid

Rectus
muscle

Fig. 55. — ^Diagram of a section through the eyeball. (From Wolcott, Ln\r)xal

Biology, McGraw-Hill Book Company, inc.)

considers the presence of a phototropic substance which is called
visual purple, as an essential factor. The rods and cones seem to take
different roles in dim light and white light, but these differences are
interpreted as varying amounts of visual purple in them. The
theories of color vision utilize the presence of this chemical also.
According to this idea, color blindness would be due to deficiency
of visual purple or a peculiar distribution of it.

Sense of Hearing. — The ear, which is the organ of hearing, is com-
posed of external ear, middle ear, and internal ear. The first is com-
posed of the pinna, which is funnellike, and the audit oj-y meatus, or

i

PHYSIOLOGY

187

canal, leading into the middle ear within the skull. The pinna serves
to direct the sound waves to the meatus. The internal end of this
canal is covered by a thin, taut membrane which is called the tym-
panic memhrane or eardrum. This membrane is set into vibration by
the sound waves which reach it. The middle ear or tympanic cavity
is just internal to the eardrum and within temporal bone. It houses
three small bones or ossicles, the malleus (hammer), the incus (anvil),
and the stapes (stirrup). The malleus is attached to the eardrum and
the three bones are in contact with each other in such a way that they
conduct the sound vibrations through this cavity to a membrane at
the entrance to the inner ear, or fenestra ovalis, A tube, called the
Eustachian tube, leads from the cavity of the middle ear to the

Fig-. 56. — Diagram of a section through the right human ear. B, semicircular
canal; G, 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 tvibe ; S, cochlea; T, membrana
tympani ; Vt., scala vestibuli. (From Zoethout, TexthooTc of Physiology, The C. V.
Mosby Company, after Czermak.)

pharynx, and it serves in the transmission of air between the cavity
and the exterior in order that the pressure may be kept at a balance
on the two sides of the tympanic membrane. This avoids interference
which might be caused by undue stretching of the membrane with
changes in barometric pressure.

The internal ear contains the sensory apparatus, both for reception
of sound waves and for equilibrium. The organ of Corti in the
cochlea performs the first function and the semicircular canals, the
latter. The cochlea is a coiled tube filled with perilymph, a fluid,

188 ESSENTIALS OF ZOOLOGY

which conducts the vibrations finally to the organ of Corti. The latter
is supported by a membrane dividing the cochlea throughout its
length. Endings of fibers from the eighth (auditory) cranial nerve
reach the cells in this organ and are stimulated by the vibrations of
the fluid. The average human ear is said to receive wave lengths of
frequencies ranging between 30 and 40,000 per second, but other
animals including the rat likely perceive wave lengths of a different
range.

Sense of Equilibrium. — The three semicircular canals which were
mentioned above stand in three different planes and all join the
vestibule or general cavity at their bases. One is anterior- vertical in
position, another posterior-vertical or at right angles to the first, and
the third is in the lateral horizontal position. There are sensory
endings of branches of the auditory nerve in the ampullae of these
canals, and the canals are filled with a lymphlike fluid. Any change
of level or position of the head causes this fluid to move in the canals
and stimulate certain of the endings. The individual is able to sense
the position and orientation of the body from the impulses produced
by these stimulations.

Excretion

A certain result of the oxidation necessary for metabolism is the
production of end-products (principally nitrogenous) which are not
only of no further use to the protoplasm but may be a distinct
menace to the welfare of the organism because of their toxic effects.
The substances are usually dissolved and removed as a waste liquid
or occasionally as crystals by special parts of the body.

In Protozoa this function is performed by general diffusion
through the plasma membrane and in many forms by the contrac-
tile vacuoles. The quantity of water which passes through the pro-
tozoan in tw^enty-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 accumu-

PHYSIOLOGY 189

lated excretion down the tubule. The waste liquid of the surround-
ing tissues diffuses into this cell. The main excretory ducts open to
the surface of the body by excretory pores. This arrangement is
sometimes called a protonephridial system.

The nephridial system is found in Annelida and has been studied
in connection with the earthworm. Here a coelomic cavity is present,
and a series of segmentally arranged pairs of coiled tubes or
nephridia extend through the wall to the exterior. The excreted
wastes accumulate in the coelomic cavity and are moved into the
nephridia through the ciliated funnellike internal end, kno^^^l as the
nephrostome. This coelomic 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 coelomic epithelium. The echino-
derms make use of direct diffusion as well as intracelhdar excretion
by which excreted materials are taken up from the coelomic cavity
by the numerous phagocytic, amoeboid cells of the coelomic 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 modifica-
tion and condensation from segmentally arranged nephridial tubules.
The fact that in vertebrate embryos as well as in low^er 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

190

ESSENTIALS OF ZOOLOGY

to the exterior of the body are the MalpigMan corpuscles^ each made
up of a glomerulus and a Bowman's capsule, and the coiled urinifer-
ous 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 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

Fig. 57. — Diagram of Malpighian corpuscle and renal secretion. G, Malpighian
corpuscle with two layers of Bowman's capsule enclosing the space i; af, vas
afferens vessels supplying the glomerular capillaries which fill the capsule to line
j, to which the inner wall of the capsule is closely applied ; ef, vas efferens which
drains the glomerular capillaries and supplies the capillaries ; ca, surrounding the
convoluted tubule, C; a, blood pressure in glomerular capillaries; c, the urine filtrate
pressure in the tubule ; r, the reabsorption from the uriniferous tubule into the
capillaries, ca ; h, the urine on its way to the colleCLin^ tubule: b, the restraining
osmotic pressure' of the protein molecule. (From Zoelhout, Textbook of Physiology,
The C. V. Mosby Company.)

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 no 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

PHYSIOLOGY 191

excreted by the skin. Most of the water to be excreted is taken
from the blood in Malpighian 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.

Reproduction

A living organism is in numerous ways similar to a machine, but
reproduction of new units of living material 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 division is the fundamental basis
for all reproduction. For centuries before the invention of the
microscope it was commonly believed that living things arose spon-
taneously 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 pres-
ent conception is that the protoplasmic substance of the new indi-
vidual is but a continuation of the specific protoplasm peculiar to
an earlier individual or in sexual reproduction to two individuals.
Therefore, under ordinary circumstances the structural and physio-
logical complexities which arise through embryonic development
must be generally similar to those of the predecessors.

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

192 ESSENTIALS OF ZOOLOGY

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 may be
found 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
vertebrates. Studies made on the reproduction of Hydra have
shown that the gonads are temporary, both being formed by ag-
gregations 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
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 and ac-
cessor}^ structures capable of producing only one kind of germ cells.
In some of the types of animals, individuals of the two sexes simply
deposit the mature germ cells in the same vicinity and at about

PHYSIOLOGY 193

the same time. Under the section on reproduction in starfish such
a procedure is described. In animals like the toads and frogs,
a special provision is made to bring the individuals of the op-
posite sexes together 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. 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 the
female, and the ova are fertilized within the urinogenital tract of the
female.

In birds and reptiles after the addition of nutritive and protective
coats the egg passes to the outside to develop and hatch (oviparous
animals) but in all mammals, except monotremes, it is retained
within the uterus during the period of embryonic development, and
the young are born 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 or
vice versa but the necessary materials are allowed to diffuse through
the tissue of the placenta in which both systems are distributed.

Parthenogenesis.- — In some species of invertebrates, sexual repro-
duction 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 partheyiogenesis. 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-

194 ESSENTIALS OF ZOOLOGY

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 prickling, 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 genetic characteristics of the male parent.

Other features of reproduction are given in the chapter on Sexual
Reproduction and the Development of the Individual.

References

Burton-Optiz, R. : An Elementary Manual of Physiology, Philadelphia, 1936, W.
B. Saunders Company.

Heilbrunn, L. V.: An Outline of General Physiology, Philadelphia, 1937, W. B.
Saunders Company.

Howell, W. H.: Textbook of Physiology, Philadelphia, 1936, W. B. Saunders
Company.

Macleod, J. J. R. : Physiology in Modern Medicine, St. Louis, 1938, The C. V.
Mosby Company.

Rogers, C. G-. : Textbook of Comparative Physiology, New York, 1938, McGraw-
Hill Book Company.

Stiles, P. G. : Human Physiology, Philadelphia, 1939, W. B. Saunders Company.

Zoethout, W. D.: Textbook of Physiology, St. Louis, 1938, The C. V. Mosbj
Company. •

CHAPTER IX

THE ENDOCEINE GLANDS AND THEIR FUNCTIONS

The great complexity of the structure of organisms, particularly
of those animals in the higher ranks of the aniiyial kingdom, makes
necessary a means of regulation and coordination of the functions
of the organ systems individually and a means of intercommunica-
tion between them. This work is cared for in part by the nervous
system; but another agency of regulation is present in all higher
organisms and many of the lower forms of life, which is of first
importance in this respect, one so complex and interwoven with
other organs in carrying out its functions that it is not thoroughly
understood at the present time. The group of organs doing this
work we designate as the endocrme glands. They manufacture and
furnish the body with chemical compounds called hormones, a term
which refers to compounds formed in the body and capable of
encouraging or exciting activity in another part of the body; they
are chemical messengers. The endocrine glands pour their hormones
directly into the blood stream without the assistance of ducts, and
the blood carries them to other parts of the body ; but they are never
carried to the outside of the body directly, as in the case of secre-
tions from the glands of external secretion, and for this reason the
endocrine glands are often referred to as organs of internal secretion,
or ductless glands.

The functions of the hormones are numerous, and current research
and investigation in the new field of endocrinology are constantly
presenting new activities and interrelationships of these secretions.
The manner in which the endocrine organs produce their secretions,
the hormones, the manner in which they choose the raw materials,
combine them, and give forth products which are vital to the welfare
and happiness and often to the very existence of an organism, is of
most intense interest and of the greatest importance.

The hormones may be classified, arbitrarily, according to function
in three main groups. The first of these groups includes those hor-
mones which arouse specific responses in particular organs or in
localized parts of the body. A second group is composed of those
hormones which affect the general metabolism of the body. The

195

196 ESSENTIALS OF ZOOLOGY

third group is that of the hormones which affect, in particular, the
growth and development of the organism. Another general method
of classifying substances of this nature terms those substances
which serve as excitors or accelerators ''hormones" and those which
mhihit or depress activities as ''chalones. " Illustrations of each of
these will be found in the discussion of the individual organs and
their functions.

Hormones are found in plants as growth hormones, in the inverte-
brate animals, and are best known in vertebrate animals. The follow-
ing examples will serve to demonstrate the nature of endocrine func-
tion in invertebrates: Earthworms whose testes are completely de-
stroyed, do not develop the clitellum, the band-like organ which func-
tions during and following copulation. In BonelUa, another annelid
worm, there seems to be a relation between sex determination and cer-
tain hormones, in that the week-old larvae are indifferent sexually,
but those that attach to the proboscis of the female parent become
males due to some agent received there. All others develop to become
normal females. Among the crabs there is a parasite which attacks
and destroys the gonads, and it is found that the parasitized male
crab will take on distinctly female characters due to the lack of some
humoral agent lost with the destruction of these glands. Too, using
radium to destroy the gonads of Asellus, another crustacean, seems
to indicate that the development of the brood pouch is controlled by a
substance produced in the normal ovaries. Molting and metamorpho-
sis both are regulated by hormones in insects. The supraesophageal
ganglion seems to produce a substance which initiates pupation activi-
ties in moths. The endocrine glands of vertebrates which are best
known and most clearly understood at the present time are the thy-
roid gland, the parathyroid glands, the suprarenal bodies, the pitui-
tary body or hypophysis, the thymus, the gonads, and the pancreas.

The Thyroid Gland

The thyroid gland is the most familiar of the endocrine organs
to the layman. It is a body of two lobes of about the size of wal-
nuts, slightly flattened, placed one on each side of the upper part
of the trachea just below the larynx, or voice box, the two lobes
being connected by a saddle-shaped isthmus. The thyroid is well
supplied with blood, receiving in proportion to weight, three and
one-half times as much blood as the brain; this permits an easy
access of the hormone to all parts of the body. The thyroid is

ENDOCRINE GLANDS AND THEIH FUNCTIONS 197

normally not visible externally, but the pathogenic condition of
the organ caused by overgrowth, and kno^vn as goiter, is familiar
to everyone.

The functions of the thyroid gland, as recognized today, are two:
that of control, in conjunction with other endocrine organs, of the
growth and development of the body ; and, second, a most important
role in the regulation of metabolism. Any upset of the normal
functioning of the gland or the removal of the gland results in
serious physical and mental disorders, if not in death itself. The
hormone produced by this organ, designated thyroxine, has been
prepared from the fresh thyroid glands removed from various ani-
m.als and has also been manufactured synthetically. Chemical diag-
nosis of the hormone reveals the presence of iodine in its compo-
sition, and the amount of iodine available in the body seems to be
the determining factor in the degree of control which this hormone
exerts on bodily functions. In regions of the world in which iodine
is scarce in water and food, goiter is a prevalent disorder, and only
in recent years has the understanding of the cause been complete
enough to suggest as a remedy and a preventive the introduction
of additional iodine into the diet by use of iodized salt. Since the
amount of iodine required for normal purposes is almost infinitesimal,
a sufficient quantity is supplied by this means. The correction of
disorders due to a deficient functioning of the thyroid gland by use
of natural thyroxine or by use of the synthetic product is quite com-
mon today among human beings.

The removal of the thyroid gland in animals which have not ob-
tained their complete growth results in delayed or arrested develop-
ment. An interesting example of this is found among certain
amphibians. The proper functioning of the thyroid gland is essen-
tial for the accomplishment of metamorphosis of the frog tadpole
into the adult frog. The tadpole Jacking sufficient thyroid extract
may grow to an unusually large size, but metamorphosis never
occurs without the encouragement of this hormone. In the develop-
ment of human beings, a deficient supply of thyroxine in early
years results in a condition known as cretinism. The growth of
the bones does not take place, the entire body is stunted and de-
formed, mental development ceases, the facial features are misshapen,
growth of hair is scant, and the development of the sexual organs is
incomplete. Before work on the thyroid gland was undertaken,
cretins were often seen in certain parts of Europe and occasionally in

198 ESSENTIALS OF ZOOLOGY

America; but the present understanding of the hormone and its use
has made possible the prevention and cure of most cases of this nature.

Improper functioning of the gland in adult years results often
in a condition known as myxedema, in which there is a thickening
and drying of the skin, a puffiness of the eyelids and lips, loss of
hair due to the condition of the skin, a slowing down of metabolism
and heartbeat, a depression of body temperature, the deposition of
large quantities of fat, and a final result, in many cases, of im-
becility. The administration of thyroxine, especially in the early
stages, accomplishes a complete, or at least a temporary, remedy.

An overfunctional thyroid gland, in which the condition is known
as hype7'thyroidism, results in an increased metabolic rate, a loss of
body fat, and a condition of hyperirritability of the nervous system.
It seems, therefore, that the difference between an overly energetic and
a sluggish person, and a lean and an obese person, may often be traced
directly to the degree of functioning of the thyroid gland. Hyper-
thyroidism is accompanied by increased excretion of calcium. It dif-
fers from the calcium upset due to parathyroid disturbance in that in
hyperthyroidism its concentration in the blood remains normal.

Work on the lower vertebrate groups suggests the probability of
an important function of the thyroid gland in determining the hiber-
nation periods of certain animals. The thyroid performs additional
functions in conjunction with other of the endocrine glands; e.g.,
the control of sexual activity; but these interrelationships will not
be discussed here. Peculiarly, thyroxin, although an accelerator of
oxidation in vertebrates, has a depressor effect on cell division and
differentiation in such invertebrates as Paramecium, sea urchin, and
the hj^droid Pennaria. This has not been completely explained.

The Parathyroid Glands

Connected with the thyroid body are four little glands about the
size of small peas, so insignificant in appearance that they were
overlooked for many years. The removal of these small bodies along
with the thyroid gland in certain operations provoked such startling
results, however, as to attract attention to their presence and to
evoke considerable interest in their investigation. A complete re-
moval of the parathyroids results in unbalancing the blood calcium
and in a type of convulsion known as tetany; death is the usual result.
A deficient supply of parathormone or parathyrin, the hormone of the
parathyroid glands, may be responsible for defective growth of the

ENDOCRINE GLANDS AND THEIR FUNCTIONS 199

bones and for deficient formation of enamel and dentine of the teeth.
Calcium is needed for both the teeth and the bones and the introduc-
tion of either calcium or parathormone into the body is made to sup-
plement a deficient supply of calcium due to malfunctioning of the
parathyroids. Removal of the parathyroids also brings about a fall
in the renal excretion of phosphorus, and the injection of parathor-
mone causes an immediate rise in the level of renal phosphorus.
Parathyroid activity is particularly useful to laying hens where so
much calcium is needed in shell formation. There is evidence of a
close functional relationship between the pituitary gland and the
parathyroid. In dogs the removal of the pituitary causes atrophy of
the parathyroids, particularly if the pancreas has been removed also.
Conversely it has been noticed that injections of extract from an-
terior pituitary raises blood calcium in some species because of in-
creased activity of the parathyroids.

Too great an activity on the part of the parathyroid glands results
in a decalcification of the bones, and an increased content of calcium
in the blood and in the excretion of the kidneys. The final result of
this softening of the bones may cause serious disfiguration and stunt-
ing of the body. Accompanying these results are flabbiness of the
muscles, decreased irritability of the nervous system, and other un-
favorable conditions. These may be remedied by the removal of a
portion of the parathyroids.

The Suprarenal Bodies

Lying close to and slightly anterior to the kidneys are two small yel-
low or reddish masses of tissue, which play a prominent role in the
regulation of the body and one of such complexity that much is yet
to be learned concerning its method of functioning. These adrenal
glands, or suprarenal bodies, are made up of an inner and an outer
portion, the medulla and cortex respectively. The former secretes a
substance designated as adrenalin (epinephrine or adrenin), which
acts upon various organs and raises the level of their functioning.
Adrenalin, at times of excitement or emergency, may cause con-
striction of blood vessels, increased rate of heartbeat, a greater
discharge of glucose from the liver to provide additional energy,
erection of hairs, stimulation or inhibition of the various visceral
muscles, etc., to provide greater efficiency. The range of control of
the medullary portion of the adrenals is thus wide and complex,
definitely interrelated with the functions of the sympathetic nervous

200

ESSENTMLS OF ZOOLOGY

system, with other endocrine glands, and numerous processes of the
body, so that its true importance is difficult to estimate. Adrenalin
has been obtained from various animals for use in the treatment of
certain disorders, as an anesthesia in minor operations, and to stop
small hemorrhages. It has also been successfully employed in the
relief of asthma and similar troubles.

A hormone known as cortin has been isolated from the suprarenal
cortex, and, while the removal of this portion of the adrenals results
in death, the exact functions of the hormone produced therein are
not entirely understood. It does relieve the condition known as
Addison's disease. There is, without doubt, a close relationship
between the cortex and sexual development; and some workers
believe that the cortex regulates the normal flow of blood, which

Fig-. 58. — The suprarenal glands, N, against the anterior portion of the kidneys
and close to the diaphragm. (From Stiles, Human Physiology, W. B. Saunders
Company. )

would account for the fatal results of its removal. An extract has
been secured from the adrenal cortex of cattle in particular and is
used for the treatment of conditions resulting from malfunctioning
of the cortical portion of the adrenals in other organisms. Some
have shown cortin to have capacity for delaying the onset of scurvy
in vitamin C deficiency.

After complete bilateral removal of the adrenal cortex, the fol-
lowing were the average survival periods for these several animals:
opossums, six days; guinea pigs, seven days; dogs, ten days; cats,
twelve days; while rabbits and rats may live on quite normally,
because of the presence of accessory inter-renal tissue. Symptoms
of insufficient cortin are loss of appetite with particular distaste for
fats, vomiting, diarrhea, muscular twitching, tetanic convulsions,

ENDOCRINE GLANDS AND THEIR FUNCTIONS 201

lowering of blood pressure and body temperature, and decline in
urine secretion and heart rate. Lack of cortin is said to disturb
the salt relation in the blood (particularly sodium chloride and
potassium), the water metabolism and redistribution in the body,
the metabolism (especially absorption) of intermediate carbohy-
drates and fats, and milk production in females (at least in cats).

The Pituitary Gland

The pituitary gland in the human being is a body weighing about 0.5
Gm., lodged in a depression at the base of the brain. It consists of two
principal parts: the anterior lobe and the posterior lobe. These
two portions have distinct functions. The vital importance of this
body was not realized for many years, but a series of observations
has placed it in a position oi such importance that it has been
referred to as the regulator of the glandular system. The pituitary
gland is now known to be the source of a number of hormones, and
their functions are so closely connected with those of other endo-
crine secretions that they apparently have a part in all hormonal
processes of the body.

The secretions of the anterior lobe of the pituitary affect the
growth and development of the organism in general, the general
metabolism of the body, the development of the sex organs, and
work with other hormones in controlling additional processes, in-
efficiency in the anterior lobe, furnishing the body with either too
much or too little of the growth-promoting hormone (phj^rone) re-
sults in the production of giants or dwarfs. A decreased supply
of the hormone in an immature individual, if the condition is not
remedied by administration of the hormonal extract, retards the
growth of the body and may cause a complete cessation of growth.
The dwarfs of the circus furnish examples of this unfortunate con-
dition, although not all dwarfism must be thought due to this cause.
For example, the cretin described previously is the result of thyroid
disorder and is usually a mental dwarf as w^ell as a dwarf in body,
while dwarfism resulting from deficient phyrone is accompanied in
most cases by a normal mental development.

An overfunctional anterior pituitary results in a marked increase
in the growth of bones, although the general development of the
individual thus affected may be symmetrical and the physiological
processes may be normal in every respect. Cases of this type are
commonly seen. One such instance may be cited in which a nine-

202

ESSENTIALS OF ZOOLOGY

year-old boy measured six feet and one inch and weighed 178 pounds.
His mental condition was normal.

Another condition, known as acromegaly, may result from an over-
functional anterior lobe of the pituitary during adult years. The'
facial bones enlarge, particularly in the ridges above the eyes, the
nose, and the lower jaw, and the soft tissues of the face undergo an
overgrow^th resulting in a coarsening of the features. The hands
and feet may enlarge, also. The disease may prove fatal if it con-
tinues sufficiently long.

Fig. o9. — A pituitary dwarf at the age of nine and one-half years, compared
with a normal child of the same age. (From Zoethout. Textbook of Physiology,
The C. V. Mosby Company, after Engelbach.)

A second hormone secreted by the anterior lobe has a direct in-
fluence on the sex organs. This gonadotropic honnone (prolan)
stimulates growth and activity of the gonads, the testes and ovaries,
and therefore controls the production of the gonadal hormones, which
will be discussed later. The absence of gonadotropic hormone results
in an atrophy of the testes and ovaries, and the cessation of the pro-
duction of the spermatozoa and the ova; its injection increases the
activity of the sex organs.

The posterior lobe of the pituitary also produces more than one
hormone, although pituitrin is the one concerning w^hich we have

ENDOCRINE GLANDS AND THEIR FUNCTIONS 203

the most definite knowledge and which is commonly associated with
this portion of the organ. Pituitrin is known to stimulate the mus-
cles of the arterial system, increasing or decreasing the blood pres-
sure according to the amount of the hormone released in the blood.
It is also a stimulant for the musculature of the uterus and the
intestinal muscles.

It is concerned also with the regulation and disposal of carbo-
hydrates in the body. The body is able to use an increased quantity
of sugar when the secretion of pituitrin is reduced; and, on the
other hand, when the quantity of the hormone is more than normal,
the body needs less sugar; carbohydrates not actually needed are
stored as fat, resulting often in abnormally fat people, extreme cases
of which are seen in the circus. The posterior lobe of the pituitary
probably does not affect the development of the bones, but the func-
tion just discussed is quite definite.

The posterior lobe of the pituitary is concerned also with the
regulation of the secretion by the kidneys. A diseased condition,
known as diabetes insipidus, in which the patient voids large quan-
tities of urine, is treated by injection of the postpituitary hormone.
It appears that the hormone probably enables tissues to utilize and
store larger amounts of water than is possible in its absence.

Still another effect of the secretion is found in the case of certain
amphibians and reptiles ; that is, its effect on the pigmentation of
the skin of these animals. The removal of the pituitary gland of
a frog results, among other things, in the bleaching of the animal
and the inability of the frog to alter the color scheme of its skin
to agree with the surroundings. When in a strong light or on a
light background the retinas of the frog's eyes are stimulated by
light rays, and some of the impulses reach the pituitary 's posterior
lobe, resulting in a suppression of its secretion, and consequently
a lightening of the frog's skin. When the light is decreased the
pituitary increases its secretion and the frog has a darker pigmenta-
tion. These reactions probably do not occur so directly in the higher
groups of vertebrates. Removal of the pituitary gland tends to
cause atrophy of the other endocrine glands.

The Thymus Gland

The thymus, a small glandular structure located in the chest be-
tween the upper part of the sternum and the pericardium, is a tem-
porary organ, which normally atrophies in human beings by the time

204 ESSENTIALS OF ZOOLOGY

of the onset of puberty. When the gland is too active, a condition
is found in children in which an enlargement of the organ results,
and breathing is rendered difficult. No distinct hormone has been
obtained from this gland.

It has been claimed recently that accruing acceleration in the rate
of growth and development occurs when successive generations of
rats are given daily injections of thymus extract. In third and
fourth generations, the rats at twelve days of age compared favor-
ably with controls of twenty days. Introduction of thymus extract
in young tadpoles causes them to grow rapidly to the size of the
adult frog but still retain their tadpole form and appearance. The
disappearance of the thymus at the time of puberty permits the
differentiation of mature animals and particularly the onset of
activity of the sex glands. Some workers claim that the thymus
contributes to the orderly and proper development of the bones of
the skeleton.

The Gonads and Sex Hormones

In addition to the usual function of producing germ cells for re-
production, the gonads produce hormones which influence the devel-
opment of secondary sexual characters and which have a regulatory
effect on the reproductive processes and activities. Sex differences
are caused in part by various hormones which have a selective
action on the male or on the female secondary and accessory sex
characteristics. The earlier concept that the male sex produces
''male" hormones exclusively and the female produces only ''fe-
male" hormones is no longer held. For example, extracts have been
prepared from the urine of women as well as from men which on
injection into capons caused growth of the comb which ordinarily
fails to develop as it would in the cock. The sex hormone sub-
stances affecting the male are spoken of as androgenic and those
affecting the female as esti^ogenic. The important sex hormones are
androsterone, testosterone, theelin, and prog ester 07ie (progestin). The
first two are male hormones, and the others are female.

Androsterone is found in male urine and can be crystallized from
it. It has a stimulating effect on development of secondary sex char-
acteristics and a definite regenerative effect on accessory organs
(seminal vesicles, prostate glands, and penis) of castrated male ani-
mals. Testosterone is produced in the interstitial tissue of the testis
but is absent from the urine. This hormone is several times as

ENDOCRINE GLANDS AND THEIR FUNCTIONS 205

effective as androsterone in bringing about regeneration of acces-
sory sex organs in castl-ated males. Recently androgenic hormones,
which are potent enough to affect the growth of comb in the capon,
have been found in the urine and ovaries of female animals.

Theelin (oestrin, estrogen, folliculin, menoform, progynon) has
been isolated from liquor folliculi, pregnancy urine, the placenta,
and amniotic fluid. This substance causes (1) increased growth of
the accessory female organs (uterus, oviducts, etc.), including changes
in the glands of their linings and in vascularization; (2) contraction
of the smooth muscle of the uterus; (3) initial growth of mammary
glands and nipples; and (4) sudden lowering of theelin concentra-
tion in blood (suggested as cause for bleeding during menses). The
secretion of theelin is influenced by the gonadrotropic principle of
the anterior pituitary.

Progesterone (lutein hormone, corporin, luteosterone progestin) is
a female hormone produced by the corpus luteum, the yellow body
of material which forms in the ruptured Graafian follicle after the
escape of the ovum. It produces the following effects: (1) sensitiza-
tion of the lining of the uterus so that implantation or attachment
of the zygote may take place in case of fertilization; (2) develop-
ment of placenta ; (3) arrest of rhythmic contractions of the smooth
muscle of the uterus; (4) inhibition of ovum production and uterine
bleeding.

The influence of other hormones of the body is noted in the repro-
ductive and sexual processes. The effect of the pituitary has already
been mentioned. The thyroid secretion probably plays a role, not
clearly understood, in the female reproductive processes, since the
thyroid always enlarges at puberty and during pregnancy. Another
hormone is thought by some investigators to be formed in the pla-
centa during the development of an embryo. The interrelationships
of these hormones are involved, and doubt exists in some cases as
to their exact functions.

The Pancreas

This is one of the organs serving a dual purpose in the body; it
secretes from a group of its cells, called the islands of Langerhans, a
hormone designated as insulin. Experiments have shown the action
of insulin to be concerned with the metabolism of carbohydrates
and fats. Its presence facilitates the combustion of carbohydrates,

206

ESSENTIALS OF ZOOLOGY

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208 ESSENTIALS OF ZOOLOGY

regulates the rate of sugar production by the liver, and promotes
the storage of sugar as glycogen in the muscles. It therefore de-
creases the amount of sugar in the blood.

Extracts of insulin are obtained from the pancreas of animals and
are used commercially for the treatment of the condition known as
diabetes. This disorder is due to a disturbance of the metabolism
of sugars, provoked by a deficiency of insulin. The blood contains
too great a percentage of sugar, but this is not turned into needed
energy, and much water is excreted by the kidneys in order to elimi-
nate the excess sugar. The patient suffers, therefore, from fatigue,
excessive hunger, and thirst. The injection of insulin subcutane-
ously assists in regulating the condition by restoring the power to
transform glucose into glycogen in the muscles. The patient re-
gains strength and weight as a result, but the treatment does not
perfect a complete cure and additional insulin must be injected at
intervals to maintain normal health. Overdoses of insulin result in
very serious disturbances, which may be relieved by ingestion of
glucose.

Among the functions of the hormone, adrenalin, is the accelera-
tion of the production of glucose in the blood ; it is apparent, there-
fore, that the hormones, insulin and adrenalin, are antagonistic in
their effect on sugar metabolism, and an upset of the normal pro-
duction of these secretions results in metabolic disturbances.

Thus we see the complexity of the performance of these secre-
tions in the body. When they are present in correct proportions,
the organism enjoys a smooth functioning of its physiological proc-
esses, regulated by the hormones. The malfunctioning of any one
endocrine organ results, however, in a disturbance of those processes
which it specifically regulates and usually of others which it influ-
ences in conjunction with its sister glands of internal secretions
or with other regulatory organs of the body. Each gland is called
upon, therefore, to function in full cooperation with the other glands
as well as to perform duties peculiar to itself.

References

Allen, Edgar: Sex and Internal Secretions, Baltimore, 1939, Williams and Wilkins

Company.
Cameron, Alex. T.: Recent Advances in Endocrinology, Philadelphia, 1936, P.

Blakiston's Son & Co.
Schafer, E. A.: The Endocrine Organs, ed. 2, London, 1916^ Longmans^ Green &

Company.

CHAPTER X

SEXUAL REPRODUCTION AND DEVELOPMENT OF

THE INDIVIDUAL*

Eeproduction is one of the fundamental capacities of all living-
things. New individuals of the same kind can be produced sexually
by the activity of special germ cells (gametes) provided for that
purpose in metazoan animals. The germ cells of the two sexes are
quite different. The processes involved in sexual reproduction and
embryology are generally similar in different groups of animals but
different in matters of detail.

The male germ cells or spermatozoa are produced in the male
gonads or testes. The internal structure of more highly developed
testes consists largely of extensive seminiferous tubules imbedded in
interstitial tissue. The cells composing the inner lining layers of
these tubules produce the spermatozoa by a process of maturation.
The female germ cells or ofa are produced in the female gonad or
ovary. Certain cells of the inner surface of the epithelial covering
of the ovary become differentiated as germ cells. They become im-
bedded in the stroma of the ovary in the process of development.
The original epithelial cells from which germ cells arise are known
as primordial germ cells in either sex.

The maturation (gametogenesis) or development of germ cells
consists of a series of cell divisions which is modified at one point
to bring about a fusion of chromosomes and subsequently a reduc-
tion in their number. In the case of development of spermatozoa
the maturation process is completed within the gonad, but in devel-
opment of ova the last division or in some cases last two divisions
are completed after leaving the gonad and uniting with a sperma-
tozoon. The maturation process prepares the germ cells for fertili-
zation. Development of the male germ cell is known as sperma-
togenesis, and the development of the female germ cells is oogenesis.

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 more complete study of comparative embrj^ology, the student is referred
to Richards, Outline of Comparative Embryology, published by John Wiley & Sons,
Inc., or to the chapter by Dr. Richards on comparative embryology in Potter, Text-
book of Zoology.

209

210

ESSENTIALS OF ZOOIiOGY

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
instances each of these cells divides once more. Next, each of these
oogonia passes through a groivth 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

Oogenesis

Primordial f^

qermcell 'W'i

Spermatoqenesis

Primary Yff W

oocyte \^//

fi)\\ Primordia]

Spermato-
'-" gonia

— Chromosome

7/i\ Primary

(( ^T sperTT)atocyt<i

Secondary A/x

ndcvtz ((^j^

oocyte

"—^ /Sc formation

1st. polar body

Zr'd- polar body-"-'

rertiliied ovum (Zygote) [^ '^

Fig. 60. — Maturation of germ cells. Oogenesis includes the maturation divisions
in development of the female germ cells or ova, and spermatogenesis is a similar
process of division in the development of mature male germ cells or spermatozoa.

{ l\ Secondary
f ( J spermato-

cyte

Mature

'" spermabo-
^oon

these oocytes divides by meiotic division, the fused chromosomes
dividing as though they were single ones in normal division. This
division, therefore, results in cells with half the sQ^matic (diploid)
number of chromosomes and is spoken of as the reduction division.
The cytoplasm does not divide equally ; nearly all of it goes to one
of the 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 signifi-

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 211

cance after carrying 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 maturation period of the process. The
ovum containing the haploid number of chromosomes is now pre-
pared to unite with a mature spermatozoon in fertilization.

Sper7natoge7iesis is completed within the tubules of the testis, and,
like oogenesis, is a series of cell divisions. The primordial germ
cells divide by mitosis to form spermatogonia, and this process con-
tinues just as it does in oogenesis, until the division of the primary
spermatocytes which have developed during the growth period.
When the primary spermatocytes divide, the division is an equal
meiotic 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 spermatozoon with
its half number or, in this case, four chromosomes. The change
from spermatid to spermatozoon does not involve a cell division but
simply rearrangement. The spermatozoon is a slender, motile cell
composed of head, middle piece, and tail. It is now able to swim in
fluid and prepared to unite with a mature ovum.

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. These modified divisions of the oocytes in
oogenesis and spermatocytes in spermatogenesis, w^hich bring about
the reduction of chromosomes, are termed meiosis. 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 swims to the egg 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
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 finallj^ fuse

212 ESSENTIALS OF ZOOLOGY

to form the fusion nucleus, and the fertilization is complete. The
s^nmrance of fertilization is largely centered around two important
functions First, it is the impetus for the development of an embryo
omX egg under most normal ci-mstances ; however, par hen -
genesis replaces this function in some cases. Secondly, it brings

v^^v-zv^Vt-irtfei^:^":

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■nm

• • • -• •••-•...•. . • • • • . • •

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F,g 61.-Fertn,zation in the sea. urchin Jo J0P-««-- ,S.n^^^^^^^^^

m, fransiormation of ^l-iroi^f °%'" ?S |d by S«™'s= °" ^'°'" °"'Srt "/ ^
°;:,.Sf;rit^^o'SV^Saraif S"'t^ii^y^ i'na son. Inc.. after W.ison.,

about the means for inheritance of characteristics from two different
toes of ancestry. This union also restores the diploid number of

chromosomes.

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 213

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
divisions extend completely through tiie 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

s

3- itS^'. ...... • ..•>Z\-

Fig. 62. — Cleavage in the embryo of Asterias (starfish). 1, fertilized ovum
(zygote); 2, two-celled embryo following first cleavage division; 3, the four-cell
stage; J,, the eight-cell stage; 5, the sixteen-cell stage; 6/morula stage (solid);
7, blastula stage (hollow) ; 8, early gastfula stage (infolding of cell layer at one
side) ; 9, later stage of gastrulation. The infolded layer is the endoderm. (Drawn
by Titus C. Evans.)

smaller parts. This process provides for the rapid increase in the
number of cells and growth of the embryo which is necessary before
any special parts can be formed. Cleavage has been described briefly
in an earlier chapter under the discussion of the development of
the frog.

As divisions proceed, a Uastula is formed by the development of
a cavity (blastocoele) within the spherical mass of cells, the wall of

214

ESSENTIALS OF ZOOLOGY

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.

nut.

bid.

GASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK— AMPHIOXUS.

ea.

:— - blp.

GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE

AMOyNT OF YOLK— AMPHIBIA.

ent

gastrocoele

GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE

AMOUNT OF YOLK— BIRDS.

Fig- 63.— Gastrulation in three types of embryos. Each type begins with the late
blastula at the left, blc, blastocoele ; bid., blastoderm; bZp., blastopore; ect., ecto-
derm; ent., endoderm; mit, cells undergoing mitosis. (From Patten. Emhryoloay
of the Chick, by permission of P. Blakiston's Son and Company.)

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 2l5

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 genu layers, ectoderm and
endoderm. In sponges and coelenterates development stops here.

In higher forms, immediately following gastrulation, a third germ
layer, the mesoderm (middle layer), 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
portion to organize 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 geryn layers is determined as cell division and
development continue. Cell 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

216

ESSENTIALS OF ZOOLOGY

,Body Form

Since the development of the frog has been discussed briefly in
the chapter dealing with the bullfrog, it will not be included here.
In a developing chicken, which is easily studied, the head fold ap-
pears in a fertile egg that has been incubated about twenty hours.
The neural groove (primitive spinal cord) begins its formation at
about this time. At about twenty-one or twenty-two hours of incuba-
tion, there appear on each side of midline and lateral to the neural

area opaca
vitellina

proamnion

ectoderm of head

mesench5nne

neural fold

neural groove
notochord

border of fore-gijt

subcephalic pockct

margin of ant.
intestinal portal

area pellucida

.unsegmented
mesoderm

border of mcsodcrtn

Fig. 64. — Dorsal view of entire chick embryo of about 24 hours of incubation.
Notice that the fourth pair of somites is almost developed. (From Patten, Em-
bryology of the Chick, P. Blakiston's Son and Company.)

groove blocklike thickenings in the mesoderm. These are somites,
and thej^ are paired opposite each other, marking segmentation in
the body. At twenty-four hours of incubation there are four pairs
of these somites, and they increase with growth until at thirty-six
hours there are fourteen, at sixty hours thirty-two, and approximately
forty at four days. Similar somites develop in a somewhat similar
way in other vertebrates including mammals. Rabbit, cat, rat, calf,
or man is no exception. In a pig embryo 6 millimeters long there

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 217

are thirty-two somites and the body form is becoming defined. The
human body can barely be discerned in embryos of one month, and
the embryo must be nearly two months of age before it can be identi-
fied definitely as human on the basis of morphological features.

The younger embryos of different groups of vertebrate animals are
so similar that it is impossible to distinguish them from each other.
The illustrations in Fig. 203 in a later chapter will bear this out.
At a stage early enough, the embryos of the human being, the pig,
the rat, the rabbit, the alligator, the salamander, and the fish all ap-
pear very similar. The gill slits and segments are conspicuous on all
of them. Much later, the limbs develop from lateral limb buds in
the mesoderm. The hind limb buds develop first, and the front ones

follow.

Organs and Systems

After the three germ layers, ectoderm, endoderm, and mesoderm,
have been established in the embryo, the next step is the differentia-
tion of these layers each in various ways for the formation of par-
ticular organs and systems of organs. The development of the
typical vertebrate systems is generally similar in all of the different
classes of this group. The development of the different organs in
the body is often referred to as organogenesis.

Digestive System. — This system takes its beginning in the arcJien-
teron which is formed as a new cavity at the time the endoderm be-
comes differentiated from the germ plate of cells. The endoderm
becomes the lining of the alimentary canal and its outgrowths, with
exception of the mouth and anal cavity. And the splanchnic portion
(medial sheet) of the mesoderm comes in to cover it. The muscles,
connective tissue, and blood vessels in the wall of the canal are pro-
vided by this mesoderm sheet. Immediately posterior to the mouth
is the pharynx, which has lateral gill pouches or pharyngeal pouches.
In the higher vertebrates there are typically five pairs of these
pouches. In fishes and amphibians these pouches break through to
the exterior, forming gill clefts. In higher forms these pouches are
transient structures which remain to call attention to existence of
.functional gills in their phylogenetic ancestors. Certain adult struc-
tures do arise from these pouches even in mammals. Such structures
as the middle ear. Eustachian tube, hyoid, thyroid, thymus, tonsil,
and others are products of these pouches.

The tubular canal is inflated and curved in the formation of the
stomach some distance posterior to the pharynx. The small and the

218 ESSENTIALS OF ZOOLOGY

large intestines develop from the posterior portion of this origi-
nally straight tube with relatively slight modification. The liver
appears as a tubular outgrowth from the duodenum as likewise does
the pancreas.

The Respiratory System. — The trachea develops as a posterior ex-
tension of the pharynx, which is separate from the esophagus. It
begins as a groove, extends posteriorly, and soon bifurcates, forming
the bronchii from which the two lungs develop. This original struc-
ture, therefore, is endoderm, and the internal lining of the adult
trachea and lungs is the portion derived from this. Again, the
splanchnic mesoderm has contributed the muscle and connective tis-
sue outside of the lining.

Nervous System and Sense Organs

At the end of gastrulation the ectoderm has become a distinct
layer. The portion of it which will become the mid-dorsal area of
the body thickens to become the neural plate. This plate sinks,
forming a neural groove with a neural fold as each lateral boundary
of it. The groove deepens, and the folds grow to meet each other
over the cavity thus forming the neural tube. This tube is the fore-
runner of the spinal cord and brain.

The Brain. — The anterior end becomes swollen and thickened for
the development of the brain. This brain structure divides by con-
strictions into the anterior forehrain (prosencephalon), midbrain
(mesencephalon), and the posterior hind brain (rhombencephalon).
Later, the forebrain divides into two regions, the anterior cerebral
hemispheres (telencephalon) and the posterior diencephalon. The
midbrain, or mesencephalon, remains undivided to become optic lobes
and related structures. The hind brain divides to furnish the cere-
bellum (metacephalon) from its anterior portion and the medulla ob-
longata (myelencephalon). During the time of this development the
brain becomes bent or flexed ventrally on itself so it becomes short-
ened and compact.

Sense Organs. — These organs all develop from the ectoderm. The
organs of four of the principal senses develop from the peripheral
ectoderm, but those of the fifth, the sensory portions of the eyes, are
essentially processes of the ectoderm wall of the brain.

The sensory portions of the eyes form as hollow evaginations (out-
growths) from the sides of the forebrain (diencephalon portion)
growing laterally toward the peripheral ectoderm of the head.

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 219

These expanded outgrowths are known as optic vesicles and the nar-
rowed connection of each to the brain is an optic stalk. The lateral
wall of the vesicle inverts into its cavity. The indented portion thus
forms the optic cup, the internal lining of which becomes the retina.
Slightly later, the crystalline lens is formed by a thickening of the
peripheral ectoderm which covers the optic cup. Along the ventral
side of the optic stalk is a furrow, the choroid fissure in which blood

myclencephalon
visceral arch III
bulbo-conusarteriosus

auditory vesicle
, / endolymphatic duct

ganglion IX/ / ganglion VII-VIII

atrium

ventricle

liver
prominence

hyomandi bular cleft
mandibular arch

ganglion V

antenor
appendage bud i^

omphalo-
mesenteric
vein

. meten—
cephalon

mesen-
cephalon

optic cup

lens

diencephalori

epiphysis
telencephalon

olfactory pit

"S — omphalo-
mesenteric
artery.

allantois

Fig. 65. — View of right side of entire chick embryo of about four days of in-
cubation. Heart, brain, sensory organs, gill slits, and appendages are in the
process of development. (From Patten, Embryology of the Chick, P. Blakiston's
Son and Company.)

vessels and the optic nerve will later lie. The choroid, sclera, ciliary
process, and muscles all develop from surrounding mesoderm, just a
little later.

Olfactory organs begin as two areas of thickened ectoderm on the
head, anterior to the oral aperture. The plates of ectoderm sink to
form olfactory sacs which later open through to the mouth cavity,
or pharynx in higher vertebrates. The walls of the chamber thus

220 ESSENTIALS OF ZOOLOGY

formed become deeply folded, and sensory cells are distributed in
the epithelial lining.

The ear originates in the individual as a thickened plate of periph-
eral ectoderm on each side of the hind brain region of the head.
These plates invaginate (fold in), forming an auditory vesicle or sac
on each side and are later freed from the external ectoderm. Each
becomes a membranous lahy7^inth of the inner ear, which includes the
sensory cochlea and the semicircular canals. The middle ear develops
later from the first gill pouch forming a tympanic cavity. The
ossicles or ear bones follow later from the mesoderm. The connection
of this pouch to the pharynx is retained and becomes the Eustachian
tube on each side. The tympanic membrane develops as the meso-
dermic sheet between the outer extremity of the gill pouch and the
depression of the external cleft, which becomes external meatus. The
external ear arises as a mesodermal outgrowth beneath the skin.

The taste-buds develop in the epidermis of the tongue and else-
where in some forms. Thus they are ectodermal.

Circulatory System. — This system develops in the mesoderm. The
formation of the earliest blood vessels has not been studied in man
because they have already formed in the earliest embryos critically
studied. But in cats and other vertebrates they are formed from
small spaces which develop among the mesenchymal cells. These
spaces extend and fuse, usually developing along the longitudinal
axis. The heart develops as a pair of amnio-cardiac vesicles in the
mesoderm, one on each side of the wall of the foregut, which at that
time has not closed vent rally. As the digestive tube closes ventrally,
the vesicle from each side moves ventrally until it meets its fellow.
By this time each vesicle has extended lengthwise to become a tube
which is the omphalomesenteric vein. The anterior portions of these
two cavities join to form the tube which is the primitive heart. This
tube then bends on itself with one limb of the fold becoming the
atrium, which receives blood from the veins, and the other limb of
the fold becoming the ventricle and bulbus, which deliver blood to
the arteries. This represents the primitive condition as found in the
two-chambered heart. As development progresses a septum forms in
the atrium dividing it into right and left chambers and similarly an
interventricxdar septum forms to produce right and left ventricles.
These septa are completed at about the time of birth.

In the embryo, the gill arches are supplied with branches of the
ventral aorta, which leads anteriorly from the heart. These branches

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 221

extending through the gill arches are called aortic arches (Fig. 204).
The typical number of these arches in the vertebrate is six, but in the
embryos of most higher vertebrates the fifth one is quite transitory.
The first and second pairs degenerate, except for remnants which
unite with the third in forming the carotid arteries. The fourth pair
of arches becomes the systemic arteries in amphibia and reptiles, but
in birds only the right side of the arch persists while in mammals
only the left. The sixth pair of arches give rise to the pulmonary
arteries.

Myc.

A

Bui.

Vent.

S.K

Tr.Ari.

V.ao.

Tr. Art.

At.J..

Fig. 66. — Ventral view of the development of the heart of the pig-. The stages
shown bring out the changes involved in converting the primitive tubular vessel
into a four-chambered heart.

A, from 7 somite embryo ; ^, 13 somite embryo ; G, 17 somite embryo ; D, 25
somite embryo ; E, from embryo 3.7 mm. in length ; F, from 6 mm. embryo. At,
atrium (r, right; I, left) ; Bid, bulbus arteriosus; Endc, endocardial tubes; Myc,
cut edge of epimyocardium ; S. V., sinus venosus ; Tr. A7-t., truncus arteriosus ; V.
ao. r., ventral aortic roots; Vent., ventricle (r, right; Z. left) ; V. O. M., omphalo-
mesenteric veins. (From Patten, Embryology of the Pig, P. Biakiston's Son and
Company. )

. Urogenital System. — It is convenient to present the excretory and
genital systems under one head because certain parts are developed
in common and others in close relation to each other. The principal

222 ESSENTIALS OF ZOOLOGY

structures of this system arise from the middle portion (mesomere)
of the mesoderm germ layer. This portion of the mesoderm de-
velops as a fold of tissue just lateral to the midline on each side,
extending from the thorax to the pelvis and projecting down into
the body cavity slightly. The earliest and most anterior part of this
fold is the pronephros. In higher vertebrates this is strictly an em-
bryonic structure, though it does develop some Malpighian corpuscles
which exist for a time. The pronephric duct which develops and
leads from the pronephros to the cloaca persists, later becoming the
Wolffian duct. Next follows the mesonepliros, which is more elaborate
and really functions as a kidney for a time in the embryo of reptiles,
birds, and mammals, and throughout the life of fishes and amphibia.
It partially overlaps the posterior portion of the pronephros and ex-
tends several segments farther posteriorly. The collecting tubules,
which receive urine from the Malpighian corpuscles, reach the surface
of the mesonephros and join the pronephric duct leading to the cloaca.

In the reptiles, birds, and mammals, the permanent kidney arises
from still a third development, the ynetanephros. This is formed at
the free end of the metanephric duct which is an outgrowth of the
pronephric duct near the cloaca. The metanephros overlaps the pos-
terior portion of the mesonephros. There are numerous Malpighian
corpuscles in the tissue of this body, and the metanephric duct be-
comes the ureter. The Wolffian duct becomes the vas deferens of
the male but degenerates in the female except for a vestige. The
urinary bladder develops as a ventral do^vnpushing from the cloaca.

A genital ridge develops along the ventral margin of each meso-
nephros before it degenerates, and from this fold a gonad arises.
Regardless of future sex, there develops also in each individual, a
Miillerian duct from the ventrolateral portion of the mesonephric
body. This duct becomes the functional oviduct of the female, but
it degenerates in the male except in animals like the leopard frog.

The ovaries develop in their permanent location, but the testes of
most male mammals are derived from the mesonephros in the dorsal
part of the body cavity and at a certain age descend through an
inguinal canal to the scrotum which is outside the body Avail.

The external genital organs develop as general structures with no
distinction as to sex until about the end of the second month in the
human being. At that time the penis takes form from the phallus
or genital tubercle, if the individual is to be male. In the female

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 223

the clitoris, a relatively small body of erectile tissue, which develops
ventral to both the genital and urinary apertures, is homologous to
the glans portion of the penis. The labia, which are folds of tissue
at each side, are homologous to the scrotum of the male.

Intrauterine Development

There are several groups of animals, particularly among chordates
in which the zygotes are retained within the uterus for a consider-
able portion of the embryonic and fetal development. In such
chordates as some of the common sharks ; a few of the bony fishes,
as Gamhusia the mosquito minnow; certain snakes and lizards; and
in all mammals except the lowest group, the Prototheria, there is some
form of intrauterine development. In the groups mentioned, other
than the mammals, the egg bears an abundant supply of yolk, and
the developing embryo, although retained in the uterus for protection,
depends on the yolk supplied by the egg for nourishment. In mam-
mals, including the human being, the egg has a very meager supply of
yolk. It becomes attached (implanted) in the epithelium lining the in-
ternal surface of the wall of the uterus, which is highly vascularized.
In all forms where intrauterine development occurs, as well as in many
others, the ovum is fertilized within the genital tract of the female by
introduction of spermatozoa in the act of copulation or coitus. Among
the mammals the penis of the male is well developed as an intromit-
tent organ which is received in the vagina of the female where the
semen, carrying spermatozoa, is discharged. The motile spermatozoa
swim up the oviduct and meet the ovum shortly after it enters. Fer-
tilization occurs here, and the zygote moves down the oviduct, de-
veloping as it goes. By the time it reaches the uterus the embryo
is in the gastrula stage of development. The embryo is attracted to
the wall of the uterus, where it is partially embedded and attached.
This procedure is knoAvn as implantation. Due to the influence of a
sex hormone, the uterus becomes highly vascularized and increases in
size. The extra-embryonic membranes develop rapidly to cover the
embryo. These embryonic membranes unite vdth the epithelial lining
of the uterus at the point of contact to form the placenta. The uterine
portion of this membrane receives an abundant supply of maternal
blood, while the embryonic portion receives a rich supply from the
embryo by way of the umbilical vessels which pass through the um-
bilical cord. In some mammals the layers of the placenta con-
tributed by parent and embryo become inseparably fused, but in

224 ESSENTIALS OF ZOOLOGY

others, las for example the pig, they separate and only the embryonic
portion is shed at time of birth. In the other forms, some of the
uterine portion is shed along with the embryonic, and together they
are known as ** afterbirth. "

The circulation of blood in the placenta is by distinct maternal and
embryonic vessels which do not join each other. The materials in
solution in the blood of either can reach the other by diffusion only,
as the vessels and sinuses come in intimate contact in the membrane.
The embryo then does not receive the mother's blood as such, because
there i* not a direct connection between the two systems. The em-
bryo, or later the fetus, receives its nourishment and oxygen through
diffusion in the placenta and disposes of carbon dioxide and nitrog-
enous wastes by similar diffusion in the opposite direction.

The time elapsing from time of conception, or fertilization, to time
of birth is called the gestation period. In the human being this
period is of nine months' duration.

In the female mammal there is a definite cycle of changes which
occur in the uterus, in other sexual organs, and in physiologic rela-
tions of the body. This cycle is called the oestrus cycle. In the
human being the period required for this complete cycle averages
twenty-eight days. At one point in this cycle there occurs rather
severe degeneration of the uterine lining and some bleeding. This
period in the cycle is known as menstruation period. It was thought
earlier that the ovum was discharged from the ovary at this time and
entered the oviduct, but at present it seems to be more commonly
accepted that ovulation occurs during the 13th to 15th day following
the onset of menstruation. The length of the oestrus cycle varies
widely in different mammals. In most of them the female will mate
only at a certain period in the cycle. This is designated in common
terms as the ''heat" period.

Biogenetic Law, or Theory of Recapitulation

It is almost impossible to study the embryology of vertebrate
animals of the different classes without observing some features of
the apparent progressive development in regard to particular or-
gans and systems. In a number of respects the more highly de-
veloped forms seem to recapitulate or pass through the stages Avhich
existed through the long history of their ancestors. The appearance
of gill arches and even part of the slits in certain stages in the em-
bryonic development of mammals indicates that these animals pass

SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 225

through a stage comparable in this respect to adult fish. There are
many examples of such comparisons among animal groups. It is
not likely that any animal in its development repeats every stage
in its racial development or phylogeny, because many are fused and
new ones are introduced. In general, however, the statement, ''On-
togeny recapitulates phylogeny/' is recognized as having some appli-
cation. Ontogeny refers to the embryonic development of the in-
dividual.

Homology. — The principle of homology is another concept which
is employed in studies of development as well as in taxonomy and
anatomy. Structures are said to be homologous when they have simi-
lar morphological nature and similar embryonic origin. For example,
a pectoral fin of a fish, the wing of a bird, and the arm of the human
are homologous. The eye of the fish and that of man are homologous,
but the eye of a crayfish is analogous rather than homologous to these.
Its function is similar but its structure and embryonic origin are
different.

Theory of Metabolic Gradients (Axial Gradients). — In connection
with the orientation and polarity of developing embryos, there is
definite and normal order in the physiological phenomena involved.
Dr. C. M. Child has related this to regions along the principal axis
in forms which possess one. He holds that there is a relative meta-
bolic dominance of certain regions of such a body over succeeding
regions with a center or centers of high rate of metabolism and a
gradation to lower rates as progressing away from the center. The
animal pole of the embryo is such a center; this area sets the pace
in the development. In vertebrates the head structures develop
most rapidly and tend to dominate the developmental activity pos-
teriorly along the axis of the body. The posterior portions are
slower to develop. According to this theory, the organization and
orientation of developing parts are determined by the interaction of
chemical substances of cellular origin, with the rate of metabolism
as the most important factor in infiuencing the sequence of appear-
ance of parts.

References

Arey, Leslie B.: Developmental Anatomy, Philadelphia, 1934, W. B. Saunders

Company.
Richards, A.: Comparative Embryology, New York, 1931, John Wiley & Sons,

Inc.
Wieman, H. L. : An Introduction to Vertebrate Embryology, New York, 1930,

McGraw-Hill Book Company.

CHAPTER XI

ANIMAL ANOMALIES

It is rather generally understood among students of biology that
no two animals, even of the same species, are exactly counterparts of
each other. There is a certain normal range of variation in size and
structure as well as in functional efficiency. Any variations beyond
these rather fixed limits are classified as malformations or anomalies.
A study of such abnormalities is known as teratology. Abnormalities
may occur at almost any stage in the life of the individual, but a
large majority result from abnormalities in the process of develop-
ment. Many are due to failure of development and some to over-
development. In turn, much of this is due to malregulation resulting
from failure of balance in the functioning of the endocrine system.
The occurrence of striking malformations in newborn human babies is
in approximately the ratio of one to 165. Fortunately, many of the
most grotesque anomalies do not reach full development and the mon-
sters are born dead.

Causes of Anomalies. — There are both internal and external agen-
cies which bring about malformations in the individual. Both em-
bryology and pathology contribute to the explanation of the causes
of these abnormal conditions. The development may be perfectly
normal and a subsequent disease may be the cause of striking ab-
normality. On the other hand, certain diseases of the parent will
influence the normal development of the fetus. Even twinning by
itself is an abnormal process in most animals. It, however, is not
usually thought of under the title of anomaly, except when they are
physically connected or the individuals are otherwise malformed.
The causes of anomalies mav be summarized as follows :

1. Internal Causes.

a. The germ plasm carries hereditary causes for some, and these
characteristics are transmitted as are normal traits. Color blind-
ness and hemophilia (bleeder condition) are examples.

b. Diseases which cause abnormal growths and conditions, as
elephantiasis.

226

ANIMAL ANOMALIES

227

c. Unbalancing the chemical regulators (hormones) which are
produced by the endocrine glands. (Overactivity of the hypophysis
causes gigantism; cretinism, a dwarf condition, results from deficiency
in thyroid activity.)

d. Fortuitous abnormalities whose causes are not apparent or are
influenced by certain variations in other organs. Such anomalies
show up most frequently in the circulatory and nervous systems.
Venous or nervous connections to organs are often modified. Another
example is the rearrangement brought about by diaphragmatic hernia.

2. External Causes.

a. Environmental agencies may affect almost any individual whose
development occurs outside the body of its parent. Exposure to
radium or x-ray radiation, sharp variations in temperature, exces-
sive salt content, or contact with toxic substances may all be respon-
sible for various degrees of abnormality. The same factors are also
effective, if present, in the uterine environment of the placental
type of animal.

Fig, 67. — Grasshoppers at time of diapause, showing some of the abnormalities
which very infrequently occur in their natural development. 1, Normal embryo :
2, embryo with two extra heads and mouth parts ; 3, embryo with lateral twin
joined at the abdomen ; ^, almost complete twins back to back ; 5, embryo with a
double abdomen. (From Evans, Effects of Roentgen Radiation, Physiol. Zool., Vol.
X.)

b. Mechanical factors, such as abnormal pressure, blows, and falls,
may cause some abnormalities.

c. Abnormal implantation in the wall of the uterus resulting in
deficiencies in nourishment and support of the fetus.

d. Such diseases as syphilis, which may be transmitted from mother
to offspring, are responsible for some types of defects, as impaired
vision.

e. Developmental inhibition or arrest brought about by deficiencies
in metabolism at a time when the rate or efficiency should be high.

228

ESSENTIALS OF ZOOLOGY

The work of several embryologists seems to indicate that the pro-
duction of twins either as normal individuals or otherwise is re-
lated to this condition.

Harelip and Cleft Palate

These two defects are related and are sometimes found together.
The lateral palatine processes may fail to complete growth and
unite properl}^, thus leaving a gap in the roof of the mouth which
opens directly into the nasopharynx above.

Fig. 68. — A case of harelip due to arrested development.

Harelip (cheiloschisis) is a very common defect and is due to the
failure of union of the nasal and maxillary processes. There may
be double harelip or single harelip. In connection with double hare-
lip it sometimes happens that the premaxilla projects beyond the
outline of the face to form what is called luolf-snoiit. Harelips are
frequently remedied by a surgical operation early in life.

Diaphragmatic Hernia (Open Diaphragm)

An extreme case of this was found in a cat which was being used
for dissection purposes in Baylor University. The animal had lived
an apparently normal life and had been killed for laboratory study
without showing evidence of its abnormality until dissected. From

ANIMAL ANOMALIES 229

all appearances the diaphragm had not completed its development,
but had formed a fringelike projection which reached inward about
half an inch from the thoracic wall and extended the entire cir-
cumference of the inside of the thorax. The aperture in its center
measured one and three-fourths inches in diameter.

Due to this condition the arrangement of several visceral organs
was greatly affected. The thorax was somewhat elongated, and the
right side of the cavity comprised about two-thirds of the space
of the chest. The mediastinum (supporting median mesentery of
heart and lungs) had its attachment more than half an inch to the
left of the midline.

Almost the entire liver was turned forward to occupy the right
two-thirds of the chest cavity, and this placed the gall bladder at
the level of the junction of the auricle and ventricle of the heart.
Approximately half of the spleen, the pyloric portion of the stom-
ach, and a large portion of the omentum had been drawn through
the aperture in the diaphragm. The right lung was extremely
crowded and small.

Polydactyly (Extra Digits)

There are numerous abnormal variations in the number and ar-
rangement of digits, ranging from a stumplike structure of no digits
through the ''lobster claw" condition of two or three, to as many
as two more than normal. The polydactylous condition is rather

Itl

1*l:;l. •#

Fig. 69. — Front feet of a half-grown kitten with six digits. The claws are pro-
tracted on the left foot and retracted on the right. There were five toes on the
hind feet of this cat.

frequently found in cats. In fact, Wilder once reported that pos-
sibly 25, per cent of the cats of the vicinity of Ithaca, N. Y., possessed
an extra digit on each foot. The forefeet shown in Fig. 69 are
from a living cat in Waco, Texas, which came from a litter of four.

230

ESSENTIALS OP ZOOLOGY

two of which showed these complete characteristics on all feet, one
of the litter had only an extra toe on one foot, and the fourth was
normal. Both hind feet of this animal had five well-developed
digits instead of the usual four. This condition is a hereditary one
and is brought about by partial duplication of elementary structures.

Conjoined Twins

Instances occur in which individuals of about equal size or of
unequal size are fused together. If this occurs at the hips with
dorsal sides together, the condition is usually known as Siamese

'=»f__,,'.*^r-

^%

4^

Fig. 70. — Conjoined twins of cat showing single head, but double trunk and ap-
pendages.

twins. There are a number of different varieties of fusion, includ-
ing the head region, chest region, or complete fusion of trunk into
a single body with two heads and vice versa. A chicken with two

ANIMAL ANOMALIES

231

pairs of legs and wings, two backs, and a single head has been
dissected and described by Dr. F. L. Fitzpatrick. The feather tracts
were double on the neck and trunk. Internal dissection showed
the single digestive system to extend between the two necks, follow
through the combined body cavity with some modifications, and

Fig. 71. — Conjoined human twins showing- single hips but double trunk and head.
(Redrawn and modified from Arey, Development Anatomy, W. B. Saunders
Company. )

empty into the cloaca of the right back region. No cloaca was pres-
ent in connection with the left back. There was a single heart, two
very unequal lungs, two pulmonary arteries and veins to the larger
lung, while there was only one common carotid artery, that being

232

ESSENTIALS OP ZOOLOGY

the right. Two trachea were present, the right being rather nor-
mally developed while the left was rudimentary. The brain was
normal, but joining the medulla were two separate and complete
spinal cords, one passing to each back region. Most of the internal
organs ''favored" the right side, except the lungs, of which the
left was much more developed.

Dr. F. L. Fitzpatrick reports upon the anatomy of a double pig
also. This freak had two tracheae leading to two sets of lungs.
The brain consisted of a three-lobed cerebrum, two cerebellums,

Trunh from other heart —
R. subclavian-

R. carotid

L. carotid

-L. subclavian

Ductus
arteriosus

Parietais-

Dorsal
aorta

Porietals

Allantoic-
(lef t back)

Renal

-Utero -ovarian

^ External ill ac

-'"^^'^ Allantoic r left)

Caudal

Fig. 72. — Dorsal aortae and branches in embryo pig which had a single head and

two bodies. (After Fitzpatrick.)

and two medullas. There was a spinal cord in each of the two backs.
A third or median eye was present on a ventral stalk beneath the
third or transverse lobe of the cerebrum. Two pairs of kidneys, two
bladders, four ovaries, double sets of oviducts and uteri, as well as
a double cloaca, were present. There was a single but modified heart
and two aortas leaving it. Externally the animal had two sets of
limbs, two tails, two sets of mammae, two anal apertures, and two
urinogenital openings.

ANIMAL ANOMALIES 233

It is suggested that such individuals have developed from a single
zygote which underwent an abnormal cleavage. Examples of these
conditions are shown as they occur in cats. Similar fusions occur
in human beings as is illustrated in the accompanying diagrams.

Hermaphroditism

There are abnormal cases of sexual development in A^ertebrates,
including man, in which the organs of both sexes are combined in
one individual. In true cases there are present both ovaries and
testes in the one individual, but no cases are known in which both
have functional capacity. The external organs are partly male and
partly female, and the secondary sexual characteristics, such as
voice, mammae, stature, and beard, may be of a mixed nature.
Hermaphroditism is the normal condition in the coelenterates, flat-
worms, annelid Avorms, and mollusks. Here both sets of organs are
capable of function. Birds and mammals are rarely subject to this
condition.

A condition spoken of as false hermaphroditism involves the pres-
ence of gonads of one sex but the secondary sexual characteristics
and external genitalia of the other. In masculine hermaphroditism
testes are present but not usually descended into the scrotum, while
""he external genitalia and secondary characteristics are those of
the female. In feminine hermaphroditism ovaries are present, even
descended into a scrotum in rare cases, but the clitoris is enlarged
and labiae are fused to resemble the penis of the male. In some
instances the lips of the slitlike urinogenital aperture on the under
side of the penis fail to fuse.

There is a very close homology in the organs of the two sexes.
The external genitalia are indifferent or sexless until the end of the
seventh week of embryonic development in human beings. Then
the determination comes, causing normally the modifications of de-
velopment to form the organs of one sex or the other. The develop-
ment of the external organs of the two sexes is strictly parallel.
It is likely that this development is controlled by hormone rela-
tionships, and it is in cases in which this balance is disturbed that
hermaphrodites occur. There is still much to be learned concerning
the causes of this condition.

234 ESSENTIALS OF ZOOLOGY

Cardiac Anomalies

Transposition of the heart to the right side of the midline of the
body is known as dextrocardia and occurs rarely. It is usually asso-
ciated with displacement of other visceral organs.

Due to faulty development, an incomplete interventricular septum
is occasionally found in the four-chambered hearts. The failure of
complete development of the septum between the auricles to close
the foramen ovale is a more common anomaly. It has been reported
that this occurs in some degree in one case in four for the human
being. Actual mixing of auricular blood sufficient to interfere with
normal function is much less common, however, because of the over-
lapping of the membranous walls which are pressed together by
the pressure of contraction. In a small number of human cases the
aerated blood and unaerated blood from the two auricles do mix
and produce a purplish colored blood which in turn affects the color
of the skin. Such a case is known as a ''blue hahy'* and often
leads to early death.

Abnormalities of Brain and Sense Organs

Encephalocoele is due to the protrusion of a sac of the meninges
and part of the brain through a defect in the roof of the cranium.
An abnormally large brain, which is usually associated with the
distention of the cranium by superabundance of cerebral fluid, is
known as hydrocephalus, or the size may be macro cephalus. The
opposite extreme in which the head and brain are abnormally small
due to failure of development is known as microcephalus.

There are cases of cleft nose in which the nostrils are in inde-
pendent projections. This condition is usually associated with hare-
lip and cleft palate. With regard to the eyes there are several
possible abnormalities. Cyclopia is a condition in which there is
a single median eye like that of the Cyclops instead of the usual
paired arrangement. In such cases the nose is usually at the base
of the forehead, above the eye, and cylindrical in shape. Failure
of complete development of the iris or chorioid, thus leaving a gap
or open sector in the margin of the pupil, is known as coloboma.

il

CHAPTER XII

GENETICS AND EUGENICS

(By Frank G. Brooks, Cornell College, Iowa)

The History of a Great Discovery

Like father like son" is a very ancient adage. Since man has
been able to think, he has pondered the problem of heredity. Al-
though very early he had observed evidence of the inheriting of
parental characters by the offspring of the various forms of life with
which he was familiar and was convinced by these observations that
heredity did take place, he has not known the ''how" or ''why" of it
until recently. The science of heredity, therefore, is very new. The
fundamental law on which it is based was announced by Gregor
Johann Mendel in 1866. However, Mendel, an Austrian monk, pub-
lished his discovery in an obscure journal and it did not receive
general recognition until its rediscovery in 1900.

Mendel's success in finding the underlying principle of heredity
was due, in part, to his choice of an experimental unit. Instead of
following the usual trend by considering how a parent conveys his
various traits to his offspring, thus making the individual the unit of
observation, he chose a definite inheritable character and considered
how it was transferred from many parents to all their offspring. In
addition to this wise choice of an investigational unit the patience,
mathematical ability, skill as a gardener, and analytical insight of the
investigator contributed also to the success of the research.

Mendel chose the garden pea as the material for his work. This
was a fortunate choice for, although the law that he was to discover
underlies practically all inheritance, it is not always as free from
complications as it is in the case of those traits of the pea which he
investigated.

Mendel's Law

For one of his projects he planted seeds from stock that had been
known to produce nothing but tall plants for many generations. He
planted also seeds from stock that had produced nothing but dwarf
plants. When the two types of plants were in blossom, he transferred
the pollen from the stamens of the one to the stigmas of the other.
The seeds that resulted from the cross were collected and planted.

235

236

ESSENTIALS OF ZOOLOGY

To the amazement of the investigator they produced nothing but tall
peas ; peas as tall as the original tall parents. Dwarf ness seemed to
be lost and tallness was certainly dominant. A less thorough investi-
gator might have called his experiment finished and have proclaimed
as a rule, the fact that a trait can disappear when it is crossed with
its opposite. But not so with the patient monk. He pollinated these
plants with their own pollen and planted another generation. This
time three-fourths of the plants were tall and one-fourth of them
were as short as the original dwarf parents. Not being ready even
yet to formulate a law, he self -pollinated his plants for several more
generations and got results that required his best mathematical skill
to interpret. The dwarf peas produced only dwarf peas. Of the tall

0 0 0J0 0 0

TT

Td

Td

dd

Fig. 73. — Diagram to show the result of crossing- tall and dwarf peas, TT, at
upper left, is an individual which is pure dominant for tallness. At upper right,
dd is pure recessive for dwarfness. When two such individuals are bred together,
or crossed, all of their offspring will carry both characters, but appear tall, smce
tallness is dominant. The Td individual in the upper middle is representative of
this generation, which is the Fi (1st filial). W^hen two of these mixed, Td individ-
uals are crossed, the offspring (F2 or 2nd filial generation) present what is known
as the 1:2:1 ratio, i.e., one pure tall (TT) individual (at left), two mixed (Td) in-
dividuals (in center), and one pure dwarf individual (dd) at right. The bottom and
next to bottom rows show (at left) continued generations of crossing pure tall TT
with TT to consistently produce pure tall offspring. The middle of the rows shows
the consistent production of the 1:2:1 ratio when the mixed Td individuals are
crossed. The pure dwarf individuals dd at the right in these lower rows show suc-
ceeding generations of breeding together to produce all pure recessive dwarf (dd)
offspring.

GENETICS AND EUGENICS 287

peas, one-third of them (constituting one-fourth of the whole num-
ber) produced tall peas without deviation, but the other two-thirds
(half of the whole number) produced stock that repeated the produc-
tion of tall and dwarf peas in the same proportion as did the preced-
ing generation. Here, then, was a definite ratio of 1 :2 :1. Mendel
tested his findings with six other traits of the pea and with more than
a dozen other kinds of plants, and after verifying his results he was
able to announce the following law : When mernhers of a species hav-
ing contrasting characters are crossed, all the immediate 'Offspring
will show the trait that is dominant, hut if the members of this gen-
eration are bred to themselves, one-fourth of the offspring will show
the dominant trait and breed true for it; one-fourth will shoiu the
recessive trait and breed true for it; the remaining two- fourths of
the offspring will show the dominant trait, but will reproduce the
contrasting characters in the same proportion as did their immediate
parents.

Derivatives of Mendel's Law

From the fundamental law which Mendel discovered, several cor-
ollaries can be drawn, based on factors responsible for the distribu-
tional behavior of inheritable characters. These corollaries are :

Principle of Dominance. — The determiner for one member of a
pair of contrasting characters (which we shall call an allelomorph)
may take precedence over the other member when the two have been
brought together in a cross between unlike parents. Which trait is
dominant and which is recessive can be determined only after a cross
has been made, and the determination holds only for the species ob-
served. Thus, tallness may be dominant over dwarfness in one species
of plant and recessive in another. In many cases, dominance is not
complete, and in a few classic instances it is lacking altogether. These
cases will be discussed later in this chapter.

Independence of Unit Characters. — The determiner for any given
character acts as an independent hereditary unit as it is passed along
from generation to generation. In each generation it may have a
different set of associates, but its associations in one generation do not
affect the company it may keep in the next. Thus tallness in peas may
be associated with yellow pod and with wrinkled seed coat in one
generation, but it may form a combination with green pod and smooth
seed coat in the next. This important principle is responsible for the
great variation we find in plants and animals and makes it possible

238 ESSENTIALS OF ZOOLOGY

for a breeder to bring about any desired combinations of the traits
possessed by the species of plant or animal with Avhich he is working.
Thus the modern Shasta daisy has a combination of the three impor-
tant characters, size of flower, gloss of petal, and prolific growth,
each of which was procured from an original variety having one, but
not the other two, of these traits.

Principle of Segregation. — Since unit characters are independent,
it follows that they can go into various combinations and are free
to segregate out again. This was illustrated by dwarf ness in Mendel's
peas which went into a cross with tallness, but segregated out again
to form peas that were as consistent for dwarfness as was the orig-
inal dwarf stock. To illustrate both the principles of unit characters
and the related phenomenon of segregation we might use an analogy.
If a gallon of white marbles were mixed with a gallon of black
marbles, they would make a mass of marbles that, at a distance at
least, would look gray. But the mixture is not irrevocably gray as
would be the case if a gallon each of black and white paint had been
mixed. The white marbles and the black marbles can be segregated
out again if it is so desired and either variety can then be put into
combination with marbles of any other kind. Inheritable traits, like
marbles, can make various combinations which last for one generation
only, after which they can make new combinations with other traits.

The Physical Basis

With the simple facts in mind of how heredity works, it is desirable
to consider the germinal background of the laws governing it.

The tiny sperm cells of both plants and animals and the egg cells
which, deprived of their food-containing yolk, are almost equally
small, contain within them something that determines all the traits
which the individual resulting from their combination will develop.
In fact, the body of the sperm cell and the nucleus of the egg con-
tain little else than a material that carries the combined inheritable
traits of one generation over to the next. This important material is
called chromatin and during mitosis it becomes arranged in series of
hereditary units called genes. Each gene has a definite causal rela-
tionship to an inheritable trait. Both the sperm cell and the egg
cell contain a complete gene complex, and each, under favorable
conditions, could produce a complete individual.

When the two totipotential gametes are brought together at fertili-
zation, the resulting zygote contains two genes for each allelomorph.

GENETICS AND EUGENICS

239

When the two genes are for the same trait, the resulting individual
is said to be homozygous for that trait and, of course, shows it. When
each of the two genes is for a different member of the allelomorph,
in which case we use the term Jieterozygous, one gene takes precedence
over the other, but the recessive gene is in no way destroyed; it sim-
ply lies dormant and bides its time. How these genes are arranged
into a limited number of packages called chromosomes and how
chromosomes are distributed in the process of sperm and egg forma-
tion have been explained in a previous chapter.

Plotting Crosses

The Monohybrid Cross. — When the genetic constitution of paren-
tal stocks is known, it is possible to plot out the results of various
kinds of crosses. The checkerboard is the simplest device for such

T d

TT

Td

Td

dd

Fig. 74. — The outcome of a monohybrid cross between two heterozygous (Td)
individuals is according to the ratio 1:2:1, i.e., (TT), (Td), (Td). (dd).

T d

n

dY

Tg

dg

g

Fig. 75. — The checkerboard may be used to determine the possible gene combina-
tions in the ova and sperm of a possible dihybrid cross. A tall green-podded pea
(Tg) is crossed with a dwarf yellow-podded pea (dy).

computations. The genetic constitution of the various kinds of male
gametes is set down along the ordinate and that of the various kinds
of female gametes along the abscissa. These values are then copied
into the squares horizontally and vertically and their sums give the
values of the various kinds of gametes that will result from the cross.
In the Fg (second filial generation) cross of Mendel's experiment

240

ESSENTIALS OF ZOOLOGY

with tall and dwarf peas, half of the male gametes and half of the
female gametes contained a gene for tallness (T) and half in each
case contained a gene for dwarfness (d). The outcome, therefore,
is shown in Fig. 74.

The Polyhybrid Cross. — It is often desirable to know the outcome
of a cross in which two or more characters are considered together
as in the case of crossing a tall, green-podded pea (Tg) with a dwarf,
yellow-podded pea (dY) (Fig. 75). The determination of the pos-
sible kinds of gametes may be simplified by first making a small
checkerboard for them. Thus we find that there are four kinds of
male gametes and four kinds of female gametes which may be listed
as TY, Tg, dY, dg. Now we make a checkerboard of sixteen squares
and proceed as we did before.

Ty

Tg

dY

dg

TY

Tg
TY

dY
TY

dg
TY

TY
Tg

Tg
Tg

dY

Tg

dg
Tg

TY
dY

Tg
dY

dY
dY

dg
dY

T5f
dg

Tg
dg

dY
dg

dg
dg

TY

Tg

dY

dg

Fig. 76. — The outcome of a dihybrid cross between two heterozygous individ-
uals is according to the ratio of 9 :3 :3 :1. The four kinds of male and female gam-
etes, tall yellow (Ty), tall green (Tg), dwarf yellow (dy) and the recessive dwarf
green idg) are employed as they would combine in this ratio.

The computation of the dihybrid cross (Fig. 76) indicates that
nine-sixteenths of the progeny will show the two dominant traits.
Three-sixteenths will show the dominant trait of the first allelomorph
and the recessive trait of the other. Another three-sixteenths will show
the recessive trait of the first allelomorph and the dominant trait of
the second. One-sixteenth of the offspring will show both recessive
traits. It will be noticed, however, that the nine squares showing indi-
viduals that will be tall and yellow vary with each other in regard to
their entire content. Further examination of the squares will indicate

GENETICS AND EUGENICS

241

that there are nine different combinations of letters and that in only
one case as manj^ as four are exactly similar. Individuals whose genes
cause them to look alike are called phenotypes ; those whose genes are
exactly alike are called genoty,pes.

A trihybrid cross, such as one between a tall yellow-podded pea with
a wrinkled seed-coat (TYw) and a dwarf green-podded pea with a
smooth seed-coat (dgS), can be plotted by using a checkerboard of
sixty-four squares. There are eight different kinds of male and female
gametes found in this arrangement. The ratio of phenotypes of a tri-
hvbrid cross is 27 :9 :9 :9 :3 :3 :3 :1.

TT

TT

Td

Td

Td

Td

dd

dd

Fig-. 77A.

Fig. TIB.

Fig. nA. — A cross between heterozygous (Td) and homozygous tall peas (TT)
produces a 2:2 ratio. The resulting individuals are two pure tall (TT) and two
phenotypically tall or mixed (Td).

Fig-. 77B. — A cross between heterozygous tall (Td) and homozygous dwarf peas
(dd) also produces a 2:2 ratio. The resulting individuals here are two pure dw.arf
(dd) and two phenotypically tall or mixed (Td).

The Back Cross. — The checkerboard is also useful in computing
the results of a cross between a heterozygous and a homozygous in-
dividual as would be the case in which the offspring of a cross between
a homozygous dominant and a homozygous recessive individual is
bred back to either of the parental stocks, a procedure often followed
in practical animal and plant breeding. When such a cross is made
to the homozygous dominant stock the results, in terms of pea traits,
are TT, TT, Td, Td.

When the heterozygous stock is bred back to the homozygous reces-
sive parental stock a 2 :2 ratio is also produced, but between heterozy-
gous and homozygous recessive individuals as : Td, Td, dd, dd.

Complications of Mendelian Inheritance

If all inheritable characters followed Mendel's law in as simple a
fashion as do the pea traits that Mendel first investigated, the science
of genetics would be much more elementary. Unfortunately this is
not the case. Although Mendel's law is found so consistently as the

242 ESSENTIALS OF ZOOLOGY

underlying principle of heredity that it can be said to be practically
universal in its application, it is often modified and complicated in
various ways.

Multiple Allelomorphs. — Instead of the allelomorph consisting of
the usual two factors, a larger number of alternatives sometimes ap-
pear. Dominance in these cases occurs in a graded series, each mem-
ber, between the extremes, being dominant to the lower members and
recessive to the higher members of the series. A simple case of mul-
tiple allelomorphs is found in the inheritance of color in rats where
there are three factors ; namely, ordinary pigmentation, ruby-eyed
dilution, and albinism. In a cross betw^een heterozygous parents any
two of the three factors may be brought together in the offspring. If
ordinary pigmentation is present with either of the other two, it will
determine the color, since it is first in the series of dominance. If the
other two are the ones present, the color will be ruby-eyed dilution,
the second member of the series.

A large series is presented by eye color in the common fruit fly,
BrosopMla inelanog aster, in which case there are eleven members
which are, in the order of their dominance : red, apricot, coral, ivory,
ecru, buff, tinged, blood, cherry, eosin, and white. Each parent may
have any two of these in its germ plasm and transmit either of them
to the offspring.

Plural Genes.* — Several cases formerly interpreted to be simple
blending inheritance not conforming to Mendel's law have been ex-
plained by the discovery that there can be more than one allelo-
morphic pair concerned with the inheritance of the trait. Thus, in-
stead of there being one gene located at some definite place on a
chromosome, there are two or more genes variously located. Cases of
plural genes fall into two categories. In the one, each gene inherited
as a dominant, produces part of the result, and the effects are cumula-
tive. In the other type of case, the inheritance of one dominant gene
produces the entire effect.

A common example of the cumulative type is the inheritance of skin
color in man. It has long been known that a cross between a white
man and a negro produces offspring of a medium shade called mulatto,
and that a cross between two mulattoes will produce offspring with a
range of color varying from intense black to a shade that may allow

♦Other names used for this are multiple factors and duplicate grenes. I avoid the
first term because it is lil^ely to be confused by the beginning student with the term
multiple allelomorph and I object to the second because of the connotation of only
one repetition.

GENETICS AND EUGENICS

243

the person to pass for white. This is explained by the fact that there
are two allelomorphic pairs concerned with the inheritance of color,
the dominant of either of which will produce a certain amount of pig-
ment per square millimeter of skin surface. The negro of pure skin
inheritance is homozygous for both pairs. Using P and P' to repre-
sent the dominant factors of these two allelomorphs and p and p' the
recessives, the homozygous negro would have PPP'P' while the ho-
mozygous white person would have ppp'p' and the mulatto resulting
from the first cross would have PpP'p'. The cross between two mulat-
toes would be a dihybrid cross which could be plotted by a checker-
board similar to that of Fig. 78. The accompanying summary shows
that there caij be four, three, two, one, or no dominant factors present,
thus accounting for the series: ''negro," ''chocolate," "mulatto,"
"quadroon" and "pass for white."

ppt Ppt ppt ppt

ppt
??•

negro

PP'

ppt

choc.

pPt
ppt

choc.

PP*
ppt

mulat

ppt
choc.

Pp'
mulat

pP»

mulat

ppt

quadr

ppt

pP»
choc.

Pp»

PP'
mulat

pPt

pP'
mulat

pp»

pP»
quadr

ppt

PP*
Imulat

Pp'

pp»

Quadr

pP»

PP'
quadr

pp'

pp»
white

ppt

Pp

pp

PP*

Fig-. 78. — The inheritance of color by children of mulatto parents can be shown by

the checkerboard for a dihybrid cross.

The other type of case in which plural genes occur is exemplified by
a certain brown-seeded variety of oats in which there are two pairs
of genes concerned with the determination of color. Here the pres-
ence of the dominant gene of either pair produces the entire effect.
This case, too, can be plotted on the dihybrid checkerboard, but the
result is a 15 :1 ratio, for fifteen of the sixteen squares would show at
least one dominant gene. Crosses between a red and a white variety
of wheat in which there are three pairs of genes for color, produce
a 63 :1 ratio. This is a trihybrid cross in which each dominant gene
can produce the entire effect.

244 ESSENTIALS OF ZOOLOGY

Complementary Genes. — In a number of cases color is produced
by two genes, the dominants of which must react on each other to
produce the color effect. In the sweet pea, if the dominant factor of
the red allelomorph (Rr) is acted on by the dominant factor of the
color allelomorph (Cc), then red is produced. If either of these
dominant genes is present without the other, white flowers are pro-
duced, and if two white-flowered plants of the genetic constitution
Re and rC are crossed, a part of the progeny will be red. A student
familiar with elementary chemistry may understand this better if he
thinks of the red factor as being represented by a colorless solution of
phenolphthalein and the color factor as a colorless alkaline solution.
When both solutions come together, red is produced, but neither can
produce the color without the other.

Supplementary Genes. — But sometimes purple sweet peas appear
when two white varieties are crossed or when a red variety is crossed
with a white variety. Purple is produced by a third gene for blue
which intensifies red if it is already present, but has no effect unless
the RC genes are likewise inherited. Thus we find that its effect is to
supplement that of the two genes R and C which are complements to
each other in the production of red. The accompanying list shows
some of the surprising crosses that may be made with sweet peas. Of
course the combination of genes shown in the last column is not the
only one that will come about in each of the various cases, but it is
the one that emphasizes the point desired.

White + White = Purple RcB + rCb = RGB

Red + White = Purple RCb + RcB = RCB

Purple + Red = Red RCB + RCb == RCl)

Purple + White = Purple RCB + rcb = RCB

Purple + White = White RCB + rcb = rcB

Purple + White = Red RCB + rcb = RCb

Lack of Dominance. — In several classic cases, some of which are
among plants and others among animals, neither factor of an allelo-
morph is dominant over the other. In the Fj generation, these cases
seem to produce perfect examples of blending inlieritance, but the F2
generation exemplifies the 1 :2 :1 ratio so beautifully that these ex-
ceptional cases are often used to explain Mendel's law to beginning
students.

The Andalusian breed of chickens includes both black and white
individuals. When black fowl are crossed with white fowl, all the off-

GENETICS AND EUGENICS 245

spring are of a slate color technically known as ' ' blue. ' ' When a blue
chicken is crossed with another blue chicken, one-fourth of the progeny
is black, one-fourth is white and the remaining two-fourths are
blue. The blacks and the whites are homozygous for their respective
colors while the blues are heterozygous. The inheritance of color in
short-horn cattle, and of the color in the four-o'clock flower are also
examples of this. In still other cases in which there is not neutrality
of dominance, the dominant effect may not be complete. Very often
the heterozygous individuals can be picked out from the homozygous
dominant ones by casual inspection.

Inheritance of Sex

In certain lower animals and in the early embryos of mammals, both
male and female reproductive systems are present in each individual.

^ — ■

Fig. 79. — Diagram to show the combinations in an XY sex chromosome pattern.
Half of all possible fertilization combinations of sex chromosomes will be male
(Xy, at left) and half will be female iXX, at right).

Typically in higher forms, one of these systems is repressed, and we
speak of the individual as being either male or female. The deter-
mination of whether the male or the female system will develop is an
inherited trait. The chromosome that carries the gene of sex deter-
mination has come to be designated , as the X-chromosome. In typical
cases, the female has in each of her germ cells, two of these X-chromo-
somes and the male has only one. Therefore, when, in the process of
spermatogenesis, the chromosomes match with their homologous chro-
mosomes and separate again to give each of the resultant cells one
complete set of chromosomes, the X-chromosome cannot pair with an-
other like itself, and when the chromosomes are distributed in sets,
half the sets will lack an X-chromosome. The sperm cells that do not
have an X-chromosome will produce males while those that have such
a chromosome will produce females.

246 ESSENTIALS OF ZOOLOGY

In other cases that include man and Drosophila, the X-chromosome
will pair with a Y-chromosome which has been described as ' ' an empty
case " or a ' ' ghost chromosome. ' ' In cases in which a Y-chromosome is
present, we cannot say that the male has one less chromosome than
the female, but it practically amounts to that since the Y-chromosome
seems to contain not more than a few genes (Fig. 79).

Linkage

Since the number of inherited traits of an animal or plant is very
great while the number of chromosomes is quite small, it is apparent
that a single chromosome must contain many genes. This being so,
there is not absolutely free assortment of traits as they are passed
from parent to offspring, but rather the offspring must inherit his
traits in groups that are not, ordinarily, broken up. Therefore, when
two genes, A and B, are on the same chromosome of one of the par-
ents, if the offspring inherits gene A, he must inherit gene B. In
Drosophila we know that the gene which determines whether or not
the wing will be fringed occurs on the same chromosome as the one
that is responsible for the body being or not being black. Now if one
of the parents is fringed-winged and black-bodied, the offspring will
have to inherit both these traits from that parent if he receives either
one of them. This phenomenon is called linkage.

Sex Linkage

Since there are other genes than those concerned with sex on the
X-chromosome, it is to be expected that their inheritance will differ
somewhat in the two sexes. This is strikingly illustrated by certain
abnormalities of man in whom the abnormal condition is inherited as
a recessive ; for example, color blindness, blindness resulting from
atrophy of the optic nerves (Leber's atrophy), and hemophilia or
bleeder's disease, in which great loss of blood occurs even from slight
wounds because clotting will not take place. Since the abnormality is
a recessive trait produced by a gene on the X-chromosome, its effect
can be offset by a dominant for normal in the female, but not in the
male, because in the latter case there is not another X-chromosome
present. Therefore, the daughter of a color-blind father will not show
the defective trait but may pass it to her son.

Fig. 80 shows a cross between a color-blind father and a normal
mother. Two kinds of male gametes are produced, half of which carry
the father 's gene for color blindness on the X-chromosome, and half of

GENETICS AND EUGENICS

247

which, have no X-chromosome and therefore, have no such gene. All
female gametes have one X-chromosome bearing a gene for normality.
The various possible combinations of the two kinds of sperm with the
single type of egg will produce two types of offspring; namely, sons
who will be strictly normal in regard to the trait in question and
daughters who will inherit the gene, but who will not show the trait
because its effect will be overcome by the presence of a gene for nor-
mal. Therefore, the daughters will be heterozygous for the trait and
might be spoken of as carriers. Fig. 81 shows a cross between such a
daughter and a normal male.

Fig. 80. — In the cross between a color-blind male (at left) and a normal female
(at right) the results (N, cb), (N, cb), (N), (N) indicate that all daughters, being
heterozygous, will not show the defect though they might transmit it to their off-
spring, while the sons are entirely free from it. iV2V=normal female individual ;
cb = Color-blind individual ; and N, c&=normal function but carrier of the color-
blind gene.

Fig. 81. — Results (N, N), (N, cb), (N), (cb). In a cross between a normal
male (at left) and a female who is heterozygous for color blindness (at right),
half of the sons will be normal and half will be color-blind. Half the daughters will
be homozygous for normal and half will be heterozygous.

Inspection of this figure will show that such a cross will produce
four types of offspring; namely, normal daughters, ''carrier" daugh-
ters, normal sons, and color-blind sons. Thus, we see that half the
grandsons of a color-blind man will receive the trait through their
mothers, and that all daughters and half the granddaughters of the
color-blind male will be carriers of the trait, but will have normal

248

ESSENTIALS OF ZOOLOGY

color vision. Of course, from the mating of such a person with a color-
blind man, all the sons and half the daughters will be color blind, but
a young woman who is used to having her father buy orange-colored
neckties, thinking they are blue, is not likely to marry a man similarly

defective.

Crossing Over

Genetics might be defined as an exact science which has exceptions
to many of its rules. A previous paragraph describes the principle of
linkage which provided for the inheriting as groups of those traits
whose genes occur together on a chromosome. Certain gametogenetic
accidents bring about occasional exceptions to this principle. It will

A

C A

B

C A

D D

B D

B

Fig. 82. — Crossing- over occurs when chromosomes break apart after synapsis.
AB and CD are the original chromosomes while AD and CB are the chromosomes
resulting from the cross-over.

be remembered (Fig. 60) that at synapsis the chromosomes pair
and the homologous chromosomes become loosely attached to each
other. At times, the pairing chromosomes will even twist around each
other. When they come to separate in the maturation division, instead
of making a complete separation they sometimes break off at some
point So that a part of each will be joined to a reciprocal part of the
other, as is shown in Fig. 82. Thus, as a result of crossing over, in-
stead of traits A and B, and C and D being associated in the offspring
as they were in the respective parents, A will be associated with D,
and C with B.

Crossing Over, a Useful Tool. — The process of crossing over has
proved to be a valuable means of determining something of the nature

GENETICS AND EUGENICS 249

of genes and chromosomes and, especially in Drosophila, it has enabled
us to locate the positions of many of the genes on their respective
chromosomes. If two linked traits are separated every time crossing
over occurs, it is obvious that their genes lie at opposite ends of the
chromosome, but if they are separated only half the times, it is ap-
parent that half the length of the chromosome lies between them.
Therefore, by computing the percentages of such separations and
noting which traits are affected, it has been possible to make chromo-
some maps showing the positions of the various genes and the distances
separating them. The chromosome maps for Drosophila are quite
complete.

Mutations

When chromosomes separate from each other at synapsis, aside from
breaking so that a piece of one will adhere to the correlative piece of
the other, various additional ''chromosomal accidents'^ will occur
which change the organization of the chromosome. A piece of chro-
mosome, for example, might break off from the end of one chromosome
and adhere to the end of its homologue. Any disarrangement of the
genes of a chromosome results in structural or physiological changes
in the organism into which it goes. Such changes are inheritable and
are called mutations. Since there are places on the chromosomes where
these accidents occur more frequently than at other places, there is a
tendency for certain mutations to occur with fairly determinable
regularity, say once in every hundred thousand cases. Some mutations
are useful and are preserved to the species to such an extent that De
Vries believed they were the principal factor in bringing about evolu-
tion. Other mutations are a disadvantage to the species, and the or-
ganisms possessing them are eliminated in Nature's fierce struggle
for existence unless they are saved from that cruel fate by man's
interference.

Human Heredity

Man, very naturally, is interested most in the heredity of man. In
spite of this supreme interest, his knowledge of his own heredity is
much more limited than his knowledge of inheritance in any of a num-
ber of other organisms. There are two reasons for this : namely, he is
not free to experiment with his own kind; and, as would be expected,
the application of Mendel's law to inheritable traits in this, the most
complex of all living forms, is correspondingly complicated. Although
there are cases in which the 1 :2 :1 ratio occurs in as simple a form as

250

ESSENTIALS OF ZOOLOGY

in Mendel's peas, there are many more cases in which the Mendelian
principle is manifested as multiple allelomorphs, plural genes, modi-
fying factors, complementary factors, incomplete dominance, change-
able dominance, etc. There is a tendency among some to depreciate
our knowledge of human heredity on the ground that there is so much
that we do not know. It is scientific to admit the extent of our lack
of knowledge, but it is wise to give proper credit to our present store
of information and to take cognizance of the rapidity with which the
gaps in our knowledge are being filled in. One by one the complicated
problems of human heredity are being solved by patient investigators,
and by putting what we already know to use, we stimulate the acces-
sion of more data.

Some Cases of Human Heredity. — To give a complete summary of
our knowledge of human heredity would be beyond the scope of this
chapter. The accompanying table gives some of the data that have
been accumulated:

Data on Human Heredity

TRAIT

HOW INHERITED

DOMINANCE

General Physique and Skeleton

Stature

A composite

Many genes for shortness

character

are dominant

Body build

Plural genes, two

Factors for stoutness are

or more pairs

dominant

of factors

Polydactyly (extra digits)

Simple

Abnormality dominant

Brachydactyly (short digits and

Simple

Abnormality dominant

limbs)

Symphalangism (fused fingers and

toes)
Zygodactyly (webbed fingers and

toes)
Lobster claw (split hands and feet)

Simple

Abnormality dominant

Simple

Abnormality dominant

Simple

Abnormality dominant

Exostoses (outgrowths of long bones)

Simple

Abnormality dominant

Abnormal fragility of bones

Simple

Abnormality dominant

Amputation (entire absence of hands

Simple

Abnormality dominant

and feet)

Skin and Hair

Simple. Lethal

Ichthyosis (scaly skin)

when homozy-
gous

Abnormality dominant

Keratosis (thickened skin on palms

Sex-linked

Abnormality dominant

and soles, suggestive of hoofs)

Normal pigmentation

Plural genes

Cumulative

Piebald (spotted skin)

Simple

Piebald dominant

Premature grayness

Simple

Abnormality dominant

Blaze (a white forelock)

Simple

Blaze dominant

Epidermolysis (excessive formation

Simple

Abnormality dominant

of blisters)

GENETICS AND EUGENICS

251

Data on Human Heredity — Cont'd

TRAIT

Skin and Hair — Cont'd

Albinism (lack of pigment in skin,

eyes, and hair)
Beaded hair

Ovoid hair form (kinky or wavy)
Alopecia (baldness)

Hair color (brunette or blonde)

Eed hair

Eyes
Eye color

Hereditary cataract
Color blindness

Night blindness

Atrophy of optic nerve
Large, irregular pupils

Ear Structure and Hearing

Complete absence of external ears
Cup ears (small, deformed, inverted

pinnae)
Otosclerosis (progressive hardening

of ear drums)

Nervous System
Feeblemindedness
Amaurotic family idiocy
Huntington's chorea

Manic depressive insanity

«

Dementia praecox (schizophrenia)

Dipsomania

Diseases and Diatheses

Hemophilia

Allergy (predisposition to hay fever,
asthma, eczema, migraine, etc.)

Gower's muscular atrophy

Diabetes insipidus (excessive produc-
tion of urine)

HOW INHERITED

Simple

Simple

Probably simple
Sex-linked

Plural genes,
probably two
pair

Probably simple

Modifying fac-
tors and varia-
tion in domi-
nance

Simple

Sex- linked

Sex-linked

Sex-linked
Simple

Simple
Simple

Simple

Can be simple

Simple

Simple

Plural genes

Plural genes, 32
pair

Probably simple

Sex-linked
Simple

Simple
Simple

DOMINANCE

Recessive to all types of

pigmentation
Abnormality dominant
Partial dominance
Dominant in men
Recessive in women
Dark shades dominant

Red pigment dominant

Dark shades tend to be
dominant over blue

Abnormality dominant
Abnormality usually

recessive
Abnormality usually

recessive
Abnormality recessive
Abnormality, an irregu-
lar dominant

Abnormality dominant
Abnormality dominant

Abnormality dominant

Abnormality recessive
Abnormality recessive
Abnormality dominant

Abnormality chiefly dom-
inant

Abnormality seems to oc-
cur as an incomplete
recessive

Abnormality recessive

Abnormality recessive
Abnormality dominant

Abnormality recessive
Abnormality dominant

252

ESSENTIALS OF ZOOLOGY

Matings Among^ Defectives

It is obvious that persons who have inherited scaly skin, lobster
claws, amputated hands and feet, exostotic bones, or who might have
any of many other inheritable defects that incite pity or repulsion
Avill find difficulty in securing mates. When these abnormalities occur
as mutations, the afflicted persons are not likely to marry on their
own social level, but will probably mate with others who are of lower

Fig. 83. — Family from Brazil showing hereditary absence of hands and feet.
The man at the right rear is the uncle of the children shown. The father, who is
dead, had the same deformity. Of the twelve children born, six were normal and
six were deformed. (From Holmes, Human Genetics and Its Social Import,
McGraw-Hill Book Company, Inc.)

grade mentally or who have other abnormalities that make them ob-
jectionable to normal people. Thus there is a tendency for defective-
ness to be precipitated to a social group that can clearly be called
dysgenic. By her process of eliminating the unfit who could not sur-
vive the fierce struggle for existence. Nature formerly kept this group
at minimum size. Today its numbers are being added to, not only by
recruits from higher groups who have a poor heredity either by un-
fortunate segregation of undesirable genes or by the occurrence of

GENETICS AND EUGENICS 253

such mutations as have been mentioned, but also by increased repro-
duction by the members of the dysgenic group itself.

The Diflferential Birth Rate

Nature keeps her creatures fit by giving reproductive advantage to
the best members of each species. Various dioecious animal forms
produce from dozens to millions of young per pair from which, on an
average, two individuals are selected to replace the parents. As a rule,
the two selected are the ones that are strongest and the most free of
defects — these are usually the ones that are best adapted to their en-
vironment. Man has, in the case of his own kind, preserved the weak
and defective individuals that Nature would have eliminated had it
not been for the application of medical science, together with public
health and other measures that have come with the development of a
humanitarian consciousness. Nothing but praise should be given to an
altruism that saves lives and relieves suffering, but the effect on our
race of man's present practice of preserving individuals that Nature
would have destroyed, without safeguarding the reproductive advan-
tage of the fitter group, is worthy of consideration.

It has been shown by Lorimer and Osborn* that certain large groups
are increasing so rapidly while others are so diminishing, that the
surviving children of a million women of reproductive age of the first
category will be twice as numerous as the surviving children of a
similar group of women of the second classification. Carried on at the
same rate for three generations (which is only a long lifetime) the
descendants of the first groups will be sixteen times as numerous as
those of the second groups.

Casual observation will make evident that such grouping is likely
to be on a basis of eugenicity and dysgenicity. A number of studies
have been made of the reproductive rate of groups classified by voca-
tion. These studies reveal that passing from the professional and suc-
cessful business classes through the various occupations to that of the
unskilled, transient, agricultural laborer, the number of children per
family rises steadily.

Family Size in Eugenic Groups

The vocational group made up of college teachers might be taken as
an example of a profession whose members have a low reproductive

♦Lorimer, F., and Osljorn, F. ; Dynamics gf Population, Macmillan.

254 ESSENTIALS OF ZOOLOGY

rate. A recent study made by Kunkel* shows that 4,567 college teach-
ers have 5,932 living children, an average of 1.3 children per teacher.
Dividing the teachers surveyed into three groups according to age, he
found that in the oldest group there is an average of 1.6 children, the
middle group averages 1.42, and in the youngest group, which consists
of those less than forty-three years of age, there are 0.86 children
per teacher. Since the families of this last group are not complete, the
average of the other two groups, or about 1.5 children per family,
might be taken to indicate the reproductive rate of college professors.

A correspondingly low birthrate is found among other groups whose
members would be expected to possess traits that should be preserved
for our race. Cattell reports that the average number of children in
the families of the persons listed in American Men of Science is 1.88.
But small families are not limited to college professors and scientists,
for those distinguished people whose names are recorded in Who's
Who in America have families averaging only slightly more than two.

Since the families from which college students come can reasonably
be taken as a eugenic group, several studies have been made of the
sizes of the families represented on the campuses of various American
colleges and universities. The author kept a record for a ten-year
period of the sizes of the families represented by the students of a city
university of the Southwest. The average number of children in those
families was found to vary from year to year from slightly under to
slightly over three. Since there were no childless families represented,
these figures are high for the social stratum concerned. What effect
college education may have on familj' size may be inferred from other
studies. Harvard graduates whose year of graduation would give us
reason to suppose that their families are complete, have produced 1.9
children per married alumnus ; allowing for the members of the group
who did not marry, the average falls to 1.6 children. Corresponding
averages for Yale are 1.9 and 1.5, for Swarthmore 2.15 and 1.9, and
for Vassar 2.15 and 1.25.

A false sense of eugenic security might be prompted by the belief
that the figures exceeding two in the foregoing citations indicate that
the parents are being replaced and that any residual value represents
a gain. But in the cases of two-child families, what assurance have we
that those children will live to reproductive age, that they will marry,
and if they marry that they in turn will have children? Considering

♦Kunkel. B. W. : A Survev^ of College Faculties, Bulletin of the Association of
American Colleges 23: No. 4, Dec, 1937.

GENETICS AND EUGENICS

255

these possibilities, it is evident that fertile families must provide for
more than replacement if the group to which they belong is to be per-
petuated. Various computations have been made of the average num-
ber of children per fertile family necessary to maintain the numerical
strength of a group. These estimates range from 3.1 to 4. Consider-
ing that the current incidence of childless families in America is
20 per cent, probably the higher number is more nearly correct, and
it is not safe to place the figure at less than 3.5.

Family Size in Dysgenic Groups

Various studies have shown that larger families occur among
people who have but a poor store of those qualities of intelligence,
stability, and physical traits that go to make up racial excellence.

o

11

III

6

m

LEGSND

o

Uale without trait

Female without trail

IT

o o

o

VaIo and female with trail;

Sex unknown; ? inheritance unlSOVIL

Pig-. 84. — Standard pedigree chart. Charts such as this can be used in tracing
a trait through several generations of a family. This particular chart shows the
inheritance of a dominant trait starting with a cross between a male who was
heterozygous for the trait and a female in whom it was homozygously recessive.

256 ESSENTIALS OF ZOOLOGY

Lorimer and Osborn found in their study of the school children of
selected eastern cities that children with the lowest intelligence scores
(I.Q. below 60) came from families that averaged nearly six chil-
dren; that those with medium scores (I.Q.'s 90 to 110) came from
families averaging less than four; and that the children of superior
intelligence (I.Q.'s 140 and over) came from families that averaged
less than two and a half children.

The author's study of nearly a thousand improvident families of a
type well known to the social workers of southwestern cities revealed
that in this group the number of births occurring in completed fam-
ilies averaged 7.9 and the average number of children born to mothers
of all ages was 5.7. The average number of surviving children of the
two groups was 6.1 and 4.6 respectively. The significance of these
data in comparison with those concerned with the size of the families
from which college students come can be realized from the fact that
if the reproductive rates in these two groups continue for ten genera-
tions, the descendants of one hundred families of the dysgenic group
will number more than twenty-eight thousand, while in the same gen-
eration, one hundred families of the present-day college-student group
will be represented by eleven persons.

What Can Be Done?

It must be admitted that the present racial situation has been
brought about through the advance of humanitarianism and science.
The cure for the situation must come through the application of
greater humanitarian and scientific measures. The many corrective
panaceas that have been suggested must be tested carefully and ap-
plied cautiously. Some reforms can be applied at once and are being
applied today to some degree; others must wait until the time for
their application is more opportune.

Some Eugenic Measures

The step that would be most fruitful of racial betterment is general
education along eugenic lines. If a sufficient number of thoughtful
citizens were informed about racial trends, a eugenic consciousness
would be developed that would cause every proposed social or eco-
nomic change to be considered from the standpoint of its genetic
significance.

The marriage and divorce laws of the various states should be uni-
fied and rewritten along eugenic lines. They should provide for ad-

GENETICS AND EUGENICS 257

vance notification of applications for marriage licenses as is provided
for by the California laws, and should contain provision for health
examinations as already enforced in Illinois. The latter practice
should be extended to include the examination of family histories.

Many positive measures have been proposed for granting aid of
\^arious kinds to large families. At the present time most of these
proposals are impractical, but we might look forward to wage adjust-
ment to family size starting with government employees, and rental
rates of government-owned houses based on a fixed percentage of the
family income regardless of the size of the house required.

It has been proposed that those who are clearly unworthy of parent-
hood should be segregated in colonies of their own sex. The expense
of this as well as the probability of many social and other problems
that would arise in such a situation challenges the wisdom of such a
measure beyond the degree it is now being practiced in our eleemosy-
nary and punitive institutions.

Twenty-nine of our states have adopted laws providing for the
eugenic sterilization of such persons as those who have been committed
to sanitaria for mental cases because of an inheritable type of insanity
and who are to be returned to their families. A few states provide also
for such sterilization of habitual criminals and those who are clearly
feebleminded. Eugenic sterilization consists of vasectomy and sal-
pingectomy— operations that bring about sterility without interfering
with endocrine function or normal sexual reactions. Over twenty-
five thousand such sterilizations have been performed under the
present laws.

It is evident from the data previously discussed that family limita-
tion is being practiced by the eugenic group. It is suggested by many
who are facing squarely the problems of racial welfare that those who
are mentally and in other ways far below the majority of our people
should have made available to themHhe means of similarly limiting
the sizes of their families. The number of clinics where such measures
are made available to persons who will make proper use of them al-
ready number in the hundreds.

Some of these eugenic measures are questioned on the grounds that
they violate human rights. We should also be considerate of the rights
of the unborn. It is reasonable to say that every child that is to be
born into this world has :

The right to he horn with a sound mind.

258 ESSENTIALS OF ZOOLOGY

The right to he horn with a strong and normal hody.
The right to he horn into an environment in which his inherited po-
tentialities will have a fair chance to develop.

References

Baur, E., Fischer, E., and Lenz, F.: Human Heredity, New York, 1931, The
Macmillan Company.

Bowen, E.: An Hypothesis of Population Growth, New York, 1931, Columbia
University Press.

Castle, W. E.: Genetics and Eugenics, Cambridge, Mass., 1930, Harvard Univer-
sity Press.

Holmes, S. J.: Studies in Evolution and Genetics, New York, 1923, Harcourt
Brace & Company.

— : Human Genetics and Its Social Import, New York, 1933, Harcourt Brace &
Company.

Lindsey, A. W.: A Textbook of Genetics, New York, 1932, The Macmillan
Company.

Newman, H. H.: Evolution, Genetics, and Eugenics, Chicago, 1932, University
of Chicago Press.

Popenoe, P., and Johnson, R. S. : Applied Eugenics, New York, 1933, The Mac-
millan Company.

Shull, A. F.: Heredity, New York, 1931, McGraw-Hill Book Company.

Sinnott, E. W., and Dunn, L. C: Principles of Genetics, New York, 1932, Mc-
Graw-Hill Book Company.

Walter, H. E.: Genetics, New York, 1930, The Macmillan Company.

CHAPTER XIII

CLASSIFICATION OF ANIMALS

For practical reasons it is necessary to classify the animals of the
animal kingdom, listing and arranging them in a definite, orderly,
systematic fashion in order that they may be recognized and named.
Since there are at least 840,000 different species of animals already
described and more being added continually, this becomes signifi-
cant. In addition to the matter of convenience in cataloguing ani-
mals, our present system of classification, which is based primarily
on structures, brings to light many of the phylogenetic relations
between groups. The study of classification is called taxonomy, of
which some mention has already been made in the introductory
chapter.

The usual method of classification of animals originated with Lin-
naeus who lived from 1707-1778. He recognized groups of four dif-
ferent values : class, order, genus, and species. Since his time several
other divisions of the classification have been added, as the phylum,
subphylum, subclass, suborder, family, subfamily, subgenus, and sub-
species. The Linnaean system designates the species by two Latin or
Latinized names ; the first is the generic name and the second is the
specific name. For example, we have Rana catesheiana, the bullfrog,
and Rana pipiens, the leopard frog, as two species of frogs belong-
ing to the same genus. Because of the necessity for two names for
the designation of each species this is known as the binomial system
of nomenclature. In cases where subspecies or varieties have been
recognized, the genus, species, and subspecies names all are used,
making the arrangement trinomial instead of only binomial.

The basis for our present classification is largely relationship of
structure with groups formed because of genetic relation and descent
from common ancestry. Animals whose characteristics are similar
and whose relationship is evidently very close are placed in the same
species. Species which are quite similar are placed in the same
genus. Families are each composed of genera which resemble each
other in certain respects much more than they resemble other
genera. Orders are composed of related families, classes of related

259

260

ESSENTIALS OF ZOOLOGY

orders, and phyla of related classes. These groups are regarded as
having arisen from successively more remote ancestors. To the ex-
tent that evolution has proceeded in the different groups, this sys-
tem of classification carries a fairly accurate outline of the phylo-
genetic background of any modern animal. Forms possessing ho-
mologous structures (structures of similar form and similar em-
bryonic development) are described as of close relationship. Since
similarity in embryonic development is regarded as most important
evidence of homology, the theory of recapitulation (individual in its
life lives over the history of its race) enters strongly in determining
relationships of animals and animal groups.

Chordata
(Frog)

Annelida
{lumbhcus)

Bryo^oa
(Bugulaj

Rotifera

Platyhdminthei
{Tapeworm)

Arthropoda
(Crayfish)

MoUusca
(Snail)

Ecblnodermata
(Jtarfiih)

Nemathelmintbe5
(Ascans)

Per if era
(icypha)

Coelenteraba
(Hydra)

Ctenophora

(Beroe j
sea walnut

Protozoa
(ParameciurT))

ANIMAL KINGDOM

Fig. 85. — Phylum relations in the animal kingdom.

Rules of Nomenclature

In order that each kind of animal may have a valid and recognized
scientific name, and not several different names, and also that two
different animals will not have the same name, some definite rules
of nomenclature have been organized into a code. The code which
is most generally used is the hit er national Code of Zoological Nomen-
clature. It was adopted by the International Zoological Congress and
is administered through a Commission on Nomenclature.

CLASSIFICATION OF ANIMALS 261

Some of the important rules embodied in this code are given below :

1. The first name proposed for a genus or species prevails, provided

it was published along with ample description or definition and
the principles of binomial nomenclature have been applied. This
is known as the law of priority.

2. The author of a genus or species is the one who first publishes

the name in connection with the description, definition, or in-
dication of the organism referred to, and his name appears in
full or abbreviated as a part of the name. An example would
be Rana pipiens Schreber.

3. In citations the generic name of an animal is written with the

initial letter capitalized, while the initial letter of a species
name or sub..pecies name is not.

4. If a species is transferred to a genus other than its original one,

or if the name of a genus is changed, the author's name is in-
cluded in parentheses.

5. One species constitutes the type of the genus, it serves as being

typical for the genus. One genus constitutes the type for a
family or a subfamily. The type is indicated by the describer,
or, if not, it is done by some other author.

SUMMARY OF THE CLASSIFICATION OF ANIMALS^^^

Below is given a classification of the principal groups of animals
from phylum through orders.

Phylum I. Protozoa (Single-Celled Animals)

Class I. Mastigophora

Subclass A. Phytomastigina

Order 1. Chrysomonadia — Synura uvella

Order 2. Cryptomonadia — Chilomonas

Order 3. Dinoflagellata — Ceratium

Order 4. Euglenoidia — Euglena viridis

Order 5. Phytomonadia — Volvox glohator ^

Subclass B. Zoomastigina

Order 1. Pantastomatida — Mastigamoeba

Order 2. Protomonadina — Codosiga (clioanoflagellate)

Order 3. Polymastigina — Trichomonas

Order 4. Hypermastigina — Trichonympha campanula

Class II. Sarcodina (Khizopoda)

Order 1. Amoebina — Amoeba proteus
Order 2. Heliozoa — Actinophrys

•Modified with permission from Shull. Principles of Animal Biology, published
by McGraw-Hill Book Company.

Subclass

A

Order

1.

Order

2.

Order

3.

Subclass

B.

Order

1.

Order

2.

Subclass

C.

Order

1.

Order

2.

ass IV.

In

Order

1.

Order

2.

Order

3.

Order

4.

Order

5.

262 ESSENTIALS OF ZOOLOGY

Order 3. Foraminifera — Glohigerina
Order 4. Eadiolaria — Thalassicolla
Order 5. Mycetozoa — Mucilago

Class III. Sporozoa

Telosporidia
Coccidiomorpha — Eimeria stiedae
Gregarinida — Gregarina blattarum
Haemosporidia — Plasmodium vivax

Cnidosporidia
Myxosporidia — Myxobolus
Microsporidia — Nosema

Acnidosporidia

Sarcosporid ia — Sarcocystis Undemanni
Haplosporidia — Ichthyosporidium

fusoria

Holotrichida — Paramecium caudatum
Heterotrichida — Stentor coeruleus
Oligotricha — Halteria
Plypotricha — Stylonychia
Peritricha — Vorticella

Class V. Suctoria — Podophrya gracilis

Phylum II. Porifera (Sponges)

Class I. Calcispongiae (Calcarea)

Order 1. Homocoela — Leucosolenia
Order 2. Heterocoela — Scypha (Grantia)

Class II. Hyalospongiae (Hexactinellida) — Euplectella aspergillum

Class III. Demospongiae

Order 1. Tetraxonida — Geodia

Order 2. Monaxonida — Spongilla

Order 3. Ceratosa — Euspongia (bath sponge)

Order 4. Myxospongida — Ealisarca dujardini

Phylum III. Coelenterata (Jellyfishes)

Class I. Hydrozoa

Order 1. Anthomedusae — Hydra

Order 2. Leptomedusae — Obelia

Order 3. Trachomedusae — Trachynema

Order 4. Narcomedusae — Cunocantha

Order 5. Hydrocorallinae — Millepora

Order 6. Siphonophora — Physalia

Class II. Scyphozoa

Order 1. Stauromedusae — Tessera

Order 2. Peromedusae — Periphylla

Order 3. Cubomedusae — Chiropsalmus

Order 4. Discomedusae — Aurellia

Class III. Anthozoa

Subclass A. Alcyonaria

Order 1. Stolonifera — Cornulariella
Order 2. Alcyonacea — Alcyonium
Order 3. Gorgonacea — Corallium
Order 4. Pennatulacea — Eenilla

CLASSIFICATION OF ANIMALS 263

Subclass B. Zoantharia

Order 1. Edwardsiidea — Edwardsia

Order 2. Actinaria — Metridium

Order 3. Madreporaria — Astrangia

Order 4. Zoantliidea — Zoanthus

Order 5. Antipathidea — Antipathes

Order 6. Cerianthidea — Cerianthus

Phylum IV. Ctenophora (Sea Walnuts)

Class I. Tentaculata

Order 1. Cydippida — Fleurolrachia
Order 2. Lobata — Mnemiopsis
Order 3. Cestida — Cestus

Class II. Nuda — Beroe

Phylum V. Platyhelminthes (Flatworms)

Class I. Turbellarla

Order 1. Rhabdocoelida — Stenostomum
Order 2. Tricladida — Planaria
Order 3. Polycladida — Planocera

Class II. Trematoda

Order 1. Monogenea — Gyrodactylus
Order 2. Digenea — Fasciola
Order 3. As^idoaotylesi—Coty lapis

Class in. Cestoda

Order 1. Bothriocephaloidesi—Diphyllolothrium
Order 2. Tetraphyllidea — Crossohothrium
Order 3. Cyclopliyllidea — Taenia
Order 4. Trypanorhyncha — Ehyncolothrus

Class IV. Nemertinea — Cerehratulus

Phylum VI. Nemathelminthes (Roundworms)

Class I. Nematoda

Order 1. Ascaroidea — Ascaris

Order 2. Strongyloidea — Necator

Order 3. Filaroidea — Filaria

Order 4. Dioctophymoidea — Dioctophyme

Order 5. Tricliinelloidea — Trichinella
Class II. Gordiacea (Nematomorplia)— G^or^Zms
Class III. Acanthocephala — Echinorhynchus

Phylum VII. Annelida (Segmented Worms)

Class I. AvchiSinne\id2i.—Polygordius
Class II. Chaetopoda

Order 1. Polychaeta — Nereis

Order 2. Oligochaeta — Lumlricus

Class III. Hirudinea — Hirudo

Class IV. Gephyrea (Frequently classified as independent groups)

Order 1. Echiuroidea — Echiurus

Order 2. Sipunculoidea — Sipunculus

264 ESSENTIALS OF ZOOLOGY

Phylum VIII. Echinodermata (Starfish, Sea Urchin, Etc.)

Class I. Asteroidea

Order 1. Phanerozonia — Astropecten
Order 2. Spinulosa — Solaster
Order 3. Forcipulata — Asterias

Class n. Ophiuroidea

Order 1. Ophiurae — Ophioderma
Order 2. Euryalae — Gorgonocephalus

Class III. Echinoidea

Order 1. Cidaroida — Eucidaris

Order 2. Centrechinoida — Arhacia

Order 3. Clypeastroida — Clypeaster

Order 4. Spatangoida — Moira

Class IV. Holothuroidea

Order 1. Actinopoda — Thyone

Order 2. Paractinopoda — Leptosynapta

Class V. Crinoidea — Antedon

Phylum IX. MoUusca (Clams, Snails, Etc.)

Class I. Amphineura

Order 1. Polyplacophora — Chiton
Order 2. Aplacophora — Neomenia

Class II. Pelecypoda

Order 1. Protobranchiata — Nucula

Order 2. Filibranchiata — Mytilus

Order 3. Eulamellibranchiata — Lampsilis

Order 4. Pseudolamellibranchiata — Ostrea

Class III. Gastropoda

Order 1. Prosobranchiata — Helicinia
Order 2. Opisthobranchiata — Aplysia
Order 3. Pulmonata — Helix

Class IV. Scaphopoda — Dentalium

Class V. Cephalopoda

Order 1, Tetrabranchiata — Nautilus
Order 2. Dibranchiata — Loligo

Phylum X. Arthropoda (Insects, Crustacea, Etc.)

Class I. Crustacea

Subclass A. Branchiopoda

Order 1. Phyllopoda — Branchipus

Order 2. Cladocera — Daphnia
Subclass B. Ostracoda — Cypris
Subclass C. Copepoda — Cyclops
Subclass D. Cirripedia — Balanus
Subclass E. Malacostraca

Order 1. Nebaliacea — Nehalia

Order 2. Anaspidacea — Anaspides

Order 3. Mysidacea — My sis

Order 4. Cumacea — Diastyli,$

CLASSIFICATION OF ANIMALS

26f5

Order 5. Tanaidacea — Tanaia
Order 6. Isopoda — Oniscus
Order 7. Amphipoda — Gammarus
Order 8. Euphausiacea — Euphausia
Order 9. Decapoda — Cambarus
Order 10. Stomatopoda — Squilla

Class II. Onychophora — Peripatus

Class III. Myriapoda

Order 1. Pauropoda — Eurypauropus
Order 2. Diplopoda — Julus
Order 3. Chilopoda — Lithohius
Order 4. Symphyla — Scutigera

Class IV. Insecta

Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order
Order

Class V.

Order
Order
Order
Order
Order
Order
Order
Order
Order
Order

1. Thysanura — Lepisma

2. Collembola — Sminthurus

3. Ephemerida — Ephemera

4. Odonata — Lihellula

5. Plecoptera — Pteronarcella

6. Embiidina — Emhia

7. Orthoptera — Melanoplus

8. Isoptera — Termes

9. Dermaptera — Anisoldhris

10. Coleoptera — Lachnosierna

11. Strepsiptera — Xenos

12. Thysanoptera — Thrips

13. Corrodentia — Atropos

14. Mallopliaga — Menopon

15. Anoplura — Pediculus

16. Hemiptera — Eeduvius

17. Homoptera — Cicada

18. Neuroptera — Corydalis

19. Trichoptera — Enoicyla

20. Lepidoptera — Pieris

21. ^Mecoptera — Panorpa

22. Diptera — Musca

23. Siphonaptera — Pulex

24. Hymenoptera — Apis

Araclmida

1. Scorpionidea — Hadrurus

2. Pedipalpi — Admetus

3. Phalangidea — Phalangium

4. Palpigradi — Koenenia

5. Araneida — Aranea

6. Acarina — Sarcoptes'^

7. Solifugae — Bhagodes

8. Chernetidia — Ohisium

9. Xiphosura — Limulus

10. Eurypterida (Extinct) — Eurypterus

Groups of Nonchordates Whose Systematic Position Is Uncertain

Group 1. Nemertina — Cerehratulus

Group 2. Cliaetognatha — Sagitta

Group 3. Kotifera — Philodina

Group 4. Bryozoa — Bugula

Group 5. Plioronidea — Phoronis

Group 6. Brachiopoda — Magellania

266 ESSENTIALS OF ZOOLOGY

Phylum XI. Chordata (Man, Etc.)

Suljphylum I. Enteropneusta (Hemicliorda)

Order 1. Balanoglossida — Dolichoglossus
Order 2. Cephalodiscida — (None in America)

Sutophylum II. Tunicata (Urochorda)

Order 1. Ascidiacea — Molgula
Order 2. Thaliacea — Salpa
Order 3. Larvacea — Oikopleura

Subphylum III. Cephalocliorda
Subphylum IV. Vertebrata

Class I. Cyclostomata

Subclass A. Myxinoidea — Myxine
Subclass B. Petromyzontia — Entosphenus

Class II. Elasmobranchii

Subclass A. Selachii

Order 1. Euselacliii — Car char odon

Order 2. Cyclospondyli — Squalus

Order 3. Batoidea (Kaji) — Kaja
Subclass B. Holocepliali — Chimaera

Class III. Pisces

Subclass A. Teleostomi

Order 1. Crossopterygii — Polypterus

Order 2. Chondrostei — Polydon

Order 3. Holostei — Lepidosteus

Order 4. Teleostei — Ameiurus
Subclass B. Dipnoi — Protopterus

Class IV. Amphibia

Order 1. Caudata (Urodela) — Necturus
Order 2. Salientia (Anura) — Bana
Order 3. Apoda — Ichthyophis

Class V. Reptilia

Order 1. Testudinata — Pseudemys

Order 2. Rhynchocephalia — Sphenodon

Order 3. Crocodilia — Alligator

Order 4. Squamata — Phrynosoma

Class VI. Aves

Subclass A. Archaeornitlies (Extinct) — Archaeornis

Subclass B. Neornithes
Order 1. Hesperornithiformes (Extinct) — Hesperornis
Order 2. Ichtliyornithiformes — Ichthyornis
Order 3. Strutliioniformes — Struthio
Order 4. Rheiformes — Bhea
Order 5. Casuariiformes — Casuarius
Order 6. Crypturiformes — Tinamus
Order 7. Dinornithiformes (Extinct) — Palapteryx
Order 8. Aepyornithiformes (Extinct) — Aepyornis
Order 9. Apterygiformes — Apteryx
Order 10. Sphenisciformes — Spheniscus
Order 11. Colymbiformes — Gavia
Order 12. Procellariiformes — Diomedea
Order 13. Ciconiiformes — Botaurus
Order 14. Anseriformes — Anas

CLASSIFICATION OF ANIMALS

267

Order 15. Falconiformes — Cathartes

Order 16. Galliformes — Gallus

Order 17. Gruiformes — Fulica

Order 18. Charadriiformes — Oxyechus

Order 19. Cuculiformes — Geococcyx

Order 20. Coraciiformes — Ceryle

Order 21. Passeriformes — Passer

Class VII. Mammalia

Subclass A. Prototheria
Order 1. Monotremata-

-0 rnithorhynchus

Metatheria
Marsupialia — DidelpJiis

Eutheria

Insectivora — Scalopus
Dermoptera — Galeopithecus
Cliiroptera — Eptesicus
Carnivora — Felis
Pinnipedia — Callorhinus
Primates — Homo
Edentata — Dasypus
Rodentia — Battus
Lagomorpha — Lepus

Artiodactyla — Sus

Perissodactyla — Equus

Proboscidea — Elephas

Hyracoidea — By rax

Pholidota — Manis

Tubulidentata — Orycteropus

Sirenia — Ealicore

Odontoceti — Phocaena

Mystacoceti — Balaena

References

Comstock, J. H.: The Spider Book, Garden City, N. Y., 1912, Doubleday Page

Company.
Comstock, J. H., and Comstock, A. B. : A Manual for the Study of Insects, Ithaca,

N. Y., 1930, Comstock Publishing Company.
Dickerson, Mary C. : The Frog Book, Garden City, N. Y., 1906, Doubleday Page

Company.
Ditmars, R. L. : The Reptile Book, Garden City, N. Y., 1908, Doubleday, Doran

& Company, Inc.
Jordan, D. S.: A Manual of the Vertebrates, New York, 1929, World Book

Company.
Pratt, H. S.: "A Manual of the Common Land and Fresh-Water Vertebrates of

the United States, Philadelphia, 1937, P. Blakiston's Son & Company.
— : A Manual of Common Invertebrates, Chicago, 1937, A. C. McClurg & Co.

Subclass B.

Order

1.

Subclass

C.

Order

1.

Order

2.

Order

3.

Order

4.

Order

5.

Order

6.

Order

7.

Order

8.

Order

9.

Order

10.

Order

11.

Order

12.

Order

13.

Order

14.

Order

15.

Order

16.

Order

17.

Order

18.

CHAPTER XIV

PROTOZOA 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 universally placed first
when animal groups are placed in the order of complexity, begin-
ning with the simplest. It has been supposed, and with reasons to
support the supposition, that modern Protozoa have descended, with-
out changing their single-celled candition, 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 exception
of one class the Protozoa have characteristic locomotor organs.

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.

1. Class Mastig-ophora (mas ti gof 6 ra), Avhich 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

268

PROTOZOA IN GENERAL

269

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. The class also has a close relationship
with plants in that many of its representatives possess chlorophyll.
These forms are frequently classified as plants by 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,

Cercomonas

Monoaiga

Chilomoms

Codorjosi'ga.

i^^^

Phaous

Trachelmonas

Peranema Maatigamoeba

jPig. 86. — Group of representative Mastigophora. (Reprinted by permission from
Curtis and Guthrie, Textbook of General Zoology, John Wiley and Sons, Inc.)
(Figure of Chilomonas modified.)

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 body of
other animals, as in the intestine or the blood stream.

A large number of Mastigophora live in quiet streams, ponds, lakes,
and in the ocean. Euglena is a very commonly 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.

270

ESSENTIALS OF ZOOLOGY

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 out 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, Trichomonas, Chilomastix, Retortamonas and En-
teronomas are all genera with representatives occurring in the diges-
tive-tract of man.

2. Class Sarcodina (sar k6 di' na, fleshy) or Rhizopoda (ri zop' 6 da,
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 animal is able to accomplish locomotion by extending the proto-
plasm into these pseudopodia. The representatives of this class

Dijflugia

Arcella

Actinophrys

Fig. g7. — Group of typical Sarcodina. (From Curtis and Guthrie, Textbook of Gen-
eral Zoology, John Wiley and Sons, Inc.)

include many free-living forms as well as numerous parasitic ones.
A number of the representatives of class Sarcodina secrete an ex-
ternal shell of lime, silicon, chitin, cellulose, or some bind in sand or
other solid substances with one of the secretions. The class is com-
monly divided into five orders, (a) Amoehina 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. Endamoeha histolytica is
the most common parasitic form. Arcella, which secretes its shell,
and Diffliigia, which constructs its shell of sand cemented together by
a secretion, are two of the most commonly observed shell-bearing
forms, (b) Foraminifera is an order of shelled forms whose pseudo-
podia are very slender and reticular. The pseudopodia are extended
through small pores in the shell. Only a very few of this group
live in fresh water. The vast majority are marine, and Glohigerina

PROTOZOA IN GENERAL

271

is a typical example. The disintegrating calcareous shells of this or-
ganism 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 characterized as being able to produce enormous
Plasmodia containing hundreds of nuclei and contractile vacuoles, as
well as having ability to reproduce by spore formation. They live
quite commonly in masses of decaying vegetable material upon which

Fig. 88. — Shells of different Foraminifera. A, Rhabdamina abyssorum (X4.5) ;
B, Nodosaria hispida (Xl8); C, Globigerina buUoides (X55). (From Borradaille
and Potts, The Invertebrata, The Macmillan Company.)

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) Radiolaria is a
marine group with fine, raylike pseudopodia and a shell composed
largely of silica. The pseudopodia extend through the relatively large
apertures in the shell.

272

ESSENTIALS OF ZOOLOGY

3. Class Infusoria (in fii so' ri ^, 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 Yorticella are
the commonly studied Infusorians. The class may be divided into two
subclasses, Ciliata and Suctoria. The first, Ciliata, is composed of

Coleps

Bidimum

STEHTOK

VOI^TICCLLA.

Fig. 89. — Group of typical Infusoria. (Courtesy of General Biological Supply

House. )

0

four orders (or five by some), (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 common genus living in fresh water. Didinium, Frontonia,
Chilodon, and Coleps are other common forms. Opalina is a well-
known parasitic genus which inhabits the large intestine of the frog,
(b) Heterotrichida possess a well-developed undulating membrane
in the cytopharynx. The body cilia are small or partially absent,

PROTOZOA IN GENERAL

273

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 Balantidium is a parasite in
the intestine of man and some other mammals. This order is often
divided into two orders, forming a second by the name of Oligo-
trichida, of which Halteria is typical, (c) Hypotrichida possess cirri
or structures formed by fusion of cilia ; these are found principally on
the ventral side. The cell is flattened dorsovent rally and most of the
genera use creeping as their means of locomotion. Stylonychia,

Prorodoti

Fronton I a

I

'5

:i

BiplGtes

Stylonyohk

I- -f / ,'.•/•

lacrymaiia

Fig. 90. — Representatives from class Infusoria. (Reprinted by permission from
Curtis and Guthrie, Textbook of General Zoology, John Wiley and Sons, Inc.)
(Figure of Frontonia modified.)

Oxytricha and Euplotes are common fresh-water genera. Kerona is
parasitic and is often found creeping over the external surface of
fresh-water Hydra, (d) Peritrichida is an order composed of seden-
tary 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. Vorticella is probably the most
common living genus. Epistylis and Carchesium are well-known

274 ESSENTIALS OF ZOOLOGY

colonial genera. Vorticella and Carchesium have contractile stalks
while Epistylis is attached by noncontractile branching stalks.

The second subclass, Suctoria or Tentacnlifera, 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. This subclass is frequently
given the standing of a class, giving the phylum five classes.

4. Class Sporozoa (spo rO zo' k, 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
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 another.
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-
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, commonly called
gregarines, inhabit the cells (cystozoic) of earthworms, cockroaches,
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, arthropods,
and vertebrates, including man. The life history involves a period
of asexual reproduction (schizogony) and a period of so-called sex-
ual reproduction which ends in spore formation (sporogony). (c)
Haemosporidia. The representatives of this order live chiefly in the

PROTOZOA IN GENERAL

275

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 ar^ 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.

Ep'itheUam
Early sta^e

^.HOOr-C

.Intermediate sta^c

haturc Sta^e

Fig. 91. — Gregarina attached to an epithelial cell of a host's intestine. Other stages
of its development are shown within adjacent cells.

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 corpuscle, (b) Haplosporidia are single cells,
each with a single nucleus, and they have a relatively simple struc-
ture. Individuals of this order parasitize fishes and certain insects,
notably the cockroach.

Plasmodium, the malaria parasite, is one of the H aemosporidia,
and its life cycle will be given to illustrate the intricate life history

276 ESSENTIALS OF ZOOLOGY

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- tw^o 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. 176) 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 the 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
this development to go on in the mosquito. These little parasitic
sporozoites make their way to the salivary gland of the mosquito

PROTOZOA IN GENERAL

277

where they may remain for weeks. When this 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
of these forms only two cells adhere, but in others the cells may

Codoposi^a

fandonna

Carchejium

Fig. 92. — Different types of colonial Protozoa. Eudorina, a simple colony ;
Pandorina, a colony within a gelatinous capsule ; Ceratium, a linear colony ;
Carchesmm, a stalked infusorian colony ; Codonosiga, a stalked flagellate colony.
(Drawn by Joanne Moore.)

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 ceUs
remain. 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-
ranged in a line form a linear one, and colonies of irregular arrange-

278 ESSENTIALS OF ZOOLOGY

ment are spoken of as gregaloid. The 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
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 metazoa, the cells are specialized and distributed, so
that certain groups carry out a definite portion of the entire metab-
olism. They are classified into general body (somatic) cells and
reproductive (germ) cells. Certain of the spheroid colonies, such
as Volvox, have a rather striking resemblance to the blastula stage
in the early development of metazoans. Both are spherical organi-
zations 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 currents,
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

PROTOZOA IN GENERAL 279

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
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 oi this origin
are more prevalent in the tropical and subtropical regions of the
earth. Such diseases may attain sufficient 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 infection, if
allowed to continue, will be carried to the liver where serious ab-
scesses 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, Endamoeha histolytica (see Fig. 174),
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 be-
ings, but, so far as known, they are not pathogenic. Endamoeha coli,
Endolimax nana, and Endamoeha 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 Glohigerina 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.

280 ESSENTIALS OF ZOOLOGY

Kadiolaria 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 dis-
ease 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,
and the disease is limited to that area in Africa where this fly is
found. The organisms (Fig. 181) live free in the blood and collect
in the lymph glands, spleen, liver, and other organs. In final phases
it collects and attacks the brain.' The infection will bring about
loss of appetite, severe emaciation, extended coma, which ends in
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.

Chagas' 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 symptoms 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 flrst 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 century 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 Manson independently sug-
gested that the organism might be transmitted by some blood-suck-
ing insect. After several years more of investigation, Major Ronald

PROTOZOA IN GENERAL 281

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

Reference

Calkins, Gary N. : Biology of Protozoa, Philadelphia, 1926, Lea and Febiger.

CHAPTER XV

REPRESENTATIVE PROTOZOA— EUGLENA,
AMOEBx\ AND PARAMECIUM

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 of the Euglenae possess
chloroplastids which give them the possibility of photosynthesis. They
are usually found living in the surface waters of ponds, sluggish
creeks, and lakes. Euglenae are sometimes classified as plants by
botanists because of the presence of chlorophyll. Euglena is a form
which illustrates certain plant characteristics and animal character-
istics 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 ex-
tension of the cytoplasm. The superficial layer of the cell or ectosarc
(ectoplasm) is covered by an extremely thin portion, the cuticle.
Most of the eugienoid forms have spiral markings (striations) on
the surface of the body. The mouth of the cell is near the anterior
end, and extending inward from it is the gullet or cytopharynx.
Beside the cytopharynx is the reservoir or large vesicle. Just anterior
to this is the stigma, which is red in many individuals of E. viridis.
Bodies of collected protein material may be seen in connection with
most of the chloroplasts which are distributed through the cytoplasm.
These bodies are called pyrenoid bodies. Within the inner portion of
the cell or endosarc (endoplasm) is located the nucleus. It is usually
obliterated from view by the abundant chloroplasts. Small contractile
vacuoles empty from the endoplasm into the reservoir.

282

EUGLENA, AMOEBA, AND PARAMECIUM

283

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
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

CONTRACTILE VACS:
RESERVOIR

CHROMATOPHORES _jJw»T-ai f- ^^.- „^

CUTICLE- ^^S-d^^i%l^

MUCLEU3

Fig. 93. — Euglena viridis, a chlorophyll-bearing flagellate. (From Parker anil
Clarke, Introduction to Animal Biology, The C. V. Mosby Company.)

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.
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

284 ESSENTIALS OF ZOOLOGY

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.

Respiration and Excretion

Respiration is carried on through the general surface of the cell
membrane. There may be some utilization of the carbon dioxide pro-
duced in the metabolic activity by the process of photosynthesis in
forms where it exists. Likewise, some of the excess oxygen produced by
photosynthesis may be used in metabolism. Water and waste products
collect in the several small contractile vacuoles which empty into the
reservoir, a permanent 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.

EUGLENA, AMOEBA, AND PARAMECIUM 285

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 Flagellar 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 advan-
tage of this movement. This activity is known as Euglenoid move-
ment. 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.

AMOEBA, OF CLASS SARCODINA

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 as 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,

286

ESSENTIALS OF ZOOLOGY

some simpler and some more complex than Amoeba 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. Amoeba 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 tanning pits, in streams
where the water runs over rocky ledges, and wherever there is abun-
dant aquatic vegetation. It is often found on the surface of sub-
merged lily pads. A mass of pond weed may be brought into the

rooo

VACUOL.E
CONTRACTIL-E
VACUOL- E

NUCL.EUS

ENOOPL,ASM

^ PSEUDO PODIUM

ECTOPL-ASM

Fig. 94. — The structure and appearance of living Amoeba proteus.

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-moving,
fine particles within. When it is active, the outline is constantly
changing.

Structure

Amoeba proteus is one of the largest of the fresh-water forms.
Its average diameter is about %oo inch (0.25 mm.), while its ex-
treme diameter is %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.

EUGLENA, AMOEBA, AND PARAMECIUM 287

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 cyiosome, cell
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 ; mitochon-
dria; fat globules; and crystals. Some authors distinguish two types
of protoplasm in the endosarc; the inner more fluid, plasmasol, in
which the streaming movements take place and, surrounding this a
more viscous, passive portion, the plasmogel. 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 pro-
toplasm and its concurrent oxidation (catabolism). It includes all
activities necessary for maintenance of itself and its race. These
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 conflnes 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.

Ingestion. — ^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.

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

288

ESSENTIALS OF ZOOLOGY

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
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

liio'^

Eosjfion

'etion

1. Excretion

Fig. 95. — Diagram showing the phases of the metabolic process as it occurs in
amoeba. (Redrawn by permission from Wolcott, Animal Biology, McGraw-Hill
Book Company, Inc.)

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

EUGLENA, AMOEBA, AND PARAMECIUM ' 289

of the decaying vegetation present. Amoebae may live several hours
in water from which the oxygen is removed before asphyxiation
occurs. The contractile vacuole likely assists in discharging COg.

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-
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 accomplished
by addition to the protoplasm. If food is plentiful, more material is
added to the protoplasm than is used up in the oxidation which pro-
duces 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 many-celled animals to be studied later
includes a series of changes from q^^, through embryo state, to
adult. In Amoeba the cycle is likely only partly known, because it
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?

290

ESSENTIALS OF ZOOLOGY

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
its diameter; while its surface increases only according to the
square of its diameter. In other words, the amount of material in

Nucleus

Contractile
iracaole

Fig. 96. — Binary fission in amoeba. A, Beginning of the process; B, nearing the
completion of two new cells. (Drawn by Joanne Moore.)

New cells huclear fragment

Fig. 97. — An amoeba encysted and undergoing the process of sporulation. (Drawn

by Joanne Moore.)

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

EUGLENA, AMOEBA, AND PARAMECIUM 291

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 the Protozoa,
states that Amoeba starts out as a tiny pseudopodiospore Avhich 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.

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
XIV. 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

292 ESSENTIALS OF ZOOLOGY

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 an 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
amoeboid movement. It is so named from the perfect exemplification
of such activity by Amoeba. Locomotion is accomplished by the

~" "^ Particle - ""

Pseudopodtum

Fig. 98. — Successive positions in the movements of an amoeba viewed from the
side. Notice the formation of new pseudopodia and the engulfing of the particle on
the surface. (Modified from photographs by Bellinger, 1906, Journal of Experi-
mental Zoology.)

pseudopodia, and the process of their formation in most Amoebae.
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 endoplasm (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-

EUGLENA, AMOEBA, AND PARAMECIUM 298

per side of the body ; it continues to move forward to the tip of the
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.

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 or cabbage
leaves and allowed to stand about ten days. These animals occur abun-
dantly in any water which contains^considerable decaying organic mat-
ter. 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

294

ESSENTIALS OF ZOOLOGY

the heel part ; while the posterior portion, which is generally broader
but pointed, represents the sole portion.

At one side is a depression, the oj^al 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

CIUI A

CON T R ACTlt_e
VACUOLE

ORAU GROOVe
MACRO—

nucl.e:us

MICRO-

Nuci_eus

MOUTH
SUL.t_ET
FOOD VACUOl-E
ANAL. PORE

CO NT R AC T I 1_'e
VACUOLE

TRICHOCYSTS
PELLlCue

Fig. 99. — Paramecium, much enlarged to show structure.

Evans. )

(Drawn by Titus C.

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
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

EUGLENA, AMOEBA, AND PARAMECIUM 295

the gullet are fused together into a sheet, forming the undulating
membrane.

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 mth a semifluid substance and each
opens to the outside through the pellicle. The endosarc, composed
of 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 cyclosis. 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 outline of 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).

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
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.

296 ESSENTIALS OF ZOOLOGY

Digestion, 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-
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 (as A and B in Fig. 100). 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

EUGLENA, AMOEBA, AND PARAMECIUM

297

(U »y4— MICRONOCLEOS

MEGANUCLEUS

Cy -FUSION NUCLEUS

Fission

FUSION NUCLEUS

FISSION

"C"cONJUGANT CARRIES ON

MMMAL OmmiN* OF C

Fig. 100. — A series of diagrams to show the progressive changes involved in con-
jugation in Paramecium caudatiim. The process begins as shown in the upper lett
of the illustration and follows down the column and then to the top of the next
column. (From Parker and Clarke, Introduction to Animal Biology, The C v.
Mosby Company.)

298 ESSENTIALS OF ZOOLOGY

nuclei of each undergo changes. The micronucleus enlarges and
divides, forming two micronuclei, while the macronucleus undergoes
disintegration and final disappearance. Each of these two new micro-
nuclei again divides to form four, three of which disintegrate, but
the fourth divides again, forming one large and one small micro-
nucleus (as in those at the top of the 2nd column in Fig. 100). Some-
times 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 ani-
mal 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 (C
of the illustration) ; these divide, forming four nuclei in each ani-
mal, and these four divide to form eight. The descriptions of the
subsequent events vary somewhat. At least it is known that four
of the eight nuclei enlarge and become macronuclei; three of the
others degenerate, and one remains as a micronucleus. This micro-
nucleus divides, and almost immediately the entire animal divides by
binary fission with two macronuclei and one micronucleus going to
each cell. These daughter cells then divide to produce a total of four
Paramecia which have the typical number of one micronucleus and
one macronucleus of the active phase. Following this comes the
long series of generations formed, one after the other, by transverse
binary fission.

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

EUGLENA, AMOEBA, AND PARAMECIUM

299

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.

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 renew the vitality
of the individuals. Very recently, not only sexual reproduction but
also distinct sexes have been described for Paramecium.* These
results are all possibilities.

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

' ' . ' • ■.♦*•.

. . .. '•

A.

* : ' ' ^ . :

-• L.

B.

Fig. 101.— 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. (From Jennings, be-
havior of the Lower Organisms, The Columbia University Press.)

material, contact and other minor reactions. Its reactions to stimuli
are somewhat similar to those described for Amoeba; however, it

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

300

ESSENTIALS OF ZOOLOGY

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
to sodium chloride, positive to weak acetic acid, and positive to th«
negative pole of a weak, galvanic electric current. The optimum
temperature for Paramecium ranges between 24° and 28° C. (71°
F.). 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

■

a

.. -»•« ^-X-- ,_-H/ X*- »«/,/

'.'»%»"• '•* ♦''. ,' » l-x~.

>.*••- X ' • * •' . ' » - T ' • • *

;;.. •..•.--,'..■•,;.-;,;,-.-;-/:•.-:•.•,;-:

' '. . . ^ - ' . / • . - . '^ .^ X - «" . X , N , X ' , \ - . »

19' 19'

■

■

I

26' " — JS-

c

10'

25'

Fig. 102. — Reactions of paramecia to temperature, a, Paramecia in a trough
with a uniform temperature of 19° C. The animals are evenly scattered through
the water. In &, the temperature is held at 26° C. at the left end and 38° C. at the
right. The animals are congregated at the end of the lower temperature. In c the
temperature is 25° C. at one end and 10° C. at the other and the animals have all
collected in the region of the higher. An optimum temperature for these animals is
evident. (From Jennings, Behavio7' of the Lower Organisms, The Columbia Univer-
sity Press.)

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.
Such successive attempts to gain the result desired constitute what
is kno^vn as the *' trial and error'' mode of behavior.

EUGLENA, AMOEBA, AND PARAMECIUM

301

^?^^»

W^

Fig. 103. — Diagram of the course and movement of Paramecium through the
water. Notice the spiral path. (From Jennings, Behavior of the Lower Organisms,
The Columbia University Press.)

302 ESSENTIALS OF ZOOLOGY

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.

References

Hegner, E. W., and Taliaferro, W. H.: Human Protozoology, New York, 1924,

The Macmillan Company.
Kudo, Richard R. : Handbook of Protozoology, Springfield, 111., 1931, Charles C.

Thomas.
Marshall, C. E.: Microbiology, Philadelphia, 1926, P. Blakiston's Son & Company.

CHAPTER XVI

HYDRA, OF PHYLUM COELENTERATA

The phylum name, coelenterata (sel en ter ata), 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 diploUastic. 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 gastrovascular 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
readily available, easily collected and handled, and is representative

303

304

ESSENTIALS OF ZOOLOGY

of multiceUular 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.

Olassificatioii 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 group of corals, some smaller
jellyfish es, and the fresh-water polyps.

Fig. 104. — 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, The C. V. Mosby Company.)

Order Leptolina (may be divided into Anthomedusae and Leptome-
dusae) — a group which has a sedentary or sessile polyp stage. Such
examples as Hydra, Ohelia, Gonionemus, Campanularia, Tuhularia,
and Craspedacusta are well-known forms.

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 exumbrella is the convex upper, or aboral side while the
subumbrella is the concave, lower, oral side. A short stalklike part,
the manubrium, hangs down from the center of the subumbrella. At

HYDRA

305

Obella hab'ib

Mouth ^

Hydrotheca — .

Coelenteron '-

intodeirn)

ictode rm

Qonotheca
MedusQ'bud
Bla5to^ty/e -

Radiol canah,

Reptoduciive
Ofgan---

Houbh #^^;;

Jtatocyjfc— -
Tentacles-

hledusa

Obelia

Fig. I05.-Ohelia, hydrozoan colonial coelenlerate f owing ^se^usA gneratjon
:ual generation (medusa), structure, and h.-ibit of life. (Courtesy ot c^enerai

sexual generatio
Biological Supply House.)

306

ESSENTIALS OF ZOOLOGY

its distal end is the mouth, bordered by four oi^al lobes. The mouth
is the aperture leading into the internal or gastrovascular cavity
which has four radial branches or canals.

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 hydroi^hiza. They are distributed in the Atlantic Ocean and
Gulf of Mexico out to forty fathoms in depth. The colony begins

Medusa

Mature
(jonancjium.

Position of.
mai

sjtion Of
rture colony

//ew
colony

Sperm from
another
medusa

(o^^_ .Terbilijed ecja
V i^ycjote)

\^ ^ ^^> Cleovacfe

Blastula.^

'^ \arva ^

Fi,?. 106. — 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 stashes are free swimming. (Re-
drawn and modified from Wolcott, Animal Biology, McGraw-Hill Book Company,
Inc.)

as a single individual which buds, but they do not separate from the
preceding or parent generation. This may continue for several gen-
erations.

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 jellyfish
form. The medusae arise as buds from the special individuals,

HYDRA

307

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.
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

Fig. 107. — Physalia, the Portuguese man-of-war, a floating colonial coelenterate.
(From Hegner, College Zoology, The Macmillan Company.)

disintegrates, and after producing germ cells, the medusae die. This
process, involving alternation of generation, is called metagenesis.

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

Order Hydrocorallina. — This group resembles the corals by produc-
ing strong calcareous skeletons. They have extensive, branched hy-

308 ESSENTIALS OF ZOOLOGY

drorhiza and powerful nematocysts (stinging bodies). Millepora, the
staghorn or stinging coral, as it is called, is a good example.

Order SiphonopJiora. — 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 (pneumatophore)
with a superior crest. The polyps hang down into the water beneath
this float. 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 ex-
ample. Its sting is quite poisonous; bathers coming in contact with
the trailing tentacles, 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. 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 visu-
ally lack marginal sense bodies (tentaculocysts). Tessera, Lucernaria,
and Haliclystus are usually cited as examples.

Order Peromedusae. — These are cup-shaped, free-swimming forms
with four interradial tentaculocysts. They occur in the open sea.
Pericolpa and Periphylla.

Order Cuhomedusae. — Forms which have rather cubical shape, four
perradial tentaculocysts, interradial tentacles, and are chiefly tropical.
Chiropsalmus and Charyhdea are examples.

Order Discomedusae. — Scj^hozoa whose medusae are dominant,
saucer-shaped and almost transparent. Some of them are more
than seven feet in diameter. 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 com-
mon examples.

Aurellia* is the typical example, and, like most jellyfishes, is
composed largely of water. This is a common one and ranges from

•This spelling is according to Mayer's monograph. The generic name first pro-
posed by Peronas Le Sueur was so spelled.

HYDRA

309

New England to the Gulf of Mexico. It may reach a foot in diam-
eter and has the appearance of a transparent umbrella. Many of
these animals produce a calcareous external skeleton called coral.
Both sexual and asexual reproduction are common.

Class Anthozoa. — This class, of the polyp form, has two subclasses.
The group includes the corals and sea anemones. -

Subclass Zoantharia. — This group has numerous paired septa,
typically occurring in multiples of six, and plain tubular tentacles.
It includes sea anemones and corals.

CiJitsutiacQ. of body wall

Stomach

Qonad

Sab-qenital p]t

-\ Upper portion of

manubrium

r — Lateral mouth

r^; Radial canal

- - Jcib-umbrcl/a
space

— Circular
muscle

t^anubriurr)'"
(cut surface)

Central mouth
Oral tentacles

Fig. 108. — Cabbage-head jellyfish, Stomolophus, a very common form in the Gulf of

Mexico. Bisected to show internal structure.

Order Actinaria. — These anemones are usually solitary polyps; they
have many complete septa and numerous tentacles but no skeleton.
Sagartia, Ceria7ithus, and Metridium 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

310

ESSENTIALS OF ZOOLOGY

expanded and contracted, and it can change its location by ''scooch-
ing" on its basal disc (attached end). The mouth is located in the
center of the crown.

Asexual reproduction by budding from the margin of the basal
disc is practiced by this animal. Occasional lougitudinal 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.
Mature ova and spermia are discharged into the water of the cavity
and escape through the mouth to unite in fertilization outside.

P«riit<

MoutK

p T«nttcle«

Tertiary Mesentery

Conad

Primary Meaeniery
B

Meienterial filament
5econdary Meientery

Cincladc. vrith
Acontium protrudiftf-

Stereogram of Anthozoan Polyp

Meaenterie fiUmeni

Doraal

5)pKo no glyph

£:amo(laeum

Primary
Mcaenleriea

Seedndary
.Mesenteriea

Tertiary
**Mcacnterict

...-. Hollow
Tent* da

' Sphincter

Diagrammatic T. S. of Anthozoan Polyp ^t level A*A

Endoeoel. chamber between
two metenteriet of «
the tame pair ^

Csoeoel. chamber
between paira ^
of meacntcrica *^

Di raciivca
(Ventral 5cr of
Primsry Mcaenlen^a) **

Meientefie
, filament

Seeonilafy
, Mesentcrica

^ Primary

" Mocntcriea

**«^ Terli«ry
Metcnteriea

Diagrammatic T 5. of Anthozoan Polyp at fevel B-B

Fig. 109. — Diagrams showing structure of the anthozoan. Metridium, (Courtesy of

Pacific Biological Laboratories.)

Order Madreporaria. — The representatives of this order secrete an
external limestone skeleton; most of tliem are colonial. Astraiigia,
Madreporay and Oculina are examples.

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. In time continually growing colo-

HYDRA 311

nies of these animals can produce enormous stony barriers (reefs) in
the sea. 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.

Many authors recognize Zoanthidea, C erianthidia, and Edwardsi-

idea as orders in this subclass also.

Subclass Alcijonaria.—The features of this division include eight
hollow, feathered tentacles, eight mesenteries, and one siphonoglyphe.
Colonial and polymorphic 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. Organ pipe coral belongs in this order.

Another order, Stolenifera with Cornulariella as an example, is
frequently set apart.

Order Gorgonacea.— This is another colonial coral which is sessile
and has a calcareous axial rod. The common sea fan. Gorgonia, as
well as the precious Corallium ruhrum are well known examples.

Order Pennatulacea. — Another colonial form whose body is modi-
fied so that one portion is submerged in the substratum. The colony
takes a bilateral form, and the individuals are born on a disc or axial
stem which is supported by a hard skeleton. Renilla and Pennatula,
sea pens and sea feathers, are typical examples.

Habitat and Behavior of Hydra

Hydra (ChloroJiydra) viridissima is likely the most common hydra
available. It is the small green hydra which is very active
and has short tentacles. Most of the hydras 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

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

312 ESSENTIALS OF ZOOLOGY

from place to place. It keeps the tentacles extended, ready to grasp
any food which may come into its reach. Nematocysts or sting
bodies are discharged when the tentacle comes in contact with
potential food, and if it chances to be a small animal, it will likely
be paralyzed by the toxin which is injected by the nematocysts.
The prey is then carried to the mouth and tucked into it by the
tentacles. Frequentlj^ hydra is able to stretch its body over articles
of food which are actually larger than the hydra usually is in nor-
mal 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 re-
ported that a hungry hydra will perform the characteristic feeding
movements when only beef extract is in solution in the 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
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 common tropismSj 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. As
pointed out previously above, both chemotropism and thigmotro-
pism are concerned in food-taking. Contact stimuli are of consider-
able significance in a sedentary animal like this. It remains at-
tached in contact with some solid body most of the time. Sudden
mechanical stimulation like stirring the water or jarring the attach-
ment of the animal will cause it to contract vigorously.

Locomotion is accomplished in at least four ways. Gliding from
one point to another by partially releasing the basal disc and slipping
it to a new location is common. Or the animal may bend over and
cling to the substratum by the tentacles, release the basal disc, then
draw the body toward this point of the tentacles, where the basal disc

HYDRA

313

is reattached. This process is consecutively repeated and is called
'^ looping." Occasionally the animal bends over, holds by the tentacles,
then turns a ''handspring" or ''somersault" to attach the basal disc
beyond the attachment by the tentacles. 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.

FI&. 110. — Locomotion in hydra. Succes'sive positions taken when progressing
by somersaults. (From Jennings, Behavior of the Lower Organisms, The Columbia
University Press.)

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

314 ESSENTIALS OF ZOOLOGY

(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 extend free in the water. Tentacles may-
stretch out to be slender threads five to seven centimeters in length.
They are very useful either singl}^ 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.
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, w^hich secretes an adhesive substance
which helps the animal in attaching to objects. From one to several
huds 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 //. viridissima. Occasionally there
may be observed rounded projections on the side of the column which
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.
Sedentary and sessile animals very commonly have radial symmetry,
while the motile or free-living organisms tend toward bilateral sym-
metry.

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 gastrovascidar 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

HYDRA

315

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
extending 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 nematocysts, stinging cells or nettle cells. These are dis-
tributed over all the body except the basal disc, but they are much

TENTACLE
MOUTH

TESTI S

GASTRO-
VASCUI_AR
CAVI TV

ECTODERM

MESOGUOEA

EINDODERM

OVARY

Fig. 111.-

-Diagrammatic longitudinal
and typical cell layers.

BASAI_ DISC

section of hydra, showing mature gonads
(Drawn by Titus C. Evans.)

more numerous near the distal part of the column and on the ten-
tacles. The nematocysts are usually ^contained in little raised tuber-
cles 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 lar^ge barbed type is the most conspicuous, it will
be described here. Within the cnidoblast it is principally a sac of
fluid within which is inverted a stalk with some barbs and 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-

316

ESSENTIALS OF ZOOLOGY

cyst. Chemicals, such as weak iodine, acetic acid, or methyl green,
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 nematocj^st 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

Nemotocyst

Nucleus 3

Barb

Stalk --

Bag

Bemq'ins of
Cnlaoblasb

Barbless nematocyst

Fig. 112. — 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, In-
sect 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-

HYDRA

317

regular, slender, neuro-epithelial 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.

EPITHEL_IO-

MUSCUL.AR

CEt_l_

I MTER-

STITfAU

CEl_l_

NEMATOCYS"
CfMIDOBl-AS"

MESOC31_OEA

DIGESTIVE

CEUUS

C3l_ANO
CEl_U

Fig. 113. — Cross section through the column of hydra. The central space is the
gastrovascular cavity. (Drawn by Titus C. Evans.)

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 Avhich 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 enguK
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 them-
selves 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

318 ESSENTIALS OF ZOOLOGY

other cells of this layer. The general morphology of the adult
animal is very similar to the gastrula stage of the developing embryo
of more complex metazoans.

Metabolism

The food 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-
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 adsorbed 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.
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.

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

HYDRA 319

known as egestion. Respiration 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 several 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 neuro-epithelial 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 efficiently and quite rapidly. This process is
essentially reproduction by somatic cell division. The bud first ap-
pears as a slight superficial bulge. The cell division at this point is
very rapid, involving considerable activity in interstitial cells. An

320

ESSENTIALS OF ZOOLOGY

extension of the enteron extends into the bud, which is essentially
an outgrowth of the body wall. Tentacles appear as outpushings of
ectoderm and endoderm, and in the terminal position a mouth is
developed. After the bud has attained some size, a constriction oc-
curs between it and the parent, finally separating the two to form
a free individual.

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

Embryo

HydrAr <§exu&l i^production

Fig. 114.

-Methods of reproduction in hydra. (Courtesy of General Biological

Supply House.)

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
the female gonad and spermatogonia in the male. All phases of
maturation (gametogenesis) may be observed 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

HYDKA

321

develops at the expense of the other oogoiiia, which are engulfed
bodily and used for food. This one cell grows rapidly, and when
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 hlastula of many cells is formed. Then follows the formation
of the gastnda by a shedding of cells into the cavity (blastocoele)

Fig-. 115. — Development of hydra. 1, Fertilized ovum ; 2, two-cell stage ; S,
blastula stage; 4, gastrula, showing ectoderm (ec) and endoderm {en) ; cc, cleavage
cavity (blastocoele); m, cyst; p.b., polar bodies. (After Tanreuther, Biological
Bulletin, Vol. 14.) .^

from the inside of the original layer of cells. These new cells on the
inside become organized as an endoderm layer, while the original
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, :t increases in length
within the cyst; when it has attained some size it breai^s out, after

322 ESSENTIALS OF ZOOLOGY

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 made use of as food by
some of the useful fish. The corals are of importance both posi-
tively 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.

f

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.

References

Hegner, E. W.: Invertebrate Zoology, New York, 1933, The Macmillan Company.
Hickson, S. J.: Coelenterata, Cambridge Natural History, New York, 1906, The

Macmillan Company.
Ward, H. B., and Whipple, George C: Fresh-Water Biology, New York, 1918,

John Wiley & Sons, Inc.

CHAPTER XVII

THE FLATWORM, PLANARIA, OF 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 (the mouth) to
the exterior. The excretory sj^stem is composed of a pair of longitudi-
nal 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 sys-
tems 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 (ter be la' ri a— little stirring).— This class
consists of a group of soft-bodied, elongate and usually free-living
forms. There are both land and water forms. Four orders (may be
combined into three) are known: Acoela, Rhabdocoelida, Tricladida,
and Polycladida. Planaria and Stenostomtim are examples.

Class Trematoda (tre ma to' da — having pores). — These animals,
commonly called flukes, have no epidermis but a thick nonciliated
cuticle. This entire class is parasitic, and the immature stages fre-

323

324 ESSENTIALS OF ZOOLOGY

quently make use of snails and crabs as hosts for a phase of their
life history. This group is divided into only three orders : Monogenea,
Digenea, and Aspidocotylea. CoUjlapsis, Paragonimus, Clonorchis,
Fasciola 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
muscular tissue of several different animals. The class includes four
orders : Bothriocephaloidea, Cyclophyllidea, Tetraphyllidea, Trypano-
rhyncha. Taenia, Diphyllohothrium, Hymenolepsis 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 newly
developed blood vascular system, the alimentary canal, two apertures,
and cilia over the body are all characteristic of this type. There are
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 glands in the
skin which may produce a tubelike dwelling for the worm. The
larva is usually called pilidium. Prostonia, Cerehratulus, Tetrastema
are representatives.

Habitat and Behavior of Planaria

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
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

FLATWORM (pLANAKIa) 325

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 proboscis 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 attracted
to those points.

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 w^hich essentially lays a smooth track for the

Rye Genital pore

Side of head • v„„ Ji, Proboscis

Pharynx sheath

Fig. 116. — Entire planaria with pliarynx extended in position for feeding. (From
Hegner, College Zoology, The Macmillan Company, after Shipley and McBride. )

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 sj^stem
which in turn transmits an impulse to an efferent cell carrying it to
a muscle qr gland. The planarians respond to several tropisms. They
possess negative phototropism and thermotropism (as regards high
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
Planaria maculata, P. agilis, and P. dorotocephala.

326 ESSENTIALS OF ZOOLOGY

External Anatomy

The body is elongated, flat, broadly wedge-shaped at the anterior
and tapering to a point at the posterior end. It is triplohlaslic
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 bilateral. 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 proboscis which is used in feeding. Posterior to the
mouth is a small, constricted, scarlike aperture, the genital 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 repro-
ductive 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
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

FLAT WORM ( PLAN ARIA )

327

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

nc

ex

A.

B.

Fig. 117. — Structure of planaria. A, Digestive and excretory systems; B, nervous
system; 6r, cephalic ga iglia (btain) ; ex, excretory pore; int, intestine; l.ne, longi-
tudmal nerve; mo, mouth; o.ph, opening of pharynx; /j/i, pharynx. (From Parker
and Haswell, Textbook of Zoology, The Macmillan Company.)

328

ESSENTIALS OF ZOOLOGY

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.

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 body, which receive many small branches and open by minute
pores located just posterior to the eyespots, and by several other
pores along the length. All of the smaller branch-tubules have at
their blind ends a flame cell which is hollow and contains a mass of

Nucleus

Cilia

^^ Excretory tubule

Pig. 118. — Flame cell of planaria.

long cilia that are continually beating in a direction toward the
tubule, the movements appearing something like a flickering flame.
The cellular walls of the tubules as Avell 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,

FLAT WORM (pLANARIA) 329

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 vesiclies,
and converge to form the penis or cirrus, the copulatory organ. This
opens into the common cavity called the genital atrium or genital
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
by way of the vagina. There are numerous yolk glayids joining each
oviduct along its length; a glandular structure of questionable func-
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
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 (it breathes through
its skin); no coelom or body cavity; and no circulatory system;

330

ESSENTIALS OF ZOOLOGY

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.

Aurick

CerebraL
(janglion

Lonqibudinal

nerve cord

Testis

Vasa X-?

efferentia

interal
nerve

Vas

deferens

Mouth \-^^^

Seminal

vesicle

Sewlnaf

{receptacle

_\ Ovary

^^Yolkcjlands

Lateral nerve

Jniestine
Diverticula

<c:^-^ lumen of

pharynx

__/nfccjfc/ne
Oviduct

J^aryn(jcal

chamber

^.Qen'italpore

Fig. 119. — Reproductive system of planaria. Male organs are shown on one side

only.

FLAT WORM (pLANARIA) 331

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 necessary 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
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
they are thought to be stored. Cross- fertilization or self-fertilization
may occur, due to the fact that both the cirrus and the vagina open
by way of the genital atrium. Planarians have been observed to
copulate with an apparent exchange of spermatozoa in the form of
spermatophores. In copulation the cirrus or penis is protruded
through the genital pore and enters the genital pore and on into the

332 ESSENTIALS OF ZOOLOGY

Literus 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 fertiliza-
tion 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 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, gastrulation and even later stages
before the cocoon ruptures and the small wormlike planarians escape
into the water.

. — Young olanaria
\ hacchina

Icjcj capsule or cocoon

Fig. 120. — Planarian cocoons with young emerging from one.

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
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

1

FLAT WORM ( PLAN ARIA)

338

again follows 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 inay 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
(metabolic gradient) by Dr. Child. When the animal is young, it is

Fig. 121. — Fission in Planaria dorotocephala.

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 t)rganism. 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" dominance 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

334 ESSENTIALS OF ZOOLOGY

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.

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.

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.

References

Hegner, R. W. : Invertebrate Zoology, New York, 1933, The Macmillan Company.
Hyman, L. H.: Studies on the Morphology, Taxonomy, and Distribution of

North American Triclad Turbellaria, 1928-31, Tr. Am. Micro. Soc, "Vols.

47 and 50.
Ward, H. B., and Whipple, George C. : Fresh-AVater Biology, New York, 1918,

John Wiley & Sons, Inc.

CHAPTER XVIII

THE ROUNDWORM, ASCARIS, OF PHYLUM
NEMATHELMINTHES

This group is kno^vn as the unsegmented roundworms or thread-
worms. Some of the Nemathelminthes (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 ^^' to four feet. They lack respira-
tory 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 cnake," hookworm, pinworm,
Trichinella, Filaria, Guinea worm, whipworm, and eye worm.

Classification

Three classes are usually recognized in the phylum, 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 Y^q 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.

Order Ascaroidea. — This group includes both parasitic and free-
living forms. Ascaris, 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 lumhricoides will be discussed later as a typical example of
Nemathelminthes.

335

336

ESSENTIALS OF ZOOLOGY

Order Strongyloidea. — 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

Fig. 122. — Hair "snake," Gordius^ an aquatic roundworm.

muscle of chordate animals, and transmitted by certain insects.
Guinea worm, eye worm, and Filaria are the common human para-
sites. Some species cause elephantiasis through occlusion of blood
and lymph vessels. This disease results in enormous swelling of the
affected 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

ROUNDWORM (aSCARIS) 337

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 and the whipworm are well-known
examples.

Class Gordiacea (gor di a she a, a knot). — Superficially these
animals 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 they are para-
sitic on larval 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
the water of some stream, puddle, or watering trough. These fe-
males 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. Gor dins aquaticus and Paragordius varius are
the common examples of the group.

Class Acanthocephala (a kan tho sef a la. thorn head) includes a
group, known as "spiny-headed worms," which is absolutely parasitic
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 cvlindrical
shape when removed to some solution outside the body. The protru-
sible proboscis 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 constitutes
the central nervous system

Habitat and Behavior of Ascaris

Ascaris lumbricoides frequents the digestive tract of men and hogs.
It is entirely dependent on its host for furnishing suitable food and
environment. The only time this organism is at the mercy of the

338

ESSENTIALS OF ZOOLOGY

elements of. nature is during the egg stage, when it may remain
potent for months or even years if it falls in an environment unsuit-
able 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

Fig. 123. — External anatomy of Ascaris lumhricoides, male and female.

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

ROUNDWORM (ASCARIS) 339

lips have small papillae on their surfaces, two on the dorsal and
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 approxi-
mately one-third of the length of the body from the anterior 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 longitudinal
muscle fibers, whose medial margins are rather baggy. There 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
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
at the posterior portion of the body. The two laterally located,
longitudinal ducts 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.

340

ESSENTIALS OF ZOOLOGY

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.

J770at^ circumes>ophagecc/

aterus

oyanes

CJtcretort/ p/?ari/nx fer?iCa/ pore //7C>e3C/r7e, docfi/ iva// a/9a^

0)^/CfiJCt

oyar^

uterc/5

va^/rfoc

pseuotocoe/

teatM

cte/ctas ofeferc/7S

sem/ncf/ ves/ch

e/aca/atorf/ dc/ct

cat/c/e ] tntesUne,

pseudocoeJ

ep/oferm/e\ bod^ ivcr//

y77a3c/e ) ^c/7?//7(y/ yes/b/e
cut/cJe

Inteet/ne'

Gxcretoru cana/

pert/cff
o\r/duct 5e^Jae"

seta/ sac
rectu/r?

oyar^
ner/e cord

ar?c/i

s

Pig. 124. — Internal anatomy of Ascaris lumhricoides. A, Diagram showing
lateral view of dissection of female ; B, cross section through the mid-region of the
body of the female ; C, longitudinal section of the posterior portion of the male ;
D, reproductive system of female; E, reproductive system of male. (From Curtis
and Guthrie, Textbook o/ General Zoology, John Wiley and Sons, Inc., modified
from Leuckart.)

Reproduction and the Life Cycle

The animals copulate, and 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-

ROUNDWORM (aSCARIS) 341

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 chenopodium, santonin, and hexylresorcinol
have been used successfully under physicians' directions as a cure.
Effective sanitary disposal of fecal material is the most successful
preventive.

References

Chandler, A. C. : Introduction to Human Parasitology, New York, 1940, .Tobn

Wiley & Sons, Inc.
Faust, E. C. : Human Helminthology, Philadelphia, 1929, Lea & Febiger.

CHAPTER XIX

EARTHWORM, OF 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, nonjointed, 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 n. Archiannelida
Class III. Hirudinea
Class IV. Gephyrea

Order 1. Echiuroidea

Order 2. Sipunculoidea

Class Chaetopoda (ke top' 6 da, hair and foot). — This class in-
cludes the most commonly known forms of the phylum. There are
marine, fresh-water, and terrestrial forms ; and they all possess setae

342

EARTHWORM 343

(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 there-
fore useful in locomotion. The coelom, which surrounds the straight
digestive tract, is divided between the segments by partitions known
as septae. Typically, each coelomic space possesses a pair of nephridial
tubules which communicate with the coelom at one end by means of
a ciliated, funnellike opening, the nephrostome. The other end opens
to the outside by means of a nephridiopore. The nephridia remove
nitrogenous waste materials from the coelomic cavities and from the
blood.

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 supra-pharyngeal 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 sub-
pharyngeal 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 gr connectives. The dorsal ^vessel exhibits wavelike contrac-
tile movements (peristaltic contractions) which force the blood an-
teriorly. The latter passes through the hearts, which also pulsate,
then backward through the ventral vessel to the skin, intestine, and
other organs. Hemoglobin is suspended in the blood plasma of some
Chaetopoda; in others, a green pigment known as chlorocruorin is
found ; in still others no known blood pigment occurs. The principal
vessels and hearts have valves on their inner surfaces which prevent
the blood from flowing in the opposite direction.

344 ESSENTIALS OF ZOOLOGY

The class Chaetopoda may be divided into two orders; namely,
(1) the Polychaeta and (2) the Oligochaeta.

Order Polychaeta, — The polychaetes (many bristles) are typically
marine Chaetopoda. One of the most widely known forms of this
group is Nereis virens or the clamworm. It possesses many setae
(chaetae) located in fleshy parapodia. In this case the parapodia
with their setae constitute the segmented appendages. 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 prosto-
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
pair of cirri or tentacles. The pharynx is equipped with muscles by
which it can be everted, and a pair of chitinous jaws which protrude
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 cirri extending posteriorly.

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
passes posteriorly through the ventral one. Its movement is effected
by wavelike contractions in the walls of the dorsal vessel.

Each segment of the body except the peristomium has two nephridia
(excretory tubules) opening directly from the coelom to the outside.
The nephridia serve to convey the excretory 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 trochophore 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-
ynissures 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.

Order Oligochaeta.— The best known example of the order Oligo-
chaeta is Lumhricus terrestris, the common earthworm, which is used

EARTHWORM

345

almost universally as a laboratory specimen. Lumbricus is not as
common in the Southwest as are other large forms of earthworms,
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.

External Anatomy of the Earthworm

The body of Lu7nhricus varies from six to fourteen inches in
length and gives the appearance of a series of rings joined in a

Prosforrn'um

X3cvr-'

XXXE-i

xxxvu-

oTOviducf

!..

...,

1. . -HL

!.. .-4

1

\, ..11

Br. . 01^

f,, . , 4

^Opefiit70 (rf
yens cfeferens

"Sem/nctf

CI/^e//urrf

'Sefcne

Anus

Fig-. 125. — External anatomy of earthworm, ventral view, segments numbered
in Roman numerals. (From Wolcott, Animal Biology, McGraw-Hill Book Com-
pany, Inc.)

linear arrangement. The rings are the body segments, or meta-
meres, and vary in number up to 150. In the adult the number of
segments from the anterior end to the posterior end of the clitellum

346 ESSENTIALS OF ZOOLOGY

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

The peristomium 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
mouth through its ventral side. 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 de-
posited during reproduction.

Each segment, except the first and last, bears four pairs of chitin-
ous setae, which are fine, stiff bristles. They are moved by pro-
tractor and retractor muscles and serve to help the worm move
through the soil. A pair of nephridiopores (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
serves as a protection against physical and chemical injury to the
animal's body and as a respiratory membrane.

Internal Anatomy

The body of the earthworm is in the form 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).

EARTHWORM

347

Reproductive Organs

The earthworm is hermaphroditic, the organs of both sexes be-
ing 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

nn ^i^'tu,? ^~^^?''°^^^/^^'® organs and nervous system in segments VIII to XV of
nnH YT f^ I^*,^ portion of the seminal vesicles has been removed in segments X
ova rvV: o^ disclose the testes and sperm funnels, es. egg sac ; nc, nerve cord ; ov,
Hpf^ro«5^ ' semmal funnel; sm, septum between two somites: Sji, sperm duct (vas
vAcfJiJ^ f openmg in the fifteenth somite r sr, seminal receptacle; sv, seminal
C? V Mosby Com ^ oviduct. (From White, General Biology, published by The

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 efferentia
coming from the vesicles on each side forms a single pair of vasa

348

ESSENTIALS OF ZOOLOGY

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. 127. — Dorsal dissection of an earthworm in the region of segments I to
XXL be, buccal cavity ; eg, calciferous glands ; cr, crop ; dv, dorsal blood vessel ;
eo, esophagus ; g, gizzard ; n, nephridium ; sb, subpharyngeal ganglion ; st, intestine ;
pe, peristomium ; II-XXL somites; ph, pharynx; p, prostomium. (From White,
General Biology, The C. V. Mosby Company.)

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
calciferous glands, the secretions of which help to neutralize the acid
organic matter taken as food. The esophagus opens into the ci'op,

EARTHWORM

349

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 the intestine is covered with a layer of brown cells, known as
cliloragogen cells, whose function is doubtful. They are generally be-
lieved, however, to play a part in the excretion of nitrogenous wastes.

dv

n^

Fig. 128. — Cross section of an earthworm through a posterior segment, ch,
chloragogue cells ; cir, circular muscle fibers ; coe, coelom ; cu, cuticle ; dv, dorsal
blood vessel ; ep, epidermis ; int, intestine ; la, lateral neural vessel ; Id, parietal
vessel ; Ion, longitudinal muscle fibers ; n, nephridium ; nc, nerve cord ; sb, subneural
blood vessel; se, seta; Ty, typhlosole ; vv, vejitral blood vessel. (From White, Gen-
eral Biology, The C. V. Mosby Company.)

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
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 near the burrow, and feed on dead organic matter,
such as leaves. Food is drawn into the mouth by suction produced

350

ESSENTIALS OF ZOOLOGY

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 mate-
rials is accomplished.

Circulatory System

The blood of the earthworm consists of a 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 hemoglobin

Beart

:^-Dorsal vessel

Intestlno-tegumeatai7
- — vessel
.-Ventral vessel

Sub-neural vessel

Septa
Dorsal vessel

Septa j^

Intestino-tegumentary
vessel

VIII
Dorsal vessel

CEsopbagus

Ventral vessel

6ub-neural vessel B

Efferent Intestinal vessel

Nephridium
Lateral-neural vessel

D

Ventral
vessel

Sub-neural vessel

Afferent intestinal vessel

Dorsal vessel
Typblosolar vessel

Ventral vessel
Sub-neural vessel

Parietal vessel

Fig. 129. — Circulatory system of the earthworm. A, Longitudinal view of vessels
in somites VIII, IX, and X : B, transverse section of the same region ; C, longitudinal
view of the intestinal region; D, transverse section of the same region. (From
Hegner, College Zoology, The Macmillan Company, after Bourne, after Benham.)

EARTHWORM 351

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 wavelike 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 nephridia, 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-intestinal vessels. An-
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.

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
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-

352 p:ssentials of zoology

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.

The Nervous System

The brain of the earthworm consists of the suprapharyngeal
(janglion, two cirmimpharyngeal connectives, and the suhpharyngeal
ganglion. The ventral nerve cord extends posteriorly the length of
the bod}^ with a ganglion and three pairs of nerves in each segment.
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 subpharj^ngeal ganglion lies ventral to the pharynx in the
fourth segment. Nerves from these two ganglia innervate the first
three segments and the prostomium.

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
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

EARTHWORM

358

vesicals and vas deferens to the dosed passage and move through it
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 received
sperms from the other, they separate by working themselves through
the bands secreted by the clitella. This leaves each animal with a
band which is gradually worked off toward the anterior end. As
the band passes over the openings of the oviducts, eggs are released
into it, and as it passes the openings of the seminal receptacles,
sperms, which came from the reproductive mate, are released.

fipertures of flpertare of •

Seminal receptacles Vas deferens CliteUam

Seminal droove Pore of ovidact

Dorsal
blood vessel

Body wall

Intestine

3en?ina/
groove

Band of
macus
secreted by
a clitellum

Fig. 130. — Reproduction in earthworm showing copulation and cocoon. A, Two
worms enclosed in bands of mucus during copulation ; B, transverse section show-
ing the seminal grooves ; C, cocoon.

Both ends of the band close, forming a cocoon in which fertiliza-
tion and development take place.

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

354

ESSENTIALS OF ZOOLOGY

and anus. The mesodermal cells which fill the space between the
ectoderm and endoderm develop segmental cavities which are the
coeloms of the metameres. This constitutes a tube within the worm
from which eventually develop the organs of the adult individual.

Blastocoe/

flesob/ast
cell

tl^soblast
tissae

hesoblast
cell

Ectoderm A

fndoderfT)'

flesob/ast
cell

Unas .

Ectoderm

Endoderfi)

Ectoderm
Coelom

Hesoblast
Cijme

3of7)atic
esoderm

EododeriT).

Coelom

Splaachnk
mesodau)

Ectoderm
Have cord

Endoderm

Coelom'ic
cavities

hesodermk
tissue

Ectoderm

i^noatl?

Fig. 131. — 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; C and D, showing- stages in the beginning 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, gastrula in
cross section ; /, longitudinal section of a young worm after formation of the
mouth and arms ; J, same as I but in cross section ; K, cross section of later stage.
(After Wilson, Embryology of the Earthworm, Journal of Morphology, 1889.)

EARTHWORM 355

Regfeneration

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 paropodia. 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 tentacles on
the prostomium. Two ciliated pits are present as a retention of
juvenile characters. The trocJiophore larva is common to the entire
group.

Class Hirudinea (hi ru dm' 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 fprmed 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,
intestine, and ectodermal rectum. Blood which is sucked from an-

356 ESSENTIALS OF ZOOLOGY

other animal receives a ferment from the salwary glands of the
pharynx, which prevents it from coagulating. It is then stored in
the diverticulae of the crop. The animal is capable of ingesting three
times its own weight in blood, and, since several months may elapse
before it is all digested, frequent feedings are not necessary. The
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 clitellu7n. Two nephridia are pres-
ent. The nervous system is typical of the annelids.

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 afPord pro-
tection. In this class, the representatives of 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
male has no proboscis, is ciliated, and lives as a parasite on the pro-
boscis of the female. Representatives of the order Sipunculoidea have
no prostomium in the adult.

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

EARTHWORM 357

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.

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 swim along, biting
off that part of the worms protruding from the mud or sand. In-
stead of dying, the injured worms simply 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.

References

Beddard, F. E.: Earthworms and Their Allies, London, 1901, Cambridge Uni-
versity Press.

Borradaille, L. A., and Potts, F. A.: The Invertebrata, New York, 1932, The
Macmillan Company.

Hegner, E. W. : Invertebrate Zoology, New York, 1933, The MacMillan Company.

Parker, T. J., and Haswell, Wm. A.: Textbook of Zoology, Vol. I, New York,
1930, The Macmillan Company.

Olson, Henry W. : The Earthworms' of Ohio, Ohio Biol. Survey Bull. 17, Vol. IV,
No. 2.

CHAPTER XX

STARFISH, OF PHYLUM ECHINODERMATA, AND

OTHER ECHINODERMS

The Echinodermata (e kl no dur' ma ta — hedgehog skin) consti-
tute 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 and 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

358

STARFISH AND OTHER ECHINODERMS 359

are some species which do not adhere to this pentamerous condition
and have up to forty rays. The rays are not usually sharply con-
stricted from the central disc. Asterias, Astropecten, SolasteVy Greas-
ier and Echmaster are representative genera.

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 ventrally. The
ability of autotoimj (self -mutilation) is so well developed here that
arms will become detached by merely grasping them. Ophioderma,
OpMura, Ophiothrix and Gorgonocephalus are common Atlantic and
Gulf genera.

Fig. 132. — Oral view of a basket star belonging to class Ophiuroidea. (By courtesy

of General Biological Supply House.)

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
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-

360

ESSENTIALS OF ZOOLOGY

lacral rows. These rows correspond in position to the ambulacral
grooves of the starfish. 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

Madreportfe

7

p/afes

ppp^

Afrfbu/crcraf
p/afes

Fig. 133. — Dried test of the sea urchin, Arbacia. A, Shows arrangement of the
plates on the aboral side; B, oral view showing mouth and perioral area. (From
Wolcott, Animal Biology, McGraw-Hill Book Company, Inc.)

removing algae from rocks for food. This arrangement constitutes
Aristotle's lantern, and the esophagus leads internally from its
aboral part. The principal organs of respiration are the interradial
pouches and the tube feet. The nervous system is composed of a
circumoral ring with radial cords extending into the ambulacral

STARFISH AND OTHER ECHINODERMS ii6J

areas. Strongylocentrotus, Arbacia, Trip7ieustes, Clypeaster, and
Echinarachinus (sand dollar) are representative genera of the gronp.
Class Holothurioidea. — The echinoderms 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
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. Representative
genera of this class include Thyone, Holothuria, Cucumaria, Lepto-
synapta, Aphelodactyla and Caudina.

Fig. 134. — Thyone, the common sea cucumber.

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. The mouth is located in the upper-
most center of the calyx and is surrounded by the anus. The anus
is also to be found on the oral side within the enclosure made by the
arms. Neocomatella, Pentacrimis, Rhizocrinus, Metacrmus and Ante-
don are representative genera.

Habitat and Behavior of Starfish

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

362

ESSENTIALS OF ZOOLOGY

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

Fig. 135. — The ochre starfish, Pisaster ochraceus, an abundant form along the
Pacific coast. (Johnson and Snook, Seashore Animals of the Pacific Coasts The
Macmillan Company.)

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.

STARFISH AND OTHER ECHINODERMS 363

. 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 branchiae.
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 blades 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
constitute the bivium. 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 ambu-
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.

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 arranged
with the flat sides together, like cards in a filing case. The two
middle rows of ossicles are called ambulacral plates. Ambulacral
pores, through which the tube feet project, are located between these
plates. The outer rows of plates, forming the margin of the groove,
are shorter and are known as adambulacral plates. Five flat oral
ossicles surround the mouth.

Within the body wall and extending into the arms is a large coelom
which is lined by a peritoneum and filled with coelomic fluid. In

364

ESSENTIALS OF ZOOLOGY

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-

Fig. 136. — 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 bivium
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. Avi, ambulacral
groove; cs, cardiac stomach; D.B., dermal branchiae; E, eyespot ; G, gonads; M.
madreporite ; Os., ossicle ; P, pedicellariae ; P.C., pyloric caeca ; Py, pyloric sac ;
R, rectal gland ; Sp., spine ; T.F., tube feet ; T.F.z, arrangement of tube feet in skeletal
ray. (From White, Genera? BioZofiry, The C. V. Mosby Company.)

STARFISH AND OTHER ECHINODERMS

365

gus and is separated aborally from the pyloric portion by a marked
constriction. A large pair of branched glandule structures, known
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
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

Fig 137. — Longitudinal section through the central disc and one ray of a star-
fish, a, Anus ; am, ampulla ; car, cardiac stomach ; coe, perivisceral coelome ; ey,
eyespot ; hca, hepatic caeca ; i, intestine ; m, mouth, mth madreporic plate ; nr,
nerve ring ; oe, esophagus ; os, ambulacral ossicle ; Py, pyloric sac ; 7'a, radial canal ;
re, ring canal ; rca, rectal caeca ; rn, radial nerve ; Sc, stone canal ; sp, spme ; tf,
tube feet. (From W^hite. General Biology, The C V. Mosby Company.)

through its mouth and is spread over the tissues of the prey. An
abundance of digestive fluid secreted over the food causes it to be
digested in its own shell, and it 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 starfish in a week. It has also been shown
that a starfish may survive after months of fasting. After feed-
ing, the stomach is withdrawn into the body cavity by five pairs
of retractor muscles, one pair extending 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 between adjacent arms.

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

366

ESSENTIALS OF ZOOLOGY

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
foot. The ampulla is bulblike and is located above the roof of the

Radial canal

Madreporite
Stone canal

Tiedemann's body

Ci ream -or a
canal

Tube foot

Fig. 138. — Diagram of water-vascular system of the starfish.

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
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
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

STARFISH AND OTHER ECHINODERMS

367

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 ex-
tended. 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

Fig. 139. — Starfish "walking" on glass. Viewed from the oral surface. Notice the
extended tube feet. (Courtesy of General Biological Supply House.)

in that it absorbs the digested food and distributes it. This fluid
bears amoehocytes 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 Tiedemann's bodies and have migrated to
the coelomic cavity. The rectal caeca serve in respiration to some

368 ESSENTIALS OF ZOOLOGY

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
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 pigmented eyespots at the tips
of the arms are photosensitive and sensitive to touch. The pedi-
cellariae 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 bipin-
naria in which there are several extended symmetrical processes,
is known as the hrachiolarian stage. Following this condition 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.

TaA.

t).

Fig. 140.— Development and metamorphosis of the starfish. A, J^°^!,al vie\v of
early%iliated larva, showing ciliated bands and coelomic pouches ;Bvgitral^^^^^^^
of bipinnaria larva showing the extension of the left and ^ightcoelomic pouches .
G, dorsal view of the same larva showing the left j^^dreponc pore and water t^^^^
aAd the fusion of the left and right coelomic Pouches to fo^^l^fi^^i'/.^f'^^bes f?Sm
D, dorsal view of an older larva showing the budding of the ^ve water tubes trom
the left coelom; E, view of left side of a stijl older larva showing the water a^^^^
lar system developing from water tubes, and the rays of the adult starfish develop
ing on the dorsal side; P, Brachiolaria. larva . in the Profess of metamorphosis
The larva has settled on the preoral region which is g^^eatly shortened G.aboral
view of a young starfish showing the developmg sP^^es; a anus ac, anterior
coelom; ad, anterodorsal arm; b, brachiolar arms ; cj, amoral ciliated band cij-,
dorsal surface developing rays ; es, esophagus ; /, point of fixation tnf, i^^stine .
I, lateral arm; Ic, left coelomic pouch; m mouth ; md, median doisa arm n^^^^^
madreporic pore and water tube; vd, posterodorsal arm; ^'o- Po^toral ^liated band
pr, preoral ciliated band ; re, right coelomic pouch ■ sp, sPines;sf stomach to, five
water tubes of water vascular system. (Modified from Wilson and McBride. Ke
produced by permission of The Macmillan Company.)

370

ESSENTIALS OF ZOOLOGY

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

I Madreporite
Reqeneratinq rctj

Fig. 141. — Regeneration in the starfish. Above, a starfish regenerating part of
the central disc and three arms ; below, arm of starfish regenerating the remainder
of the body.

arms of the starfish may be lost and the missing parts regenerated.
An arm with a small portion of the central disc will regenerate the
missing parts under favorable conditions. A mutilated arm or one
caught in the grip of some enemy may be cast off by breaking loose

STARFISH AND OTHER ECHINODERMS 371

at the constricted point wkere it joins the central disc. This ability
of self-mutilation is known as autotomy. Following 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, how-
ever, 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 particularly 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 futility 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 de-
licious food by many people.

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

CHAPTER XXI

FRESH-WATER MUSSEL AND THE SNAIL, OF

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 rasp-
ing food.

Approximately 78,000 species of moUusks 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 diver-
sity of habitats, occurring abundantly on land, in fresh water, and
in the sea. Although most of the species live in moist surroundings,
a few inhabit arid regions. Some species, such as the cuttlefish, are
strictly carnivorous; many of the snails are herbivorous, 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
annelids is a matter for conjecture since some morphologists regard

372

FRESH-WATER MUSSEL

373

this similarity in larval forms as an example of adaptive parallelism
in a similar type of environment. Certainly, morphological evidence
shows a close relationship.

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, vvliich are found abundantly on rocks between
tide marks along the Atlantic and Pacific Coasts. 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
conspicuus.)

Class II. Pelecypoda

Includes the bivalve mollusks, 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 gill characters. (Clams.)

Class III. Gastropoda

Includes the snails and slugs. Approximately fifty-five thousand
species have been discovered and described. Shell, if present,
univalve. (Snails.)

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 known 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, LoUgo, Polypus.)

Habitat and Behavior of the Clam

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
anterior end of the shell they plow their way slowly through the
-stream or pond bed, feeding on the microscopic organisms in the

374

ESSENTIALS OF ZOOLOGY

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.

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.

Anodonta
Jtewort/ana

Quadrula
forsheyi

Proptera
purpurakQ

Amblema
costata

Leptod(?a
fracjilis-

Canjnculina ^Musojlium
texasensis remssi

Fig. 142. — Some common fresh -water bivalves.

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.
The oldest part of the shell is the umbo which is usually a rounded
protuberance near the top of the valves and is frequently eroded

FRESH-WATER MUSSEL

375

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.

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.

Ligamentous hinqe Umbo

.^jQrowihlims

Ventral siphon
I Dorsal siphon

Expanded

^°^^^fee'^'^"'^l_----^^^^C l^r^^nor cdduztor

■^^-a^0K:k J:' r'^ir/ protractor
''^'^^■^.:^:0:.i^:^^py& retractor
muscle

Pallial Unz Q

Fig. 143. — Exteinal (A) and internal {B) shell features of Lainpsilis anodontoides.

Internal Anatomy
(Detailed description based on Lampsilis)

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-
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.

376

kssentialS of zoology

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
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

Mantle cut frco.

Pericardial cavity Rectum

Auricle

Ant. retractor

Anterior i
adductor •

Post, retractor
Post. adductor
I tx.slpbon

Foot'

Protractor txt. labial palp

Left gill plate

in. siphon

"wffljfeii^-

Fig. 144. — Lampsilis anodontoides with the left mantle partially removed and

turned back to expose the underlying organs.

facilitates the digestion of carbohydrates. The food, having been
mostly digested and partly absorbed in the stomach, is passed on
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.

FRESH-WATER MUSSEL

377

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

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 branchial veins; after puri-
fication in the gills it is returned to the auricles by way of the
efferent branchial 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 cerebropleural gan-
glion, the two ganglia being connected by means of a cerebral com-
missure which passes above the esophagus. Each ganglion gives

Vertical ^'^^^- oddacbor M
water tubes \ Hidnoy

Exholant
Siphon

Arjus

Pericardial wall Reno -pericardial pore

Post, aorta ! Ventricle j Excretory pore

Auricle I | Ant.aorta Liver

I ! 1 Stomach \ cerebral
I I j I ; commissure

I I
I '

i;

Ant. adductor
muscle

Inhalant
siphon

Visceral Q. Qonod \ } root '. ^°"^^

Intestine Pedal Q. Cerebro pleural &,

Fig. 145. — Internal anatomy of Lampsilis anodontoides.

378

ESSENTIALS OF ZOOLOGY

off two nerve cords, one of which passes ventrally and posteriorly
to the pedal ganglion situated at the junction of the visceral mass
with the foot. The other nerve cord extends backward, terminating
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 stato-
lith, 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 excretory pore
through which wastes are discharged to the outside.

Reproductioii 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-

FRESH-WATER MUSSEL

379

3

c
o

O

C

o

t-

o
o

o

o

53 H y >t: o c *-

>*7 >- — ^

g

O

O

§ y t3 >t: d
^.i2 !2'^ ^-O

Cl Ox:
O

^

-- "^ M ii Pi '-J

69

to

o

HO

o
'«
o

•«»

09

a.

e

►^

o

CO

a;

bo

iil'llM

S\M

380 ESSENTIALS OF ZOOLCKiY

tory thread, the byssus. In most clams the glochidia are discharged
to the outside through the dorsal siphon. They fall to the floor of
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 Avith the glochidium, it will close
down upon it and remain attached if the fish is the suitable host
for the particular species of clam. The tissues injured due to the
attachment of glochidia produce by proliferation new cells which
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 tfeeir fish-hosts.

THE SNAIL

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
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 gyriym 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,
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

THE SNAIL

381

rocks, and there begin their period of hibernation. During this
condition of torpidit}- the body of the snail may be well protected
by one or several thin parchmentlike membranes called epiphragms
which are stretched across the shell aperture. When warm weather
arrives, the membranes are broken and the snail resumes its activities.

Water snails are active all four seasons, provided open water is
available. Naturally their movements are slow^ed down in the winter
due to cold, but when the pond or stream is frozen over, the move-
ments of Lymiiaea, Physa, or Helisoma 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
palustiis), the same as in land snails; these structures probably func-
tion in retarding water loss.

hesodermal
band

npical or^an
Zye

E^opha§

Head kidney.
Qtoc/st

he^enchyme

Stomach

Pre oral

ciliated
rin§

Blastocoef'z

Anal vehicle

Apical or^an

Endoderm
Embryonic muf'CiC

Prototrocb

f'l&soderm

Telatroch

Fig. 147. — 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.)

At least a few species of land snails possess a homing instinct.
Helix aspersa, H. po7iiatia, and Polygyra roemeri have all been ob-
served to occupy as ''home" a definite place and go out from this
''home" on nocturnal foraging trips, then return by sunrise the next
morning.

The life span seems to vary considerably in snails; some of the
aquatic genera, such as Lymnaea and Helisoma may live two to four
years 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 sqiimnosiim and the crustacean Gebia stellata.

382

ESSENTIALS OF ZOOLOGY

b/mnoeQ

buUmoides

techella

Physa
humeroso

Tropicorbis
liebmanni

Lymnaea .
stagnalis

[Lymnaeidae)

Physa
, anatina
( Physidae)

Menebus
dllatatus

Lymnaea
palusbris

/""^

Ferrissia
excentrica
(Ancylidao)

Helisomo

trivolvis

lentum

(Planorbidae)

Fig. 148. — Some common fresh watei- pulmonate snails.

humboldUona
chisosensis

Potyqyra
roemen

Bulimulus
dealbatus
li^Qbilis

Fig. 149. — Common terrestrial snails.

,Ruwlna
decollata

THE SNAIL

383

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. (Detailed Description, based on Helix.)

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,
some shells are shaped like house rooms {Patella that lives in the sea

Po\yqyra , Polyqyra
texasiana dorfeuilUana

Helicina

orhiculata

tropica

Retinella
indentata
pauc/llrata

_ .... Euconulas

Zonjtoides chersinus

arboreus trochulus

Strob]]ops (^asbrocopta
labyrinthica armifera
texQsiono

Succinea Zuqiandina Pupoides Philqmycu^
Q\/ara sincjleyana. marqinatus carolinensis

Fig. 150. — Common terrestrial snails.

or Ferrissia, a fresh-water form). The worm shell {VermeUis
spiratus) is so loosely coiled that it superficially resembles a worm.
Some shells, such as those belonging" to the genus Miirex, may have
long peculiarl}^ 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.

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.

384

ESSENTIALS OF ZOOLOGY

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,

Wrrnetus spiratms
(Worm shell) -

Limacina australb
(Ptcropod)

Murex tzmispina
(Venus's comb)

, Urosalpinx.
(Oyster dnll)

. Aeolis
(Sea slug)

Teredo naval is.
(Ship worm)

Fig. 151. — Marine mollusks.

snails living in an acid bog have thin transparent shells, whereas
the same species inhabiting an area rich in lime salts have thicker,
perhaps opaque shells. Certain species, such as Polygijra roemeri,
P. albolahris and P. teooasiana are capable of repairing broken shells
if the damage is not too severe.

THE SNAIL

385

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
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

Respiratory
apertum

Velum
I

A

I I »

GQnital aperture ' ^V^,

Tentacle

Mouth

Respiratory aperture

&qe of mantle
Foot — -

stalked eye

I

I

'Tentacle

-Mouth

Genital aperture

Fig. 152. — Fresh-water and land snails with bodies expanded. A, fresh-water

snail, Lymnaea; B, land snail, humboldtiana.

snails {Lymnaea, PJiysa, Helisoma) have their eyes situated at the
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.

386 ESSENTIALS OF ZOOLOGY

Just ventral to the moutli is the opening of the pedal gland which
deposits a highway of mucus over which the snail usually glides;
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
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
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
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 HeliXy 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 oxygen. The pulmonary sac

THE SNAIL

387

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 arc gill breathers, and some, such as the sea slugs, have
external feather-like gills.

Fig. 153. — Arrangement of teeth in the radula of a snail.

-Hermaphroditic Duct
'' ^.O^ofesf-is

,'i5emino/ Pecepfac/Q
///nfestine
/ 'Albumen G/anc/

Vas Defer

Sac

CIojtk
una
ivar^ C/anc/

7/}/s

Anterior
/Tenfaclz

—Pharynx
--Mouth

^^^ "Cerehral Gono/ion
^5afi>'ary Over

Fig. 154.— Internal anatomy of Helix. Shell removed.

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

388 ESSENTIALS OF ZOOLOGY

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
ventricle through a common aorta which divides into two branches,
one of which supplies the head and foot, and the other carriCvS 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 grmglia, 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 Mchiey 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 foi-med, followed by the production of ova.

THK SNAIL

389

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-

Albumin qtand -t^

DVer

hermaphroditic qtand

Crop

f^it Oviduct

r-JI Oart sac

Wf- MucotAS qlands

— . bfZ

Pig. 155. — Genitalia of Helix aspersa; act of union. (Modified, after Cooke, Cam-
bridge Natural History. By permission of The Macmillan Company.)

testis into the hermaphroditic duct, and from there into the oviduct
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.

390

ESSENTIALS OF ZOOLOGY

B

D E F

Fig. 156. — 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 texasiona (terrestrial; eggs in cluster) ; F. egg capsules of Busycon.

Qoniobasis comQlenjiS
(Pleuroceridae)

Cowpeloma decisurr)
(Viviporidac)

(§ (^ ^

Amnicoia comalensls

Cochliopo teKona
(AmnicolidQe)

Fig. 157. — Some common fresh-water branchiate snails.

FRESH-WATER MUSSEL AND THE SNAIL 391

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 cltfsters in well-pro-
tected places, such as in rooten wood or beds of humus. The eggs
are covered with thin shells which i^revent 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 in Helix.

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 w^orld. 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 artificiall}^ 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 alwavs 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

392 ESSENTIALS OF ZOOLOGY

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
(Fasciola hepatica) whose intermediate host is the small fresh-water
snail, Lymnaea bulimoides, causes the disease, liver rot in livestock,
particularly in the sheep.

Since shells are easil}^ fossilized they serve as excellent guides to
the geologists in determining the type of rock formation and relative
age of the strata.

References

Baker, F. C. : The Fresh Water Mollusca of Wisconsin, Madison, 1928, Wisconsin

Geol. & Nat. Hist. Survey.
Morris, Percy A.: A Field Guide to the Shells of Our Atlantic and Gulf Coasts,

Boston, 1947, Houghton Mifflin Co.
Tryon, G. W., and Pilsbry, H. A.: Manual of Conchology, 1885 to date, Acad.

Nat. History, Philadelphia.
Ward, H. B., and Whipple, Geo. C. : Fresb-Water Biology, New York, 1918, John

Wiley & Sons, Inc.

CHAPTER XXII
THE CRAYFISH, A CRUSTACEAN ARTHROPOD

Arthropoda (ar throp' 6 da, joint iootj is the name of the largest
known phylum of animals, and the crayfish is a member of this group.
As the name implies, all representatives of the phylum have paired,
jointed appendages and a definite tendency toward specialization of
them. Their bodies are triploblastic, segmented, bilateral, and covered
by a chitinous exoskeleton. The coelom is modified by a marked re-
duction 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 differentiation than was the case in annelids. In forms where
there is little or no differentiation of segments, the condition is re-
ferred to as homonomous, while a highly differentiated condition of
segments as found in most arthropods is spoken of as heteronoynous.
This group has fairly distinct head, thorax, and abdomen. The ap-
pendages 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 open 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
adai^tation 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.

393

394

ESSENTIALS OF ZOOLOGY

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

Class I. Crustacea, crayfish, crab, pill bug, barnacle, water flea,
etc.

73

O

O
<M

w
>

<V ^^
CO .

o
c ^

<v o

6X

1=

2 '^

O —I

ft

3-

O d

> >>

CO r >,
■M

o5
£o

2^
c c
H '^

.2 3

^>>

<v^
m w

at-

QJ O
1-1

«

00

bo

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.

CRAYFISH 395

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.

Class IV. Insecta, beetles, bees, locusts, etc., all with three pairs
of thoracic appendages and most of them with wings.

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

Fig-. 159. — Cambarus clarkii, the swamp crayfish, couimon through southern United

States.

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

This summary of the classification of the phylum has been in-
cluded here in order that the student may realize its magnitude and
the great variety of animals included. A more complete outline
may be found in the chapter on animal classification. The number
of species described under the phylum is approximately one-half
million, and there are large numbers still undescribed and unnamed.

Since the crayfish 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 two genera Cambarus and
Potomohms 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.

396 ESSENTIALS OF ZOOLOGY

Habitat and Behavior of Crayfish

For the most part crayfishes (crawfishes, crawdads, 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 swimming habits are rather peculiar in that they dart back-
ward 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 them.
The crayfish is quite well protected, due to its protective color which
matches the background, its chitinous skeletal covering, and its
pinchers. In spite of this, they are captured by water snakes, alli-
gators, turtles, fish (such as bass and gars), frogs, salamanders,
herons, and raccoons in particular. Many have been exterminated
by the drainage of swamps, and by their use as food for man.

External Structure

The chitin-covered body is divided into cephalothorax, abdomen,
and appendages. The cephalothorax is a compound division of the
body including the thirteen most anterior segments and is divisible
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

CRAYFISH 397

portion, while the portion posterior to the groove is the thorax. The
anterior end of the cephalothorax is drawn out to almost a point, and
this portion is called the i^ostrum. 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 branchial areas or hranchiostegites, 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 abdomen, 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 sternum; 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.

The appendages are paired, with one pair attached to each typical
segment. There are nineteen such pairs. They are all developed 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 swimmerets
or pleopods and all have the fundamental parts consisting of a basal
protopodite composed of coxopodite, joining the body and the basi-
podite; the exopodite or lateral branch and the endopodite or medial
branch each have many joints. The lirst 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 pos-
terior pair of swimmerets, attached 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 pos-
terior 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

398

ESSENTIALS OF ZOOLOGY

A2

Endopodite
Protopodite.:,

External opening
of nephudium.

^...•j Protopodite

A.l

Exopodite

Endopodite

Mx, 1

M.

_. Endopodite
Endopodite

Exopodite
Endopodite

Mp,l

Epipodite

Endopodite
Protopodite.,...

Mp.3

Endopodite .

.. Exopodite

Mx.2

■••Epipodite

Protopodite

Epipodite

Endopodite,,
Protopodite k

Epipodite

Fig-. 160. — Cephalic and thoracic appendages of the crayfish, ventral view. A.l,
antennule ; A. 2, antenna; L.i, fourtli walking leg: M, nianaioie: Alj/.l, firyt iiia\il-
liped ; M23.2, second niaxilliped ; 3I2J.3, third maxilliped ; Mx.l. first maxilla: .1/*S.
second maxilla. (From Newman, Outlines of General Zoology, The Macniillan
Company, after Kerr.)

CRAYFISH 399

endopodite. Joining the coxopodite (first segment of protopodite) is a
sheetlike structure whicli 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, offense, and de-
fense. The three anterior segments of the thorax bear three pairs of
biramous maxiUipeds. 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 bailer or
scaphognathite which fits over the gills and by its movement helps
circulate the water for respiration. The 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
with teeth and a fingerlike endopodite, which is tucked under the
anterior edge of the former. This appendage is used for chcAving.
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 antenmdes
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 compaTed, show different structure and
origin but similar function. During early development each of the
appendages is similar to all others. Some become modified with
development. Other illustrations 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 structures are found in manj^ ani-
mal groups and are used in establishing relationships. It has been
suggested that the parapodia of Nereis represent possible forerun-

400

ESSENTIALS OF ZOOLOGY

ners of crustacean legs. They are both typically biramous and both
take about the same position on the body, as well as having a simi-
lar segmental distribution. There is also considerable 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.

Carapace
removed

Sterna/

artery
f^asclc —

kntra/ thoracic
artery

Ventral sinus

Pericardial sinus

heart

Ostium

hasc/e

Cxonad

Intestine
Digestive §land
Etferent /esse/
GUI

fierve cord
Carapace

Vis 161 — Diagram of a cross section thirough the posterior thoracic region of a
crayfish. Arrows indicate the direction of flow of blood in the spaces and vessels.

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:
pleurobranchiae, attached to the sides of the thorax; podobranchiae,
arising from the epipodites of the thoracic appendages; and arthro-
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 Avay 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

CRAYFISH

401

402 ESSENTIALS OF ZOOLOGY

over the gills to facilitate the exchange of oxygen and carbon di-
oxide between the blood in the capillaries of the gills and the sur-
rounding water. The aerated blood is then carried to all of the
tissues of the body. .

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-
ment 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 Mood which circulates. It consists of the fluid plasma containing
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,
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 wlivch arise from the anterior end of the heart. The large dorsal
abdominal 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

CRAYFISH

403

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 abdominal 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

_ - Suproesophoqeal qanqlion

Circumesophoqeal connective

<^-Suhesopbageal qanqlion
)^yL Thoracic qanqWon

f?/ng for st<^rm\ artery

—PJ- abdominal gnngl/on

Loteml nerve

j:^^ Ventral ner\/e cord

Tegmenta/ division

Terminal qanqlion

Fig. 163. — Dorsal view of the nervous sj'^stem of crayflsli. The subesophageal
ganglion merges with tlie anterior ilioracic ganglia.

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.

404 ESSENTIALS OF ZOOLOGi'

The excretory system consists principally of a pair of large bodies
(green glands) 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 excre-
tory 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-
missures. This portion, which consists of the fused ganglia from
segments three to seven, is known as the suhesophageal ganglia.
Nerves go from it to the mouth parts, first and second maxillipeds,
green glands, esophagus, and muscles of the thorax. Each segment
posterior to the suhesophageal ganglia possesses a segmental ganglion
with branches to its respective appendages and muscles. The sense
organs include antennae, antennules, sensory hairs, statocysts, and
eyes. The antennae are tactile organs (sensitive to touch), the end-
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-
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

CRAYFISH 405

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 animal has an im-
paired sense of equilibrium. Experimenters have placed only iron
1^ 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
t through numerous peculiar contortions in attempting to respond to
pthese stimulations of orientation. Besides the above functions the

antennules provide the chemical senses of smell and taste,
p 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 hy 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
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 tl^e 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
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
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

406

ESSENTIALS OF ZOOLOGY

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
tei-med a modified api^iendage because an antennalike structure will
regenerate in case an eye is mutilated.

■* Cornea

"' Corneagen cells

- - • Crystalline cone ..--•-

Distal retinal
pigment cells

MM

{'..r.-W'-i

":-.y^

^^^^i

:.*4'^^

^^■■^

f'-HA't

'A ■: .. !

1 :< ■:'..■'.!

mU

m

iv;a;i

?^-|.V

%t::^

'i-'i.-i

■ !"i

t ■= -I';

<i 3

..•«•»!

M

li4

Tv-

'J-

vt^

».'•>:<

Jy^'i

^n:a?\i

?4 U; ^

Proximal retinal
pigment cells

------ Rhabdome ■

M

a

Basement
— membrane -

^ -(- — -Nerve fibers '-

Fig. 164. — Longitudinal section of ommatidia from eye of crayfish, a. Position
of pigment when iiglu is present : b, position ui pigmeni when in iIil- dark. i\u«.iee
tliat in tlie latter the dis al pigment is in the outward position and the proximal
pigment is concentrated inwardly. (From Hegner, College Zoology, The Macmillan
Company, after Bernhards. )

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

CRAYFISH 407

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 with 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 fertilized in the
fall may not be laid before spring.

In the case of Camharus 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 feynale 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 w^alking 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

408

ESSENTIALS OF ZOOLOGY

Pi. H. M

Pi.

B.

D.

Fig. 165. — Development of the crayfish. A, Young crayfish clinging to swini-
niereis of mother. B, Second larval stage {2) attached by its chelipeds to hairs
(PIH.) on swimmeret (PI.) of the parent. The molted shell of the first larval
stage (i) is clinging bv chelipeds. A portion of the egg-membrane (m) 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 (Tf). The second larva is
similarly attached to the molted shell of the first by filament Af. By means of these
filaments the young remain fastened to the mother during development. C, first
larva hatching through shell. D, the second larva. (Reprinted by permission aftt-f
Andrews, 1916, Smithsonian Contributions, Vol. 35.)

CRAYFISH 409

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. When the mature eggs are later 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 cemented to the
swimmerets hy 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 egg 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 davs after
hatching they pass through the first molt or ecdysis; that is, they
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 this 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.

410 ESSENTIALS OF ZOOLOGY

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
nviinerous smaller genera, like Daphnia, Cyclops, Cypris, Gammarus,
Aselhis, 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
ones concerning Crustacea on the Texas coast of the Gulf of Mexico. In
the Mississippi Valley and on the Pacific Coast the crayfish is used ex-
tensively 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.

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 very 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 sharpl}^ downward. The crabs are quite dilferent in shape
in that the cephalothorax is broader than it is long, the abdomen
is poorly developed, and folded sharpl}^ beneath the thorax. Crabs
of different kinds vary in diameter from a few millimeters to several
inches. There are four species of swimming crabs in the Gulf of
IMexico, of which the hhte or edible crab (Callinectes sapidus) is the
most important and best known. The lady crab and calico crab
are also interesting species. When the blue crab is captured at
molting time it is called the soft-shelled crab. At other times it is
the hard-shelled crab. They may be caught in baited nets or on
pieces of meat on a line with which they are brought to the surface
and lifted out in a dip-net. The hermit crab (genus Pagurus) is
smaller and lives in empty gastropod shells by backing into the shell

CRAYFISH 411

and carrying it around. Dae to the cramping and inactivity the ab-
domen has become soft and partly degenerate. The fiddler crab
(genus Uca) 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 side-
wise, and they 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 occasionally a large number
of these little crabs will be seen raising and lowering these enormous
pinchers in concert.

Asellus commu7iio is a common fresh-water form found in streams
and pools. A salt-water genus, Idotea, is found in the ocean. The pill

Fig-, 166. — Asellus, a common fresh-water crustacean. (Courtesy of General

Biological Supply House.)

bug (Armadillidiun) and the sow bug (Oniscus asellus or Porccllio)
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
reason they must live in moist places. They are a garden pest in
that they eat leaves of delicate plants. There are a number of
aquatic 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 around
on the bottom in fresh water. Gammarus is the best knoAvn fresh-
water form. The legs of representatives of this order are divided

412 ESSENTIALS OF ZOOLOGY

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 (Euhranchipus) are delicate, transparent and feathery
appearing. They are about three-fourths of an inch in length. They
swim with the ventral side up and their long, leaf-life appendages
hang from the body. These appendages serve also as respiratory
organs. They live in cool streams in the spring and fall, but the
summer is passed in the egg, which can withstand complete dryness.
Many of them are parthenogenetic, hence, males are rare. The com-
mon marine form is Artemia, often called brine shrimp.

The water fleas including Daphnia of order Branchiopoda, Cyclops
and Diaptoynus 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
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. Diaptomiis, 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. Arguhis is a genus
of Copepods w^hich 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-sv/imming in the
larval stage. Their entire life is spent in marine waters. There

CRAYFISH

413

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. SaccuUna 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 or Biog-enetic Law

A statement of this idea, which was developed by von Baer,
Haeckel, and others, and is so well illustrated by the comparison

Fig. 167.

Meg^IopiS

Fig. 168.

iSchizopod

Fig. 169.

(Courtesy of General

Rinfrlfi^o-^^o^^'^^T^T^^^ ^^^^^ ^^ the barnacle, Balanus.
Biological Supply House.)

theS^staJesTn °Hfpir^!^l.i^^^^^°P.^ ^.V'^^^^ ^^ developing: Crustacea. Crabs include
mese stages in their development. (Courtesy of General Biological Supply House.)

in tv^if; 169.— Schizopod or mysis stage through which the shrimp and lobster pass
in their development. (Courtesy of General Biological Supply House ) '"^^^^^^ ^^^^

of the phylogenic and embryonic stages of certain Crustacea, may
well be mentioned again at this point. This theory maintains that
certain developmental stages or structures of the individual are re-

414 ESSENTIALS OF ZOOLOGY

lated to ancestral conditions. That is, the individual in its develop-
ment tends to repeat in an abbreviated fashion the history of the
development of the race. Briefly stated ontogeny recapitulates 'phy-
togeny. There is still some doubt as to the validity of this generali-
zation in direct application.

A classical example Avhich is frequently cited is that of the devel-
opment of the shrimp, Penaeus, which hatches out as a nauplius larva,
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 a])pendages. The next molt brings on
segmentation and some change in form. This stage is called the Zoea
and resembles very closely the adult Cyclops of modern Copepoda.
The Zoea transforms 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 ^lalpighian tubules (in-
sects) as excretory organs, (8) organization of social life.

References

Greaser, E. P.: Decapod Crustaeeans of Wisconsin, 1932, Tr. Wisconsin Acad.

Sc, Arts, and Letters, No. 27.
Hay, W. P.: Crustacea of Indiana, ISOH, Proc. Indiana Acad. Sc.
Hegner, R. W. : Invertebrate Zoology, New York, 193.'}, The Macmillan Company.
Ward, H. B., and Whipple, Geo. C. : Fresh- Water Biology, New York, 1918, John

Wiley & Sons, Inc.

CHAPTER XXIIl

THE LOCUST, A EEPRESENTATIYE OF INSECTS
(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 sw^amp; from the tundra of
the north to the tropical pampas ; in trees ; on and in animals, as well
as irian, many of which are carriers of disease. They ravage our
crops and damage guv 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 of the Phylum Arthropoda has been upon
the earth from the Pennsylvanian 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 environ-
mental complex, and the success with which they have met the chal-
lenge is quite evident today.

Aside from the chitinous 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?

Insects are Arthropoda in which the body is divided into three
regions, the head, thorax, and abdomen. The head, whiqji 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-

415

416

ESSENTIALS OF ZOOLOGY

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. •

■•^,

'^ro*^/^;-

^iijljrM "*^

Fig. 170.— V^estern lubber grasshopper, Brachypephus magnus. .^'^^}^'^9^
B, female. This form is found on the plains. (Photographed by Leo r.
Murray. )

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

t^ocasT

417

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 may serve to illustrate
the general structure of the class Insecta.

13 1.4 15 16

Fig-. 171. — External features of a grasshopper. 1, maxillary palp.; 2, mandible;
S, labrum ; .',, clypeus ; 5, f rons ; 6, compound eye; 7, ocellus; 8, vertex; .0, antenna;
10, gena ; 11, pronotum ; 12, wing, mesothoracic ; IS, spiracle, thoracic; U, spiracle
of first abdominal segment; 15, auditory apparatus; 16, wing, metathoracic ; 11,
supra-anal plate; 18, podical plate; 19, cercus ; 20, ovipositor; 21, labial palp; S2,
femur of prothoracic leg; 23, coxa of metathoracic leg; 2Jf, trochanter; 25, femur;
26, tibia; 27, tarsus; 28, femur of metathoracic leg; 29, spiracle; 30, sternum; Si,
tergum. (Courtesy of General Biological Supply House.)

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 chitin. In many places two or more of these
rings have grown together, or are fused. Again, in certain regions

418 ESSENTIALS OF ZOOLOGY

of the body, parts of the segments may be lost. Regardless of the
amount of variation in this respect, we lind that the segments are
always grouped into three regions, Imown 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, Avhile the front of the head between the eyes is
called the frons.

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 wdth 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 below 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 lahrum, 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

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Fig. 172. — 1, The external structure of the grasshopper, Dissosteira spurcata.
al., Hind angle of lateral lob ; on., eresc ol the luatazune . c.p., crest of the i luzun*,* ;
g., gena ; g.g., genal groove ; I.e., lateral carina of the matazone ; yn.p., maxillary
palpus; t.i., transverse Incision. 2, Front view of the head of the grasshopper,
Dissosteira spurcata. a.g., Antennal groove ; ant., antenna ; c.c, lateral carina ;
c.e., compound eye ; c.f., central foveola ; e.g., carina of the antennal groove ; cl.,
clypeus ; c.o., central ocellus; fas., fastigium of the vertex; f.c, frontal costa ; g.,
gena; la., labrum ; I.e., lateral carina of the fastigium; l.p., labial jalpus ;
man., mandible ; m.c, median carina of the fastigium ; m.p., maxillary palpus ;
O.C., ocellus ; s.c, sulcation of the frontal costa ; t.f., tempora, temporal foveola ;
ver., vertex. (From Henderson, by permission of the Utah Agricultural Experiment
Station. )

420 ESSENTIALS OF ZOOLOGY

end ; the outer lobe or galea; and the maxillary palpus. The caudal
part of the mouth parts is the lower lip or labium, which is composed
of the suhmentu7n which acts as a hinge on the epicranium above;
a mentum; labial palpi, and two large outer flaps, the ligulae.

The prothorax is the segment to which the head it 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.

The next two segments, the mesothorax and metathorax, are made
up of sclerites that are intimately associated. 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 wdth the first ab-
dominal segment. The mesothorax and metathorax form a strong,
boxlike structure for the support of the w^ing and leg muscles. Like
the prothorax these segments are made up of separate plates, held
together by a tough, connecting membrane. These plates may, how-
ever, be divided into three groups : the tergum, 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 leers.
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 segment, the tihia, is slender, but about the same length

LOCUST 421

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 fanlike over
the abdomen when not in use. The metathoracic wings are used
in flight.

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 entirely 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.

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 suhgenital plate which forms the most
posterior ventral plate of the body.

422

ESSENTIALS OF ZOOLOGY

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LOCUST 423

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
pair. When these four pieces are brought together, their points are
in contact, forming a sharp organ by means of which the female
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 circulatory 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

424 ESSENTIALS OF ZOOLOGY

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
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-
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.
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
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 tubules, and
perform a similar function to that of 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.

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

LOCUST

425

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

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Fig. 174. — Digestive system of Rhomaelia microptera. A, anus ; C, crop ; Co.,
colon; G.C., gastric caeca; Int., intestine; M, mouth; M.2\, Malpighian tubules;
Oe., esophagus; R, rectum; Sal., salivary glands; St., stomach. (From White,
General Biology, The C. V. Aiosby Company,,

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Fig. 177.

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C. V. Mosby Company.) ,„ * * it rt

Fig. 176.— Male reproductive organs of ie?^omaeZ{amicropier-a ^e.; tes^e^ ; V.JJ„
vas deferens. (From White, General Biologu, The C. V. .viubb> Company.)

! deterens. (.i^rom vviiiLe, ixe/n:'/ i^t jj^^^^i/ nf - — ~-. . - „

Fig. 177.-Female reproductive organs otRnomaeliami^^Vtera.C^^^^
„•• O T., ovarian tube with eggs; Ov., oviduct; Ya., va^ma. (i<iom wmie, u-e/i.
~erai Biology, The C. V. Mosby Company.)

sac

LOCUST 427

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
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.

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 located
dorsal to the intestines. The sperms are borne in ducts w^hich
communicate with the penis, which consists of ehitinous styles used
in copulation with the female. In the female there are two ovaries,
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 duct.
The female is able to dig a hole in the ground with the ovipositor
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 metamoi'phosis.

References

Comstock, J. H.: Introduction to Entomology, Ithaca, N. Y., 1933, Comstock
Publishing Company.

Essig, E. O.: The Insects of Western North America, New York, 1926, The
Macmillan Company.

Fernald, H. G.: Applied Entomology, New York, 1926, McGraw-Hill Book
Company.

Folsom, J. W., and Wardle, E. A. : Entomology With Reference to Its Ecological
Aspects, Philadelphia^ 1934, P. Blakiston 's Son and Company.

Lutz, F. E.: A Fieldbook of Insects, New York, 1921, G. P. Putnam's Sons.

Snodgrass, R. E. : The Principles of Insect Morphology, New York, 1935, McGraw-
Hill Book Company.

CHAPTER XXIV

ANIMAL PARASITISM
(By Sewell H. Hopkins, Texas A. and M. College)

SOCIAL RELATIONS OF ANIMALS

It has been explained in the previous chapter that no animal is
ever entirely independent of others, since all plants and animals
are influenced, directly or indirectly, by all the other organisms in
the community. Most animals, however, can and do catch and eat
their own food, and such animals are said to be ''free-living."
But there are thousands of species which depend either completely
or partially on others to provide them with a livelihood. The vary-
ing degrees of dependence are called commensalism, mutualism, and
parasitism. The term symbiosis is applied to all cases of two different
kinds of animals living together, and thus includes commensalism,
mutualism and parasitism.

In commensalism,- one animal receives all of the benefit from the
association while the other is neither benefited nor harmed. The
jackal which follows the tiger and cleans up the carcass of the prey
when the tiger has eaten his fill, the small fishes which accompany
sharks and feed on the scraps wasted by the shark in feeding, and
the oyster crab which lives inside the oyster's shell and feeds on the
organisms brought in by the oyster's feeding movements, are examples
of commensalism.

Mutualism is the kind of symbiosis in which both animals receive
benefit from their association. One species of hydra (Hydra viridis)
is green in color because a certain species of green alga lives within
its cells; the alga receives protection and some nourishment from
its host, while the hydra benefits from the food manufactured by
the green plant. A case of mutualism so far developed that the
two animals cannot live separately is the relationship between
wood-eating termites and their intestinal protozoa. The termite
cannot digest the wood which it eats; the protozoa in the termite's
intestine break down the wood into a form in which it can be used
by the host ; on the other hand, the protozoa are absolutely depend-
ent on the termite for food and the proper environment; neither
termite nor protozoan can live without the other partner.

428

ANIMAL PARASITISM 429

The word parasitism in its broad sense applies to all cases in which
one animal depends on another to furnish it with food; for instance,
ornithologists call cowbirds parasites because they lay their eggs in
the nests of other birds and leave the foster-parents to feed and
care for the young cowbirds. Most zoologists, however, use the
word parasitism only for cases in which the parasite lives in or on
the body of its host; for example, lice live on the bodies of many
animals, and tapeworms live in them.

Origin of Parasitism

How did it happen that some animals became dependent on others
to furnish their food, that is, how did parasitism arise f There is
a considerable amount of evidence for the belief that all parasites
are descendants of free-living ancestors, and that these descendants,
in the course of generations, gradually became more and more de-
pendent on certain hosts, until in some cases they are now abso-
lutely unable to make their own living. For example, certain species
of nematodes which are free-living inhabitants of the mud at the
bottom of ponds and streams are able to live in the large intestine
of a frog if they happen to be swallowed by a frog. Other species,
very similar to the mud-dwelling nematodes, have found the intes-
tines of frogs such a good habitat that they live nowhere else ; in
other words, they have become parasites. Some intestinal parasites,
in the course of many generations, have lost their locomotor struc-
tures or even their digestive organs and yet continue to thrive
because there is little or no need for locomotion or digestion when
all food is brought to the parasite already digested by the host's
intestine. Since such degenerate parasites are unable to secure food
elsewhere, they are condemned by their peculiar structure to live as
parasites in the intestine of their host.

Degrees of Parasitism

Free-living animals which sometimes become parasites when they
get into another animal (by being swallowed, for instance) are
called accidental or occasional parasites, as in the case of the mud-
dwelling nematodes mentioned above. ''Vinegar eels," nematodes
in vinegar, sometimes establish themselves as harmless parasites in
the human urinary bladder. Facultative parasites are able to live

430 ESSENTIALS OF ZOOLOGY *

almost equally well as free-living animals or as parasites; many
leeches are facultative parasites. Obligate parasites, on the other
hand, cannot live without the host. Parasites which are free-living
during part of the life cycle, as in the case of horsehair worms and
some ticks and mites, are called temporary parasites, while animals
like Acanthocephala and tapeworms which are parasitic during the
entire life cycle are called permanent parasites.

The Successful Parasite

Like all other ways of living, successful existence as a parasite
requires certain modifications or adaptations in structure and func-
tion. Parasites which live on the outside of the host's body are
called ectoparasites; they must have special organs for attachment
in order to maintain their hold on the host ; for example, lice have
hooklike feet with which they hold on to the skin, hair, or feathers
of the host, and ectoparasitic trematodes have either muscular
suckers or chitinous hooks for attachment to the outside skin or to
the gills of the fishes on which they live. On the other hand, ecto-
parasitic insects have no need for wings, so fleas and bedbugs con-
tinue to thrive without them. Many ectoparasites, such as fleas, lice,
bedbugs, mites, and ticks, also have specially constructed mouth
parts for piercing their host's skin and sucking blood. Endopara-
sites, which live inside their hosts, also require special adaptations.
For maintaining their positions in the intestine or other organs they
must have some sort of attachment organ, such as the muscular
suckers of trematodes and tapeworms and the hooks of thorny-
headed worms. On the other hand, they live in the dark so eyes
may be entirely lacking without inconveniencing the endoparasite ;
usually all sense organs are either absent or very poorly developed.
There is little or no need for rapid locomotion, so most endo-
parasites have locomotor structures much reduced or even entirely
lacking. Many endoparasites also have less of a digestive system
than their free-living relatives ; parasites in the liver, lungs, blood
vessels, etc., usually have some sort of digestive apparatus, but
many intestinal parasites, such as tapeworms and thorny-headed
worms, have no sign of digestive organs whatever, but depend on
the host to furnish them with food already digested and ready for
absorption. Most endoparasites have their reproductive organs
enormously developed, sometimes so much so that 90 per cent

ANIMAL PARASITISM

431

of the body is taken up by the reproductive system. This is in
keeping with the general rule that animals whose offspring have
the least chance to survive usually produce the largest number
of ova. In the case of a tapeworm, for instance, the chance of any
one egg being eaten by the right kind of host, so that it can

Fig. 178. — Arthropod para.sites. A, human itch mite, female. Sarcoptes scahiex,
ventral surface; B, ventral surface of male icCh niite ; C, body louse (coolIc),
Pediculus humanus corporis; D, head louse, P. humanus capitis; E, crab louse,
Phthiriiis pubis. (From Sutton and Sutton, Diseases of the Skin, The C. V.
Mosby Company.)

develop into another tapeworm, is only one in a million, and tape-
worms would have become extinct long ago except for the fact
that each tapeworm produces many millions of eggs. The peculiar
habitat and mode of life of endoparasites also make necessary

432

ESSENTIALS OF ZOOLOGY

peculiar adaptations in the functions or physiology of the parasite.
A parasite in the intestine, for instance, must be able to carry on
respiration in almost complete absence of oxygen, must secrete
substances to counteract the digestive juices of the host in order
to prevent its being digested, must be adapted to a high concen-
tration of salts, acids, and other substances in solution in the fluid
around it, and if in a warm-blooded animal must be able to live
at a relatively high constant temperature. The fact that no host
is immortal makes it necessary for a parasite to have some special
provision for its offspring to escape to another host, in order to
maintain the existence of the species; this necessity is met by various
peculiar adaptations in the life cycle or development, such as the
complex succession of larval stages in the endoparasitic trematodes
and cestodes. i

Fig. 179. — Diagram of the tunnel of an itch mite in human skin. The female
animal is depositing eggs. (Reprinted by permission from Introduction to Human
Parasitology by Chandler, John Wiley and Sons, Inc. Adapted from Riley and
Johannsen. )

Some parasites are able to carry on parasitic activities without
injuring their hosts, while others may weaken or destroy the host.
Parasites which injure their hosts are said to be pathogenic (disease-
producing), while those which cause no appreciable injury are said
to be nonpathogenic or commensal. Since most parasites cannot live
without their hosts, a parasite which shortens the life of its host
destroys its own home and means of livelihood; nonpathogenic para-
sites are more likely to be successful in the long run, and are there-
fore more abundant. Some parasitologists consider pathogenic
parasites to be imperfect parasites because they are not quite per-
fectly adapted for successful parasitic life, while nonpathogenic

ANIMAL PARASITISM 433

species are considered perfect parasites. However, no hard and fast
line can be drawn between the two. Many parasites which are so
perfectly adapted to their customary host that they produce no ill-
effects have been found to be strongly pathogenic to other hosts
where the adaptation is less perfect; for example, certain trypano-
somes which are harmless to the antelopes of Africa, their natural
hosts, produce the highly fatal African sleeping sickness when in-
jected into men.

Means of Infection and Transmission

Many different means of transfer from host to host have been de-
veloped by the various kinds of parasites. These may be classified
as below :

A. Passive transmission.

1. In food or in water.

2. By bite of insects.

3. By sexual intercourse.

4. By direct contact.

- B. Active invasion under own power.

By ''passive transmission" is meant the transfer of eggs or larvae
from one host to another without any action of their own. For ex-
ample, the eggs of cestodes and of some nematodes, such as Ascaris,
pass out of the host's intestine in the feces; if food of other animals
is contaminated by these feces, animals which eat this food will
swallow the eggs, which hatch into larval worms within the diges-
tive system of the second host and thus establish a new infection.
Sheep may become infected with liver flukes by eating the encysted
larvae on grass or swallowing encysted larvae while drinking water.
Trichina larvae encysted in hog nieat develop into adult worms in
. the human intestines if infected pork is eaten raw or improperly
cooked.

The second means of passive transmission is also widely used.
Malaria parasites, the trypanosomes which cause African sleeping
sickness, and many parasites of domestic and wild animals are car-
ried from infected individuals to new hosts by biting insects which
suck up the parasites with the blood and inject them into the new host
w4th their salivary secretions.

434 ESSENTIALS OF ZOOLOGY

The third means is used by only a few parasites, most of them
Protozoa. " The spirochete, Treponema, which causes syphilis, a flagel-
late protozoan called Trichomonas vaginalis, and a trypanosome para-
sitic in the reproductive organs of horses are examples. Some
nematode parasites of insects are also transmitted in this way.

The fourth means includes a few cases, such as the acquiring of
dog tapeworm by people who kiss dogs and the transmission of pin-
worm from the hands of infected people who do not have cleanly
habits.

Hookworms and the human blood flukes, called schistosomes, are
examples of parasites that invade new hosts under their own power.
Their larvae are able to penetrate the skin.

Parasitism and Host Specificity.

Since the beginning of the scientific study of parasitism, it has
been recognized that different animals have different parasites; for
instance, the parasites found in and on goats are nearly all different
from those of man. Some of the early parasitologists leaped to the
conclusion that each species had its own peculiar species of parasites
found nowhere else, and carried this idea of species specificity so
far that they considered presence in different hosts to be sufficient
evidence of specific difference of the parasites.

Modern knowledge reveals that while some parasites are actually
species specific others have a wide range of hosts. Thus the beef
tapeworm, Taenia saginata, is found in the adult stage in man only,
but the fish tapeworm, Diphyllohothrium latum, seems to be able to
live in nearly all mammals which eat fish.

Three main factors determine whether a parasite will infect any
given host: (1) opportunity for infection of host, determined by
habits or mode of life of parasite and host (malaria parasites may
be injected into any land animal by bite of mosquito, but strictly
aquatic animals, such as fish, would not be bitten) ; (2) the environ-
mental condition of the habitat furnished by the body of the host,
involving such factors as body temperature, nature of outside sur-
face, size, chemical content of internal organs, etc. (intestinal para-
sites of birds are seldom found in mnmmals, which have lower body
temperatures, and parasites adapted to the oxygen-rich interior of
a frog's lung can find no suitable habitat in a lungless fish) ; and
(3) ability of the parasite to adapt itself to the wide range of environ-
mental conditions found in different hosts ; thus Z>. latum, though it

ANIMAL PARASITISM

435

finds its optimum conditions in the human intestine, is adaptable
enough to survive under the very different chemical conditions found
in the dog, while Taenia saginata is usually unable to survive under
these conditions.

PARASITES AND THE GROUPS IN THE
ANIMAL KINGDOM

What kinds of animals are infested by parasites? Not only all
phyla and classes, but all known species and probablj^ all individuals
of higher forms serve as hosts for some kind of parasite. Even in

Fig. 180. — Giardia lamblia, an intestinal flagellate. A^ face view ; B, semiproflle
view ; C, cyst. (Reprinted by permission trom Introduction to Human Parasitology
by Chandler, John Wiley and Sons, Inc.)

the microscopic Protozoa many individuals harbor still smaller proto-
zoans. For example, several species of parasitic Protozoa are found
in Amoeba proteus.

To which of the main groups of animals do parasites belong? All
animal phyla, except Echinodermata, include some species which
lives as parasites, but the great majority of parasites belong to one
of these four phyla: Protozoa, Platyhelminthes, Nemathelminthes,
and Arthropoda.

Protozoa. — Of the four classes in this phylum, one, Sporozoa, is
entirely parasitic; the other three (Sarcodina, Mastigophora, In-

436

ESSENTIALS OF ^iOOLOCY

fusoria) also contain a number of parasitic forms. Examples of
parasitic Sarcodina are the three common human amoebae, Enda-
moeha histolytica, which invades and destroys the intestinal lining,

7b cerebrospinal ffuld causing sleeping
sickness and death -

Transmission by
bite of tsetse fJLy.

4^ Man, Antelope, etc.

Trypanosomes

in human blood causing
Trypanosomc jz>/zr-

Transmission by btbe
of tsetse f>y.

Tsetse Fly

Forms in salivary glands
ready for re. infection.
\20'''-30*'^ day)

Crltbidial /brms ia

salivacy jglands
(2. or o doya later)

Forms in mtdgut,(VSM
after infective lyieal).

newly arrived form in
;3afivar;y aland.
iJ<a*^to20^''dqya.)

Long ^lender forms In prove ntriculu.3.
("about I0*>'tol5*''days)

Fig. 181. — Life history of Trypanosoma gatnhiense. (Reprinted by permission
from Introduction to Human i^arasitologii by Chandler, John Wiley and Sons,
Inc.)

thus causing amoebic dysentery ; Endmnoeha coli, a harmless com-
mensal in the intestine ; and Endamoeba gingivalis, a very common
parasite in the human mouth, usually harmless but sometimes ap-
parently injurious to the gums. Examples of parasitic Mastigophora

ANIMAL PARASITISM

43-

are the human intestinal flagellate, Giardia lamhlia, and the blood-
inhabiting trypanosome, Trypanosoma rhodesiense, causative agent of
African sleeping sickness which is carried by the tsetse fly. Ex-
amples of parasitic Infusoria are the human intestinal ciliate, Balan-
tidium coli; the various species of Opalina, and related genera found
in the excretory bladder or cloaca of frogs and toads. Of the thou-
sands of species of Sporozoa, all of which are parasitic, probably the
best known are the three species of the genus Plasmodium, which cause
human malaria, and Babesia higemina, which produces Texas tick
fever of cattle.

Fig. 182. — Tsetse flv, Glossina, the transmitting agent for trypanosoma. which
causes African sleeping sickness. (Reprinted by permission from Introduction to
Human Parasitology by Chandler, John Wiley and Sons, Inc.)

Platyhelminthes.— This phylum also contains four classes, two of
which, Trematoda (flukes) and Cestoda (tapeworms), are all para-
sitic, while the other two, Turbellaria and Nemertinea, are mainly
free-living but contain some species which are parasitic on aquatic
invertebrates. Among the best known examples of Trematodes are
Fasciola hepatica, the sheep liver fluke; Clonorchis sinensis, the
Chinese human liver fluke ; and Schistosoma haematoUum, one of the
three species of human blood flukes. Probably the best known tape-
worms are Taenia saginata, the beef tapeworm. Taenia solium, the
pork tapeworm and DiphyUohothrium latum, the broad fish tape-
worm, all three common parasites of the human intestine, and Echino-

438

ESSENTIALS OF ZOOLOGY

COCCUS granulosus, a dog and wolf tapeworm whose larval stage is
the cause of a horrible human disease.

Nemathelminthes. — The single class Nematoda includes at least
95 per cent of the species in this phylum ; most of them are free-
living, but there are also thousands of parasitic species. Examples
of parasitic species are the human hookworms, Necator americanus,
the American hookworm, and Ancylostoma duodenale, the Old World
hookworm ; Ascaris lumhricoides, the large intestinal roundworm of
hog and man ; Dracunculus medinensis, the Guinea worm, often over
a yard long, which crawls around under the human skin (believed
by some to be the ''fiery serpent'' mentioned in Exodus) ; Trichinella
spiralis, which causes the often fatal human disease, trichinosis, when

Fig. 183. — Balantidium coli, an infusorian parasite of the intestine. Active form
from intestine. c.v., anterior contractile vacuole; cyt., cytostome ; f.v., food
vacuole; n, nucleus. (Reprinted by permission from Introduction to Human Para-
sitology by Chandler, John Wiley and Sons, Inc.)

its larvae, encysted in pork, are eaten by man; and Wuchereria han-
crofti, the filaria which is injected into the human blood by certain
tropical mosquitoes and causes elephantiasis, a disease in which the
infected limbs may become larger than the body of the victim.
Onchocerca volvulus, transmitted by certain biting flies, is a com-
mon cause of blindness in some parts of Mexico. Besides the human
nematodes there are thousands of others parasitizing lower animals,
both vertebrates and invertebrates. . The other two classes of the
phylum Nemathelminthes are entirely parasitic ; the Acanthocephala,
or thorny-headed worms, are common intestinal parasites of many
vertebrates, including the hog and occasionally man ; the Gordiacea

ANIMAL PARASITISM

439

or horsehair worms are parasites of insects until nearly mature; they
crawl out of their insect hosts when the latter fall into water, be-
come sexually mature, and lay their eggs.

Arthropoda, — All of the classes in this phylum are predominantly
free-living, but several classes also include parasitic species. The
class Hexapoda or Insecta contains, besides several hundreds of

B

Fig, 184. — Elephantiasis, some extreme cases. A, of legs and feet; B, of scrotum ;
C, varicose groin gland ; D, of scrotum and legs ; E, of mammary glands. ( Re-
printed by permission from Introduction to Human Parasitology by Chandler, John
Wiley and Sons, Inc. A and B sketched from photographs from Castellani and
Chalmers ; C, D, and E from Manson. )

thousands of free-living insects, the parasitic fleas, lice, and bedbugs ;
the class Arachnida, characteristically free-living, contains the para-
sitic ticks and mites, and the class Crustacea, though mostly- free-
living, includes a number of species parasitic on fishes and other
aquatic animals. While most of the parasitic arthropods are ectopara-

440

ESSENTIALS OP ZOOLOGY

sites, there are also a few endoparasitic species. For example, the
horse bot, Gastrophilus, which is the larva of a fly, is parasitic in the
stomach of horses ; long wormlike arachnids known as Linguatulids
or tongue worms are found in the intestines of some reptiles and
mammals ; and Sacculina, a crustacean, parasitic on crabs and lobsters,
sends rootlike outgrowths all through the body of its host, although
the saelike bodj^ remains on the outside.

Some Representative Parasites

Protozoa. — The very small amoeba-like protozoans of the genus
Endamoeha are examples of parasites only slightly modified for
parasitic life. There are two distinct stages in the life cycle, the

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■---nb.c.

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cVtr.W-

"^^Mi.

vi

^^■■■■

■"-n.

Fig. 185. — Amoeba histolytica, one of the important protozoan parasites. It is
the causal agent of amoebic dysentery. A, Stained vegetative amoeba ; B, cyst with
four nuclei ; n, nucleus, showing peripheral chromatin granules and central karyo-
some ; r.b.c, ingested red blood corpuscles ; chr.b., chromatoid body. (Reprinted by
permission from Introduction to Human Parasitology by Chandler, John Wiley
and Sons, Inc., after Dobell. )

active form being much like a small amoeba except that the pseudo-
podia are shorter and move more slowly; these active forms iinaJly
round up and become surrounded by a semirigid, resistant cyst wall.
In this encysted condition the Endamoeba is passed from the host
with the feces or other body excrements. While in the encysted con-
dition the parasite divides b}^ binary fission into first two, then four,
and finally (in E. coli) eight little amoebae. If the cyst is swallowed
by another host, the cyst wall is dissolved and the four or eight
young amoebae come out to begin the active stage again. The com-
mon Endamoebae of man are E. gingivalis which lives in the mouth

ANIMAL PARASITISM

441

and is usually transmitted by kissing, and the two intestinal species
E. coli (nonpathogenic) and E. histolytica. The latter species breaks
down the cells of the intestinal lining by means of enzymes which it
secretes, and then ingests the broken cells in the same way that the
common free-living amoebae take in their food. The disease caused
by E. histolytica is kno^vn as ''amoebic dysentery." Infection occurs
as the result of eating food or drinking water which has been con-
taminated by the feces of infected people, as in the case of the
Chicago hotels where contamination of drinking water by water
siphoned up from the toilet drains into the water pipes caused a
serious outbreak in 1933.

Fig. 186. — Endamoeba coli. A, stained vegetative amoeba; Bj cyst with eight
nuclei ; n., nucleus, showing coarse peripheral chromatin granules, chromatin
granules in "clear zone" between periphery and karyosome which is eccentric in
position; chr.b., remnant of chromatoid body. Numerous food vacuoles in vegeta-
tive form. (Reprinted by permission from Introduction to Human Parasitology by
Chandler, John Wiley and Sons, Inc., after Dobell. )

The malaria parasites, of which there are three species infecting
man (Plasmodium vivax, P. falciparum, and P. malariae, each caus-
ing a different form of malaria), are Protozoa belonging to the class
Sporozoa, and are very highly modified for parasitic life. The adapta-
tions for parasitism and for transmission from host to host involve
a very complex life cycle. The two main phases of the life cycle are
the vegetative or schizont stage (merozoites) and the sexually repro-
ductive or sporont stage. The biology of this parasite has been dis-
cussed in the earlier chapter on Protozoa under class Sporozoa and
Economic Relations of Protozoa.

442

ESSENTIALS OF ZOOLOGY

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ANIMAL PARASITISM 443

Nematodes. — Of the thousands of species of parasitic nematodes,
space permits mention of only a few which are particularly impor-
tant because of their danger to man.

Hookworms. — In the United States the most important human
nematode parasite, from the public health viewpoint, is the American
hookworm, Necator americanus. Although called the ''American hook-
worm" this species probably came originally from Africa and was
introduced into America by the negro slaves. The pioneer work on
hookworm in the United States was done by Dr. Charles W. Stiles
in 1901. Hookworms are slender threadlike nematodes about one-
half inch long; the females are tapered to a point at each end, while
the slightly smaller males have on the posterior end a fanlike ex-
pansion, the copulatory bursa, with curved riblike supports. Both
sexes have a large mouth containing hooklike chitinous teeth by means
of which they tear holes in the walls of the intestine and start blood
flowing from the w^ounds. A muscular esophagus leading back from
the mouth cavity gradually broadens into a large muscular bulb ; by
means of rhythmic contractions and expansions of the bulblike esoph-
agus blood is drawn into the mouth and forced down into the straight
intestine where some of it is digested and the rest passes on through
and out of the anus near the posterior end. Because of the large
number of worms present there is a serious loss of blood, resulting in
anemia and lack of energy ; in children the growth is stunted or re-
tarded by hookworms, and often there is also a lack of proper mental
development. Individuals, very heavily infected during childhood
and early youth, may fail to develop sexually. Treatment is fairly
easy, hookworms being easily killed by doses of anthelmintics, such
as carbon tetrachloride and hexylresorcinol (which are poisonous and
should be taken only under doctor's supervision).

Each female hookworm produces -9,000 eggs per day; these eggs
pass out with the feces of the host ; if the infected person defecates
on the ground, the eggs hatch and the larvae crawl around in the
soil; there they develop into infective larvae w^hich live for several
months on the surface of the ground. If bare human skin comes in
contact with these microscopic worms they bore through it to the
blood vessels, are carried by the blood to the lungs, then migrate up
to the trachea and pharynx, into the esophagus, then down through
the esophagus and stomach to the small intestine, meanwhile increas-

444 ESSENTIALS OP ZOOLOCJY

iiig in size, so that on arrival in the small intestine they are ready
to attach themselves to the wall of the intestine, feed on the blood
of the host, and become adults.

Since the larval hookworms must go through part of their develop-
ment in the soil, and a person can become infected only by direct
contact with contaminated soil, the distribution of hookworm in the
United States is determined by the following factors: (1) freezing
of soil in winter (kills the larvae) ; (2) texture of soil (hookworm
larvae live best in light, sandy loams) ; (3) moisture (hookworm
larvae can live only in damp soil) ; (4) customs of the people in dis-
posal of feces; the bad hookworm districts are sections in which sani-
tation is very primitive and sanitary toilets are not in universal use;
deposit of feces on the ground is particularly conducive to spread of
these animals. i

From the public health standpoint hookworm disease is a social
problem rather than a medical problem. Few if any people are killed
by hookworm, and infected individuals are easily cured if they go
to a physician for treatment. On the other hand, such a large pro-
portion of the population, in hookworm territory, are kept in bad
health and a listless condition that the social welfare of the whole
community is injured. Prevention of hookworm disease is theoreti-
cally easy: hookworms could be killed out of a community in a few
months if everyone would defecate only in sanitary toilets, if every-
one would take treatments for hookworms at the same time, or if
everyone would wear good shoes ; but so far, it has been impossible to
get the cooperation of all the people in hookworm districts. The work
of the medical profession, with the help of certain state agencies, has
reduced hookworm disease in the United States, but there are still
considerable districts in which over 20 per cent of the population are
infected. In parts of East Texas 33 per cent of the people examined
have hookworm, even now.

Trichina. — Trichinella spiralis, commonly known as Trichina, is an
example of a nematode with an alternation of hosts and a passive
means of transmission. The microscopic larvae are encysted in the
muscles of various meat-eating animals, being particularly common
in hogs and rats ; within the cyst, the larva is coiled in a tight spiral,
which gives the species its name. If pork containing trichina cysts
is eaten by a man, the cysts are digested off in the stomach, the larvae
become active and penetrate the mucosa of the small intestine to

Fig-. 188. — Ancylostoma duodenale, female and male, with head of Necator
americanus drawn to same scale, an., anus ; h, bursa ; b.c, buccal capsule ; cem.gh,
cement gland ; ceph.gl, cephalic gland ; cerv.gl, cervical gland ; cerv.p., cervical
papilla ; cL, cloaca ; c.sj)., caudal spine ; ex.d., so-called excretory duct ; int., in-
testine ; n.ceph.gl., nucleus of cephalic gland ; n.7\, nerve ring ; oes., esophagus ; ov.,
ovary; ovej., ovejector ; sp., spicules; t, testes; ut., uterus; vag., vagina; v.s.,
yesicula seminahs. (After Looss from Chandler, Hookworm Disease. Reprinted
by permission from Chandler, Introduction to Human Parasitology, John Wiley and
Sons, Inc.)

446

ESSENTIALS OF ZOOLOGY

moult. They soon become full grown, sexually mature adults, some-
times beginning copulation only forty hours after being swallowed.
The adult worms are short-lived and are not harmful to the host, but
each fertile female, about a w^eek or ten days after the infected meat
was swallowed, begins to produce thousands of microscopic larvae
which she deposits in the walls of the intestine, usually directly into
a lymph vessel or blood vessel. The larvae are carried by the blood
or lymph to the heart, and from there they are carried by the blood
to all parts of the body. The larvae w^hich reach voluntary muscle

Fig. 189. — Larvae of Trichinella spiralis, encysted in voluntary muscle. The
adults are parasites in the intestine, (Photomicrograph by Albert K. Galigher,
Inc.)

enter the muscle fibers and coil up into spirals, grow rapidly to a
length of about 1 mm., and in a few weeks become enclosed in a cyst
of connective tissue which grows around them. Trichina larvae are
likely to be most abundant in active muscles, such as the diaphragm,
the intercostals, and the muscles of the larynx, tongue, and eye. The
symptoms of trichinosis (the disease caused by trichina worms) vary
according to the stage of the infection. In the first week, when the
adult females are burrowing into the walls of the intestine to de-
posit their larvae, the symptoms are so much like those of typhoid

ANIMAL PARASITISM 447

fever that many cases are diagnosed as typhoid by good physicians.
Nine or ten days after the beginning of infection, when the larvae
are migrating to the muscles, there are severe muscular pains and
aches (sometimes diagnosed as rheumatism), and the inflammation of
the muscles used in breathing, chewing, etc., may interfere with these
functions. As the parasites become encysted, about six weeks after
infection, pains become worse and swelling of the infected parts
occurs, accompanied by anemia and skin eruptions. If the victim
survives this period he usually recovers, as the parasites are now
walled off by cysts of connective tissue formed by the host ; later
calcium carbonate is deposited in these cysts, walling off the parasites
so completely that they die, but the calcified cysts remain as hard
grains in the muscles and may cause some rheumatic pains for years.
Until the worms are completely walled off fever is caused by poison-
ous substances produced by the larval worms. Recent studies of the
cadavers used in medical schools have revealed that about 20 per
cent of the American population probably have cases of trichinosis
in some degree at some time during life, since about this proportion
of the cadavers had trichina cysts in the muscles.

All danger of trichinosis can be avoided by cooking pork thor-
oughly before eating it, as the larvae are killed by a temperature of
55° C. (131° F.). Investigations of the United States Bureau of
Animal Industry, in which the author assisted, indicate that pre-
pared sausages seldom contain living worms, most of them being
killed by the salts and seasoning or by long-continued cold storage ;
the greatest danger is from fresh pork. Contrary to popular im-
pression. Federal inspection does not guard against trichina, as there
is no effective way to inspect meat for trichina on the large scale that
would be necessary.

Trematodes. — Schistosoma haematobium, a Human Blood Fluke. —
The blood flukes (Family Schistosomatidae) are distinguished from all
other trematodes by having separate sexes. The male has a thick body
with the lateral edges bent ventrally, thus forming a long groove on
the ventral surface, the gynecophoric canal. The female is long and
slender, almost threadlike in some species; when an adult male and
female happen to come in contact, the male folds his body around
the female so that she is held fast in the gynecophoric canal, and the
pair begin copulation. After once becoming paired they remain in
copula during the rest of their lives. Three species of the genus

448 ESSENTIALS OK ZOOLO(JY

Schistosomurn are human parasites: Schistoso7na mansoni, found in
Africa and the tropical parts of the New World, 8. japonicum of
Japan and China, and S. haematobium, the Egyptian blood fluke
found in north Africa and southwestern Asia. Schistosoma haema-
tobium is a parasite in the large blood vessels of the rectum and
urinary bladder. The female lays an enormous number of eggs,
which collect in the capillaries of the bladder and intestinal walls
and block the flow of blood, causing the infected parts to become
swollen and ulcerlike. Eventually the eggs are released into the
lumen of the rectum or bladder and pass out of the host's body in
the feces and urine, along with considerable quantities of blood from
the torn tissues. Aside from the pain caused by the egg-filled swell-
ings, the loss of blood is the most serious effect of blood-fluke infection.
Infected individuals are kept in a run-down anemic condition. The
parasite is a serious public health problem in Egypt, where 80 per
cent of the population of the Delta region is infected.

The egg must fall into water for the embryo to develop into a
ciliated larva, the miracidium. When the miracidium is fully de-
veloped, the cap or operculum on the end of the egg shell is pushed
open and the liberated miracidium begins to swim around in the
water. Certain species of snails seem to emit a chemical which at-
tracts schistosome miracidia; if a snail of the right species is in the
vicinity, the miracidium swims to it and enters its body. If no suitable
snail is available, the miracidium dies after swimming a few hours.
After penetration into a snail the miracidium loses its cilia and de-
velops into a long sausage-shaped sac, the sporocyst, without any
recognizable organs. Germ cells within the sporocyst develop into a
number of young sporocysts, which escape from the mother sporocyst
into the tissues of the snail, grow to full size, and then in turn give
birth to a new generation of sporocysts, or, under some conditions,
these sporocysts may give rise to a different kind of larva, the
cercaria, which has a. long, forked, muscular tail, a pair of eyespots,
two suckers (one anterior and one midventral), and a rudimentary
digestive system. When fully developed, the cercaria forces its way
out of the snail and begins to swim through the water. If unsuccess-
ful in finding a host, the cercaria dies in a few hours, but if it comes
in contact with the skin of a man, or any other mammal, the cercaria
enters the skin, with the aid of glands in the head region which seem
to be used in digesting or destroying the skin tissues. After pene-
trating the skin the cercaria soon finds its way into a blood vessel

ANIMAL PARASITISM 449

and begins a voyage through the circulatory system, carried along by
the current of the blood stream, meantime growing into an adult
fluke. The mating of males and females usually occurs in the larger
veins, and the pair moves to the veins in the walls of the rectum and
bladder, where egg-laying begins.

Irrigated districts, such as the Nile Delta, are especially favorable
for the development of blood flukes because the eggs have more
chance of getting into water and because the field workers often get
into the water while working around the irrigation ditches. The
chances of infection are increased by the customs of defecating and
urinating into the water, and using water from irrigation ditches for
drinking and washing. In Japan the number of human blood fluke
cases has been greatly decreased by improved sanitation and by
killing the host snails. In Egypt, public clinics (by injecting fuadin

Fig-, 190. — GlonorcMs sinensis. Oriental human liver fluke, showing male and
female reproductive organs. (Photomicrograph by Albert EJ. Galigher, Inc.)

into the blood) treat thousands of cases of this disease, but it will
probably remain a public health problem for years because of the
refusal of the Egyptian peasants to change their old customs.

Clonorchis sinensis, the Chinese Liver Fluke. — This is an impor-
tant human parasite in parts of the Orient. Clonorchis sinensis also
occurs in other fish-eating mammals, including dogs, cats, and pigs.
The adult worm lives in the bile passages of the liver. In man it
often causes enlargement of the liver, diarrhea, jaundice, anemia,
and extreme weakness, sometimes resulting in death. Hundreds of
worms may be found in a badly infected man. The eggs laid by the
adults pass from the liver to the intestine of the host by way of the
bile duct, then pass from the body in the feces. Snails probably be-
come infected by swallowing the eggs, while feeding on fecal matter
in the water. After hatching, the miracidia migrate into the lymph
spaces of the snail and develop into elongated sporocysts, each of
which gives birth to a number of redia (differing from sporocysts by

450 ESSENTIALS OF ZOOLOGY

possessing a pharynx and a rudimentary gut). Each redia gives
birth to six or eight cercariae, which emerge from the snail and swim
around in the water by means of a very large, undivided tail. When
a cerearia comes in contact with a fish, it enters the skin and encysts
either in the skin or in the muscles just below the skin. It is now
called a metaceixaria, or agamodistoynum. Man becomes infected by
eating these metacercariae in poorly cooked fish. When swallowed,
the cysts are dissolved by digestive juices of the host, the larva
escapes into the duodenum, migrates up the bile duct to the liver,
and there develops into an adult. There is evidence that the adult
Clonorchis may live as long as twenty years in the liver of man.

Treatment of clonorchiasis is not very satisfactory. Prevention is
simple : avoid eating fish which are not thoroughly cooked.

It will be noted that the Clonorchis life cycle involves three hosts :
a mammal as the final host, a snail as the first intermediate host,
and a fish as the second intermediate host. Infection of the fish is
by active invasion of the cercariae, and infection of the final host
is passive.

Other Trematodes. — One of the best known parasites of domestic
animals is the sheep liver fluke, Fasciola hepatica, which occurs in
all sheep-raising countries in which wet pastures are common. It is
also a common parasite of goats and cattle. Like Clonorchis, Fasciola
lives in the bile passages, and its eggs pass out with the feces of
the host, but unlike Clonorchis, the eggs hatch in water and the
free-swimming miracidium actively seeks and penetrates the snail
host. Sporocysts in the snail give rise to rediae which produce
cercariae, but the cercariae encyst on any surface, including grass
blades and even the surface film of the water. Sheep become infected
by eating grass bearing encysted larvae or by swallowing floating
cysts while drinking water. Fascioloides magna, the large liver fluke
of cattle and sheep in Louisiana, Arkansas, and Texas, is very similar
in structure and life history.

Other important flukes are the human intestinal fluke, Fasciolopsis
huskii, which is common among the Chinese, who become infected by
eating the cysts on various aquatic food plants; the human lung
fluke, Paragonimus westermanii of eastern Asia, where the natives
become infected by eating the encysted larvae in fresh-water crabs
and crayfishes; C otyloplioron cotylophorum, a stomach parasite oi
cattle in Louisiana, and Dicrocoelium lanceatum, a common liver fluke
of herbivorous mammals in Europe and Asia.

ANIMAL PARASITISM

451

Numerous ectoparasitic trematodes, of the Subclass Monogenea,
occur on the skin and gills of fish and are often of economic im-
portance because they kill goldfish and other aquarium fishes, and
also young fishes in state fish hatcheries.

Proboscis (extruded)

Nerve-ganglion

Egg

7ol]c-cell8

.< Intestine

-Genn-celte

Cercarl»

Vectnl sucker.
Mephridium

'Genn>eellf

spoSfyst^-^Vfn^H^ ;iwLi^o^%-^"''^^^ Tiepaftca. A, egg; B, miracidium ; C,
l^SSt^fkn^^^.iJ'r^'^?'^< ^^^"^^^^^ '.y' ^"cy«ted stage- i/, adult (showing

dWf^Qfivo orwi ^AC^r' '^"''^^' ^\' v.ci^d,ija,; tr, eiicystea stage; ±t, adult (showing
Company ^^^^^^tory systems). (From Hegner. College Zoology, The Macmillan

452

ESSENTIALS OF ZOOLOGY

The Tapeworms (Class Cestoda). — Cestoda differ from Trematoda
in the complete lack of a digestive system. In fact, cestodes never
have any sign of a digestive organ at any time during life; they
receive their nourishment by absorbing through the surface of their
bodies the food already digested for them by the host. Most cestodes

Fig. 192. — Life history of the liver fluke. Fasciola hepatica. A, adult in liver
of sheep ; B, freshly passed egg as it leaves the body of the host ; C, developing
embryo, ready to hatch in the water ; D, ciliated miracidium einbryo in the water
and about to enter the pulmonary chamber of snail E ; F, sporocyst containing
rediae ; G, redia containing daughter rediae ; H, redia of the second generation contain-
ing cercaria ; /, cercaria ; J, same having emerged from snail into water ; K, cercaria
encysted on blade of grass : L, cercaria liberated from cyst after ingestion by sheep ;
M, young fluke developing in liver of sheep. (Reprinted by permission frorn
Chandler, Introduction to Human Parasitology, John Wiley and Sons, Inc.;)

ANIMAL PARASITISM

453

also differ from trematodes by having the body divided into a series
of segments, one behind the other, each segment having a complete
set of reproductive organs. This structure characteristic of tape-
worms is usually referred to as segmentation of the body, but it is

N^whM*******

'■C-'f !>>^JSa.«S' ■M ^t^

Fig. 193. — Structure of tapeworm to show different stages of maturity. At
lower left, Taenia pisiformis scolex with hooks and sucker discs. At lower right,
mature proglottids. Above, scolex and proglottids of three ages. The reproductive
organs and pore are shown in most proglottids. (Courtesy of Ward's National
Science Establishment, Inc.)

454

ESSENTIALS OF ZOOLOGY

probably more correct to consider a tapeworm as a linear colony, in
which the segments are really individuals in various stages of maturity.
Taenia solium, the pork tapeworm of man, may be taken as an
example to illustrate the structure and life history of a cestode. The
adult tapeworm consists of a scolex or head provided with four mus-
cular suckers and a snoutlike 7'ostellum surrounded by a row of
chitinous hooks, which serve as means of attachment to the wall of
the human intestine; a narrow unsegmented neck behind the scolex,
and then a series of several hundred proglottids (the segments) be-
coming progressively larger as they get farther from the scolex. The

AC D

Fig. 194. — Development of tapeworm. A, six-hooked embryo ready to become
embedded in muscle ; B, cysticerus, or bladder worm as encysLCd ; C, section tlirou^^h
developing scolex in cysticercus ; D, later stase ; E, scolex everting as it protrudes
from bladder ; F, extension of scolex from bladder ; G, later stage ; H, formation of
proglottids. (From Parker and Haswell, Textbook of Zoology^ The Macmillan
Company, after Jijima and Hatschek.)

whole chain of proglottids is called the strohilus. New proglottids
are constantly budded off from the neck; consequently, the youngest
proglottid is the first one back of the neck and the oldest one is
the one at the end of the strobilus farthest from the scolex. The
youngest proglottids contain no recognizable structures, except the
paired longitudinal nerve cords and longitudinal excretory vessels

ANIMAL PARASITISM

455

which run the full length of the strobilus, and a transverse excre-
tory vessel in each proglottid. As the proglottids become older and
are pushed farther away from the scolex, the reproductive organs
begin to develop; each proglottid develops a complete set of both
male and female reproductive organs; when these become mature
and ready to function, the proglottid is a mature proglottid. Each
mature proglottid is capable of copulating with itself, by bending
the cirrus down into its own vagina, or it may copulate with another
mature proglottid of another tapeworm if one is present close by.

Head

iHa*.*i:urc seginen-fcs

vr^«*»*;

r.*7,ir

ti"^*'

M

■ViV

^V^

Fig. 195. — Common tapeworms, showing different regions of the body. At
the left above — scolex of Taenia saginata, beef tapeworm : left below — pro ilottlds of
Moniezm, sheep tapeworm ; middle — scolex and proglottids of Taenia solium, pork
tapeworm ; right — scolex and proglottids of Dipylidium caninunxt a dog tapeworm.
(Courtesy of General Biological Supply House.)

After copulation the male organs begin to degenerate, the uterus
becomes filled with fertilized eggs and takes up more and more
space, then the ovaries, vitellaria, and other female organs degen-
erate and leave the uterus to occupy nearly the whole proglottid;
in this condition the proglottid, now hardly more than a sack of
eggs, is called a gravid proglottid. The gravid proglottid at the ex-
treme end of the strobilus breaks off and passes out of the host's

456 ESSENTIALS OF ZOOLOGY

intestine with the feces. It continues to live and crawl slowly
through the feces like an independent animal for a few hours, then
usually dies, but the embryos within the proglottid remain alive
much longer. If gravid proglottids or separate eggs are eaten by
a hog, the six-hooked hexacanth larva hatches in the hog's intestine,
bores through the intestinal wall, and migrates to other parts of
the body where it changes into a bladder- worm or cysticercus, which
is a saclike larva with an inverted scolex. The cysticerci remain
in the flesh of the hog until the pork is eaten by man; under the
influence of human digestive juices the cysticerci become everted
so that the scolex is on the outside of the saclike part, then the
scolex attaches itself to the wall of the human intestine, proglottids
begin to bud off from the neck, and an adult tapeworm is formed
within a few weeks.

Taenia saginata, the beef tapeworm, has a similar life history, but
uses cattle instead of hogs as intermediate hosts. Taenia serrata,
a common dog tapeworm often used as a laboratory specimen, is
very similar to the two human species in structure and life cycle.

Other important cestodes are Hymenolepis nana of man and mice;
the broad fish tapeworm, Biphyllohothrium latum, which man gets
by eating raw or poorly cooked fish ; the peculiar Echinococcus granu-
losus, adult in dogs, whose cysticercus stages are dangerous parasites
of man (man being the intermediate host in this case) ; and many
tapeworms of domestic animals, such as Dipylidium caninum of dogs,
Thysanosoma and Moniezia of sheep, goats, and cattle, and many
others.

References

Chandler, A. C. : Introduction to Human Parasitology, New York, 1940, John

Wiley & Sons, Inc.
Ewing, H. E. : Manual of External Parasites, Springfield, 111., 1929, C. C. Thomas.
Faust, E. C. : Human Helminthology, 1929, Lea & Febiger.
Riley, W. A., and Christensen, R. B. : Guide to the Study of Animal Parasites,

New York, 1930, McGraw-Hill Book Company.

CHAPTER XXV

THE ANIMAL AND ITS ENVIRONMENT
(By A. 0. Weese, University of Oklahoma)

Ecology is that division of biology which has to do with the rela-
tions between organisms and their environment. The environment
of an organism, for convenience, may be divided into two parts, the
nonliving, including physical, chemical, and climatic factors, and
the living, including other organisms of the same and of different
kinds. The science of ecology, in a sense, is as old as man, because
from the very beginning of his conscious existence it was necessary
that man take cognizance of the fact that his environment was made
up in part of plants and animals and that these organisms in turn
had relations to their environment. It was not until comparatively
recently, however, that ecology came to be recognized as a separate
department of biology. Modern ecology may be said to have begun
with the recognition of the community. Plants and animals are
distributed as they are over the surface of the earth, not because
of any chance coincidence, but because of a combination of circum-
stances, one of which is the fitness of the physical environment for
the proper completion of their life histories and another of which
is the presence of such other organisms as are necessary to furnish
food and to provide other requisite conditions. We can think of a
community of organisms in much the same way as we think of a
community of human beings. The analogy cannot be followed too
closely, for, after all, human beings are much alike while the organ-
isms in an ecological community are of many different kinds, having
different requirements in detail as- to food and environmental con-
ditions. We shall arrive at a better concept if we think of a human
community as made up, not only of butchers and bakers and candle-
stick makers, each with their particular functions as producers and
as consumers, but also of the domestic animals which furnish mate-
rials for food or clothing or which perform labor, the household
pets and pests, the cultivated plants which are utilized in the manu-
facture of food, clothing or shelter, and the host of wild animals
and plants which enter into some relation with those previously

457

458

ESSENTIALS OF ZOOLOGY

mentioned. The characteristics of such communities vary from time
to time and from place to place. A difference in climate may be
sufficient to change almost every component of the community. The
domestic animals and plants associated with man in the tropics are
quite different from those in arctic regions. Perhaps one species,
the dog, almost as adaptable as man himself, might be considered
as a member of both communities. The removal of a single species
or the addition of a new one may alter profoundly the aspect of
the community. Consider, for example, a human society from which

Fig. 196. — Diagram to show food relations in a hypothetical prairie community.
(Redrawn and modified from Shelf ord. Animal Communities in Temperate America,
University of Chicago Press.)

all cows were removed, or the changes made in the life of certain
sections of the United States with the introduction of the cotton
boll weevil.

Some idea of 4:he complexity of the relationships involved in a
community of organisms may be gained by citing Charles Darwin's
example of the dependence of clover on cats, or Thomas Huxley's
extension of the chain of cause and effect to the responsibility of
the old maids of England for the supremacy of that nation on the

ANIMAL AND ITS ENVIRONMENT

459

seas. Obviously, since old maids are fond of cats, the number of
the latter is greater when old maids are numerous. Cats eat field
mice, which in turn prey upon the nests of bumblebees. Thus, a
large cat population is favorable for the development of clover
which is fertilized by the bumblebee. Clover is fed to cattle and
it is well known that Britain's sea power is due to ''the roast beef
of old England."

Most of the relationships suggested in this series have to do with
food. It is often possible to gain a better idea of relations within
a community by the use of a diagram indicating the more obvious
influences. The accompanying figure, modified from Shelf ord (Fig.
185), represents food relationships only in a hypothetical prairie
community and may be called a "food chain diagram." Many
very obvious food chains are omitted in order to avoid a complexity

"--Lithosphere"'
---Hydrosphere - -
---Atmosphere - -

Fig-, 197. — Diagram to show the relationship of the general areas of our planet.

too great for easy reading, and, of course, any real community
would include many more kinds of animals. The chart is very in-
complete, also, in that no relations other than those directly con-
cerned with food are indicated.

Life exists on the earth only in a relatively very limited space.
We might represent the relationship of the various parts of the
planet on which we live by a series of concentric circles, the area
within the inner circle corresponding to the ''solid" portion of the
earth, the lithosphere. If the lithosphere were uniform in diameter,
it would be covered by a layer of water, the hydrosphere, which is
in turn surrounded by a gaseous layer, the atmosphere. It is only
where the components of these three layers interact that life is pos-
sible. Living organisms cannot exist in the lithosphere without the
presence also of water, and of oxygen from the atmosphere. Simi-
larly life in water is possible only where it contains in solution both

460

ESSENTIALS OF ZOOLOGY

solids and oxygen. Air is habitable only temporarily and animals
spending time there must return frequently to water or soil. Free
interaction between the three components of the earth is made pos-
sible by the fact that the lithosphere is not of exactly uniform
diameter so that certain portions project above the hydrosphere,
forming continents (and islands). These elevated areas constitute
a rather small proportion of the whole surface of the earth, but it
is with these areas that we shall be concerned, not because there

Fig. 198. — Distribution of "precipitation effectivity" on an ideal continent. (Modified
after Thornthwaite. Drawn by Edward O'Malley.)

is no life in the submerged areas, but because the space available
in this chapter is too small for us to consider the great subject of
marine ecology.

The inclination of the earth's axis of rotation to the plane of the
earth's orbit about the sun and the relative positions of the conti-
nents are factors which influence the distribution of climates. The
two principal climatic factors are, of course, temperature and mois-
ture. If the effect of altitude is not considered, the former is distrib-
uted rather uniformly, so that the familiar torrid, temperate, and

ANIMAL AND ITS ENVIRONMENT

461

frigid zones express rather well the comparative temperature con-
ditions on a continent. Many factors, however, operate in the con-
trol of the amount of precipitation and its distribution. In general,
the distribution of available rainfall on an ''ideal" top-shaped conti-
nent without mountain ranges might be expected to follow the plan
of Fig. 198, in which the darker areas indicate maximum rainfall
and the unshaded areas represent very arid regiqns. Each climatic
type makes possible the occupancy of the region concerned by a

[m P<zppdua\ snom or ice
DE Tundpa
GH) Evergreen ioresi
1=] Deciduous forest

■^ Grassland

dl] Scrub Csagebrush)

EM Deszpf

O Tropical forests savanm

Fig-. 199. — Hypothetical distribution of biotic communities on an ideal continent.

definite type of biotic community which can be most easily desig-
nated by the character of the conspicuous or dominant vegetation.
Taking into consideration the seasonal distribution of available
moisture and the annual variations in temperature, the ''ideal con-
tinent" might be expected to present an aspect something like that
represented in Fig. 199. It will be seen at once that in the regions
of deficient temperature heat is the determining factor, and that in
warmer regions the amount of moisture is the major influence. How
closely this expectation is realized in the case of North America
may be seen by a comparison with Fig. 200. As will be seen later,

462 ESSENTIALS OF ZOOLOGY

the animals of a climatic region are as distinctive as the plants. The
soil, which is the result of interaction between the climate, the plants
and animals, and the rock substratum, also is characteristic of each
climate. Thus, in the area influenced by each climatic type, there is
a definite biotic community or formation, characterized by plants
and animals whose relations to their environment are similar or
equivalent.

The Principal Biotic Formations

A rough classification of the climatic formations of North America
includes the following:

1. The Tundra Formation

2. The Coniferous (Evergreen) Forest Formation

3. The Deciduous Forest Formation

4. The Grassland Formation

5. The Sagebrush Formation

6. The Desert Formation

In addition there are transitional communities difficult of repre-
sentation on a map because of their discontinuous distribution or
small area. Some of these may be referred to, provisionally, as
Woodland, Desert Scrub, Chaparral, and Swamp communities.

1. The Tundra Fonnation (Sedge-Musk Ox Biome). — This is the
community of the arctic ''barren grounds," beyond the northern
limit of trees and between it and the polar region of perpetual snow
and ice. Included, also, is the region above timber line on high
mountains, where climatic conditions are similar to those of the
far north. The vegetation of the tundra is composed mainly of
grasses, sedges, and lichens (e.g., reindeer moss), although there
are also patches of dwarf willows and other woody plants from a
fraction of an inch to a few feet in height. Typical animals of the
Arctic tundra are the musk ox and the Arctic caribou which feed
on the low vegetation. Here, also, are found great hosts of rodents,
including lemmings, whose enormous increases in numbers and
periodic migrations in Europe have been known for many hundreds
of years. Arctic hares and Arctic ptarmigan, with those animals
previously mentioned, furnish food for the Arctic fox and the Arctic
wolf. Many of these animals, including the Arctic fox, the Arctic
hare, the ptarmigan, and the collared lemming, are adapted to the
climatic rhythm of their environment by changing in color from

ANIMAL AND ITS ENVIRONMENT

463

white in winter, when the entire landscape is snow-covered, to a
darker color during the summer. On the tundra nest vast numbers
of migratory birds, some of which, like the golden plover, fly to
winter homes in South America.

2. The Coniferous Forest Formation (Spruce-Moose Biome).—
South of the Arctic tundra and below the high mountain tundra
along the Rockies and the Sierras is the great coniferous forest
whose dominant vegetation is evergreen, composed chiefly of spruces,

Fig. 200. — Distribution of the major biotic regions in Northi America.

firs, and pines of many species. The moose and the woodland caribou
are characteristic animals over the greater part of the area. They
feed upon grass and shrubs, chiefly in open areas near streams or
lakes. The principal carnivore is the timber wolf. The Canada
lynx is also a creature of the evergreen forest. Black bears are
numerous but are widespread, also, Deyond the borders of this for-
mation. The wolverine and the red fox are lesser carnivores. The
varying hare is similar to the Arctic hare but lives in more protected
situations.

464 ESSENTIALS OF ZOOLOGY

3. The Deciduous Forest Formation (Oak-Deer Biome). — Most of
the United States lying east of the Mississippi River is characterized
by deciduous forest. . In this area the dominant vegetation consists
of trees whose leaves are shed during the unfavorable winter sea-
son, such as oaks, beech, maple, hickory, and elm. The most wide-
spread large herbivorous animal is the Virginia deer, although the
wapiti (elk) was formerly of frequent occurrence. The wolf and
the cougar were once widely distributed but are almost extinct now.
The bay lynx, a close relative of the Canada lynx, preys upon the
numerous rodents and birds. Other characteristic animals are the
opossum, the raccoon, the flying squirrel, and the woodchuck. Many
of these inhabit open areas in the woods rather than the dense forest.

4. The Grassland Formation (Grass-Bison Biome). — No single
grass is included in the ''common" name of this community because
it is probable that the name of no single genus of grass is sufficiently
familiar. Important grasses in the great central grassland area are
the bluestems (or beard grasses), June grass, porcupine grass, grama
grass, and buffalo grass. The grassland region was the habitat of
the bison which once ranged over the prairies from Texas to Sas-
katchewan in enormous numbers. It is estimated that at the com-
ing of the white man there were over 75 million of these animals,
upon which the plains Indians depended for food, clothing, and
shelter. The American pronghorn or antelope was also abundant.
Both were held in check by the gray wolf. The black-tailed jack
rabbit is an animal of the open grassland while the cottontail oc-
cupies wooded or at least bushy areas. In the drier Avestern portion
of the grassland the prairie dog and the badger are common. There
are many small rodents, including the pocket gopher, the ground
squirrels, and prairie deer mice.

5. The Sagebrush Formation (Sagebrush- Jack Rabbit Biome). —
This community is centered in the Great Basin of Nevada, Utah,
and neighboring states. It is an area of deficient rainfall, and
sagebrush is a prominent constituent of the vegetation. Most of
the larger animals belong to groups which are also found in neigh-
boring formations, although the smaller mammals are mostly of
different subspecies. Rabbits and rodents are especially abundant.

6. The Desert Formation (Creosote Bush-Kangaroo Rat Biome). —
It is difficult to characterize this community by the name of a domi-
nant plant and an important mammal, as there are many types of

ANIMAL AND ITS ENVIRONMENT 465

desert associations. They have, however, one characteristic in com-
mon— a great deficiency in available water. Plants must guard
against water loss and conserve water between the infrequent moist
periods. Some of the adaptations which meet such conditions are;
reduction of leaves (thorns), fleshy stems, thick cuticle, extensive
underground organs, etc. Animals conserve water by remaining
underground or in sheltered places during the hotter parts of the
day and coming out only at night or at dusk. Reptiles and birds
are able to reduce water loss by the absence of sweat glands and
by giving off their nitrogenous excretion in the form of uric acid
which does not need to be dissolved in water.

Adaptation

From what has been said above, it appears that each particular
set of climatic and other environmental conditions accommodates
a particular group of plants and animals. The activities and struc-
ture of these animals are such as enable them to survive best under
the conditions in which they are found. The barren ground caribou
is not fitted for life on the central grassland area nor is the bison
able to survive on the tundra. Each is said to be adapted to the
particular conditions under which it exists. Many so-called adap-
tations, however, seem to be worthless. One might think, for ex-
ample, that the shovellike brow tines of the caribou would be a
remarkable adaptation for shoveling snow from the vegetation on
which the animal must feed during the winter — ^but the antlers are
shed in the late autumn and do not grow out again until the fol-
lowing spring. One must be careful not to assign adaptive func-
tions to animal structures without careful consideration of the habits
and life histories of the animals concerned.

Succession

It must not be thought that, within the areas of the great climatic
formations, there is entire uniformity in the communities of plants
and animals. This is far from true. There exist minor differences
of climate, soil, and physiographic conditions and of biological his-
tory which result in differences in plant and animal population.
There are always areas which, for example, have been denuded by
physiographic or other processes. Fire, fiood, and human utilization
are among the more common influences which may cause partial or

466 ESSENTIALS OF ZOOLOGY

total destruction of the biotic communities of a region. When
denudation occurs, a region does not long remain unpopulated, but
the slow process of redevelopment of a community characteristic
of the existing climate begins as soon as the destructive force has
ceased its action. This process of development is called succession
because it is characterized by the appearance, first, of pioneers,
which give way to other groups of organisms which, in turn, alter
conditions in such a way that still others find conditions favorable
for their existence. The final result, after a long time, is the de-
velopment of a community which is in equilibrium with its en-
vironment and will not change unless the environment clianges.
Such a community is called a climax community. An analogy might
be sought, again, in human experience. The human pioneers who
first penetrated into the broad plains of the ''old west" conquered
the wilderness to such an extent that some who required more of
the comforts of civilization were able to find suitable homes. Many
of the original pioneers, however, moved on to new frontiers wlien
the land became too thickly settled. The "climax human commu-
nity" has not yet been attained, as man is forever seeking more
perfect adjustment to his environment.

It is often possible to study the stages in the development of a
community without waiting for the entire process to take place in
one locality. For example, the broad flood plains of many of onr
rivers are subject to frequent inundations. Such inundations may
remove all living things from their paths, but since the flood ph-iins
are so broad and the course of the river is never the same from year
to year, the same tracts are not covered each spring, and areas mny
be found which have been undisturbed for a few weeks, a few
months, a year, two years, five years, fifty years, etc. In tlie flood
plain of the Canadian River (in Oklahoma) mud flats from Avliich
the water has receded recently are soon occupied by an assemblage
of organisms including blue-green algae, two small beetles, and a
fly. Tiger beetle larvae come a little later, and then seedlings of
willow and cottonwood. During the course of a year or so blowing
sand is deposited so that the level of the ground is raised several
feet, willows and cottonwoods increase in size, and other plants
are added. Soil and sand continue to be deposited, the trees in-
crease in size, and the ground becomes more thickly covered by
vegetation. Finally, after a hundred years or so, a still "higher level
is attained and the willows and cottonwoods give way to oaks and

ANIMAL AND ITS ENVIRONMENT 467

elms. The table below indicates some of the insects to be found in
each stage of this developmental series. After the name of each
insect is given, in parentheses, its principal food.

Succession on tpie Canadian Eiver Flood Plain. (Data From Hefley.)

STAGE in development 12 3 4 5 6

Paralimma appendiculatum x*

Fly

Heterocerus pallidus x

Beetle (Algae and detritus)

Bendidion laevigatum x x

Beetle (Algae and detritus)

Cincindela hirticollis x x x

Tiger Beetle (small insects)

Cinindela cuprascens x x x

Tiger Beetle (small insects)

Cinindela punctulata x

Tiger Beetle (small insects)

Mutillidae x

Velvet ants (insects and spiders) '

Apion pennsylvanicum x

Weevil (cocklebur)

Haltica bimarginata x x x

Beetle (willow)

Phalacrus politus x

Beetle (willow)

Stictocephala lutea x

Tree hopper (willow, cottonwood)

Cicadella gothica x

Leafhopper (willow)

Dorytomus squamosus x

Weevil (willow)

Strongylocornis stygicus x

Bug (coral berry)

Epitrix brevis x

Flea Beetle (Miscellaneous plants)

Brief descriptions of stages:

1. Mud flat with blue-green algae, later liverworts.

2. Sedges, willow and cottonwood seedlings in addition to 1.

3. Second level. Sand, sedges, willows, cottonwoods, cockleburs.

4. Third level. Grasses, willows, cottonwoods.

5. Third level. Cottonwoods, fewer willows.

6. Fourth level. Elm-oak forest with shrub undergrowth.

•x indicates the presence of the species as an important member of the com-
munity.

468

ESSENTIALS OF ZOOLOGY

Animal Populations

A great deal can be learned about the relations of organisms by
quantitative methods making possible an estimate of the numbers
of various species present in a given community at any one time.

Fig. 201. — Numbers of insects collected in average catch with 100 sweeps of
an insect sweep net in a prairie ravine (Oklahoma) arranged according to orders
through the season of the year. (Data from Carpenter.)

Comparisons may then be made between the populations of com-
munities differing in some observable respect, or between the popu-
lation of the same community at different times of the year. It is
comparatively easy, although somewhat tedious, to determine the

ANIMAL AND ITS ENVIRONMENT

469

relative numbers of various plants in a given region by blocking
off sample areas and counting the plants. Most animals, however,
w^ill not remain stationary while a census is being taken, and vari-
ous less exact methods of obtaining population data must be em-
ployed. Some of these methods may be suggested. The number of
birds may be estimated by the number seen within a given area
or during a certain time, or a census of nesting pairs may be taken
by counting nests. Larger mammals may be counted by experienced
observers, burrows of rodents may be counted (but it is also neces-
sary to determine the average number occupying a burrow) or the
rel'ative frequency of tracks, fecal pellets, or other evidences of the
presence of the animals may give an idea of their numbers. The
relative number of rodents is often estimated by the frequency with

Fig-. 202. — Total number of insects collected in average catch with 100 sweeps of
insect net In prairie ravine (Oklahoma). (Data from Carpenter.) The scale for
this g-raph is one-fourth of that in the preceding figure.

which they are caught in traps. The most frequently used method
of estimating the number of insects is the use of the insect net.
A net is swept through vegetation a definite number of times and
all insects caught are counted. While all such methods are neces-
sarily inaccurate, a great deal of valuable information may be
obtained.

A list of the ten most abundant groups of insects found in two
adjacent areas, one heavily overgrazed and the other lightly grazed,
is given in the following table. The figures indicate their relative
abundance in ten collections of fifty sweeps of the insect net, each.
These collections were taken over a period of a month and indicate
a very real difference in the insect populations of the two areas.

470

ESSENTIALS OF ZOOLOGY

Relative Numbers of Ten Most Abundant Insects From Overgrazed and
Normal Grassland. (Wichita Mountains Wildlife Refuge.)

GENUS

common NAME

normal

OVERGRAZED

Melanoplus

(grasshopper)

11.0

220.0

Campylencliia

(tree hopper)

0.5

144.0

Mermiria

(grasshopper)

14.0

138.0

Scolops

(plant hopper)

10.0

120.0

Ellesclms

(weevil)

0.9

80.0

Poeciloscytus

(leaf bug)

1.2

54.0

Agallia

(leaf hopper)

8.0

38.0

Harmostes

(plant bug)

0.5

17.0

Deltoceplialus

(leaf hopper)

8.0

3.4

Bruchomorpha

(plant hopper)

11.0

0.4

Seasonal Changes

It is a matter of common observation that the animals observed
in any one place differ greatly from season to season during the

Fig. 203. — Fluctuations in populations of four Protozoa in an artificial lake in

Oklahoma during the months of the year,

year. This is not only true of those animals which, like many birds,
migrate southward on the approach of winter and return to north-
ern climates for nesting. Some animals hibernate or aestivate, and

ANIMAL AND ITS ENVIRONMENT 471

others spend a part of the year in an inactive stage, such as the egg,
or the pupa. The life cycle of an organism must be adjusted to the
annual climatic cycle of the climate in which it lives. Some idea
of the variations in the number of insects during the year may be
gained from a study of the accompanying charts (Figs. 201 and 202)
giving the average catch with one hundred sweeps of an insect net
at different seasons.

A study of the abundance of Protozoa in an artificial lake (Fig.
203) shows a similar difference in the time of abundance of the
various species.

Summary

This chapter has considered very briefly the distribution of the
biotic communities of North America in relation to climate. The
phenomena of succession and seasonal fluctuation of populations
have been discussed, with examples. Attention has been directed
toward the community rather than toward the individual organism
or the species. Similarly the sum total of physical environment as
expressed in climate has been stressed rather than single factors,
such as moisture, temperature, light, etc. The animal in nature is
subject always to the action of a complex environment and its dis-
tribution and reactions are the result of its response to the whole.

References

0

Chapman, R. N. : Animal Ecology, New York, 1931, McGraw-Hill Book Company.
Elton, Charles S. : Animal Ecology, London, 1927, Sedgwick and Jackson.
Hesse, Richard, et al.: Ecological Animal Geography, New York, 1937, John

Wiley & Sons, Inc.
Pearse, A. S.: Animal Ecology, New York, 1926, McGraw-Hill Book Company.
Shelf ord, V. E. : Animal Communities in Temperate America, Chicago, 1927,

University of Chicago Press.
Welch, P. S.: Limnology, New York, 1935, McGraw-Hill Book Company.

CHAPTER XXVI

ANIMAL DISTRIBUTION

(By Willis Hewatt, Texas Christian University)

Life Regions and Zones of the Earth

Every area of the earth has its animal and plant life. The ap-
parently barren sun-baked desert, the ice-capped polar regions, the
highest mountain tops, and the tropical rain forests are all inhabited
by their faunas and floras. Life in the seas extends from the shore
line to the greatest depths. After extensive taxonomic surveys have
been made in practically all areas of the earth and these data are
studied, it is found that the earth can be divided into fairly definite
horizontal regions and vertical zones. These two phases of the dis-
tribution of organisms on the earth are very closely related to each
other but are usually studied separately as geographic or horizontal
distribution and hathymetric or vertical distribution, respectively.

Geographic Distribution. — Many attempts have been made to
divide the earth into horizontal life regions based upon the distri-
bution of various groups of animals and plants. The exact bound-
aries of the regions are not fully agreed upon since several groups
of animals have been used as criteria for the division into life re-
gions. P. L. Sclater (1829-1917), the earliest zoogeographer, worked
with the perching birds. These forms were well adapted to such a
study since they have little power of flight, they are widely dis-
tributed, and they had been very closely studied. The early (1876)
classic work of the Englishman, Alfred Russell Wallace, in which
the earth was divided into six primary regions, is the one most com-
monly followed by modern biologists. Wallace based his divisions
upon the distribution of the mammals. Other significant studies in
geographic distribution have been based upon the geographic ranges
of mollusks, earthworms, moths, butterflies, spiders, fresh-water fishes,
reptiles, and many others. From works dealing with such varied
groups of animals it is readily understood that any classification of
the regions of the earth will depend greatly upon the group of ani-
mals used as a criterion. On the other hand, there are many corre-
lations found in the studies of the geographical ranges of all of these
groups of animals. The zoogeographical regions of Wallace and the

472

ANIMAL DISTRIBUTION

473

general boundaries of these areas are given below. The boundaries
of the regions do not necessarily follow the outlines of continents as
will be seen in Fig. 193.

1. The Palaearctic Region includes all of Europe, Africa north of
the Sahara, and all of Asia north of the Himalaya Mountains.

2. The Nearctic Region embraces the North American continent
as far south as Mexico.

3. The Ethiopian Region consists of Africa south of the Sahara,
southern Arabia, and Madagascar.

Fig-. 204. — Map showing the life regions of the earth.

A. J. Nystrom Company.)

(Base by permission of

4. The Oriental Region includes Asia south of the Himalayas, and
that portion of the Malayan Archipelago which lies northwest of
Wallace's line. This famous line dividing the Oriental and Aus-
tralian Regions passes east of the Philippines between the islands of
Bali and Lombok, and between Borneo and Celebes. Bali and Lom-
bok are separated by a very narrow but deep strait, and there is a
very remarkable contrast between the faunas of the two islands.

5. The Australian Region is made up of Australia and the neigh-
boring islands as far west as Wallace's line. It includes the great
number of small islands east of Australia.

474 ESSENTIALS OF ZOOLOGY

6. The Neotropical Region consists of Mexico, Central America,
and South America.

The present distribution of animals on the earth gives us many
evidences concerning the past history of the earth. The North Ameri-
can fauna is notably different from the South American fauna. This
fact apparently indicates that North and South America were com-
pletely separated from each other by an extensive water barrier until
a relatively recent period. The great contrast between the faunas of
the islands of Bali and Lombok indicates that the Australian continent
and its neighboring islands have been separated from the Asiatic
mainland for a relatively long period of time. The study of distribu-
tion also gives many clues to the ancient changes of climate on the
earth.

Bathjmaetric Distribution. — In discussing the vertical or altitudi-
nal distribution of animals two fundamental types of habitats must
be considered; namely, the water habitat or Hydrohios and the land
habitat or Geobios. The inhabitants of these two realms differ
greatly, but some few forms occupy both regions.

The hydrobios iiicludes both fresh- and salt-water realms. The
variation in vertical distribution of fresh-water animals is relatively
insignificant when compared to that of marine animals. The seas
have been variously classified into vertical zones. The divisions
most commonly recognized are :

1. The Littoral which is the area between tide marks. It is charac-
terized on rocky shores by growths of barnacles, mussels, and snails.
Life in sandy littoral regions, such as that along the Texas Gulf
Coast, is relatively scarce and consists chiefly of annelid worms and
other boring forms.

2. The Suhlittoral includes the subtidal region to a depth of ap-
proximately 100 meters. This is about the lowest depth at which
plants can grow abundantly. It is the most productive region on
the earth as far as variety and numbers of animals are concerned.

3. The Abyssal zone is marked by the absence of light and extends
to the greatest depths of the oceans. Life is not very abundant in
this zone. Plants cannot live at such depths, therefore, the animals
which live in the abyssal region are dependent upon other animals
as a source of food, or they subsist upon the floating plant life which
is continuously dying and sinking to lower levels.

ANIMAL DISTRIBUTION

475

The vertical or altitudinal distribution of terrestrial animals de-
pends primarily upon the vertical distribution of the vegetation.
The variation in the kinds of plants found at different altitudes de-
pends to a large extent upon the climatic belts which extend around
the earth between the equator and the poles. The vertical belts of
plants encountered in traveling from a lowland to a high mountain
top correspond very closely to the climatic zones surrounding the
poles of the earth. The communities of animals and plants found in
each vertical zone are similar throughout the world.

The seven vertical life zones and a few of the characteristic animals
and plants found in each zone, as it is represented in the Southwest,
are here given.

MUDSONIAM

,. vo*'%n ><>>itt»i>n

UA t-IVlL

ALi»ise ^'

HUOSONIAH 12-

CANADIAN

TRANSITION "*

10-

9-

»-
7*
6-
5-

4-
J-

2-

1-

Fig. 205.— Diagrammatic sections of the Grand Canyon and San Francisco peak
showing the vertical life zones. (After N. N. Dodge and Merriam. )

1. The Alpine zone (usually above 10,500 feet) is the area above
the timber line. This highest zone is represented in the Southwest
on the top of San Francisco Mountain in Northern Arizona and in
the Sangre de Cristo range of the Rockies in Northern New Mexico.
This zone is characterized by the presence of few plants, such as
saxifrages and dwarf willows. Among the fauna are found the
golden eagle, some weasels, and mountain sheep.
■ 2. The Hudsonian zone (9,000-10,500 feet) consists of a forest of
spruces and some firs, and harbors the dusky horned owl, bears,
shrews, and red squirrels. The Hudsonian zone is found in the same
region as the Alpine zone.

3. The Canadian zone (8,000-9,000 feet) is distinguished by the
presence of the Douglas fir and the aspens. Common animals of this
zone are the three-toed woodpecker, one species of shrew, and two

476

ESSENTIALS OF ZOOLOGY

species of field mice. This zone is found in the Rockies as far south
as southern New Mexico. There is some evidence of Canadian fauna
in the Chisos Mountains of Texas.

4. The transition zone (7,000-8,000 feet) is found in the same
regions as the above zones. It is also found in the Davis and Chisos
and Guadalupe Mountains of Texas. It is covered with pine trees

Fig-. 206. — Map of the vertical life zones of Texas and adjacent areas. (Aftej-

Bailey. )

(mainly Pinus ponderosa). Abert's squirrel is found in the Transi-
tion zone as is one species of horned lizard.

5. The Upper Sonoran zone (6,000-7,000 feet) is the pifion belt
distinguished by the nut or pifion pine and cedars. The zone extends
from western Texas through New Mexico, Colorado, Arizona, and
into southern California. Characteristic animals are the piiion jay
bird, and the large rock squirrel.

ANIMAL DISTRIBUTION 477

6. The Lower Sonoran zone is less homogeneous in its flora and
fauna than are the other zones. In the Southwest it includes most
of Texas, Oklahoma, southern New Mexico, southern Arizona, and
southern California. The eastern third of Texas is typical of the
lower Sonoran of the southeastern states which are characterized
mainly by long-leafed pines, magnolia, and live oaks. Some animals
of the eastern portion are mockingbirds, painted buntings, and wood
rats. The western lower Sonoran is inhabited by typical desert and
semidesert flora and fauna.

7. The Tropical zone is of very little significance in the south-
western United States, but many tropical plants, such as the Texas
])alms and bananas, grow in a narrow strip of the lower Rio Grande
Valley.

Migration of Animals

The migrations of many animals are not well understood but in
most cases they involve breeding habits, food, or shelter. Most of
these migrations are seasonal, but many permanent changes of loca-
tion have been made by groups of animals as a result of permanent
changes of climate on the earth.

The seasonal migrations of the North American caribou and the
bison are among the most noted examples of migrations of animals
which move about in search of food. The caribou migrate south-
ward in Canada during the winter and follow the melting snow
northward in the summer.

The remarkable migrations of the eels of the Mediterranean area
and of the eastern coast of the United States are among the best
known examples of breeding migrations. The eels from the rivers
of these two regions migrate into the Caribbean Sea and spawn.
Although the two spawning territories overlap, the young eels of
each species journey hundreds of miles back to the rivers inhabited
by their ancestors. A great many species of birds migrate from the
tropics and semitropics and breed in northern regions. The scarlet
tanager spends the winter in northwestern South America and mi-
grates across the Gulf of Mexico into northeastern United States and
southern Canada and breeds during the summer months. The upland
plover is familiar in the Southwest, since it migrates through this
region twice each year on its journeys between Argentina and British
Columbia.

478 ESSENTIALS OF ZOOLOGY

Means of Dispersal and Barriers

The wide distribution of species of animals on the earth depends
largely upon their means of dispersal or means of being carried from
one place to another and upon the barriers which they encounter.
Among the members of a species as well as among the related and
nonrelated species of animals there is a continuous struggle for
existence. Those forms which, by some means, are able to enter new
environments where competition is less severe will have the better
chance at survival.

In practically all marine organisms there exists a means of loco-
motion during some stage of the life history. In a great number
of the forms, especially those which are sessile in the adult stage,
there is a free-swimming larval stage: planula larva of the coelen-
terates ; trochophore larva of annelids ; and the various free-swim-
ming larvae of mollusks, echinoderms, and crustaceans. The plank-
tonic larvae and adults, i.e., those which swim or float free of the
bottom, depend greatly upon oceanic currents for their wide dispersal.

The dispersal of oceanic animals is also greatly enhanced by the
large numbers of eggs and larvae produced by these forms. For
example, one investigator found that a **sea hare," a marine gas-
tropod, deposited 478 million eggs during one four-month spawning
period. The eggs were laid at the rate of 41,000 per minute.

Marine animals are limited in their distribution by such barriers
as temperature, land masses, and salinity of the water. The marine
animals on each side of the narrow Isthmus of Panama are entirely
different. The extreme changes in temperature prevent most of
these species going around the southern tip of South America. Lit-
toral animals are often limited in their distribution by large river
mouths which empty great quantities of fresh Avater into the oceans.
Only specially adapted forms can live in these brackish waters.

Fresh-water faunas are restricted in their distribution by land
barriers which usually separate the bodies of Water in which they
live. These forms depend mainly upon other animals, such as birds
and insects, for the dispersal of their eggs and dormant stages.
Many fresh-water animals, such as snails and clams, attach them-
selves to the bodies of birds or insects and are carried into new
habitats.

Among land animals the birds and flying insects appear to be
least restricted in their ranges. Even these, however, are often

ANIMAL DISTRIBUTION 479

limited by mountains, deserts, and large rivers which act as the
most effective barriers to the wide distribution of land animals.
Many of the birds and mammals found on the south rim of the
Grand Canyon of the Colorado River are not able to reach the north
rim of the Canyon. For example, the Abert squirrel is found in
New Mexico and Arizona, south of the Canyon, while the Kaibab
squirrel occurs only north of the Canyon. Small islands, which are
located great distances from the mainland, are often inhabited by
small mammals and reptiles similar to those found on the mainland.
Such forms have probably reached the islands by way of floating
rafts of vegetation, and hollow trees which are known to be carried
several thousands of miles by oceanic currents.

Wind currents may carry birds across the Atlantic Ocean. It is
not uncommon for American birds to land on the coast of England
after severe storms. These same currents also carry spores of Pro-
tozoa, small insects, and the eggs of many invertebrates.

Effects of Man Upon Distribution

The advent of man upon the earth and the development of his
more efficient means of travel have greatly enhanced the wide dis-
tribution of many species of animals. Marine invertebrates attach
themselves to the bottoms of ships (ship fouling) and are carried
to practically all parts of the earth. Rodents and insects are acci-
dentally distributed from one country to another by ships. During
the past man has purposely transported animals from one country
to another. In some instances the animals multiply more rapidly
in the new environment than they did in the original habitat.
English sparrows were introduced into North America in 1859, and
since that time they have become so numerous as to be a great pest
in this country. The starling, which was introduced at about the
same time, is rapidly increasing in numbers and in recent years has
been reported as far south and west as Central Texas. By cultivat-
ing extensive prairie lands, man has created insurmountable bar-
riers to grazing animals which once roamed these areas. Deforesta-
tion of wooded regions has destroyed the shelter necessary for such
animals as deer, foxes, wolves, bears, and many others.

References

Newbingin, Marion: Animal Geography, London, 1913, Oxford University Press.
8charf, R. F.: Origin and Distribution of Life in America, London, 1911, Con-
stable & Co., Ltd.

CHAPTER XXVII

THE THEORY OF EVOLUTION

It is likely that no student of modern biology has gone far before
he has realized something of a progressive relationship between the
various groups of animals in the animal kingdom. Phylogeny refers
to the background of what has gone before in producing a race or
phylum and incorporates the thought that different species have
arisen from common ancestors. The general idea of it, as now ac-
cepted by most biologists, is that all living organisms have been
derived through normal reproduction and variation by adaptation
from simpler, more generalized ancestors. Phylogenetic relation-
ships of organisms and the origin of species have long been topics
of exceptional interest to biologists. Two questions which have
intrigued the thought of many are : first, whence came living ma-
terial? and second, what has been its course of events since the
time of origin? The theory of organic evolution provides the his-
tory of living things as interpreted by the biologist.

The observations and thought of Charles Darwin gave the first
substantial support to the idea of all modern species originating
from preexisting organisms. Results of his careful studies were
presented in 1859 in his book The Origin of Species, which is now
a classic in the field. There is still lack of agreement among scien-
tists concerning the details of origin of life and species, but there
seems to be little doubt that species, genera, and even larger groups
have progressively developed since life originated on earth. The
ideas of continuity, development, and differentiation of living organ-
isms are quite generally accepted by those who have studied these
problems. The lines of kinship of animal groups are traced only
through common ancestry, and it is not ordinarily assumed that
direct relationship exists between representatives of modern groups.

Sources of evidences of relationships are quite widespread, coming
from such studies as zoogeography, comparative anatomy, embry-
ology, physiology, paleontology, and even others. By combination
of information gained from such sources much has been learned of
the past history of many groups of organisms. Even these available

480

THEORY OF EVOIiUTION 481

sources give incomplete and sometimes inconclusive evidence of the
history of any particular organism. The individuals are recognized
as being only points on a long line of modifications. The two ends
of this progressive line are seldom recognized with any finality.
Darwin, and many others since his time, recognized three funda-
mental facts that bring about a progressive continuity among living
things : heredity, with the tendency for organisms to resemble their
parents; variation, in that no two organisms are exactly alike, hence
the resemblance between parent and offspring is not absolute; and
constant production of more offspring than can survive. The phylo-
genetic relations of animals show all indications of having been
brought about by the operation of the above-mentioned factors in an
orderly, evolutionary progress.

Variation is one of the most obvious facts and consistent phe-
nomena in the animal kingdom. Darwin held that variability is
axiomatic among living things. As long as differences among indi-
viduals of a group are minor or are irregularly distributed, the
group is likely to be considered a species. However, the species is
not rigidly bounded, and it must be remembered that continuous
variation is in progress within this group as one generation follows
another. It is becoming apparent to man that new groups have
arisen and are arising within old groups of animals, also that old
groups have broken up, some to become new ones and others to be-
come extinct. These new groups were separated by small differ-
ences at first but gradually they attained greater and greater diver-
gence from the common form. This is particularly likely where the
different groups have become widely separated geographically or
exist under distinctly different conditions of life. Frequently, it is
only the widely separated extremes of the group which carry on
the posterity, the intermediate forms having died out. In this way
rather widely separated specieg may have arisen from common
stock, and the lack of intermediate forms may accentuate the wide
divergence between them. During the millions of years which life
has existed on earth, it seems evident that this process of divergence
between groups has been in operation until there now exist many
definable groups with distinct lines of relationship. Our natural
system of classification is based on the relations and differences
established for the different animal groups. Comparative studies of
numerous animal groups will help to show some of the relationships.

482

ESSENTIALS OF ZOOLOGY

Colony Formation in Certain Protozoa

Due to incomplete separation of cells following division in Pro-
tozoa, colonies are begun. In some instances groups of cells in some
such colonies, e.g., volvox, become specialized as gametes for repro-
ductive purposes and the other cells remain as somatic cells. Be-
cause of the similarity between various mastigophoran colonies and
blastula stages in metazoan development, this group has frequently
been cited as the predecessors from which Metazoa at some time
originated.

Development of the Gastrula

The primitive gastrula is thought to have had its origin from a
spherical colony of Protozoa by a more rapid growth of the cells
at one pole which brought about an infolding or invagination to
produce a new cavity from the exterior. This development estab-
lishes a body composed of two general layers. Modern adult coe-
lenterates, like the hydra, and others demonstrate precisely this
condition today, with the outer ectoderm and the inner endoderm,
each composed of cells serving vital general functions. This allows
all cells surface exposure either directly on the exterior or to the
cavity. By this means larger and more complex organisms were
developed and have lived.

Trochophore Larva

In Platyhelminthes, Annelida, Molluscoidea, Trochelminthes and
Mollusca there occur larval forms of the trochophore type. Al-
though the surface ciliation and some other superficial features
differ somewhat, a direct relationship among these forms is trace-

f^esode
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View. (After Shearer.)

B, Veliger larva of Patella (a marine snail), frontal section. (After Patten.)
(Drawn by Joanne Moore.)

THEORY OF EVOLUTION 483

able til rough the larva. The fundamental morphology of the cteno-
phore is similar to that of the trochophore larva, and the Platy-
helminthes are generally thought to have been derived from Cteno-
phores. The regular arrangement of gonads and the even, orderly
distribution of the diverticula from the intestines of the flatworms
are interpreted by many to indicate preparation for segmentation
as it appears in annelids.

Peripatus and the Wormlike Ancestry of Arthropoda

Peripatiis, the only representative genus of class Onycophora, was
considered a segmented worm for a long time because of its shape, its
even, conspicuous segmentation, and possession of nephridia for ex-
cretion. But later it was discovered that the respiration is carried
on by tracheae, and the body cavity serves as a blood space, both
of which are typical arthropod features. In addition, the paired
legs are jointed, although similar in appearance to paropodia of
Annelida. There are two jointed antennae on the head and some

Fig. 208. — Peripatus capensis. Natural size. (After Moseley from Folsom's

EntovioLogy. Redrawn by Nelson A. Snow.)

jawlike plates in the mouth. Situated as it is, midway between
annelids and arthropods, this form seems to show an immediate
transition from the one group to the other. Upon this basis it is
usually held that the arthropods have a wormlike ancestry.

Interestingly enough, there exists another idea of arthropod an-
cestry from the fact that all of the lower forms of Crustacea pass
through a characteristic stage known as the Nauplius (Fig. 167).
This larva does not correspond very closely to any strictly annelid
stage, but with its short body and three pairs of appendages it re-
sembles a modified trochophore larva. The nauplius larva has some
features in common Avith the rotifers which authors feel may have
arthropod tendencies.

Echinoderms arid Their Larval Relations

Although the adult echinoderms possess radial symmetry, that
seems not to have been in the immediate phylogenetical background.

484 ESSENTIALS OF ZOOLOGY

The larvae of all echinoderms possess bilateral symmetry. These

larvae all have definitely arranged bands of cilia over the body.

Embryologists have pointed out the fact that these larvae are not

directly related to the trochophore. They show more resemblance to

the Tornaria larva of Balanoglossns than to any of the nonchordate

forms.

Ancestry of the Vertebrates

The establishment of relationships between the chordate and non-
chordate animals has been one of the perplexing problems in the
study of phylogeny. Different students of this problem have in-
vestigated the possible relationships of such nonchordate groups as
the flatworms, nemertine worms, annelids, arachnids, insects, and
echinoderms. Their investigations have resulted in the formulation
of a number of theories putting forth the various ones of the above-
mentioned groups as the progenitors of, or claiming common an-
cestry with, chordates.

Some authors insist that the chordates have arisen from some
segmented form ; others conclude from their evidence that this is
not necessarily true or essential. All of the theories establish their
relationships to the vertebrates through the protochordates, which
are represented by Amphioxus, the tunicates, and Balanoglossns.
Each of these has been considered as ancestral stock, which has
contributed to the origin of vertebrates. It is usually conceded by
authorities in this field of study that Amphioxus is a modified an-
cestor of the vertebrates, due to the clear-cut and well-defined con-
dition of the distinctive characteristics of the chordates and the
presence of a midventral endostyle. Next in the line of thought
would come the possibilities of ancestors of Amphioxus. The tuni-
cates have been given this distinction by some. The adult has lost
most of its typical primitive characteristics, but the larva possesses
the distinctive characteristics of chordates and also the ventrally
located endostyle, very similar to these structures in adult Am-
phioxus. It has been suggested that probably the adult tunicate
once existed as an animal similar to its larva of today, and that
its forebear was not only the ancestor of modern tunicates but also
the form from which the Amphioxus group has descended.

Balanoglossns, which is usually considered the most primitive of
chordates, is regarded as a possible ancestor of, or as possessing
common ancestral stock with, tunicates and Amphioxus. As will

THEORY OF EVOLUTION 485

be remembered from the previous study of Balanoglossus, it possesses
gill slits, a support in the base of the proboscis which may be
homologous to the notochord, and four longitudinal nerve cords of
which the dorsal is the most highly developed. The above proto-
chordate relations are rather generally conceded, but there is much
less agreement concerning their origin, and several theories have
arisen of which the following are important.

Annelid Theory. — The segmental condition of this group, the re-
lationship of the digestive system to other circulatory and nervous
systems, and presence of the coelom with related nephridia, present
a close comparison to what is found in the embryonic development
of the vertebrates. It has been suggested by some scholars that by
inverting the body of the nonchordate annelid, the fundamental
systems are brought to resemble their relative locations in verte-
brates. A fibrous cord has been found in some groups of the an-
nelids, and this structure is held to take the place of a notochord in
function and position. These fibers are found just dorsal to the chain
of ganglia in the annelids.

Arachnid Theory. — Such forms as scorpions, Limulus, and other
arachnids have been favorably compared to vertebrates. By com-
parison of these arachnids with the extinct, fishlike ostracoderms,
an elaborate theory of the possible origin of the vertebrates from
this ancestry is derived.

Echinoderm Theory. — This theory of vertebrate descent goes again
to the Balanoglossus. The developing egg of this animal becomes
a larva known as Tornaria (Fig. 47), which floats in marine waters,
has bilateral symmetry, is almost transparent, and possesses bands
of cilia used in locomotion. This larva is almost exactly like that of
the starfish and other echinoderms which live in the same habitat
(Fig. 140). The close correspondence of features of these two groups
of larvae has suggested the conclusion that these two types of ani-
mals have descended from a common ancestor which was similar to
these larvae. The line of descent of one branch of this stock has
presumably passed through Balanoglossus, Tunicata, Amphioxus, and
Vertebrata. The nonchordate ancestors are not yet conclusively
determined, but the foregoing theories suggest the thinking and evi-
dence along that line.

Within the class Vertebrata the relations are somewhat more evi-
dent, but the phylogenetic sequence is rather obscure at some points.

486 ESSENTIALS OF ZOOLOGY

Cyclostomes, the simplest vertebrates, are most closely related to
Amphioxus, Avhich has been suggested as the protochordate most
similar to vertebrates. These very primitive fish have an eellike
body without paired fins and without jaws. They have from seven
to fourteen branchial (gill) apertures in different species, and all
of them possess skeletons composed of cartilage. The cartilaginous
skull is not entirely closed dorsally but resembles a trough with
bars over the roof. The anterior end of the nerve cord has expanded
to become a brain. Next in order of complexity are the Elasmo-
branchii, which possess well-developed, paired appendages (fins)
and jaws. They also have a cartilaginous skeleton, but the skull
is much more complete dorsally. The number of gill arches is re-
duced to five, but the apertures are uncovered as in cyclostomes.
The number of aortic arches has been reduced from the sixty to
ninety pairs of Amphioxus or seven pairs of lamprey to five pairs.
The group of ganoid fishes, which was the dominant Devonian ani-
mal, is generally conceded to have Elasmobranch ancestry. Most
ganoids have more or less cartilage along with the bony structure
of the skeleton. Their gills are covered with an operculum, and
there are only four aortic arches. The bony ganoids are usually
thought to be the ancestors of true bony fish.

There is an extinct form of Amphibia, Stegocephalia, which shows
relations to the ganoid fishes and for this reason the ganoids are
usually named as ancestors of Amphibia. Some authors hold that
the lungfishes, which represent an independent branch of the Elas-
mobranch group, are the ancestors of Amphibia because of their
ability to breathe air and live out of water.' However, the former
view of the phylogenetic relation between fish and amphibia is most
commonly held. Amphibia have well-developed bony skeleton with
paired appendages for locomotion on land. Lungs have appeared
as a means of aerial respiration, and the aortic arches have been
reduced to three.

Reptiles are supposed to have descended from Stegocephalia also,
with most modern reptiles coming by way of Rhynchocephalia which
is represented by one living species, Spkenodon punctatum. The
snakes, lizards, crocodiles, and the extinct dinosaurs have probably
branched frODi this group, while the turtles are thought to have
descended through Therornorpha, another extinct branch of Stego-
cephalia. The dinosaurs are credited with the ancestry of birds by

THEORY OF EVOLUTION 487

way of a toothed, feathered, extinct form known as Archaeopteryx.
It was essentially a flying reptile. The mammals probably descended
from the reptilian group Tlieromorpha by way of our modern mono-
tremes which lay eggs, hatch them out, and then suckle the young
with milk from mammary glands. The marsupials, such as kan-
garoos and opossums, are next in order, and from these it is thought
the Placentalia have arisen. Within this group some authorities
hold the view that the Primates, the order including man, have
arisen from Insectivora. The apes and monkeys belong to the Pri-
mate group, and there has been some misunderstanding among lay-
men generally in regard to the possible relationship of man and the
apes. Most people have the misconception that this is a linear
descent in which the most advanced member of the lower group
represents the immediate ancestor of the next higher group. As a
matter of fact, the theory is not that the higher monkeys are in
the process of becoming apes and the higher apes becoming men, but
that all three of these groups have had origin as different lines from
a common primitive form.

Recapitulation Theory. — In the early part of the nineteenth cen-
tury von Baer observed that the early stages of vertebrate embryos
of different classes had a very close resemblance to each other. He
did not subscribe, however, to the recapitulation theory when it
was formulated later. Haeckel, coming a little later, became con-
vinced that the developing embryo lives over again the stages
through which its whole race has passed, and he formulated the
recapitulation theory or biogenetic law from this idea. In other
words, the organism in its individual life tends to recapitulate the
different stages through which its ancestors have passed in their
racial history. Briefly this same statement is, ontogeny recapitulates
phylogeny. The rehearsal of the phylogeny is in rather slurred form
in some details, but the basis for the idea is readily seen. In brief,
the theory is applied by comparison. Nearly all metazoan organ-
isms begin life by the union of the two germ cells to form a single-
celled zygote which is the new organism. At this time it is com-
parable to the Protozoa. During the ensuing cleavage divisions a
colonial form is represented. Following this, when one side infolds
to form a gastrula with two germ layers, the embryo is almost
identical to the diploblastic coelenterates as represented by Hydra.
Following this, the third germ layer forms between the others and
results in the triploblastic metazoan.

488 ESSENTIALS OP ZOOLOGY

Basis for the Theory of Evolution

One who lias thoughtfully studied the field of zoology soon realizes
many relationships or homologies in structural make-up of certain
different groups of animals ; much similarity of embryonic develop-
2iental processes, and fundamental coincidences in physiological
activities in all living material. Too, it is recognized that fauna
and flora are not the same in all parts of the earth at the present,
and have not been the same in the past as now. These realizations
and other similar ones have been based on scattered sources of evi-
dence, much of which is indirect. The evidences which have been
discovered, however, have led to the formulation of the theory of
evolution and its rather general acceptance as a working hypothesis
among biologists. There has been confusion regarding this subject
through failure to distinguish between the existence of progressive
evolutionary changes, the course of these changes, and the cause of
such a progressive series of events. The latter point has been the
basis of most of the questions concerning the whole idea, and it is
the most speculative of the three. These three are separate though
related factors.

In general, scientists are convinced, according to the evidence
they have examined, that organic evolution exists in the form of a
progressive change, which has generally proceeded from simpler to
more complex organization through a long period of time. Although
some sections of the course of this series of developmental events
cannot be charted as completely as a ship's course at sea, numerous
landmarks serve to indicate what has taken place. Biologists are
free to admit that the causes of this process are not understood, and
treat it strictly as a scientific problem.

Most estimates of the time when living organisms first came into
existence range between 60,000,000 and 1,200,000,000 years ago,
coincident with Pre-Cambrian deposits. The manner of origin of
protoplasm is purely hypothetical. It is suggested that when con-
ditions became suitable, as to chemical elements present, tempera-
ture, pressure, etc., a relatively simple colloidal, protoplasmic mix-
ture arose, having properties of life though perhaps in a very ele-
mentary way. It is assumed that all organisms which have lived
are descendants of this simple origin. Cellular organization, with
division of labor between nucleus and cytoplasm, is taken to be a
first step in the development of protoplasm. It is the simplest vital

THEORY OF EVOLUTION 489

unit now recognized. If this speculation is followed further, it may
be supposed that the unicellular organisms of modern times have
descended from such early cells without changing from the simple-
celled state, but have developed many specializations as individual
types, while metazoans have advanced from certain of these simple
forms to a more complexly organized cellular condition.

From the nature of protoplasm and from the evidence in the geo-
logical records, it is usually assumed that organisms first appeared
in the water near the shores of the primitive oceans. Presumably
the first oceans were boiling hot, and the land at that time was a
molten volcanic mass. If this is true, there must have been a long
period after the formation of the atmosphere, seas, and land before
protoplasm, as we know it, could have existed. After these cooled
and became favorable for life the seas are thought to have supported
an abundant life before the land became suitable for its existence.
Along with these several speculative aspects of the subject there
have been offered several forms of evidence to support the existence
of an evolutionary progress of development in organisms.

Geological Evidence. — Paleontology is the study of fossil remains
of organisms deposited in the strata of the earth's crust. Shells
and other hard parts are mineralized or petrified; in other cases
mud impressions, or tracks, or pitch-preserved individuals, such as
insects, and a few frozen forms constitute the majority of fossils.
It must be kept clearly in mind that the geologist is able to deter-
mine quite accurately the sequence of time or chronological suc-
cession of the layers of the earth's crust. The geological time scale
shows a long period before any life existed, then the appearance of
unicellular plants, then unicellular animals, then colonial forms,
simple many-celled forms, and then the more complex ones. Such
a timetable estimates the relative period of time during each era
and shows some fusion and overlapping of certain types of life.
Certain types of unicellular forms are continuous through the en-
tire scale.

The principal facts shown by the fossil record may be summa-
rized: (a) The fossil forms are not strictly identical with any living
species, and the remains of plants and animals of each geologic
stratum are at least specifically different from the forms in any
other stratum, but they may belong to the same genus; (b) the
oldest strata containing fossils have represented in them most of

490

ESSENTIALS OF ZOOLOGY

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THEORY OF EVOLUTION

491

the simple forms of nonchordate animals, while the upper strata
contain fossils of all groups more nearly like modern forms, includ-
ing chordates; (c) in studying these in sequence, there may be ob-
served a gradual progression from simpler and generalized types
toward more specialized and complex forms as one proceeds from
the older toward the upper or newer strata; (d) only the more gen-
eralized varieties have persisted within the groups that, as a whole,
have become specialized; many of the others have long since reached
their climax of specialization and have become extinct; (e) many
of the dominant groups of organisms have arisen near the close of

Wr/st

Wrist-

Fig-. 210. — Positions of the human hand to show the comparative stages of
elevation of the horse's foot to the tip of the middle toe. (Courtesy of American
Museum of Natural History.)

a period during which gieat climatic changes were taking place and
have enjoyed dominance during the following period because such
a group probably arose in response to the conditions; (f) although
many nonchordate phyla had reached an advanced stage of develop-
ment in the early Cambrian period, where early fossil records occur,
many ancestral sequences have been observed, and these have sup-
plied information making possible the detailed description of the
course of events that has led to the surviving animals of modern
times; (g) the developmental changes of the chordate groups are
more completely read in the fossil record, with the history of the

492 ESSENTIALS OF ZOOLOGY

mammals in fullest outline, partly because they are relatively recent
and partly because the mammalian skeleton is readily fossilized. The
most complete pedigree in fossils has probably been worked out for
the horse, and a great deal of its material has been located right in
the southwestern part of the United States.

The Rise of the Horse. — Primitive horselike animals are thought
probably to have arisen from an extinct group called Condylarthra,
which had five toes on each foot and a large part of the sole resting
on the ground. The first unquestionable horselike form found in
America is the small Eohippus which was about one foot tall and
the fossils of which came from the rocks of Eocene times. It had
the outer four digits complete on the forefoot, but no trace of the
thumb, while the hind foot had three complete digits with vestiges
or splints of the first and fifth. Following the foxlike Eohippus,
later in the Eocene period came the OroMppus with an enlarged
central digit in the forefoot and the loss of the splints in the hind
foot. Mesohippus, about the size of a large dog with a three-toed
foot both in front and behind but with the side toes much reduced
and a trace of the fifth digit in the forefoot, appeared during the
Oligocene epoch. Merychippus of Miocene times and Pliohippus of
the Pliocene epoch show a continuation of this reduction to a one-
toed type which leads to Equus cahallus of modern times. The
modern horse walks on the tip of the middle toe of each foot with
the vestiges of digits 2 and 4 persisting as splints.

Arthur Dendy, in 1911, wrote that the horse is an example of the
adaptation of a lowland type to become a plains type, as the ex-
tensive, dry, grass-covered plains developed. The adaptation has
proceeded along two lines. The limbs have become elongated by
the elevation of the heel, thus putting the animal on tiptoe and
fitting it for rapid flight from its enemies on the grass-covered open
prairie. At the same time the neck and head became elongated to
enable the animal to graze the ground without bending the legs.
Along with this the teeth changed from a carnivorous form to a
complex, broad grinding type for feeding on grass. In addition to
these changes the brain advanced.

These changes took place gradually through millions of years and
the intermediate forms give the paleontologist a graphic picture of
the history of this modern species. The sequence of these stages
seems to fit in exactly with the theory that each has been derived

THEORY OF EVOLUTION

493

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494 ESSENTIALS OF ZOOLOGY

from the preceding by a continued adaptation to the changing con-
ditions of life. The horse's pedigree is essentially similar to that
of numerous other forms, such as the elephant, camel, and certain
birds, which have been worked out.

The Rise of Man.- — The human group constitutes one of the groups
of the class Mmnmalia along with many of the common larger ani-
mals which are well known. In fact, all of the vertebrate animals
which have hair and mammary glands fall in this same class. Man
is classified in the order Primates in which is grouped also the apes
and their relatives. There can be no question but that the structure
of the human body is that of a mammal or even a Primate. The
suborder is AntJiropoidea ; the family, Hominidae; the genus, Homo;
and the species, sapiens. According to studies and discoveries com-
ing from many parts of the earth, it seems quite evident that there
has been a long progressive development of manlike animals for
hundreds of thousands and possibly millions of years to produce the
being known as modern man.

The oldest known representative of the human family is Sinan-
thropus pekinensis (Peking man). His remains were found in a
cave of Pleistocene times approximately forty miles from Peiping,
China. A complete cranium of this form Avas found in 1929 and
since that time parts of approximately a dozen others have been lo-
cated. They are always found along with the remains of the type
of elephant, rhinoceros, horse, saber-toothed tiger and others of the
Pleistocene epoch which is estimated to have been between 500,000
and 1,000,000 years ago. Anthropologists have agreed that this
ehinless apelike skull with a receding forehead, heavy brows, and
heavy jaw, belonged to a primitive man who could use tools and
build fires. Another important discovery was made in Java in 1891
when a skull, a jaw bone, a few teeth, and a femur were found.
This specimen is now known as Pithecanthropus erectus (Java man),
and from the shape of the femur, it is definitely indicated that he
walked erect. The brain cavity is intermediate in size between that
of the largest apes and savage man. It is estimated that this form
is somewhat less than a million years old. In 1907 a jaw bone found
near Heidelberg, Germany, was determined as having come from a
primitive man. A few years later, in England, the remains of
Eoanthropus dawsoni (Piltdown man) were discovered. The brain
cavity was larger than that of Java man and the forehead was

THEORY OF EVOLUTION 495

wider and higher. The Heidelberg and Piltdown men are considered
as contemporaries of about 500,000 years ago.

A more recent type of European man is known from remains
(skullcap and several long bones) found near Dusseldorf, Germany,
about the middle of the last century. A rather complete collection
of the skeletal parts has since been made. This form is called Homo
neanderthalensis (Neanderthal man) and is thought to have de-
scended from the Heidelberg man. There is division of opinion as
to destiny of this man. One group holds that he became extinct
sometime between 100,000 and 200,000 years ago, during the Great
Ice Age in Europe. Another group of anthropologists contends that
the negroid and Australoid races of men are the descendants of this
man.

There is a more recent Asiatic group known as the Cro-Magnon
man dated with the Old Stone Age of perhaps 40,000 years ago.
Some students of early man hold that modern races have originated
from this group. Some even suggest that Eskimos may be rather
direct descendants. There are fewer of the apelike features in this
type than in any of the others and apparently there have been no
striking changes in human features since the time of this group.

From the paleontological evidence which remains, it seems in all
probability that the more primitive prehistoric man was apelike.
Too, it seems likely that men and apes have originated from a com-
mon ancestor. The heavy jaws, receding forehead, strong orbital
ridges over the eyes, pointed ears, presence of hair over the ears
and parts of the body are exhibited in some groups of modern primi-
tive men. There is a close resemblance between the skeleton of the
gorilla and the human skeleton. Other notable features of compar-
ison may be seen in that the arms of the human infant are propor-
tionally long and the grasp of the hand is exceptionally strong.
Also the large toe is freer and more prehensile in the infant human
than in the adult.

Man is a rather generalized type of animal as compared with
some other mammals. Most of the parts of the body have remained
relatively unspecialized, and as a result there is a high degree of
adaptability, making it possible for men to live under many diverse
conditions and climates. However, man has advanced among ani-
mals to a greater degree than others because of the development
and use of intelligence. This development of the brain and its func-

496

ESSENTIALS OF ZOOLOGY

A.

ms 212— A Face and skull of Pithecanthropus erectus. B, Face and skull of
Eolnfkrlfus dat^sSS ^Udapted from McGregor Fr^m r^^e ^^oZugou Earth
and Man, edited by G. A. Baitsell and published by Yale University Fress.)

THEORY OF EVOLUTION

497

A.

Fig. 213. — A, Face and skull of Homo neanderthalensis. B, Face and skull of
Homo sapiens (Cro-Magnon man). (Adapted from McGregor. From The Evolu-
tion of Earth and Man, edited by G. A. :p£|,itsell ^.nd published by Yale University
Press. )

498 ESSENTIALS OF ZOOLOGTY

tions has even given man the advantage of being able to make use
of other animals for his own needs. The use of fire, tools, weapons,
clothing, shelter, and storage of food is all contemporary and
corollary to the development of the human brain. As development
along this direction progressed, the power of speech and later the
ability to write have become exclusively human traits. Through the
development of intelligence the human kind has been able to control
certain forces in nature to his own advantage. This superiority of
brain and intelligence is man's direction of specialization, and it
distinguishes him from the other animals.

The molluscan, echinoderm, and arthropod fossils give similar
stories among invertebrates. The numerous fossils of fish. Amphibia,
Reptilia, particularly dinosaurs, and birds with the famous Arche-
opteryx, all have their testimony to contribute.

Distributional Evidence. — The paleontological distribution of ani-
mals is considered vertically while geographic distribution is in
horizontal plane. It is impossible to separate these two completely.
The study of geographical distribution gives essentially a cross
section of the vertical distribution, thus giving a sort of ''still pic-
ture" of the complex developmental relations of animal groups at
one moment in geologic time. In studying this subject, it is neces-
sary to have in mind two fundamental conceptions. The first is that
the ancestors of related genera first appeared or originated in a
locality which is designated as the common center of origin. A sec-
ond conception is that as the ancestral form became established and
multiplied, migration in search of food and more suitable conditions
occurred. Barriers, many of w^hich were geographic, determined
the direction and extent of this migration. Large bodies of water
blocked the passage of terrestrial animals, as of course land was a
barrier to aquatic animals. High mountains or deserts were barriers
to all animals unable to withstand low temperatures and altitudes
on the one hand, or high temperature and dryness on the other.
These forces, and others, are believed to account for the natural
distribution of animals. There are the cases of the camel group,
originating in North America, migrating to South America and Asia
by the land connections of the Eocene to Pliocene epoch, and the
tapirs, which are represented by distinct species in two widely sepa-
rated regions, South Asia territory, and the Central America-South
American territory. Here again paleontological data show that in

THEORY OF EVOLUTION 499

the Pliocene epoch tapirs were distributed over nearly all of North
America, Northern Asia, and Europe. Following that time they
were gradually decreased due to discontinuous distribution until
the one-time world-wide distribution is now isolated in two widely
separated regions. Long isolation of genera in different environ-
ments will bring about definite specific differences. As an example,
a litter of foreign rabbits was introduced to the islands of Porto
Santo during the fifteenth century and by the middle of the nine-
teenth century the descendants had become so distinct from the
original ancestors that they were described as a new species. There
are numerous instances of this effect, demonstrated by isolation on
ocean islands. The islands are either continental, with fauna similar
to those of the nearby continent from which the animals have come,
as the British Isles, or they are oceanic, with a very bizarre assem-
blage of animals which have either drifted in or have been carried
there, as the Haw^aiian Islands. Many of the animals on these
oceanic islands are peculiar and are found nowhere else on earth.
Australia has a group of animals which are very different from those
of Asia because the two have been so long separated. Europe, Asia,
Africa, and North America have been connected with each other by
land bridges in recent- enough times that the mammals show simi-
larity. The distribution, of the species of a genus often radiates
from the more generalized species which occupy the center of the
range of the genus, and the more specialized species are found in
the scattered outskirts of the range.

From the preceding statements concerning distribution it seems
that any given species originates in a definite locality, that it multi-
plies there and migrates in all possible directions. It modifies as
it goes in response to the various new conditions prevailing and
becomes divided into local varieties which in the course of time
become species. Thus the working- method of animal distribution,
as it has been presented, is the principle of dtscent from preceding
generations with modification.

Morphological Evidence. — Classification of the animals shows in
fact something of the morphological evidence, since current classi-
fication is based chiefiy on anatomical features and comparative
anatomy. The groups of the classification are established largely
on anatomical similarities. The differences existing among the rep-
resentatives of all the classes of vertebrates are relatively slight

500 ESSENTIALS OF ZOOLOGY

when set over against the fundamental similarities. Closely related
groups show numerous similarities in the form of homologies. The
flipper of the whale, the wing of the bat or the bird, the foreleg of
the cat and the arm of man all show the same general type of struc-
ture in spite of certain specific differences. There is seldom any
question of their phylogenetic relationship.

The presence of series of similar parts on different segments of
the same animal and the various specializations of these parts show
a progressive development. The highly specialized walking legs,
uropod, claws, and feelers of crayfish, for example, have all devel-
oped from the simple swdmmeret type of appendage. They form a
serial homology and are also homologous to the appendages of all
other Crustacea a^ well.

In higher forms of animals, such as man, there are numerous struc-
tures which seem to be useless and are even harmful in some in-
stances. These are spoken of as vestigial structures. Such parts
correspond in structure and plan to functional parts in other re-
lated forms, but are reduced morphologically and without the origi-
nal function. In man, one probably thinks first of the vermiform
appendix of the colon as such a structure. This is apparently func-
tionless in human beings and can be removed with no loss, but this
same organ in rabbits, some birds, and other animals is an extensive
and functional digestive organ. Man has a coccyx or vestigial tail,
and the frog has the urostyle. Pythons and porpoises, neither of
which has the least use for them, have vestigial hind limbs similar
to the functional ones of their relatives. The salivary glands of
certain snakes have become adapted as poison glands, certain sweat
glands have become milk glands, and gill arches have become sup-
porting structures of the tongue, larynx, and throat of adult ter-
restrial animals. The blood supply and nerves both follow the
phylogenetic changes of these organs.

Most animal types seem not to have originated in their present
forms, but they seem to have undergone changes through the long
periods of geologic history. The explanation offered by modern
biologists for the anatomical relations and resemblances between
animals is that the individuals in any group have inherited a similar
plan of structure from the ancestors which were common to all mem-
bers of the group. In a group, such as the vertebrates, there have
been numerous modifications of various fundamental structures in

THEORY OF EVOLUTION 501

different subdivisions in relation to the particular habits of life ; still
they remain fundamentally alike because they have developed from
the underlying plan of organization found in the ancestors. The
seal and the bird, although quite different, show similarities in habits
and otherwise because of common ancestry long ago. The conclu-
sion of biologists of today is that all of the animals in a group, such
as the vertebrates, have arisen by descent with change from a primi-
tive organism which possessed the fundamental organization as shown
from cyclostomes to man.

Embryological Evidence. — Evidences from this field really con-
tinue directly from the previous discussion. The animal is to be
thought of as an individual from the single-celled zygote stage to
the mature stage of old age, no matter what its complexity is.
Intimately related types of animals parallel through a large portion
of their development to diverge somewhat in adult condition, more
remotely related forms take separate developmental courses rather
early in life, and unrelated forms may be different almost from the
beginning. In numerous instances the developing stages of more
advanced forms resemble very closely the mature stages of the less
advanced types in a serial fashion. The history of the individual
animal often corresponds in a general way to the history of the
advances of the animal kingdom, up to its state of development.
This apparent repetition of the ancestral development in individuals
was what led Haeckel to formulate the recapitulation theory, ex-
pressed briefly: ontogeny repeats or recapitulates phylogeny, as has
been discussed earlier in the chapter.

In the vertebrate group these apparent relations are much shown.
Most of the embryos of this group are so similar that it is nearly
impossible to distinguish them. They pass through the identical
stages of development. Systems, such as the circulatory, nervous,
digestive, and respiratory, follow the same course of development in
all of the vertebrates, no matter how simple or complex. In earlier
stages the similarity is strikingly close.

The course of development and the modifications shown in the
aortic arches or main arteries leaving the heart and passing through
the gill regions is a specific example of the manner in which a spe-
cific set of structures follows out a repetition of ancestral stages in
the development of the individual. Fig. 204 show^s a comparison
of the arrangement of the branchial (aortic) arches from the primi-

502

ESSENTIALS OF ZOOLOGY

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THEORY OF EVOLUTION

503

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504

ESSENTIALS OF ZOOLOGY

tive set of six through the fishes, Amphibia, reptiles, to the birds
and mammals, with a modified condition of three arches.

Along with the aortic arch situation are other examples of similar
stages of development in all forms of the group. Gills are present
in all chordates at some time. In the primitive ones the gills are
functional throughout life; in the more advanced types they are

Fig. 215. — Diagram to show the modifications of aortic or branchial arches in
different vertebrate types. A, primitive scheme ; B, lungflsh ; C, primitive amphibian
(urodele) ; D, frog; E, snake; F, lizard; G, bird; H, mammal, ec, ic, external and
internal carotids ; va, ventral aorta ; da, dorsal aorta ; db, ductus Botalli ; p, pul-
monary artery; s, subclavian; c, coeliac. Vessels carrying venous blood are black;
those with mixed blood are shaded ; those which disappear are dotted outlines.
(After Boas, Reprinted by permission from Kingsly, Comparative Anatomy of
Vertebrates, P. Blakiston's Son and Company.)

only transitory gill structures. In frogs and toads, the tadpole stage
is essentially fishlike, and they become amphibian at metamorphosis.
The notochord is a definite structure throughout the life of the

THEORY OF EVOLUTION 505

primitive chordates, but is present as such in only the embryo stage
of all Others. The development of the heart from the tubular con-
dition through the two-chambered, three-chambered, and finally four-
chambered condition, illustrates the same progressive development.

Physiological Evidence.— The fact that all protoplasm possesses
the same set of fundamental properties or capacities as contractility,
irritability, metabolism, etc., is in itself a definite indication of rela-
tionship of all organisms, since they are all composed of protoplasm.
Too, all protoplasm acts under similar lav7S and conditions.

Such natural substances of animal bodies as the hormones, or anti-
bodies or even enzymes are almost universal in their reactions among
chordates and even among nonchordates. Most of them are inter-
changeable from one animal group to another. A deficiency of pep-
sin or adrenalin in man may be supplied from a cow, a hog, a cat,
a rat, or a dog. The blood of all vertebrates has certain physiological
similarities and some specific differences. The blood of large groups of
human beings will mix without agglutination and is said to ''match."
Other individuals' blood may not ''match" in this type but will mix
with blood from another group. The agglutination (clumping of red
corpuscles) when blood from two individuals is brought together is
due to the reaction of two substances produced by the corpuscles in
one or both of the blood samples. The designation of the blood
groups depends upon the presence or absence of one or both of these
substances. One blood group (0) contains neither, another (A)
contains one substance, a third (B) the second substance only, while
a fourth group (AB) contains both substances. Serum of the first
group will agglutinate corpuscles when mixed with any of the other
three. The groups with the single substance will agglutinate cor-
puscles in blood having only the other substance or blood with both
substances. The serum from blood with both substances will not
cause agglutination in any of the others.

In lower mammals some similar blood groups have been found,
but it is only in the apes that the groups correspond to those in the
human being. This is an indication of the rather close relationship
of these animal groups. In the human being and in other animals,
this blood characteristic is permanent in the individual, and it is
hereditary.

Serum (blood minus corpuscles and fibrinogen) studies also estab-
lish certain relationships among animals. If small quantities of

506 ESSENTIALS OP ZOOLOGY

human blood serum are introduced at intervals into the blood stream
of a rabbit, in time there will be developed in the blood of the rabbit
a substance (antibody) which, when mixed with normal human
blood will cause precipitation of the proteins here. Serum from such
a rabbit is called antihuman serum. When this serum is mixed with
serum in a certain dilution taken from human, chimpanzee, gorilla, or
monkey, it will cause precipitation. If the dilution is increased,
there is no precipitation when mixed with monkey serum; at still
higher dilution there is no precipitation when mixed with gorilla
blood; and higher none for chimpanzee blood, until finally none for
human blood. On the basis of these sera precipitation tests the chim-
panzee is closest in its relationship to man, then the gorilla, and then
the monkeys. '

In making similar tests on other vertebrate groups, it is found that
crocodiles are more closely related to birds than are the other reptiles ;
also among reptiles, that snakes and lizards are more closely related
to each other than to turtles. Too, crocodiles show a closer relation-
ship to turtles than to the other groups of reptiles. Blood studies of
the various groups of vertebrates indicate that there is more similarity
in blood of closely related forms than of others. At the same time, it
is seen that a chemical relationship persists in the blood throughout
the chordate phylum.

The breeding of plants and animals through long series of genera-
tions of domestication and laboratory experiments has yielded much
information concerning the ways of adaptation and phylogenetic
development. A significant result is the demonstration of changes
occurring in animals and plants. From such studies it seems quite
obvious that organisms now living have come to be what they are
by gradual change from generation to generation through a course
of descent from preexisting and varied ancestors, rather than by a
sudden and completely new development. Most of the various
breeds of cattle, chickens, dogs, horses, sheep, crops, etc., have been
developed in each case from a preexisting common ancestor.

Darwin and Studies of Evolution. — Most discussions of organic
evolution usually begin with mention of Darwin's monumental work
on this subject, and difficult it is to get away from his fundamental
basic thinking on the subject. He was the first to survey thoroughly
the fields of morphology, embryology, and paleontology, and to re-
late logically the data found there to the theory of evolution. From

THEORY OF EVOLUTION 507

his studies, many of which were done along the east and west shores
of South America while he was naturalist of a British Naval ex-
pedition on the ship Beagle, Darwin formulated a clear-cut and
definite argument for evolution on the basis of natural selection. Be-
ginning with Malthus' law of population, published in 1838, which
stated that since man reproduces in a geometric ratio, the earth would
be overpopulated in a few generations except for such checks as the
arithmetic ratio of increase in food production, disease, war, flood,
earthquake, fire and other natural catastrophes reducing population,
Darwin formulated the theory of natural selection. This theory in-
cludes among other things the application of Malthus' law to all living
organisms. The four basic points on which this theory is developed
may be named in order as follows: (1) overproduction, (2) struggle
for existence, (3) variation and heredity, and (4) survival of the
fittest (natural selection).

Overproduction is in operation in all thriving normal species. A
single codfish will produce several million eggs in one season. If
every codfish egg were to be fertilized, to reach maturity, and to re-
produce with no loss from one generation to the next, it would not be
more than a dozen years until the entire face of the earth would be
covered with codfish and all other animals would be crowded out of
existence completely. Even a form like the elephant, which lives to
be ninety or a hundred years of age and averages only six progeny,
could soon occupy all of the standing room on the face of the earth.
Beginning with one pair of elephants and providing every individual
lived and reproduced even at the slow average rate mentioned above,
19,000,000 individuals would be produced in 750 years. If every
elephant alive today were to enter into a program like that, both food
and space would become quite scarce before "many generations. How-
ever, this doesn't happen on a large scale. All plants and animals
tend to produce more offspring thafi can ever reach maturity and
reproduce.

The struggle for existe^ice is ever present because there are more
individuals produced than the habitat will support. The two most
fundamental needs for which organisms struggle are (1) food and (2)
opportunity to reproduce and rear young. Of these two, the struggle
for food is very immediate and the food supply is an important
limiting factor on population from season to season. Since the food
supply, on the average, remains quite constant, it is evident that

508 ESSENTIALS OF ZOOLOGY

only a limited number of the increase in individuals can be sup-
ported in a particular habitat ; thus a struggle ensues with each in-
dividual attempting to secure the necessities of life. Not only is
there a struggle for food but also with many factors in the environ-
ment like climate, geographic changes, etc.

Survival of the fittest was the outcome which Darwin saw resulting
from such a struggle. Those individuals which were best adapted to
the environment into which they were born have been the ones to
win out in the struggle and leave offspring for a future generation.
The inheritance of favorable or unfavorable pharacters influences very
strongly the success of the individual in maintaining itself. The sur-
vivors in any generation are those which inherit the most favorable
combination of variations. Many variations, both favorable and un-
favorable to the success of the individual, are hereditary.

When changes in environment come and bring about new living
conditions, the animals in the particular habitat must either meet
these changes, be able to migrate, or perish. The standard for fitness
has changed under such circumstances, and animals with somewhat
different characters and adaptations may now be the ' ' fittest. ' ' The
individuals whose variations have brought them to most nearly fit
the requirements for life in their particular habitat will be the ones
most likely to obtain sufficient food supply and adequate provision
for reproduction to increase their population rapidly. As one group
is able to do this, it invariably reduces or perhaps entirely eliminates
other species in the locality. Evolutionary changes result from sur-
vival of the fittest, since those individuals which succeed have done
so because of an accumulation of favorable variations in each of suc-
cessive generations. Those individuals or races which have not been
as well adapted to conditions of the habitat have become inconse-
quential or extinct. The appearance of characters in an animal is a
matter of chance as far as the individual is concerned. Natural
selection may act as an eliminating agent and may determine whether
the character or trait shall survive after it appears.

Mutation Theory. — Hugo de Vries is the name most prominently
connected with the origin of this theory. He was a Dutch botanist
and in 1886 found some evening primrose plants (Oenothera lamarki-
ana) which exhibited discontinuous variation or sudden appearance
of new characters. These sudden, sharp variations came to be known
as mutations. There are two types of variations which have been

THEORY OF EVOLUTION 509

recognized : ( 1 ) continuous or fluctuating variation, such as height of
individuals of a species where they are expected to fall within a nor-
mal range thus allowing a degree of variation among individuals of
the species, and (2) discontinuous or sport variation (mutation)
where the variation falls outside the normal range of variation and
not connected with it by intermediate changes.

In the course of seven generations of this primrose and involving
approximately 50,000 individuals, six different mutations were found.
The new characters which appeared were quite different from those of
the typical species and were hereditary as well. Since this was the case,
de Vries concluded that he was observing the origin of new species.
He was sufficiently convinced of this to discount Darwin's conception
of the development of new species by the gradual accumulation of
continuous variations through natural selection. He pointed out
that mutations are due to changes occurring in the germ plasm
while the continuous variations, individually, are due to changes in
somatic cells.

Following de Vries' work there has been much study of mutations,
and numerous ones have been found in nature. Too, it has been
found that spontaneous mutations could be produced in Drosophila
(fruit fly) by x-ray radiation. It is thought that mutations come
as the result of physiological changes in the chromosomes or genes.

While this mutation theory of origin of new species has prompted
much study and thought on evolution it seems not to have displaced
Darwin's general conception of the origin of new species. So far as
Darwin's theory is concerned, the occurrence of mutations only
hastens the process of evolution since they produce quick, abrupt
variations instead of the slower, smaller, continuous variations.
Natural selection will operate with either. Biologists now consider
both small and large variations as mutations, and have turned back
to Darwin's idea of natural selection as the most likely explanation
of the development of new kinds of animals.

References

Lindsey, A. W.: Problems of Evolution, New York, 1931, The Macmillan

Company.
Lull, E. S. : Organic Evolution, New York, 1929, The Macmillan Company.
Newman, H. H. : Evolution Yesterday and To-day, Baltimore, 1932, Williams &

Wilkins Company.
Wilder, H. H.: History of the Human Body, New York, 1909, Henry Holt &

Company, Inc.

GLOSSARY*

Abdomen (ab do'men), the portion of the trunk posterior to the thorax of an
animal.

Aboral (ab 6'ral), opposite the mouth.

Absorption (ab sorp shun), the process of taking in soluble foods by the circu-
latory medium or by the protoplasm directly.

Accommodation (a kom 6 da'shun), the power of adjustment of the eye to near
and far objects.

Acetabulum (as e tab'ti lum), the socket in each side of the innominate bone
of the pelvic girdle into which the femur fits.

Achromatic figure (ak ro mat'ik), the mitotic figure without chromosomes.

Aestivation (es ti va'shiin), state of torpidity induced by heat and dryness.

Allelomorphs (a le'16 morfs), a pair oi corresponding genes in homologous
chromosomes, but each produces a different character.

Allergy (al'er ji), acute sensitiveness to a foreign substance, as foreign protein
in the body.

Alternation of generation (al ter na'shun of jen er a'shun). (See Metagenesis.)

Altricial (al trish'al), in reference to birds which are hatched without feathers
and in a helpless condition.

Alveolus (al ve'6 ICis), a small cavity or pit, such as the air sacs in the lung
of a higher vertebrate.

Ambulacral (am bu la'kral), area of echinoderm related to tube feet.

Amino acids (am'i no), organic acids with a (NHj) radical, and derived from
complex proteins.

Amitosis (amito'sis), direct cell division, occurring without chromosomal ac-
tivity.

Amnion (am'ni on), inner embryonic membrane of terrestrial vertebrates.

Amoeboid movement (a me'boid), the streaming of protoplasm in a cell to ex-
tend the cell in some direction with the formation of pseudopodia.

Amphiaster (am'fi as ter), the complete mitotic, figure of a dividing cell.

Amphiblastula (am fi blas'tu la), the free-swimming larval form in sponges.

AmpMmixus (am fi mik'sis), union of nuclear material from two different
cells, as in fertilization.

Ampulla (am pul'a), a bulblike dilatation.

Amylopsin (am i lop'sin), a pancreatic enzyme capable of converting starch
into sugar.

Anabolism (an ab'6 liz'm), the building up of living protoplasm.

Analogous (a nal'o gus), differing in structure and origin, but similar in func-
tion.

Anaphylaxis (an a fi lak'sis), acute reaction of the body to foreign protein
materials which have a toxic effect; this may be an increased sensitivity
to the material because of previous contact with it.

Anatomy (a nat'6 mi), the science that treats of the structure of organic bodies.

Anus (a'nus), the posterior opening of the alimentary canal.

Appendicular skeleton (ap en dik'u lar), skeleton of the paired fins of Pisces.

Archenteron (ar ken'ter on), the cavity of the gastrula which is the primitive
digestive cavity.

Artery (arteri), the larger blood vessels leading away from the heart.

Asexual reproduction (a sek'shu al), reproduction without sex cells.

Assimilation (as sim i la'shun), the transforming of digested food into proto-
plasm.

Asymmetry (a sim'et ri), a condition in which the two sides of an animal are
dissimilar.

•Phonetics according to Webster's New International Dictionary.

510

GLOSSARY 511

Autonomic nervous system (6 to nom'ik), that portion of the nervous system

controlling the involuntary muscles.
Autotomy (otot'omi), self -mutilation.
Axial gradient (ak'si al gra'di ent), the graduation of the rate of metabolism

along the principal axis of an axiate animal.
Axone (ak'son), a nerve liber serving to conduct impulses away from a nerve

cell body.

Barrier (bar'i er), any physical, chemical, or biological obstruction that pre-
vents migration of animals.

Benthos (ben'thos), life of the deep sea bottom.

Binary fission (bi'na ri fish'un), division of a cell into two daughter cells.

Biramous (bi ra'mus), a two-branched condition.

Bisexual (bi sek'shob al), a condition in which both male and female organs
are present in one individual.

Bivium (biv'ium), one side of an echinoderm including a pair of ambulacra.

Blastocoele (blas'to sel), the cavity present in the blastula stage of develop-
ment; also cleavage of segmentation cavity.

Blastomere (blas'to mer), one of the segments first formed by the division of
the ovum.

Blastula (blas'tu la), a sphere of cells with a hollow cavity resulting from
cleavage of the zygote.

Blepharoplast (blef'a ro plast), the body in a cell from which a flagellum
arises.

Brachium (bra'ki um), arm.

Branchial (brang'ki al), pertaining to gills or branchiae.

Buccal (buk'al), pertaining to the mouth cavity.

Budding (bud'ing), reproduction involving the branching of new individuals
from the external surface of the old one.

Byssus (bis'us), a tuft of fiberlike threads which attach certain mussels to the
substratum.

Caecum (se'kum), a blind pouchlike p6cket of the intestine; usually at the

junction of the small and large intestines.
Calcareous (kal kar'e us), composed of lime or calcium salts.
Calciferous (kal sif'er us), glands which are thought to secrete an alkaline

secretion into the esophagus of the earthworm.
Canaliculus (kan'a Hk' u lus), one of the tiny canals extending from lacuna

to lacuna to distribute nutriment in bone.
Capillary (kap'i ler i), a microscopic branch of an artery which extends into

a tissue and finally joins a small vein.
Carapace (kar'a pas), shell-like external covering.
Carbohydrate (kar bo hi'drat), organic compound of carbon, hydrogen, and

oxygen, such as starch or sugar.
Cardiac (kar'di ak), pertaining to the heart.
Carnivorous (kar niv'6 rus), flesh eating. .
Caste (kast), any group of distinct forms within a species, as found in some

insects.
Catabolism (ka tab'o lizm), process of oxidation or break-down of protoplasm;

destructive phase of metabolism; dissimilation; the oxidation of organic

substances of the body to release kinetic energy and heat.
Catalysis (ka tal'i sis), the initiation or acceleration of a chemical reaction

by the presence of a substance which itself does not enter into the

reaction, as an enzyme.
Caudal (ko'dol), pertaining to the tail.
Cell theory (sel the'6 ri), the theory that all living things are composed of

cells.
Centimeter (sen'ti me'ter), one-hundredth of a meter and the equivalent of

0.393 inch; or 1 inch equals 2.54 centimeters.

512 GLOSSARY

Central nervous system (sen'tral), that portion of the nervous system composed

of the brain and the spinal cord.
Centriole (sen'tri 61), a small granule within the central part of the aster in

the mitotic figure; also known as centrosome.
Centrolecithal (sen'tro les i thai), refers to the type of egg with the yolk mass

in the center, as the egg in insects.
Centrosome (sen'tro som), usually considered synonymous to centriole.
Cephalic (se fal'ik), pertaining to the head.
Cephalothorax (sef'alo tho'raks), a fusion of the head and thorax or chest,

as in crayfish.
Cerebellum (ser e bel'lum), the large lobe of the hind brain, in front of and

above the medulla.
Cerebrum (ser'e brum), the anterior division of the brain.
Cercaria (sur ka'ri a), a tailed larval stage of the liver fluke.
Ctenoid scale (te'noid), a type of fish scale with spines at the free margin.
Cervical (sur'vi kal), has reference to the neck region.
Chaeta (ke'ta), one of the bristlelike structures in the body wall of many

annelids, used as organs of locomotion.
Chelicera (ke lis'er a), an anterior pair of appendages in arachnids.
Cheliped (ke'li ped), most anterior thoracic leg of crayfish; large pincher.
Chemotropism (ke mot'ro piz'm), response of an organism to chemical changes.
Chlorophyll (klo'ro fil), the green coloring matter in plants and a few ani-
mals which is active in photosynthesis.
Chromosomes (kro'mo somz), bodies formed in the nucleus during mitosis which

constitute the physical basis of inheritance.
Chitin (ki'tin), the hard material composing the exoskeleton or shell of

Crustacea, insects, and others.
Chloragogue cells (klo'ra gog), compose the outer layer of the intestine of

the earthworm.
Chondrin (kon'drin), the material of which cartilage is composed.
Chorion (ko'ri on), the outer embryonic membrane of mammals.
Chorioid (ko'ri oid), middle or vascular coat of vertebrate eyeball.
Chromatin (kro'ma tin), dark-staining substance of the nucleus of the cell.
Chromatophore (kro'ma to for), a colored pigment cell.
Chromidia (kro mid'i a), scattered chromatin granules through the cytoplasm

of some cells.
Chromonemata (kro mo nem'ata),' threads of chromatin distinguishable within

chromosomes during mitosis; seen in the resting phase of some.
Chyme (kim), artially digested food material which is in semiliquid con-
dition.
Cilia (sil'ia), hairlike cytoplasmic processes, used by certain protozoans for

locomotion.
Cirrus (sir'iis), a bristlelike appendage.
Cleavage (klev'ij), the cell divisions changing the zygote into an organism

of many cells.
Clitellum (klitel'um), a broadened area in the earthworm about one-third of

the length of the body back from the head. It is glandular and serves

in producing the cocoon.
Cloaca (klo a'ka), the common chamber into which the intestine, and urinary

and genital canals discharge in some forms.
Cnidohlast (ni'do blast), the type of cell of the coelenter^te in which the sting

cell or nematocyst develops.
Cochlea (kok'le a), a coiled structure of the inner ear in which is located the

sensory ending of the auditory nerve.
Cocoon (ko kobn'), a covering which protects a larva, pupa, or even the adult

stage of certain animals.
Coelohlastula (se 16 blas'tii la), blastula having a hollow center.
Coelom (se'lom), or coelome (se'lom), the space between the walls of the body

and the inclosed viscera.

GLOSSARY 513

Commensalism (ko men'sal izm), an association of different species of ani-
mals in which at least one benefits without injury to the other.

Commissure (kom'i shobr), a strand of nerve fibers or nerves joining two cen-
ters or ganglia.

Conditioned reflex (kon dish'iind), a reflex action which is modified or estab-
lished by previous experience.

Conductivity (kon'duk tiv'i ti), the power of conducting or of receiving and
transmitting.

Congenital (kon jen'i tal), conditions existing at birth.

Conjugation (kon job ga'shiin), a temporary union of two individuals with ex-
change of nuclear material.

Copulation (kop fi la'shun), union of genital regions of two individuals during
which spermatozoa are transferred from one to the other.

Corium (ko'ri um), the deeper layer of the skin or dermis.

Cornea (kor'nea), transparent coat of modified epithelial tissue over the front
of the eye.

Cortex (kor'teks), superficial portion or outer layer, as of the brain or kidney.

Cranial (cra'jiial), pertaining to the portion of the skull enclosing the brain.

Cretin (kre'tin), a defective individual due to abnormality of the thyroid
gland.

Cutaneous (ku ta'neus), pertaining to the skin.

Cuticle (ku'tikl), the outer surface of the skin of many animals.

Cycloid scale (si'kloid), a scale which is thin and shows concentric lines of
growth without serrated margin.

Cyclosis (siklo'sis), the rotation of the endoplasm of protozoan forms.

Cyst (sist), an organism enclosed by a resistant wall.

Cysticercus (sis ti sur'kus), the bladder worm or encysted stage in the life
history of the tapeworm.

Cytology (sitol'o ji), the science that treats of the minute structure of cells.

Cytopharynx (si to far'inks), channel from surface to endoplasm in Euglena.

Cytoplasm (si'to plaz'm), the protoplasm of the cell not including the nucleus.

Dactyl (dak'til), refers to finger.

Daughter cells (do'ter selz), the two cells resulting from a division of one cell.

Delamination (delami na'shun), the formation of a new layer of cells parallel

to the old by the division and migration of cells of the primary germ

layers.
Dendrite (den'drit), a nerve fiber which carries impulses toward the nerve

cell body.
Dermis (dur'mis), same as corium.
Dialysis (dial'isis), separation of dissolved materials in crystalloids and

colloids by means of semipermeable membrane.
Diaphragm (di'afram), a muscular partition between the abdominal and

thoracic cavities in mammals.
Diastase (di'astas), the class of enzymes capable of bringing about conversion

of starches to sugars.
Diencephalon (di en sef'alon), a region of the brain just posterior to the

cerebrum.
Differentiation (dif er en shi a'shiin), the formation of special parts, tissues, or

cells from the primitive unspecialized layers.
Diffuse (dif us'), to mix with or to spread through completely and thoroughly

' another substance.
Digestion (di jes'chun), the conversion of complex unabsorbable food materials

into a form capable of bodily absorption.
Dihyhrid (dihi'brid), progeny or offspring of parents differing in two charac-
ters.
Dominance (dom'inans), a condition in which one of two characters present

in the individual appears to the exclusion of the other.

514 • , GLOSSARY

Dimorphism (di mor'fizm), difference of form between members of the same
species.

Dioecious (die'shus), the male and female germ cells being produced by dif-
ferent individuals.

Diploblastic (dip 16 blas'tik), composed of two germ layers.

Diploid (dip'loid), having the base number or double number of chromosomes,
as in somatic cells.

Dissimilation (disinii la'shun). (See Catabolism.)

Diurnal (diur'nal), active by day.

Diverticulum (di ver tik'u Mm), a blind tube branching out of a larger one.

Duodenum (du 6 de'niim), the part of the small intestine between the stomach
and the jejunum.

Ecdysis (ek'disis). (See Molt.)

Ecology (ekol'oji), the science of the relation of an organism to its environ-
ment.

Ectoderm (ek'to durm), the outer cell layer of the wall of a gastrula and its
later derivatives.

Ectoplasm (ek'to plaz'm), substance of the outer layer of cytoplasm or ecto-
sarc of a protozoan animal.

Ectosarc (ek'to sark), the superficial layer of cytoplasm of a single-celled
animal.

Egestion (ejes'chun), the casting out by the body of indigestible food ma-
terial.

Electrolyte (e lek'tro lit), a substance whose molecules dissociate into ions.

Electrotropism (elek'trot'ropiz'm), response of an organism to electric cur-
rents.

Embryology (em bry ol'o ji), the science of the origin and development of the
individual.

Endocrine system (en'dokrin), a system including those ductless glands which
secrete hormones.

Endoderm (en'do durm), the inner cell layer of the wall of the gastrula and its
later derivatives.

Endomixis (en do mik'sis), nuclear reorganization within a protozoan which
does not involve conjugation.

Endoplasm (en'do plaz'm). (See Endosarc.)

Endopodite (en dop'6 dit), the internal or principal branch of a biramous ap-
pendage of Crustacea.

Endosarc (en'do sark), the area of cytoplasm within a cell which is surrounded
by ectoplasm; substance of this is endoplasm.

Endoskeleton (en do skel'e tun), the bony, cartilaginous, or other internal frame-
work of an animal.

Endothelium (en do the'li um), the mesodermic lining layer of such closed spaces
as blood vessels and lymph spaces.

Enteric (en'terik), adjective form of enteron.

Enterocoele (en'ter 6 sel), a portion of the coelomic cavity that arises by out-
growth from the enteric cavity.

Enteron (en'ter on), a digestive cavity or tube. >

Entomology (en'tomol'd ji), the branch of zoology that deals with the study of
insects.

Entozoic (en'to zo'ik), forms which live within the bodies of other animals.

Enzymes (en'zimz), substances that bring about chemical transformation. ,

Ephyra (ef'i ra), tlie free-swimming larval form of the Scyphozoa.

Epiboly (epib'oli), posterior growth of a fold of the blastoderm over the sur-
face of an embryo in the formation of the enteron during gastrulation.

Epigenesis (ep i jen'e sis), the conception that the parts of the organism arise
from an undifferentiated germ cell.

Epithelium (ep'i the li um), a sheet of cells covering an internal or external
surface of the body.

GLOSSARY 515

Equatorial (ekwa to'ri al> plate, the platelike arrangement of chromosomes

lying in the plane of the equator of the mitotic spindle during cell

division.
Erepsin (erep'sin), an intestinal enzyme which splits peptones into amino acids.
Estivation (es ti va'shun), a dormant condition adopted by certain animals during

summer.
Eugenics (u jen'iks), the science of genetics applied to human kind, usually for

the purpose of improvement.
Euglenoid (u gle'noid), resembling a Euglena, as euglenoid movement.
Eustachian (usta'kian) tube, the tube extending from the middle ear to the

pharynx.
Evagination (e vaj'i na'shun), the unequal growth outward of a surface layer,

one of the processes by which , differentiation of organs is produced.
Eviscerate (e vis'er at), to remove or cast out the internal organs.
Exopodite (eks op'o dit), the external branch of the appendages.
Exoskeleton (ek so skel'e tun), the hardened parts of the external integument of

an animal.
Expire (ek spir'), to expel water or air in the process of respiration.

Factor (fak'ter), one of several interacting elements in a complex process.

Agency influencing the development of an individual, as those carried

in the genes of chromosomes.
Fauna (fo'na), a term referring to animal life.
Feces (fe'sez), the indigestible portion of the food which passes through the

alimentary canal and is discharged by way of the anus.
Fertilization (fur tili za'shun), the union of a mature ovum and a mature

spermatozoon to produce a zygote.
Fetus (fe'tus), an advanced stage of the embryo of a mammal before birth.
Fibrin (fi'brin), the fibrous material in a blood clot; formed when fibrinogen

of the blood is exposed to air.
Filtrable virus (fil'trab'l vi'riis), an organism too small to be seen with the

microscope and usually within cells of other organisms.
First filial (furst fil'i al) generation, the individuals arising from a particular

mating.
Fission (fish'un), division of an organism into approximately equal parts.
Flagellum (flajel'um), a whiplike locomotor structure of a cell or single-celled

animal.
Follicle (fol'ik'l), a cellular sac or pocket.
Fragmentation (frag men ta'slmn), a process by which individuals of certain

Protozoa and simple Metazoa may divide internally to form several new-
individuals.
Freemartin (fre'mar tin), a modified female member of a pair of cattle twins

which shows certain male features.

Gametes (gam'ets). (See Germ cells.)

Gametogenesis (gam e to jen'e sis), the series of cell divisions in the develop-
ment of germ cells.
Ganglion (gang'lion), a group of nerve cell bodies outside the central nervous

system.

Ganoid scale (gan'oid), rhombic in shape, composed of an inner layer of bone
and outer layer of enamel.

Gastrula (gas'trob la), the two-layered stage in the development of an embryo.

Genes (jenz), the units of material which function in the transmission of char-
Jictcrs in liGrGQitv.

Genetics (jenet'iks), the science that treats of variation, resemblances, and their
inheritance from parent to offspring.

Genotype (jen'otip), the genetic constitution of genetically identical organisms.

Genus (je'nus), pi. Genera (jen'era), a division of the classification, a sub-
division of a family, and is divided into species.

516 GLOSSARY

Geotropism (je ot'ro piz'm), response of an organism to gravity.

Germ cells (jerni) (gametes), cells specialized for reproduction.

Germ layer (jerm), one of the primary cell layers in an embryo.

Germ plasm (jerm plazm), the hereditary material of an organism, the chromatin.

Gills (gils) (pharyngeal clefts), a series of paired slits in the wall of the pharynx

and body.
Gonads (gon'ads), reproductive organs.
Gonophore (gon'ofor), a reproductive individual which bears gonads, as in

Hydroids.
Glochidlum (gl6 kid'ium), tlie larva of a fresh-water clam.
Glomerulus (glo mer'u lus), a body of capillaries enclosed at the end of eacli

kidney tubule of the mesonephric and metanephric types of kidneys.
Glycerol (glis'erol), one of the alcohols which enters into the composition of

fats; glycerin.
Glycogen (gli'ko jen), a form of carbohydrate food material as formed and stored

by the liver.

Habitat (hab'i tat), the place or area in which an animal or species lives.

Haploid (hap'loid), the reduced or half number of chromosomes of the mature
germ cells.

Heliotropism (he li ot'ro piz'm), a response of an organism to light.

Hemoglobin (he mo glo'bin), a protein pigment substance of the blood which is
capable of absorbing oxygen and is red when combined with it.

Hemolysis (he mol'i sis), disintegration of red blood corpuscles.

Hepatic (hepat'ik), pertaining to the liver.

Herbivorous (hur biv'6 riis), herb- or plant-eating animals.

Hermaphroditic (hur maf ro dit'ik) (monoecious), having both male and female
germ cells produced in one individual.

Heterozygote (het er 6 zi'g5t), an organism which is carrying sets of unlike char-
acters in its genetical constitution.

Hibernation (hi ber na'shun), the cessation of activity or dormancy of an animal
during the winter season.

Histogenesis (his to jen'esis), the development and differentiation of tissue cells.

Histology (histol'oji), the science that treats of the microscopic structure of
the various parts of the animal body.

Holoblastic (hoi 6 blas'tik), having the type of egg structure in which cleavage
divides the entire egg.

Holozoic (hdlozo'ik), the animal nutrition, the ingestion and digestion of or-
ganic material.

Homoiothermal (h6 moi'6 thur'mal), having a temperature regulation.

Homolecithal (ho mo les'i thai) (isolecithal), eggs having a uniform distribution
of the yolk.

Homologous (homorogus), similar in structure and origin, but different in
function.

Homonomous (ho mon'6 miis), slight or no differentiation of body segments.

Homozygote (homo zi'got), a zygote or resulting organism in which the corre-
sponding genes are alike.

Hormone (hor'mon), the essential substance of an internal secretion which serves
to help in metabolism regulation. Produced by endocrine glands and
carried by the blood.

Hyaline (hi'alin), semitransparent or glassy.

Hybrid (hi'brid), a cross, or offspring of parents differing in genetical con-
stitution.

Hydrolysis (hi drol'i sis), chemical rearrangement of a substance by combining
with water.

Hydrostatic (hi dro stat'ik), a type of organ which regulates the specific gravity
of an aquatic animal.

Hypertonic (hi per ton'Tk), possessing greater osmotic pressure than some related
substance.

GLOSSARY 517

Hypostome (hi'po stom), a conical projection around and below the mouth in
coelenterates.

Hypothesis (hi poth'e sis), an idea as it first develops as the result of prelimi-
nary observation and experiment.

Ileum (il'eum), the posterior and longest part of the small intestine.

Ilium (il'ium), dorsal bone of pelvic girdle of terrestrial vertebrates.

Immunity (i mu'ni ti), freedom of susceptibility to disease.

Ingestion (in jes'chiin), the taking in of food material by an organism.

Insectivorous (in sek tiv'6 rus), insect-eating animals.

Inspire (in spir'), the drawing in of water or air in the respiration.

Instar (in'star), the period between molts in insect development.

Insulin (in'sti lin), a hormone produced by the pancreas and essential to the

proper metabolism of carbohydrates.
Integration (in te gra'shun), development and correlation to give unity in an

organism.
Integument (in teg'u ment), the outer covering of the body.
Intracellular (in tra sel'u ler), within the cell.
Intracellular differentiation (in tra sel'u ler dif er en shi a'shun), the presence of

a variety of cells within one l^ody.
Invagination (in-vaj i na' shun), the unfolding process by which the primary

endoderm is withdrawn into the blastular cavity and becomes enclosed

by the primary ectoderm.
Irritability (ir i ta bil'i ti), the capacity of protoplasm for responding to changes

in environmental conditions or to external stimuli.
Isolecithal (i so les'i thai). (See Homolecithal.)
Isotonic (isoton'ik), possessing the same osmotic pressure in related substances.

Jejunum (je jobnum), the middle division of the small intestine, between the
duodenum and the ileum.

Karyokinesis (kar i 6 ki ne'sis), mitotic cell division.
Karyolymph (kar'i o limf), the more fluid material of the nucleus.
Karyoplasm (kar'i 6 plaz'm), the protoplasm which constitutes the nucleus.
Karyosome (kar'iosdm), a -''net knot" or a part of the chromatin which forms

a distinct body in the nucleus.
Katabolism (ka tab'6 liz'm). (See Catabolism.)
Keratin (ker'atin), a nitrogenous substance forming the chemical foundation

of hair, horn, feathers, nails, claws, etc.
Kinetic energy (ki net'ik), energy inherent in motion of a body.

Labium (la'bium), posterior boundary or lower lip of an insect's mouth.
Labrum (la'brum), the exoskeletal anterior boundary or upper lip of the insect's

mouth.
Lacrimal (lak'rimal), pertaining to tears.
Lacteal (lak'teal), pertains to milk; refers to lymphatics of the intestinal

region because of their light colo"!* following absorption of fat.
Lacuna (lakii'na), a cavity or space, particularly that of bone, which contains

the bone cells.
Lamella (la mel'a), a thin layer or plate.

Larva (lar'va), the young stage of an animal, which changes form during life.
Larynx (lar'inks), the expanded upper end of the windpipe or trachea; voice

box.
Lethal (le'thal), capable of producing death.
Leucocyte (lu'kosit), a white blood corpuscle.
Ligament (lig'ament), a band of white fibrous connective tissue connecting

structures other than muscles; particularly joining bones at the joints.
Limpet (lim'pet), a small type of gastropod (Mollusca) with a simple uncoiled

shell,

518 GLOSSARY

Linin (li'nin), the delicate threadlike structure which supports the chromatin
granules in the nucleus.

Linkage (lingk'ij), the constant association of particular genes in certain
chromosomes.

Lipase (li'pas), a fat-splitting enzyme.

Lipin (li'pin), fatty substance.

Lipoid (lip'oid), fatlike substance.

Lophophore (lo'fofor), a disc which surrounds the mouth and bears the tenta-
cles of the Bryozoa.

Lumbar (lum'ber), pertaining to the region usually known as the small of the
back.

Lumen (lu'men), internal cavity of a tubular duct, gland, vessel, etc.

Luminescence (lu mi nes'ens), the emission of light from the body.

Lymph (limf), the blood plasma and white corpuscles in the lymph spaces about
the tissues.

Lymphatic (limfat'ik), a vessel which carries lymph. In general, pertaining
to lymph.

Macronucleus (mak ro nu'kle us), the large nucleus of certain protozoans sup-
posed to control vegetative functions.

Madreporite (mad're p6 rit), the strainerlike external aperture of the water-
vascular system of echinoderms.

Malpighian (malpig'ian) body, a structure in the cortex of the kidney, com-
posed of a glomerulus and Bowman's capsule which serves to take urine
from the blood.

Mantle (man't'l), a fold of the body wall which partially envelops the body;
present in most mollusks and here secretes a shell.

Marsupial (mar su'pi al), having a pouch for carrying the young.

Marsupium (mar su'pi um), an external pouch used in carrying the young, as
in the kangaroo or opossum.

Matrix (ma'triks), the mother substance, such as that which encloses anything;
the intercellular material of cartilage or other sustentative tissue.

Maturation (mat u ra'shun), the series of changes occurring in the development
of germ cells before fertilization, including a reduction in the number of
chromosomes in the cells.

Maxilla (maksil'a), the major bone of the upper jaw of vertebrates or the
accessory mouth part just back of the mandibles in many invertebrates.

Medulla (medul'a), posterior portion of the vertebrate brain; also the median
area of many organs.

Medullary (med'ii ler i), pertaining to the medulla.

MeduUated (med'ii lat ed), term used in reference to a nerve fiber which pos-
sesses a fatty or myelin sheath.

Medusa (medii'sa), a free-swimming individual coelenterate, such as a jelly-
fish.

Meiosis (mi 6'sis), the reduction division in maturation of germ cells.

Meridional (me rid'i o nal), a condition in which planes extend from pole to
pole of a spherical body.

Meroblastic (mer 6 blas'tik), having the type of egg structure in which cleavage
is only partial, owing to the accumulation of yolk in the egg.

Mesencephalon (mes en sef'a Ion), the third region of the vertebrate brain, com-
monly called midbrain.

Mesenchyme (mes'eng kim) (Parenchyma), undifferentiated mesoderm composed
of large cells.

Mesoderm (mes'odurm), the middle germ layer and its later derivatives.

Mesoglea (mesogle'a), a jellylike substance found in Coelenterata between the
ectoderm and endoderm.

Mesonephros Cmes 6 nef'ros), the vertebrate kidney of forms from lamprey to
amphibians inclusive.

Mesorchium (me sor'ki um), the mesentery or membrane supporting a testis.

GLOSSARY 519

Mesothelium (mes 6 tlie'li um), the mesodermic, membranous lining of the peri-"
toneal cavity.

Mesovarium (mes 6 va'rium), the mesentery in which the ovary is suspended.

Metabolism (me tab'6 liz'm), the building up of living protoplasm and its con-
current oxidation.

Metagenesis (met a.ien'e sis), an alternation of sexual and asexual generation
in the life cycle of an organism,

Metameres (met'a mers), one oi a series of similar parts that follow one another
in a vertebrate or articulate animal.

Metamerism (me tam'er iz'm), serial symmetry or succession of segments.

Metamorphosis (met a mor'fo sis), the transformation of one developmental stage
into another without intermediate steps.

Metaphase (met'a faz), the phase of mitosis involving the longitudinal splitting
of the chromosomes on the equatorial plate.

Metazoa (met'a z5 a), animals whose bodies consist of few or many cells func-
tioning as a unit.

Micronucleus (mi kro nu kle us), the small nucleus of certain protozoans sup-
posed to control reproduction.

Micropyle (mi'kropil), the small opening in the egg where sperm enter in cer-
tain forms of animals.

Milt (milt), the light-colored spermatic fluid of male fisli.

Miracldium (mi ra sid'i fim), the early larval stages in the flukes.

Mitochondria (mit 6 kon'dri a), small structures in the cytoplasm of animal
cells; their significance is not entirel}^ understood.

Mitosis (mito'sis), indirect cell division, involving the formation and splitting
of chromosomes and their equal distribution to daughter cells.

Molt (molt), a complete or gradual shedding of the outer covering.

Monodelphia (mon 6 del'fi a), having a placenta.

Monoecious (mone'shiis). (See Hermaphroditic.)

Monohybrid (mon 6 hi'brid), an offspring of parents which differ by only one
character.

Morphology (mor fol'o ji), the science that treats of the form and structure of
the bodies of animals.

Morula (mor'ula), a type of blastula characterized by the absence of a seg-
mentation cavity.

Mucosa (miiko'sa), a cellular membrane lining such cavities as those of the
digestive tract.

Mucus (mii'kus), a viscous secretion which contains mucin (mu'sin). Mucous
is the adjective form.

Mutation (muta'shun), a heritable change in an organism due to changes in
one or more genes of germ cells.

Mutualism (mu'tii al iz'm), animals of different species associating together for
the mutual advantage of each.

Myelencephalon (mi e len sef'alon), the fifth or most posterior division of the
vertebrate brain; the medulla oblongata of the adult.

Myelin (mi'elin), fatty substance surrounding the axone in medullated nerve.

Myoneme (mi'onem), contractile fiber or strand in the cytoplasm of certain
protozoans.

Myotomes (mi'otoms), segmental divisions of the muscles.

Nares (ua'rez), the openings into the nasal cliambers in vertebrate animals.

Nauplius (no'plins), a larval stage of certain Crustacea.

Nekton (nek'ton), the pelagic aquatic animals which are independent of the

effect of wind and waves.
Nematocysts (nem'a to sists), stinging bodies found in the tentacles of certain

coelenterates.
Nematode (nem'a tod), a roundworm belonging to class Nematoda of phylum

Nemathelminthes.
Neoteny (neot'eni), the indefinite persistence of the immature condition of an

animal.

520 GLOSSARY

Nephridium (ne frid'i um), a form of excretory organ, as found in the earth-
worm.

Nephrostome (nef'ro stom), the funnel-shaped aperture at the medial end of a
nephridium.

Neural (nu'ral), pertaining to the nervous system or to a nerve.

Neurilemma (nu rilem'a), the membranous outer coat of a nerve fiber.

Neurold transmission (nu'roid), primitive transmission of impulses from cell to
cell.

Neuron (nu'ron), a nerve cell together with its processes.

Notocliord (no'tokord), a flexible rod extending anterior to posterior in the
longitudinal axis of the body dorsal to the digestive tube and ventral
to the nerve cord in chordates.

Nocturnal (nok tur'nal), reference to night. Contrasted to diurnal which per-
tains to daytime.

Nodes of Ranvier (ranvya'), constrictions in meduUated nerve where the myelin
sheath is interrupted.

Nomenclature (no'men kla ttir), a system of naming objects or ideas.

Nondisjunction (non dis jungk'shun), the failure of homologous chromosomes to
separate after synapsis and both go to one daughter cell with none to the
other.

Nucleolus (nu kle'6 Ids) (Plasmosome), a body within the nucleus containing
material that is not chromatin.

Nucleus (nti'kleus), a typically spherical body within the cell that contains the
chromatin.

Nymph (nimf), the larval stage of an insect which undergoes incomplete meta-
morphosis; also the larval stage of a few vertebrates.

Ocellus (6 sel'us), a simple type of eye, as in some insects.

Ommatidium (6m a tid'i um), one of the numerous rodlike units of the com-
pound eye.

Ontogeny (ontoj'eni), the entire development and life history of an individual
organism.

Oocyte (o'o sit), the female germ cell before maturation is completed.

Oogenesis (o 6 jen'e sis), the maturation of the female germ cell.

Oogonium (6 6 go'nium), the female germ cell during the multiplication and
growth stages of maturation.

Operculum (6 pur'kuliim), a fold of skin, bone, and scales, which covers the
gills of fishes and certain Amphibia; also the bony structure closing the
aperture of certain snail shells.

Organ (or'gan), an arrangement of two or more tissues as a part of the body
which performs some specific function or functions.

Organism (or'gan iz'm), any independent living being.

Orthogenesis (or tho jen'e sis), the theory which holds that animals tend to
develop along lines leading constantly in the same direction because
they are determined by internal factors.

Osmosis (osmo'sis), diffusion of substances dissolved in fluid, through a semi-
permeable membrane.

Ossicle (os'ik'l), a small bony structure.

Ostium (os'tium), a mouthlike opening or entrance.

Otocyst (o'tosist), the primitive organ of hearing.

Ova (6'va), mature female germ cells. Sing., ovum (o'vum).

Ovary (6'vari), the female gonad.

Oviduct (6'vidukt), the duct for the passage of ova from the ovary to the
exterior of the animal.

Oviparous (6 vip'a riis), pertaining to those animals which lay eggs that hatch
after exclusion from the body.

Ovipositor (o vi poz'i ter), an organ of female insects and others which serves
in helping to deposit the egg.

GLOSSARY 521

Ovoviviparous (6 v6 vi vip'a ids), a condition of retention of the. egg in the
mother's body where It is nourished by the yolk of the egg.

Ovulation (o vu la'shun), the process of discharging mature eggs from the ovary.

Oxidation (ok si da'shiin), a chemical combination of oxygen with another
element.

Paleozoology (pale 6 z6 ol'o ji), the science that treats of the animals of the

past as represented by fossil remains.
Parasite (par'asit), an organism that lives on or within and at the expense of

another organism.
Parenchyma (pa reng'ki ma). (See Mesenchyme.)
Parietal (pa ri'e tal), pertaining to the walls of the coelom.
Parthenogenesis (par the no jen'e sis), the development of an egg without

fertilization.
Pathology (pathol'oji), the study of abnormal structures and abnormal func-
tioning of life processes.
Pedal (ped'al), pertaining to the feet.
Pedicellaria (ped i se la'ri a), pincherlike structures found over the surfaces of

sea urchins and starfishes.
Peduncle (pedung'kl), the stemlike attachment of certain shells and barnacles

to other objects.
Pelagic (pelaj'ik), floating near the surface of water.
Pericardial (peri kar'di al), situated around the heart.
Periosteum (per i os'te um), the membranous covering of bone.
Peripheral nervous system (pe rif'er al), that part of the nervous system ex-
clusive of the brain and spinal cord.
Peristaltic (per i stfil'tik), forcing the food along the intestine by rhythmical

contractions of the intestinal wall.
Peritoneum (per i to ne'um), the membrane that lines the coelom of vertebrates.
Phagocyte (fag'osit), a white corpuscle which engulfs and destroys bacteria

and other foreign material.
Pharynx (far'ingks), the region between the mouth and the esophagus.
Pharyngeal (fa rin'je al), pertaining to the pharynx.
Phenotype (fe'no tip), a type of organism possessing a complex of characters

in its external features.
Phenotypic (fe no tip'ik), pertaining to phenotype.'

Photosynthesis (fo to sin'the sis), the process by which green plants manufac-
ture starch from raw materials.
Phototropism (fo tot'ro piz'm), response of an organism to light.
Phylogeny (filoj'eni), the study of the origin and relationships of the different

groups and races of organisms.
Physiology (fiz i ol'o ji), the study of the function of the parts of an organism

as well as its living processes as a whole.
Pia mater (pi'ama'ter), the membrane which is the immediate covering of the

brain and spinal cord.
Pilidium (pilid'ium), helmet-shaped larva of certain forms.
Pineal (pin'e al) body, a dorsal projection from the diencephalon and thought

to be the vestige of a third or me'dian eye in vertebrates.
Pituitary (pi tu'i ter i) body, a glandular structure attached to the neutral side

of the diencephalon of the vertebrate brain. It is an endocrine organ.
Placenta (plasen'ta), the vascular membrane which connects the embryo with

the parent.
Placula (plak'ula), a type of blastula in which the animal and vegetative halves

are somewhat compressed toward each other.
Plankton (plangk'ton), the small pelagic organisms which are at the mercy of

the waves.
Plasma (plaz'ma), the fluid portion of the blood.

Plasmagel (plaz'ma jel), the viscous 'or semisolid portion of protoplasm.
Plasmasol (plaz'ma sol), the more fluid phase of protoplasm.

522 GLOSSARY

Plasmosome (plaz'mo som). (See Nucleolus.)

Pleural (pidbr'al), pertaining to the cavity which contains the lungs.

Plexus (plek'sus), a network.

Polar (po'ler) body, a small nonfunctional cell or oocyte produced during the
maturation divisions of the female germ cell.

Polarity (polar'iti), referring to a condition in which points or poles of con-
centration or dominance are established in a body.

Polocyte (po'16 sit), a technical name for a polar body.

Polyandry (pol'i andri), the practice of one female mating with several males.

Polygamy (p6 lig'ami), having more than one mate at the same time.

Polygyny (polij'ini), the practice of one male mating with several females.

Polymorphism (pol i mor'fiz'm), the occurrence of two or more forms of indi-
viduals within a species.

Polyp (pol'ip), the attached phase of the life history of a coelenterate animal.

Precocial (pre ko'shal), type of bird which leaves the nest and has downy cover-
ing at time of hatching.

Predaceous (pre da'shus) animal, one which preys on others.

Predatism (pred'a tiz'm), the practice of one animal preying on another.

Primordial (pri mor'di al), the first or primitive form.

Proboscis (probos'is), an extension of the head or mouth parts. May be nose,
as in elephant; mouth parts, as of moth; or pharynx, as of planaria.

Proctodeum (prol: to de'um), the pocket in the ectoderm ventral to the posterior
part of the enteron of the embryo; primordium of the anus.

Proglottid (pro glot'id), one of the sections or individuals of the chain making
up the body of a cestode, such as the tapeworm.

Pronephros (pro nef'ros), the first kidney structure to form in the developing
vertebrate.

Pronucleus (pro nu'kle us), one of the two nuclei within a fertilized egg before
cleavage occurs.

Propagation (prop a ga'shiin), the production of new individuals.

Prophase (pro'faz), the preparatory stages of mitosis during which the forma-
tion occurs of spindle, spireme, and chromosomes.

Proprioceptor (pro prio sep'ter), the receptor or end organ of the nervous sys-
tem located within a certain tissue receiving stimulations in reference
to bodily position or orientation.

Prostate (pros'tat) gland, one of the male reproductive organs producing part
of the semen.

Prostomium (pro sto'mi iim), portion of the anterior segment of annelids which
overhangs the mouth.

Protein (pro'tein), one of the organic compounds found in protoplasm. It con-
tains the elements carbon, oxygen, hydrogen and nitrogen.

Protoplasm (pro'to plaz'm), the living matter of which all organisms are com-
posed.

Protopodite (pro top'o dit), the proximal section of the crustacean appendage.
It includes coxopodite and basipodite.

Prototroch (pro'to trok), the band of cilia extending around the equatorial region
of trochophore larva.

Protrusible (pro trob'si b'l), the ability to be put out or extended from the body.

Proventriculus (pro ven trik'ulus), the anterior, secretory portion of the stom-
ach in certain animals, as the bird.

Pseudopodia (su do po'dia), protoplasmic processes (false feet) formed by cer-
tain protozoans and used for locomotion.

Ptyalin (tl'alin), the starch-digesting enzyme of saliva; a diastase.

Pupa (pu'pa), the encased, inactive stage between the larva and adult condition

in many insects and other animals.
Pyloric (pilor'ik), pertaining to the pylorus.

Pylorus (pilo'rus), the junction of the posterior portion of the stomach with
the small intestine.

GLOSSARY 523

Radial symmetry (ra'dial sim'etri), applied to a body that can be equally

divided by several radial planes.
Radiant energy (ra'di ant ener ji), inherent power or energy transmitted through

space, as that from the sun, radium, or x-ray.
Radula (rad'ula), the sheetlike rasping structure of the mouth of gastropods;

used in mastication of food.
Recessive (reses'iv), in reference to a gene which is carried in the cell without

expressing its character unless there is absence of its dominant mate.
Recapitulation (re kapit u la'shun), repetition in development of an individual

organism of its phylogenetic history.
Redia (re'di a), second phase of the life history of the fluke.
Reflex (re'fleks) action, automatic reaction to a stimulus from a receptor neuron

and passed on to an adjustor neuron; performs an involuntary, appropriate

act.
Regeneration (re jen er a'shun), the replacement of mutilated parts or an entire

animal from a portion of one.
Renal (re'nal), pertaining to the kidney.
Rennin (ren'in), an enzyme constituent of gastric juice of mammals and capable

of coagulating the protein portion of milk.
Reproduction (re pro diik'shun), the production by an organism of others of its

kind.
Respiration (res pi ra'shim), the exchange within an organism of oxygen enter-
ing the protoplasm and carbon dioxide leaving it.
Response (re spons'), the reaction of an organism to a stimulus.
Rete (re'te), a limited meshlike arrangement or network.
Reticulum (re tik'u lum), a fibrous or tubular network.
Retractile (re trak'til), that which can be withdrawn.
Retrogression (ret ro gresh'un), going behind or moving backward.
Rhabdites (rab'dits), special structures found interspersed among the epidermal

cells of flatworms.
Rheotropism (re ot'ro piz'm), response of an organism to mechanical currents.
Roe (ro), ovary and eggs of fish.
Rudiment (roo'di ment), partially developed or embryonic structure; usually

without function.
Rugose (rob'gos), possessing many ridges and folds.
Ruminants (rob'mi nants), animals which chew the cud.

Saprophyte (sap'rofit), an organism which absorbs nonliving organic matter

in solution directly through the surface of the body.
Sarcolemma (sar ko lem'a), the delicate membrane immediately enclosing the

striated voluntary muscle cell.
Sarcoplasm (sar'ko plaz'm), the cytoplasm of muscle cells exclusive of the sar-

costyles or fibrils.
Sarcostyles (sar'ko stils), cytoplasm fibrils in the structure of cytoplasm of

voluntary muscle cells.
Schizogony (ski zog'6 ny), asexual reproduction.

Sclerotic (skle rot'ik), the dense fibrous outer coat of the vertebrate eye.
Scolex (sko'leks), knoblike "head'.' on anterior proglottid of cestode.
Scute (skut), a plate on the ventral side of the body of the snake.
Sebaceous glands (seba'shus), small subcutaneous glands, usually connected

with hair follicles.
Sedentary (sed'en ter i), unattached forms which remain in one place.
Semen (se'men), fluid which carries the spermatozoa in the males of most

animals.
Seminal (sem'inal), pertains to semen.
Semipermeable membrane (sem i pur'me ab'l), one which permits the passage ot

solvents through it but not solutes, unless they dissolve in the membrane.
Senescence (senes'ens), period of old age and its effects.
Septum (sep'tum), a wall dividing two cavities.

524 GLOSSARY

Serial homology (ser'i al ho mol'o ji), presence of structures of similar origin

and form in different segments of the same animal.
Serosa (sero'sa), membrane covering peritoneal surface of internal organs;

one which secretes a watery fluid.
Sertoli cells (ser t5'le sels), modified, supporting or nurse cells for forming

spermatozoa in the testes.
Serum (se'rum), the fluid part of the blood which remains after clotting.
Sessile (ses'il), attached directly and incapable of locomotion.
Seta (se'ta), hairlike spine or bristle, found in animals.

Sinus (si'nus), a cavity in a bone or other part, or a dilated vessel or canal.
Siphon (si'fon), a canal or passageway, as the waterways in clams or tunicates.
Somatic cells (somat'ik), the cells of the body exclusive of the germ cells;

body cells.
Somites (so'mits), segments of the body of a segmented animal.
Spermatogonium (spur ma to go'nium), a male germ cell during the period of

multiplication and growth in maturation process.
Spermatocyte (spur'ma to sit), the male germ cell before its maturation is

completed.
Spermatogenesis (spur ma to jen'e sis), the maturation of the male germ cells.
Spermatozoa (spur ma to zo'a), mature male germ cells.
Sphincter (sfingk'ter), a muscular band surrounding a tube or aperture which,

by its contraction, closes the lumen.
Spicule (spik'ul), one of numerous needlelike, solid structures found supporting

the tissues in the body wall of sponges.
Spiracle (spir'ak'l), openings of air tubes in insects, or modified opening of

first gill slit of certain fish.
Spireme (spi'rem) thread, the coiled bead-like string of , chromatin material

that appears during the prophase of mitosis.
Splanchnic (splangk'nik), has reference to the visceral organs.
Spongin (spun'jin), the skeletal material of a sponge.
Sporulation (spor u la'shun), production of spores by division of a protozoan

while encysted.
Statohlast (stat'6 blast), an encased, asexual winter bud of a bryozoan.
Statocysts (stat'6 sists), sense structures assisting in maintaining equilibrium

in certain forms.
Steapsin (steap'sin), one of the pancreatic enzymes which is capable of chang-
ing fats to fatty acids and glycerin.
Stereohlastula (ster e o blas'tu la), a blastula in which all the cells are in close

contact and no blastocoele is formed.
Stigma (stig'ma), a sensitive pigment spot of Protozoa or the opening of a

spiracle of insects.
Stomatodeum (sto ma to de'um), the opening of the developing alimentary tract

in an embryo.
Stratum (stra'tum), a layer of a series.

Striated (stri'ated), a type of muscle with more dense areas across the fibers.
Strobila (strobi'la), a series of individuals produced by linear budding, as cer-
tain Scyphozoa and tapeworms.
Succession (suk sesh'un), the successive occupation of a given area by several

species, either hourly, daily, or seasonally.
Sustentative tissue (sus'ten ta tiv), binding together or supporting the various

parts of the body.
Suture (su'tur), to sew together; a line of junction.
Symbiosis (sim b! 6' sis), the living together of two organisms for their mutual

benefit.
Synapsis (sinap'sis), the pairing of the chromosomes in the germ cells at one

stage of maturation.
Syncytium (sin sish'i dm), a mass or layer of protoplasm with numerous nuclei

but without distinct cell boundaries.

GLOSSARY 525

Syngamy (sin'ga mi), union of mature gametes to form a zygote.
System (sis'tem), an aggregation of organs to perform some general function
of life.

Taxis (tak'sis), a tropismal response involving movement of an organism as a
whole.

Taxonomy (taks on'6-mi) (systematic zoology), the classification or orderly ar-
rangement of organisms according to their natural surroundings.

Tegumentary (tegumen'ta ri), referring to the skin.

Telencephalon (tel en sef'a 15n), the anterior division of the vertebrate brain.

Telolecitlial (tel 6 les'i thai), type of egg with abundant yolk unequally dis-
tributed.

Telophase (tel'o faz), the final stage in mitotic divisions.

Tentacle (ten'tak'l), flexible, armlike extension of the body of many non-
chordates.

Terrestrial (ter res'tri al), a land form; living on or in the ground.

Testis (tes'tis), male gonad.

Thermotropism (ther mot'ro piz'm), response of an organism to temperature.

Thigmotropism (thig mot'ro piz'm), response of an organism to contact.

Thoracic (tho ras'ik), of or pertaining to the thorax or chest.

Thorax (tho'raks), the middle region of the body.

Threshold (thresh'old), the minimum strength of stimulus necessary to get a
response.

Thrombin (throm'bin), the substance of the blood which plays an important
part in clotting.

Thyroxin (thi rok'son or -sin), the hormone which is produced by the thyroid
body.

Tissue (tish'ii), an organization of similar cells into a layer or group for the
perfoi'mance of a specific fifnction.

Toxin (tok'sin), any poisonous substance.

Trachea (tra'kea), the windpipe or a tube for conveying air to the lungs; air
tubes in insects.

Trichocyst (trik'6 sist), saclike structure in the ectosarc of Paramecium.

Triploblastic (trip 16 blas'tik), composed of three germ layers.

Trivium (triv'ium), the three anterior ambulacra of Echinodermata, collec-
tively.

Trochophore (trok'o for), a semispherical type of larva with cilia; found among
flatworms, annelids, mollusks, etc.

Tropism (tro'piz'm), the movements of an organism in response to a stimulus.

Trypanosome (trip'a no s6m), genus of parasitic Protozoa (Mastigophora) in-
cluding the causal agent of African sleeping sickness.

Trypsin (trip'sin), a pancreatic enzyme which converts proteins to amino acids.

Tsetse (tset'se) fly, a species of fly which serves to transmit the causal agent
of African sleeping sickness.

Tundra (tobn'dra), level plains region of the arctic region.

Tympanum (tim'paniim), cavity of the" middle ear or more generally any organ
serving to receive sound waves.

Umbilical (umbil'ikal) cord, the cordlike connection between the fetus and
the placenta.

Umbilicus (iim bil'i kus), the navel or the point of attachment of the umbilical
cord to the abdomen.

Uncinate (lin'sinat), in the shape of a hook.

Ungulate (img'gulat), hoofed.

Unguiculate (un gwik'lat), having claws.

Urea (iire'a), a nitrogenous compound which is produced as a protein by-
product in metabolism.

Ureter (iire'ter), the duct which conveys urine from the metanephric kidney
to the cloaca or bladder.

526 GLOSSARY

Urethra (ure'thra), the duct which leads from the urinary bladder to the

exterior of the body.
Uropods (u'ro podz), the sixth pair of abdominal appendages of a crustacean.

Vacuoles (vak'uolz), small cavities in a cell filled with water, gases, or oils.
Vagina (vaji'na), the cavity between the uterus and the external genital

aperture of the female in many animals.
Vascular system (vas'kuler), the circulatory system.
Vascular tissue (vas'kuler), fluid tissue consisting of cells known as corpuscles

in a fluid medium, plasma.
Vein (van), the larger blood vessels leading to the heart.

Ventral (ven'tral), side away from the back; literally belly; opposite to dorsal.
Villus (vilTis), a fingerlike, vascular process of the internal lining of the small

intestine.
Vitamins (vi'tamins), substances which occur in small amounts in numerous

foods and are essential regulatory substances for the animal body.
Vitelline (vitel'lin), the outer membrane of an egg.
Vitreous (vit'reus), glassy in appearance.
Viviparous (vi vip'a rus), the retention and development of the egg in the

mother's body and nourishment of the embryo from the blood of the

mother.
Volant (vo'lant), able to fly.

Zoogeography (z6 6 je og'ra fi), the study of the geographical distribution of

animals.
Zoology (zo ol'o ji) (animal biology), the study of the science which treats of

animals.
Zygote (zl'got), a fertilized egg, or embryo, after fertilization.
Zymogen (zi'mojen), a pre-enzyme; a substance which is produced in a gland

cell and becomes an enzyme when it is discharged and activated by some

other substance, perhaps another enzyme.

INDEX

• A

Abducens, 92
Abducent nerve, 137
Abnormalities, 226

of brain, 234
Absorption of food materials 69, 173
Abyssal zone, 474
Acanthocephala, 337, 430
Acetabulum, 87, 115
Acnidosporidia, 275
Acoela, 323
Acromegaly, 202
Actinaria, 309
Actinophrys, 271
Adaptation, 465
Adrenal glands, 199
Adrenalin, 199
Adrenin, 199
Aedes, 281
Afterbirth, 224
Agamodistomum, 450
Agassiz, 35

Agriculture and zoology, 29
Alcvonacea, 311
Alcyonaria, 311
Alisplienoid, 110
Allelomorphs, 237

multiple, 242
Alpine zone, 475
Alternation of generation, 63
Alveoli, 80, 132
Amblema, 374

Ambulacral groove, 363, 366
Amino acids, 46, 171, 172
Amitosis, 50

Amnio-cardiac vesicle, 220
Amoeba, 285
Amoebic dysentery, 279
Amoebina, 270
Amoebocytes, 367
Amoeboid movement, 292
Amphiaster, 50
Amphibia, 148

classification of, 103

economic importance of, 102

families of, 104
Amphineura, 373
Amphioxus, 147, 152

circulatory system of, 156

habitat of, 152

in evolution, 484, 485

reproductive system, 157

respiratory system, 157

structure of, 152

Amphiplexus, 193
Amphipods, 411
Amylase, 69
Amylopsin, 69, 125

action of, 171
Anabolisni, 166
Analogous structures, 399
Anaphase, 51
Anastomosis, 57
Anatomy, gross, 19
Ancestry of vertebrates, 484
Ancylostoma, 427, 438-445
Andalusian chicken, 244
Androgen, 207
Androgenic hormone, 204
Androsterone, 204
Anemia, 341
Annelid theory, 485
Annelida, 23, 342
Annelids, importance of, 356
Annulus, 355, 397
Anodonta, 374
Anomalies, 226

cardiac, 234
Anopheles, 276, 281
Antebrachium, 109
Antedon, 361
Antelope, 464
Antennata, 395
Antennules, 398, 399, 401, 404
Anthelmintics, 443
Anthozoa, 309
Anthropoidea, 494
Antibodies, 180
Antigen, 180
Antipathes, 311
Antipathidea, 311
Antihemorrhagic vitamin, 176
Antineuritic vitamin, 174
Antipellagric vitamin, 175
Antirachitic vitamin, 175
Antiscorbutic vitamin, 175
Antisterility vitamin, 176
Antixerophthalmic vitamin, 174
Anus, 109
Aortic arches, 221

development of, 501, 504
Appendages, 109

of crayfish, 397
Appendicular skeleton of frog, 86
Aqueduct, cerebral, 136

of Sylvius, 136
Arachnid theory of vertebrate ancestry,
485

527

528

INDEX

Arachnida, 395

parasitic, 439
Arachnoidea, 395
Arbacia, 361
Arboroid colony, 277
Arcella, 270

Archaeopteryx, 487, 498
Archenteron, 100, 215
Archiannelida, 355
Argulus, 394, 412
Aristotle, 31
Aristotle's lantern, 360
Armadillidum, 411
Artemia, 412
Arteries, afferent branchial, 156

of rat, 127
Arthrobranchiae, 400
Arthropod parasites, 431
Arthropoda, 23, 393, 415

parasitic, 439

phylogenetic advances of, 414
Arthropodial membrane, 397
Ascaridia, 335

Ascaris, 23, 335, 337, 420, 438
Ascaroidea, 335
Ascorbic acid, 175
Asellus, 411
Aspidocotylea, 324
Assimilation, 27

in Amoeba, 288
Asterias, 359, 364

cleavage, 213
Asteroidea, 358
Astrangia, 310
Astropecten, 359
Atmosphere, 459
Atriopore, 154
Atrium, 219, 220
Attributes of life, 24
Auditory capsule of frog, 94

meatus, 109

nerve, 137

vesicle, 220
Aurellia, 308
Auricles, 126

Auriculo- ventricular aperture, 77
Australian region, 473
Autonomic nerves, 93
Autotomy, 359, 371

in crayfish, 409
Aves, 148

Avoiding reaction, 285
Axial gradient, 225, 326, 333

skeleton of frog, 83
Axone, 57
Axons, 137

Babesia, 437
bigemina, 281

4fi

Back cross, 241
Balance in nature, 24, 25
Balanoglossus, 148

in evolution, 484, 485
Balantidium, 437, 438
coli, 272, 273, 437
Bali island, 473
Band worms, 324
Barnacle, 394, 412
Barriers, 22, 478, 498
Basipodite, 397
Basisphenoid, 110
Basket star, 359
Bass, 148

Bathymetric distribution, 456, 472, 474
Beagle, the ship, 507
Beche-de-mer, 371
Beetle, Japanese, 26
Behavior, Amoeba, 291
Belfrage, 38
Beri-beri, 175
Bile, 173

duct, 125
Binary fission in Amoeba. 290, 291
Binomial nomenclature, 35

system, 259
Biogenetic law, 224, 413
Biological sciences, IS
Biome, 462
Biotic formations, 462

regions, map, 463
Biotin, 175
Bipinnaria, 368
Birth rate, 253
Bivalve, 374
Bivium, 363

Bladder worm, 324, 454, 456
Blastocoele, 99
Blastomeres, 98
Blastopore, 99
Blastula, 213

of frog, 99

of hydra, 321
Bleeders, 180
Blending inheritance, 244
Blind spot, 186
Blood, coagulation of, 180

cells, characteristics, 179

matching and evolution, 505
Body form, 216
Boll, Jacob, 38
Bonellia, 196, 356
Bot fly, 440
Bothriocephaloidea, 324
Bowman's capsule, 140
Brachial plexus, 93
Brachiolarian larva, 368
Brachiopoda, 24
Brachium, 109
Brachypephus, 416

INDEX

529

Brain, development, 218
functions of, 183

Branchial chamber, 400

Branchiata, 394

Branchiostegites, 397

Branchiostoma, 147, 152

Branchipus, 394

Breeding and evolution, 50(j

Bronchi, 132

Bronchioles, 132

Brooks, Frank G., 235

Brown, Robert, 40

Brownian movement, 43

Bryozoa, 24

Buccal cavity of frog, 67

Buffon, 36

Bulimulus, 382

Bullfrog, arteries, 72
circulatory system, 70
digestive system, 67
excretory system, 81
external structure, 65
habitat of, 64
heart, 76

respiratory organs,
skeletal system, 83

80

veins

73

Bursaria, 273
Busycon 390, 391

C

Caeca, hepatic, 365

pyloric, 365
Caecilianella, 380
Caecilians, 64
Calcaneum, 115
Calciferous glands, 348
Calcium metabolism, 198
Callinectes sapidus, 410
Calorie, 173
Cambarus, 395
Campanella, 307
Campanularia, 304
Campeloma, 390, 391
Canadian zone, 475
Canal, ring, 366
Capillaries, 70
Carapace, 396
Carbohydrates, 45
Carbon cycle, 27

tetrachloride, 443
Carchesium, 273, 277
Cardiac anomalies, 234
Cardo, 418
Caribou, 462
Carotene, 174
Carotid arch, 71

common, of rat, 127
Carpals of rat, 114

British Navy,

of rat, 128
50
316

39

Carpus, 109
Carriers, 247
Cartilage bones, 110
Casein, 122
Catabolism, 166

in Amoeba, 289
Catalyst, 46
Cats, bumblebees, and

458
Cattle, short-horn, 245
Caudata, order, 104
Caudina, 361
Cecum, 124
Celiac artery
Cell division,

interstitial,

principle.
Cell theory, 35

typical animal, 47
Cells, chloragogen, 349

daughter, 51

epitheliomuscular, 315

flame, 23, 188, 323, 328

germ, 54, 209

nutritive-muscular, 317

size of, 47

somatic, 54
Cellular differentiation, 54

organization, 55
Centipedes, 23

Central nervous system of rat, 133
Centrosome, 48
Cephalic ganglion, 329
Cephalization, 158
Cephalochorda, 147, 152
Cephalodiscida, 147
Cephalopoda, 373
Ceratium, 277
Cercaria, 448, 450
Cerci, 421

Cerebellum, 91, 134, 218
Cerebral hemispheres, 218

peduncles, 134

vesicle, 154
Cerebratulus, 324
Cerebrum, 134
Cerianthidia, 311
Cerianthus, 309
Cervical enlargement, 136

groove, 396

plexus, 139

region, 109
Cestoda, 324, 452
Cestodes, 323
Chaetognatha, 24
Chaetopoda, 342
Chagas disease, 280
Chalones, 196
Chambers, Robert, 49

530

INDEX

Chaos chaos, 286

drffluens, 286
Charybdea, 308
Cheatum, Dr. Elmer P., 372
Checkerboard, 241
Cheiloschisis, (Harelip), 228
Chela, 399
Chemotropism, 278
Chenopodium, 341
Chevron bones, 110, 113
Chick embryo, 216
Child, Dr. C. M. 225, 332
Chilodon, 272
Chilomastix, 270
Chilomonas, 269
Chimaeras, 148
Chinese liver fluke, 449
Chiropsalmus, 308
Chisos mountains, 476
Chitin, 56, 393, 417
Chitons, 373
Chloragogen cells, 349
Chlorocruorin, 343
Chlorohydra, 311
Cholesterin, 173
Chondriosome, 48
Chordata, 24

characteristics of, 146

classification of, 147

phylogenetic advances of, 148
Chordates in general, 146
Chorioid plexus, 92, 136

fissure, 219
Chromatin, 49
Chromatophore, 283
Chromonemata, 49
Chromosomes, 239
Chyme, 69, 122
Ciliata, 272
Circulation, function of, 178

of mammal, 162
Circulatory system, development, 220

of rat, 126
Circumoral canal, 365
Circumpharyngeal connectives, 352
Cirri, 344
Cirripathes, 311
Cirripedia, 412
Cirrus, 329
Cisterna magna, 81
Clams, 23, 373

Classification of animals, 259
Clavicle, 114
Cleavage, 213

in crayfish, 409

in frog, 98
Cleft palate, 228
Climax community, 466
Clitellum, 345
Clitoris, 142, 223

Cloaca of frog, 67
Clonorchis, 324

sinensis, 437, 449
Clypeaster, 361
Clypeus, 418
Cnidoblasts, 315
Cnidosporidia, 275
Coccidia, 274
Cochlea, 164, 187
Cocoon, 356

of Planaria, 332
Codonosiga, 269, 277
Coelenterata, 23, 303

phylogenetic advances of, 322
Coelom, false, 339
Coitus, 193, 223
Coleps, 272
Colloidal emulsion, 42
Coloboma, 234
Colon, 124

Colonial protozoa, 277
Colony formation, 482
Color blindness, 246

of children, 247
Comb jellies, 23
Commensalism, 428
Conception, 224
Conductivity, 43, 132
Condylarthra, 492
Condyles, 115

Coniferous formation 462, 463
Conjugation, 296
Conjunctiva, 184
Continent, ideal, 461
Contractility, 43
Contractions, peristaltic, 318
Conus arteriosus, 76
Convolutions, 134
Coordinated movement, 183
Copepoda, 412
Copulation, 223
Coracoid, 87
Coral, 310

Corallium rubium, 311
Cornea, 94

Corneagen cells, 405, 406
Corpora quadrigemina, 134
Corpus callosum, 134
Corpuscle, Miescher's, 275
Corpuscles, 130, 161, 179, 351
Cortex, 140

Corti, organ of, 164, 187
Cortin, 200
Cotylapsis, 324
Cotvlophoron, 450
Coxa, 417, 420
Coxopodite, 397, 399
Crab, 394

edible, 410

fisheries, 410

INDEX

531

Cranial nerves of frog, 92
Cranium, 83
Craspedacusta, 304
Crayfish, 23, 394, 395

development, 407

habitat and behavior, 396

internal structure, 400
Creosote bush-kangaroo rat biome, 464
Cretinism, 197
Crinoidea, 361
Crocodiles, 148

Cro-Magnon man, 479, 495, 497
Crop, 348

Cross-fertilization, 331
Crosses, 239
Crossing over, 248
Crura cerebri, 91
Crystalline cone, 405, 406

iens, 219

style, 376
Ctenophora, 23
Cubomedusae, 308
Cucumaria, 361
Cuvier, 35
Cycle, nitrogen, 28
Cyclophyllidea, 324
Cyclopia, 234
Cyclops, 394, 412
Cyclosis, 178, 295
Cyclostomata, 147
Cynthia, 147
Cypris, 394

Cysticercus, 324, 454, 456
Cytology, 19
Cytopharynx, 282
Cytoplasm, 48
Cytosome, 287

D

Daphnia, 394, 412
Darwin, Charles, 36, 480

and evolution, 506
Darwin, Erasmus, 36
Davis Mountains, 476
Decapoda, 410
Deciduous formation, 464
Deer, Virginia 464
Deltoid ridge, 114
Dendrites, 137
Dendy, Arthur, 492
Dental formula, 112
Dentalium, 373
Depressor, 11

Dermal branchiae, 363, 367
Dermatitis, 175
Desert formation, 462, 464
Development, intrauterine, 223

of aortic arches, 501, 504

of crayfish, 407

Development — Cont 'd

of different vertebrates, 502

of individual, 209
deVries, Hugo, 508
Dextrocardia, 234
Diabetes, 208

insipidus, 203, 206
Diaphragmatic hernia, 228
Diaptomus, 412
Diastase, 125, 169
Diastema, 112
Diastole, 180
~ Dicrocoelium, 450
Didinium, 272
Diencephalon, 90, 218
Differential birth rate, 258
Difflugia, 270
Digenea, 324
Digestion, as function, 168

in clam, 376

in frog, 69

gastric, 170

intestinal, 170
Digestive enzymes, 169

system, development, 217
of rat, 121
of vertebrate, 159
Digits, 109

extra, 229
Dihybrid cross, 240
Dilator, 116
Dillinger, 293
Dioctophyme, 337
Dioctophymoidea,
Dioecious organisms, 62
Diphyllobothrium, 317, 324, 434

latum, 434, 437, 456
Diploblastic 303
Diploid, 210

Dipylidium caninum, 455, 456
Discomedusae, 308
Dispersal of animals, 478
Distribution, animal, 472

bathy metric, 472, 474

effects of man upon, 479

geographic, 472

of biotic communities, 461

vertical, 472, 474
Diverticula, 328, 356
Diverticulum, 149
Dolichoglossus, 148
Dominance, lack of, 244

principle of, 237
Dominant factor, 237
Dorsal aorta of rat, 128
Dourine, 281
Dracunculus, 438
Drosoyhila, 30, 242
Duct, Wharton's, 125

Wolffian, 222

532

INDEX

Ductus eholedoclius, 125
Dujardin, 39
Duodenum, 124, 160

of frog, 67
Dura mater, 92, 133
Dwarf, 201, 206
Dysentery, amoebic, 279
Dysgenic, 252, 255

E

Ear, development, 220
Eardrum, 187
Earthworm, 23, 342

development, 353, 354
Ecdysis, 393, 409
Echinarachinus, 361
Echinaster, 359

Echinococcus granulosus, 437, 450
Echinoderm theory, 485
Echinodermata, 23, 358
Echinoderms, larval relations, 485
Echinoidea, 359
Echiuroidea, 356
Ecology, 21, 457
Economic importance of Amphibia, 102

relations of starfish, 371
Ectoderm, 215, 314
Ectoparasites, 430
Ectosarc, 282
Edwardsiidea, 311
Eel worm, 335
Elasmobranchii, 148
Electrotropism, 278
Elephantiasis, 439
Embryo, chick, 216
Embrvological evidence of evolution,

501
Embryology, 21, 209

of frog, 96
Emulsoid, 42

Encephalocoele, 234

Endamoeba, 440
coli, 279, 418
gingivalis, 279, 436
histolytica, 270, 279, 436
nana, 279
endocrine glands, 195

Endoderm, 215, 296, 310

Endolimax nana, 279

Endolymph, 95

Endomixis, 298

Endoparasites, 430

Endopodite, 397

Endosarc, 282, 287

Endoskeleton, 110, 167

Endostvle, 156

Enterobius, 335

Enterobinase, 171

Enteron, 326

Enteropneusta, 147
Entomostraca, 412
Environment, animal and, 457
Enzymes, 45, 122, 167
Eoanthropus skull, 494, 496
Eocene epoch, 498
Eohippus, 492
Epicranium, 400
Epididymis, 142, 144
Epiglottis, ]22, 131
Epinephrine, 199
Epistylis, 272, 277
Epitheliomuscular cells, 315
E'poophoron, 142
Equatorial plate, 51
Equilibrium, 188
Equus caballus, 492
Erepsin, 125, 171, ♦172
Erythrocytes, 57, 70, 130
Eskimos, 495
Esophagus, 122
Estrogen, 205, 207
Ethiopian region, 473
Ethmoids, 112
Eubranchipus, 410, 412
Eudorina, 277
Eugenic measures, 256

security, 254
Euglena, 275-282
Euglenoid movement, 285
Euplotes, 273
Eupomotus, 464

Eustachian tube, 94, 131, 183, 220
Evolution and blood matching, 505

and breeding, 505

and serum, studies, 505

and vestigial structures, 500

annelid theory, 485

arachnid theory, 485

echinoderm theory, 485

embryological evidence, 501

geological evidences of, 489

morphological evidence, 499

of the horse, 492

physiological evidence of, 505

theory of, 480
basis for, 489
Excretion, function of, 188
Excretory system, of bullfrog, 81
of rat, 140
of vertebrate, 162
Exopodite, 397
Exoskeleton, 167
Expiration, 132
Exumbrella, 304
Eye, muscles of, 185

structure and function, 185
Eyes, 218

of crayfish, 405

of frog, 93

INDEX

533

Facets, 418
Family size, 253

in dysgenic groups, 255
Fascia, 88, 121
Fasciola, 392

hepatica, 437, 450, 451
Fascioloides, 450
Fasciolopsis buskii, 450
Fat bodies, 9(1
Fats, 45
Fatty acids, 125
Fauces, 122
Femur, 88, 115, 420
Fenestra oralis, 187
Fertilization, 211

membrane, 98
Fetus, 224
Fibrin, 79, 180
Fibrinogen, 131, 162, 180
Fibula, 115
Fiddler crab, 411
Filaria, 336
Filaroidea, 336
Filum terminale, 136
Fimbria, 143

Fisheries and zoology, 30
Flagellar movement, 285
Flagellum, 282

Flame cells, 23, 188, 323, 328
Flatworms, 23
Flemming, 50
Flukes, 323, 447
Food in assimilation in Euglena, 283

chain, 25, 459

relations in prairie community, 458
Foramen magnum, 83

of Monro, 136

ovale, 234
Foiaminifera, 270, 279
Forebrain, 218
Fox, Arctic, 462
Frenulum, 121
Frog {see also Bullfrog)

later development, 101
Frons, 418
Frontonia, 273
Fruit fly, Mediterranean, 30
Fulcrum, 117
Fundus, 122

G

Galea, 420
Galen, 32
Gametes, 61, 209
Gametocytes, 276, 442
Gametogenesis, 209
Gammarus, 394, 411

OAO

314

Ganglia, 163

sacral, 140
Ganglion, cephalic, 329

cervical, 139
Gasserian ganglion, 137
Gastric digestion, 170

glands, 125

mill, 402
Gastrophilus, 440
Gastropoda, 373, 380
Gastrovascular cavity,
Gastrula, 157

development of, 482

of hydra, 321
Gastruiation, 99, 213, 214, 215

in earthworm, 353
Gebia, 381

Geiser, Dr. S. W., 38
Gel state, 42
Genae, 418
Gene, 238
Generic name, 259
Genes, 49

complementary, 244

pleural, 242

supplementary, 244
Genetics, 21

and eugenics, 235
Genotype, 241
Geobios, 474

Geographic distribution, 472
Geologic evidence of evolution, 489

time scale, 490
Geotropism, 278
Gephyrea, 356
Germ" cells,- 54, 209

layer, theory, 35

layers, 215
Gestation, 224
Giardia, 270

lamblia, 437
Gill pouches, 217
Gizzard, 349
Gland, parotid, 124

sublingual, 125

submaxillary, 124
Glands, calciferous, 348
" ductless, 195

of internal secretion, 195
Glans, 109, 143
Glenoid fossa, 114
Globigerina, 270, 279
Glochidium, 378
Glomerulus, 190
Glossary, 510
Glossina, 437

Glossopharyngeal nerve, 92, 137
Glottis, 80, 122, 131
Glucose, 45, 170, 171
Glycogen, 45, 161

534

INDEX

Glvcogenase, 173
Goiter, 197
Golgi elements, 48
Gonad, 209
Gonads, 55

and hormones, 204
Gonangium, 306
Goniobasis, 390
Gonionemus, 304
Gordiacea, 337, 438
Gordius, 336, 337
Gorgonacea, 311
Gorgonia, 311
Gorgonocephalus, 350
Gradient, metabolic, 333
Granulocytes, 79
Grass-bison biome, 464
Grasshopper, 415

external features, 417

internal structure, 423
Grassland formation, 404
Gray matter, 136
Gregaloid colony, 278
Gregarina, 275
Gregarinida, 274
Growth, 210
Guinea worm, 438
Gynecophoric canal, 447

H

Haeckel, Ernst, 35
Haemosporidia, 274
Haliclystus, 308
Halteria, 273
Haploid, 211
Haplosporidia, 275
Harelip, 228
Harvey, William, 32
Heart, development of, 220

of bullfrog, 70

of rat, 126
Heat, 224

Heidelberg man 495
Heliozoa, 271
Helisoma, 380
Helix, 380

Hemichorda, 147, 148
Hemispheres, 134
Hemocvanin, 178

Hemoglobin, 45, 79, 130, 179, 343, 350
Hemolymph, 178
Hemophiliacs, 180
Hepatic portal system, 75

vein, 129
Heredity, human, 250

physical basis of, 238
Hermaphroditism, 192, 233
Hermit crab, 410
Hernia, diaphragmatic, 228

239

Heterodont, 113
Heterotrichida, 272
Heterozygous individual,
Hewatt, Dr. Willis, 472
Hexylresorcinol, 341, 443
Hilum, 140
Hind brain, 218
Hippocrates, 31
Hirudinea, class, 355
Hirudo, 355
Histology, 19
History of zoology, 31
Holothuria, 301
Holothurioidea, 361
Holotrichida, 272
Holozoic, 269
Homo, 20

neanderthalensis skull, 495, 497
sapiens, 494, 497
Homologv, 225, 399

serial, 399-
Homonomous, 393

Homozygous individual, 239

Hooke, 39

Hookworm, 23, 438, 443, 445
treatment of, 443

Hopkins, Dr. Sewell H., 428

Hormones, 195
gonadal, 204, 207

Horse bot, 440
evolution of, 492

Horsehair snake, 335,
worm, 23

Hudsonian zone, 475

Human heredity, 250

Humboldtiana, 383

Humerus, 114

Humor, aqueous, 94
vitreous, 94

Huxley, Thomas H.,

Hyaloplasm, 48

Hydra, 303

external anatomy,

Hydrobios, 474

Hydrocephalus, 234

Hydrocorallina, 307

Hydrolysis, 170

Hydrorhiza, 306

Hydrosphere, 459

Hydrozoa, 304

Hymenolepsis, 324, 456

Hyoid, 217

Hyperbranchial groove, 156

Hyperthyroidism, 198

Hypnotoxin, 316

Hypobranchial groove, 156

Hypoglossal nerve, 137

Hypophysis, 91

Hypostome, 314

Hypotrichida, 273

336

17

313

Index

1335

Ileocolic valve, 124
Ileum, 124

of frog, 67
Ilium, 87, 115
Implantation, 223
Incisors, 112
Incus, 187
Infusoria, 293

class, 272
Ingestion, 287
Inguinal canal, 145
Inheritance of sex, 245
Innominate bone, 115
Insect catch, average, 468
Insecta, 395
Insects, 23, 415
Inspiration, 132
Insulin, 205, 207
Integument, 158
Intelligence quotient, 256
Intercalary disks, 57
Interventricular septum, 220
Interzonal fibers, 51
Intracellular digestion, 303
Intrauterine development, 223
Invertase, 171, 172
Inverting enzymes, 169
Iodine, 197
Iris 94

Irritability, 42, 132
Ischium, 115
Ischnochiton, 373
Itch mite, 431, 432

Java man, 494
Jellyfish, 23
Jellyfishes, 303
Jennings, 292

K

Karyolymph, 49
Karyoplasm, 49
Karyosomes, 49
Kerona, 273
Kidneys, 140
function of, 189

Labrum, 418

Labyrinth, membranous, 220

Lacinia, 418

Lactase, 171, 172

Lacteals, 79

Lamarck, 35

Lampsilis, 375

internal anatomy, 375

life history of, 379
Lancisi, 280
Land habitat, 474
Langerhans, islands of, 205
Larva, hexacanth, 456

tornaria, 485

trochophore, 344, 355, 482

veliger, 372, 482
Larval echinoderms, 483
Larynx, 131
Laveran, Dr., 280
Law, Mendel's, 235
Leber 's atrophy, 246
Leech, 23

Leeuwenhoek, 34, 39
Lepton, 381
Leptolina, order, 304
Leptosynapta, 361
Leucocytes, 57, 70, 79
Levator, 110
Levers in body, 116
Life, 24

origin of, 488

regions, 473

zones, 475
Lincecum, 38
Linin net, 49
Linkage, 246
Linnaeus, 20, 34, 259
Lipase, 69, 169, 170, 171
Liriope, 307
Lithosphere, 459
Littoral area, 474
Liver fluke, 23
Chinese, 437
oriental, 449

functions of, 173

of rat, 125
Lizards, 148
Locomotion, 168
Locust, 415
Loligo, 373
Lombok island, 473
Louse, body, 431
crab, 431
head, 431
Lubber grasshopper, 416
Lucernaria, 308
Lumbricus, 344
Lung fluke, 450

Lymnaea, 380, 381, 382, 385, 390, 392
Lymph glands, 131

nodes, 131
Lymphatic system of frog, 79

of rat, 131
Lymphocytes, 79
Lynx, Canadian, 464

536

INDEX

M

Macrocephalus, 234
Macrogametes, 276
Madrepora, 310
Madreporaria, 310
Madreporite, 358, 363
Malaria, 280, 441
Malformations, 226
Malleus, 187
Malpighian corpuscles, 140, 190

tubules, 189
Malpighi, 32, 39
Maltase, 125, 172
Malthus' law, 507
Maltose, 170, 171, 172
Mammalia, 148
Man, Cro-Magnon, 495, 497
Heidelberg, 495
Java, 494, 496
Peking, 494
Piltdown, 494
rise of, 494 ,,

Manson, 280
Mantle, 151
Manubrium, 114, 304
Manus, 109
Marsupials, 487

Mastigophora, 268, 282

Matings among defectives, 252

Maturation, 210

Maxillae, 112, 399

Maxillipeds, 399

Means of dispersal, 478

Meatus, 220

Meckel's cartilage, 85

Mediastinum, 229

Medulla oblongata, 91, 136

Medusa, 305, 307

Medusae, 63

Megalops, 413

Meiosis, 211

Meiotic division, 210

Meissner's tactile corpuscles, 184

Melanophore, 66

Melanoplus, 417

Membrane bones, 110

Membranous labyrinth, 94

Mendel, Gregor J., 37, 237

Mendel's law, 237

Meninges, 133

Menopause, 145

Menstruation, 224

Mentum, 420

Merozoites, 276, 441

Merychippus, 492

Mesencephalon, 134

Mesenchyme, 326
Mesoblast cell, 353

Mesoderm, 215

formation in frog, 100
Mesoglea, 317
Mesohippus, 492
Mesomere, 222
Mesonephric duct, 81
Mesonephros, 142, 222
Mesorchium, 95
Mesothelium, 346
Mesothorax, 420
Mesovarium, 143 •
Metabolic gradient, 225

processes, 27
Metabolism, 43, 166
in Amoeba, 287
in crayfish, 406
in Hydra, 318
in Paramecium, 295
in Planaria, 331
Metacarpals, 114
Metacercaria, 450
Metacrinus, 361
Metagenesis, 63, 307
Metameres, 147
Metamerism, 157
Metanephros, 142, 222

Metaphase, 51

Metapleural folds, 154

Metathorax, 420

Metazoa, 22

Metazoan organization, 54

Metridium, 309

Microcephalus, 234

Microdissector, 49

Microsporidia, 275

Midbrain, 134

Miescher's corpuscle, 275

Migration of animals, 477

Millepedes, 395

Millepora, 308

Miracidium, 448

Mitochondria, 48

Mitosis, 50

Mitral valve, 126

Molars, 112

Molgula, 147, 150

Mollusca, 23, 372
classified, 373
economic relations of, 391

Molluscoida, 24

Molt, 409

Monecious organisms, 62

Moniezia, 455, 456

Monocytes, 79

Monogenea, 324, 451

Monohybrid cross, 239

Morphologic evidence of evolution, 499

Morphology, 18, 19

Mosquito, 442

Mother-of-pearl, 391

INDEX

537

Movement and locomotion, 168
Mulatto parents, 243
Miiller, Johannes, 35
Murex, 383
Muscle, striated, 121

voluntary, 115
Muscles, action of, 115
Muscular system of frog, 88

of vertebrate, 159
Mussel, fresh-water, 372
Mutation theory, 508
Mutations, 249, 508
Mutualism, 428
Mycetozoa, 271
Myelencephalon, 218
Myocomma, 154
Myofibrils, 121
Myology, 116
Myosepta, 154
Myotomes, 101, 154
Mvriapoda, 395
Mysis, 413, 414
Mystacial vibrissae, 108
Myxedema, 198
Myxidimn, 275
Myxobolus, 275
Myxosporidia, 275

N

Nagana, 281

Nares, 122

Natural selection, theory of, 507

Nauplius, 413, 483

Nautilus, 373

Nearctic region, 473

Necator, 438

americana, 443
Negro, 243

Nemathelminthes, 23, 335
Nematocysts, 23, 308, 315
Nematoda, 438
Nematodes, 443
Nemertina, 24, 324
Neocomatella, 361
Neotropical regions, 474
Nephridia, 23, 157, 189, 343, 351
Nephridiopore, 343, 346, 351
Nephrostome, 80, 343, 351
Nereis, 344
Nerves, cranial, 137

peripheral, 137
Nervous conduction, 182

functions, 180

system, development, 218
of frog, 90
of rat, 132
of vertebrate, 163
Neural groove, 100, 216

Neural — Cont'd

plate, 218

tube, 100, 218
Neurites, 137
Neurocoele, 92, 136, 154
Neuro-epithelial cells, 181
Neurone, 57, 133
Neurones, 181
Nicotinic acid, 175
Nicitating membrane, 66
Nipples, 109
Nitrogen cycle, 28
Noctiluca, 269
Nomenclature, 259

binomial system, 35, 259
Notochord, 146, 154

rudimentary, 149
Nuclear sap, 48, 49
Nucleus, 48

discovery of, 40

O

Oak-deer biome, 464

Obelia, 63, 299, 304, 305, 306

Occipital condyle, 84

Ocelli, 418

Ochre starfish, 362

Octopus, 23

Oculina, 310

Oculomotor nerve, 92, 137

Oenothera, 508

Oestrin, 205

Oestrus cycle, 224

Olecranon, 114

Olfactory epithelium, 93

organs, development, 219
Oligochaeta, 342
Ommantidium, 405, 406
Omphalomesenteric vein, 220
Onchocerca, 438
Oniscus, 411
Ontogeny, 21, 225, 414
Onychophora, 395, 483
Oocyst, 276
Oocyte, 210
Oogenesis, 209
Oogonia, 210
Ookinete, 276
Opalina, 272, 437
Operculum, 448
Ophioderma, 359
Ophiothrix, 359
Ophiura, 359
Ophiuroidea, 359
Optic chiasma, 90

cup, 219

lobes, 91

stalk, 219

vesicles, 219

538

INDEX

Oral groove, 294

ring, 368
Orbicularis oris, 116
Oreaster, 359
Organ, 58
Organogenesis, 217
Organs and systems, development, 217
Oriental region, 473
Origin, common center of, 498

of species, 37, 480
Orohippus, 492, 493
Osphradium, 378

Ossicles, 356, 384, 187, 220, 363, 402
Osteomalacia, 175
Ostium, 143
Ostracods, 412
Otocysts, 152
Ova, 55, 142, 209
Ovaries of frog, 95
Overproduction, 507
Oviducts, 96, 143
Oviparous animals, 193, 391
Ovipositor, 423
Ovulation, 145, 224
Oxidase, 169
Oxyhemoglobin, 176, 179
Oxytricha, 273
Oyster, 373

drill, 391

Pagurus, 410
Palaearctic region, 473
Palate, cleft, 228
Paleozoology, 22
Pancreas, 124, 205, 207
Pancreatic juice, 171, 172
Pandorina, 277
Pantothenic acid, 175
Papillae, adhesive, 151
Papula, 356
Paradidymis, 142
Paragonimus, 324, 450
Paragordius, 337
Paramecium, 293
Paramylum, 284
Parapodia, 344
Parasite, the successful, 430
Parasites, accidental, 429

facultative, 429

infection and transmission, 433

obligate, 430

perfect, 433

permanent, 430

protozoan, 435, 440

representative, 440

temporary, 430
Parasitic Arthropoda, 439

Nemathelminthes, 438

Platyhelminthes, 487

Parasitism, and host specificity, 434

animal, 428

degrees of, 429

factors of, 435

origin of, 429
Parathormone, 198
Pal-athyrin, 198
Parathyroid glands, 198
Parenchyma, 326
Parietal connective, 351
Parthenogenesis, 62, 193
Pasteur, Louis, 37
Patella, 115
Pathology, 21
Pectoral girdle, 86
Pedal ganglia, 388
Pedicellariae, 358, 363
Pedigree of horse, 492
Peking man, 494
Pelecypoda, 373
Pellagra, 175
Pellicle, 295
Pelmatohydra, 311
Pelvic girdle of frog, 87

of rat, 114
Penaeus, 414
Penial setae, 339
Penis, 109, 143, 222
Pennatula, 311
Pennatulacea, 311
Pentacrinus, 361
Pepsin, 69, 122, 169
Peptones, 122, 171
Perch, 148
Pereiopods, 397, 398
Peribranchial grooves, 156
Pericardial sinus, 402
Pericardium, 76, 126
Pericolpa, 308
Perineal region, 109
Peripatus, 395, 483
Periphylla, 308
Peristalsis, 159
Peristaltic contractions, 318
Peristome, 363
Peristomium, 344
Peritoneum, 69

of earthworm, 346

of starfish, 363
Peritrichida, 273
Perivisceral sinus, 403
Peromedusae, 308
Phagocytosis, 180
Phalanges of rat, 114
Pharyngeal clefts, 146
Pharynx, 122
Phenotypes, 241
Phoronidea, 24
Photosynthesis, 28
Phototropism, 278, 325

INDEX

r):]9

Phylogenetic advances of Annelida, 357

development of horse, 492

relations, 480
Phylogeny, 21, 225, 414, 487
Phyrone, 201, 206
Physa gyrina, 380
Physalia, 307, 308

Physiologic evidence of evolution, 505
Physiology, 21, 166
Pia mater, 90, 92, 133
Pigworm, 335
Pilidium, 324
Pill bug, 394, 411
Pineal organ, 90
Pinna, 108
Pinworm, 335
Pisaster, 362
Pisces, 148

Pithecanthropus skull, 494, 496
Pituitary gland, 91, 201, 206
Pituitrin, 202
Placenta, 143, 223
Placentalia, 487
Planaria, 23, 323

reproduction, 329
Plantigrade, 109
Planula, 306, 307
Plasma, 57, 70, 129, 350

membrane, 48
Plasmalemma, 287
Plasmasol, 287
Plasmodium, 275, 280, 441

falciparum, 276

malariae, 276

vivax, 276
Plasmogel, 287
Plasmosomes, 49
Platyhelminthes, 23

phylogenetic advances, 334

phylum, 323
Pleopods, 397
Plethodontids, 64
Pleural ganglia 388

genes, 242
Pleurobranchial, 400
Pleurocera, 388
Pleuron, 397, 420
Plexus, cervical, 139

lumbar, 139

sacral, 139

sciatic, 139
Pliny, 31

Pliocene epoch, 498
Pliohippus, 492
Pneumatophore, 308
Podical plates, 421
Podobranchial, 400
Podophyra, 274
Polar body, 210
Polychaeta, 344

!3

Polycladida, 32.'

Polydactyly, 229

Polygordius, 355

Polygyra, 381

Polyhybrid cross, 240

Polyp, 303

Polypus, 373

Pons, 134

Population, seasonal changes, 470

Populations, animal 468

Porcellio, 411

Porifera, 22

Porospora gigantea, 47

Portuguese man-of-war, 307, 308

Postcava, 74

of rat, 129
Potomobius, 395
Precipitation effectivity, 460
Prepuce, 109, 143
Presphenoid, 110
Primrose, evening, 508
Proboscis, 148, 324, 325, 327, 337
Progeny, 240
Progesterone, 205, 207
Proglottids, 324, 454
Prolan, 202, 206
Pronator, 116
Pronephros, 222
Pronghorn, 464
Pronotum, 420
Pronucleus, 211
Prophase, 50
Proptera, 374
Prosencephalon, 218
Prostate gland, 145
Prostomiuin, 344, 345
Prostonia, 324
Protandrous condition, 388
Proteases, 169
Proteins, 45
Prothorax, 420
Protochordata, 148
Protogynous condition, 388
Protoneurones, 181
Protoplasm and cell, 39

characteristics of, 41

chemical nature, 44

physical nature, 43

properties of, 42
Protopodite, 397
Prototracheata, 395
Protozoa, 22

colonial, 277

economic relations, 279

in general 268

representative, 275
Protractor, 116
Prozoea, 414
Pseudopodia, 287, 292
Pseudopodiospore, 291

540

INDEX

)2

Ptarmigan, 46i
Ptyalin, 125, 172
Pubis, 115

Pulmocutaneous arch, 73
Pulmonary veins, 76
Pulvillus, 421
Purkinje, 39
Pylangium, 77
Pyloric caeca, 365

valve, 160
Pyrenoid bodies, 282
Pyridoxine, 175

Quadroon, 243
Quadrula, 374

Q

R

Radial nerve, 368

symmetry, 303
Radiata, 358
Radiolaria, 271, 280
Radula, 386, 387
Ramus communicans, 140
Ranacatesbeiana, 65

grylio, 65
Rat, 107

external structure, 108
muscular system, 115
reproductive system, 142
Rattus alexandrinus, 107
norvegicus, 107
rattus, 107
Ray, 34
Rays, 148
Recapitulation theory, 163, 224, 413,

487, 501
Recessive, 237
Rectal caeca, 365
Rectum, 124
Rectus abdominis, 90
Redia, 449

Reduction division, 210
Reflex, 325

arc, 133, 182
Regeneration in crayfish, 409
in earthworm, 355
in Hydra, 322
in Planaria, 333
in starfish, 370
Regions of life, 473

zoogeographical, 472
Relations of animals and plants, 26
Remak, 50
R-enal pelvis, 140

portal system, 7(5
Renilla, 3il
R^nnin, 122, 169
Reproduction, Ascaris, 340
bisexual, 192

Reproduction — Cont 'd

development of sexual, 61

function of, 191

Hydra, 319

in Amoeba, 289

in crayfish, 407

in earthworm, 352

in Euglena, 284

in Paramecium, 290

of starfish, 368

sexual, 209
Reproductive system of frog, 95
of rat, 142
of vertebrates, 164
Reptilia, 148
Respiration, aerial, 177

as function, 176

external, 160, 177

in Euglena, 284

internal, 160, 177
Respiratory system, development,

of vertebrate, 161
Resting cell, 50
Retina, 94
Retinula, 405
Retractors, 116
Rhabdocoelida, 323
Rhabdome, 405, 406
Rhabdopleura, 147
Rheotropism, 278, 325
Rhizopoda, 270
Rhomaelia, 422
Rhombencephalon, 218
Rhynchocephalia, 486
Riboflavin, 175
Ribs, false, 114

true, 113
Rickets, 175
Rise of horse, 492

man, 494
Rodentia, 107 •
Roe, 371

Ross, Major Ronald, 280
Rostellum, 454
Rostrum, 397
Rotifers, 24
Roundworms, 23, 335
Rumina, 382

S

Sacculina, 413, 440
Sacculus, 94
Sagartia, 309
Sagebrush formation, 464

jack rabbit biome, 464
Sagitta, 24
Salientia, order, 105
Salivary glands, 124
Salpa, "147
Salts, inorganic, 47

218

INDEX

r,4i

Sand dollars, 359

Sanders, Ottys, 64

Santonin, 341

Sarcode, 39

Saroodina, 270, 285

Sarcolemma, 121

Sa re op] asm, 121

Sarcosporidia, 275

Scallops, 374

Scaphognathite, 399

Scapliopoda, 373

Scapula, 86

Schistosoma haematobium, 437, 447

Schizogony, 274

Schizont stage, 441

Schizopod, 413

Schleiden, 35, 40

Schneider, 50

Schneiderian membrane, 184

Schwann, 35, 40

Sciatic plexus, 93

Sclera, 185, 219

Sclerite, 417

Scolex, 324, 454

Scorpions, 23

Scrotum, 109, 143

Scurvy, 175

Scyphozoa, 308

Sea cucumber, 23, 361

eggs, 371

urchin, 23, 359

walnuts, 23
Secretin, 69

Sedge-musk ox biome, 462
Segregation, principle of, 238
Self, Dr. J. Teague, 342
Semicircular canals, 94, 164
Semilunar valves, 73, 77, 126
Seminal receptacles, 347

vesicle, 145, 347
of frog, 95
Seminiferous tubules, 144
Sense organs, development, 218
function of, 183
of frog, 93
of rat, 139
of vertebrates, 164
Septa, 343, 346
Serial homology, 399
Serous membrane, 132
Serum, 162

and evolution, 505

antihuman, 506
Setae of earthworm, 346

penial, 340
Sex, inheritance of, 245

linkage, 246
Sexual reproduction, development of, 61
Sharks, 148
Sheep tapeworm, 455, 456

Shipworm, 392
Shrimp fisheries, 410
Siamese twins, 230
Sinanthropus pekinensis, 494
Sinu-auricular aperture, 76
Sinus venosus, 74
Siphon, atrial, 151

branchial, 151
Siphonoglyphe, 310
Siphonophora, 308
Sipunculoidea, 24, 356
Size, family, 253
Skeleton, appendicular, 114, 159

human, 158
Skull, 159

Sleeping sickness, 280
Slugs, 23
Snails, 373, 380
Social relations of animals, 428
Sol state, 42
Solastar, 359
Somatic cells, 54

mesoderm, 100
Somatoplasm, 55
Somites, 216
Sonoran zone, lower, 477

upper, 476
Sparrow, English, 26
Spermatheca, 427
Spermatic cord, 145
Spermatids, 210, 211
Spermatocytes, 210, 211
Spermatogenesis, 210, 211
Spermatophores, 331
Spermatozoa, 55, 142, 209, 211
Sphaeriidae, 378
Sphenodon, 486
Spheroid colony, 277
Spicules, 339
Spinal cord, functions of, 183

ganglion, 93

nerves of frog, 92
Spiny-headed worms, 337
Spiracles of grasshopper, 423
Spiral valve, 77
Spireme, 50
Spleen of frog, 69
"Bponges, 22
Sporoblasts, 276
Sporocyst, 448, 449
Sporogony, 274
Sporont stage, 441
Sporozoa, 274
Sporozoites, 62, 276
Sporulation, 276, 291
Spruce-moose biome, 463
Stalk, optic, 219
Starfish, 23, 358

reproduction, 368
Starling, 479

542

INDEX

Statocyst, 404
Stauromedusae, 308
Steapsin, 69, 125, 169, 171
Stegocephalia, 486
Stegomyia, 281
Steu-mother, 62
Stenostomum, 824
Stenson's duct, 124
Stentor, 272
Sternebrae, 114
Sternum, 114, 397, 420

of frog, 86
Stiles, Dr. Charles W., 44?,
Stipes, 418
Stolenifera, 311
Stomach, cardiac, 122

pyloric, 122
Stomolophus, 308, 309
Stone age, 495

canal, 366
Strobilus, 454
Strongylocentrotus, 361
Strongyloidea, 336
Strongyloides, 335
Struggle for existence, 507
Styloid process, 114
Subclavian artery of rat, 128
Subgenital plate, 421
Sublingual gland, 125
Sublittoral area, 474
Succession, 465
Succus entericus, 171
Suctoria, 274

Suprapharyngeal ganglion, 352
Suprarenal bodies, 199
Surra, 281

Survival of fittest, 508
, Suture, 110, 417
Swimmerets, 397
Symbiosis, 428
Symmetry, bilateral, 326
Sympathetic nerves, 93
Symphysis, pubic, 115
Synangium, 77
Synapse, 57, 182
Synapsis, 210
Syngamus, 336
Syphilis, 434
Systemic arch, 71

veins, 74
Systems, 59
Systole, 126, 180

T

Tadpole, 101
Taenia, 324, 434

pisiformis, 453

saginata, 434, 455, 456

solium, 434, 454
Tanner, Dr. Vasco M., 415

Tapetum, 185

Tapeworm, 23, 452
economic relations of, 334
of sheep, 455, 456

Tarsus, 88

Taste buds, 164, 220

Taxis, 278

Taxonomy, 19

Teats, 109

Teeth, deciduous, 112
permanent, 112

Tegmina, 421

Telencephalon, 134, 218

Telophase, 51

Telosporidia, 274

Temporal, 110

Tendons, 56, 115

Tentacles, 314

Teratology, 226

Teredo, 392

Tergum, 420

Tessera, 308

Test, 359

Testes, 144

of earthworm, 347

Testicle, 144

Testosterone, 204, 207

Tetany, 198

Tetrastema, 324

Texas fever, 281

Thecodont, 113

Theelin, 205, 207

Theory, mutation, 508
of natural selection, 507
of recapitulation, 224, 487
Therm orpha, 487
Thermotropism, 278, 325
Thiamin, 174
Thigmotropism, 278, 325
Thoracic basket, 159

lymph duct, 131
Thorny-headed worms, 438
Threadworms, 23
Threshold, 278
Thrombin, 131
Thrombocytes, 70
Thymus gland, 203

of frog, 70
Thyone, 361
Thyroid, 217
gland, 196
of frog, 70
Thyroxine, 197
Thysanosoma, 456
Tibia, 115, 420
Tiedemann's bodies, 365
Time scale, 490
Tissues, 55
epithelial, 55
muscular, 56

INDEX

r)4r{

Tissues — Cont'd

nervous, 57

sustentative, 56

vascular, 57
Tocopherol, 176
Tongue, 121
Tonsil, 217

Tornaria larva, 150, 484
Toxocara, 335
Toxopneustes, 212
Trachea, 131, 218
Tracheata, 394
Trachylina, 307
Trachynema, 307
Transition zone, 476
Transverse septum, 76
Trematoda, 323

Trematodes, 323, 429, 437, 447
Trembly, 322
Trepang, 371
Treponema, 434
Triatoma, 280
Trichina, 444
Trichinella, 438, 444
Trlchinelloidea, 337
Trichinosis, 438
Trichocysts, 295
Trichomonas, 270, 434
Tricladida, 323
Trigeminal nerve, 137
Trigeminus, 92
Trihybrid cross, 241
Triploblastic structure, 326
Trivium, 363
Trochanter, 115, 420
Trochelminthes, 24
Trochlea, 114
Trochlear nerve, 137
Trochophore larva, 344, 355, 365, 372,

382, 464, 482
Tropical zone, 477
Tropicorbis, 382
Tropisms, 278, 312
Trypanorhyncha, 324
Trypanosoma, 437

cruzi, 280

gambiense, 280

rhodesiense, 280, 437
Trypsin, 69, 125, 169
Trypsinogen, 171
Tuberosities, 114
Tubularia, 304
Tundra formation, 462
Tunic, 151
Tunicata, 147
Turbellaria, 323
Twins, conjoined, 230
Tympanic cavity, 220

membrane, 109
Tympanum, 66

Typhlosole, 349
Typical animal cell, 47

U

Uca, 411

Ultramicroseope, 43

Umbo, 375

Undulating membrane, 295

Unit characters, 238

Urchins, sea, 359

Urea, 69, 81, 173, 191

Ureter, 140, 190, 222

Urethra, 140

Urinary bladder, 140

Urine, 191

Uriniferous tubules, SI, 190

Urochorda, 147

Urogenital system, development, 221

Uropods, 397

Urosalpinx, 391

Urostyle, 85

Uterus, 143

Utriculus, 94

Vacuoles, contractile, 289

food, 287, 294, 295
Vagina, 143, 223
Vaginal orifice, 109, 143
Vagus, 137
Valve, bicuspid, 126

semilunar, 126

tricuspid, 126
Variation, 481

continuous, 509

discontinuous, 509
Vas deferens, 145
Vasa efferentia, 95
Veliger larva, 372, 381, 482
Velum, 156
Ventricle, fourth, 136

third, 136
Vertebral column of frog, 85

of rat, 113
Vertebrata, 147, 157
Vertebrates, ancestry of, 484, 485
Vesalius, 32, 33
Vesicle, amnio-cardia, 220

optic, 219
Vestibule, 121

Vestigial structures and evolution, 500
Vibrissae, 1 08
Virchow, 53

Visceral skeleton of frog, 84
Vitamins, functions of, 174
Vitelline membrane, 97, 211
Vitrina giacialis, 380
Viviparous animals, 193, 391

544

INDEX

Vliet, 38

Vocal cords, 80, 131
Von Baer, 487
Von Mohl, 39
Vorticella, 272, 273

W

Walker, 38

Wallace, Alfred Russell, 37, 472

Wallace's line, 473

Water, 46

flea, 394, 412

habitat, 474

-vascular system, 365
Webb, 38

Weese, Dr. A. O., 457
Wharton 's duct, 125
Wolf-snout, 228
Wolffian duct, 222
Worms, spiny-headed, 337
Wuchereria, 438

X

X-chromosome, 246
Xerophthalmia, 174
Xiphoid process, 114

Y-chromosome, 246
Yolk plug, 100

Zoantharia, 309
Zoanthidea, 311
Zoea, 413, 414
Zones of life, 475
Zoogeography, 21
Zygapophyses, 86, 113
Zygote, 61, 211, 223
Zymogen, 169