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McGRAW-HILL PUBLICATIONS IN THE
ZOOLOGICAL SCIENCES
A. FRANKLIN SHULL, Consuuringe Epiror

ANIMAL BIOLOGY

SELECTED TiTLEsS FRom

McGRAW-HILL PUBLICATIONS IN THE
ZOOLOGICAL SCIENCES

A. FRANKLIN SHULL, Consulting Editor

Battsell - Human Biotocy

Burlingame . HEREDITY AND SocraL PROBLEMS

Chapman - ANIMAL EcoLoay

Goldschmidt - PHystoLoGicaL GENETICS

Graham - Forest ENTOMOLOGY

Haupt - FUNDAMENTALS OF BIOLOGY

Hyman - Tue INVERTEBRATES: PROTOZOA THROUGH CTENOPHORA
Metcalf and Flint - Insect Lire

Mitchell - GENERAL PHYSIOLOGY

Mitchell and Taylor -LaBoratory MANUAL OF GENERAL PuysI-
OLOGY ;

Pearse - ANIMAL EcoLoay
Reed and Young : Laporatory STUDIES IN ZOOLOGY
Riley and Johannsen - MepicaL ENTOMOLOGY
Rogers - TEXTBOOK OF COMPARATIVE PHYSIOLOGY
LABORATORY OUTLINES IN COMPARATIVE PHYSIOLOGY
Senning - LABORATORY STUDIES IN COMPARATIVE ANATOMY
Shull - EVOLUTION
HEREDITY
Shull, LaRue, and Ruthven - PRINcIPLES OF ANIMAL BIOLOGY
Simpson and Roe - QUANTITATIVE ZOOLOGY
Snodgrass - PRINCIPLES OF INSECT MORPHOLOGY
Van Cleave - INVERTEBRATE ZOOLOGY
Welch - LImNOLOGY
Wieman - GENERAL ZOOLOGY
AN INTRODUCTION TO VERTEBRATE EMBRYOLOGY
Wolcott - ANIMAL BIoLoGy

There are also the related series of McGraw-Hill Publications in
the Botanical Sciences, of which Edmund W. Sinnott is Consulting
Editor, and in the Agricultural Sciences, of which Leon J. Cole is
Consulting Editor.

ANIMAL BIOLOGY

BY,

ROBERT H. WOLCOTT

Late Professor of Zoology, University of Nebraska

Seconp EDITION
SEcoND IMPRESSION

McGRAW-HILL BOOK COMPANY, Inc.
NEW YORK AND LONDON
1940

CopyriGuHT, 1933, 1940, BY THE
McGraw-Hitt Book Company, INc.

PRINTED IN THE UNITED STATES OF AMERICA

parts thereof, may not be reproduced

in any form without permission of
the publishers.

THE MAPLE PRESS COMPANY, YORK, PA.

ACKNOWLEDGMENT

The members of the author’s family wish to express their very deep
appreciation and gratitude for this revision of Animal Biology.

The work has been accomplished by the staff of the Department of
Zoology at the University of Nebraska as a memorial to a former
friend and colleague.

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PREFACE TO THE SECOND EDITION

In deference to the original plan of the author, the general form of
this revision of Animal Biology has not been materially changed.

We have particularly avoided the use of text figures of laboratory
studies in order to direct the student’s attention to his own dissections,
observations, and conclusions.

The sections on the worms have been rewritten to harmonize with
more recent knowledge of the subject.

About sixty new illustrations have been added, many of which have
been generously contributed by friends and former students and sugges-
tions from many outside sources have been incorporated.

THE STAFF OF THE DEPARTMENT OF ZOOLOGY,

UNIVERSITY OF NEBRASKA.
February, 1940.

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PREFACE TO THE FIRST EDITION

The fundamental propositions behind this text—the platform, so to
speak, upon which it has been written—are as follows:

1. Life has a chemicophysical basis.

2. Life phenomena are the outgrowth of organization.

3. The central fact in life is metabolism.

4. Animals may be arranged in a progressive series with reference
to organization.

5. The most complex animals are the most effective and also the most
efficient from a metabolic standpoint.

6. Man, as the highest of animals, can learn by the study of animal
life the principles of the most effective living.

7. He can also understand more fully his place in nature and can more
justly judge the actions of his fellows; this in turn may contribute to his
intellectual and spiritual development.

8. Every problem concerned with living is essentially a biological
problem and capable of analysis and solution by the application of
biological principles.

The book has been prepared for use as a class textbook, not as a work
of reference, and contains an amount of material which experience has
shown can be covered in three recitation periods a week for one year.
Since it will generally be used in beginning classes in which the majority
of the students are freshmen and sophomores, an effort has been made to
present the material in such a manner that it can be easily handled by
such students with normal preparation. In other words the idea is to
give the student an amount of material which he can cover in a way he
can understand. Also since the majority of the individuals in such
classes do not intend to specialize in the field of zoology, technicalities
have been minimized and emphasis placed upon the broader aspects
of the science and the general significance of the data presented, leaving
to subsequent courses the filling in of details for students majoring in
the subject.

Feeling that the place to acquire a knowledge of the structure of
animals is in the laboratory and not in the classroom, the author has
reduced the amount of morphological material. In the case of those
types handled in both class and laboratory, the facts given here are
intended to tie the two together or to summarize the knowledge gained
in the laboratory. In the University of Nebraska the ‘‘types method”’

x PREFACE TO THE FIRST EDITION

is followed in the laboratory work and two courses are offered, one carry-
ing a credit of ten semester hours and the other six, and differing in the
number of types covered. In the longer course, three recitation periods
a week are required; in the shorter, two.

It is suggested that in the selection of material for a shorter course
the lightening of the load be done by taking in the classroom only a brief
survey of Chaps. XV to XVII, XXV to XX XIX, and XLI to LX, inclu-
sive, picking out sections here and there for the particular attention of
the students and letting the rest be merely read for the general impression
gained. The numbering of the sections makes possible the assignment of
certain ones for more intensive study and of others for consideration in
connection with the laboratory work.

In the topics handled in Part V, three aims have been in view: (1)
To give a general survey of the field of zoology with a fairly even emphasis
upon the various aspects; (2) to review many of the facts presented in
previous parts, putting them in a different setting, and developing on
the part of the student a broader view and a greater ability to apply
these facts; and (3) to establish points of attachment to which advanced
courses in the department may be articulated. It is felt by the author
that these chapters afford a means for more ready correlation between
the general subject and such special courses. Cross references facilitate
the development of the habit of thoughtful reviewing and the perception
of analogies and homologies, resemblances and differences, that form a
part of the basis for true scholarship.

Since correct spelling and exact pronunciation are among the clearest
indications of careful training, the pronunciations of phylum and class
names are given in the body of the text and the pronunciations of words
in the Glossary are given. That the student may be led to observe the
derivations of technical terms those of the phylum and class names are
given and many common Greek and Latin roots are included in the Glos-
sary. Italics are used in the text to indicate emphasis and also to call the
attention of the student to words the definitions of which are to be learned.

In the preparation of the book the author has made free use of other
texts and of works of reference, particularly of Parker and Haswell’s
‘““Text-book of Zoology,’ a copy of which should be available to every
teacher, and of the volumes of the Cambridge Natural History Series.
In connection with illustrations borrowed from other books acknowledg-
ment is made of their sources and of the courtesy of the different pub-
lishers in granting permission. Of the figures, seventy-two are from
borrowed engravings or were reproduced photographically by F. H. Shoe-
maker. ‘Two of the original drawings (Figs. 108 and 115) were made by
S. Fred Prince. With these exceptions all of the illustrations, either
redrawn or original, are from drawings by the author’s son, Robert A.
Wolcott.

PREFACE TO THE FIRST EDITION xi

The author desires to acknowledge the help of many colleagues who
have generously responded to requests for information and assistance,
and, particularly, the advice and suggestions received from those asso-
ciated with him in the zoological department at the University of
Nebraska—D. D. Whitney, I. H. Blake, H. W. Manter, Otis Wade,
E. F. Powell, H. E. Low, and G. E. Hudson. In the preparation of
the manuscript he has profited by the intelligent cooperation of his
assistant, Elmer Palmatier.

THE AUTHOR.
Lincoutn, NEBRASKA
August, 1933.

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CONTENTS

Paan
PREFACH TO THE SECOND FIDITION= =) ara eis Ge ee vil
PREFACE TO THE First EDITION... ..- =... => + = + © « = 9 # © © = «@ « 1x
PART I
FUNDAMENTAL PRINCIPLES
CHAPTER I
Tue FIELD OF ZOOLOGY ..... . Pa ae tiie WI Fe Se a She 3

Appeal of zoology—Number of animals—Variety of animals—Size of
animals—Distribution of animals—Relations of animals—Definition of
zoology—Divisions of the subject—Scope of general zoology—Animal
biology.
CHAPTER II
MATEBRS5eyiven ~ Lells Kiersey Sees Sue ees ytd es

Definitions Constitution of ae HT Oe ead a Seg
bases, and salts—States of matter—Surface films—Mixtures—Ionization—
Colloids—Colloidal emulsions— Reactions.

CHAPTER III
HINERGYen es yale kei sas an: Shula dram nieces (1K)
Forms of Age errr ep ee yee of ieunodpmacics

CHAPTER IV
Livine AND Nonutvinc MaTTER..... . re Ns bk ae Sp OL ee ae leas

Contrast between living and nonliving athens rests of life,

CHAPTER V
PROTOPLASM. . . . Se Eke tb leet tee ee ree Sk eee
Historical Fete ce ermienl hamsotti of Seathplaai—Physidal savatteviatios
of protoplasm—Microscopical structure of protoplasm—Appropriateness of
protoplasm as living substance—Life is a consequence or concomitant of
organization.
CHAPTER VI

Definition—Vital forcee—Vitalism and mechanism—Origin of life—Possi-
bility of creating life.
CHAPTER VII
OP oUt Se Re Omer | Ree mnern ea ent Ra Wen Bar chen roars ae BT ers ge OO)
Desnition--Sines sid shapes of sue eniibes of Piette of cells—
General physiology of the cell—Development of knowledge of the cell—Cell
theory and cell doctrine.
xiii

XIV CONTENTS

Page
CHAPTER VIII
MrrABOnISMy) — lou ee a Shee hv ave Wasson eet ge: Wen ee 5 eR
Definition—Food—Steps in motabolintn—Tnigetion pine: tiou team
tion—Circulation—Inspiration—Assimilation— Dissimilation—Secretion—
Excretion—Expiration—Elimination—Egestion— Respiration—Anabolism
and katabolism—Vitamins—Energy changes in metabolism—Uses of differ-
ent foods—Storage— Metabolism the central fact in life.
CHAPTER IX
PLANTS AND ANIMALS ....... 2 EN ue ree aan oa ee A
Comparison between plants el aiivaals= Biology =D tteute pees
plants and animals.
CHAPTER X
GROWTH AND REPRODUCTION. .... . reer beg | A!

Growth cycles—Limit of Adin Peat Mint

CHAPTER XI
INETTOSIS! cy 885, Sede Ae GES Phe SR ry oh rr 47

Normal cell division—Significance of mitosis—Amitosis—Continuity of cell
life and chromatin—Growth of the cell.

CHAPTER XII
FORMS OF ANIMAUSe—. So... 3 =A SUVA Ss Bo We DS.

Asymmetry—Spherical, or universal, symmetry—Radial symmetry—
Bilateral symmetry—Metamerism—Appendages—Homology and analogy.

CHAPTER XIII
BEHAVIOR... . : : ; Je OO

Stimuli—Direct Be Gain Ree sled ag nee nervous sys-
tem—Physiological state.

CHAPTER XIV
CLASSIFICATION AND NOMENCLATURE. ..........:... . +. SnnDS
Definition—Arrangement of groups of animals—Nomenclature.

PAR Tk

PROTOZOA

CHAPTER XV
AMEB Aa. sera tertile) fs sculeee a War Ane Lt eerts ME ls a). OS

Occurrence and appdarande-“Stpietire— NietabollannTetent eae”
havior—Reproduction.

CHAPTER XVI
PARAMECIUM. ....... on her fb cab er Be, <a> 8 nh ot ee Bee OS

Occu irons “Stenetiive-“Metabelieth=<Lotomnecien- “chat meaner
tion—Conjugation—Endomixis.

CHAPTER XVII
PROTOZOA IN GENERAL. .. . bn ats ee i ee cee Preah baeChss

Gladification—“Mactivoch om -Satcodina! “see andiliy Beasley
facts—Sexual reproduction in protozoa.

CONTENTS XV

PaGE

CHAPTER XVIII
BO TIOZONMANIDIDISWABE % (cp) seca. cc ds cha 1 coe siya ae oy ok oe ok oy ee TOD

Pathogenic protozoa—Malarial parasite.
PAR Gil
METAZOA IN GENERAL

CHAPTER XIX

NAT OAs. ook oo ca eteond eltogew bas pogiiistet-<slae eee 695

Differentiation—Division of labor—Somatic and germ cells—Potential
immortality of germ cells.

CHAPTER XX
SAUTE en ates Ae pes is od ets, SPE es toh Se ae SEP age ies oe OS
Definition—Epithelia—Supporting and connective tissues— Muscular tissues
—Nervous tissues.
CHAPTER XXI
ORGANS AND SYSTEMS... . . 104
Pe ee ela ec Oneake Hoang to » different ie Oates ae
of the body.
CHAPTER XXII
REPRODUCTION IN THE METAZOA ....... .- ae aerate athe elles 9) V7/
Methods of reproduction in i ae a sepasaetion=_Uainarental
reproduction—Types of fertilization—Oviparity and viviparity— Metagene-
sis.
CHAPTER XXIII
WriciniGmrim Sexes Cmts. les eo) eh Veh ete ee Poe ee ee OO
Gametogenesis—Spermatogenesis—Oogenesis—Comparison and contrast
between spermatogenesis and oogenesis—Division of labor between the germ
cells—Variations in gametogenesis.

CHAPTER XXIV
FERTILIZATION ..., . NS Satis Ae Ficoe cash a ee tate oat Ll Cea cote
Steps in Sere crmarerns ean clini = -Sisnifiranes of synapsis.

CHAPTER XXV
Mm RVOGMNY- rascal. Aiyl Oaks angeteiols aia t= nents 4 seslesty 21S
Types of egg cells—Forms of cleavage—Steps in embryogeny— Variations in
embryogeny—Germ layers—Coelom.

PART IV

METAZOAN PHYLA

CHAPTER XXVI
SPoNGES. .. . : : AST ce eyaled
Relationship of Pe eee rea on = eiratt re Catal eames okales
ton—Histology—Metabolism—Behavior—Reproduction— Uses of sponges
—Cultivation of sponges—Relations to other animals.

Xvl CONTENTS

CHAPTER XXVII
Hypra. see ls 8 ed Os 0 Rye PS OP ee] eee ng een See en Se
External nee Te ena atrmvoturnec/Nendntsey ches Ne aka sae rien
mechanism— Metabolism—Behavior—Reproduction—Symbiosis— Regen-
eration.
CHAPTER XXVIII
CoELENTERATES IN GENERAL. ......- -
Polyps and Wicditeae—-Cleatifies laste Hy drcasan -Sevptotsa== nites
Color—Polymorphism—Metabolism—Behavior—Reproduction—Metagen-
esis—Corals—Distribution and economic importance.

CHAPTER XXIX
PuyLuM CTENOPHORA. :
Structure—Advances in toa pine tivities.

CHAPTER XXX
FRESH-WATER PLANARIAN..... . RA eae ce kad Bea
Structure—Internal stivoGate”Botappli¢n = Reproduction=- Berean
Regeneration.
CHAPTER XXXI
PHYLUM PLATYHELMINTHES.

Classificatio Bl re 4 atte Teer Gd Canto da <M atabolemee een
duction—Occurrence and economic importance.

CHAPTER XXXII

PARASITISM . : ‘ , 5 : : Pe ie iad cht
Structure of a eee ive fluke—Life Gato ee the sheep liver
fluke—Life history of a tapeworm—Behavior of parasites—Practical aspects.

CHAPTER XXXIII

PuyLuM NEMATHELMINTHES . Te ch spmcecomiseyt luedreene ciwon SE

Structure of an ascaris—Characteristics and adivatitee Claes Reatia

Free-living nematodes—Metabolism—Reproduction—Life history of the

pig ascaris—American hookworm—Trichinella—Filaria—Hairworms—
Spiny-headed worms—Economic importance.

CHAPTER XXXIV
OrHER UNSEGMENTED WORMS .

Phylum Nemertinea—Phylum Gicioriatlie=Phelion ‘Rotifem Ele
Bryozoa—Phylum Brachiopoda.

CHAPTER XXXV

STARFISH. Bae A Ve, ee ES Oe Ee eas. Thee c

External Waa ee hah and saudeulseure—Wateemusenie system—

Internal organs—Feeding and metabolism—Nervous system and behavior—
Reproduction—Regeneration and autotomy—Economic importance.

CHAPTER XXXVI

PHYLUM ECHINODERMATA . : ; a eee at nt :

etre ceetiore™ Spebinlinations -- Caasineatibn-“ Aetemides- Ophiaemiem

Echinoidea—Holothurioidea—Crinoidea—Reproduction—Behavior—Color
—Occurrence and economic importance.

PaGE

. 136

. 144

. 156

= Log

. 166

172

4 218

186

. 194

. 203

CONTENTS

CHAPTER XXXVII

FRESH-WATER MUSSEL. : ae: 5 be: | Gilvaced ha ede ob aw ox AODEER IE

External Barren he="Snen=an dated anatomy—Body mass and ot

Mantle—Gills—Digestive system and metabolism—Circulatory system—

Excretory system—Musculature—Nervous system—Behavior—Reproduc-
tion—Other fresh-water mussels.

CHAPTER XXXVIIT
MOo.Luvusks IN GENERAL .

Classification— henna GREG AME. cs | coada Palenrpada
Cephalopoda— Metabolism—Behavior—Reproduction and regeneration—
Economic importance.

CHAPTER XXXIX

EARTHWORM . Bre cee a ee Nee egies ae OSA, cae o Came Seer ge

External pele Ar ee structure—Alimentary canal and metabo-

lism—Circulatory system—Excretory system—Musculature and locomotion

—Nervous system—Behavior—Reproduction system and reproduction—
Regeneration—Economic importance.

CHAPTER XL
REFLEX ACTION.
Nervous finehions= Reflex ee areerios Hoare

CHAPTER XLI
ANNELIDS IN GENERAL.

Classific ation—Archiannelida—Chaetopoda—Hirudinea—Gephyrea—
Metabolism—Behavior—Reproduction—Occurrence and economic impor-
tance.

CHAPTER XLII

CRAYFISH . ee es a ae Ge ee Re crake

External Aue etaibe ee SOT ANE a ib See ae

Feeding habits—Behavior—Reproduction—Regeneration and autotomy—
Economic importance.

CHAPTER XLIII
CRUSTACEA.

Pe ries i thi patincu= Bohavion = Renmed uehon™Reanornte: impor-
tance—Biogenesis.

CHAPTER XLIV
ONYCHOPHORA AND MyRrIApopA.

Onychophora—Myriapod a ee ete ne etic Re cine eitia in
myriapods.

CHAPTER XLV
Cuass INSECTA .

External Se eee eats mae OTR Ore of inesta Raatwate)
tion—Autotomy—lInjuries due to insects—Benefits from insects—Injurious
types—Combating injurious insects—Beneficial insects—Social insects.

XVli

PaGE

. 212

. 221

. 234

. 243

. 255

. 265

273

. 276

XVill CONTENTS

Pacn
CHAPTER XLVI
Cuass ARACHNIDA. .... . Boek. nh ge aks eyelet ae cars “1 nA RSs 02
External structure of spiders—Internal izcctatee—ihie Gasol einen ee
tion—Spinning activities—Behavior—Economic importance—Scorpions—
Mites—Other arachnids.
CHAPTER XLVII
ARTHROPODS IN GENERAL... .- fy Arama eee oe <a) sy a! de: 2 antes Sonera
Characteristics and adv dices Holst ROR BAe
CHAPTER XLVIII
PHYLUM: CHORDATA: <f.4 0: tachyarh-sney cals : eee ». «ithe gious
Characteristics—Advances Ait by ihe erate = Clesifesuans
CHAPTER XLIX
LOWER CHORDATES ... . : : won tbe a a WS cs, SE es |
sr ovsibhiosias ide=titocierdatd +-Cephalockondata=’weonomte value.
CHAPTER L
SUBPHYLUM VERTEBRATA. .... . ne bes! : 2 en s22

Distinguishing ie eerie eae Alan<-Bkin-—-Skeleton==Midscule sys-
tem—Digestive system—Respiratory system—Circulatory system—Excre-
tory system—Nervous system—Sense organs—Ear—Eye—Reproductive
system—Advances shown by vertebrates—Classification.

CHAPTER LI
CUASSICVCUHOSTOMAT AGE) rn) cue-luemel oul : sue. see is cates say eae
Eee ere Snide Tan eevee! Roleionein of the cyclostomes—
Economic importance.
CHAPTER LII
Cuiass ELASMOBRANCHII ... . . 344
Dogfish sharks—Other shores eiiates aa saenpnet clastiobeanceee
Economic facts.
CHAPTER LIII
Cuass Piscgs. .. . Laer : Bye. at : . 350
Gisele sation oC toaedntesell “Chondueetel— Holestee otolecsts otra
—Body form—Scales—Fins—Locomotion—Air bladder—Forms of tails—.
Colors of fishes—Internal anatomy—Food of fishes—Respiration—Senses
of fish—Behavior—Reproduction—Ages of fish—Deep-sea fishes—Remark-
able fishes—Economic relations.

CHAPTER LIV
Vora, \asimmoyeNIOS 6 5 Gb op Oh 61s) 6b id a 6 5 8c : 5 hoe eeOoOO
Changes incident to the acquirement of a terrestrial Dee e of i eee of
terrestrial adaptations.
CHAPTER LV
Crass AwpHBrass 0° 3 6 ln ake oe GAA ees “Sees eee ee
Classification—Urodela—Salientia—Apoda—Food—Color changes in
amphibia—Nervous system and sense organs—Behavior—Reproduction

and development—Neoteny and pedogenesis—Regeneration—Hibernation
—Economic importance.

CONTENTS X1X

Pagn

CHAPTER LVI
REPTILES AND BIRDS ..... . LBA NE Tbs et Se ®t Ie Melee s S82

Structural cHurdetetietioManrinaponte dashes Hovis near nigh =
Allantois—Body coverings.

CHAPTER LVII
Cuass REPTILIA. .. . ee isla : fabs Ge : , sana a 386

Ras Re LAW malt ree dary tea sede a eres es
Snakes—Venomous snakes—Rhynchocephalia—Crocodilia—Testudinata—
Economic importance.

CHAPTER LVIII
CUASSEAVES Une Seiwa en he tale Se Pe ee, i ee Seer ee rd OS

External characteristics—Feathers—Internal structure—Classification—
Origin of birds and of flight—Flight—The bird as a flying animal— Modifica-
tions of birds—Plumage—Songs—Migration of birds—Reproduction—
Economic importance. .

CHAPTER LEX
CRASSHIVIAMMAT TA capo? culcptrlcang. Srikeptd cuacdona chet <i cutee a Saree ES. ofl ea PAE

External characteristics—Hair—Internal structure—Classification—Ori-
gin of mammals—Monotremes—Marsupials—Unguiculata—Primates—
Ungulata—Cetacea— Hibernation— Reproduction—Economic importance.

CHAPTER LX
ANTHROPOID APES AND Man... .. . Pa TS ak ee ae: ree . . 449

Manlike apes—Erect emote aheteent in man of eanee nase eanl life—
Intermediate forms—Fossil men—Present-day man.

PARE -V

GENERAL CONSIDERATIONS

CHAPTER LXI
ANIMAL ORGANISMS .. 2... 2.5.54. : att ees af) Lung 400
The pent ietinitzon“tasine re aiitve= pimobonimtanes Tategey:
tion—Centralization—Chemical control—Individuality—Life cycle in birds
and mammals—Other life cycles—Practical considerations—Organismal
concept.

CHAPTER LXII
STRUCTURE) OF ORGANISM ys. x3 as) = 04s. lease 80 3 SRE Gres | ae . . 463
Grades of organization—Germ iors and Giasubes= RElatonellip of eels in
metazoans—Organs and systems—Tegumentary system—Skeletal system—
Digestive system—Glands—Respiratory system—Circulatory system—
Excretory system—Reproductive system—Muscular system—Nervous sys-
tem—Convergence and divergence.

CHAPTER LXIII
DEVELOPMENT OF THE ORGANISM... . Nard 8s RecA cals acre oe : ao AS

Germ cells—Origin of germ allaeavtatatatin of the germ Sie eee.
Fertilization—Cleavage—Blastula—Gastrulation—Mesoderm formation—

XX CONTENTS

PAGE

Tissue formation and organogeny—Postembryonic development—Poten-
tial immortality of germ cells.
CHAPTER LXIV

Inwumay CHANGES IN ORGANISMS 2.5 2 26 <4 8 > © 7) ee
Chemical changes in the body—Organism compared to a fire—Organism
compared to an engine—Organism more than a machine—Individuality—
Rhythmicity—Uses of foods—Planes of metabolism—Body heat— Heat
regulation—Warm-blooded and cold-blooded animals—Temperature of the
human body.

CHAPTER LXV

FUNCTIONS OF ANIMAL ORGANISMS (0's 1° 52 20+ see 3 os pare
Chemical cycles—Water— Digestion and absorption—Circulation—Respira-
tion—Secretion—Internal secretions—Excretion and elimination— Motor
functions—Nervous activities.

CHAPTER LXVI

BEHAVIOR OF ANIMAL ORGANISMS. VEEAMO 65 GY ee cee
Memory—Types of animal behavior—Direct response—Simple reflexes—
Instincts —Habits—Learning—Intelligence—Reasoning—Combinations of
modes of behavior—Behavior of lower and of higher animals—Mind and
consciousness.

CHAPTER LXVII

ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT. fete Some
Facts of ecology—Relations of animals to plants—Physiological life histories
_Habitat—Ecological factors—Reactions of the animal—Communities—
Suecession—Rhythms— Marine faunas—Fresh-water animals—Terrestrial
faunas—Mimicry and protective resemblance.

CHAPTER LXVIII

ANIMAL ORGANISMS IN HEALTH AND DISEASE. ene oa (oO 0c

Definitions—Health in a protozoan—Comparison of protozoan and metazoan

cells—Conditions of health—Causes of disease—Effect of individuality—

Self-regulatory tendency in the body—Toxins and antitoxins—How the

body fights disease—Immunity—Anaphylaxis and allergy—Maintenance of
health in human beings.

CHAPTER LXIX

RELATIONS BETWEEN ANIMAL ORGANISMS . :
Solitary life—Associations of animals of the same species—Mating—
Families—Colonies—Societies—Associations of animals of different species—
Gregariousness—Epizoic associations ——Commensalism— Mutualism—Sym-
biosis—Parasitism—Predatism.

CHAPTER LXX

DISTRIBUTION OF ANIMALS. . . - - - + s+: > ee hace GaP eee
Present distribution—Past distribution—Place of origin—Dispersal of
animals—Factors hindering dispersal— Modification of types—Periodic
migration—Altitude—Oceanic distribution—Island faunas—Faunal divi-
sions of the earth—North American life zones.

. 480

. 486

. 497

. 505

. 515

. 521

. 527

CONTENTS XXxi

PaGcnE

CHAPTER LXXI
Past DIsTRIBUTION OF ANIMALS ..... . eee Oe dae ee eee ea 9 f:
Fossils—Stages in Palincer <Coolocieal A cesinenal time scale—
Metamorphism—Animals of the past.

CHAPTER LXXII
EVOLUTION OF ANIMALS @ fos s 2 2 5. 45 2 4% 6s ee Es se oa
History of evolution—Evidences of Oe rates of evolution—
Methods of evolution—Evolutionary series.

CHAPTER LXXIII
[INHURIDANCHE INI ORGANISMS! © 2 24 2) ess) Gene Gee ee se aes es 555
Organisms from the genetic viewpoint—Determiners or genes—Behavior of
chromosomes in maturation and fertilization—Effect of chromosome reduc-
tion—Allelomorphs—Mendel—Mendelism—Hybrids—Distribution of char-
acteristics in hybrids—Checkerboard diagrams—Multiple hybrids—Actual
cases—Breeding the test for characters—Variations in inheritance—
Breeding for certain characteristics—Inbreeding and crossbreeding—Inherit-
ance of acquired characters—Inheritance of disease and abnormalities—
Sex determination—Twins— Determination of sex in parthenogenesis—Sex-

linked characters—Linkage and crossing over—Eugenics.

CHAPTER LXXIV
CLASSIFICATION OF ANIMALS .... . ee es Nae en eee ee RO LO)

Eerie sch ae teed “Papp niouahitme “Pleats of Aha eatidee Basis of nomen-
clature—Rules of nomenclature—Phyla—Phylogenetic tree.

CHAPTER LXXV
EIISTORYAORLZ O OL OG Yar ne ee ee Boy Pee ce lane ae en pee ome) nae em aeRO Of

Greeks— Dark ages—Vesalius—-Harvey—Microscopists—Comparative anat-
omy—Physiology—Cell theory—Embryology—Taxonomy—Evolution and
genetics—Pasteur—Recent advances.

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PART I
FUNDAMENTAL PRINCIPLES

ELMS
Sec uis Lea hava,

CHAPTER I

THE FIELD OF ZOOLOGY

A wealth of animal life about us challenges our attention. Birds
travel the highways of the air. Vegetation of all kinds swarms with
animals. The surface of the earth is alive with crawling things, and
under many objects lying on the ground an animal community is hidden.
The ground itself teems with life. And not only are fresh waters abun-
dantly populated, but the sea, which has been supposed to mother all
life, is occupied by a host of forms, greater in variety than those of any
other environment.

1. Appeal of Zoology.—Among this vast assemblage are animals
which appeal to us because of their beauty or oddity of appearance;
others to which we are attracted by their remarkable and interesting
activities; still others whose varied and complex relationships to one
another excite our wonder and suggest a multitude of questions; and,
finally, many whose relations to ourselves, either beneficial or injurious,
demand our serious consideration.

2. Number of Animals.—The number of individuals which at any
given time is living in this world surpasses calculation and is beyond the
power of the imagination to conceive. That large number, subject to
the modifying influences of changing seasons and affected from time to
time by an altering of the balance between animals of different kinds, is
constantly maintained. Naturally great differences exist between differ-
ent regions of the earth’s surface, which, in very different degrees, offer
the conditions favorable for animal life.

3. Variety of Animals.—The number of kinds of animals is not yet
determined and probably never will be precisely known. ‘Those living
which have been previously described and named have been estimated
at approximately 600,000; and there is no doubt that if all still unknown
were added, the total would far exceed a million. The exact number,
however, is subject to constant change, since some kinds of animals are
probably continually becoming extinct and new ones are probably as
continually being developed. This enumeration also takes no account
of the millions of species which have lived in the past and have perished,
some without leaving any trace, others represented more or less com-
pletely by fossils. Then, too, opinions differ greatly as to what con-
stitutes the difference between two kinds or, in other words, what
constitutes a species. The words type and form are frequently used in
the same sense as species, or kind.

4 FUNDAMENTAL PRINCIPLES

4. Size of Animals.—The smallest animals are invisible without the
aid of the microscope and the largest are the 100-foot whales weighing
about 150 tons. These huge water animals are probably the largest that
have ever existed and far exceed in size the largest terrestrial 614-ton
elephants.

5. Distribution of Animals.—Animals are found everywhere on the
earth’s surface, except perhaps on the glaciated tops of the highest
mountains and a very few at the poles. On these mountains creeping and
flying forms pass the margins of the snow fields, and the areas of ice and
snow at the poles are constantly invaded by such forms as are able to
venture into them. Animal life is found throughout the waters of the sea
and even penetrates to the deepest parts of the oceans. Animals burrow
below the surface of the ground to considerable depths and also follow
fissures still deeper to reach the farthest recesses of the most extensive
caverns. Finally, myriads of living creatures live within the bodies of
other living things, both plant and animal.

Fic. 1.—Diagram showing the relative size of a small sperm whale, Physeter catodon,
of 30 tons and a herd of 24 horses, Equus caballus. The largest whales may be four to five
times this size.

6. Relations of Animals.—Animals are related in various ways to
other animals, to plants, and to their physical environment. Between
parents and offspring the relation is that of descent. Between other
animals the relation may be nutritive, one living upon the other; repro-
ductive, where two join in the production of young; locomotor, where
one attaches itself to another for the purpose of being transported from
one place to another; or any one of many other relations which might
also be named. Plants serve as food for animals, afford them conceal-
ment, and are useful to them in other ways. A solitary existence, in
which one animal lives without any relationship to any other, is possible
but rarely occurs in nature. Animals possessing sex associate together
for a longer or shorter time as mates. Many of the same kind live
together, forming a colony or, as in the case of ants, bees, and wasps,
are organized into a society. The relations of animals to their physical
environments are manifold. Some are confined to the land, others to
the water, and still others may be at home in both. Aquatic forms may
be restricted to fresh waters, others may be only marine, while there
are also those that pass from one to the other environment. Animals
exist that spend all of their lives in the soil, and others that enjoy the
power of flight pass much of their active existence in the air.

THE FIELD OF ZOOLOGY 5

7. Definition of Zoology.—A study of animals from every aspect
constitutes the science of zoology. This broad field is capable of being
divided into many of less extent depending on the various aspects from
which animals may be viewed, whether considered in whole or in part,
as to structure or function, in relation to the inorganic environment or
to other animals and the plants about them, or from the standpoint of
the principles and laws which underlie and determine the phenomena
exhibited by animal life.

8. Divisions of the Subject.—Zoology may be divided, in accordance
with the manner in which animals are studied, into two great sub-
sciences; these are morphology, which deals with animals as to form
and structure, and physiology, which deals with them as to their functions.

Under the head of morphology are included anatomy, which is con-
cerned with structure as made out by dissection; histology, which treats
of structure as determined by the microscope; taronomy, which is the
study of the laws and principles of classification and which is based upon
structure; geographical distribution, or zoogeography, the study of the
geographic distribution of animals; and paleozoology, which deals with
the fossil remains of animals. The reason for the placing of zoogeography
under morphology is that in this field animals are treated as species or
groups, a morphological basis.

Physiology includes physiology in the narrow sense, which deals with
the functions of the different parts of which the body is composed;
ecology, which deals with the functional relations of animals to their
environment; psychology, which is the study of the mental life of animals;
and sociology, which is the study of animal societies.

Three other fields belong to both morphology and physiology—
embryology, which deals with the early development of animals; pathology,
which relates to the diseases that affect them; and parasitology, which
is concerned with animals that live at the expense of their fellows. No
effort has been made up to the present time, however, to separate the
structural and the functional aspects in any of these three fields.

Not only may zoology be divided into the two broad subsciences first
named and their various divisions, based upon the manner of approach
and method of investigation, but it also may be separated into many
restricted sciences, each of which deals with a particular group. Among
these are protozoology, which deals with the lowest, one-celled animals;
helminthology, which is concerned with the worms; entomology, which is
the study of insects; conchology, the study of mollusks; zchthyology, the
study of fishes; herpetology, the study of reptiles and amphibians; ornz-
thology, the study of birds; and mammalogy, the study of mammals.
Many other sciences, which concern less extensive groups and are less
familiar, might be added to this list, but only one need be spoken of and
that is anthropology, which is the study of man as to his physical nature.

6 FUNDAMENTAL PRINCIPLES

Other divisions of zoology are evolution, which seeks to explain the
origin and modification of the different species, and genetics, which deals
with the laws that underlie inheritance.

It is evident from what has been said that the divisions of the subject
cross one another. Anatomy, taxonomy, geographical distribution,
ecology, and all of the other fields mentioned as differing in the point
of approach or method will, for example, deal more or less with birds.
Ornithology, on the other hand, may be considered from a morphological,
physiological, taxonomic, distributional, or ecological aspect. The
same is true of any other group of animals.

In addition to the fields which have been mentioned, a long list of
practical applications might be added which would greatly increase
the list of the divisions of zoology. Among such applications are animal
husbandry, which deals with the cultivation of the domesticated higher
animals, apiculture, with that of bees, and aquaculture, with that of fish
and other aquatic forms; medicine, which is concerned with disease and
the methods used in its treatment; and hygiene, which presents the
' principles involved in the maintenance of health. All of these involve the
study of animal life and should really be included in zoology in its widest
extent.

9. Scope of General Zoology.— Within the scope of a beginning course
in zoology, it is impossible to discuss more than the most general principles
of the subject and the broader phenomena of animal life. None of the
fields enumerated above can be more than barely introduced to the
student; their further cultivation must be left for special courses.

10. Animal Biology.—To many persons the word zoology is associated
with the structure and classification of animals, while the word biology
conveys the implication of life and activity. This is an unwarranted
connotation; but because in this text the emphasis is on the latter aspect
of the subject rather than on the former, it has been entitled ‘‘ Animal
Biology.”’ Properly speaking, the term biology is applied to the combined
sciences of botany and zoology.

CHAPTER II

MATTER

Proficiency in all of the different divisions of zoology cannot be
attained without considerable knowledge of physics and chemistry,
though the different fields differ greatly in the demands they make upon
such knowledge. An adequate grasp of even the most general and most
fundamental zoological principles, however, requires a familiarity with
the broad conceptions which underlie those sciences; and since many
approach this subject lacking such acquaintance, it is necessary to review
briefly these conceptions. Logically, the first subject to be considered
is the nature of living matter. To understand this it becomes necessary
to define what is meant by matter in general and to state some facts in
regard to it.

11. Definitions.— Matter has been defined as that which occupies
space. We commonly refer to all of our experiences as either material
or spiritual. Those which are material presume the existence of matter;
those which we term spiritual have no essential relation to it.

12. Constitution of Matter—Matter differs in kind, exists in various
forms, and exhibits a great variety of phenomena. The study of matter
with respect to kind is in the field of chemistry; that of matter without
regard to kind, including the phenomena of matter in general, belongs to
physics.

A few common forms of matter consist of only one kind of matter, |
such as a mass of gold, silver, iron, the liquid mercury, and the gases,
nitrogen and oxygen, in the atmosphere. Most matter with which we
are familiar does not consist simply of one kind of matter but is of the
nature of a compound, consisting of two or more different kinds. A
piece of any ordinary compound substance, as, for instance, a piece of
chalk, is termed a mass and may by being broken into two parts be
divided into two masses. These may be again broken, and the process
may be continued, resulting in masses of smaller and smaller size, each
still remaining chalk. This division may be carried beyond the limit
of visibility by the unaided eye and even far beyond that by the micro-
scope. The masses become smaller and smaller, but each bit remains a
mass. Finally a fragment may be conceived that can no longer be
broken and the portions be alike. This smallest particle of any com-

8 FUNDAMENTAL PRINCIPLES

pound substance is termed a molecule. When molecules are separated
into smaller fragments these are unlike and are definite in number for
every substance. These fragments are termed atoms. A molecule of
chalk is divisible into five atoms—one of carbon, one of calcium, and three
of oxygen. It has been found that an atom may be further subdivided
into much smaller particles, one or more of which lie at the center and are
termed protons, while the others, either associated with the protons in a
nucleus or distributed at distances about it, are known as electrons.
When, however, atoms are divided into these finer particles, they are
found to be all of the same nature, and so all matter in this finely divided
state becomes alike. Atoms of different kinds differ only in the arrange-
ment of these component particles with respect to each other.

13. Elements and Compounds.—This division of matter into mole-
cules, atoms, protons, and electrons belongs to physics. Chemistry,
strictly speaking, deals only with atoms classified according to their
kind and with molecules considered with respect to the kind and arrange-
ment of the atoms of which they are composed. Each kind of atom is
known as an element. Compounds are classified with respect to their
composition in terms of elements and also with respect to the manner in
which they react, or change, when brought in contact with other com-
pounds or with elements. Chemists now recognize about 92 different
elements, some of the most common of which are carbon, hydrogen,
oxygen, nitrogen, iron, calcium, phosphorus, sodium, and potassium.
To economize time and space in referring to these elements they are
designated by symbols, which may be the initial letter of the name of the
element, either in its English or in its Latin form, or two letters when
it is necessary to distinguish between elements having the same initial.
Thus, C represents carbon; Ca, calcium; H, hydrogen; N, nitrogen; and
Fe (from the Latin ferrum), iron.

14. Acids, Bases, and Salts——The elements are divided into two
categories. Metals, which number more than three-fourths of the total,
include gold (Au), silver (Ag), lead (Pb), copper (Cu), and iron (Fe), and
also calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na).
The nonmetals include oxygen (QO), nitrogen (N), carbon (C), sulphur (S),
silicon (Si), phosphorus (P), chlorine (Cl), and iodine (1). Hydrogen
(H) is not a metal but in chemical combinations acts like one. Metals
combine with oxygen to form bases which, in solution in water, color
litmus more or less strongly blue—that is, they are alkaline. Nonmetals,
when combined with hydrogen, yield acids which, in aqueous solutions,
are sour to the taste and color litmus red. All acids contain hydrogen.
A substance resulting from the union of a base and an acid is called a
salt. Examples are table salt, or sodium chloride (NaCl); lime, or cale‘'um
carbonate (CaCO3;); and blue vitriol, or copper sulphate (CuSO,). In
all chemical combinations the number of atoms of each element in a

MATTER 9

molecule of the compound is indicated by a figure written as a
subscript.

15. States of Matter.—All molecules, and also smaller particles, are
believed to be in continual motion, but this motion is restrained by the
attraction which molecules or other particles exert upon each other.
This attraction is proportional to the sizes of the particles and inversely
proportional to the squares of the distances between them. Thus it
follows that particles at a great distance exert an attraction which is
practically negligible, but as they approach each other the attraction
increases at a constantly accelerated rate.

The relation between the molecules in a mass determines the character
of the mass which they form, and thus we get the different states of
matter. If the molecules are sufficiently close together that the attrac-
tion between them holds them in the same relative position with respect
to each other, the mass preserves constantly the same form and is termed
a solid. If, however, the attraction is insufficient to preserve this form
and the mass tends to change its shape, it is called a flucd. If the mole-
cules of a fluid tend to remain together but there is so nearly a balance
between the force of motion and the force of attraction that the mass
easily changes shape, it is called a liquid. Under the influence of gravity
the molecules in a liquid seek the lowest level and the upper surface of
the mass becomes a plane surface. Finally, if the molecules of a fluid
are so far apart that the attraction of one for another fails to keep them
together and they tend to move in all directions, the mass expands and
fills all available space and is termed a gas. Thus both liquids and gases
are included under the term fluid. Some gases are heavy and expand, or
diffuse, slowly; others are light and diffuse rapidly.

Since there may be any degree of attraction, depending upon the
sizes of molecules and the distances between them, these states of matter
are not sharply defined but pass into one another through an indefinite
number of gradations. A mass which is not a perfect solid but which
may be made to change its shape gradually is termed viscous. Under
the influence of varying degrees of heat and pressure, substances may be
made to pass from one state to another. All elements in the sun are in
a gaseous state due to the high temperature, but without exception, all
kinds may be solidified at lower temperatures. Under ordinary condi-
tions some solids seem to pass directly into the gaseous state without
appearing at any time as liquids, as snow may evaporate without wetting
the surface on which it lies.

In this passage from one state into another none of the material is
destroyed, nor is there any new material created. This principle is known
as the conservation of matter. At certain degrees of temperature water
may be in the form of a gas, a liquid, or a solid but the same amount is
present in each state.

10 FUNDAMENTAL PRINCIPLES

16. Surface Films.—In a liquid mass, as has just been said, the parti-
cles are free to move; but if the mass is at rest, they do not do so, being
equally attracted by other particles all about them and in a state of
balance. This being true they are easily pushed aside by an object which
passes through the liquid. The surface of the mass, however, is formed
of a layer of molecules which are not in balance but which attract one
another and are attracted to those below them to a degree which makes
the penetration of this layer a matter of overcoming a certain amount
of resistance (Fig. 2). This layer of molecules is called a surface film.

It is the presence of this film and

( \<—>( | ) the resistance it offers to penetration

b VN which makes it possible, with suffi-
@ @ cient care, to lay a dry needle upon

the surface of a liquid and for it to

@ O remain there. The resistance which

this film offers to the penetration of

VL any object is also responsible for the

2 © Cree@ dimpling of such a surface when a
Lis dry object is pressed down upon it.

Fic. 2.—Diagram illustrating the bal- stantly increasing force the dimple
anced attraction exerted upon a molecule

in a mass of liquid (a), and the unbalanced becomes gradually deeper until
attraction upon a molecule at the surface finally, when the force becomes suffi-

mutual attraction between two molecules. cient to rupture the film, the object
enters the liquid and is wet by it.
Then the liquid, as a result of adhesion between the particles of the object
and those of the liquid, rises on the surface of the object in a characteristic
way which is familiar to everyone—that is, unless the liquid is a very
heavy one, like mercury, in which case the surface film does not rise
but is depressed next to the surface of the object, to which, in other
words, the liquid does not adhere. Some animals, like the water striders,
move freely about on water supported by the surface film, while others,
like snails, cling to this film from below and move about hanging to it.
The surface film thus serves as a highway which may be traveled on
both its upper and its under surfaces. The strength of the surface film
of a thick liquid causes a drop of it to stand up on a dry surface and
assume the form of a flattened sphere, whereas the weakness of that of a
thin liquid results in its spreading out over a considerable area in a thin sheet.
17. Mixtures.— Masses of different kinds may be associated in what
may be termed, in general, mixtures. Two solids reduced to a state of
fine division may be mixed; liquids also may be mixed, and gases as well;
and any two of the three, or all three, may be mixed.
If a solid is mixed with a liquid and remains in masses of greater size
than molecules, the liquid is more or less turbid and the mixture is termed

MATTER ila!

a suspension. If, however, the solid is reduced to particles of the size
of molecules or of atoms, the mixture will become as clear as the liquid
itself and such a mixture is termed a solution. The liquid is called the
solvent and the dissolved solid the solute. In the same way one liquid
may be mixed with another liquid and form a suspension or a solution,
depending on the size of the particles. A suspension of minute globules of
one liquid in another is termed an emulsion. Gases, too, go into solution
and also form suspensions, but such suspensions do not persist. A gas
and a liquid may be shaken up together and a suspension created, the gas
being distributed through the liquid in the form of bubbles, but the gas
quickly escapes from the mixture except for what becomes dissolved.
Gases also escape from solution unless there is just as much gas over the
liquid as there is in an equal volume within it. This passage of gas either

Fic. 3.—Diagram showing the diffusion of a substance as salt, sugar or copper sulphate
in a glass of water. A, when the substance is first put into the glass of water. B, a few
minutes later the particles are leaving the main mass of the substance and moving in all
directions through the water. C, later (an hour, more or less) all the particles of the mass
have separated and become evenly distributed throughout the water.
into solution cr out of it, depending on whether the gas pressure is greater
without or within, explains why animals take in oxygen and pass out
carbon dioxide. This exchange, which is called respiration, takes place
through extremely thin membranes which separate the air from the liquids
in the body and which allow the gases to pass through freely. Differ-
ences in gas pressure also account for the constant entrance of oxygen into
water to replace what aquatic animals have taken from it in respiration,
and the constant escape into the water and then into the air of the carbon
dioxide which they have produced.

18. Ionization.—Whenever acids, bases, or salts go into solution in
water, there is a tendency for the molecules to separate into the compo-
nent atoms or into radicals, which are groups of atoms. The atoms or
radicals then exist free in the solution. These solutions conduct elec-
tricity and are known as electrolytes. Free atoms or radicals in such a
solution are found to carry minute electrical charges and are called ions.

12 FUNDAMENTAL PRINCIPLES

Those ions which are metallic in nature carry positive charges, and those
which are nonmetallic carry negative charges; they are termed, therefore,
positive or negative ions. Table salt (NaCl) in solution separates into
sodium (Na) and chlorine (Cl) ions; sodium sulphate (Na.SO,), into
Na and SO, ions, SO4 being a radical. Na is a positive ion; Cl and SO.
are negative. When, by evaporation of the solution or by precipitation,
the substance which is in solution is made to reappear again in solid form,
the ions combine, and the charges neutralize one another and disappear.
This separation of ions in solution is known as electrolysis, or dissociation;
different substances show great differences in the degree to which this
occurs. Sugar and other substances which are nonconductors do not
show much dissociation. Acids by dissociation produce H ions; bases,
OH ions.

19. Colloids—Many thin membranes occur in the bodies of animals
in which openings exist of very minute size; similar membranes can be
artificially produced. Whenever two different liquids are in contact
with the two sides of such a membrane, the liquids tend to mingle by the
passage of molecules or atoms through the openings. When one or both
of these two liquids contain solids, other liquids, or gases in solution, the
particles of these substances also may pass through the same openings.
By the use of these membranes we can distinguish two categories of
substances. Those substances which when they are in solution are
capable of passing through such a membrane are said to be crystalloidal
and if they normally exhibit this character are often called crystalloids,
while those which will not pass through are termed colloidal and called
colloids. But many crystalloids may be made to assume a colloidal
condition. Of course it is a matter of relative size of particles and
openings, but in general it is true that crystalloids are substances which
exist in atoms or as molecules of very small size in a solution, while
colloids are substances which exist as particles and molecules of larger size,
are dispersed in liquids in the form of suspensions, and do not form true
solutions. These colloidal suspensions are thicker or more like glue than
are crystalloidal solutions. From crystalloidal solutions the substances
are easily obtained in crystalline form, but this is not true of colloidal
suspensions. Oils and fats and proteins, such as the albumen which forms
the white of eggs, are colloidal. This separation of colloidal from crystal-
loidal substances by membranes is known as dialysis and the membranes
which effect the separation, as dialyzing membranes, or dialyzers.

Other membranes are found in the body which under similar conditions
permit some liquids or gases to pass through them and prevent others
from so doing, but the passage takes place as a result of solution in the
colloidal membrane and not through openings init. Sucha membrane is
termed a semipermeable membrane. This process is known as osmosis and
the force behind the movement is called osmotic pressure. In the human

MATTER 13

blood there is about 0.7 per cent of salt in solution. If the blood is diluted
with water, the red blood cells will immediately absorb additional water,
swell, and may even burst. If, however, salt is added to the blood the red
cells will shrink and their surrounding membranes will become very
wrinkled or crenated. This swelling and shrinking of the blood cells
caused by water entering or leaving them rapidly are phenomena of
osmosis and osmotic pressure (Fig. 4). Osmosis is a very important
phenomenon in all living things. The intake into living tissues of oxygen,
digested foods, and water and the elimination from living tissues of carbon
dioxide gas and other waste products are brought about by the passage of
molecules through a semipermeable membrane from a place of higher con-
centration of these molecules to a place of lower concentration. In this
physical manner chemical substances necessary for life activities come
into living tissues, while the harmful chemical waste substances that are
produced in living tissues are eliminated from them.

B
A
Cc
.7% Salt Sof,
Water
.7 % salt jn More water Less wofer
plasma Less sa/t More sa/r

Fie. 4.—Diagrams illustrating osmosis and osmotic pressure in red blood cells by
increasing or decreasing the water in the blood plasma. A, water and salt molecules in
the same ratio on the inside and outside of the semipermeable cell membrane. B, by
increasing the ratio of water molecules over the salt molecules on the outside, the water
molecules migrate from a place of higher concentration through the cell membrane to the
inside of the cell, thereby causing the whole cell to swell. C, by decreasing the ratio of
water molecules on the outside, the water molecules on the inside again migrate from a
place of higher concentration of water molecules through the cell membrane to the outside
of the cell and cause the whole cell to shrink and become crenated.

20. Colloidal Emulsions.—In case the suspended droplets in an
emulsion are colloidal, the mixture is termed a colloidal emulsion. The
liquid in suspension is dispersed and is called the disperse phase, while
the other liquid is called the dispersion medium, or the continuous phase.

A colloidal emulsion is more or less jelly-like. It may at one time
become thinner and assume the condition of a sol or at another time
become thicker and assume the condition of a gel. This may be due to
the transfer of liquid from the disperse phase into the dispersion medium,
or vice versa, without the addition of more liquid from without. The
two phases, in other words, tend to change places, one being at one time
dispersed in the other, at another time the other in the one. When the

14 FUNDAMENTAL PRINCIPLES

colloidal droplets are scattered in the watery dispersion medium, it is a
thin jelly; but when they swell, press upon each other, and the dispersion
medium is restricted to the crevices between them, the whole becomes
thick and tends to set or become firm (Fig. 5). The ability to change
from one state to another and back again, over and over, causes a colloidal
emulsion to be called reversible. A gelatin suspension in water forms such
an emulsion as well as the edible fruit jellies.

Cy SFI

1Oser
P

} 4 Xi } |
wo

er-24,
) prs:

Fig. 5.—Diagrams to illustrate the change of a colloidal emulsion from sol to gel. In
A the droplets of the disperse phase (not stippled) are shown scattered through the dis-
persion medium (stippled) and the emulsionisasol. 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 continuous and to have become 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.

21. Reactions.— Whenever two substances are brought together and
a change occurs which involves a recombination of the atoms in a manner
different from that which previously existed, this change is termed a
chemical reaction. Reactions vary in speed and in the results attendant
upon them with the character of the substances reacting. Some sub-
stances have the power to cause a reaction without themselves entering
into it or being affected by it. Such substances are known as catalyzers,
catalytic agents, ferments, or enzymes; the effect is called catalysis, ferment
action, or fermentation. A small amount of a digestive ferment is capable,
if given time, of causing the digestion of any amount of the substance on
which it acts and would itself be found undiminished in quantity and
unchanged in character at the end of that time. Each ferment acts on a
particular substance or on similar substances and is most active at a
certain temperature or in a medium of a certain degree of acidity or
alkalinity.

CHAPTER III
ENERGY

Energy is usually defined in physics as the capacity to do work.
It may be more simply expressed as that which is behind all action in
this universe. Every change in the state of matter, in the form or
position of a mass of matter, or in the chemical composition of matter
involves a change in energy.

22. Forms of Energy.—Energy appears in two forms: potential, or
fixed; and kinetic, or free. Potential energy is energy of position. Every
particle of matter on this earth possesses an amount of potential energy
varying with its size and with its distance from the center of gravity of
the earth. Gravitational attraction, if all restraint were removed, would
cause it to fall to that center; in measure as it approached the center its
potential energy would be changed to kinetic, and if it could be conceived
as having arrived at that center it would possess no potential energy at all
with respect to this terrestrial system. However, it would still have
potential energy as a part of a larger system, the solar system, of which
this terrestrial system is a part. It would also have potential energy as a
part of the largest system, which is the universe. A molecule is an
energy system, as is also an atom. In the free movement of a bit of
matter its potential energy is changed into kinetic energy, or energy of
motion. Any mass possesses potential energy because of the relation
of its particles to one another. Energy of motion is manifested not only
when masses change position but also when changes occur within them.
It is also manifested when the atoms or molecules of which matter is
composed cause by their movement certain characteristic phenomena such
as heat, light, and the passage of an electric current. These are all forms
of kinetic energy.

23. Chemical Energy.—In many cases, when elements are made to
combine to form a compound, the application of kinetic energy is neces-
sary to bring them into the proper relationship to each other. Part
of this energy, at least, is represented by the potential energy which
these particles possess by virtue of this relation. This form of potential
energy is known as the energy of chemical union or simply as chemical
energy. Every substance has an amount of chemical energy proportion-
ate to the complexity of its structure—that is, to the number and variety
of atoms which make it up. A very simple substance has a small amount

of such energy; a very complex one may have a great deal. In general,
15

16 FUNDAMENTAL PRINCIPLES

inorganic compounds, as water, have relatively small amounts of chemical
energy, whereas organic compounds, as gasoline and other oils, coal
and wood, usually have large amounts. When a compound is resolved
into its component atoms, the chemical energy reappears as kinetic
energy. This is the source of the charges carried by the ions in an ionized
solution. Because of their greater complexity, a greater amount of
kinetic energy can be produced in an animal body by the disintegration
of organic compounds than by that of inorganic compounds. This
kinetic energy may appear in various forms, as movement, light, heat,
and electricity.

24. Law of Thermodynamics.—Thermodynamics is a part of physics
which deals with energy transformations. One law has long been con-
sidered fundamental in this field: It is the law of the conservation of energy,
which states that the sum total of energy in this universe is the same at
all times, being neither created nor destroyed but simply changed from
one form to another. Machines are mechanisms for transforming one
form of energy into another. When coal is burned in an engine, its
potential energy is changed into the kinetic form of heat and then changed
again into the kinetic form of mechanical energy. This may drive a
dynamo and be transformed into electric energy, which in turn may be
transformed into the radiant energy of light.

CHAPTER IV
LIVING AND NONLIVING MATTER

Living matter or matter which has been arranged under the influence
of life is termed organic; other matter is inorganic. Any living thing may
be called an organism. No element is found in living matter which is not
found also in nonliving, but only a small part of those which are found in
nonliving matter occur in living. As will be seen later, no more than
about a dozen of the 92 elements known can be looked upon as normal
constituents of living matter. Moreover, so far as is known, no forces
operate in living matter differing from those operating in nonliving mat-
ter, but all of the phenomena of living matter involving energy may be
explained in terms of the same physical forces which operate in all inor-
ganic matter throughout the universe.

25. Contrast between Living and Nonliving Matter.—Many points
of contrast have been enumerated between living and nonliving matter,
some of which are more significant and others less so. None of them is
capable of being applied in every case successfully and in such a manner as
to yield an immediate result. It is frequently stated that both definite-
ness of size and definiteness of form distinguish living things, but this
distinction is only a very general one. Some crystals conform very closely
to a certain size and approach with great mathematical exactness a
typical form, but, generally speaking, masses of inorganic matter vary
much more in size and shape than do masses of living matter. Con-
formity to type form is one of the most effective means of distinguishing
the species of living organisms. The more important contrasts between
the two kinds of matter are the following:

1. Chemical Composition.—Living matter, while varying in its precise
chemical structure, approaches very closely, both in the number of
elements contained and in the proportions between them, a certain
definite composition. So close is this agreement that living matter is
recognized as made up of a particular substance to which is given the
name protoplasm. ‘This does not occur in nonliving matter.

2. Organization.—Protoplasm possesses an internal organization,
evidenced not only by its appearance under the microscope but also by
certain chemical relationships, the presence of which is necessary in
order that it may carry on the phenomena and exhibit the reactions
which are associated with life.

3. Metabolism and Growth.—Protoplasm also possesses the power of
waste and repair and of growth. The carrying on of life activities
involves the breaking down of living matter with the formation of wastes,

18 FUNDAMENTAL PRINCIPLES

such as urea, water, and carbon dioxide, and the liberation of kinetic
energy, mainly evident as heat and movement. Living matter possesses
the power to rid itself of this waste and also to take in new matter and, by
adding it to its mass, to repair the loss which it has suffered. All these
changes together constitute metabolism. If the material taken in and
added to the mass is greater than the amount lost by waste, the result is
an increase in the size of the living mass which is called growth. At the
same time, mere increase in bulk does not necessarily imply growth.
The taking up of water and the swelling that living matter undergoes
under certain conditions are not growth. This word is properly applied
only when new matter is added to the substance of protoplasm itself.
This addition occurs in living matter in a manner different from that in
which it occurs in inorganic. The latter usually increases in bulk by
additions to the surface, or growth is by accretion; while in the former
growth oceurs by the ‘ntroduction of new particles among those already
present, which is erowth by intussusception.

4. Reproduction.—Partially due to metabolism and growth, living
things have the power to reproduce themselves by the formation of other
masses similar in every respect to the parent mass. Sometimes this
similarity is perfect from the first. At other times a fragment from the
parent mass gradually assumes the size and form of the parent. Non-
living things do not possess this power.

5. Irritability —Living things, generally speaking, have the power of
responding to changes in their environment, such changes acting as
stimuli. This quality is termed zrritability, or reactiveness. The response
evidences itself in the movements of animals and results in the various
ways in which animals adjust themselves to the conditions of their exist-
ence. The organism is not itself modified in any essential respect by the
reaction and may, under proper conditions, reassume precisely the character
it possessed before the reaction occurred. Nonliving things may also be
affected by changes in the environment, but the modification is not in the
nature of adjustment, is destructive in its effect, and the thing cannot of
itself regain its former character.

96. Tests of Life.—Living matter, however, is not always to be recog-
nized by any characteristics which it possesses. A living seed may
appear as inert as any bit of inorganic matter, and some animals may
exist dried up and apparently without any of the attributes which belong
ordinarily to living things. The test which may be applied in such cases
to determine whether or not life is present, or is possible, is to place the
object under such conditions of warmth and moisture as experience has
shown tend to develop life activities and observe if under these conditions
the distinctive phenomena of life are manifested. If they are, the infer-
ence is that the object was alive or that the drying up occurred in such a
way as not to destroy the organization that is behind all life phenomena.

CHAPTER V

PROTOPLASM

Protoplasm is invariably associated with life, and so far as is now
known life can exist in no other substance. When protoplasm ceases to
be living, it quickly undergoes destructive chemical changes which reduce
it to other and simpler compounds.

27. Historical Facts.—Protoplasm was first determined to be a
particular substance, with characters of its own, in 1835, when it was
described as existing in animals, by a Frenchman, Dujardin, who called
it sarcode. The protoplasm of plants was first described in 1846 by a
German, Von Mohl, who attached to it the name of protoplasm. This
name had been used six years before by Purkinje but in a very restricted
sense. That sarcode and protoplasm were one and the same substance
was most thoroughly demonstrated by Max Schultze in 1861, and to this
living substance, common to both plants and animals, is now applied the
name given by Von Mohl.

28. Chemical Character of Protoplasm.—Protoplasm eludes exact
chemical analysis. The chemical composition of an animal body may be
determined, but this includes the skeleton, stored food, and other mate-
rials which are nonliving. To secure a mass of perfectly pure protoplasm
is difficult, and the exact analysis impossible. Nevertheless, certain
general statements can be safely made.

1. It is almost entirely composed of 12 elements, including the
following:

Element Symbol 2 (aL
percentage

ORV CET ee ella!) fhe ee ces tee O 65
Carbon: e cts ees. 434s Ae C 20
fiydrerentie. .0055..4 tne eee H 10
INGETOS Tyg as oh horas. kee erst N 3
PAUL Te see co uk es ee Me S
J ROY A he ees SD AI Ad hh hla Fe
Calctumaeta hese tee ee eee Ca
MVR OMRERI UTS Yee. ees) Mg 9
SOG HITE AesHs Hie 2. Ao SH Na
IROtASSTUTN Aah, reyopy ease event the K
PANGS PHOTIUS 35 Meme '5 See oe Gs Pp
Chlorine Serie ee ee en Cl

20 FUNDAMENTAL PRINCIPLES

There are very minute quantities of several other elements in the bodies
of higher animals, including fluorine in the enamel of the teeth and also
silicon and iodine, but these are found only in certain tissues and are
not considered normal constituents of protoplasm. Minute amounts of
copper, manganese, zinc, and bromine have been found in marine inverte-
brates. The first five elements named in the table above are always
present and a certain number of the last group are also, but some of them
may be lacking.

Though some elements are present in small amount, this small amount
is none the less vital to the performance of the functions of living matter.
Certain of these elements are especially abundant in particular forms of
protoplasm, such as iron in the protoplasm of red blood corpuscles and
phosphorus in the protoplasm of the nerve and reproductive cells.

2. These elements are in combination in a variety of compounds which
may be classified as follows:

Organic Compounds Inorganic Compounds
Proteins Salts
Fats Water
Carbohydrates Gases

Proteins are, so to speak, foundation substances; about them is built
up the complex aggregate called protoplasm. They contain four ele-
ments—carbon, oxygen, hydrogen, and nitrogen—together with sulphur
and in some cases phosphorus, and have certain peculiar properties.
They are colloidal and have a tendency to coagulate on heating. The
protein molecule is very large and is built up of many amino acids, which
are acids containing the amino group NH2. The number of different
proteins in different forms of protoplasm is very great; different species of
animals have different types of protein, and particular tissues in one
animal contain proteins which are not found in the other tissues of the
same animal.

Fats are also colloids and consist of carbon, hydrogen, and oxygen,
the oxygen being very small in amount as compared with the carbon
and hydrogen. This fact makes them susceptible of a great amount of
oxidation, with the consequent production of a large amount of heat.
There are three different types of fats: (1) true fats, which include com-
pounds of glycerin and fatty acids, mostly oleic, palmitic, and stearic;
(2) lipoids, including the phosphorus-containing fats, an example of which
is lecithin, and the sugar-containing fats, or cerebrosides, found in
nervous tissue; and (3) sterols, including cholesterol, which seems to
occur in every animal cell.

Carbohydrates, which include sugars and starches, may also be in
a colloidal condition, although neither they nor fats show any tendency
to coagulate with heat. They also consist of carbon, hydrogen, and
oxygen, but there is relatively a much greater amount of oxygen than

PROTOPLASM 21

in fats, the proportion of hydrogen and oxygen being the same as in
water, or two to one. Only two kinds can be shown to exist as such in
the tissues of the body, dextrose and glycogen.

The water present in protoplasm maintains many substances in solu-
tion. Water, however, is important as a constituent of protoplasm,
not only as a very effective solvent but also because of its high specific
heat, because of its comparatively high surface tension, and because of
the fact that its presence gives to protoplasm the necessary consistency
and enables it to vary this consistency. The high specific heat of water
makes necessary the application of a large amount of heat to raise its
temperature and allows it to give off a correspondingly large amount of
heat in cooling, thus enabling it to exert a protective effect against sudden
and extreme temperature changes in the living body.

Salts are present in considerable number and are to a large degree
ionized, though the degree varies with different salts. They aid in the
maintenance of certain other substances in solution and take part in some
of the reactions which are characteristic of living matter. Some of these
salts are the chlorides, phosphates, and carbonates.

3. The various substances enumerated above are associated together
in a chemical aggregate or mixture which contains several thousands of
atoms combined into many kinds of molecules. Thus protoplasm is not
so simple as other familiar substances but represents a complex of sub-
stances all associated together in a certain definite fashion.

This definite arrangement of substances in protoplasm may be termed
its chemical organization and is one of the most striking of its characters.
What this organization means may be illustrated in the following way:
One might go to a jewelry store and ask the jeweler to give him every part
which enters into the formation of a watch. The jeweler could place
in his hand the necessary number and kind of wheels, screws, pinions,
and jewels as well as the hands, dial, case, and so on, so that within his
hand he would hold everything necessary to make a complete watch.
He would not, however, have a watch. This assemblage of parts does not
become a watch capable of performing the service expected of it until
these parts have all been arranged in a certain very definite relation of one
to the other. So it is with the chemical substances which make up proto-
plasm. Without the necessary organization the assemblage of parts
named above is not a watch, and in the absence of the chemical organiza-
tion which is a property of protoplasm that substance cannot be said to
exist. This organization is made possible by the fact that protoplasm is a
colloidal emulsion and the various constituents may be distributed among
the droplets of the disperse phase and through the continuous phase.

4. Protoplasm is very unstable. It alters in composition in response to
every change in the environment about it and when active remains for no
two consecutive moments the same.

22 FUNDAMENTAL PRINCIPLES

5. Protoplasm is also exceedingly variable. There is a probable differ-
ence chemically between the protoplasm of one species of animal or plant
and that of another. Moreover, there is also a difference between the
forms of protoplasm contained in the various structures found in the body
of each animal and plant. With all the individuality that exists in living
things it is conceivable that no two bits of living matter are ever precisely
alike. This characteristic is back of all the adjustments of living matter
to its environment.

6. Protoplasm also undergoes an orderly sequence of chemical reac-
tions which we call metabolism, and as long as life is being manifested the
cycle of such reactions is being repeated over and over.

29. Physical Characteristics of Protoplasm.—Protoplasm has the
following physical characteristics:

1. It is viscid and gelatinous in consistency, differing in viscosity in
various forms of life, in the various structures of the body, and under
various conditions.

2. Its texture, generally speaking, is more or less granular.

3. It is colorless in pure form. All colors which it seems to possess are
due to the presence of colored bodies within the living substance.

4. It is more or less translucent, being never perfectly transparent,
and this translucency gives to it in mass a grayish appearance.

5. It is of the nature of an emulsoid, or colloidal emulsion, the various
substances of which it is composed being distributed through the disper-
sion medium and in the droplets of the disperse phase. Being colloidal in
character and being reversible, it is possible for water and substances in
solution to enter protoplasm from without, causing it to become more
fluid, or to pass out from it, resulting in its becoming firmer in consistency.
To the same fact and to the fact that the internal surface films in such an
emulsion may act like semipermeable membranes is due the possibility of
water and substances in solution passing in both directions through
the walls of the droplets, causing them to swell or to shrink. As they
swell and crowd together the whole tends to become a gel, and as they
shrink and move with greater ease in the more fluid dispersion medium
it tends to become a sol. This transfer of water may be the consequence
of chemical changes taking place in either the substance of one phase or
that of the other.

This ability of protoplasm to change from sol to gel and back to sol
over and over again is behind many vital activities, including all move-
ment. The entrance of water and substances in solution into the mass at
certain times and the giving up of water and other substances in solution
at other times make possible the taking in of food and the giving out of
waste. The passage of materials through the internal films, which are
the walls of the droplets of the disperse phase, possibly plays a part in

PROTOPLASM 23

the orderly sequence of reactions which takes place in living matter and
makes possible growth by intussusception.

6. The salts in protoplasm are to a very great degree ionized, and the
state of conization contributes to the speed of chemical reactions within the
mass.

30. Microscopical Structure of Protoplasm.—The droplets of the
disperse phase in protoplasm are mostly too small to be within the range
of microscopic vision, and those which are not too small are hardly visible.
The structure of protoplasm as exhibited under the microscope, however,
reveals the presence of firm granules of different sizes; of fibers, which may
form a network, or reticulum; and of droplets, or alveoli. None of these
is evidence of the emulsoid nature of protoplasm but all may have a bear-
ing on the function of the cell. Emphasis upon one or another of these
different elements has formed the basis for three theories of the normal

Fic. 6.—Semidiagrammatic sketches illustrating the different appearances exhibited
by protoplasm. 4A, the granular type; represents cells from the liver of the mouse. 8B, the
fibro-reticular type; represents a nerve cell with its fibers cut away. C, the alveolar type;
represents a portion of alveolar protoplasm. (Figs. A and C modified from Wilson, ‘‘The
Cell,” A, after Altmann, by the courtesy of The Macmillan Company.)

structure of protoplasm, which has been thought to be (1) granular, (2)
fibrillar or reticular, or (3) alveolar. Though most protoplasm now seems
to be alveolar in character, a granular appearance is often exhibited in
gland cells, and fibers are prominent in nerve cells, muscle cells, and some
epithelial cells (Fig. 6).

31. Appropriateness of Protoplasm as Living Substance.—It is
evident from what has been said as to the chemical and physical char-
acteristics of protoplasm that among these are many which contribute
directly to the carrying out of life activities, and are necessary in a
substance in which life can manifest itself. Its physical characteristics,
its chemical nature, its organization, its proneness to change, its ability
to assume an almost infinite variety of forms, and its capacity constantly
to carry on metabolism all make protoplasm the only appropriate living
substance. Its exceedingly great complexity offers the possibility of
almost infinite internal change and adjustment, while at the same time

24 FUNDAMENTAL PRINCIPLES

the total chemical composition and the general character remain practi-
cally the same. Life itself is ceaseless change. When this protoplasmic
organization becomes fixed and no longer capable of change, it has suffered
that which we call death. Very soon after death reanimation under any
conditions becomes impossible because changes superveue which destroy
the very organization itself.

It is true that seeds and even some animals may be dried under proper
conditions and exist for a long time in a dormant state. That the organi-
zation remains intact and its capacity to undergo the changes which
accompany life is unimpaired are shown by the fact that when placed
under suitable conditions life activities are soon resumed.

32. Life Is a Consequence or Concomitant of Organization.—Living
matter, then, is not living because it contains certain elements, for
none of these elements is characteristic of life. All the chemical sub-
stances which enter into protoplasm might be collected theoretically
in proper proportions, but the mixture would not be protoplasm. Life
is possible only when the organization which has been referred to above
is effected; and when that organization is brought about, the other
chemical and physical characteristics of protoplasm also become added to
it. Protoplasm has been termed the ‘physical basis of life” and it is such
in the sense that it furnishes the physical organization and the attendant
conditions that make life possible.

CHAPTER VI

LIFE

As indicated in the preceding topic, life is always associated with a
certain type of organization of matter. It can be defined neither in
terms of the chemical elements which enter into it, none of which is
peculiar to it, nor in terms of the forces which act through it, since those
forces are the same as those which also act through nonliving matter, the
results being different only because of the organization.

33. Definition —Life might be defined as the functioning of a certain
type of organization or as embracing certain phenomena. It might be
conceived of as energy manifested in a manner made possible by its organi-
zation. A precise definition is difficult to give in a form with which all
would agree, but the following is suggested: Life 1s a continual series of
reactions in a complexly organized substance known as protoplasm, by
means of which the organization tends to adjust itself to a constantly
varying environment. According to this definition a dormant mass of
protoplasm, such as that in a seed, might possess the capacity to exhibit
life but would demonstrate this only under certain favorable conditions.

34. Vital Force.—The theory has been held in the past that a mysteri-
ous vital force acts through living matter and is responsible for the
characteristic phenomena of life, but every attempt to demonstrate the
existence of such a force has ended in failure. As knowledge of life phe-
nomena has increased, it has constantly become more evident that all
such phenomena can be explained by reference to the same forces which
also operate in nonliving matter and, as far as is known, throughout the
universe.

35. Vitalism and Mechanism.—Those who have believed in this vital
force have been termed vitalists, and their view vitalism. Mechanism,
on the other hand, is the view that all forces operating within the organism
are wholly physical or chemical in nature. Those who have contended
for this view have been termed mechanists. While vitalism is not a
tenable conception today, the most extreme form of mechanism also does
not appeal to the greater number of biologists, who observe phenomena
which are distinguished as vital. The view of the majority might be
stated as a modified form of mechanism. It is true that there is nothing
peculiar in the chemical elements or the physical forces in living matter
as distinguished from nonliving matter, but that does not mean that the
chemical changes in protoplasm are precisely the same as those occurring

26 FUNDAMENTAL PRINCIPLES

in nonliving matter, nor does it mean that none of the phenomena
associated with life is peculiar to living things. The differences, however,
are the outgrowth of the organization and are not due to any supernatural
force which animates living bodies.

36. Origin of Life-—The question of the origin of life on this planet
has been a source of speculation from early Greek times, if not before.
Empedocles, a Greek, about 500 B.c. presented a theory of the origin of
life which was that owing to attractive forces elements were combined into
the parts of which plants were composed, and then under the influence
of the same forces these parts were assembled in such a manner as to
form whole plants. Animals were supposed to have originated in the
same way as did plants, parts being formed first which later came together
to form the animals. This theory, fantastic as it now seems, is the first
definite theory of the origin of life and has earned for its author the
title of father of evolution. Aristotle, another Greek who lived in the
fourth century before Christ, had a theory more in harmony with present-
day conceptions. He believed that living matter originated as a Jelly
formed at the shore of the sea and that out of this evolved first plants and
then animals. The simplest forms developed first, followed in order by
others of gradually increasing complexity up to man.

The Mosaic, or special-creation, theory of the origin of life appears in
the first chapters of Genesis and was the legendary explanation accepted
by the Jews. According to this theory each kind of animal was created
in the beginning with the same character it has today, or, in other words,
each was the result of a special creative act. Because it is in the Bible this
theory has been thought of as necessarily involving the idea of a divine
providence and for that reason different from any other theory. Asa
matter of fact, however, the conception of a deity need not be associated
with any one of the theories of the origin of life to the exclusion of its
association with others. One who believes in a creative and ruling spirit
or force in the universe will attribute to it the creation of life no matter
what his theory may be as to how creation actually occurred, while one
who does not believe in such a force will leave it out of whatever scheme of
creation he holds.

Spontaneous generation implies the repeated creation of life whenever
favorable conditions occur. A theory of spontaneous generation was
held by the Greeks, who believed that various living forms found in fresh
water died each fall and were recreated each spring. The observations
of Aristotle and others showed this belief to be incorrect in the case of
many familiar forms. Gradually the number of animals thought to be
spontaneously generated was reduced, until, in 1680, Redi, an Italian,
effectively disproved the spontaneous-generation theory held at that time
when he showed that fly maggots were not spontaneously generated in
decaying meat. This theory was again revived, however, when the

LIFE 27

microscope revealed the minute forms of plant and animal life which
exist and which were immediately conceived by many to be spontaneously
generated when the right conditions occur. The work of Pasteur in
France and Tyndall in England during the latter part of the last century,
however, disproved the possibility of spontaneous generation of even these
minute forms. There are those today who entertain a belief in the
possibility of the spontaneous generation of life, but there is no existing
evidence to support their views.

Another theory of the origin of life held by the physicists, Kelvin
and Helmholtz, who also lived in the last century, explains the presence
of life on the earth by stating that it was brought here on meteorites
through the interstellar spaces from some other world. This meteoritic
theory is frequently coupled with the conception that life has always
existed in this universe and is simply passed from one world to another
from time to time. ‘This is unsatisfactory to biologists, because it puts
the whole problem beyond the possibility of human explanation, and
because the conditions which would have to be withstood by life coming
to this planet in that way are apparently beyond the limits of endurance
of living matter. For this reason the theory has been believed in by
only a few, and these not biologists.

Another theory of the origin of life, formulated by a German, Pfliiger,
in the latter part of the last century, has been known as the cyanogen
theory. According to this theory the earth was once exceedingly hot and
the elements were in a free state. As it cooled a temperature and other
conditions were reached which caused the union of carbon and nitrogen
into a substance known as cyanogen, the formula for which is CN. As
the earth cooled still more, water was formed by the union of hydrogen
and oxygen; and then by the combination of cyanogen and water, cyanic
acid (HCNO) was produced. The characteristics of cyanic acid resemble
in many ways those of protoplasm: (1) It is a liquid which is transparent
at low temperatures, but it tends to coagulate and become opaque at high
temperatures. (2) It can increase in bulk by a process essentially like
that of growth by intussusception. (3) Its molecules can be rearranged to
form urea and it can be decomposed into carbon dioxide and ammonia.
By the addition of sulphur and other elements to cyanic acid, proteins
might have been formed and thus life might have developed.

Another, known as the bacterial theory, is that of Osborn, who places
the origin of life at a time when there was no soil on the surface of the
earth, when all the water was fresh, and the air contained more carbon
dioxide than at present. At that time the earth was shrouded in a dense
cloud through which the sun never penetrated. This cloud was main-
tained by constant evaporation from the heated surface of the earth.
The air was warm and saturated with moisture. Lightning played
constantly through the clouds and rain descended in torrents. Under

28 FUNDAMENTAL PRINCIPLES

these conditions nitrates are conceived to have been produced in rain-
water pools due to the discharges of electricity in the water-saturated
atmosphere, and ammonia also appeared in volcanic waters. Such
conditions favor the growth of bacteria, and Osborn has suggested that
the first life may have developed in the form of these minute organisms.
Able to make use of inorganic food, very resistant to destructive agencies,
and capable of exceedingly rapid multiplication, they were able to main-
tain existence and gradually evolved into higher but still simple forms
from which both plants and animals have come. This theory suggests,
the conditions under which life may have arisen and the nature of the
earliest organisms but does not successfully solve the problem of the
origin of life.

Numerous other theories have been proposed, one involving the
development first of ferments and then, under the influence of these
ferments, the organization of living matter. Another theory is that an
inherent tendency exists for simple compounds, under proper conditions,
to unite themselves together and form more complex compounds and
that as a consequence of this tendency, and in a favorable environment,
protoplasm was gradually built up or synthesized out of the various com-
pounds which it contains. As some of the disease-producing viruses
seem to exhibit characteristics not only of nonliving but also of living
material, it has been suggested that perhaps they have originated in this
manner and are the connecting links between nonliving and living
matter (Sec. 547).

None of these theories has proved satisfactory to biologists generally,
and it must be confessed that at this time it is not possible to explain how
life on this earth originated. That life must have appeared at a time
when conditions were favorable goes without saying, but most biologists
believe that at only one time in the history of the earth has there been
such a fortunate concomitance of favorable conditions as to bring about
this creation. From the life created at that time all living things which
have ever existed on this earth have descended.

37. Possibility of Creating Life.—The creation of life by human agency
has been the dream of men in the past, and the idea will surely continue to
’ be entertained in ages still to come. In the present state of human
knowledge, however, a realization of the dream seems to be out of the
question. It appears hardly probable that the conditions which existed
on the surface of the earth at the time when life first originated will ever
be repeated in the laboratory. Theoretically it would be possible to
assemble in the proper proportions those substances which exist in
protoplasm, but the crucial thing—the bringing about of the organization
which exists in living matter—seems beyond human power when the
limitations under which men work are considered. Yet the idea is con-
ceivable and efforts to bring it to fulfillment will probably never cease so
long as the human race continues to exist.

CHAPTER VII

CELLS

Living protoplasm always exists in the form of minute masses known
as cells, which possess a characteristic structure. Organisms may consist
of but one cell or of many, but in either case the cell may be considered
the unit of structure, or the morphological unit.

38. Definition.—A cell may be defined as a mass of protoplasm in
which can be distinguished a portion called the nucleus. A distinetion
may be drawn between the substance of the nucleus, which is termed
nucleoplasm, and the protoplasm of the rest of the cell, which is called
cytoplasm.

39. Sizes and Shapes of Cells.—Cells vary greatly in size. The most
minute animal cells are one-celled blood parasites which are invisible,
or only barely visible, under the highest powers of the microscope.
Most cells cannot be seen by the unaided eye. There are cells, however,
which are relatively gigantic. A one-celled organism, parasitic in the
alimentary canal of the lobster, reaches a length of two-thirds of an inch;
and egg cells, with the yolk which they contain, may even exceed this in
diameter and contain a much greater amount of substance. Some nerve
cells, the main cell body of which is not proportionately very large,
possess fibers, which are parts of the cells, that may even reach a length of
several feet.

Cells also vary greatly in shape. The typical form, unaffected by
environment or unmodified for the production of any particular function,
is spherical, but the pressure of adjacent cells or from other structures
may crowd these cells into a variety of shapes, such as polygonal, cubical,
columnar, or flat and platelike. Other cells, particularly muscle cells,
become greatly elongated and assume the form of fibers, while still others
become very complexly branched (Fig. 8).

40. Numbers of Cells.—As has been previously stated, an organism
may consist of but one cell; however, most organisms are made up of
more, the numbers in the largest organisms running into the trillions.

41. Structure of Cells.—A cell (Fig. 7) consists of a mass of jelly-
like cytoplasm inclosing a nucleus. The surface of this cytoplasm is
covered by a plasma membrane, or cell membrane, which is living and
semipermeable. Outside it may be a cell wall composed of material which
is not protoplasmic and is nonliving, being a secretion formed by the cell.
In animal cells this wall is often absent.

30 FUNDAMENTAL PRINCIPLES

The nucleus, which is set off from the cytoplasm by a nuclear mem-
brane, shows a fine network of fibers known as linzn fibers; and scattered
throughout the nucleus, adhering to these linin fibers, are masses of
another substance known as chromatin. 'This name was given to this
substance because it takes dyes or stains to a very high degree and when
the cell is subjected to these, the chromatin stands out as scattered,
deeply stained particles. A body which shows plainly in the nucleus is
known as a nucleolus. Nucleoli, however, are of various kinds. Some
are called plasmosomes or true nucleoli. Sometimes such a body is made
up of granules of the chromatin massed together and is called a karyosome
or chromatin nucleolus. The more fluid portion of the nucleoplasm

Cell membrane
or wall

Plasma membrane

Golgi

Central body con- bodies

taining two
centrio/es

Plasrmosome
Nucleus

Nuc/ear
membrane

Linin
Chromatin

Nuc/ear
SAP

Vacuole
Plastid

Metaplasrm Mitochondria

Fic. 7.—Composite diagram of a cell having the form of a typical cell and containing
all the structures generally recognized as normal in cells not modified for any particular
function.

between these structures which have been enumerated is often called
nuclear sap.

In the cytoplasm appear several characteristic structures. A body
appears, under certain conditions, near the nucleus, known as the central
body, or centrosome, containing one or two granules called centrioles.
More or less solid particles in the cell include living portions of the proto-
plasm which have some particular function to perform, such as the chloro-
phyll bodies which give the green color to plants. These have been given
the general term plastids. Included in plastids are mitochondria, or
chondriosomes, which are fiber-like and more compact structures, the
nature of which is in question; and Golgi bodies, which may be scattered
through the cell or collected around the central body. Bits of food or
waste particles which have collectively been called metaplasm may be
present in the cytoplasm. Vacuwoles are transparent droplets seen regu-
larly in certain cells or at certain times in other cells.

CELLS 31

42. General Physiology of the Cell.—There is a division of labor in the
cell among the structures which have been named. The nucleus is, in a
sense, the vital center. Cytoplasm alone is unable to carry on all vital
activities and its life is brief after it is separated from the nucleus. Prob-
ably under the influence of substances formed by the nucleus and passed

P a
‘e

x
Ron
Loe

Red
corpuscles

White
corpuscle

Fig. 8.—Various types of cells. A, epithelial cell shed from the lining of the human
mouth; a is a side view of the cell. x 300. B, human egg cell, nearly mature. X 200.
C, motile human sperm cell. X 1300. D, diagram of a nerve cell. E, a bone cell; some-
what diagrammatic. Xx 700. F, human blood corpuscles. > 1000. G, nonstriated
muscle cell from mammalian intestine. > 640.

out into the cytoplasm the latter does most of the ordinary work of the
cell, including the taking in of food, the carrying on of many of the chemi-
cal and physical changes associated with life, the passing out of waste, the
reception of all stimuli, and the movements which occur in response to
them. The chromatin is the medium by which hereditary characters are

32 FUNDAMENTAL PRINCIPLES

transmitted, and therefore it determines the character of the cell. The
central body with its centriole, or centrioles, is active in cell division.
Plastids are living structures with active functions and are more numerous
in plant than in animal cells; the chlorophyll bodies, which are one form of
plastids, utilizing the energy of the sun’s rays, build up carbohydrates
from carbon dioxide and water. As indicated in the preceding topic some
of the other structures play only a passive role, while the functions of
others are not definitely known.

43. Development of Knowledge of the Cell.—The history of this
development may be briefly summarized as follows: Hooke, an English
microscopist, discovered in 1665 that cork was divided into little com-
partments which, because they reminded him of the cells in a monastery,
he called cells. In 1833, or 168 years later, Brown, also an Englishman,
discovered the nucleus, and it was then supposed that the cell consisted
of a living wall inclosing a nonliving, watery substance in which floated
the nucleus, also living. It was not until 1835 that Dujardin, a French-
man, as has already been stated (Sec. 27), discovered that this watery
content of the cell was a substance of peculiar character and that it, too,
was living. From this time the cell was believed to contain these three
elements, which were found to be common to both plants and animals.
It was discovered after a time, however, that cells existed which did not
possess a cell wall. Thus the wall, which was at first supposed to be the
essential part of the cell, was finally eliminated as a part of it and the word
cell became really a misnomer. The most important contribution to the
modern conception of the cell was that of Max Schultze, who, in 1861,
showed that the substance of all cells, plant and animal, was similar, and
who defined a cell as a “small mass of protoplasm endowed with the
attributes of life.”’

44, Cell Theory and Cell Doctrine.—The cell theory was due to the
work of Schleiden, a botanist (1838), and of Schwann, a zoologist (1839).
Each of these men had found cells in all living matter which he had
studied, and they presented, each in a publication in his own field, a
hypothesis which has been known as the cell theory, to the effect that living
matter always exists in the form of cells. It was to them a theory, but in
the time that has elapsed since the dates mentioned it has been found to
hold good for all living substance which has been studied. Thus today we
no longer consider it a theory but rather a fact, and so it has come to be
known as the cell doctrine. This conception when first presented had a
most profound effect upon biological thought, and its influence has been
equaled only by that exerted by Darwin’s theory of evolution.

CHAPTER VIII

METABOLISM

Reference has been made previously to the fact that one of the char-
acteristics of living matter is its ability to carry on metabolism—that is,
its ability to take material into the body and work it over in such a way
as to make it a part of the living organization and from it to secure the
energy with which to carry on the processes of life.

45. Definition.—All living things, in the performance of their various
activities, exhibit physical and chemical changes. A result of the former

ration
p

-.
os

7. Dissirmnilation

11. E/imination
10. Expiration

Fic. 9.—Diagram showing the steps in metabolism as they occur in an ameba.

is the liberation of the needed kinetic energy, and of the latter the forma-
tion of waste materials which are thrown away. To replace the material
so used and to provide a source for more kinetic energy, food must be
taken into the organism and incorporated in the organization. The sum
total of all the chemical and physical processes involved is termed metab-
olism. The discussion which follows applies particularly to animal
organisms, but plants carry on metabolism by a series of steps which,
considering the difference in structure, parallel those in animals.
33

34 FUNDAMENTAL PRINCIPLES

46. Food.—The food of the organism, in the broadest sense, must
include all of the compounds which enter into the chemical organization
of protoplasm—that is, proteins, fats, carbohydrates, salts, and water.
It must also include certain substances termed vitamins which seem to
play a necessary part in the carrying on of metabolic activities. It is
also essential that this food shall supply energy in such a form as to be
available to the organism.

)

1. Ingestion

5
2
==

a Sw
ais

10. Expiration
5. Inspiration

4. Circulation In every cell

of the body ,

6. Assimilation
7. Dissimilation
8. Secretion

9. Excretion

9. Excretion of

2. Digestion
nitrogenous waste

8. Secretion of
gastric Juice

4.Circulation

3. Absorption

INTISAIN|

1. Elimination

Ga 12.Egestion

Fic. 10.—Diagram to suggest the steps in metabolism as they occur in the human body.
For comparison with Fig. 9.

47. Steps in Metabolism.— Metabolism takes place in the organism by
a series of very definite steps (Figs. 9 and 10), all of which are necessary
in the metabolism of the higher animals, but certain ones of which are
simplified or dispensed with in the case of the very simple animals.
These steps are referred to in terms that are more or less in popular use
with very loose and uncertain meanings. The words excretion, secretion,
elimination, and assimilation are frequently met but are usually used with
an uncertain significance. It will be necessary, therefore, for the student

METABOLISM 35

to learn these words in this connection as scientific terms, each with a
very precise meaning, and to keep this meaning separate from that which
he may have hitherto attached to the word.

48. Ingestion.—The first step in metabolism is the taking in of food—
ingestion. ‘This may occur in certain one-celled animals through any
point on the surface of the body. While the same may be true to a cer-
tain degree in the case of higher animals, most of them, and some of the
one-celled ones, take food through a particular opening on the surface of
the body, called the mouth. Under Fatty acids
the head of ingestion also occurs all Glycerin \ ats
mechanical processes such as chewing
and swallowing which precede any
chemical change.

49. Digestion—As soon as the
food is in position to be acted upon (GE avis)
by digestive fluids, these are secreted
into the cavity which contains it. By
their action a series of chemical and
physical changes is initiated, which
results in reducing the solid food to
liquid form and changing part of it
chemically so as to render it capable of
being absorbed. This process is called Bic) iii —“piaarans operas Maat
digestion. Inthehigher animals diges- the finger-like projections in the small
tion may begin in the mouth and be pie acre i ee iesae ae
continued in the stomach and intes- The lymph vessels are in solid black, the
tine. Only organic foods need to be ocd vessels stippled.
digested, the other foods being capable of absorption without undergoing
this process.

50. Absorption.—The passage of the digested food from the food
vacuole into the protoplasm of one-celled animals is termed absorption.
In higher animals the same process takes place by the food entering the
cells forming the lining of the alimentary canal and being then passed
into the blood or lymph contained in blood vessels or lymphatics which
lie behind these cells (Fig. 11). In vertebrates absorption occurs mostly
in the small intestine. The digested food is not further changed during
this process, though it may suffer a change as soon as the process is
complete. In the process of digestion fats, for example, are broken down
into fatty acids and glycerin but are changed back to fat in the cells into
which they are absorbed.

51. Circulation—Whether the animal is one- or many-celled, the
food cannot be all utilized at the point of absorption but must be cir-
culated throughout the living body for use in various parts. This
circulation may take place within the cell, by osmosis from cell to cell,

Amino acids
(Profein)

36 FUNDAMENTAL PRINCIPLES

or by means of a circulatory system, generally the blood circulatory
system.

52. Inspiration.—Oxygen, as well as food, is constantly needed by
the body. Its entrance into the body is termed inspiration. This may
occur through all points on the surface of the body or may occur only
through certain particular organs set aside for the purpose, such as lungs
or gills. Upon entrance into the body oxygen is circulated in the same
manner as food and taken up by the tissues as needed. This passage of
oxygen into the tissues is termed internal inspiration; its entrance into
the body, external inspiration (Fig. 12).

Fic. 12.—Diagram to illustrate external and internal respiration.

53. Assimilation.—The food, having been brought to the point in
the body where it is to be used, is taken up by the protoplasm and more
or less intimately incorporated into the living mass, becoming, at least
for the time, a part of the organization. This process of addition of new
material to the existing material of the body is termed assimilation. This
material, no longer food but a part of the protoplasm, may be soon used
or it may remain for a greater or less length of time as a part of the cell
before actually becoming involved in chemical changes.

54. Dissimilation.—Sooner or later chemical changes occur which
collectively are called dissimilation, as a result of which protoplasm and
the more complex food substances associated with it are broken down
into simpler substances. Associated with these chemical changes is a
transformation of part of the potential energy represented by these
substances into kinetic energy, which appears mostly in the form of heat
or movement.

55. Secretion.—If the substances produced in dissimilation can be
utilized in any way by the body as a whole, they are termed secretions,
and the process involved in their passing out of the cell which produces
them is termed secretion. These may be passed out upon the surface
of the body, into any cavity in the body, or into the blood and body

METABOLISM 37

fluids. Examples of such substances are the tears, which when poured
out upon the surface of the eyeball serve to keep it moist; other fluids,
which also serve to moisten or lubricate internal surfaces; the digestive
secretions, which when passed into the alimentary canal assist in the
digestion of food; and also substances known as internal secretions.
These internal secretions are carried over the body and perform various
functions in connection with the carrying on of life activities, such as the
regulation of metabolism and the control of growth processes.

56. Excretion.—Some products of dissimilation, such as urea, water,
and carbon dioxide, seem to be of no use to the organism and are termed
excretions. The process by which they are passed out of the cell which
forms them is termed excretion. In many cases the excretions are poured
out directly upon the surface and are immediately disposed of; in other
cases, however, they are formed in the organism at some distance from the
external surface and have to be transported to some particular part of
the body before they can be passed out. Here again the circulation
comes into play, it being as necessary for the carrying of waste matters
to the point where they are passed out of the organism as for the trans-
portation of food and oxygen to the cells.

57. Expiration.—The carbon dioxide formed in dissimilation is carried
by the circulation to some particular part of the organism where it
is passed out. This part may be in the lower animals the general body
surface or some particular structure within the body; in the higher ani-
mals it is the gills or lungs. This process is expiration. Here also a
distinction may be made between internal expiration, which is the passage
of carbon dioxide out of the tissues into the blood, and ezternal expiration,
which is its passage from the body (Fig. 12). Expiration relieves the
organism of its gaseous waste.

58. Elimination.—Liquid waste may be eliminated from any point
on the body surface, or it may be passed out by some particular structure.
In the higher animals the kidneys and skin are the principal organs of
elimination, though some elimination may occur through the walls of
the alimentary canal toward its posterior end. In this last case elimina-
tion should not be confused with egestion. As an example of the differ-
ence between excretion and elimination may be mentioned the fact that
urea is produced in the body in the liver, where excretion proper takes
place, but it is very largely eliminated by the kidneys.

59. Egestion.—Kgestion is the passing from the body of indigestible
materials contained in the food, which are known collectively as feces.
Feces might be referred to as solid waste, but they have not, properly
speaking, been involved in the process of metabolism as have the sub-
stances which are expired or eliminated. The material egested has been
passed through the body but has at no time been a part of it. Egestion,

38 FUNDAMENTAL PRINCIPLES

again, may take place from any point on the surface of some of the one-
celled animals or may take place through the posterior opening of the
alimentary canal in higher forms.

60. Respiration.—The processes of inspiration and expiration taken
together constitute respiration, which includes all gaseous interchanges
in the organism.

61. Anabolism and Katabolism.—The processes beginning with
ingestion and ending with assimilation are collectively termed anabolism.
Anabolism may be defined as the sum of all processes involved in the
building up of the body. The processes beginning with dissimilation
and ending with expiration and elimination are collectively termed
katabolism. Katabolism may be defined as the sum of the processes
having to do with the breaking down of the body and the getting rid
of the waste matter resulting from it. Egestion, for reasons given in a
preceding paragraph (Sec. 59), does not belong under either anabolism
or katabolism.

62. Vitamins.—It has been found recently that providing the organ-
ism with the necessary kinds and amounts of proteins, fats, and carbohy-
drates or of salts and water is not sufficient. Something else is needed
to enable it to assimilate the organic foods, and that is the presence of
vitamins. 'These are organic chemical substances that occur in certain
natural foods. Vitamin A (anti-infective or antixerophthalmic vitamin)
is present in many animal fats, milk, butter, yolk of eggs and also in
spinach, lettuce, cabbage, sweet potatoes, and soy-bean sprouts and is
not destroyed by ordinary cooking. It helps maintain a normal condition
of the covering membranes of the eye and respiratory canals. Vitamin
B, (antineuritic vitamin) occurs in fruits, meats, milk, yolk of egg,
covering of grains and other seeds, and yeast, and promotes growth and
guards the body from certain inflammatory conditions of the nerves.
Vitamin C (antiscorbutie vitamin) is found in fresh vegetables and citrus
and other fruits and prevents scurvy. Vitamin D (antirachitie vitamin)
occurs in fish oils as cod- and halibut-liver oils. It prevents rickets and a
deficiency of it leads to an inability to form a properly calcified skeleton.
Vitamin E (antisterility vitamin) occurs in green vegetables, wheat germs,
egg yolk, and milk and prevents sterility in certain animals. Vitamin G
(antipellagric vitamin) is found in milk, eggs, and fresh vegetables and
prevents pellagra.

63. Energy Changes in Metabolism.—The food taken into the organ-
ism represents a supply of potential energy. One object of dissimilation
is to change part of this into kinetic form in order that the organism can
make use of it. This kinetic energy appears mostly as heat and as the
mechanical energy exhibited in movement; a small part appears as electri-
cal energy; and in some cases, in very small part, as light, shown in the
luminescence of some organisms. Some of this kinetic energy is neces-

METABOLISM 39

sarily used in the securing of additional food, but some is also used in
growth, in reproduction, and in carrying on other activities. Among
the lower animals the portion of energy used in the securing of additional
food is much larger than in the higher animals. The development of
efficiency among the latter is, to a considerable degree, connected with
the possession of more effective food-securing devices, which leaves a
proportionately larger part of the total energy of the organism to be used
in other ways. Man has solved this problem far more successfully than
any animal below him, and the advance he has made to a dominating
position in the animal kingdom may to a considerable degree be attrib-
uted to this fact.

64. Uses of Different Foods.—The different foods serve different
purposes in the organism. The protein food is in part used to replace the
protein of living tissue when that is used up. Carbohydrates furnish
the mechanical energy expended in muscular movements. Fats are
used chiefly as a source of heat. All dissimilative changes in the body
liberate heat, but from fats, owing to the fact that they contain a very
small amount of oxygen and are therefore susceptible of a great deal of
oxidation, may be produced more heat than from any other food. Water
must be maintained in large amount in the organism, both because it is
needed to give the required consistency to the protoplasm and because
it serves as a vehicle for other substances in solution. Salts are essential
constituents of protoplasm, also participating in the metabolic changes
and exerting a regulatory effect upon them. Oxidation processes take
place in all of the cells of the organism, the extent of such processes in any
given cell determining the amount of activity carried on by the cell.
They do not occur in the blood except in the blood corpuscles, which are
cells.

65. Storage.—The organism does not in all cases make immediate use
of the food absorbed, in which case it may be stored against future need.
Fats are thus accumulated in the form of fat. Since carbohydrates are
the chief sources of muscular energy and since the body must at all
times have not only a ready supply but also a large volume in storage to
be used as needed, there is in the liver an abundant supply of stored
carbohydrate ready to be given out to the blood and circulated to all
parts. An excess of carbohydrates may be changed to fats and stored
as such. In the chemical changes in the organism, carbohydrates may be
derived from substances resulting from protein decomposition, and fats
may in some cases be changed to sugar, but neither of them can be con-
verted into proteins, since these contain nitrogen, which is lacking in
carbohydrates and true fats. Proteins are not stored, but any excess is
immediately broken down and the waste products eliminated. Storage
should not be confused with growth, since the stored food is not a part
of the protoplasmic organization.

40 FUNDAMENTAL PRINCIPLES

66. Metabolism the Central Fact in Life.—All life activities result
from metabolism in the living organism, and therefore life might be
defined as the orderly series of metabolic changes which occur in matter
possessing the necessary protoplasmic organization. In last analysis all
of the functions of the living body may be described in terms of metabo-
lism. ‘The animal organism may be conceived as an energy system, and
it has also been likened to a chemical machine the product of which is
kinetic energy. The plant organism, likewise, may be conceived as an
energy system and as a chemical machine, but its product is largely
the complex organic compounds which form the basis of the food of
animals.

CHAPTER IX

PLANTS AND ANIMALS

Two great groups of living things exist, plants and animals. The
higher forms of the two are readily recognized, but the simpler ones lack
the characteristics which serve to distinguish the higher types. Many
simple living things cannot be satisfactorily assigned to either category.
A German named Haeckel suggested as a way out of this difficulty that an
intermediate group, which he termed Protista, be recognized. This sug-
gestion, however, has not been followed, because it would simply double the
difficulty—instead of having to draw one line of demarcation which is very
uncertain, it would be necessary to draw two lines, both as uncertain.

67. Comparison between Plants and Animals.—In many respects
plants and animals agree. The protoplasm of which both are composed
is, as far as can be seen, essentially the same. Although plant and animal
cells have certain features which aid in their discrimination, those fea-
tures are not essential characteristics of the protoplasm of which they
are composed, and as far as present knowledge goes, the protoplasm of
the two is indistinguishable. Indeed, it is generally assumed that there
was but one creation of life on this earth and that from that first created
life both plants and animals have sprung. This makes quite intelligible
the difficulty in distinguishing between the simpler forms of the two.
Metabolism is carried on essentially in the same manner in plants as in
animals. Plants and animals have many activities in common and
those in which they differ are developed gradually in passing from lower
to higher forms. The lower plants are termed protophytes and the
higher metaphytes, while the lower animals are called protozoans and
the higher metazoans. The unit of plant structure is the cell, as is also
the unit of animal structure, and plant cells present the same phenomena
in connection with their multiplication as do animal cells. They are
affected in a similar manner by various external forces. The higher
forms of both possess sex. The mechanism of inheritance is the same and
the phenomena connected with inheritance are also quite comparable in
plants and animals.

68. Biology.—Because of the fact that there are many things in
common between plants and animals, the subjects of botany and zoology
are frequently considered as parts of one larger subject termed biology.
It has to do with all that concerns living things in general and may be
conceived of as divided into the two fields botany and zoology. In the
further division of these two fields each can be divided into a series of

42 FUNDAMENTAL PRINCIPLES

subsciences which in general correspond. Thus one can speak of plant
morphology and animal morphology, of plant ecology and animal ecology,
of plant physiology and animal physiology, and the same is true of
taxonomy, pathology, embryology, and so on.

69. Differences between Plants and Animals.—The higher plants
and higher animals, as has already been stated, present distinctions which
are sufficient in all cases to enable us to assign a living thing to either one
category or the other. Among these distinctions are the following:

1. Movement.—Broadly speaking, the higher plants lack the power of
movement and in all cases are without the power of locomotion. On the
other hand, almost without exception, animals are possessed of both.

2. Manner of Growth.—In a general way it may be said that the
plant grows by the addition of parts externally, such as the addition of
leaves and twigs. There is also evidence of internal growth, as is seen in
the gradually expanding trunk and constantly thickening branches,
the new wood being added just underneath the bark. In animals, on the
other hand, few parts are seen to be added externally, though some ani-
mals show at times a gradual increase in size of wings, horns, and other
visible parts; growth is mostly internal and the body simply increases in size.

3. Cells —Plant cells usually possess a distinct cell wall, composed of
cellulose, wood, which gives to the cell rigidity of form and to which is due
the immobility of the plant body. Animal cells, on the contrary, often
possess no wall of any kind, and the walls, when they are present, are
generally thin and permit the cell to change shape. This fact contributes
to the power of movement and of locomotion possessed by animals.

4. Food Securing and Metabolism.—There are minor differences
between plants and animals connected with the metabolism of the two,
but the steps are essentially the same (Fig. 13). However, the oxidation
changes in the cells which are included in this text under the term dis-
similation are by botanists termed respiration. Plants, in addition to
carrying on the same type of metabolism as has been described for
animals, have the power of manufacturing their own complex foods. By
virtue of their possession of plastids and of their ability to utilize the
energy of the sun’s rays they can take simple substances from the earth
and air and out of them synthesize complex substances such as proteins,
fats, and carbohydrates. . These processes are known as photosynthesis.
After producing these substances plants make use of them in the same
fashion as do animals, but animals being incapable of manufacturing
such foods have to get them from plants or by eating other animals.

Since plants take these simple substances in gaseous or liquid form
there is no solid waste left as a result of plant metabolism, and conse-
quently egestion does not occur. Also, since plants build up proteins,
which they add to their substance, and make immediate use of any
nitrogenous matter liberated in protoplasmic activity, they do not

PLANTS AND ANIMALS 43

produce urea, a characteristic excretion of animals. In photosynthesis
plants use up the available carbon dioxide and give off an excess of oxygen,
while animals always utilize all the oxygen they can get and give off
carbon dioxide as waste.

Liquid wastes

a Elimination
&\ Water and Urea

_ Dissimilation _

Inspiration Assimilation

(oxygen)

Digestion

Organic food

a. Animal

Practical!
ro waste

Dissimilation
(Respiration)

Assimilation

Excess of
oxygen

7 S
a
Water, Salts Food materials. oS

and C02

b. Plant
Fic. 13.— Diagrams contrasting the metabolic and food-manufacturing processes in plants
with the metabolic processes in animals.

Because of the difference in metabolism of plants and animals plants
have often been referred to as predominantly anabolic in their activities,
while animals are characteristically katabolic. Also, since in the presence
of light the processes concerned with photosynthesis outweigh those
concerned in the metabolism of protoplasm, during the day plants use
more carbon dioxide than they produce and produce more oxygen than
they use. At night when photosynthesis is arrested they are, from a
metabolic standpoint, on the same plane as animals.

CHAPTER X

GROWTH AND REPRODUCTION

Whenever during the lifetime of an animal assimilation exceeds
dissimilation, there results an increase in the actual amount of protoplasm
in the body; this increase is termed growth. When the reverse is true and
dissimilation exceeds assimilation, the body shrinks in size; this process
is known as emaciation. Many animals continue to grow throughout
their lifetimes, although growth is more rapid at the beginning and slows
up more and more with advancing age. This is true of many cold-
blooded vertebrates, in the case of which size is a fairly clear index of
age, other conditions being equal. Of course, in this case care must be
taken to judge of the amount of available food, for in an environment in

|
|
|
|
Adolescence Maturit Senescence

Birth Age 21 Age 45 Age 60 Death

Fic. 14.—Diagram illustrating the growth cyclein man. This is intended to be typical,
but individual growth cycles vary greatly, both as to the span of the whole and the propor-
tionate lengths of different periods.

{
|
|
|
|
|

!
|
|
|
|
|
|
|
|

which food is limited a limit is also set to the size of the animal, and no
matter how old it may grow it will never equal in size an animal living
under more advantageous conditions.

70. Growth Cycles.—The life cycle of an animal comprises the whole
series of phenomena from the time development begins to the death of the
organism. Among the various aspects in which this can be studied is
that which involves the growth cycle. This varies greatly with different
animals. As has just been stated, some animals never cease to grow;
others grow only during the early parts of their lives. The latter is true
of insects, none of which ever grows at all after the adult condition is
reached. The higher vertebrates, however, including man, have a
regular growth cycle involving youth, maturity, and old age (Fig. 14).
Growth is most rapid in this case at the beginning of life and remains
still rapid until the end of the period of youth, when the individual has

practically attained full stature. A very gradual growth still continues,
Ad

GROWTH AND REPRODUCTION 45

becoming constantly less rapid, until the maximum size is attained, which
is usually somewhat beyond middle life. There is then a slow decrease
in weight. Throughout the rest of the period of maturity this decrease
continues, becoming gradually more pronounced; after the individual
passes into old age, however, there is a more rapid emaciation, which
ends with death. The youthful condition is termed adolescence; that
accompanying old age, senescence.

71. Limit of Size—No matter what the character of the life cycle
may be or when growth takes place or for how long, there is in the case of
all animals a size limit which is not surpassed. While in one-celled
animals this varies somewhat in different lines of descent, in any one
line it is rather closely approximated. In such forms there seem to be
some metabolic relations in the cells, which, as this limit is approached,
give rise to changes which automatically result in the division of the cell
and the production of smaller organisms. The same thing is true of the
individual cells which compose the bodies of higher animals, but the
result is to produce a larger body and not new individuals. In some
higher forms the process of cell multiplication practically ceases as the
individual becomes adult; in other forms it stops in certain parts of the
body but goes on in other parts. The organs of the central nervous
system reach full size early in life. Bones and muscles continue to grow
until the animal becomes adult. The skin, however, from the outer
layer of which dead cells are continually shed, grows throughout life by
the multiplication of living cells in the deeper layers. The size of many-
celled animals is also limited by various factors such as inheritance, the
available food, and the activity of glands the secretions of which favor
or hinder growth.

72. Reproduction.—Multiplication by cell division, which is the
most common way among one-celled animals, is not possible to those
which are many-celled, since the different cells which make up the latter
became varied in form and structure and also become specialized for the
performance of one or a few functions out of the many that are possessed
by the body as a whole. Consequently such cells cannot reproduce the
complete animal. Under these conditions certain generalized or unspe-
cialized cells are set aside for the purpose of reproduction and are relieved
from the performance of any other duty. They serve as cells from which
the development of another individual may be initiated, transmitting to
that individual the characters of the parents in the bodies of which they
have been produced. Such cells are termed sez cells or gametes. Of these
there are two types which in the higher animals are known as egg cells
and sperm cells, or sperms. The animal which produces egg cells is called
female and the one which produces sperm cells, male. Generally speak-
ing, egg cells are relatively large in size and sperm cells relatively very
small, so that the former may be termed macrogametes and the latter,

46 FUNDAMENTAL PRINCIPLES

microgametes. These terms are common to both botany and zoology,
the same being true of another term, zygote, which is applied to the cell
resulting from the union of a sperm cell and an egg cell in fertilization.
This union of two sex cells is also known as syngamy. This sexual type
of reproduction is characteristic of higher animals, while asexual reproduc-
tion, which is any type not involving these sex cells, occurs chiefly in
lower ones.

CHAPTER XI

MITOSIS

The necessity of ultimate cell division in the cases of cells which
continue to grow has been explained in the previous chapter (Sec. 71).
If the cell thus dividing is itself a one-celled animal, then cell division
and reproduction occur at the same time. If, however, the cell is only
one of the cells in a many-celled animal, then division does not in most
cases result in reproduction, which is the formation of a new individual,
but simply in an increase in size of the individual of which the cell is a
part. It makes no difference, however, as to the precise manner in which
the division is carried out except that in one-celled animals it may be a
relatively simple process while in many-celled animals it is more complex.

73. Normal Cell Division.—The ordinary way in which a cell divides is
by a series of steps (Fig. 15) which do not occur in every case in precisely
the same order, but all of which are passed through before the division
is complete. These steps may be outlined as follows: First, the central
body, if it has not before been visible, comes into view beside the
nucleus and both it and its centriole divide into two. These two central
bodies begin to separate, and as they do so fibers appear between them
which form a spindle-shaped figure. At the same time the chromatin
granules in the nucleus, which have been scattered irregularly upon the
linin network, begin to collect together into a slender and very much
tangled thread, or spireme, and the linin network as such begins to dis-
appear. This thread shows itself early to be a double thread. Radiating
rays, termed astral rays, appear about each central body, forming star-
shaped figures known as asters. As the central bodies gradually separate,
each with an aster about it, the spindle fibers between them seem to
press against the nuclear membrane. The chromatin thread shortens
and thickens, forming a much less involved tangle, while the linin com-
pletely disappears. This shortened, double chromatin thread breaks
crosswise, producing a number of pieces which are known as chromosomes
and which, because the spireme was double, are in pairs. During these
changes, while the central bodies still continue to separate, the nuclear
membrane disappears and the spindle swings into the area occupied
hitherto by the nucleus. The central bodies come to lie on opposite
sides of what was the nuclear area and the spindle stretches across this
area from one central body to the other. The chromosomes arrange
themselves in a double row across the center of the spindle, forming in
some cases what is known as an equatorial plate. End views at this time

show that the chromosomes may form a ring about the equator of the
47

48 FUNDAMENTAL PRINCIPLES

spindle but that more often they are distributed through it. The asters
reach a maximum development, their rays extending sometimes to the
periphery of the cell. At this time the mitotic figure is called the amphi-
aster, and the stage the amphiaster stage. ‘The chromosomes are in pairs
and the two of a pair are similar. Now the two of each pair of chromo-
somes begin to separate, one being pulled along the rays of the spindle

2. Spindle
Central body x rie
and. Cef- Eas
trioles

(CQ
RS POR

Spireme shor-
tens and thickens

Nuclear
membrane
disappears

Nuclear Asters.
rrermbrares and spindle
appear disappear

Fic. 15.—Diagrams representing the steps in a typical mitosis. The steps numbered
1 to 4 represent the prophase, 1 being a resting cell. 5, the metaphase. 6, the anaphase.
And 7 to 9, the telophase, 9 showing the two daughter resting cells.

toward one central body and the second being pulled toward the other
central body. As the two separating groups of chromosomes approach
the central bodies these chromosomes become scattered about the respec-
tive ends of the spindle in an irregular fashion. Then a series of steps
occur which in a general way are the reverse of the steps occurring at the
beginning of the process. The separate chromosomes become irregular
in shape and fuse, forming a meshwork, from which finally are produced

MITOSIS 49

granules of chromatin scattered over a new network of linin, all formed
out of the chromatin. A new nuclear membrane appears about each
group and thus two nuclei take form; this membrane is also formed
from the chromatin, though the cytoplasm may assist in its formation.
The spindle fibers gradually disappear, as do the astral rays. A constric-
tion appears in the cytoplasm of the cell in the same plane as the equa-
torial plate, frequently as early as the stage in which the two groups of
chromosomes begin to separate. This constriction grows deeper and
deeper while the two new nuclei, or daughter nuclei, are being formed.
Finally, as the spindle fibers disappear, the constriction cuts clear through
the cell, which thus forms two new cells, each with a nucleus, containing
one-half of the original chromatin material and each half the size of the
parent cell.

The whole process of cell division which has been outlined is divided
into four phases. All of the steps from the beginning to the time when
the chromosomes line up in the equator of the spindle are termed collec-
tively the prophase. The steps involving the splitting of the chromo-
somes into pairs in the equatorial plane are termed the metaphase.
The process of division may be arrested for a time at this point. The
period of migration of the daughter chromosomes from the equator of
the spindle to the poles is termed the anaphase, and the series of steps
involved in the division of the cytoplasm of the cell and the construction
of two separate nuclei is termed the telophase.

These steps do not always occur in the same order and any general
outline such as has been given will have to be modified in a great variety
of ways to suit different cases. There may be two centrioles in the
resting cell before mitosis begins. Variations occur in the time of the
splitting of the chromatin thread and its division into chromosomes.
In some cases the spireme is single and breaks transversely into chromo-
somes during the prophase. These line up on the equator of the spindle
and split longitudinally in the metaphase. The two of a pair so formed
are similar to the parent chromosome, and it is assumed that each pair of
chromosomes formed from the double thread is derived from a parent
chromosome, to which they are similar. Sometimes the linin seems to
disappear; at other times it seems to assist in the production of the
spindle; and in still other cases it seems to contribute to the formation of
the chromosomes. In some cases, especially when the chromosomes form
a ring around the spindle, a distinction may be drawn between the chro-
mosomal or traction fibers which connect the chromosomes with the
poles of the spindle, and which are also called mantle fibers because they
are on the outside of the spindle, and the continuous fibers that run
from one pole to the other. This mode of cell division has been called
karyokinesis, meaning nuclear movement, or, more commonly, mitosis,
from the Greek word for thread, referring to the chromatin thread.

50 FUNDAMENTAL PRINCIPLES

The whole process may be conceived as a play in four acts in which
there are no pauses between the acts. In the first act, the prophase,
the characters are introduced and several scenes are presented which lead
up to the second act, the metaphase. This is a grand tableau which
shows the stage fully set and the characters in formal array. In the
third act, the anaphase, a parade of the daughter chromosomes of each
pair results in their separation and the division of all the characters con-
tained in the original chromosomes into two groups at the opposite sides
of the stage. In the fourth act, the telophase, the characters in each
group adjust themselves to the changed conditions and find their proper
places in the new order.

74. Significance of Mitosis.—The universality of this process in the
division of both animal and plant cells, and the regularity with which in
every case the various steps occur suggest that the process is of vital
importance. The great care with which the chromatin is equally divided
between the two cells seems to show that this division is, of all these steps,
the most significant. Recognizing in chromatin the substance which
bears the hereditary qualities from cell to cell, and in the case of sex cells
from animal to animal, this splitting has been conceived as having for its
end the passing on of the hereditary qualities to each of the two daughter
cells. Thus not only do these become structurally alike but each also
possesses the same inherited characteristics as the other and as the parent
cell. The various modifications of the process do not seem to affect this
judgment. The equal division of characteristics is explained on the
assumption that these characteristics correspond to units which are
arranged in a longitudinal series from one end of the chromatin thread to
the other, so that a longitudinal splitting of the thread involves the equal
division of every unit and therefore the sharing of every characteristic.

75. Amitosis.—In contrast to the process just described, there has
always been recognized another type of cell division known as amitosis,
or direct cell division. Amitosis has been described as involving simply
the constriction of the cytoplasm into two portions, this constriction also
affecting the nucleus and dividing it into two portions, so that the whole
cell becomes divided into two parts containing equal amounts of cyto-
plasm and nucleus. It seems to occur only in cells which are highly
specialized, lacking in vitality, or undergoing degeneration.

76. Continuity of Cell Life and Chromatin.—Two conceptions flow
directly from a consideration of the phenomena of cell division. One is
that all cells must be derived from previously existing cells, just as all
living things receive their life from previously existing living things.
This fact has been recognized for a long time and expressed in the apho-
rism omnis cellula e cellula, or ‘“‘every cell from a cell,’’ which we owe to
the German pathologist, Virchow. Another conception, based upon the
equal division of chromatin qualitatively and quantitatively between the

MITOSIS 51

two daughter cells, is expressed in the phrase “continuity of chromatin”
and is that all chromatin has come from previously existing chromatin
back to that of the first created protoplasm. ‘This implies that in the
first life created on this earth were inherent all the possibilities which
have been realized in all living things that have since come from that life.

77. Growth of the Cell.—The two daughter cells resulting from cell
division, each precisely similar to the parent cell except in size, grow and
tend ultimately to reach the same size as the parent cell. This involves
growth in all portions of the cell, and being growth by intussusception it
is accomplished by the slipping in of particles of new material among
particles already definitely arranged. Thus the organization, which is
the central fact in protoplasm and is behind all life phenomena, is passed
on unchanged.

CHAPTER XII

FORMS OF ANIMALS

Animals of various kinds seem to present a great variety of forms,
but when these are carefully studied it becomes possible to recognize
a small number of distinct types.

78. Asymmetry.—Symmetry is regularity of form and involves the
existence of corresponding parts. In the case of some animals, particu-
larly the more simple ones, there seems to be no symmetry; this condition
is termed asymmetry, and the animals are spoken of as being asymmetrical
(Fig. 22).

79. Spherical, or Universal, Symmetry.—An ideal form of symmetry
which is rarely approached in nature would be a form in which an indefi-
nite number of planes might be passed through the center of the animal
and each plane exhibit a structure precisely similar to that of every other,
as well as dividing the animal into symmetrical halves. This is termed
spherical, or universal, symmetry. This is most nearly attained in some of
the one-celled animals (Fig. 31 D).

80. Radial Symmetry.—Another form of symmetry which is presented
by many of the lower many-celled animals is one in which the body can
be divided by a number of radial planes into parts that are similar to
each other. This type of symmetry is termed radial symmetry and the
parts, since they are opposite around the center, are termed antimeres
(Fig. 108).

81. Bilateral Symmetry.—Other animals are capable of being divided
by a single median plane into similar right and left halves, the one being
a mirror image of the other. This is termed bilateral symmetry and is
characteristic of higher forms.

82. Metamerism.—Many bilaterally symmetrical animals have a
body which is not divided into similar parts other than the right and left
halves. Others, however, including the highest animals, are divided into a
series of parts arranged in a linear series which, because they are placed
one behind another, are termed metameres, or segments, the condition
being termed metamerism. If these metameres are, generally speaking,
similar to each other, it is known as homonomous metamerism, which is
well illustrated by the common earthworm or angleworm (Fig. 144).
If, however, these parts are dissimilar, the condition is termed heterono-
mous metamerism; an example of this type is the crayfish (Fig. 162), in

FORMS OF ANIMALS

Fic. 16.—Outlines of the fore limbs and their skeletons,
bird (D), to illustrate homology.

-3
a —-------- --— ~~

-
2

of man (A), horse (B), bat (C), and

The dotted lines pass through corresponding joints.

54 FUNDAMENTAL PRINCIPLES

the case of which different metameres bear different types of appendages,
such as feelers, or antennae, mouthparts, and legs.

83. Appendages.—If the body possesses such structures as have
just been named or others which might be added, these are termed
appendages. An appendage may be
CS defined as a projecting part, capable
of movement and performing some °
Leg active function. Immovable horns,
age ET spines, hairs, and scales are not
a termed appendages, but movable
spines, tentacles, legs, wings, and

tails could all be recognized as such.

84. Homology and Analogy.—
Whenever parts of the body, whether
in the same or in different animals,
are similar in plan of structure, they
are termed homologous and the con-

dition is referred to as homology.

This usually involves similarity in

origin and in mode of development.

Whenever parts which are structur-

Fic. 17.—Outline of the wings of a ally or morphologically different in

butterfly and she vine in hem for eomP8"~ PIam perform the same function, they

are analogous, and the phenomenon

is referred to as analogy. Thus the foreleg of a horse, the arm of a man,

the wing of a bat, and the wing of a bird are all homologous parts (Fig.

16). This homology concerns not only: the division into segments which

correspond to upper arm, forearm, wrist, and hand but also involves the

skeleton, muscles, blood vessels, and nerves. The wing of a bird and the

wing of a butterfly, however, possess nothing in common structurally but

perform the same function, and the case is recognized as one of analogy.

The wing of a butterfly (Fig. 17) is formed by an outfolding of the surface

layers of the body wall, mostly of the skin, becomes Jointed at the point

of attachment, and is moved by muscles within the body. Since all

likeness may be expressed in varying degrees, homology and analogy can
be spoken of as being more or less complete or perfect.

CHAPTER XIII

BEHAVIOR

By behavior is meant the sum total of an animal’s movements. Noth-
ing else can be included in behavior, since only by the movement of the
body as a whole or of some part of it can an animal convey any indication
of change within it. This also holds true with human behavior, since
posture, facial expression, speaking, or any other mode of communication
all involve movement. The movement of animals when involving no
change in location of the animal is termed motion, but when change in
location occurs it is locomotion.

85. Stimuli AI] movement is referred to some cause, either outside
the animal or within it, which is termed a stimulus. If the cause is
purely external the phenomenon is called external stimulation; if, on the
other hand, the cause exists within the body of the animal, it is recognized
as internal stimulation. The stimulus initiates a change, chemical and
physical, within a part or whole of the animal which results in either
motion or locomotion and which is called a response. External stimuli
are either continuous or discontinuous. The first application of a con-
tinuous stimulus produces a movement, while the continuation of that
stimulus, if it is maintained at the same intensity, has no effect. When
the operation of the stimulus ceases, movement is again observed. A
discontinuous stimulus may be looked upon as a series of similar stimuli
each of which produces the appropriate response on the part of the animal,
both when it begins and when it ends. Since the response takes a certain
length of time, discontinuous stimuli, if following each other with suffi-
ciently brief intervals between them, may produce a continuous effect.
An example is the production of a tetanic, or prolonged, muscular con-
traction by a discontinuous electrical current.

86. Direct Response.—If the response which the animal gives immedi-
ately follows the application of the stimulus and seems to be determined
by the nature and force of the stimulus, the response is termed direct and
is called a tropism. Two words have been used in this connection:
tropism, which means simply a turning, and taxis, which implies move-
ment of the animal as a whole in response to the stimulus. Since the
difference between these two is one determined by the locomotor ability
of the organism, the word taxis has given way to the more general term
tropism.

56 FUNDAMENTAL PRINCIPLES

Tropisms are named with respect to the stimulating agent. The
following are generally recognized:

1. Thigmotropism, or response to contact.
Thermotropism, or response to temperature.
Phototropism, or response to light.
Chemotropism, or response to chemical stimulation.
Rheotropism, or response to mechanical currents.
Electrotropism, or response to currents of electricity.
. Geotropism, or response to the force of gravity.

Animals in nature are subject to all of these forms of stimuli except
that of the electric current, which is purely an artificial stimulus.

If the response is such as to cause the animal either to turn toward the
source of stimulation or to approach it, it is termed a positive response.
If, however, it is such as to cause the animal to turn away from the source

NOP WN

Fig. 18.—Sketch showing a long, rectangular glass aquarium partly covered by an
opaque hood, and containing three species of organisms responding differently to light.
The direction of the light is shown by the arrows and the positions assumed by the animals
at + (positive), — (negative), and + (gradient).

of stimulation or to avoid it, it is termed a negative response. ‘The mini-
mum strength of the stimulus which is necessary to produce an effect is
termed the threshold of stimulation.

Some animals do not seem to respond positively either to the strength
of the stimulus immediately above the threshold or to a maximum
strength of the stimulus but do seem to be attracted to a position in which
they receive the stimulus in a degree intermediate between the maximum
and minimum. In this case they are said to exhibit a response to a
gradient of the stimulus, corresponding to an intermediate strength
expressed in degrees of temperature, intensity of light, or concentration of
chemical solution (Fig. 18).

87. Conductivity—Though the stimulus may be received at a particu-
lar point on the animal, the effect is not limited to that point but is con-
ducted more or less throughout its body. This power of living matter
to transmit the effect of the stimulus is termed conductivity. Irritability,
or reactiveness, which is the power to respond to stimuli, as well as con-

BEHAVIOR 57

ductivity, which is the power to transmit this effect, are both properties
of living matter.

88. Part Played by the Nervous System.—lIn animals possessing no
nervous system, behavior is summed up in the responses to stimuli or
in the tropisms which the animal exhibits. In animals that possess a
nervous system, the structure relations within the nervous system modify
the responses in a variety of ways. Two or more cells are involved
between the reception of the stimulus and the response, which accordingly
is said to be indirect. The presence of the nervous system also makes
possible more numerous and more varied effects due to internal stimuli.
The result is the production of the complex forms of behavior character-
istic of the higher animals.

89. Physiological State——The character of the response which an
animal will give to a stimulus is determined not only by the kind and
strength of the stimulus but also by the condition of the animal and
depends upon the state of the metabolic processes within its body. Thus
a, one-celled animal in the body of which there is no food, which is hun-
gry, and which is at the end of a cycle of metabolism may give a response
different from that of an animal which has recently fed and in the body
of which the metabolic cycle has just begun. In higher animals different
parts contribute to the physiological state of the whole. Animals which
possess a nervous system exhibit physiological states dependent upon the
varying conditions of that system, which in turn have a metabolic basis.
Repeated and abnormal stimulation may throw an animal into a condition
of excitement in which it acts in a manner quite unusual. The different
feelings of which we are conscious at different times, the mental attitudes
which dominate us, and our varying ability to carry on our different
activities are all connected with different physiological states. Physio-
logical states are back of what is called temperament, or mood, and
explain one’s ability to excel on one occasion and his inability to perform
creditably on another. It is a certain physiological state resulting from a
change in our ordinary routine which causes us to feel and act differently
after a holiday or after an unusual experience. This explains “blue
Mondays” and “off days.”’ The word psychological is sometimes used
instead of physiological when the nervous system is involved.

CHAPTER XIV

CLASSIFICATION AND NOMENCLATURE

Whenever one has to deal with a great many objects of varied char-
acter it becomes necessary to arrange them in such a manner that any
additional object can at once be put in its proper place with respect to the
others, that any particular object can at once be found, and that they may
be referred to by groups. Such arrangement and grouping are called
classification. It is imperative in the arrangement of a library and
increasingly so as the library grows in size, is necessary in every mercantile
establishment, and to a degree even desirable in the handling of objects in
our homes.

90. Definition.— Zoological classification may be defined as the group-
ing and arrangement of animals in such a way as to facilitate reference
tothem. If this grouping is based only upon the place where animals live
or upon their form and structure, without regard to any relationships
which may exist, it is termed artificial classification. If, however, an
arrangement is secured such as to bring out the degree of relationship,
assuming all animals to be related and to have evolved from the living
matter first developed upon this earth, it could be called a natural
classification. The basis of zoological classification is essentially artificial.
but in so far as knowledge permits, zoologists endeavor to make it
natural.

91. Arrangement of Groups of Animals.—The groups into which
animals are arranged present a graded series, beginning with the whole,
leading through those of gradually diminishing extent, and ending with
each particular kind, which is a collection of like individuals. Generally
speaking, any particular group will contain several groups of the next
lower rank. It is evident that the characteristics of any one group will
involve details of structure less fundamental than those of the next higher
group and that, on the other hand, the characteristics of any group will be
only those in which all the lower groups contained in it will agree. The
names of the most widely used of these groups, in order of rank, illustrated
by reference to a particular species of animal, are as shown on page 59.

These group names in their plural form may be treated either as Latin
or as English words, as, for example, phyla or phylums, and subphyla or
subphylums; or they may be given only their Latin plural, as genera;
or only their English plural, as in the case of all the other groups above.
The word species is the same in both singular and plural forms.

CLASSIFICATION AND NOMENCLATURE 59

Kingdom: Animalia (all animals)
Subkingdom: Metazoa (many-celled animals).
Phylum: Chordata (chordate animals).
Subphylum: Vertebrata (vertebrates).
Class: Mammalia (mammals).
Order: Carnivora (carnivorous mammals).
Family: Canidae (doglike carnivores).
Subfamily: Caninae (dogs and their relatives).
Genus: Canis (dogs).
Species: familiaris (the domesticated dog).

If the number to be handled in any group is very large, for convenience
in classification groups are introduced between those given. A common
method is by the addition of the prefix sub-, by which device, for instance,
several subspecies may be included ina species. Ina similar manner may
be formed subgenera, subfamilies, suborders, subclasses, and, as given
above, subkingdoms and subphyla. Another device consists of the intro-
duction of words with the prefix super-, a superfamily being a group lower
in rank than a subclass but one which includes several families. If these
devices do not reduce the groups to convenient size, a variety of other
words has been employed, such as series, division, and legion. It has been
found in nature that many widely distributed and variable species can be
divided into smaller groups based upon geographical range, color, form,
and other characteristics, and these may be called races and varieties.
Various aspects of classification will appear as the different animals
which make up the animal kingdom are treated more in detail, and the
whole subject will later be reviewed.

The names of the groups of animals above families vary in form, but
the name of the family always ends in -idae and that of a subfamily in
-inae. In each case the name of the family or subfamily is derived from
that of the genus in that family or subfamily which is taken as the type
genus, the names Canidae and Caninae, for example, being both derived
from the name Canis. The names of groups higher than the genus are
always written with a capital initial letter when used in a taxonomic sense
but are not italicized. Generic names are capitalized and usually itali-
cized, while specific and subspecific names are italicized but not capitalized.

92. Nomenclature.—In order that each animal shall have a distinctive
appellation and that this may be the same throughout the world, it
is necessary to avoid common names, which differ in different localities
and often have a very uncertain application, and also to use a language
which is the common language of scholars everywhere. For this reason
each animal bears a scientific name which is Latin in form, though it
may not be in origin, and which includes the names of the genus and
species to which the animal belongs. For the purpose of exact reference,
and since different authors may have referred to different species under
the same name, to the generic and specific names is added the name of the

60 FUNDAMENTAL PRINCIPLES

individual whom we recognize as the original authority for the name used.
Thus the scientific name of the dog is written Canis familiaris Linnaeus,
since Linnaeus bestowed this name upon this particular species. The
name of the author is not italicized. If the form of the name is not the
exact one the author used, his name is enclosed in parentheses. In such
cases the species name has been removed from its original genus to
another. For example, the name Sorex aquaticus was given by Linnaeus
to the common mole, but since it belongs to a more recently established
genus, Scalopus, the name is written Scalopus aquaticus (Linn.). Authors’
names are often abbreviated, as Linn. for Linnaeus. The date when the
author proposed the name is often added and follows the name of the
author. For example, Canis familiaris Linnaeus, 1758. Uniformity
in nomenclature is attempted by application of the International Rules
of Zoological Nomenclature sponsored by international congresses of
zoologists. The name of the subspecies may be added to that of the genus
and species, as Scalopus aquaticus machrinus (Rafinesque), the subspecies
of the common mole found in the upper Mississippi valley. Sometimes
the common name of an animal is also the scientific name of the genus,
in which case the difference is shown by italicization and capitalization.
For example, paramecium and hydra are the common names of animals
belonging respectively to the genera Paramecium and Hydra.

PART II
PROTOZOA.

CHAPTER XV

AMEBA
A SIMPLE PROTOZOAN

This animal may serve as a type of the more simple one-celled animals,
which are themselves the simplest forms in the animal kingdom.

93. Occurrence and Appearance.—Amebas occur commonly in fresh
water. There are also numerous marine forms. The fresh-water species
may be collected in a great variety of situations, such as watering troughs,
spring pools, dams, stretches in streams where the water runs over
rocky ledges, and wherever there is abundant aquatic vegetation. They
are found gliding over the surfaces of submerged plants and of algae-
covered mud, rock, or planking. One method which has been suggested
for securing them is to collect a mass of pond weed, place it in a flat dish,
and cover it with water. A brown scum which gathers on the water in a
few days will be found to contain amebas if they were present in the pond
from which the weeds were obtained. Not all appropriate habitats con-
tain this animal, however, and one frequently has to search for a con-
siderable time before coming upon a supply.

When found, amebas are in the majority of cases only to be discovered
by the use of the microscope. The very largest specimens of Amoeba
proteus Leidy, however, are visible to the eye as minute whitish dots when
seen against a dark background while the giant amoeba, Chaos chaos, is
often 5mm. (1% inch) inlength. Under the microscope an ameba appears
like a mass of colorless jelly, irregular in form, and, when active, con-
stantly changing in outline. The generic name given to this animal is
Amoeba, and under the rules of nomenclature it cannot be changed, but
the common name is now quite generally spelled ameba.

94. Structure.—The animal (Fig. 19) owes the irregularity of its out-
line to the fact that from the surface of the main mass extend numerous
and variously shaped projections known as pseudopodia. These are
constantly varying in length and thickness and may at any time be
entirely withdrawn.

An ameba has no cell wall but possesses an extremely delicate outer
layer known as a plasmalemma. This is too thin to be seen even with
higher powers of the microscope, but by the movement of particles on its
surface it may be shown to exist. Below this is a clear layer of cytoplasm,
the ectoplasm, much thicker than the plasmalemma but still forming a
thin layer over the surface of the cell. Within this and forming the bulk

of the body is the more granular endoplasm. The layer of ectoplasm is
63

64 PROTOZOA

relatively thickest at the anterior end and at the tips of the projecting
pseudopodia and is thinnest at the side away from the direction in which
the animal may be advancing. Within the endoplasm are seen numerous
vacuoles and a nucleus.

The vacuoles are of three types: (1) Food vacuoles, which appear like
colorless drops of water inclosing particles of food. They are spherical in
form if the food particle is small and compact but assume somewhat the
form of the particle if it is large and elongated in any dimension. Food
vacuoles disappear with the egestion of the feces from the body. (2)
Water vacuoles, which appear like perfectly transparent colorless drops

Contractile Endoplasm (=<
vacuole ‘

Nucleus
i Ecfoplasm

Pseuvdopodium

Water
vacuoles

Fic. 19.—Amoeba proteus Leidy. XX 1000.

of a spherical shape. They do not change under observation. (3) Con-
tractile vacuoles, of which there may be one or more and which appear like
the water vacuoles. However, they change in size, gradually growing
larger until they reach a maximum, when they suddenly contract and
disappear, the process being regularly repeated. Because of this alternate
filling and contracting these are sometimes known as pulsating vacuoles.
They may sometimes be recognized by the possession of a faint pinkish
tint. Throughout the endoplasm may be seen numerous granules of
varying shapes and sizes, including crystals, whose nature can be deter-
mined by their crystalline form; and foreign substances, like bits of
sand.

Somewhere within the endoplasm, and usually nearer the end away
from that which is advancing, is the nucleus. It is finely granular in
texture, homogeneous in appearance, and refractive to light, which gives it
a brighter or clearer appearance than the surrounding endoplasm. It
also frequently has a faint bluish color.

AMEBA 65

95. Metabolism.—The food of an ameba consists of minute aquatic
plants and animals, though it will attempt to engulf any organism which it
can surround. Small bits of organic matter are also taken in, but inor-
ganic particles seem to be rejected unless they are accidentally taken in
with a bit of food.

Ingestion takes place (Fig. 9) with the formation of pseudopodia.
These envelop the food, which thus becomes completely inclosed in the
body. With the food is taken in a certain amount of water and so a food
vacuole is formed. Into this vacuole is secreted hydrochloric acid, which
gives to its contents an acid reaction. An enzyme which acts on proteins
is then added and the reaction becomes alkaline. No enzyme acting on
carbohydrates has been demonstrated, and the same must be said for one
acting on fats. As digestion proceeds, the digested and dissolved food is
passed into the fluid of the food vacuole, whence it is absorbed into the
surrounding protoplasm. It is then circulated to all parts of the body and
assimilated wherever needed. Dissimilation also takes place everywhere
in the body, the digestive secretions being passed into the food vacuoles
and the excretions into the contractile vacuole. This, contracting at
intervals, expels the excretions from the body, thus accomplishing elimina-
tion. After all that is digestible of the food has been removed from it and
absorbed, the undigestible residue, or feces, is permitted to pass out
through the wall of the body, the animal simply flowing away and leaving
it behind. This is egestion.

Respiration takes place everywhere through the body surface, but
the carbon dioxide may also be expelled through the contractile vacuole.
A continuous supply of oxygen is necessary for the life of the animal.
If an excess of food above the immediate needs is taken in, growth
occurs by intussusception and the animal gradually increases in bulk.
Food may not be immediately assimilated and undoubtedly some of the
granules present in an ameba which is well fed represent unassimilated
or stored food. Some of the crystals seen in the animal appear to be
excretions.

96. Locomotion.—Locomotion in the ameba is accomplished by means
of the pseudopodia, which are temporary locomotor structures. Many
theories have been given to account for the manner in which pseudopodia
are formed and the way in which they effect locomotion. Their formation
has been attributed to a lessened surface tension at the points where
they are formed or to an increased surface tension elsewhere. The
freshly formed pseudopodium has been described as adhering to the sub-
stratum, the force of adhesion becoming a maximum at the extreme tip.
However, the most careful and detailed description of locomotion in the
ameba has recently been furnished by Mast. According to him the body
of the moving ameba is divided into four parts: the plasmalemma, which
is very thin and elastic; the clearer ectoplasm; the plasmagel, which is the

66 PROTOZOA

outer motionless part of the endoplasm; and the plasmasol, which is the
central moving portion of the endoplasm. This is shown in Fig. 20, which
represents a simple type with one pseudopodium to which also the follow-
ing description applies. The plasmalemma is quite permanent and is
stationary over that part of the surface in contact with the substratum,
to which it adheres. At the side of the animal away from the advancing
pseudopodium, the plasmalemma is being carried upward and over upon
the upper surface; on this surface it is being carried forward; at the tip of
the advancing pseudopodium it is moving downward; and it is laid down
against the substratum in advance of that portion which is temporarily
stationary and of which it now becomes a part. Through the center of
the animal is a forwardly directed flow of plasmasol, which as it
approaches the advancing tip turns to the side and becomes plasmagel.

Fig. 20.— Diagrammatic representation of a simple ameba, such as Amoeba verrucosa
Ehrenberg, viewed from the side and moving by the formation of a single pseudopodium.
The arrows along the surface show the direction of movement of the plasmalemma, which is
stationary against the substratum from a to b. The arrows within the body show the
direction of flow of the plasmasol from the area of solution at the temporary posterior end to
the area of gelation near the temporary anterior end. The arrow above the figure shows
the direction in which the animal is moving.

The onflow of plasmasol continues, serving to push this tip forward more
and more. At the other side, the plasmagel is continually becoming
plasmasol and thus providing the material for the continued flow. The
forward movement of the ameba is accompanied by a continuous gelation
and decrease in volume of the protoplasm toward the tip of the develop-
ing pseudopodium and a continuous solution and increase in volume of
that at the opposite side. In the case of an ameba with several pseudo-
podia, one or more may be developing in the manner described above,
while others may be disappearing. The latter will exhibit solution toward
the tip and a flow of plasmasol back through the center into the main
body. The elastic plasmalemma will show extension or contraction in
different directions as it becomes adjusted to the changing outline. There
is a curious resemblance in the manner of functioning of the plasmalemma
to the continuous metallic belt which forms the tread in a caterpillar
tractor.

97. Behavior.—An ameba exhibits reactions to various stimuli. To
contact it responds positively if the contact is a gentle one but negatively

AMEBA 67

if it is forceful. When coming quietly in contact with food or with any
indifferent object an ameba tends to increase the amount of body surface
in contact, but when anything touches it at all violently it draws back and
moves away. It avoids a strong light but does not seek darkness, so it
selects an optimum. This reaction may be complicated by the effect of
temperature when it is exposed to the rays of the sun. To chemicals in
solution the response varies with the character of the chemical. To the
normal constituents of the water in which the animal lives it is indifferent;

Contractile
Vacuole

Nucleus

Nuclear Fragments

Pseudopodiospores

Frc. 21.—Fission and sporulation in Amoeba. Fission is illustrated in A to D, which
represent four stages in fission in a European species. (Taken from Doflein, ‘‘ Lehrbuch der
Protozoenkunde,”’ after F. E. Schulze.) E and F show sporulation in Amoeba proteus.
(Also from Doflein, by the courtesy of Gustav Fischer.)
to substances which indicate the presence of food it responds positively,
being thus brought to its food; to substances, however, which are not
normal constituents of the water and which tend, therefore, to injure
the ameba it responds negatively. Amebas have an optimum at a low
temperature. Cold benumbs them as the temperature approaches the
freezing point, while a temperature above 30°C. (86°F.) also retards their
activities.

98. Reproduction.—At intervals amebas reproduce, doing so whenever
they reach the limit of size, which in Amoeba proteus is about 0.25 mm.,
or 0.01 inch.

The ordinary form of reproduction is known as fission, or binary divi-
sion. In this process the body elongates and a constriction appears in the

68 PROTOZOA

middle. The constriction gradually gets deeper and deeper, cuts through
the nucleus, and passes entirely through the body of the animal. The
ameba thus becomes divided into two individuals, each half the size of
the parent and each with a nucleus half the size of that of the parent.
Fission involves a very simple mitosis (Fig. 21 A to D).

Another form of reproduction is that of sporulation. This occurs
only when conditions become unfavorable and is a means of carrying
the animal over to a time when normal conditions are again reestablished.
It occurs, therefore, when the body of water in which an ameba is living
dries up and apparently comes about as a consequence of the increasing
salt concentration of the water. It also occurs when the chemical com-
position of the water changes in any way so as to be unfavorable or when
other environmental conditions threaten the death of the animal. In
sporulation, the pseudopodia are first drawn in, and the animal assumes
a spherical form; a three-layered cyst is then formed about the surface,
which serves for protection (Fig. 21 #). Within the cyst the nucleus
undergoes division into a great many fragments, and the cytoplasm
becomes divided in such a way that a small amount surrounds each
fragment of the nucleus. Thus are formed what are termed pseudopodio-
spores (Fig. 21 F). When the pond is again filled with water or when
normal conditions are restored, the encysted mass takes up water, bursts
its wall, and the liberated pseudopodiospores develop into little amebas.

CHAPTER XVI

PARAMECIUM
A MORE COMPLEX PROTOZOAN

The paramecium may serve as an example of a one-celled animal in
which there is a considerable degree of specialization. Certain parts
of the body are permanently modified for the performance of particular
functions, a process generally known as division of labor.

99. Occurrence.—Paramecia appear in abundance in any water in
which there is a considerable amount of decaying organic matter. They
abound in streams and all bodies of water polluted by the entrance of
sewage, feeding upon the bacteria which swarm in such water, and
frequently appear in ameba cultures in which there is an accumulation
of decaying animal and plant matter.

Since paramecia tend to gather at the surface and especially in contact
with floating objects, they frequently form a white scum. They may,
however, be found throughout the bodies of water in which they live.

100. Structure.—Paramecia (Fig. 22) are elongated in form and are
frequently described as cigar-shaped, the anterior end being blunt and the
posterior more pointed. Because the form is similar to that of a slipper,
the anterior end representing the heel and the posterior the toe, they have
been called slipper animalcules. A groove called the oral groove, or
peristome, starts at the anterior end of the animal and runs obliquely
backward from right to left or left to right, ending a little behind
the middle of the body. If one were to conceive of himself looking at the
paramecium from in front, this could be expressed by saying that the
groove may run in a clockwise direction, which would be the direction
corresponding to the direction of motion of the hands of a clock, or in a
counterclockwise direction, which would be the opposite. In Paramecium
caudatum Ehrenberg the groove usually runs clockwise, but in cultures in
which this is the prevailing type others may be found. Occasionally
a culture will appear in which the majority of the individuals show a coun-
terclockwise direction. The body is covered with fine hairlike czlia,
which in the species referred to are longer at the posterior end. Near
the end of the oral groove is an opening known as the mouth. This
leads into a gullet, which is a cleft running a short distance back into the
cytoplasm.

Over the whole surface of the body is a colorless, elastic membrane
known as the pellicle, or cuticle. ‘This seems to be divided by raised lines

70 PROTOZOA

into a great number of very small hexagonal areas. From the center of
each of these areas arises a cilium. Just below the pellicle is the clear and
relatively firm ectoplasm, often called the cortex, which contains a great
many spindle-shaped cavities placed with their longer axes at right
angles to the surface. These cavities are filled with a semifluid substance
and are known as trichocysts. They open to the outside along the lines
which bound the hexagonal areas. The cilia are described as having an
axial thread continuous with the cortex and a covering continuous with

Ectoplasm and
trichocysts

rs S

Endoplasm
5 Oral groove
S
Macronucleus A gate fssN—Micronucleus
eS e La S:

Mouth

Food vacuo/e <— Gullef
\

Contractile vacuole
Pellicle

Fig. 22.—Paramecium caudatum Ehrenberg. XX 400.

the pellicle. The endoplasm contains food vacuoles, two contractile vacu-
oles, and numerous granular masses various in size, shape, and character.

The contractile vacuoles, one at each end of the body, lie between
the ectoplasm and endoplasm and are made up of a central space sur-
rounded by a system of radiating canals, from 6 to 10 in number. These
radiating canals end blindly in the cytoplasm at the outer end and at the
inner end empty into the central space, which in turn opens to the outside.
After the central space becomes empty, the canals discharge their con-
tents into it and then again gradually fill while the central space is being

PARAMECIUM 71

emptied. The two vacuoles empty alternately at intervals of about 10
to 20 seconds.

Near the center of the animal or somewhat behind it, and not far
from the mouth, is the macronucleus, which has somewhat
the form of a lima bean, and a very much smaller micronu-
cleus. The micronucleus is lodged in a depression on the
surface of the macronucleus at a point which would corre-
spond to the hilum of the bean, the place where the root
comes out in germination.

101. Metabolism.—The food of paramecia consists of
bacteria and minute forms of Protozoa. It is swept into
the oral groove by the action of the cilia, carried back
through the mouth, onward into the gullet, and finally
passed into the endoplasm, where the food vacuole is formed.
The passage through the gullet is effected by means of two |
or three undulating membranes, formed by rows of cilia
placed side by side and fused. ‘These food vacuoles are in
constant circulation around the animal, following a definite
course. Digestion takes place in the food vacuoles as it
does in the ameba, and circulation, assimilation, and dis-
similation are similar. Excretions are accumulated in the
contractile vacuoles and eliminated through them to the
outside. Expiration also takes place into the contractile
vacuoles. Inspiration seems to be possible over the whole
body surface. Egestion occurs at a particular point near
the posterior end, where there is a potential opening
through the ectoplasm known as the anus, or anal spot.

102. Locomotion.—Owing to the presence of the elastic
pellicle, the body of the paramecium exhibits elasticity,
which is not observed in ameba. It can force its way ff
through a narrow passage, the body contracting as it does {
so, but on its exit from the passage it immediately A
assumes its normal shape.

Locomotion is effected by means of the cilia, which
may beat either forward or backward and by means of
which the animal swims in either direction through the
water. Normally it moves forward. The cilia, however,
do not beat directly backward but obliquely, so that the
animal rotates on its long axis. The cilia in the oral groove
also strike obliquely along the axis of the groove and

@
Fic. 23.—The spiral path followed by a swimming paramecium. Fia. 23.
(Modified from Jennings, ‘‘ Behavior of the Lower Organisms,”’ by the
courtesy of Columbia University Press.) The rotation of the animal on its axis is indicated
by the position of the oral groove. The large arrow shows the direction of motion, and
the axis of the spiral.

72 PROTOZOA

produce a swerving. As a result of the combination of progression, rota-
tion, and swerving, the animal follows a spiral path, the axis of the
spiral being the direction of motion. This axis may be a straight line
(Fig. 23). Thus the paramecium, which is asymmetrical, is enabled to
pursue a direct course though it is unable to swim directly forward or
backward.

103. Behavior.—Paramecia react to various stimuli in a manner
somewhat similar to amebas, though apparently they are not affected

Fic. 24.—Diagram to illustrate the avoiding reaction given by a paramecium to a
solid object. (Modified from Jennings, ‘‘ Behavior of the Lower Organisms,” by the courtesy of
Columbia University Press.) The arrows between the successive positions represented indi-
cate the path followed, and those drawn within the outlines of the animal the direction in
which the animal is moving. The direction of movement is also indicated by the position
of the cilia; no attempt has been made to show rotation by any other structural features.

by ordinary light. Their positive thigmotropism is seen in the tendency
to come to rest against objects in the water. It is stated that for each
chemical to which paramecia apparently give a positive reaction there
isan optimum concentration in the region of which they tend to accumulate.
The paramecium possesses a temperature optimum at about 26°C. (79°F.),
or about the temperature of ordinary pond water in summer, when para-
mecia become most abundant. The animal possesses positive rheo-
tropism, which leads it to swim against a current, or upstream.
Paramecia also seem to respond negatively to gravity, tending to come to
the top of the water in a vessel in which they are confined. ‘The electric
current affects the beating of the cilia, but this is not a natural stimulus,
occurring only in the laboratory.

One mode of response of the paramecium is known as the ‘‘avoiding
reaction, ’’ which takes place in the following manner: When a paramecium

PARAMECIUM 73

comes in contact with a solid object, assuming it to be swimming at full
speed forward, the force of the contact produces a negative response.
The animal backs up, pivots upon its posterior end, and swims forward
again. If it again strikes the object, it exhibits the same negative reac-
tion, which is repeated until on swimming forward the animal passes the
obstacle (Fig. 24). Though it is called an avoiding reaction, which
would imply a consciousness on the part of the animal, all that it is doing
is simply giving a series of negative
responses. The term ‘‘avoiding reac-
tion” thus gives rise to implications
which are unwarranted, a fact usually
indicated by inclosing the words in

ye
SS

Fic. 26.

Fic. 25.—A paramecium shown pivoting upon its posterior end and sampling the water
before starting off in a direction; which may be determined by the result of such sampling.
(From Jennings, ‘‘ Behavior of the Lower Organisms,” by the courtesy of Columbia University
Press.) The figures indicate successive positions and the arrows show the direction of
movement.

Fic. 26.—Diagram illustrating the method of trial anderrorinparamecium. (Modified
from Borradaile and Potts, ‘‘ The Invertebrata,” after Ktihn, by the courtesy of The Macmillan
Company.) At the center is shown a soluble substance (C’) which is dissolving and diffusing
into the surrounding water. The circles represent zones of equal concentration, the zone
O, in which protozoans, which might be paramecia, are gathered, being the zone of optimum
concentration. The irregular line shows the path of a paramecium which enters the area
involved and after repeated stopping, sampling, and change in direction, comes to rest in
the zone of optimum concentration at x.

quotation marks. A slowly moving organism like an ameba would not
exhibit such a response.

Paramecia also show what is known as the method of “trial and
error,”’ which implies a series of experiments on the part of the animal.
A paramecium is constantly taking water into its oral groove with sufh-
cient force to draw it from a little distance in front and to produce
a cone of movement in the water. Thus it “samples” the water just
ahead (Fig. 25). If the water is too hot or too cold or if it contains an
injurious chemical substance, the animal gives an avoiding reaction.
This may be repeated again and again. While paramecia seem to be
swimming aimlessly through the water in all directions, the repetition
of these avoiding reactions sooner or later brings them into that part
of their environment which is most favorable (Fig. 26). The result

74 PROTOZOA

is the same as that seen in the ‘trial-and-error’ mode of behavior of
higher animals, and so this term is often applied to the activity of this and
similar Protozoa.

Whenever a paramecium is responding to one stimulus it often will
-not be affected by another stimulating agent unless the second one is
very strong. It has been found, however, that the response to gravity
is always set aside whenever the animal receives any other stimulus.

The responses which paramecia give to stimuli are not always the
same, the difference being due to the different physiological states of the
animals. A paramecium which is fully fed tends to come to the surface
and remain quiet in contact with some object, though the sources of its
food supply may be, in a laboratory culture, at the bottom of the jar.
When it becomes hungry, however, it reverses its responses, swims to the
bottom, secures its food, and then once more seeks the surface. Thus one
physiological state gradually becomes changed into another, and a definite
rhythm is established in the animal’s movements. It has been found that
a response becomes more pronounced after it has taken place a number of
times. This indicates a change in physiological state and shows an effect
of one response upon succeeding ones which has been termed summation of
stimult.

When violently stimulated either by a chemical or by contact, a
paramecium frequently responds by throwing out the contents of the
trichocysts, which harden and form a barrier of fine threads. When
the trichocysts are emptied, they are refilled by material which originates
near the nucleus, probably from it, and passes through the endoplasm to
the proper points in the ectoplasm.

104. Reproduction.—Paramecia reproduce only by fisston, the animal
being divided transversely into two. During this process both the
macronucleus and the micronucleus divide, the old gullet divides into
two, and two new contractile vacuoles arise by division of the old ones.
The micronucleus divides mitotically, and perhaps the macronucleus does
also. The entire process occupies from half an hour to two hours. Sub-
sequent growth is rapid, and division occurs again after a number of hours.
Paramecia multiply with great rapidity. It has been estimated that from
one ancestor could be produced in one month, if all survived, a total of
265,000,000 individuals.

105. Conjugation.—At intervals occurs a phenomenon known as con-
jugation (Fig. 27). When this occurs, two paramecia come together,
attached by the surfaces on which the oral grooves are located. The
pellicle breaks down at the point of contact, as does also the ectoplasm,
and an endoplasmic bridge is formed between the two animals. During
this time the micronucleus of each conjugant moves from the concavity
in the macronucleus, where it has been lodged, grows larger, forms a
spindle, and divides. A second division follows immediately. Of the

PARAMECIUM 75

four micronuclei thus produced three degenerate and disappear, and
the fourth divides again, this time unequally. The smaller of the two
micronuclei of each animal now moves over the protoplasmic bridge into

1. Two paramecia come together

2. Micronucleus divides. Macronucleus begins to dis-
integrate

3. Micronuclei divide and three of four disappear

4. Remaining micronucleus divides unequally

5. Smaller micronucleus crosses into other animal

6. Each animal with its own larger micronucleus and
smaller one from other animal

7. Two micronuclei fuse

8. Two animals separate. Each exconjugant with
fusion micronucleus

This animal

hot followed LED , : ‘4
further; same 9. Fusion micronucleus divides
as other

10. Two micronuclei divide

11. Four micronuclei divide

12. Four micronuclei become macronuclei, three dis-
appear, one remains

Kee. <-0e 13. Micronucleus divides and animal divides
14. In each of two micro-
ce: > Ce D> ae aE ES ES nucleus divides and ani-
mals divide again

Fig. 27.—Diagram illustrating conjugation in Paramecium caudatum Ehrenberg.
(Slightly modified from Jennings, ‘Life and Death, Heredity and Evolution in Unicellular
Organisms,” by permission.) Macronuclei are indicated by large black bodies, micronuclei
by the smaller ones, and those micronuclei which disappear by small circles.
the other animal, passing the micronucleus from the other as it does so.
Each of these micronuclei fuses with the larger micronucleus of the other
animal, forming a fusion micronucleus. This process has been compared

76 PROTOZOA

to fertilization in higher animals which possess sex, the smaller micronu-
cleus being viewed as male and the larger one as female. However, it
cannot, properly speaking, be called fertilization, since no gametes are
involved. The macronucleus begins to degenerate soon after the micro-
nucleus leaves it, breaking up into fragments.

The two animals now separate and the fragments of the macronuclei
slowly disappear, their substance being dissolved in the endoplasm.
The fusion micronucleus in each of the exconjugants divides by mitosis
into two, these into four, and these into eight, all equal in size. Four of

1. Animal before endomixis begins

2. Micronuclei divide. Macronucleus disintegrates

3. Micronuclei again divide
4. Six out of eight micronuclei disappear. Animal divides
Ge, > 5. Each animal with one micronucleus

6. Micronucleus of each divides

he
<s2>
a <a,
CS FD <tt> 7. Micronuclei divide again.
SEEDS ae,

8. Two micronuclei in each animal become
macronuclei

9. Micronuclei divide and animals divide
E> Cis <A) Ga 10. Four ordinary individuals

Fic. 28,—Diagram illustrating endomixis in Paramecium aurelia Miller. (From Wood-
ruff, ‘‘ Animal Biology,” by the courtesy of The Macmillan Company.) Large black bodies
are macronuclei, small ones micronuclei, and small circles micronuclei which disappear.

these then become larger and develop into macronuclei. Of the four
remaining, three disappear. The fourth divides into two, and the animal
divides, each of the two individuals formed having one micronucleus and
two macronuclei. In each individual the micronucleus again divides and
this is followed by fission, producing four animals each with one micro-
nucleus and one macronucleus. This description applies to Paramecium
caudatum; the steps in the process are variously modified in other species.

The significance of conjugation is uncertain. Some investigators
believe that after a long series of fissions the animals become senescent
and conjugation serves as a process of rejuvenation which restores their
vitality. Woodruff, however, has succeeded in maintaining a culture
since May 1, 1907, without conjugation. In the thirty-second year,

PARAMECIUM ree

November 1, 1939, the culture had attained the number of about 20,000
generations. The significance may be in the determining of inheritance.
The scrapping of the old macronucleus and the development of a new one
from the fusion micronucleus suggest the need of harmonious direction
of activities in an animal differing from the conjugant parent by nuclear
material received from the other conjugant.

106. Endomixis.—An interesting phenomenon analogous to conjuga-
tion, but taking place within a single individual, has been observed
in Paramecium aurelia Miller (Fig. 28). This species possesses two
micronuclei, exhibits a definite rhythm in the rate of division, and periodi-
cally undergoes what has been called endomixis. During the process
the macronucleus breaks down and disappears and the micronuclei
undergo two divisions, producing altogether eight micronuclei. Six
of these disintegrate and disappear. The paramecium then divides and
each of the offspring receives one micronucleus. This micronucleus
divides into two and these divide again, producing four. Two of these
develop into macronuclei and two remain micronuclei. The mic-
ronuclei divide again and the entire animal divides, resulting in
two, each with two micronuclei and one macronucleus. Four individuals
have thus been produced. This process also occurs in Paramecium
caudatum and in other forms. The result of endomixis may be the same
as that of conjugation.

CHAPTER XVII

PROTOZOA IN GENERAL

The phylum Protozoa (pro td 20’ a; G., protos, first, and zoon, animal)!
includes all one-celled animals, the one cell which forms the body of the
individual carrying on in simple fashion all of the general functions which
are performed by the many-celled bodies of higher animals. This means
that though the animals included in Protozoa are simple in that they are
composed of only one cell, this cell is physiologically complex. Some of
the Protozoa always exist as single cells. Others are associated in colonies
in which they are all alike and each quite independent. In other proto-
zoan colonies, however, certain functions, such as reproduction, are
assumed by certain cells, which thus become reproductive individuals.

107. Classification.—Protozoa is the first phylum of the animal
kingdom, but, since all other phyla have characteristics which they share
and which distinguish them from Protozoa, it may be considered also
as a group of higher rank than a phylum. In this case it becomes a
subkingdom, with the same name, coordinate with the subkingdom
Metazoa, which includes all the remaining phyla.

Protozoa may be characterized as composed of animals existing as
single cells. In the case of certain types these one-celled individuals are
associated in colonies.

The phylum is usually divided into four classes, each characterized
by a distinctive locomotor structure or, in one class, by the absence of
any such structure in the final stage of the animal. These classes are:

1. Mastigophora (mis ti gdf’dra;G., mastix, whip, and phoros,
bearing), or Flagellata (fli gél 1a’ ta)—Have a limited number of long
whiplike locomotor appendages known as flagella.

2. Sarcodina (sir ko di’na; G., sarkodes, fleshy), or Rhizopoda
(ri z5p’d da; G., rhiza, root, and podos, foot).—Form pseudopodia,
which are temporary structures developed from the surface of the body
and which can be withdrawn.

3. Sporozoa (spo ro 20’ a; G., spora, seed, and zoon, animal).— Possess
no locomotor structures in the final stage, though they have them in the
earlier stages of their life histories.

1 The vowel sounds indicated in the pronunciation of this and other phyla and
class names are described at the beginning of the Glossary (p. 585). In all cases
where the nominative form of a Latin or Greek word does not contain the full root,
the genitive is given, as, for instance, podos, genitive, instead of pous, nominative,
for the Greek word for foot. If a word comes from the Greek through the Latin,
the Greek is given.

PROTOZOA IN GENERAL 79

4. Infusoria (in fi sd’ ria; L., infusus, poured into, crowded), or
Ciliata (sili 4’ ta).—Have a very large number of permanent small hair-
iike appendages known as cilia.

By some authorities a fifth class is recognized which is called Suctoria.
In the classification adopted here this is considered a subclass of the
Infusoria.

108. Mastigophora.—A type of this class is the euglena (Fig. 29),
a small greenish protozoan living in bodies of fresh water. This animal
agrees with the paramecium in possessing, in addition to the ectoplasm
and endoplasm, an elastic cuticle, which is
striated. On the anterior end is a single
long slender flagellum connected with a
granule within the body known as a bleph-
aroplast. This term is applied to any
granule in a cell with which a cilium or
flagellum is connected. The mouth is at the
base of the flagellum. A permanent vesicle,
the reservoir, into which several contractile Bre.
vacuoles pour their contents, opens into the ods. 40 Chrormarophore
gullet. Close to the reservoir is a mass of Rueae
red coloring matter called the stigma, or
eyespot. It is, of course, not an organ of
sight, though it is thought to be sensitive
to light. Near the center of the body is a
nucleus, and scattered through the protoplasm
are many bodies of bright green color called

Flagellurm

*- Contractile
vacuole

chromatophores. . ‘
Euglena is a type which possesses some ‘
of the characteristics of plants. Other Fig. 29.—Euglena viridis

Ehrenberg. (Slightl
members of this class show these to such a oe ee SY rege ce

degree that they are by botanists considered Protozoenkunde,” by the courtesy
plants and classified by them as_ such. peal ae eee) 180:
A plantlike characteristic is the ability, by means of chlorophyll in the
chromatophores, to manufacture part of its own food. This type of
nutrition is known as holophytic, in contrast with the type which char-
acterizes animals generally, which involves the ingestion of solid particles
of organic food, and which is called holozoic. This resemblance to plants
justifies their being considered the first class of the Protozoa. Many
zoologists place Sarcodina first, believing them the simplest structurally.
Euglena illustrates particularly well a reaction to an optimum stimu-
lus. When placed in a vessel, one end of which is lighted and the other
darkened, the animals gather neither at the light nor at the dark end
but in a zone between the two where the optimum of light for this animal
is found.

80 PROTOZOA

The Mastigophora (Fig. 30) are divided into two groups: (1) Those
which are animal-like and which may be holozoic, saprophytic, or ento-
zoic. Saprophytic implies the absorption of nonliving organic matter in
solution directly through the surface of the body. Hntozoic means living
within the bodies of other animals. Examples are flagellates which
live in the intestinal tract or blood stream of man or in the intestines of

Fic. 30.—Several different species of Mastigophora. A, Proterospongia haeckeli Kent.
(From Kent, ‘“A Manual of the Infusoria.’”’) XX 530. B, Giardia lamblia Stiles. (After
Wenyon, in Archiv fiir Protistenkunde, Suppl. 1.) X 2200. C, Trypanosoma gambiense
Dutton. (From Wenyon, ‘‘Protozoology,”’ by the courtesy of William Wood & Company.)
* 1330. D, Noctiluca scintillans (Macartney). (From Kent.) X 40. E, Volvox
aureus Ehrenberg. (From Doflein, ‘‘Lehrbuch der Protozoenkunde,”’ after Klein, by the
courtesy of Gustav Fischer.) X 110. A colony containing six daughter colonies, developed
from parthenogonidia. F, Uroglenopsis americana (Calkins). X 350.

insects. (2) Those which are more plantlike and which may be holo-
phytic, saprophytic, or entozoic.

An interesting form is the genus Proterospongia, which is a colony of
individuals each bearing a flagellum and around it a protoplasmic collar.
Another form, known as Giardia, the structure of which is quite complex,
lives in the small intestine of man. These are both animal-like.

PROTOZOA IN GENERAL 81

Among the plantlike types are several of interest, including Uroglena,
found in reservoirs and imparting a peculiar oily odor and fishy taste to
the water. Another form is Volvozx, a very beautiful colonial animal—
or plant—which lives in fresh water and which may consist of many
thousands of cells. As it swims the spherical colony revolves, the motion

Fic. 31.—Different types of Sarcodina. A, Rotalia freyert. (From Doflein, “* Lehrbuch
der Protozoenkunde,” after Max Schultze, by the courtesy of Gustav Fischer.) An example
of the Foraminifera. B, Difflugia urceolata Carter. (From Leidy, ‘‘Fresh-water Rhizopods
of North America.) 167. The shell is composed of sand grains. C, Actinosphaerium
eichhorni Ehrenberg. (From Kudo, ‘‘ Handbook of Protozoology,”’ by permission of the
publisher, Charles C. Thomas.) X40. One of the Heliozoa. D, Heliosphaera inermis
Haeckel. (From Bronn, ‘‘ Klassen und Ordnungen des Tierreichs,” after Haeckel.) X 350.
One of the Radiolaria. The skeleton forms a lattice work on the surface of the body.

being due to the combined action of all the flagella. Still another example
is a marine form known as Noctiluca. This animal frequently collects
on the surface of the sea in enormous numbers, the jelly-like bodies form-
ing a thick scum which has the color and appearance of thick cream-of-
tomato soup and which sometimes covers an area of many acres. At

82 PROTOZOA

night, and when stimulated, these bodies are luminescent, giving to the
water a pervading greenish white or bluish white light.

109. Sarcodina.—Sarcodina (Fig. 31) include not only the ameba but
also many other similar forms, some of which are parasitic. A number
of them secrete an external shell of chitin, cellulose, lime, or silica, or

Paras/tes

Nucleus of
corpuscle

Parasites

Fic. 32.—Examples of Sporozoa. A, a hemogregarine in the red blood corpuscle of a
frog. (From Hegner and Taliaferro, ‘‘ Human Protozoology.”’) 550. B, section through
the intestinal epithelium of a rabbit, showing infection with one of the Coccidia, Himeria
stiedae (Lindemann). (From Doflein, ‘‘Lehrbuch der Protozoenkunde,”’ after Thoma.)
Highly magnified. C, Gregarina blattarum Siebold, from the digestive tract of the cock-
roach. (From Doflein, after Hertwig.) X60. Shows an endwise union of two individuals,
a union which occurs commonly and is known as syzygy. D, section through the intes-
tinal wall of a meal worm (the larva of a beetle), infected with Gregarina polymorpha (Ham-
merschmidt). (Also from Doflein, after Pfeiffer.) Highly magnified. Immature parasites
in different stages of development are seen in the epithelium lining the intestine and one
mature individual in the lumen of the canal. (A by the courtesy of The Macmillan Company;
B, C, and D by that of Gustav Fischer.)

they build one of particles of sand and other foreign objects held together
by one of these substances.

Interesting members of this class are the Foraminifera, which are
mostly marine and which form shells of lime composed of numerous
chambers united by openings called foramina, whence the name of the
group. They occur in enormous numbers and exhibit great variety.
When the shells of dead individuals sink to the sea bottom they form a
soft mud or ooze, known as foraminiferous or Globigerina ooze, which,
when solidified, becomes natural chalk.

PROTOZOA IN GENERAL 83

Another group is Radiolaria. These have a central perforated cap-
sule of chitin and a larger inclosing shell of silica. They also are marine,
existing in great numbers in the ocean, and when their shells sink to the
bottom they form what is known as radiolarian ooze. When solidified
this produces a rock of the nature of flint. These rocks occur in strata
several hundreds of feet in thickness.

Another group of Sarecodina found in fresh water have numerous
slender, radiating pseudopodia containing axial threads of chitin.
Because of the resemblance of the animal with these radiating pseudo-
podia to the sun surrounded by its rays of light, they are frequently
termed sun animalcules, and the order to which they belong is called
Heliozoa.

110. Sporozoa.—The Sporozoa (Fig. 32) in their early stages fre-
quently are ameboid but in their final stages they lack locomotor organs
and form spores. They are parasitic in other animals and are generally
transmitted to the host in the spore form. In some cases the life of the
individual ends upon the formation of spores but in other cases spores
are produced at intervals during the animal’s lifetime.

Among these forms are the gregarines, which exist within the cells of
earthworms, cockroaches, and other insects as well as of other inverte-
brates and which in their later stages become free in the body cavities of
these animals. Those of an order known as Coccidia are found in the
liver and intestine of man and other vertebrates as well as in some inverte-
brates. Others are found in the blood or muscles of vertebrates or within
the cells of fish. One form produces the silkworm disease known as
pébrine. Pasteur discovered that this parasite is transmitted from the
silkworm moth to the eggs before they are laid and that the caterpillars
hatched from these eggs thus become infected. By showing how infec-
tion can be avoided he saved the silkworm industry of France at a time
when its existence was seriously threatened.

111. Infusoria.—Infusoria (Fig.33) occurin both fresh and salt waters,
while others are found parasitic in the bodies of higher animals. Para-
mecium is an infusorian. Opalina is a form which lives in the intestine of
the frog. Inaddition to cilia, infusoria frequently possess undulating mem-
branes or cirri, formed by the fusion of numerous cilia. The body may
be covered all over with cilia of approximately equal length, or it may
have the cilia distributed over certain portions. The cilia are upon the
ventral surface in a form known as Stylonychia and in a circle around the
blunt end of the trumpet- or bell-shaped body in the forms known as
Stentor and Vorticella. These cilia are sometimes varied in size and
shape in the different parts of the body. Several types of Infusoria
form branching, treelike colonies.

The Suctoria are attached animals the cilia of which are modified in
such a way as to make tentacles of them. These have sucking discs at

84 PROTOZOA

the tip by means of which the suctorian captures other protozoans and
passes them back to the mouth to be taken into the body.

112. General Facts.— Protozoa vary in size from minute blood para-
sites which are barely visible to the highest powers of the microscope to a
gregarine, Porospora, which lives in the alimentary canal of the lobster
and which may be 17 mm., or 24 inch, in length. Most of them are not

Nuc/eus

Nucleus

Fig. 33.—Some forms of Infusoria. A, a species of Podophyra. (After Biitschli,
“* Klassen und Ordnungen das Tierreichs.’’) To illustrate the Suctoria. Highly magnified.
B, Opalina ranarum Purkinje. (From Kent, ‘‘A Manual of the Infusoria,”’ after Zeller.)
An infusorian parasite of frogs. 80. C, a species of Vorticella. (Modified from
Hegner, ‘‘ College Zoology,’ after Shipley and Macbride.) Showing a portion of the attach-
ment stalk coiled. Highly magnified. D, Stentor polymorphus Miller. (From Kent.)
x 60. Attached individual. EF, Balantidiuwm coli Malmsten. (From Thomson and
Robertson, ‘‘Protozoology.”) X 400. F, Stylonychia mytilus Ehrenberg. (From Kent.)
x 100. (C by the courtesy of Fhe Macmillan Company; E by that of William Wood &
Company.)

visible to the unaided eye. The shapes of Protozoa are also exceedingly
varied.

The cytoplasm of protozoans usually appears alveolar and is usually
divided into ectoplasm and endoplasm. Sometimes the nucleus is scat-
tered throughout the cell in small portions known as chromidia, when it is
called a distributed nucleus. We have already noted that in certain
cases there are two kinds of nuclei, the macronucleus and the micronu-

PROTOZOA IN GENERAL 85

cleus. The former is believed to preside over the nutritive functions of
the cell, the latter is active in cell division and transmits hereditary
characters. This significance of the two seems borne out by the fact
that after conjugation the old macronucleus disappears and a new one is
formed from a micronucleus, which insures agreement in the hereditary

Adoral membranel/le

Neuromotor
center

Mouth
opening
Oral cilia
Dorsal \
membranelle Adoral
rmernrbrarne/le
Contractile
vacuole
Macronucleus
Skeletal lamina
Micronucleus
Ectoplasm Endoplasim
Contractile
yacuole
“caecum”
Cuticle
Rectum”
“nus”

Fic. 34.—Diplodinium ecaudatum Fiorentini. (After Sharp, in Univ. Calif. Pub.
Zool., vol. 13, and by the courtesy of University of California Press.) An infusorian found in
the stomachs of cattle, to illustrate the extreme of intracellular differentiation as exhibited
by protozoans. Somewhat diagrammatic, and a composite based upon the study of actual
longitudinal microscopical sections of preserved animals. X 750. The black ring around
the esophagus, the connection from it to the neuromotor center, and the solid black areas
at the bases of the membranelles form the neuromotor apparatus.
character of the two nuclei. In many cases chromosome formation has
been observed in the division of Protozoa, but in other cases it has not.

Some Protozoa, both flagellates and infusorians, show great special-
ization within the cell, and parts called cell organs, or organelles, are set
aside for certain functions. This phenomenon is called in general,
differentiation, and, since it occurs here within the cell, it is termed
intracellular differentiation. Such parts are contractile strands of proto-
plasm called myonemes, which correspond to muscles in higher animals;

conducting strands and coordinating centers, which perform the func-

86 PROTOZOA

tions of a nervous system; sensitive areas, which function as sense organs;
and supporting parts, which form a sort of skeleton (Fig. 34).

The food of protozoans consists of organic matter, both vegetable
and animal, living and dead. Their metabolism is, in general, similar
to that described for the ameba or the paramecium. Because of the
size of the animals the study of digestive enzymes is difficult, and there
is little precise knowledge. Protozoans certainly digest proteins, have
been shown to be able to use emulsified fats, and also are able to use
certain starches.

In addition to fission, or binary division, and sporulation, protozoans
sometimes exhibit a third type of asexual reproduction known as gemma-
tion, or budding. In this case individuals of smaller size than the parent
grow out from it like buds and when developed break loose, later growing
to the same size as the individual which produced them (Fig. 35).

Contractile
vacuole

Nucleus

Fic. 35.—Gemmation, or budding, in Ephelota gemmipara (Hertwig). (After Hertwig,
in Morphologisches Jahrbuch, vol. 1.) A, organism on stalk, showing two types of ten-
tacles, suctorial and prehensile, the latter with spiral ridges on the surfaces. B, an indi-
vidual showing the formation of buds, into each of which extends a portion of the nucleus.
These buds become detached and free-swimming; they possess cilia on one side but later
develop tentacles and become attached. X 120.

Bacteria, although they are one-celled organisms, are not protozoa.
They are considered as more plantlike than animal-like. Bacteria differ
from protozoa chiefly in lacking a definite nucleus, the chromatin mate-
rial being distributed throughout the cell, and in possessing cellulose in
their cell wall. They are usually very minute and may be spherical,
rodlike, or spiral in shape. Many bacteria are free-living, many are
parasitic, some are pathogenic (disease producing). Bacteriology is the
science dealing with bacteria.

113. Sexual Reproduction in Protozoa.—Some colonial protozoans
exhibit a simple form of sexual reproduction. The animals in the colony
become divided into two types: the ordinary ones, known as nutritive
individuals, or nutritive zooids, which reproduce by fission in the ordinary
way; and a second type which is represented by reproductive individuals
or gametes. These gametes exist in two forms: the larger macrogametes,
which, like egg cells, are usually not active; and the smaller microgam-
etes, which, like the sperm cells of higher animals, are active. When

PROTOZOA IN GENERAL 87

these two types of sex cells unite, a zygote is formed from which a new
colony may arise. The macrogametes may also show a type of sexual
reproduction without fertilization. When this occurs, they remain
within the colony, increase in size, divide into many cells, and finally
escape to form new colonies. These groups of cells are known as parthe-
nogonidia (Fig. 30 EZ). In many of the Sporozoa there are both sexual
and asexual generations. The zygotes produce a number of spores which
develop into sporozoites. These become nutritive individuals, or tro-
phozoites, and these in turn may form another generation of gametes.

CHAPTER XVIII
PROTOZOA AND DISEASE

Protozoa which when living in the bodies of other animals are capable
of producing disease in those animals are termed pathogenic. Many such
protozoa are known.

114. Pathogenic Protozoa.—Among the Mastigophora are the try-
panosomes (Fig. 30 C). Certain of these, found in parts of tropical
Africa, produce a disease known as trypanosomiasis, or, because it is
characterized by a loss of consciousness, sleeping sickness. These try-
panosomes are transmitted from one person to another by the so-called
tsetse flies. The sleeping sickness of Africa should not be confused
with a disease in this country which exhibits similar symptoms and
which sometimes goes by the same name; in the latter case, the loss of
consciousness is not caused by an animal parasite but is due to congestion
in the blood vessels of the brain.

Among the Infusoria are forms belonging to the genus Balantidium
(Fig. 33 F), which cause a type of dysentery known as balantidial dysen-
tery. ‘This is most common in the tropics.

Among the Sarcodina, one ameba, Hndamoeba histolytica, causes a
serious and often fatal form of dysentery known as amebic dysentery.
Very minute cysts pass from the body and may contaminate food, drink-
ing water, or towels. Infection is brought about by ingesting these cysts.
This parasite occurs in all parts of the United States but is more com-
mon in the tropics or where sanitation is poor. Several other kinds of
amebae live in the intestine of man and one in the mouth, but these
are all considered to be nonpathogenic.

There are many species of parasitic Sporozoa. Among these is
the malarial-fever parasite, which is pathogenic to man. The life
history of this organism will be given in detail to illustrate the life eycle
of a pathogenic protozoan, though it is more complex than that of many
other types (Fig. 36).

The spirochaetes, which cause syphilis and other diseases, are by
some authorities considered as belonging to the Protozoa, while others
consider them intermediate between the Protozoa and the Bacteria and
more closely related to the latter.

115. Malarial Parasite.—The malarial parasite may exist in the blood
of man, where it undergoes a series of asexual generations which may
continue for many years and even through the lifetime of the person.

PROTOZOA AND DISEASE 89

The individual parasite lives in a red blood corpuscle, into which it enters
while in the spore stage. Then it changes to a form resembling a minute
ameba. It feeds upon the contents of the corpuscle and when full grown
nearly fills it. The parasite then sporulates. The rupture of the cyst
formed in sporulation, accompanied by the rupture of the wall of the
corpuscle, liberates numerous spores into the fluid of the blood. These
enter other corpuscles and pass through a similar life history. The setting
free of spores from many infected corpuscles corresponding to the starting

nt in red blood Corp,

ne pry /
rl AD)
vA 2

iN Asexual
i} cycle
17 in man

Sexual cycle
in mosquito

Microgalmetes '

FE “Oo kinete

Fic. 36.—Diagram of the life cycles of the malarial parasite of the tertian type, show-
ing the asexual cycle in man and the sexual cycle in an anopheline mosquito. Stages 1 to 6
show the entrance of the spores into a red blood corpuscle and the growth and sporulation
of the parasite. Stage 7,the production of gametocytes. Stage 8, their transformation into
gametes. Stages 9 and 10, fertilization and the zygote. Stages 11 and 12, the change of the
zygote to an active form, the ookinete, which penetrates the wall of the stomach and encysts.
Stage 13, forming an oocyst below the outer layer of the stomach wall. Stages 14 and 15,
the development of several sporoblasts in the oocyst, the development from each of many
spores, and their dispersal into the body cavities. And stage 16, the entrance of these spores
into the salivary gland. They are introduced with the saliva into a human being, stage 17,
enter red blood corpuscles, and another cycle is begun.

of a new generation, is accompanied by the liberation of poisons in the
blood which cause an attack of chills and fever. The time between these
attacks, therefore, indicates the period between generations of the
parasite. These intervals are 24, 48, or 72 hours, corresponding to three

90 PROTOZOA

forms of the disease known respectively as pernicious, tertian, and quartan
malaria.

In addition to these spores, there are also produced within the red
corpuscles spores which become sexual in character and by means of which
the sexual cycle of the parasite may be initiated. This sexual cycle,
however, does not occur in the body of man but must take place in a
mosquito. If these sexual forms, which are the microgametocytes and
macrogametocytes, do not enter the body of a mosquito, they do not
further develop.

The mosquito which is capable of transmitting the malarial parasite
belongs to the genus Anopheles. It is distinguishable from the common
mosquitoes, which belong to Culex, by the fact that it holds its body at an
angle to the surface on which it rests. The body of a culicid mosquito
is held parallel to such a surface. When the mosquito bites, it drills a
hole through the epidermis with its proboscis and penetrates the vascular
dermis. Then it injects into the wound saliva the effect of which is to
prevent coagulation of the blood and thus permit the mosquito to suck
until filled. It is the irritation caused by the saliva that produces the
itching which is so often a feature of these bites.

If in the blood sucked up by the mosquito there are only ordinary
spores, the mosquito does not become infected and is not capable of
transmitting the infection. If, however, there are microgametocytes and
macrogametocytes, these give rise in the stomach of the mosquito,
respectively, to microgametes and macrogametes, which unite to form
zygotes. These zygotes become elongated, exhibit a gliding movement,
penetrate the wall of the stomach, and encyst just beneath the outer
layer. In these cysts are produced a great many spores, which, when
they are set free, make their way through the body of the infected mos-
quito to the salivary gland, in the cavity of which they accumulate.
When this mosquito bites another person these spores are injected into the
wound made by the proboscis, along with the saliva. In the blood
they enter the red blood corpuscles, become ameboid, and thus another
asexual cycle is begun.

It is evident from this outline of the life cycle that after biting a
malarial individual and acquiring the infection, the mosquito cannot at
once transmit the disease. It is necessary for such a transmission that
there shall be sexual spores in the blood of the person bitten and that
they shall be taken up by the mosquito. A sufficient length of time must
also elapse for the sexual cycle to be completed and for spores to form from
the zygote. This takes, on the average, about twelve days, though the
time varies with the form of the disease and environmental conditions,
such as temperature.

In the absence of man the female mosquito feeds on the blood of other
animals or upon the juices of plants. The male mosquito does not bite,

PROTOZOA AND DISEASE 91

and therefore cannot become infected and transmit the disease. The
sexes may be distinguished by their antennae; those of the male are
complexly branched and have a feathery appearance, while those of the
female are simple, straight, and hairlike.

The malarial parasite belongs to the genus Plasmodium. Three
species are recognized, corresponding to the three forms of the disease.
Plasmodium vivax (Grassi and Feletti), producing tertian malaria,
is the common one in the United States.

The malarial-fever parasite was discovered in Algeria in 1880 by a
French army doctor, Laveran, who found it in the blood of patients suffer-
ing from malaria. In 1883 a Dr. King of Washington, D. C., presented
evidence to show the transmission of the parasite by the mosquito; and
this transmission was demonstrated experimentally by Sir Ronald Ross,
an Englishman, in 1898. An Italian, Grassi, and his pupils worked out
the complete life cycle. Previous to these discoveries it was generally
believed that the disease could be acquired by the breathing of miasma
rising from swamps and marshes, and the name, meaning literally bad air,
was given because of this superstition. Owing to the work of the investi-
gators named and others, it is now a fact of common knowledge that
malaria can be conveyed to a person only through the bite of an infected
mosquito of the proper kind.

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PART IIT
METAZOA IN GENERAL

CHAPTER XIX

METAZOA

All animals which are not protozoans are included in one subkingdom
known as Metazoa. All such animals are similar in that they have a
many-celled body in which the cells are not all alike but are varied in
structure and function, the activities of the whole being the result of their
cooperative efforts.

116. Differentiation—The modification of certain parts for the
performance of corresponding functions is known as differentiation. In
the Protozoa there has been seen intracellular differentiation (Sec. 112), as a
result of which one particular structure within the cell comes to perform
one function and the other structures other functions. In the highest
of the Protozoa this results in an exceedingly complex cell. In the
Metazoa, however, complexity does not result from the complexity of
the individual cells which make up the animal but from differences
between them. This ¢ntercellular differentiation results in a great variety
of cells within one body. Differentiation which is concerned with struc-
ture alone is morphological differentiation. Accompanying this, however,
is differentiation in function, which results in cells of different structures
having different functions, appropriate in each case to the structure.
This may be termed physiological differentiation. All differentiation is
based upon modification in the metabolic activities of the cells. As a
result of differentiation the cells in the metazoan body become inter-
dependent, in contrast to the independence which exists between
protozoan cells, even in colonies. This interdependence, however, is fore-
casted in the case of certain of the colonial Protozoa such as the volvox,
in which protoplasmic bridges extend from cell to celland in which certain
reproductive cells are differentiated. It thus appears thst the line of
demarcation between the Metazoa and the Protozoa is not so sharp as
might be supposed. It may, however, be drawn on the basis of the
differentiation in the non-reproductive, or somatic, cells, which does not
occur in the Protozoa but is characteristic of the Metazoa.

117. Division of Labor.—Another term for physiological differ-
entiation is that of division of labor. The idea conveyed by this expres-
sion involves an analogy between the development of the animal body
and that of human society. Among very primitive peoples each indi-
vidual is largely independent of his fellows, doing for himself all that he
needs to have done. As society develops, certain individuals become
more proficient in the doing of certain kinds of work, and as a result a

96 METAZOA IN GENERAL

person skilled in one field exchanges the products of his labor for the
products of the labor of another who is more proficient in some other field.
This specialization in the work of the individual and the exchange of the
results of that work develop in proportion as civilization advances. In
the most highly civilized society the individual may spend his whole time
doing simply one thing, as the making of a single part for a complex
machine, most of the articles which he uses being secured by the purchase
of the products of the labor of others. In the study of animals repre-
senting various groups from the lowest to the highest a similar reduction
in the degree of independence and increase of the interdependence which
accompanies specialization may be observed.

Development CO) Developrnent CO) Developrment
of soma of soma of soma
© © @
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cel cel] ce//
ta O 7 OY
yZ
f j A
{ oe; Ow; ©
Zygote O Zygote O zygote O
O © ©
© O O
First generation Second generation Third generation

Fic. 37.—Diagram to illustrate Weismann’s conception of the continuity of the germ
plasm and the development of the somatoplasm anew in each generation. Germ cells are
black, somatic cells white.

118. Somatic and Germ Cells.—The earliest type of cells to become
differentiated from the rest are the sex cells, or germ cells. It has been
seen (Sec. 113) that they are set aside even among some of the colonial
Protozoa. All the cells in the body other than sex cells are termed
somatic cells. When a metazoan reproduces sexually, either the egg cell
or the zygote which is to participate in the formation of the new indi-
vidual separates from the body of the parent, and by differentiation the
whole organization characteristic of the particular species of animal is
developed. With the death of such an individual all the somatic cells
perish. If we call the protoplasm of the somatic cells somatoplasm and
that of the sex cells which transmit hereditary characters germ plasm,
it may be said that the thread of life continues from generation to
generation through the germ plasm alone, the somatoplasm being formed
anew in each generation from the germ plasm. Only in the case of
asexual reproduction in the Protozoa does the whole animal live on in
the bodies of its descendants—that is potential immortality.

The genetic continuity of the germ plasm was emphasized in the work

of Weismann, whose conception is illustrated in the accompanying
diagram (Fig. 37).

METAZOA 97

119. Potential Immortality of Germ Cells.—Germ cells possess a
potential immortality, since any germ cell has the capacity under proper
conditions to take part in the production of another individual, and
this may continue for an indefinite number of generations. Neverthe-
less, they perish in enormous numbers, since many eggs are never fer-
tilized and a greater number of sperm cells never find an egg cell with
which to unite. In contrast to germ cells, somatic cells present no possi-
bility of life beyond the lifetime of the individual of which they are a
part. The distinction between germ cells and somatic cells, or, more
exactly, between germ plasm and somatoplasm, was emphasized by
Weismann (Sec. 118). He also stressed the independence of the germ
cells and likened the body, or soma, to a vehicle for the nourishment
and transmission of germ cells. The hereditary units, which determine
the possibilities open to the animal, are passed from generation to
generation in the germ cells, whereas in the various types of somatic
cell, under the environmental conditions which surround each, are
realized and manifested such of these possibilities as, taken together,
equip the individual with its characteristic features.

CHAPTER XO

TISSUES

As a result of differentiation a variety of cells is produced. These
tend to be associated in groups of similar cells to which is applied, in
general, the term tissues.

120. Definition. A tissue is a group of somatic cells which are
similarly differentiated—that is, which are similar in structure and which
perform one or more functions in common—together with the structures
produced by them. In some tissues is found intercellular material which
is developed from the cells and which is very important in the performance
of the particular function belonging to the tissue.

Among the various tissues in the body are recognized four distinct
types, classified on the basis of both structure and function: (1) epithelia,
or epithelial tissues; (2) supporting and connective tissues; (3) muscular
tissues; (4) nervous tissues.

121. Epithelia—An epithelium, or an epithelial tissue, is the type of
tissue which covers any free surface, either the outside of the body or the
walls of cavities within it. In the simplest Metazoa this is the only kind
of tissue present and there may be little differentiation in it in the differ-
ent parts of the body. In the more complex animals, however, the
epithelia found in various parts of the body become quite diversified and
are named according to the shape of the cells or to the functions which
they perform. For example, an epithelium which on its outer surface
is made up of very flat cells is termed a pavement epithelium; one in which
the surface cells are in the shape of long prisms, set at right angles to the
surface, is called a columnar epithelium; and one in which the cells on the
surface bear cilia is known as a ciliated epithelium. If an epithelium
possesses only one layer of cells it is termed simple; if it has several layers,
stratified (Fig. 38). Examples of epithelia named from their function are
sensory, glandular, protective, and reproductive.

The functions which epithelia perform are several. Some serve to
protect the structures below them. Others contain sensory cells and
serve to receive and transmit stimuli from the outside. Through
epithelia all food has to enter the body, and also all waste matter has to
leave. They also produce many of the secretions which, when poured
out upon a surface, serve to moisten it, to lubricate it, or to digest food.
Reproductive cells arise from what are called germinal epithelia.

TISSUES 99

All epithelia are similar in that their cells generally possess walls,
they are relatively small and compact, are crowded closely together, and
are usually cemented to one another by an intercellular cement secreted
by the cells. The connective tissue underlying an epithelium often
forms a thin sheet called a basement membrane (Fig. 38 C to £) to which
the epithelial cells are attached. In some cases intercellular bridges of
protoplasm connect one cell to the next, and in the absence of a basement
membrane the deepest layer of cells in an epithelium may be anchored
by rootlike projections which penetrate the tissue below them. Neither
nerves nor blood vessels are ordinarily found in epithelia, though this does
not apply to nerve terminals in sensory epithelia.

Basement membrane D

Fic. 38.—Semidiagrammatic sketches illustrating different types of epithelia. A,
simple pavement epithelium, seen in surface view and in section. 8B, section of simple short
columnar, or cubical, epithelium, also seen in surface view and in section. C, sectional view
of asimple columnar epithelium. D, section of simple ciliated epithelium. £, section of a
stratified pavement epithelium. All highly magnified. Figure 8A also shows a single
very thin pavement epithelial cell.

When epithelial cells have a secretory function the secretion may be
‘accumulated in droplets within the cells (Fig. 39). In the case of
enzyme-secreting cells droplets or granules containing a zymogen are
accumulated, and when secretion occurs the zymogen is transformed to
the enzyme and passed out of the cell. These droplets or granules
become massed at the outer end of the cell and the cells consequently
become markedly granular in texture. The droplets or granules dis-
appear when the enzyme is formed and passed out through the cell wall,
to reappear during the time the gland is not secreting. Examples are
the cells of the salivary glands and the pancreas. In some cases, how-
ever, as in the case of cells which secrete mucus, the droplets flow
together and form a great mass toward the outer end of the cell. The
secretion is set free by the rupturing of the cell wall. Examples are the
mucus-secreting goblet cells of the intestines of vertebrates. In still

100 METAZOA IN GENERAL

other cases the secretion involves the destruction of the whole cell, which
pours out its contents to form the secretion; examples are milk glands
and sebaceous, or oil, glands.

When epithelial cells undergo a change which makes them hard, the
substance formed is horn, which chemically is a substance called keratin.
In this fashion true horns, claws, nails, and tortoise shell are developed.
In the case of teeth and some scales of vertebrates, however, enamel may
be the substance produced. In some cases epithelia produce a hard
covering by the hardening of a secretion; an example of such a hard
covering is the cuticula of the bodies of insects, which
contains chitin.

122. Supporting and Connective Tissues.—These
tissues are found in all parts of the body and differ
from other tissues in the fact that the character of
the tissue depends not so much upon the cells which
it contains as upon nonliving intercellular materials
formed by secretion from these cells. Examples of
such materials are fibers, bone, and cartilage. Most
of the embryonic connective tissue appears in the
form of a network of branched cells and is known as
mesenchyme.

A prominent function of these tissues is support,
either of the body as a whole or of some particular

Fig. 39.— Figures
to illustrate the secre-
tory function in epi-
thelial cells. A, cells
of such a gland as the
salivary glands or pan-
creas showing zymogen
granules accumulated
in the part of the cell
adjacent to the lumen,
or cavity, of the gland.
B, two goblet cells from
the intestine of a ver-
tebrate showing the
accumulation of mucus
and its extrusion into
the lumen of the intes-
tine.

part. Among supporting tissues having this func-
tion are fibrous tissues, which are characterized by
bundles of fibers or single fibers between the cells.
White, nonelastic fibers are usually collected in
bundles, while yellow, elastic fibers are, in most cases,
single and, since they branch and run together, tend
to form a network. The fibrous tissues also serve
to bind parts together and to hold them in place.
Another type of supporting tissue is cartilage, in
which the space between the cells is occupied by a
substance known as chondrin, or “‘gristle.” Still
another type is bone, in which there is laid down
between the cells a deposit of salts of lime which

makes the tissue very firm and capable of giving effective support to
even a large body. Special types of fibrous tissue which also serve to
bind parts together include the ligaments, which connect the parts other
than muscles, and the tendons, which serve to connect muscles to other
parts at their point of attachment (Fig. 40).

An additional function which these tissues have, and which also is a
passive function, is to store fat. Fat tissue is simply a connective tissue
in which the cells, because they are filled with great globules of fat, have

TISSUES 101

become large and crowded upon each other, while the intercellular
elements become the less conspicuous part of the tissue. The blood may
also be considered as a connective tissue in which the intercellular
elements are all fluid and form the blood plasma in which the cells
float.

123. Muscular Tissues.—Muscular tissues have as their function
motion and locomotion. As befitting cells set aside for this purpose,

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Fic. 40.—Different types of connective tissues; somewhat diagrammatic. A, bone,
showing the haversian canals which transmit the blood vessels and nerves, and the lacunae,
which lodge the bone cells, or bone corpuscles (refer to Fig. 8 Z). B, portion of subcuta-
neous areolar connective tissue, showing several tissue elements. C, fat. D, tendon in
longitudinal section, showing longitudinal fiber bundles and rows of cells crowded into the
space between them. £, section of cartilage with the cells lodged in spaces in the chondrin.
All highly magnified.

they become more or less elongated or fiber-like. In some cases, in order
to secure a greater length of the contractile fiber, it is composed not of
one cell but of many, all united into a single fiber, which gives evidence
of its composite nature only by the fact that it contains many nuclei.
The protoplasm within these cells becomes organized in a very complex
manner and in such a way as to determine the direction in which con-
traction shall take place. All muscle cells perform their function by

102 METAZOA IN GENERAL

virtue of their power to contract; their subsequent elongation is simply
a matter of relaxation and returning to a normal shape.

Three types of muscle tissue are recognized: (1) skeletal, striated and
voluntary, found, generally speaking,
in the muscles which are themselves Vi Ny Denies
organs under the control of the will f
and often attached to the skeleton; (2)
visceral, nonstriated and involuntary,
generally not under the control of the
will and forming a part of other organs;
and (3) heart muscle, a type intermediate
between the two others, found in the
heart (Fig. 41). Skeletal muscles con-
sist of large multinucleated fibers which
show a very marked cross banding or, Medullary

Brite Nee : sheath
as it is termed, cross striation; visceral
muscle fibers are individual cells and
do not show this cross striation; heart
muscle is made up of individual cells
which are involuntary but cross-striated.

Axon

Collateral

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Neurilemma

\-Nucleus of
1 “eur//emma

, =

Mofor
end
plate
Corttractile fibril Muscle
A B GC frber
Fig. 41. Fia. 42.

Fic. 41.—Different types of muscle cells. A, portion of a striated muscle fiber showing
a section in which the contractile fibrils are divided into groups by semifluid sarcoplasm.
Two nuclei are shown, surrounded by undifferentiated cytoplasm, and the whole fiber is
surrounded with a delicate sheath, or sarcolemma. 8B, three nonstriated muscle fibers, or
cells. C, several cardiac muscle cells. All highly magnified.

Fia. 42.—Diagram of a nerve cell, possessing a cell body and a medullated motor nerve
fiber, ending in a motor end plate. Such cells are characteristic of the spinal cord of
vertebrates. The medullary sheath is acquired while the axon is in the outer layers of the
cord, and the neurilemma as the fiber emerges from the cord. The nerve fiber is too long
to be shown entirely, so a break is indicated.

Of these types of muscle tissue, nonstriated muscles are found more
generally in the lower animals and the striated muscles predominate in
the higher forms. In the higher forms the nonstriated muscles are in

TISSUES 103

the walls of the alimentary canal, the blood vessels, and the gland
ducts.

124. Nervous Tissues.—Nervous tissues contain cell bodies from
which extend processes or nerve fibers which vary in length and in the
degree to which they are branched (Fig. 42). The function of these
tissues is to register the effect of stimuli and to conduct this effect from
cell to cell until it finally reaches a cell which gives the appropriate
response. The nerve cells of a brain may themselves initiate impulses
which stimulate another part of the body to action. Finally, nervous
tissues have the power to conserve the effect of stimulation and to use
it in modifying future action. The effect of a stimulus conducted along a
fiber is known as animpulse. Irritability and conductivity are properties
of all protoplasm but are developed to the highest degree in nerve tissues.
A fiber which transmits an impulse to the cell body of which it is a part
is known as a dendron or dendrite, while a fiber which transmits an impulse
in the opposite direction is an azon, or axis cylinder process. Some nerve
fibers have a fatty sheath and are said to be medullated; others, which lack
this, are nonmedullated.

CHAPTER XXI

ORGANS AND SYSTEMS

In the bodies of all Metazoa, except the lowest, tissues become
associated together in such a fashion that several contribute to the per-
formance of some function which belongs to the association as a whole.

125. Definitions.—An organ is a part of the body formed by an
association of tissues all of which contribute to the performance of some
function or functions. Many organs in the higher animals include repre-
sentatives of all of the four different types of tissues. For instance, the
heart is covered and lined with epithelium; the greater part of its wall is
made up of muscular tissue; fibrous connective tissues serve to connect
other tissues and give support; and nervous tissues receive the impulses
from the central nervous system, coordinate them, and distribute them
to the heart muscles.

Not only are tissues associated in the body to form organs, but organs
are associated to form systems. A system is a group of organs which
collectively perform certain related functions. Thus the body might
be conceived of as being built up by adding 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 whole; or it could be analyzed in
terms first of systems, then of organs, then of tissues, and finally of cells.

126. Systems.—Nine systems are recognized in higher animals. A
list of these with the most prominent functions follows:

1. Tegumentary System.—Protection, temperature regulation, elimi-
nation of a small amount of liquid waste, and external support.

2. Digestive System.—The ingestion, digestion, and absorption of food,
the secretion of digestive ferments, egestion, and elimination of a small
amount, of liquid waste.

3. Circulatory System.—The transportation of food, oxygen, and the
excretions of the body as well as the carrying about of certain internal
secretions; also internal respiration.

4. Respiratory System.—The taking in of oxygen and giving off of
carbon dioxide, or external respiration.

5. Excretory System.—The elimination of most of the liquid waste
products derived from metabolism (this would be more appropriately
named the eliminative system, but the name excretory has been univer-
sally used).

6. Skeletal System.—Protection and support.
104

ORGANS AND SYSTEMS 105

7. Muscular System.—Motion and locomotion.

8. Nervous System.—Reception of stimuli, sensation, coordination,
and causation of muscular and secretory activity.

9. Reproductive System.—Reproduction.

This enumeration does not cover all of the structures in the body.
Such a tissue as fat, the function of which is storage, does not logically
come under the head of any one of these systems. In the lower Metazoa
many functions may be carried on in part by single cells and in part by
tissues, and when finally organs and systems become clearly defined
there is a gradual increase in the complexity of both, reaching its highest
degree in the highest animals.

127. Organs Belonging to Different Systems.—Systems should
always be analyzed in terms of the organs which compose them. The
tegumentary system includes the skin and the structures contained in it,
with the exception of sense organs, which are usually considered as part
of the nervous system, and the skin muscles, which are generally referred
to the muscular system. The skin occupies a somewhat equivocal
position. It may play a part in absorption and in respiration as well as
performing the functions already given. It has often been considered
a single widely spread organ with smaller organs imbedded in it, such as
various glands, and organs of attachment, like sucking discs.

The more important regions of the digestive system in the vertebrates
are the mouth, pharynx, esophagus, stomach, small intestine, large
intestine, and rectum, which may be considered, in a general sense, a
series of tubular organs placed end to end making up the alimentary
canal. Some of these also contain other accessory organs, such as tongue,
teeth, and certain glands. Other accessory organs lying outside the
canal, as salivary glands, liver, and pancreas, also belong to this system.
The organs of the circulatory system are the heart, blood vessels, lymph
nodes, and spleen. The respiratory system includes, in various animals,
gills, lungs, air passages, and tracheae, or breathing tubes, of insects.
The pharynx, and in some cases the mouth, may be considered as belong-
ing to this as well as to the digestive system. Nephridia in the lower
forms and kidneys in the higher, the bladder, and the tubes which convey
the urine are the organs of the excretory system.

The muscles, individually, are the organs of the muscular system
(Fig. 43); they contain muscle tissue and, in addition, fibrous connective
tissue sheaths, tendons, and nerve endings. In a similar way bones and
cartilages are examples of organs of the skeletal system; a bone, besides
bone tissue, may contain marrow, a fibrous sheath, and cartilage, which
coats certain areas on the bone, forming smooth surfaces for articulation
with other bones (Fig. 43). One organ may be included within another
and become a part of it, as blood vessels and nerves in muscles and
bones.

106 METAZOA IN GENERAL

The organs of the nervous system are the brain and spinal cord, nerves,
and various sense organs. Nerves possess sheaths of connective tissue in
addition to nervous tissue and are also supplied with blood vessels and
smaller nerves.

Of the reproductive system, the most important organs are the gonads,
under which are included the testis of the male and the ovary of the
female. There are also to be added the ducts which convey the sperm

Marrow

Fig. 43.—A diagram to indicate the nature of bones and muscles as organs and the
mode of attachment of a muscle to a bone. A bone as an organ consists of several tissues,
such as bone, articular cartilage, marrow, and the fibrous periosteum, and into it enter
blood vessels and nerves. A muscle contains muscle fibers, is covered with a fibrous sheath,
ends in a tendon, and also is supplied with blood vessels and nerves. The fibers of the ten-
don are interwoven with those of the periosteum, from which other fibers penetrate the
bone, giving a firmer anchorage.
cells and egg cells and a variety of other organs, such as yolk glands and
shell glands in the female and copulatory organs in the male.

These organs and systems will be described in greater detail when the
animals which possess them are considered.

128. Other Parts of the Body.—Many divisions of the body are
recognized which have no relation to organs and systems, such as the
head, neck, trunk, and appendages. Appendages are usually not
individual organs but often contain many organs belonging to several
different systems. For instance the vertebrate limb contains organs of
the tegumentary, circulatory, skeletal, muscular, and nervous systems.

CHAPTER XXII
REPRODUCTION IN THE METAZOA

The general subject of reproduction was introduced in Chap. X,
and reproduction in Protozoa has been considered especially in Sees. 98,
104, 112, and 113.

129. Methods of Reproduction in Metazoa.—In Metazoa the usual
type of reproduction is sexual by germinal cells, although asexual repro-
duction, by somatic cells, is found in the lower forms. Fisston occurs
when the animal’s body divides into two individuals equal in size. The
process is called budding when an individual gives rise to another by the
separation of a part smaller than that which remains and which is the
parent. Both fission and budding occur in many of the lower metazoans.
Some of the lower worms also undergo what is called fragmentation.
Though not the same as sporulation, in the sense in which the word is used
in connection with Protozoa, fragmentation is a mode of reproduction
analogous to it and occurs when the body divides into a large number of
fragments each one of which becomes a complete individual.

130. Sexual Reproduction.—_Sexual reproduction in Metazoa usually
involves two parents. It is then termed biparental. In this case the two
parents usually differ from each other in their external appearance.
The one which is termed the male produces sperm cells; and the other,
called the female, produces egg cells. A species of which this is true is
termed diecious, or bisexual, referring to its existence in the two sexes.
On the other hand, particularly among lower Metazoa, there are those
species in which one individual produces both egg cells and sperm cells
and which therefore contains the organs of both sexes. Such species of
animals are represented by only one type of individual and are called
monecious, or hermaphroditic. Different species of hydra and of earth-
worms are examples of monecious animals; almost without exception the
vertebrates are diecious.

131. Uniparental Reproduction.—Another method of sexual repro-
duction is for an egg cell to develop without union with a sperm cell.
When this reproduction by females alone takes place, the phenomenon is
termed parthenogenesis. It occurs in nature in a number of diecious
animals in which exceedingly rapid reproduction contributes to the
welfare of the race. Examples of such animals are plant lice, which are
eaten by a vast number of other animals and which continue to exist only
by virtue of exceedingly great powers of reproduction, and certain

107

108 METAZOA IN GENERAL

aquatic forms, like rotifers and water fleas, which are also eaten in great
numbers by fish and other larger aquatic animals.

If rapidity of multiplication is the end reached in parthenogenesis,
this is attained to a still greater degree if the animal does not wait to
become mature before it becomes capable of reproducing. Reproduc-
tion by an immature animal is known as pedogenesis and occurs in a
number of insects; for instance, the larvae of certain gall gnats, the pupae
of some midges which produce eggs capable of developing without
fertilization, and certain animals having biparental reproduction, as is
shown by the production of young by immature salamanders, known as
axolotls.

132. Types of Fertilization—Animals which are monecious are
capable of fertilizing their own egg cells, though actually in nature few
such animals do so. When this occurs, the phenomenon is known as
self-fertilization. In the case of animals which are diecious the fertiliza-
tion of the egg cell of one individual must be by the sperm cell of another.
This is known as cross-fertilization. Cross-fertilization is not the same
as hybridization, the latter term being applied usually when the two indi-
viduals belong to different species, varieties, races or strains. Cross-
fertilization is a very general phenomenon and is practically universal
among the higher animals; hybridization is much less frequent.

133. Oviparity and Viviparity—Many animals retain for a time
within their bodies their egg cells and the embryos which develop from
them and give birth to living young. Such animals are termed vzv-
parous and the phenomenon is viviparity. On the other hand, a great
many pass the egg cell out of the body for development. These forms
are termed oviparous and the phenomenon oviparity. In oviparous
animals the egg cell when passed out of the body is usually provided with
a greater or less number of protective envelopes various in character, and
to the egg cell plus all of these envelopes is applied the term egg. In
some cases fertilization takes place within the body before these enve-
lopes are added, and here, as in viviparous animals, the phenomenon is
referred to as internal fertilization. On the other hand, the egg may be of
a character which permits fertilization after passage from the body.
Such a type of fertilization is termed ezternal fertilization.

134. Metagenesis.—There are animals in which both sexual and
asexual types of reproduction occur, and these in alternate generations.
One or more generations produced in one manner are followed by one or
more produced in the other. This phenomenon is termed metagenesis,
or alternation of generations. It is illustrated best among some marine
hydroids and jellyfishes, in connection with the study of which it will
be more fully described.

CHAPTER XXIII

ORIGIN OF THE SEX CELLS

The first step in the production of a new individual sexually is the
formation of sex cells. This takes place in gonads which arise from
the germinal epithelium, which in turn is developed from the cells lining
the coelom, or body cavity.

135. Gametogenesis.—The origin and development of the sex cells
are termed gametogenesis. This may be divided into spermatogenesis,
which deals with the male germ cell, called the sperm, sperm cell, or
spermatozoon; and oogenesis, which deals with the female germ cell,
called the egg cell, or ovum.

In all references to the male germ cell in this text it will be called a
sperm cell. The objection to the word spermatozoon is that it perpe-
tuates an error; it means, literally, ‘‘sperm animal” and was proposed at
a time when it was believed that these cells were themselves animals
living in the bodies of higher animals. The term egg cell is preferred
to ovum because the latter has been used both in this sense and also to
apply to the whole egg.

Both processes, spermatogenesis and oogenesis, begin (Figs. 44 and
45) quite early in the life of the embryo by the setting aside of a pri-
mordial germ cell from which come all of the sex cells which will be devel-
oped in that animal’s body. This cell multiplies by repeated divisions
until a very large number of cells is produced; the time during which
this occurs is called the multiplication period. In spermatogenesis these
cells are known as spermatogonia; and in oogenesis, oogonia. When the
animal becomes sexually mature, these cells undergo the processes of
growth and maturation, the growth period involving both an increase in
the size of the cell and a union of like chromosomes in pairs. This union
of chromosomes is termed synapsis. At the end of the growth period
the male cells are termed primary spermatocytes; and the female cells,
primary oocytes. From this time on the processes of spermatogenesis
and oogenesis differ.

136. Spermatogenesis.—The periods of multiplication and growth
having been completed in spermatogenesis, the maturation period fol-
lows (Fig. 44). The primary spermatocyte undergoes two maturation
divisions. The first results in the formation of two secondary spermato-
cytes, and the second in the formation of two spermatids from each of these
secondary spermatocytes, making four spermatids altogether. The

109

110 METAZOA IN GENERAL

chromosomes, which were brought together in pairs in synapsis, are
separated again in one of these divisions, in which case, instead of each
chromosome dividing, whole chromosomes pass to the poles of the spindle.
Thus the number of chromosomes becomes reduced to half the number
contained in the primordial germ cell. This peculiar type of cell division

Primordial germ cell
Multiplication he ~
period AY
re eee
cs ‘©
/

vila \

cegooacn

/ \ / / /
Pe SO On Joe a very « number of cell divisions

any one spermatogonium

Growth

period. 75, yNapsis
Primary spermatocyte
v7, \ Melotic division
Secondary
giao fei a! spermatocyte
perio

iii fr
@ (A) iS ® Spermerttals

yg ae

Fig. 44.— Diagram illustrating spermatogenesis, the haploid number of chromosomes being
four and the diploid eight.

is known as a reduction division, or meiosis. The reduced number of
chromosomes is known as the haploid number, while the larger number,
found in all somatic cells and in all immature germ cells, is called the
diploid number. Sometimes the reduction occurs in the first of these
maturation divisions and sometimes in the second.

The spermatids undergo a process of modification or ripening which
involves a change in form and also the loss of a considerable amount of

ORIGIN OF THE SEX CELLS 111

the cytoplasm. The nucleus becomes the larger part of the body, or
head, of the mature sperm cell, which in many higher animals resem-
bles in shape a tadpole with a very long tail. From a portion of the

Primordial germ cell
Multiplication 1
pe be od ban
~ =

aa (
NS @ Q re

\ Seas al
and so on for alarge number of cell divisions

wy any one oogonium
Growth

period |

TZ
D
a
S
Sg
ae
A)

-< SYNAPSs!s

Primary oocyte

[ ‘ Meiotic division
Maturation
period © First polar body

Secondary oocyte | \
Mafured \

egg cell © @ @® Polar bodies
\second polar
hody

\---Yolk accurtulated in the
egg cell

Fic. 45.—Diagram illustrating oogenesis, the chromosome number being the same as in
Fig. 44.

cytoplasm is formed the tail, and between the head and tail, connecting
one to the other, is a middle piece which contains the centrioles. This
tail, by rapid vibrating movements, can propel the sperm cell through a
liquid medium at a relatively high rate of speed.

112 METAZOA IN GENERAL

137. Oogenesis.—In oogenesis, also, after the periods of multiplica-
tion and growth have been completed, the primary oocyte undergoes a
first maturation division in which, in contrast to what occurs in sperma-
togenesis, it is very unequally divided (Fig. 45). One of the two daughter
cells is small and, while it contains half the nuclear material, has prac-
tically none of the cytoplasm. The other is much larger, receiving in
addition to half the nuclear material practically all of the cytoplasm.
The smaller daughter cell is termed the first polar body; and the larger
one, the secondary oocyte. Following this division a second division of
the first polar body may occur, giving rise to two smaller polar bodies
each equal in size to half of the first polar body. ‘The secondary oocyte
undergoes another unequal division, a second polar body being formed as
before with very little cytoplasm, while the larger cell is known as the
egg cell. As in spermatogenesis one of these two divisions is a reduction
division in which the number of the chromosomes is reduced to the hap-
loid number. The result of oogenesis, therefore, is to produce one large,
functional egg cell and either two or three polar bodies depending upon
whether or not the first polar body undergoes division. These polar
bodies die, disintegrate, and disappear. In effect, all of the cytoplasm
which would have gone to four cells if the divisions of the cells had been
equal has been accumulated in the one. This egg cell becomes still
larger by the accumulation within it of yolk and thus becomes fully
mature. This accumulation of yolk in the mature egg cell is to provide
the necessary food supply for the embryo which will develop from it until
the developing individual can secure food for itself.

It should be observed that in both oogenesis and spermatogenesis all
of the cell divisions except the reduction division are mitotic.

138. Comparison and Contrast between Spermatogenesis and
Oogenesis.—It is clear from the description of the two processes that
there are many ways in which they are alike; the more important simi-
larities may be enumerated as follows:

1. Both start with a primordial germ cell.

2. Both pass through three periods, namely, multiplication, growth,
and maturation.

3. Both undergo a process known as synapsis in the growth period.

4. Both possess two maturation divisions.

5. Both exhibit a reduction in the number of chromosomes.

On the other hand, the two processes are sharply contrasted in several
ways:

1. Spermatogenesis results in the production from each spermato-
gonium of four similar sperm cells, all of which are functional, while
oogenesis results in the formation from each oogonium of only one large
cell, the egg cell, and of three small nonfunctional cells or polar
bodies.

ORIGIN OF THE SEX CELLS 113

2. Mature sperm cells are very small and very active cells; mature
egg cells, owing to their large size, are passive.

3. Because all of the sperm cells are functional and also because of a
much greater number of multiplication divisions, the total number of
sperm cells produced is enormously greater than the number of egg cells.
The number of egg cells which become mature and may be fertilized is
only a small fraction of the number actually produced. The rest act as
nurse cells and contribute their substance to those which are to develop.
A good example of this is seen in the fresh-water hydra, in the ovary of
which a large number of egg cells are developed; only one of these, how-
ever, becomes fully mature and capable of producing another individual.

139. Division of Labor between the Germ Cells.—It will be observed
that in spermatogenesis and oogenesis there has been a division of labor
between the two types of sex cells. In order that fertilization shall occur
the two cells must come together, and to assure the development of the
embryo a large store of food is provided. If both cells had a store of
food neither would be able to move effectively and their union could not
occur, but the accumulation of a sufficient store of food in one while the
other becomes small and active makes it possible for the latter to seek
out the former and to unite with it. That practically every mature egg
cell will be fertilized is also insured by the enormous number of sperm
cells produced as compared with the number of egg cells. This greater
number, however, imposes no proportionately greater strain upon the
energy of the individual, since frequently there is no more material in
100,000 sperm cells than in one egg cell. In a certain species of sea
urchin the volume of the individual egg cell is equal to that of 500,000
sperm cells.

140. Variations in Gametogenesis.—In the details of this process
many variations occur in different animals. All of the sex cells which
the animal produces may mature at the same time; this is the rule in
insects, which, after the maturation and fertilization of the eggs, deposit
them as rapidly as possible and soon die. Maturation periods may
occur at intervals and the animal live through several breeding seasons.
Many mammals exhibit this phenomenon. Birds have an annual breed-
ing season. At other times than during the breeding season the sex
organs in these animals are quiescent and the maturation of the sex cells
is arrested. In still other animals, particularly in the male sex, matura-
tion goes on continually and the animal can breed at any time. Under
domestication the breeding season may be greatly extended. The
domestic hen in the original state had a restricted breeding season and
laid only a limited number of eggs. Under domestication the number
has been increased until 361 eggs have been produced in one year, which
means practically continuous sexual activity.

CHAPTER XXIV

FERTILIZATION

In a general sense fertilization may be defined as the union of the
sperm cell with the egg cell, though, as will be seen, the process involves
several steps, takes a certain length of time, and there may be a question

E F

Fig. 46.— Diagrams showing the successive steps in the fertilization of the egg cell of a
sea urchin, which is mature when the sperm cell enters. (From Wilson, ‘‘ The Cell,’ by the
courtesy of The Macmillan Company.) A, the entrance of the sperm cell; the maturity of
the egg cell is indicated by the two polar bodies. B, the approach of the two pronuelei,
the centriole of the sperm cell and the aster developed about it preceding the male pro-
nucleus. C, the meeting of the two pronuclei; the centriole has divided. D, the formation
of two asters about the two centrioles, now on opposite sides of the two pronuclei, which
are undergoing fusion. #, the fusion nucleus representing the two pronuclei during a
period of pause, while the asters are reduced in size. Fertilization may now be said to be
complete. fF, the first cleavage division, which follows the pause, at the beginning of the
anaphase.

as to when the union is actually consummated. Two phenomena are

involved: the activation of one cell by the other and the union of corre-

sponding chromosomes from the two parents. The former effect is

paralleled by artificial parthenogenesis. Loeb discovered in 1899 that the

eggs of starfishes and sea urchins could be caused to develop by artificial
114

FERTILIZATION 115

stimulation, and echinoderms have since been the favored types in such
experimentation. Since that time successful experiments have been
carried out with annelids and mollusks, and also with fishes, frogs and
rabbits, none of which develop parthenogenetically in nature. Several
types of stimuli—mechanical, thermal, and chemical—have been found
to be effective. The adult condition has been attained in but few cases.
In animals as high as frogs and rabbits, however, the young have devel-
oped into adults.

141. Steps in Fertilization Usually the whole sperm cell enters the
ege cell, but in some cases more or less of the tail is left outside and there
enter only a nucleus, the centrioles, and a very little cytoplasm. The

D E F

Fic. 47.—Diagrams showing the successive steps in the fertilization of the egg cell of a
round worm, Ascaris, which matures after the entrance of the sperm cell. (From Wilson,
“The Cell,” by the courtesy of The Macmillan Company.) A, the entrance of the sperm cell;
the egg cell is in the condition of a primary oocyte. 8B, the formation of the first polar body;
development of a sperm aster. C, the matured egg cell, with the polar bodies; the male
pronucleus has increased in size; from the one centriole has developed two, each with an
aster, and a spindle lies between them. D, the two pronuclei, now about equal in size and
each containing chromosomes, meet on the spindle. HH, a pause corresponding to that in
Fig. 46H. F, first cleavage division.

nuclei of the two cells, which are now called, respectively, the male and
female pronuclei, may, if both are mature, at once approach and fuse.
In this case cell division follows after a time (Fig. 46). On the other
hand (Fig. 47), the entrance of the sperm cell may take place before the
egg cell has attained the necessary maturity, in which case the male
pronucleus remains at one side until the maturation of the egg cell is
complete and undergoes a slow growth in size by absorbing the fluid

116 METAZOA IN GENERAL

from the cytoplasm of the egg cell. Then the two pronuclei approach
each other. At the same time the centrioles which were brought in
with the sperm cell become active and a spindle is produced near the
center of the egg cell. The two pronuclei meet at the equator of this
spindle. Chromosomes are formed in each, the nuclear membranes
disappear as in an ordinary mitosis, and the two sets of chromosomes
gather on the equator of the spindle, producing an amphiaster stage.
Then the steps which are seen in ordinary mitosis occur in regular order,
including metaphase, anaphase, and telophase, the final result being a
division of the cell. This division initiates the development of the
embryo. Fertilization may be said to be completed when the sperm
cell enters the egg cell, when the two nuclei fuse, or, in the case last
described, when the two nuclei cease to retain their identity and the
chromosomes which develop from them come to lie in the equatorial
plane of the spindle.

In either of the cases described above, the chromosomes from the
two parent cells appear clear and distinct and when they divide in
the metaphase, each of the two groups of chromosomes which pass to the
two poles of the spindle is half maternal and half paternal in origin.
When at the end of the telophase the nuclei of the two daughter cells
enter into a resting condition, these chromosomes lose their identity;
but in each cell division which will follow in the development of the
individual which is to be produced, the maternal and paternal chro-
mosomes again appear. Thus the individual represents a mingling of
the characteristics of the two parents, and each cell in the body has
this mixed inheritance. As might be expected, the steps given above
are varied in many ways in different types of animals but the essential
facts remain the same.

142. Chromosome Reduction.—It now becomes evident why chromo-
some reduction occurred in gametogenesis. Every species of animal has
a characteristic number of chromosomes, a number which is found in
every somatic cell in the body and remains constant generation after
generation. This number seems to have no relationship to the structure
of the animal or to its rank in the scale of animal life. For instance,
there are 2 chromosomes in a parasitic worm (Ascaris) found in the horse,
8 in the fruit fly, 28 in the spotted salamander, 48 in man, and 208 in
two species of crayfish. If the numbers of chromosomes were not
reduced in the maturation of sex cells, the fertilized egg cells would
contain twice the number possessed by the cells of the parents, and
their number would continue to double with each succeeding generation.
Chromosome reduction, however, results in passing on the same number
from one generation to the next. In all references to chromosomes up to
this point only those which act as mates in synapsis have been con-
sidered. There are odd chromosomes which in meiosis pass to either

FERTILIZATION slay;

one or the other of the daughter cells. These are associated with the
determination of sex and will not be considered until Chap. LX XIII is
reached.

143. Significance of Synapsis.—In all of the somatic cells of the body
the chromosomes of maternal and paternal origin remain separate in
cell division, and since the corresponding ones from the two parents are
similar, they appear in pairs of like chromosomes. In the process of
synapsis in gametogenesis these like chromosomes unite, and this is
followed by their separation again in chromosome reduction. In mitosis
every chromosome is divided and the two daughter cells have the full,
or diploid, number, but in meiosis whole chromosomes pass to the
daughter cells, which thus acquire the haploid number. Since it is a
matter of chance to which of the two poles of the spindle any particular
chromosome goes, there is an assorting and chance distribution of
chromosomes to the mature egg cell or sperm cell. Thus every individual’
not only represents a chance mingling of the chromosomes of its parents
but also receives a chance selection from those of previous generations,
the probable number received from each generation being progressively
smaller in going back from one generation to the previous one.

CHAPTER XXV

EMBRYOGENY

The word embryogeny may be defined as the development of an animal
from the time when the fertilized egg cell begins to divide until the
organism has acquired an organization comparable to that of the adult.
Until that time it is an embryo, but afterward it receives different names
in the different types of animals. Examples are the larvae of many
invertebrates, the tadpoles of amphibia, the chicks of birds, and the

Fic. 48.—Diagrams of homolecithal egg cells and total cleavage. A, homolecithal egg
cell with nearly uniform distribution of yolk. B, total equal cleavage resulting from the
condition shown in A. C, homolecithal egg cell in which the yolk tends to accumulate
toward the lower pole. D, total unequal cleavage of the egg cell shown in C, when the first
cleavage plane is horizontal, resulting in the production of two unequal cells. , cross
section of an eight-cell stage, resulting from cleavage in C in a case in which the first two
cleavage planes were vertical, giving rise to four equal cells, and the third was horizontal,
producing four smaller cells at the upper pole and four larger at the lower. The proto-
plasm is stippled, the yolk indicated by the outlines of globules.

fetuses of mammals. Generally speaking, the higher the place of the

animal in the animal kingdom the longer will be the embryogeny. Any

particular embryogeny is a part of a corresponding ontogeny, which covers

the development of the animal from the beginning until it reaches

full maturity. The term embryogeny should not be confused with

embryology, which is a broad science covering not only the embryogenies
118

EMBRYOGENY 119

of all animals but also a large amount of detail and generalization which
is outside all embryogenies.

144. Types of Egg Cells.—A part of the process of maturation in the
egg cell consists in the accumulation of yolk, but the amount of yolk
thus stored and its distribution in the cell are not the same in all egg
cells.

All egg cells show polarity. This is indicated by the polar bodies
being formed at or near the upper pole and by the. nucleus, which is
always more or less excentric, being located nearer this pole. The
opposite pole is called the iower pole.

In some cases the yolk is not very great in amount and is scattered
throughout the cytoplasm. Such egg cells are termed homolecithal
(Fig. 48 A and C). Most of the lower invertebrates and almost all of
the mammals possess this type of egg cell, though in mammals the
condition is not primary but secondary and is due to adaptation to the
peculiar mode of development. In some homolecithal egg cells there

Fic. 49.—Diagrams of telolecithal and centrolecithal egg cells, and discoidal and super-
ficial cleavage. A, telolecithal egg cell, in which the protoplasm is all at the upper pole.
B, the discoidal cleavage which occurs in a telolecithal egg cell. C, centrolecithal egg cell,
with a superficial layer of protoplasm, protoplasm around the nucleus, and in some cases
strands of protoplasm connecting the two. D, the superficial cleavage of a centrolecithal
egg cell; an early stage, when the nucleus has divided into several nuclei and each, with a
portion of protoplasm about it, is migrating toward the periphery. #, later stage in super-
ficial cleavage showing the nuclei and cytoplasm at the periphery, forming a superficial
layer of cells, and the yolk at the center. The protoplasm is stippled, the yolk indicated by
the outlines of globules.

is somewhat more protoplasm toward the upper than toward the lower
pole and somewhat more yolk toward the lower than toward the upper
pole.

In the egg cells of almost all vertebrates but the mammals, however,
the yolk, which is present in very large amount, is massed toward the

120 METAZOA IN GENERAL

lower pole, leaving the cytoplasm as a disc at the upper pole. In this
case the egg cell is known as telolecithal (Fig. 49 A). In such egg cells
the upper pole is called the animal pole; and the lower, the vegetal pole.
Primitive mammals have telolecithal egg cells, and the ancestors of
mammals doubtless had such egg cells.

In the insects a third arrangement is presented. Here the yolk
occupies the central portion of the egg cell, inclosing within it, at the
very center, the nucleus, which is surrounded by some of the cytoplasm,
while the greater part of the cytoplasm forms a layer over the whole
surface. Such an egg cell is called centrolecithal (Fig. 49 C). Here there
is a considerable amount of yolk, though relatively not so much as in
telolecithal egg cells.

145. Forms of Cleavage.—The process which follows fertilization
and which results in the formation of a large number of cells from the
fertilized egg cell is known as cleavage. The individual cells which are
thus formed are termed blastomeres.

The effect of the difference in the amount and distribution of the
yolk is seen in the different ways in which egg cells cleave. In an ideal
embryogeny, which may be accepted as that of a homolecithal egg cell,
the plane in which the first cell division takes place is typically meridional,
passing from one pole to the other. The second cleavage plane is also
meridional, being at right angles to the first, and results in the formation
of four similar cells. The third cleavage plane, however, passes at right
angles to the two others, and thus eight cells are produced. Of these the
upper four will be smaller than the lower four. Since the whole egg has
been involved in the cleavage, the egg is sometimes termed holoblastic and
the cleavage is called total. If the yolk is quite evenly distributed and
the cells which result from the cleavage are all approximately the same
size, it is termed equal cleavage (Fig. 48 B). If, however, the yolk is not
evenly distributed but is greater toward the lower pole of the egg cell, the
cells at the upper pole will be decidedly smaller than the other four. This
is termed unequal cleavage (Fig. 48 D and £). In some cases the two
cells first formed differ in size, the first cleavage plane being horizontal
and the smaller cell being above the larger. In other cases the difference
in the size of the cells does not develop until the third cleavage occurs, the
upper four being smaller than the lower. There are many modifications
of these different types.

In telolecithal and centrolecithal egg cells the cleavage planes do not
pass entirely through the cell but only through the cytoplasmic portion,
and thus the cleavage becomes partial; such egg cells are often termed
meroblastic.

In telolecithal egg cells the division of the cytoplasm results in an
embryonic dise at the animal pole, and accordingly the cleavage is termed
discoidal (Fig. 49 B). As development proceeds and the cells continue

EMBRYOGENY 121

to multiply in number, this disc gradually surrounds the yolk, which is
finally absorbed by the growing embryo.

In centrolecithal egg cells the nucleus in the center of the cell divides
repeatedly, each of the daughter nuclei being surrounded by a little mass
of cytoplasm. As these nuclei increase in number they migrate toward
the periphery, accompanied by the bits of cytoplasm, and enter the super-
ficial cytoplasmic layer. Now division of the cytoplasm takes place by
planes which cut it at right angles to the surface, and this for a time leaves
each cell open toward the yolk in the center. A little later the walls of
these cells become complete. Because a superficial layer of cells is in this
way formed around the yolk it is termed superficial cleavage (Fig. 49 D
and £).

146. Steps in Embryogeny.—In the ideal embryogeny (Fig. 50) pre-
viously referred to as that of a homolecithal egg cell, cleavage may be
conceived as resulting in the development of a compact mass of cells
which, because of its general resemblance to the fruit of the mulberry tree,
has been called a morula.

As the multiplication of cells continues, a cavity begins to form in the
mass. The embryo is then called a blastula. ‘This cavity increases in
size until the blastula appears like a hollow rubber ball, the cells or
blastomeres forming the wall, which is now called the blastoderm. ‘The
central cavity is variously known as the cleavage cavity, segmentation
cavity, or blastula cavity and also as the blastocoel. The blastoderm in a
typical blastula is a single layer of cells, but in certain cases it is made
up of more than one layer.

As cell division is still going on, the blastula tends to increase in size
with the increasing number of cells in the blastoderm, but these cells
differ in size and also in the rapidity of their multiplication. Those
toward the upper, or animal, pole are the smaller ones and are multi-
plying more rapidly, whereas those toward the other pole are larger and
are multiplying more slowly. This unequal growth causes an expansion
of the upper wall of the blastula and leads to an invagination of the lower
cells, the blastula thus becoming converted into a double-walled inverted
cup. As soon as this invagination begins, the embryo is termed a gas-
trula. As the gastrula develops further, the two walls come gradually
closer together until finally the cleavage cavity becomes entirely oblit-
erated. This process is called gastrulation, the cavity formed is known
as the archenieron, or primitive digestive cavity, and the opening into it
from the outside is termed the blastopore. The gastrula is thus made up
of two layers of cells; the one forming the outer wall of the cup is called,
because of its position, the ectoderm, and the one within, forming the
lining, is known as the entoderm.

Now a third layer of cells appears between the two others, being
developed in some cases from the ectoderm and in others from the

122 METAZOA IN GENERAL

entoderm. This third layer is termed mesoderm. If the mesoderm is
composed of a meshwork of scattered cells which have passed from either
of the other layers into the blastocoel, it is known as mesenchyme (Fig.
50 1). In the embryogenies of certain animals the mesoderm cells are
formed by an outpocketing of entoderm cells, which pushes into the
space between the entoderm and ectoderm (Fig. 51). In those of other
animals they are separated from the wall of the archenteron as solid
masses of cells, which later become hollow. In both of the latter cases
the cells surrounding these cavities form mesothelium (Fig. 51 I and J).

Blastocoel Blastoderm Entoderm

G Blastopore H Archenteron I

Fria. 50.—Diagrams illustrating the steps in an ideal embryogeny. A, the egg. B,
the two-cell embryo. C, the four-cell stage. D, the eight-cell stage. , the sixteen-cell
stage. F, the morula, a solid mass. G, section of the blastula, with the blastocoel. 4H,
section of the gastrula. J, gastrula in which the mesoderm cells are appearing in the
blastocoel. These mesoderm cells will form mesenchyme.

From the three germ layers tissues are developed. The tissues then
become arranged to form organs, the process being termed organogeny.

This series of stages and processes may be outlined in the following
manner:
First stage: The egg cell (normally previously fertilized).

First process: Cleavage, or segmentation.
Second stage: The morula.

Second process: Formation of the cleavage, or segmentation, cavity.
Third stage: The blastula (monoblastic, or one-layered, embryo).

Third process: Development of the archenteron, or gastrulation.

EMBRYOGENY 123

Fourth stage: The gastrula (diploblastic, or two-layered, embryo).

Fourth process: Appearance of a third layer.

Fifth stage: The triploblastic, or three-layered, embryo (not given a
particular name).

Fifth process: Tissue formation.

Sixth process: Organogeny (development of organs).

It should be remembered that the stages are not stopping points,
that each of the processes lasts for a considerable time, and that the
whole forms a continuous development. As the cleavage cavity first
appears, the embryo is spoken of as an early blastula; as it increases in
size, an older blastula; and just before invagination begins, a late
blastula. In the same way reference may be made to an early and a late
gastrula.

147. Variations in Embryogeny.—Since egg cells differ so much in
the amount and distribution of the yolk it will be clear that many varia-
tions in the course of embryogeny are bound to occur, and all the steps in
the ideal embryogeny described cannot be expected to appear in any
actual individual embryogeny. Different types of cleavage have been
previously noticed. In total cleavage the blastomeres may be spirally
instead of regularly arranged. It is then called spiral cleavage. When
the yolk is reduced to a minimum and the blastomeres are in contact
by only a small area, there may be no morula stage but cleavage may
result in the immediate development of a blastula (Fig. 51). When the
yolk is so abundant that the cleavage cavity is reduced to only a slit,
invagination becomes impossible, and the resulting overdevelopment
of the cells at the animal pole causes an outfolding. This is known as
gastrulation by epibole (Fig. 267 G). The archenteron is formed under
this fold, which may gradually grow around and envelop the whole
embryo. In the case of the mammal, as will be seen later, a very marked
change in the character of the embryonic stages results from the condition
which involves the attachment of the embryo to the wall of the maternal
uterus and its nourishment from the blood vessels of the mother.

148. Germ Layers.—Reference has been made to three germ layers.
The blastoderm, appearing in the blastula, gives rise in the gastrula to
the ectoderm and entoderm, and the mesoderm is added in the triplo-
blastic embryo. These layers, in all Metazoa but the sponges, retain
this relative position, and from each arise a certain number of tissues.

The tissues derived from the ectoderm include the epithelial covering
of the body, often known as the epidermis, which may extend inward a
short distance at the external opening or openings of the digestive cavity
or canal. They also include the epithelium lining all hollow organs the
cavities of which open to a surface covered by epidermis. This includes
such cavities as the external ear, the nasal chamber, and the spaces under
the eyelids. All nervous tissues are also derived from the ectoderm.

124 METAZOA IN GENERAL

From the entoderm is derived the epithelium lining the digestive
cavity or canal, except at the open ends; also the epithelium lining all.
hollow structures formed as outpocketings of this cavity or canal. This
latter category includes, in the air-breathing vertebrates, not only the

opie

D Nerve cord
Mesoderrn

Fig. 51.—Stages in the development of amphioxus, one of the lower chordates. (Drawn
from Ziegler models, based on the work of Hatschek.) A, the four-cell stage, polar view,
showing a crevice between the cells. B, cross section through the opposite cells of an
eight-cell stage, showing a median space. C, a median section of the 32-cell stage; the
median space is developing into a blastocoel and the embryo is becoming a blastula without
passing through the morula stage. JD, cross section of the blastula. £, invagination. F,
the gastrula has become asymmetrical and has turned on its side; the dorsal surface is
flattened, the ventral convex. G, the diploblastic embryo, showing the ectoderm growing
over and covering in the dorsal ectodermal cells which will form the central nervous system.
H, cross section of a later stage, showing the chorda, or notochord, arising as a median
dorsal outpocketing of the wall of the archenteron, and the mesoderm developing from
dorsolateral outpocketings of the entoderm. J, cross section of a later stage, showing the
central nervous system, the chorda, and the enteron in the median line, and on each side a
mesodermal pouch, containing a coelomic cavity. The walls of this pouch are mesothelium,

linings of cavities of such organs as the liver and pancreas but also the
lining of the so-called respiratory tract, consisting of the lungs and
the passageways leading from the pharynx to them.

From the mesoderm are developed all of the other tissues of the body,
including muscles, connective and supporting tissues, the blood vessels

EMBRYOGENY 25

and the blood which they contain, and the epithelial lining of all cavities
developed in this layer. The mesodermal tissues in higher forms far
exceed in bulk those from the ectoderm and entoderm combined.

It will be observed from what has been said that an epithelium may
be derived from any one of the three layers. The epidermis is ectodermal
in origin, the epithelium lining the greater part of the digestive cavity
is entodermal, and the epithelium lining the cavities within the mesoderm,
including the lining of the heart and blood vessels, is mesodermal. Skele-
tal parts may be formed not only from the mesoderm but sometimes from
the ectoderm, and even in rare cases from the entoderm.

149. Coelom.—Any cavity formed in the mesoderm and surrounded
by mesothelium is known as a coelom. When it is present the sex organs
become developed from its wall and the excretory organs open into it.
It is lacking in the lower Metazoa but it is present in most of the higher
forms, in which it may be divided into several cavities. The layer of
the mesoderm outside the coelom and lying against the body wall is called
somatic; that inside the coelom and lying against the viscera, splanchnic.

Since three cavities develop during the embryogenies of higher animals
it is well to bring them into contrast, as may be done in the table which
follows:

Name of cavity Pee Lining of wall Fate
of appearance
fuego a ee 1 ee EE eee
Segmentation or cleav-
Age eavabyae fae ee: Blastula Blastoderm Disappears as the next is
formed
Archenteron.......... Gastrula Entoderm Becomes the digestive

cavity of the adult

Coelom..............| Triploblastic | Mesoderm Becomes in the adult the
embryo (mesothelium) | body cavity and cavities
derived from it

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PART IV
METAZOAN PHYLA

CHAPTER XXVI

SPONGES
THE PHYLUM PORIFERA

In the ocean are found many animals which would not be recognized
as such by the ordinary observer, since they have neither power of move-
ment nor power of locomotion, and since they form inert masses attached
to various solid objects including the shells of other living animals. In
many cases these are sponges, though some ascidians (Sec. 340) would
fit the description. Sponges were long supposed to be plants and their
animal nature was not fully established until about 1857, since which
time they have been variously classified
in the animal kingdom.

150. Relationship of Sponges.—
In many respects sponges are like
colonial protozoans. For instance,
they possess collar cells (Fig. 52)
which are similar to the collared cells
of the colonial flagellate protozoan,
tenon: (ik 604), auntie he emerges
account the sponges were for a time chambers of a fresh-water sponge, Spon-
classified as colonial flagellate Pro- ‘yu pat Aarne ee
tozoa. They differ from them, how- Vosmaer, by the courtesy of The Macmillan
ever, in the fact that the body is Compeny:)
penetrated by a system of canals, whereas in colonial Protozoa the cells
are upon the surface of the mass formed by the colony. They also
differ in the fact that there are a number of different types of nonre-
productive, or somatic, cells which perform different functions. In this
respect sponges resemble higher animals. They have, therefore, been
considered for some time to be Metazoa.

Sponges differ fundamentally from other Metazoa by not having any
digestive cavity, which is present in some form in all higher animals
except where lost from degeneration. Instead, digestion is always
intracellular, within the cells, as in Protozoa. Neither do they have
body layers corresponding exactly to those in other Metazoa, in which
ectoderm, mesoderm, and entoderm retain from the beginning the same
relative position in the body. In the sponges the layer which appears in
the embryo as an ectodermal layer comes in the course of development
to line central cavities and to have the function of circulating water

129

Vacuole

130 METAZOAN PHYLA

through the body instead of carrying the animal about. These same
cells are also digestive. In all other Metazoa digestion is carried on by
cells of entodermal origin. In the sponges the layer which at the begin-
ning seems to be entoderm comes to lie on the surface of the body and to
perform the functions which we generally associate with ectoderm.
The middle layer is not differentiated in the way the mesoderm is in all
triploblastic animals and so is not recognized as a germ layer. The
sponges are, if anything, diploblastic, but this term is not strictly applica-
ble because of the difference in the manner of development of the two
body layers. In view of these facts it seems best to include sponges in
the Metazoa but to separate them as a distinct group from all the rest
and to call them Parazoa—literally, animals set off at one side. The rest
of the Metazoa are called Enterozoa, or animals with a digestive cavity.
The group Parazoa contains but the one phylum, the Porifera.

151. Classification.—Porifera (po rif’ ér a; L., porus, pore, and ferre,
to bear) is divided into three classes:

1. Calcarea (kal ka’ ré a; L., calcarius, limy).—Sponges which possess
spicules of carbonate of lime; all marine.

2. Hexactinellida (héx ak ti nél’ lida; G., hex, six, actinos, ray, ella,
Latin diminutive, and ezdos, form).—Sponges with siliceous spicules
having three axes; confined to the deep sea.

3. Demospongiae (dé mo spiin’ gi é; G., demos, the people, and spongia,
sponge).—Sponges with either spicules of silica, which are not triaxial,
or a supporting framework of spongin, or both; mostly marine, but with
a few fresh-water species.

152. Structure.— Various types of sponges differ greatly in their
general form, in their size, and in their plan of structure (Fig. 53). Some
are quite regular in shape, while others are irregular, being branched,
often quite complexly so, fan-shaped, or cup-shaped. Some form
raised masses, and others spread out like flat discs on the surface to
which they are attached. Some are very small and are just visible to
the naked eye, while others may be 5 feet in height. They are often
brilliantly colored, and among the different species all colors may be
seen. The shape of individuals of the same species is not always the
same, though in a general way it conforms to a certain type; it may be
much modified by environmental factors.

On the surface of the sponge are very many small openings called
ostia and a much smaller number of larger ones known as oscula
(Fig. 54). Water enters through the ostia and leaves through the oscula.
All openings are surrounded by spicules, which appear like spines, and
these may form a barrier over the ostia, protecting them from objects
which might do injury to the sponge.

Within the body of simple sponges is a gastral cavity which opens
by an osculum. In more complex sponges there may be many such
cavities, each opening by an osculum.

SPONGES 131

153. Canal Systems.—In sponges there are three principal types of
canal systems, known as the ascon, sycon, and rhagon types (Fig. 54), of
which the ascon is the simplest. The body of a sponge of this type has
a thin wall which is penetrated by simple canals that run clear through
to the gastral cavity. In this type flagellated cells line this cavity.

, ~Oscula

Fic. 53.—Different types of sponges. A, Grantia ciliata (Fabricius), one of the Cal-
carea, a simple sponge showing colony formation and budding. X 2. B, skeleton of
Euplectella sp., a hexactinellid sponge known as Venus’ flower-basket, showing the form and
general structure; the spicules are white and like spun glass. X14. C, Chalina oculata
Pallas, one of the marine Demospongiae. (From Minchin, in Lankester’s ‘‘A Treatise on
Zoology,’’ by the courtesy of A. and C. Black.) X 3¢. D, Ephydatia fluviatilis (Linnaeus),
a fresh-water sponge belonging to Demospongiae. (From Zacharias, ‘‘Die Tier- und Pflanz-
enwelt des Siisswassers.”’) XX 3%.

In the sycon type a more complex plan is presented, with incurrent
and radial canals. The ostia lead into incurrent canals which do not
open into the gastral cavity; radial canals open into the gastral cavity
but not to the outside. The two types of canals lie side by side and are
connected by minute pores. The radial canals are lined with the flagel-
lated cells.

132 METAZOAN PHYLA

In the rhagon type the animal is much larger and the whole body
forms a rather thick mass penetrated by a complexly branched canal
system. In the fresh-water sponges, which may be taken to represent
this type, the ostia lead into subdermal cavities. From these cavities
incurrent canals run to chambers lined with flagellated cells. After
the water has passed these flagellated cells it is carried by excurrent
canals into a gastral cavity, which opens to the outside by an osculum.

154. Skeleton.—The classification of the sponges depends upon the
character of the skeleton, which may be made up of spongin or of spicules.
The spicules may be either calcareous or siliceous and differ in shape in the
different forms. Spongin is a substance which chemically is similar
to silk and which is formed by cells known as spongoblasts. Spicule-

ne IX

nae Radtal canal
gt 7 igh Hlagellated PEA
chamber. Gastral cavity
ff L &. ‘3 Incurrent Se a
i ees
CF

a = canal
Yes LFS ZZ

Fic. 54.—Diagrams of canal systems of sponges. A, ascon type. 8B, sycon type.
C, rhagon type. (From Wieman, ‘‘General Zoology,’’ A and B after Minchin, and C modified
from Parker and Haswell, by the courtesy of McGraw-Hill Book Company, Inc.) The gastral
epithelium is shown by heavy black, the dermal epithelium by a light line. Arrows show
water currents.

forming cells are scleroblasts. The spicules may be straight rods with
one axis, the monaxon type; or they may have three rays in one plane
and be ¢trzradiate; or four rays lying in four planes, in which case they
are known as ¢tetraron. They may have six rays, the ends of three axes,
in which case they are triaxon; or they may have numerous rays and be
polyaxon (Fig. 55). Many modifications of each type occur.

155. Histology.—There are in the bodies of sponges a number of
different types of cells. In the outer, or so-called dermal layer, are flat
epithelial cells, contractile cells, gland cells which secrete the material
that attaches the animal to its support, and the cells which form the
skeleton. In the middle layer are reproductive cells and wandering
cells, the latter capable of ameboid movement. The cells of the gastral
layer are flat epithelial cells or collar cells. These cells, however, do not
work together to the same degree as do the cells in higher animals. The
whole is really a great colony of semi-independent cells, and individuality
is so little evident that zoologists have not agreed upon what constitutes
an individual. H. V. Wilson has found it possible, by gently squeezing
sponges through the meshes of fine silk cloth, to separate them into
individual cells. These cells will then gather together in small groups

SPONGES 133

and each group will grow into a sponge. This illustrates the semi-
independent character of the cells.

156. Metabolism.—Metabolism is carried on practically in the
same manner as it is in Protozoa. That there are, however, different
enzymes acting on proteins, carbohydrates, and fats seems to be generally
accepted.

The food of sponges consists of minute plants and animals and also
small particles of organic matter which are drawn into the ostia and
through the canals by currents produced by the movement of the flagella
of the collar cells. As this current
sweeps these objects past the collar
cells they are seized upon by the
cells and ingested by means of pseudo- e
podia. The current of water pro-
ceeds onward into the gastral cavity
and out of the body through the %
osculum. The food which has been a
taken by the collar cells is digested
in food vacuoles in the same manner
as it would be digested by proto-
zoans. Further steps in metabolism
also occur like those in protozoans. Fie. 55—Types of spicules. (From
Each cell excretes and respires for rile pane eee a ae
itself. The cells which are not collar pany.) a and b, monaxon; c, triradiate;
bells, receive. their, food more jor % 73%02; & Hinxon:, 7, polyaxou.
less directly from the latter by absorption from cell to cell, aided by the
ameboid wandering cells, which serve to carry both food and waste
matter about the body.

157. Behavior.—Little is known of behavior in sponges generally.
The larvae are ciliated and swim about, but the adults are attached
and never move from their position. Some sponges possess fiber-like
cells around the ostia and oscula which are capable of slowly contracting
and closing these openings or of relaxing and permitting them to open.
The opening and closing are so gradual, however, that they do not attract
notice unless particular attention is given to them. These openings tend
to open when the water is in motion but close when the water becomes
quiet; they also open in fresh water and in weak solutions of atropin and
close on exposure to the air or on injury to the animal or when the animal
is subject to the experimental action of weak solutions of ether and
cocaine. These fiber-like cells, since they have the function of both
receiving stimuli and contracting in response to them, are termed neuro-
muscular cells. Because groups of these cells surround openings which
are closed by their contraction, the group is termed a sphincter. (This
term is also applied to all muscles closing openings in the bodies of higher

134 METAZOAN PHYLA

animals and man, such as the muscle which shuts oif the stomach from
the intestine, that which guards the exit from the bladder, and that
which controls the passage of egested matter from the posterior end of the
alimentary canal.)

158. Reproduction.—Reproduction is both sexual and asexual. The
asexual mode of reproduction involves the gradual formation of external
buds which arise near the point of attachment of the parent. After grow-
ing for a time thus attached a bud may separate and begin an individual

Fic. 56.—Gemmules of a fresh-water sponge, Carterius tubisperma Mills. A, fragments
of old sponge and gemmules on a piece of wood. XX about 3. 8B, section of a gemmule
showing arrangement of spicules in the shell, the foraminal tubule, and the many enclosed
cells which were separated from the tissues of the old sponge. X78. (Drawn by E. F.
Powell.)
existence. If budding continues and the individuals remain together,
a colony is produced (Fig. 53 A).

In addition to external budding, some sponges have the ability
to form gemmules, or internal buds. These are groups of cells which
gather together in the middle layer and become surrounded by a siliceous
shell. They are formed when living conditions become difficult and thus
preserve the life of the organism during such periods.

The gemmules are produced in the autumn, after which the adults die;
in the spring the growing cells of the gemmule escape from the shell

SPONGES 135

through foraminal openings or tubules, or by rupture of the shells, and
develop into new sponges (Fig. 56).

Sexual reproduction also occurs, both egg cells and sperm cells being
formed in the same animal. These sex cells lie in the jelly-like middle
layer where fertilization takes place. An embryo is formed which escapes
through the wall of the body and becomes a free-swimming ciliated larva.
This later settles down and develops into a sponge.

159. Uses of Sponges.—The cleaned skeletons of those sponges which
are composed entirely of spongin are familiar because of their many
domestic uses. Among these are bath sponges and the surgeons’ sponges
used to take up blood and other fluids in surgical operations. ‘Though
today artificial sponges are made which in many cases take the place of
natural ones, there is still a large market for the latter.

160. Cultivation of Sponges.—The best commercial fibrous sponges
come from the coast of the Mediterranean Sea, from the shores of Florida
and the West Indies, and from Australia. They are gathered by means
of long-handled hooks, by dredging, or by divers. They are then allowed
to decay, are washed, dried, bleached more or less, and sent to market.

Sponge culture is now carried on in several localities but most suc-
cessfully in Italy and Florida. Commercial sponges do not flourish
where the water is cold. The place selected for this purpose must have
a clean bottom and must be exposed to currents which bring an abundant
supply of well-aerated water and food. Specimens of the variety of
sponge to be cultivated are secured, cut into small pieces approximately
one inch square, and fastened either to stakes or to sunken cement
plates. From these pieces grow complete sponges which are ready for
the market in a few years, the time depending upon the character of the
sponge grown and the conditions. When the sponge is gathered, the
part that remains after most of it is cut away will continue to grow and
develop into another sponge.

161. Relations to Other Animals.—Sponges are used as food by very
few animals, their spicules and the unpleasant character of their excre-
tions rendering them objectionable. Because of this fact many other
animals take refuge in sponges. The excretions of sponges also play a
part in the disintegration of the empty shells of mollusks, the lime of
which is thus turned back into the sea water to be used over again by
other animals.

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CHAPTER XXVII

HYDRA
A TYPE OF THE PHYLUM COELENTERATA

Hydras are abundant in bodies of fresh water everywhere and are
excluded only from those where the water is foul or the temperature too
high. They flourish in those which are clear, cool, and relatively perma-
nent. Wherever they occur they may be found attached to solid objects
in the water, such as leaves or stones on the bottom, dead tree branches
or weed stems, stakes and posts, living vegetation,
or the undersurfaces of floating inanimate objects
or plants. In any of these situations they will be
found extending at right angles to the surfaces to
which they are attached and, when hungry, with
their bodies and tentacles stretched to the limit.
When thus extended, the total length of the body
and tentacles may reach two inches or even more.
When the object to which it is attached is lifted
from the water, the animal contracts and appears
like a very small mass of green, brown, or white
jelly, depending upon the species under observation.

162. External Features——When examined.
under a hand lens, a hydra is seen to possess a
tubular body which when it is extended is of a
practically uniform diameter but which when it

ee is contracted assumes an approximately spherical
oz shape (Fig. 57). The attached end of this
_ Fic. 57.—Hydra_viri- body is known as the basal disc. The power of
a ee cn “vad, attachment is due to an adhesive substance pro-
shown in a partially ex- duced by gland cells in this dise. The free end
pen Soars eat of the body bears a ring of tentacles varying in
number. Inclosed by these tentacles is a conical
projection called a hypostome, at the apex of which is the mouth. Fre-
quently one or more buds will be seen projecting from the side of the
body, and a bud, if well-developed, may possess its own mouth, hypo-
stome, and tentacles. On rare occasions there may be observed on
the body of a hydra projections which are temporary reproductive
structures. If these are conical and are situated nearer the tentacles
136

HYDRA 137

they are spermaries, or testes; if they are more knoblike and are situated
nearer the base they are ovaries (Fig. 58).

163. Internal Structure-—When studied by means of sections
(Fig. 58) in which the structure is brought out by appropriate staining, the
hydra is seen to be made up of a body wall surrounding a large central

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Fic. 58.—Somewhat diagrammatic longitudinal section of a hydra, showing two buds
differing in age on the left, and a spermary and ovary on the right. Batteries of nemato-
cysts are to be observed on the tentacles. In the gastrovascular cavity the entoderm is
seen to be made up of flagellated cells, cells bearing pseudopodia, and gland cells. A deli-
cate cuticle covers the ectodermal layer.
cavity known as the gastrovascular cavity, or enteron, which opens to
the outside through the mouth. The gastrovascular cavity also extends
outward into each tentacle, reaching nearly to the tip, though the canal
so formed is very narrow.

The wall of the body and that of the tentacles are composed of two
layers of cells separated by an extremely thin sheet of noncellular material

138 METAZOAN PHYLA

known as mesoglea. The outer cell layer is made up of smaller cells
varying in shape, though typically cubical, the outer ends of which form
a fairly even surface covered by a delicate cuticle. This layer is known
as the ectoderm and includes a number of different types of cells which
are scattered and not associated in such a way as to form tissues. Among
these cells are epitheliomuscular cells, from which arise contractile
fibers; sensory and nerve cells, of which conducting fibers in the mesoglea
are branches; irregular interstitial cells; and on the basal disc, gland cells.
The inner cell layer, known as the entoderm, is composed of short columnar
cells of larger size and more irregular shape. The surface of the entoderm
is neither so even as is that of the ectoderm nor does it possess a cuticle.
In this layer are epitheliomuscular cells, nerve cells, and gastric gland
cells. In the tentacles the cells of both layers become much shorter and
the whole wall very much thinner.

The mesoglea, which is the layer between these two cell layers,
is composed of several elements: (1) a supporting lamella, jelly-like in
consistency, secreted by the cells of the two other layers, and giving a
certain degree of support to the body; (2) two networks of nerve fibers,
derived from the nerve cells in the two cell layers; (3) contractile fibers,
prolongations of the epitheliomuscular cells in both ectoderm and
entoderm.

164. Nematocysts.— Interstitial cells in the ectoderm of the body
in which are formed nematocysts are known as cnidoblasts. A nematocyst
(Fig. 59) consists of a sae of fluid within which is coiled a thread. When
fully developed, the ecnidoblast also possesses a projecting, sharply
pointed cnidocil. As the cnidoblasts approach full development they
migrate to the surface, and the projecting cnidocil, when it is stimulated,
causes the nematocyst to react. In this reaction the coiled thread is
thrown out, probably as a result of increased pressure within the sac.
In one kind of nematocyst this thread is hollow, sharply barbed at its
base, and carries a poison which serves to anaesthetize the animal into
which it is discharged (Fig. 59, A and B). In another type the thread
is barbless, elastic, and becomes coiled around the object against which
it is discharged (Fig. 59 #). By thus coiling around the spines and
hairs of the prey these nematocysts impede its movements. No enido-
blasts seem to originate on the tentacles themselves, but great numbers
migrate from the place of origin in the body to the tentacles, where the
nematocysts can be used most effectively in the capture of food.

165. Neuromuscular Mechanism.—While the hydra cannot be said
to possess tissues, much less organs and systems, there are developed
definite mechanisms out of the variety of cells which the body possesses.
Among these is the neuromuscular mechanism. This is made up of the
scattered sensory cells lying on the surface of both ectoderm and ento-

HYDRA 139

derm; of the nerve cells, which with their processes form conducting
networks (Fig. 60); and of the contractile fibers of the epitheliomuscular
cells. The sensory cells receive stimuli and the contractile fibers cause a
modification in the form of the body. The contractile fibers connected
with the ectoderm run longitudinally and those of the entoderm trans-
versely, the two sets thus acting like longitudinal and circular muscles.
The circular contractile fibers also tend to be concentrated around the
mouth and at the base of each tentacle, where they act like sphincters.

Fig. 59.—Sketches illustra
ing an undischarged nematocyst and possessing a cnidocil. B, the same with the nemato-
cyst discharged. (A and B from Dahlgren and Kepner, ‘‘ Principles of Animal Histology,”
after Schneider.) C, portion of a tentacle, showing the batteries of nematocysts. D, an
insect larva covered with nematocysts as a result of capture by a hydra. (C and D from
Jennings, ‘‘ Behavior of the Lower Organisms,”’ by the courtesy of Columbia University Press.)
E, last segment of the leg of a small aquatic animal, with nematocysts of a barbless type
shown coiled about its spines; this impedes the movements of the animal. (From Hegner,
‘College Zoology,’ after Toppe.) (A, B, and E are by the courtesy of The Macmillan
Company.)

The nerve cells seem to be most numerous around the basal dise and
on the hypostome, which indicates a certain degree of localization of
nervous activity. Owing to the more complete network formed by its
nerve cells, the ectoderm is more active and its movements are more
definitely coordinated than are those of the entoderm.

The nematocysts seem to be stimulated directly by chemicals in the
water, such as the secretions from the body of the animals which serve
as prey, and not by the nervous mechanism.

140 METAZOAN PHYLA

166. Metabolism.—The food of the hydra consists of any animal
sufficiently small and weak that it may be held by the tentacles, anaes-
thetized by the poison of the nematocysts, and brought to the mouth
into which it is passed. Small insect larvae and crustaceans form the bulk
of the food. After being ingested the food is digested in the enteron by
means of enzymes formed by the gland cells of the entoderm. This type
of digestion, which is met here for the first time, is termed extracellular
digestion. 'The hydra also possesses intracellular digestion, particles of
food being taken into the entoderm cells by means of pseudopodia and

Fic. 60.—Nerve net of Hydra oligactis Pallas. (From Rogers, ‘‘ Text-book of Comparative
Physiology,” by the courtesy of McGraw-Hill Book Company, Inc.) Highly magnified.

digested within food vacuoles. During the process of extracellular diges-
tion the food is carried about in the enteron and mixed with the digestive
juices; this circulation is due both to the movements of the body and to
the currents formed by entodermal flagella. After digestion the food is
absorbed into the entodermal cells and circulated by being passed from
cell to cell. Secretion is performed by certain cells in both the ectoderm
and the entoderm. Excretion is carried on by each cell for itself; and
since the body consists essentially of two layers of cells, elimination and
excretion become one process. Respiration, also, is carried on by each
cell individually. Egestion takes place through the mouth, which thus
functions also as an anal opening.

HYDRA 141

167. Behavior.—Hydras may be stationary for a time if conditions
remain uniform and food is plentiful, but with changing conditions in
the environment and the necessity of searching for food they usually
exhibit considerable locomotor activity. When hungry the hydra will
extend its body and tentacles, the latter being ready to grasp any food
which comes in contact with them. If after a time no food is encountered,
the animal moves to another location.

Locomotion may be accomplished in several ways: (1) One method is
by a gliding movement, the basal disc sliding slowly over the substratum
to which the animal is attached and at no time being free. (2) The
animal may reach over with its tentacles and after attaching them may
release the basal disc, bring it up close to the tentacles, and attach it
once more, raising itself to an erect position in the new location. If
this is repeated it represents a type of locomotion similar to that of a
measuring worm. (3) Another way has been described as a modification
of this method. The disc is released, carried clear over, and attached
again beyond the tentacles, which causes the animal to turn a sort of
handspring. (4) Finally, if the animal is dislodged, it may drop to the
bottom and use its tentacles as if they were legs. When at the bottom
of a pool it may form a gas bubble on its basal dise and by means of this
rise to the surface.

Hydra respond to several conditions in the environment. To strong
stimuli of any kind negative responses are given. To a nonlocalized
stimulus, which is one that affects the animal as a whole, it responds by
withdrawing its tentacles and contracting its body. To a localized
stimulation, such as the contact of any moderate stimulus with a single
tentacle or one particular point on the body, it responds by contracting
the area affected, which may cause the withdrawal of that single tentacle
or the bending of the body. But if the localized stimulus is a powerful
one, the other tentacles and the rest of the body will be involved, and the
reaction is then the same as that to a nonlocalized stimulus.

These animals respond to an optimum of light, which varies with
different species. The green hydra possesses an optimum at a high
degree of illumination, while the other species possess optima at a much
lower light intensity. Hydras also possess a temperature optimum which
is relatively low—that is, they flourish in cool water. They are found in
abundance under the ice-in winter but perish at a temperature which
may be reached by a shallow pool exposed to the full warmth of the
summer sun.

The response to chemicals depends upon the nature of the chemical.
The animal avoids injurious chemicals and responds positively to those
which indicate the presence of food. Chemotropism and thigmotropism
both figure in the food-taking reaction, the feeding movements being
much more vigorous if to the response due to contact with the struggling

142 METAZOAN PHYLA

victim is added the response to chemicals which may also be produced
by it.

Hydras also exhibit varying physiological states, the reaction of a
hungry hydra being distinctly different from that of one which has been
fed. When a hydra has fed, it does not seem to be affected by such
stimuli as would otherwise cause further food-taking movements and thus
does not again feed until the digestion of its food has been completed and
the undigested waste passed from the body.

168. Reproduction.—The hydra reproduces by both sexual and
asexual methods, the latter being the one most commonly observed, and
the former occurring only at times when conditions of existence become
unfavorable.

Asexual reproduction is most frequently accomplished by budding
(Figs. 57 and 58), buds being produced anywhere upon the body. They
represent outpocketings of the whole body wall, the cavity of the bud
being in direct connection with the enteron. As the bud grows, a mouth
appears at its outer end and a ring of tentacles at the base of the hypo-
stome. The bud becomes constricted at the point of attachment,
separates entirely from the parent, and begins an independent existence.
Fission, which is less frequent, is usually longitudinal, although cases of
transverse fission occur.

In sexual reproduction temporary gonads are produced (Fig. 58).
The gametes are developed from interstitial cells which accumulate at a
certain place, multiply by repeated division, and give rise to oogonia or
spermatogonia. In both the spermary and the ovary may be observed
all of the steps in a typical gametogenesis. The sperm cells produced are
exceedingly numerous and are set free in the water. In the ovary, how-
ever, one centrally located egg cell begins early to increase greatly in
size at the expense of the other egg cells, feeding upon them and taking
them into itself bodily. When this one cell becomes mature it occupies
most of the space in the ovary. The ectoderm over it is ruptured, a
sperm cell enters, and fertilization occurs. ‘This fertilized egg cell, still
in the ovary, undergoes total and equal cleavage. A hollow blastula is
formed, which becomes converted into a solid gastrula by the filling in of
the blastocoel by entoderm cells derived from the blastoderm. In the
meantime a shell has beem secreted about the embryo, which now breaks
loose from the parent and falls to the bottom. . Further changes involve
cellular differentiation and the appearance of the mesoglea. Develop-
ment proceeds at a rate varying with different environmental conditions.
Finally, when these become favorable, the embryo increases in size and,
as it elongates, ruptures the shell. Tentacles appear at one end, an
enteron and a mouth are formed by the separation of the cells within,
and the young individual gradually assumes the form and characteristics
of the adult.

HYDRA 143

The same individual may produce both spermaries and ovaries at
the same time, in which ease self-fertilization is possible. They are
usually not so produced, however, and cross-fertilization is the rule.

169. Symbiosis.—The green hydra, Hydra viridissima Pallas, exhibits
an interesting association between a plant and an animal. Each cell
of the hydra contains plant cells which are themselves individual one-
celled plants belonging to a group known as algae. These plant cells
possess chlorophyll and carry on photosynthesis. The association
therefore represents a partnership in which both partners profit, the
alga receiving carbon dioxide and nitrogen Ecfoderm
from the hydra and the hydra in turn being
furnished with oxygen. Such an association
is called symbiosis. By virtue of this condi-
tion the green hydra has its reactions some-
what modified, particularly its reaction to
light, a liberal supply of which is needed by (4% J Gestrovasculor
the algal cells. ES cavity

170. Regeneration.—Regeneration is the
replacement by an animal of any portion of
the body which has been lost. It occurs
naturally after an accident has befallen the
individual, and it can be induced artificially
by mutilation. In the hydra it readily Entoderm —
occurs, and very small fragments may thus SAS ee ee eet
developintocompleteanimals. Whileregen- showing the minute algal
eration may result in an increase in numbers, oe ae we
it is not a normal method of multiplication entoderm cells. (From Whit-
and cannot, therefore, be considered as repro- "4 Btelogical Bulletin, vol. 15.)
duction. A hydra which has been partially divided into parts may
regenerate in such a manner as to produce a compound animal with
several hypostomes, each with a mouth and a ring of tentacles. Parts
of two individuals may be grafted together, but they must be of the same
species.

The hydra was the first animal known to have the power of regener-
ation, the discovery being made in 1744 by an Englishman named
Trembley. It has been a favorite type for experimentation in this
field ever since.

CHAPTER XXVIII

COELENTERATES IN GENERAL

The hydra belongs to a third phylum, known as Coelenterata, made
up of animals which differ markedly from either Protozoa or Porifera
and which, though they are very simple, agree in the general plan of
structure with higher animals. This plan involves the existence of a
central digestive cavity or enteron with a mouth opening into it. The
wall of the body is made up of two cell layers, ectoderm and entoderm,
which occupy the same relative positions as the corresponding germ

Gastro- vascular
= cavity

A Ress B
Fic. 62.—Diagrams illustrating the comparison of the structure of a polyp (A) with that of
a medusa (B).

layers do in the embryo. For the reason that only two germ layers
are considered to be represented, the coelenterates are termed diploblastic
and may be compared to the gastrula stage in the development of higher
animals. In some cases, as in certain sense organs, collections of similar
cells form simple tissues, but there is no development of true organs.
The coelenterates have a radial type of symmetry, the number of anti-
meres varying in different groups but tending to be of an even number.
Another characteristic of all coelenterates is the presence of nematocysts,
which have already been described.

171. Polyps and Medusae.—Coelenterates exist in the form of two
general types. Those of the type known as polyps are attached to some
object. They have a mouth and almost without an exception possess a
ring of tentacles at the free end. Those of the medusa, or jellyfish,
144

COELENTERATES IN GENERAL 145

type are free-swimming. Typically they have a bell-shaped body, with
a ring of tentacles around the margin of the bell, and a mouth, which also
may be surrounded with tentacles, in the center of the lower surface.

Neglecting the many various modifications of both the polyp and
the jellyfish types, a typical polyp and a typical medusa may be directly
compared (Fig. 62). If one should imagine a polyp to be turned over
with the mouth directed downward; to be greatly broadened by lateral
extension of the body, which becomes bell-shaped; to have the tentacles
carried out to the margin of the bell; and then to show a great increase
in the amount of mesoglea, one would have an animal with some of the
marked characteristics of a jellyfish. The digestive portion of the enteron
lies more or less in a projecting manubrium which hangs down like a
clapper in the bell and at the tip of which is the mouth. ‘The increase
in the amount of mesoglea, which is almost all water, lessens the specific
gravity of the body so that it is very little more than that of water.
This enables the jellyfish to float easily. The increase in the amount
of mesoglea, however, renders necessary the development of a system
of canals to put the enteron in communication with all parts. This
need is met by radial canals leading from the central enteron outward
to a circular marginal canal, the latter in turn being in communication
with the canals of the tentacles.

172. Classification.-The phylum Coelenterata (sél én tér a’ ta; G.,
koilos, hollow, and enteron, intestine) is divided into three classes:

1. Hydrozoa (hi dré 20’ a; G., hydra, water serpent, and zoon, animal).
Includes fresh-water hydroids, colonial marine hydroids, floating hydroid
colonies like the Portuguese man-of-war, some of the smaller jellyfishes,
and the polyps which produce the stag-horn coral.

2. Scyphozoa (sifd 26’ a;G., skyphos, cup, and zoon, animal).—
Includes the larger jellyfishes.

3. Anthozoa (in thd 20’ a;G., anthos, flower, and zoon, animal).—
Contains the sea anemones, most of the coral-producing polyps, and also
those colonial forms known as sea fans and sea pens.

173. Hydrozoa.—Among Hydrozoa the polyp type prevails. The
hydra is an example of this group. Though they may be variously
modified, hydrozoan polyps are always comparatively simple. The
hydroid colonies have a pronounced superficial resemblance to plants,
which they were at one time supposed to be. This led to the name of
zoophytes—literally, animal plants—now rarely used. These colonies
are found attached to various objects in the water, sometimes completely
hiding the surface of the object and extending outward to a distance of
several inches (Fig. 63). The jellyfishes belonging to this class are
characterized by the possession of a velum, a circular shelflike fold which
runs inward from the margin of the bell and incloses a chamber below
the body (Fig. 70). The velum assists in locomotion by alternate

146 METAZOAN PHYLA

dilation and contraction which forces water out through a central opening
in it with force sufficient to drive the animal through the water. Though
a system of radial canals is developed in hydrozoan jellyfishes, these
canals remain, generally speaking, few in number and unbranched.
The hydrozoan jellyfishes are relatively small, most of them being less
than an inch in diameter and the giants among them reaching a diameter of
only 15 inches. They have a marginal row of tentacles and no tentacles
around the mouth or, at most, a limited number.

174. Scyphozoa.—The jellyfishes of this class (Fig. 64) are very
large as compared to the hydrozoan jellyfishes. There are records of
individuals 714 feet in diameter
and possessing tentacles over 100
feet in length when fully extended.
Bulky as such individuals are,
they consist almost entirely of
water and when dried form only a
thin film. There are in some cases
scattered ameboid cells in the
mesoglea but these are not con-
sidered as forming a third layer.
These jellyfishes differ from the
hydrozoan Jjellyfishes in not hav-
ing a velum; in having a com-
plexly branched system of radial
canals; in the fact that the mar-

colonial

Fic. 0o3.— Colonies of marine
hydroids. A, Pennaria sp. X about 3.

Medusa buds are shown attached to the sides
of the polyps from which they have been
developed. B, Sertularia sp. X2. Ba,
portion of a branch showing three pairs of
polyps retracted into the sessile hydrothecae.

gin of the bell is divided into sec-
tions by notches, in each of which
is a pair of marginal lappets; and,
in many cases, in the abundance

x 40. of fringed tentacles surrounding
the mouth. Many of these forms exist only as jellyfishes, generation after
generation, but in some this type alternates with a modified type of polyp.

175. Anthozoa.—Among the Anthozoa are the sea anemones, which
are polyps in which there extends downward from the margin of the
mouth into the enteron a tubular membrane forming a stomodeum, or
gullet (Fig. '65). The stomodeum, in turn, is fastened to the body wall
by radially arranged membranes called mesenteries. These divide the
enteron into a number of chambers which may be entered from below.
Between these mesenteries are shorter ones running inward from the
body wall and not meeting the gullet; thus recesses are produced on the
outer wall of the chambers. Since these incomplete mesenteries may
vary in length, they produce recesses of several degrees of depth and of
varying breadth. Openings in the upper part of a mesentery, putting
two chambers into communication, are called ostia.

COELENTERATES IN GENERAL 147

Hla

Fie. 64.—Two scyphozoan jellyfishes. A, Rhizostoma pulmo Haeckel. B, Chrysaora
hyoscella. (From Lankester, “A Treatise on Zoology,” by the courtesy of A. and C. Black.)
The first of these reaches such a size that the bell is 2 feet in diameter; in the second the
bell may be 6 inches across.

Stormodeum leading
Into enteron Lio about rrouth

Erid of av
acorntiuryt

‘e y , -
ay
ii
Citreee
“ ‘cua

Primary meseritery
Reproducti ve organ

A Edge of mesentery

Fic. 65.—A sea anemone, Metridium dianthus Ellis. (From Woodruff, ‘‘ Animal
Biology,” by the courtesy of The Macmillan Company.) A, view of polyp with one quadrant
removed. 8B, diagram of transverse section, reduced in size, showing the general plan of
the mesenteries.

148 METAZOAN PHYLA

Anthozoan polyps are much firmer in texture than are those of
Hydrozoa and Scyphozoa, and the skin, though soft, is tough. Bands of
contractile fibers lie on the surface of the mesenteries and by their
contraction enable the animal to protect itself by drawing the body
down into a compact mass with the mouth and tentacles completely
hidden from view. In some cases cells exist among these contractile
fibers, but these are not considered to form a mesoderm (See. 185). ~ The
upper surface of the polyp is covered with many hollow tentacles the

i, cavities of which communicate

ae ; with the enteron. Nematocysts

ee are found on the tentacles and

epee also on the acontia, which are
AN threadlike structures attached to
the base of the mesenteries
and capable of being protruded
through the stomodeum = or
through pores in the wall of
the body. Acontia are believed
to serve as weapons of offense
and defense, while the tentacles
are the food-securing structures.
Sea anemones usually exist as

Fig. 66.—Diagram to illustrate the forma- _.-
tion of coral by a coral polyp. (From Thomson, single polyps, though groups
“Outlines of Zoology,” after Pfurtscheller, by the may be formed by budding.
courtesy of D. Appleton & Company.) This indivaducle may attain’ ay diem
shows the formation of a basal plate and radial
septa; it does not show the external wall or theca eter of a foot or more.
al ee nes ae a eaieseeteae Bie) A coral animal, which is

usually an anthozoan polyp,
secretes lime under the basal dise and around the side of the body, form-
ing acup. The mesenteries extending inward from the outer wall of the
body are continued across the basal wall and tend to meet at the center.
Ridges of lime are secreted alternating with these mesenteries on the
basal wall and, when the coral polyp is removed, indicate the plan of
their arrangement (Fig. 66). Solitary coral polyps exist which may be
several inches in diameter, but very frequently coral animals live in
large colonies. The colonial polyps average smaller than the solitary
ones, the smallest not exceeding 4 inch in breadth. In the case of
the sea fans and sea pens a very large colony of exceedingly minute
polyps builds a skeleton of characteristic shape which suggests the com-
mon name (Fig. 67).

176. Color.—Hydroid colonies are generally whitish in color, though
they may show a slightly brown or yellow tint. Anthozoan polyps
are often very brightly colored. Many jellyfishes are perfectly transparent

Meseritéry

all re septum
“Basal plate

COELENTERATES IN GENERAL 149

and the mesoglea reflects rays of light with erystal-like clearness. Some,
however, are beautifully tinted, and a few are strongly colored. Though
all colors may be found, the prevailing colors are blue, various shades of
rose or pink, yellow, or brown. Jellyfishes are more or less luminescent
at night.

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Fic. 67.—A, sea pen from Puget Sound, Ptilosarcus quadrangularis Moroff. xX 4.
The polyps are on the edges of the leaflike folds; the stalk is imbedded in mud or sand at
the bottom of the sea when the animal is under natural conditions, but does not anchor the
animal to a particular location. B, dried skeleton of a sea fan, Gorgoniasp. X %. This
eclony is anchored to a mass of coral rock. C, portion of a sea fan colony, showing the
polyps. x 8. All from preserved specimens.

177. Polymorphism.—Polymorphism is a phenomenon which involves
the appearance of the same species of animal in different forms. It
is very generally exhibited by coelenterates. For instance, the colonies
of many of the marine hydroids have polyps of two types, one nutritive,
the other reproductive. In addition to these two forms there may also
be the medusa, which represents a third form of the same species (Fig.
70). In the case of certain colonial hydroids there are several distinct
types of polyps, accompanied by a division of labor between the indi-

150 METAZOAN PHYLA

viduals in the colony. In the Portuguese man-of-war (Fig. 68), for
example, there are polyps which are nutritive, others which are sensory,
others which contain batteries of nematocysts as weapons of offense and
defense, still others which contain male gonads, and finally some which

give rise to egg-producing medu-
AOR EMILCT sae. There remain to be added
= Nad to this enumeration polyps which
unite in the production of a gas
bag that serves to float the organ-
ism at the surface of the sea.
This kind of polymorphism, where
the unlike individuals are united
in a single organism, is very rare.
Polymorphism, however, is very
general in the animal kingdom and
may or may not be accompanied
by division of labor.

178. Metabolism.—In most
coelenterates ingestion of food
occurs by means of the tentacles,
which secure the food and pass it
into the mouth. As a hungry
jellyfish is carried along through
the water by a current, aided in
some cases by pulsations of the
bell, its tentacles trail below and
behind forming a net in which the
. prey is entrapped. These tenta-
€ cles may be spun out till they

» resemble exceedingly fine threads;
they are not strong, but, fur-

Fic. 68.—A Portuguese man-of-war, Phy- nished as they are with batteries
salia pelagica Bose. (From Packard, ‘‘Zoology,” : : :
after Agassiz.) X14. The tentacles are ca- of nematocysts which soon para-
pable of extension to a length of over 40 feet, lyze the struggling victim, they
and bear thousands of minute nematocysts. :

serve well their purpose. The
larger jellyfishes capture many animals of considerable size, including even
fish. In a similar manner sea anemones soon quiet the luckless animal
which runs or falls upon the expanded tentacles, after which it is passed
from one group of tentacles to another until it is put into the mouth.
The smaller jellyfishes cannot sting severely enough to be noticed by a
human being, but the larger ones may cause a marked effect, the sensa-
tion being similar to that following the sting of nettles.

The steps in metabolism in all coelenterates are similar to those
described for hydra.

NU M Yue

nN Ke 10) Me

| me “a
Wes

COELENTERATES IN GENERAL 151

179. Behavior.—Coelenterates respond to various stimuli, their
responses in general being similar to those of the hydra. The hydra,
however, has only scattered sensory cells, while many of the other
coelenterates develop specialized sensory structures. On the tentacles of
a jellyfish are groups of tactile cells. Between the bases of the tentacles
and along the margin of the bell are structures which are believed to
function as organs of equilibrium and hence are called statocysts. Other
groups of cells are recognized as being olfactory. Finally, there are
pigment spots which are sensitive to light.

180. Reproduction.—In coelenterates generally the same types of
reproduction occur as in the hydra. In the case of the hydra, however,

Ei 99 cell @ |,
a sa Fertilization
me sperm cell from

/ gh 5
4-cell stage 2-cell stage another arirmal

Blastula Gastrula Planula

Scyphistoma

Fic. 69.—Diagram illustrating the life history of a scyphozoan jellyfish (Aurelia). A
section of the body of a female animal is shown with gonads (A), from one of which an egg
cell is produced. This is fertilized by a sperm cell from another animal, passes through the
two-cell and four-cell stages, later becomes a blastula (B), then a gastrula (C), which is
shown in section, and finally develops into a ciliated planula larva (D). After a time this
becomes attached and changes to a scyphistoma (EF), from which is developed a strobila (F).
Each ephyra (G@) from this strobila is the young of another animal.
the one individual exhibits all these types, while in other coelenterates
asexual reproduction is often restricted to the polyps and sexual reproduc-
tion to the medusae of the same species. Budding may by its repetition
give rise to colonies consisting of many hundreds and even thousands of
individuals. Medusae remain single, are usually either male or female,
and shed germ cells into the water. While, as a rule, they exhibit sexual
reproduction, jellyfishes may produce other jellyfishes by budding from
the surface of the manubrium. When the sex cells unite in fertilization,
the embryo which develops grows into a ciliated free-swimming larva
known as a planula, which, after its free life, settles down and becomes
the parent individual of a hydroid colony.

Among the seyphozoan jellyfishes occurs an interesting type of
budding called strobilation (Fig. 69). The planula, after becoming

152 METAZOAN PHYLA

fixed, develops into an individual somewhat like a hydra, known as a
scyphistoma. This forms at its outer end a series of saucer-like buds
piled one upon another, which as they grow older gradually develop into
medusae ; each of these buds is called an ephyra. When the scyphistoma
has developed a whole series of such buds it is called a strobila. As the
ephyrae are formed successively from the outer end of the parent indi-
vidual it follows that the oldest will always be at the free end of the pile
and the youngest at the lower end next to the parent. The ephyrae,
when freed, gradually develop into mature jellyfishes.

Li fom

another
anima,

Retracted
Ln waanth

Egg- Vere zation

Youn aN
hyctrcinth\ 4

Fic. 70.—Diagram illustrating metagenesis, and also polymorphism, in the life history
of a species of Obelia. A, portion of a colony, with hydranths and a gonangium; these and
the medusa show three forms of the same species, which is polymorphism. B, sexual
medusa, produced in the gonangium by budding and set free in the water. An egg cell
from this is fertilized by a sperm cell from another animal, passes through two-cell and
four-cell stages, and in time becomes a blastula, C. This passes through a gastrula stage
and finally becomes a ciliated planula larva, D. The larva settles down, becomes attached
(EZ), and from it a new colony is formed (F and A).

181. Metagenesis.—The phenomenon of a budding generation being
followed by a generation which produces egg cells and sperm cells is
known as alternation of generations, or metagenesis. The marine
hydroids very generally illustrate this phenomenon, and Obelia may be
taken as an example (Fig. 70). A colony of Obelza consists of individuals
that have been asexually produced by budding from a parent which
in turn was developed from a sexually produced planula. These polyps
are of two types—nutritive and reproductive. The nutritive individuals,
or hydranths, provide food both for themselves and for the reproductive

COELENTERATES IN GENERAL 153

individuals, or blastostyles. The latter produce medusae by budding,
and the medusae in turn produce sperm cells and egg cells. From the
fertilized egg cells develops another generation of planulae. The
polyps thus represent the asexual generation and the medusae the sexual.
Metagenesis is also shown by scyphozoan jellyfishes, the medusae
reproducing sexually and the strobila asexually. The hydra does not
show metagenesis, because the same individual exhibits both types
of reproduction.

Fic. 71.—A, portion of a hydrozoan coral, the pepper coral (Millepora sp.), often
called the stag-horn coral. Natural’size. B, a portion of a colony with polyps of red
jeweler’s coral, Corallium rubrum. C, beads made from the skeleton.

182. Corals.—Coral is a deposit of lime formed by coelenterate
polyps. One type, the pepper coral, or stag-horn coral (Fig. 71A), is
distinguished from the rest by the fact that being produced by a simple
hydrozoan polyp its mass is relatively continuous and the pits which
lodged the living polyps are simple. On the other hand, most anthozoan
corals possess pits which are larger, deeper, and show radial ridges of
varying lengths. They are often delicately sculptured, producing a
very beautiful effect. Among such corals (Fig. 72) are those known as
the elk-horn coral, the brain coral, the rose coral, and the mushroom
coral. The organ-pipe coral and the red, or precious, coral fall in a third
group, produced by polyps related to sea fans and sea pens. Coral

154 METAZOAN PHYLA

polyps build masses of coral which after long periods of time become
very extensive and are known as reefs. These when they margin the
shore are called fringing reefs but when they lie at a distance from shore,
inclosing a lagoon, are known as barrier reefs. The Great Barrier Reef
of Australia (Fig. 74) is between 1100 and 1200 miles in length, and the

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Fic. 72.—Several types of anthozoan corals. A, brain coral, Meandrina sinuosa
Lesueur. X14. B, rose coral, Meandrina meandrites (Linnaeus). X 24. C, portion of
an elk-horn coral (Acropora sp.). X 2%. D, portion of another branching coral (Oculina
sp.). 24. E, mushroom coral (Fungia sp.). X 4%. F, part of an organ-pipe coral
(Tubipora Sor x 24. The last is made by an aural related to the sea fans and sea
pens; the rest fall ats another group, the corals of which are known as stony corals.

lagoon it incloses is in places 30 miles wide and reaches a maximum
depth of 25 fathoms. When a barrier reef surrounds a submerged island,
producing a circular reef with a lagoon in the center, it is known as an
atoll (Fig. 73).

183. Distribution and Economic Importance.—Coelenterates are
distributed generally throughout all seas. A few representatives, none
of which builds a skeleton, occur in fresh water. Coral polyps are
much more abundant in the tropical regions and disappear entirely

COELENTERATES IN GENERAL 155

north of a latitude about equal to that of the northern boundary of
this country.

———e

Fic. 73.—The Diego Garcia atoll in the Indian Ocean. It is 13 miles in length and
encloses a lagoon 100 feet deep. The land rises to about 15 feet and supports coconut
palms and other vegetation. (Based on a diagram from J. Stanley Gardiner, “ Coral Reefs
and Atolls.’’)

Coelenterates are relatively unimportant economically. Many
tropical islands, however, particularly in the West Indies, are composed
largely of coral rock built up in ages past. When first exposed this rock

; i is se 4 ; Ses = mse
Fic. 74.—A view of the Great Barrier Reef of Australia, with the mainland in the
distance. The tide is low and much of the reef is exposed displaying a great variety of

corals. (Photograph from the American Museum of Natural History, original by Saville
Kent.)

is soft, but it hardens on continued exposure to the air. While soft
it is easily sawed into blocks and used for building purposes. Precious
coral is used in jewelry but is valuable only in case it possesses a conven-
tional tint and a considerable degree of hardness. Corals of other kinds
are frequently displayed as ornaments. Floating coelenterates serve
as food for larger marine animals.

CHAPTER XXIX

PHYLUM CTENOPHORA

Bearing considerable resemblance to coelenterates because of their
jelly-like consistency, the ctenophores are considered by many zoologists

Stornodeum

SN Paddle
\~ Plate
a
Ben
ae
ey
i
il
i"
i)
pry
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NY)

J Ne tacle

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\

ZT
& EEE.

Renae

A
NX
ey
Mele —Paddle plate

Fig. 75.—A Pleuro-

cetenophore,
brachia bachet A. Agassiz, from Puget

Sound. A, the organism seen from
the side. Somewhat diagrammatic.
x 1%. B, diagram of a cross section
of the animal to show the relations of
the canals.

to be a class of Coelenterata. For
reasons which will soon appear, how-
ever, it seems more logical to put
them in a separate phylum, Cteno-
phora (té ndf’ dra; G., ktenos, comb,
and phoros, bearing). The cteno-
phores are all marine, and the species
are relatively few in number. They
are widely distributed but are most
abundant in the tropics. They are
very transparent and usually delicately
tinted with some shade of blue,
lavender, or pink.

184. Structure.—A typical cteno-
phore is ellipsoidal or nearly spherical
in form and possesses eight rows of
paddle plates running from one pole
to the other (Fig. 75). Each paddle
plate is a projecting shelf formed by
the fusion of the bases of cilia which
themselves fringe the margin of the
plate. When seen from the side the
plates resemble the teeth of a comb,
which suggests one common name for
this group—comb jellies. Because
these rows of paddle plates form
roughened ridges running from one
pole to the other somewhat resemb-
ling the ridges on a walnut, cteno-
phores are often called sea walnuts.
The mouth is at one pole and leads

into a stomodeum, which is connected with a series of canals run-
ning through the body. On each side is a sac into which may

156

aw A

PHYLUM CTENOPHORA 157

be retracted a tentacle; the tentacles, on the other hand, may be protruded
to a considerable distance and trailed behind when the animal is moving.
These tentacles bear cells which secrete an adhesive fluid and which are
therefore known as glue cells, or colloblasts. At the aboral pole of the
body is a collection of sense cells called a statocyst, which is believed to
function in the maintenance of equilibrium.

Other ctenophores differ greatly from the typical form. In some
the tentacles are lacking and the body is bell-shaped; others are pear-
shaped, and a very different type, known as the Venus’ girdle, is elon-
gated, flattened, and bandlike, being sometimes over 3 feet long.

Ctenophores are monecious, the gonads lying on the walls of the
meridional canals, which are under the rows of paddle plates, and the
sex cells being passed out through the stomodeum.

185. Advances in Body Plan.—The arrangement of the bands of
paddle plates give to ctenophores the appearance of having radial
symmetry, but the arrangement of the internal canals and the presence
of oppositely placed tentacles suggest bilateral symmetry. This com-
bination of the two is by some termed bzradial. Since bilateral sym-
metry is usually associated with the possession of a definite anterior and
posterior end and a greater degree of purposefulness in movement, the
etenophores represent an advance over the coelenterates.

Another advance is the development of a mesoderm, simple as it is
and represented merely by muscle cells. In contrast to the contractile
fibers of coelenterates, which are only parts of cells, these muscle
fibers are themselves modified cells. Also what is here termed a meso-
derm is formed from the entoderm during the gastrula stage, while the
cells which may be found in the mesoglea of coelenterates are derived
much later from the ectoderm or entoderm and do not represent a true
germ layer. Ctenophores may thus be considered triploblastic. This
advance is significant in that it is the first step toward the building up
of the powerful bodies possessed by higher forms and composed mainly
of tissues derived from the mesoderm.

186. Activities.—Like the jellyfishes, ctenophores float in the water
and are carried here and there by tide and wind. In locomotion the
animal is propelled by the paddle plates which strike from the oral, or
anterior, end backward. The movement is initiated, however, at the
posterior end and wave after wave of movement passes rhythmically
from this end forward along each row of these plates. When the animal
is seen in the sunlight this frequently gives the effect of successive series
of rainbow colors passing from one end of each band to the other. As
the animal swims the tentacles trail behind, ready to capture any small
organism that may come in contact with them and be held either by
adhesion or by the coiling of the tentacles. Though the paddle plates

158 METAZOAN PHYLA

are not able to propel the ctenophores with a speed approaching that
attained by the coelenterate jellyfishes, the animals appear to have
more definiteness of movement.

Like jellyfishes, ctenophores serve as food for many marine animals,
even for such large animals as whales. They are, however, of no direct
economic importance to man.

CHAPTER XXX

FRESH-WATER PLANARIAN
A TYPE OF THE PHYLUM PLATYHELMINTHES

If the bottom of a spring-fed pool or the vegetation in it is carefully
examined, it is probable that there will be seen a number of soft, flat,
dark-colored worms gliding over surfaces with no apparent effort. These
are planarians. They may also be found in streams and in permanent
fresh-water ponds, in which the water is pure, cool, and clear. Some
species seem to prefer being in currents while
others prefer quiet water. When at rest, pla-
narians tend to gather under stones lying on the
bottom or in other places where the light is
not bright. When abundant they may be
attracted in great numbers to pieces of meat
deposited in the water along the shore.

187. Structure.—A planarian (Fig. 76) has a
body which iselongated, flattened dorsoventrally,
blunt at the anterior end, and tapering to a
point at the posterior end. The anterior end
may be relatively square or it may be triangular
with the apex pointing forward. This portion
of the body is recognized as a head because it
goes ahead in locomotion and because it possesses
a nerve center, but it does not fulfill the ordinary
conception of a head since it does not contain a
mouth. There may be two lateral projections at Lame) CALL A

; maculata Leidy. A, dor-
the base of the head which are termed ears, but sal view. (From Wood-
these neither hear nor have any other function ee ear ones
of their own. mal turned to show features

On the upper side of the head and near each °% the ventralsurface. X 6.
other in the median line of the body may be seen two eyespots. On
the ventral side of the body as far back as the middle, or even farther,
is the mouth, through which may protrude the proboscis. The surface
of the body is soft and is covered with cilia, which aid in locomotion.

188. Internal Structure.—Careful examination shows a planarian
to possess many structures which have been found in none of the phyla

159

160 METAZOAN PHYLA

hitherto studied. The mesoderm is well developed, forms the greater
part of the mass of the body, and is composed of a meshwork of living
cells inclosing small spaces and known as the parenchyma. This paren-
chyma is covered externally by the epidermis. In it are imbedded
a variety of other structures which make up more or less definitely
organized systems.

One of these systems is the digestive system (Fig. 77 A). A mouth
opening leads into a mouth, or buccal, cavity, which contains a protrusible
pharynx. ‘The wall of the pharynx forms a fold which projects forward

Cephalic
gang/. fon

Enteron Excretory
ubes
Excretory
pore
Pharynx Nerve
cord
Open rg of
pharynx
Peripheral
Mouth Nerve
Ay j
A B Cc

Fig. 77.—Systems of a planarian. Somewhat diagrammatic. A, digestive system.
B, excretory system. C, nervous system. (From Parker and Haswell, ‘‘ Text-book of
Zoology,” after Jijima and Hatschek, by the courtesy of The Macmillan Company.)

into that cavity for nearly its full length and at its outer end is entirely
free. When protruded beyond the mouth opening, the pharynx forms
a proboscis. The pharyngeal cavity leads into another cavity called
an enteron, though sometimes referred to as an intestine. This consists
of three main trunks, one anterior and two posterior and lateral, each
of which has a large number of blind extensions, the whole representing
a very complicated gastrovascular cavity.

FRESH-WATER PLANARIAN 161

The excretory system (Fig. 77 B) consists in part of scattered flame
cells (Fig. 78), which are hollow and contain a mass of cilia extending into
the cavity of the cell. The cilia by their movements suggest a waving
flame. The cavities of the flame cells communicate directly with slender
tubes. The walls of these tubes are composed of a single layer of epi-
thelial cells or, in the opinion of some observers, the tubes are made up of
tubular cells placed end to end. These tubes lead into larger and larger
ones until finally a pair of longitudinal and much coiled tubes is reached.
One of these coiled tubes lies on each side of the body and the two are
connected with a transverse tube at the anterior end, opening by two
small pores on the dorsal surface behind the eyespots. Other openings
to the outside along the longi-
tudinal tubes also exist. These = pyefeys ibe ried Nucleus
tubes are full of fluid containing
the waste matter, and the flame
cells, by means of their cilia,
produce a current which carries
this fluid outward.

Muscle tissue is present in
the form of sheets and bundles,
the fibers of which run in differ-
ent directions. An outer cir-
cular layer lies just under the
epidermis, and below it are A

| : fvexternal linterna Fic. 78.—Semidiagrammatic sketches to
ayers of external and Imternal jhustrate the flame cells of a planarian. (From

longitudinal fibers which are Benham, in Lankester’s, ‘‘ A Treatise on Zoology,”
; mated: eet onli by the courtesy of A. and C. Black.) A, part
separate y a set Of ODNQUE oF the system of excretory tubes, showing

fibers. Bundles of muscle fibers the relation of the flame cells to them. Nuclei
= ; are seen in the walls and tufts of cilia project-

also pass through the body dorso- ing into the tubes, adjacent to the nuclei.

ventrally, between the branches 8, flame cell.

of the intestine.

The nervous system (Fig. 77 C) is very simple. Two masses of
nervous tissue containing nerve cells and fibers he below the eyespots
and form a nerve center. Such a collection of nerve cells is termed a
ganglion. From these ganglia cords of nervous tissue, which are gang-
lionic in character, pass back, one on each side of the body. Both the
ganglia and these two cords are connected by commissures. A com-
missure is a bundle of nerve fibers which in a bilaterally symmetrical
animal crosses the middle plane of the body and connects corresponding
nerve centers on opposite sides. A similar connection between centers
on the same side is known as a connective. A nerve is a bundle of nerve
fibers not in a center which conveys impulses from one part of the body
to another. From both ganglia and nerve cords, peripheral nerves are

Branches
of the cell

Excretory
tubule

162 : METAZOAN PHYLA

distributed to the internal structures of the body and to the surface,
particularly to the head, which is the most sensitive region. Since the

Ovary Testes

Yolk glands

Oviduct vasa efferentia

Vas deferens

Uterus Prostate gland

Seminal vesicle

Cirrus

Genital atrium

vagina Genital pore

Fic. 79.—Reproductive system of a planarian. Somewhat diagrammatic. (From
Parker and Haswell, ‘‘ Text-book of Zoology,’ after Jijima and Hatschek, by the courtesy of
The Macmillan Company.) The male organs are shown on the right side of the median
line, the female organs on the left, each being omitted on the opposite side.
ganglia have connective tissue sheaths which more or less protect them,
they may be recognized as simple organs. The peripheral nerves also
have similar sheaths.

The reproductive system (Fig. 79) is the most complex of all and most
fully justifies the term system. The male organs consist of numerous
spherical testes, which are masses of cells lying in the parenchyma. To

FRESH-WATER PLANARIAN 163

each of these testes is connected a tube of thin epithelial cells called a
vas efferens. All of these vasa efferentia join a large lateral tube on each
side of the body known as a vas deferens which leads to a muscular
cirrus, or penis, situated toward the posterior end of the body near the
ventral side and behind the pharynx. A seminal vesicle, in which the
sperm cells are accumulated, lies at the base of the cirrus. A number
of prostate glands pour their secretion into these passages, the function
of their secretion being to stimulate the sperm cells to activity. The
sperm cells pass from the testes down the vasa efferentia and vasa defer-
entia and are transferred by the cirrus, in packets called spermatophores,
to the uterus, either of the same or of another individual. In the uterus
they are stored until needed.

The female reproductive organs consist of two ovaries and two
tubular oviducts, one on each side, through which the egg cells pass
backward. Yolk glands scattered along the course of the oviducts add
their secretion to the egg cells as they pass, and a vagina transmits these
cells to a genital atrium, a chamber which receives the opening of the
cirrus. The pouchlike uterus is also connected with this genital atrium.
The term uterus is applied to that portion of the oviduct in which the
eggs accumulate and in which a part of the development of the embryo
takes place. The genital atrium opens by a genital pore on the ventral
surface behind the mouth.

The body is covered by a simple ciliated epithelium attached to a
basement membrane. There are no structures corresponding to cir-
culatory, respiratory, or skeletal systems. It is interesting to note that
in the development of systems the reproductive system, upon which
the perpetuation of the race depends, is in the lead, followed by the
digestive system, necessary to the existence of the individual and the
nervous system, which puts the animal in touch with its environment.

189. Metabolism.—The food of a planarian consists mostly of animal
matter, though a little plant food is taken. The animal secures the
food by means of its proboscis, through which ingestion takes place.
Digestion, as in the case of the coelenterates and ctenophores, is both
intracellular and extracellular. The food is distributed to all parts of
the body by the much branched gastrovascular cavity, the wall of which
serves everywhere for absorption. Circulation occurs by the passing
of the absorbed food from cell to cell and through the spaces between
the cells. Elimination takes place by means of the excretory system,
and egestion is through the mouth. Respiration goes on over the entire
surface, there being no structures developed for this purpose, though
some authors attribute to the excretory system an expiratory function.

190. Reproduction.—This animal reproduces asexually as well as
sexually. Asexual reproduction is usually by transverse fisszon.

164 METAZOAN PHYLA

Although both male and female germ cells are produced in the same,
individual, cross-fertilization rather than self-fertilization occurs. The
egg cells received by the atrium from the vagina are passed into the
uterus, where they are fertilized. When cross-fertilization takes place,
the cirrus of one animal is protruded and inserted through the genital
atrium into the uterus of the other. In this manner sperm cells are
transferred from the animal which takes the part of a male to the uterus
of the other which takes the part of a female. Every egg cell is sur-
rounded by a large number of nurse cells, or yolk cells, each of which
contributes its store of nourishment to the egg cell to which it becomes
attached. One or more egg cells, with the yolk cells and the yolk, are
then enveloped in a shell, sometimes called a cocoon, and deposited on
stones or on vegetation in the water. The egg cell divides into blasto-
meres which increase in number and finally form a blastoderm sur-
rounding a central cavity filled with yolk. Into this cavity are budded
off cells which arrange themselves in such a way as to form a sheet of
entoderm surrounding an innermost cavity which becomes connected
with the outside and forms the gastrovascular cavity. Between the
entoderm and ectoderm cells derived from them multiply and form the
mesoderm.

191. Behavior.—A fresh-water planarian moves with no apparent
effort, gliding over a surface and adjusting itself easily to every irreg-
ularity, this being made possible by the softness of its body. As it
progresses it raises its head and turns it from side to side as if feeling
its way. The cilia are locomotor structures, but they are so minute
as to be invisible to the eye and thus the worm seems to slide over the
surface on which it moves. The motion is rhythmic, waves of move-
ment passing backward from the head. The cilia beat in a mass of
slimy mucus secreted by glands on the under surface of the body, which
forms a track for the animal, laid down as it progresses.

The animals respond to the same stimuli which are effective in
other lower organisms—that is, to contact, to temperature, to light,
to chemicals, and to water currents. They find their food by both
chemical and contact stimuli. The chemical attraction of the juices
of the food seems to bring them to it. When found, the food is held
between the head and the substratum and compressed by the body
before the animal moves forward to bring the proboscis in con-
tact with it. A planarian reacts negatively to a variety of substances
in strong solutions and positively in weak solutions, the effective strength
varying with the substance. The eyespots seem to be light-perceiving,
though of course the animal possesses no vision. Planarians alternate
periods of activity with those of rest and are more active at night than
during the day.

FRESH-WATER PLANARIAN 165

The behavior of a planarian is of a reflex type—that is, a stimulus
received by a cell on the surface produces an impulse which is conducted
along a nerve fiber to a cell in a ganglion or in a nerve cord. This cell
in turn sends out an impulse which is transmitted either to muscle
cells, which move, or to gland cells, which secrete. In other words, the
effect is turned back or reflexed. The cell receiving the stimulus is
called a sensory neuron, the ingoing impulse an afferent impulse, the
central cell a motor neuron, the outgoing impulse an efferent impulse, and
the cell which completes the action—not a nerve cell—an effector. A
reflex act may be defined as an act involving these three types of cells or
as an act involving an afferent and an efferent impulse in
which the latter is conditioned upon the former.

Planarians are subject to different physiological states,
the character of their reactions varying with hunger, fatigue,
or nervous excitement.

192. Regeneration.—A planarian possesses a power of
regeneration hardly less developed than that of the hydra.

Pieces of their bodies may also be grafted together without
great difficulty.

A noteworthy fact is that whenever a new body is

regenerated from a piece, a head is developed on that Fic. 80.—

Diagram to

margin of the piece which was nearest the head in the
animal from which it came, while a new tail is developed
on the opposite margin. The explanation of this was for
a long time obscure but has been furnished by recent experi-
ments. These show that in an animal possessing a head
and a tail, as do planarians, there is a gradient in metabolic
activity extending from near the anterior end to the
posterior end. The rate of metabolism is greatest at the
anterior end of this gradient and decreases gradually from
this end to the other (Fig. 80). Thus it is that any
fragment will differ in the metabolic activity of its different
portions corresponding to their position with respect to the
axial gradient. Consequently, from the margin where
metabolic activity is greatest a head will develop, and from
the other margin a tail. This conception of an axial meta-

illustrate the
metabolic
gradient in a
planarian.
The change
in the width of
the black line
shows the
varying de-
grees of meta-
bolic activity
at different
levels, such as

a. [Gs Ane
effect is, of
course, not

confined to the
medianline
but extends
from one side
to the other.

bolic gradient, proposed first by Child, can also be applied in explaining
how reproduction in some worms may occur by transverse fission. It is
assumed that when the animal gets so long that the gradient becomes
exceedingly gradual, the posterior portion of the body escapes from the
dominance of the anterior portion and a new center of metabolic activity,
or another maximum in a new axial gradient, is established. Just in front
of this center appears the constriction which divides the body into two
parts.

CHAPTER XXXI

PHYLUM PLATYHELMINTHES

The phylum Platyhelminthes (plat i hél min’ théz; G., platys, broad,
and helminthos, worm), of which a planarian has been taken as a type,
represents in many respects a marked advance over previous phyla.
Bilateral symmeiry is here fully established. The animal possesses a
definite head and tail. The head represents not only the part that goes
ahead in locomotion but also the most sensitive part of the body and the
portion in which is located a nerve center. This results in a definiteness
in the movements of the animal which is in marked contrast to the
haphazard locomotion of the coelenterates or the weak but more definitely
directed swimming of the ctenophores. The presence of bilateral
symmetry is not here accompanied, however, by the presence of metamer-
ism. These animals have begun, too, to reap the advantages derived
from the development of the mesodermal layer, and there is shown, in
its inception, the plan of organization of all higher animals. This organi-
zation involves the development of tissues, organs, and systems and
greatly increases the effectiveness of the organism. In the Turbellaria
the gastrovascular cavity reaches its highest degree of development and
of efficiency. By means of numerous complexly branched canals the
food is distributed throughout the body of the animal, in spite of
the fact that the development of the mesoderm has greatly increased the
bulk and the number of cells to be reached. The nervous system in
the free-living flatworms includes a pair of ganglia below the eyespots,
which have by some been called a brain and which is a definite evidence
of centralization.

193. Classification.—This phylum is divided into three classes
as follows:

1. Turbellaria (tér bél 1a’ ri a; L., turbella, a little stirring).—Consists
of soft-bodied forms, usually free-living, with a ciliated epidermis which
contains an abundance of secreting cells and which also produces rodlike
bodies called rhabdites. The mouth is on the ventral surface but in
different species it varies in location from near the anterior end to even
behind the middle of the body.

2. Trematoda (tré mato’ da; G., trematodes, having pores).—Forms
in which the soft, ciliated epidermis is replaced by a thick, firm cuticula,
without cilia, in which the mouth is usually situated at or near the

anterior end of the body and surrounded by a sucker, and in which there
166

PHYLUM PLATYHELMINTHES 167

may also be other suckers on other parts of the body. The trematodes
are all parasites.

3. Cestoda (sés to’ da;G., kestos, girdle, and eidos, form).—The
members of this class are also provided with a thick cuticular covering.
They have a so-called head, or scolex, provided with suckers and in
many cases also with hooks. The body is divided into a series of sections
or proglottids which vary greatly in number in the different species. As
a result of parasitism, which also prevails in this group, many organs
are reduced and the digestive system is entirely absent.

194. Turbellaria.—In addition to the fresh-water planarians this
class includes a great many free-living marine flatworms. These may
be found making their way over the surface of rocks or other solid objects
in the water and at low tide are often found closely adherent to the lower
surfaces of rocks lying upon the beach. In the latter case they are
difficult to detect because they make a very thin film and the mottling
of the body is quite similar to the mottling of the surface to which they
adhere. Some of these marine forms are relatively large, reaching a
length of several inches, and are broadly oval or elliptical in outline.
On the other hand, there are some which are so small as to be microscopic.
The enteron of the more minute forms is simple and unbranched, while
that of the larger forms is divided into one or more main branches. These
in turn give rise to very complexly divided lesser branches which reach
all parts of the body. Turbellarians are not confined to water, either
salt or fresh, for in the tropics there are species which live in and on moist
earth. The fresh-water forms may be in quiet water or in swiftly flow-
ing streams; they may be collected under the ice in winter and have also
been found in hot springs at a temperature of 47°C. (116.6°F.).

The larger forms are of various shades of gray, brown, or black,
but the smaller ones are often brightly colored and may be green from
the presence of symbiotic algal cells in the parenchyma. The eyespots
may be absent, but the usual number is two; and one form, Polycelis,
has a large number. Olfactory pits and statocysts may also be present.
The turbellarians are richly supplied with glands. Some of these secrete
a slimy mucus; others, at the ends of the body, act as adhesive cells;
and still others produce material which forms rhabdites or rodlike crystal-
line bodies which are thought to serve as a means of defense or of cap-
turing food.

195. Trematoda.—This class includes animals generally known as
flukes. They are parasitic on and in a great variety of other animals,
as on the skin of the salamander and certain fishes, on the gills of fishes
and tadpoles, and in several internal organs of many vertebrates. The
number of suckers and their location differ in different types. The
pharynx is not protrusible but it is muscular and capable of suction.
The type chosen to illustrate this class is Clonorchis sinensis Cobbold

168 METAZOAN PHYLA

(Fig. 81), a fluke which lives as an adult in the bile ducts of the liver of
man, dog, and cat, and is found in China and Japan. In addition to
the firm leathery cuticle, the absence of cilia, and the presence of suckers,
all of which represent adaptations to a parasitic mode of existence, this
form in its adult condition is to be contrasted with a planarian in the
absence of eyespots and in the more highly developed reproductive
system. The single excretory pore lies at the extreme posterior end of

Yo/k Glatas

Seminal
vesicle

Uterus

Yolk duct
Seruyal VAG,
recepracle

Vas erfereris

Excretory
pore

Fic. 81.—Clonorchis sinensis Cobbold. (Redrawn from Hegner, Root and Augustine,
‘“‘ Animal Parasitology,” after Faust.) Dorsal view, showing internal structure. 8.

the body. Added to the female organs is a shell gland which secretes a
substance that hardens the shells of the eggs (Fig. 82). Almost all
trematodes pass through a complicated life cycle spending the early part
of their life in some snail or related animal before entering the host in
which they mature. There may be three or four different hosts required
for the complete cycle. Clonorchis develops first in a snail, then in a
fish, before entering man.

PHYLUM PLATYHELMINTHES 169

196. Cestoda.—This class includes the tapeworms, which are common
in the alimentary canals of vertebrates. A typical tapeworm (Fig. 834)
consists of a more or less rounded head, or scolex, and a relatively slender
neck, which together form one section of the body, and a number of other
sections called proglottids. 'The scolex bears suckers and is projected at
its free end into a rostellum, which may have an encircling row of hooks

(Fig. 84). Proglottids are constantly
being produced by transverse constric-
tion at the end of the neck. The pro-
glottids are carried farther and farther
away from the point of origin as
younger ones are produced and gradu-
ally become mature, with fully devel-
oped sex organs. After the eggs are
fertilized they develop in the uterus.
All other organs except the uterus dis-
appear and finally the gravid proglottid
becomes practically a sac of eggs, each
egg containing anembryo. Ultimately
these ripe proglottids are cast off and
are passed out of the host in the feces.
In the relation of the proglottids to
each other, there is some correspond-
ence to the relation of the individuals
in a coelenterate strobila (Fig. 69F).
The scolex and neck correspond to the
scyphistoma and the proglottids to the
gradually developing ephyrae, both be-
ing cast off when mature. Some zoolo-
gists, therefore, have interpreted a
tapeworm as representing a colony in
which the proglottids are individual ani-
mals. There is, however, considerable
unity in the organization of some sys-
tems common to the entire tapeworm.

The effects of parasitism are carried

much further in tapeworms than in the flukes.

Yolk duct

Lavrers canal

C)
Fiag. 82.—Diagrammatic sketch
showing relationships of female organs
in Clonorchis sinensis. (From a sketch
by H. W. Manter.) The egg cells are
formed in the ovary and passed into the
oviduct, where they are fertilized by
sperm cells from the seminal receptacle
and provided with yolk and shell-form-
ing material from the yolk gland. They
then go on past the shell gland, the
secretion of which hardens the shells,
and into the uterus. The sperm cells
in the seminal receptacle have come
from another animal. In this case it is
known that in copulation sperm cells
are introduced through Laurer’s canal,
which opens to the outside. In some
other flukes, however, there is no ex-
ternal opening to this canal and it seems
to be a vestigial organ.

Hooks may be added to

the suckers as organs of attachment, and the nervous system is still fur-
ther simplified; but still more striking is the complete absence of the
alimentary canal, the digested food in the intestine containing the tape-
worm being absorbed through the body wall of the parasite.

The simplest cestode lives in the body cavity of an annelid worm and
has only one section. In contrast to this form are others which may

170 METAZOAN PHYLA

possess hundreds of proglottids and which may reach a length of many
feet.

197. Metabolism.—The steps in metabolism in a free-living flatworm,
as illustrated in a planarian, have been seen to be not greatly different
in general character from those in the coelenterates. It should be noted

pocolex : Uterus

Excretory
caval

“Shell “Wolk ~=\
Vas deferens Glia Glard Ovary

Fic. 83.—The beef tapeworm, Taenia saginata (Goeze). A, the whole tapeworm,
with many portions omitted, to illustrate the change in the form of segments in different
parts of the body. (From Leuckart, ‘‘ Parasiten des Menschen.”) XW. B, scolex and
neck in an extended condition. The rostellum bears no hooks and the tapeworm is spoken
of as unarmed. (From Leuckart.) x5. C, proglottid, showing the sex organs. (Also
from Leuckart.) X7. OD, ripe proglottid, showing the uterus distended with eggs. X 6.

that though parasitism in the case of the tapeworm has resulted in the
disappearance of the digestive system and the absence of the processes
of ingestion, digestion, and egestion, all of the other processes in metabo-
lism still remain. Absorption occurs over the surface of the body;
circulation is from cell to cell; assimilation and dissimilation, secretion,

PHYLUM PLATYHELMINTHES 171

excretion, and elimination are still carried on in the same manner as in
the nonparasitic forms. Anal openings have been described in some
trematodes and openings from the branches of the gastrovascular cavity
to the outside in turbellarians, but the extent to which these ean function
in egestion is not known.

198. Reproduction.—Reproduction in this phylum occurs both
sexually and asexually. Sexual reproduction is, however, the more usual
type. Asexual reproduction in a planarian is
usually by transverse fission, but a type of frag- kostel/ur
mentation has also been described in which the
body breaks up into a number of fragments each
of which by a process analogous to regeneration
becomes a complete individual. The production
of proglottids is also asexual reproduction.

199. Occurrence and Economic Importance.—
The phylum Platyhelminthes contains a large
number of species, and a very large percentage of Tie ae fay cra rd
vertebrates is infected by the parasitic forms. pork tapeworm, Taenia
From an economic standpoint the free-living flat- 8" Linnaeus. (rom

F Leuckart, ‘‘Parasiten des
worms are of no importance, but both trema- Menschen.”) X30.
todes and cestodes produce a great deal of injury Satie Fee
to domestic animals and to man. Among trema- tapeworm. The illustra-
todes several flukes are parasitic in man. The #°D is not artistically cor-

2 rect, in that the suckers on
most serious of these are the blood flukes (genus the two sides are not
Schistosoma). _Two species of blood flukes occur Te eee es
in man in Africa, one in the West Indies, and exact side view.
another is common in parts of Japan and China.
Human tapeworms may cause serious symptoms, but usually do not, and
are rarely dangerous to life. Nevertheless the larvae of one tapeworm,
Echinococcus granulosus (Batsch), which lives as an adult in the dog, may
occur in man, where they form cysts known as hydatids and, if not
removed by operation, are often fatal. The injuries to domestic animals
caused by tapeworms, though not so serious as to cause the death of the
animal, are often sufficient to interfere somewhat with their usefulness.

CHAPTER XXXII

PARASITISM
ILLUSTRATED BY THE FLATWORMS

Parasitism is a very common association of animals of different species,
characterized in a general way by the fact that one individual, known as
the parasite, lives at the expense of another, called the host, but does not
devour it. Some parasites infect only one host, but in other cases they
pass successive periods of their lives in different hosts. In the latter
case the host in which they reach the adult condition is known as the
final host; and those in which they live during their larval development,
intermediate hosts.

200. Structure of Parasites.—The parasitic flatworms illustrate
several of the salient features of internal parasitism. The loss of
certain organs by the parasite has been noted. The organs markedly
affected are those of the digestive and nervous systems. On the other
hand, other organs become more highly developed and new structures
appear. Such structures are hooks and suckers, which serve for more
effective attachment and result in more perfect adaptation of the organ-
ism to the conditions of parasitic life. The epidermis is modified to
resist the digestive juices of the host. Of the organs which show increased
development the reproductive organs are the most prominent. There
is so much uncertainty attached to the conditions of life that if it were
not for the production of an enormous number of eggs, these parasites
would cease to exist. At several points in the life history of the liver
fluke and at two places in the life history of the tapeworm chance deter-
mines whether the life history is to come to an end or to continue. It is
stated that a single liver fluke may give rise to as many as 500,000 eggs,
and estimates of the number of eggs produced by a single tapeworm
reach 3,400,000,000 (Penfold, Penfold, & Phillips, 1937).

201. Sheep Liver Fluke.—The development of the sheep liver fluke
(Fasciola hepatica Linnaeus) is usually taken as a type of that of the
trematodes. This fluke is relatively large, being about an inch long.
The most frequent final hosts are sheep and cattle, but it may be found
in any one of a number of smaller mammals and rarely in man. The
parasite may be lodged in other tissues than the liver. In man it has
been known to live four years. This fluke is found in various parts of
the world wherever sheep are raised, and different species of snails serve
as intermediate hosts in different regions.

172

PARASITISM 173

202. Life History of the Sheep Liver Fluke.—The egg cell is produced
in the ovary of the fluke and is passed into the oviduct where it is fertilized.

Oral
; sucker

aes

Ss Eos ¥

See
=

)

ree
aR

Fig. 85.—Diagram of the life history of a sheep liver fluke. A, the adult fluke, Fasciola
hepatica, in bile duct of sheep. B, egg, much enlarged (hatches in water). C, the mira-
cidium, enlarged (free swimming). D, Lymnaea (Galba) bulimoides, a snail, the inter-
mediate host. , the sporocyst containing developing rediae, in liver of the snail. (C and
E from Leuckart, ‘‘Parasiten des Menschen.) fF, rediae in liver of the snail; one con-
taining developing daughter rediae and the other developing cercariae. G, the cercaria which
emerges from the snail and is free swimming in the water. (From Leuckart.) 4H, encysted
cercaria on vegetation. J, sheep, the final host, ingesting cysts on grass. (Figures arranged

by H. W. Manter.)
It becomes enveloped by yolk cells derived from the yolk glands, and shell
forming material also received from the yolk glands is hardened into a shell
by a secretion from the shell gland (Fig. 82). The egg now passes to the

174 METAZOAN PHYLA

uterus of the fluke, where development begins. The egg, containing an
embryo, is laid through the genital pore of the fluke and passes out of the
body of the sheep with the feces, by way of the bile ducts and intestine.

Under favorable conditions of moisture and temperature the embryo
within the egg continues its development into a larval form known as a
miracidium. This usually requires two or three weeks. The miracidium
hatches from the egg only in water. It is a microscopic, but multicellular,
ciliated creature with a pointed rostrum at the anterior end and a double
eyespot on the dorsal surface anteriorly. The miracidium swims about
in the water but cannot feed and dies within about eight hours unless it
locates a snail of the right species. Probably most miracidia weaken and
perish for want of the lifesaving snail.

Having found a snail, the miracidium enters its pulmonary chamber
and later burrows into the soft tissues of the snail to the liver. Within
the snail, the miracidium sheds its coat of cilia and becomes transformed
into a saclike form known as the sporocyst. The sporocyst is the second
larval form in the life cycle. Within it are groups of cells, usually con-
sidered as germ cells, which develop, without fertilization, into larvae
known as rediae (singular, redia). The sporocyst, therefore, carries on
reproduction. The redia is also saclike but, unlike the sporocyst, pos-
sesses a mouth and a simple enteron. Rediae escape from the sporocyst
and develop in the liver of the snail. There may be several generations of
rediae, the daughter rediae arising from unfertilized germ cells.

Finally, rediae give rise to a very different type of larva known as the
cercaria also developing from what seem to be unfertilized germ cells.
The cercaria possesses a body and a tail, the body having an oral and a
ventral sucker and a forked enteron like the adult. It is the fourth type of
larva in the life eycle. It burrows its way out of the snail into the water
where it swims about actively by lashing its tail back and forth. Because
of the reproduction by the sporocyst and rediae, a snail infected by a single
miracidium may later shed large numbers of cercariae.

The cercaria swims about for a short time only, then settles down on a
bit of vegetation, sheds its tail, and secretes a cyst wall entirely enclosing
the body portion. If this cyst is eaten by a sheep, the immature fluke is
freed from the cyst in the alimentary canal. According to recent studies,
it penetrates through the wall of the digestive tract into the body cavity
and thence into the liver, finally arriving in the bile ducts.

This curious and rather complicated life cycle is followed with some
modifications by almost all trematodes. While the life of the adult may
be monotonous, the youthful career is dangerous and eventful. Only a
few eggs are fortunate enough to hatch, only a few of the miracidia
succeed in finding a home in a snail, and most of the encysted cercariae are
doomed never to awaken for lack of the magic touch of the digestive
juices of the final host.

PARASITISM 175

An example of another trematode life cycle is shown in Fig. 86. In
this case the adult fluke lives only in the lung of a frog and during the
course of the life cycle two different intermediate hosts are required.

Fic. 86.—The life history of a lung trematode, Haematoloechus medioplexus, of the frog.
1, adult trematode from the lung of the frog, Rana pipiens. (From W. W. Cort, 1915.) 2,
egg containing miracidium. Passes from frog in feces; does not hatch until eaten by 3,
Planorbula armigera, a snail. The eggs hatch in this snail and sporocysts develop in the
liver. 4, sporocyst containing developing cercaria. 5 mature cercaria. It leaves the
snail, swims about in the water, enters the branchial basket of dragonfly nymphs. 6,
nymph of dragonfly. 7, cercaria encysted in nymph. 8, mature dragonfly containing
encysted cercaria. When eaten by a frog, the trematode larvae emerge and pass up the
esophagus into the lung. (From H. W. Manter, ‘‘ A Laboratory Manual in Animal Para-
sitology,’’ Burgess Publishing Company. Figure 7 from Krull, 1930.)

176 METAZOAN PHYLA

203. Life History of a Tapeworm.—The life history of a cestode
(Fig. 87) also involves two hosts, the final host becoming infected with
the parasite through eating the intermediate host. A typical life history
is that of the beef tapeworm, Taenia saginata (Goeze), the adult of which
is found in the human intestine, and which passes through the following
steps in its life history:

Fig. 87.—Diagram showing the life history of Taenia saginata. A, the adult tapeworm
(reduced), from intestine of man. B, the egg (much enlarged), containing six-hooked
embryo. Such eggs pass out of the host in the feces and do not hatch until eaten by
cattle. C, young larva (cysticercus) from muscles of infected cow. JD, later stage in the
development of the cysticercus. , cysticerci (about natural size) embedded in beef.
F, diagram of cysticercus (enlarged) showing invaginated scolex. G, cysticercus showing
evaginated scolex and early proglottids. (B, C, D, G from Leuckart. ‘‘Parasiten des Men-
schen”; F, from Faust, ‘Human Helminthology,” figures arranged by H. W. Manter.)

1. The egg cell is produced in the ovary, passed into the oviduct,
fertilized, supplied with yolk, inclosed in a shell, and carried to the uterus.
In the uterus there develops in this egg a six-hooked embryo. As the
proglottid becomes ripe and passes out of the body of the host with the
feces, it carries with it thousands of embryos, still inclosed in the egg
shells,

PARASITISM 177

2. The proglottids usually break or disintegrate, and the minute eggs
become scattered over the ground. If the proglottids or the eggs are
accidentally eaten by a cow, the eggs will hatch, and the six-hooked larvae
will be liberated in the intestine. The larvae bore their way through the
wall of the alimentary canal and, migrating through the tissues, reach the
voluntary muscles, especially the muscles of mastication, where they
become encysted.

3. The encysted larva in from three to six weeks develops a bladder-
like sac filled with a clear watery fluid and so becomes a bladder worm,
or cysticercus. One side of the wall of this sac gradually becomes thick-
ened, is inverted, and forms a hollow papilla projecting into the sac.
From the outer surface of the wall and in the cavity of this papilla
develops a scolex, with hooks and suckers.

4. If the flesh of the cow has been insufficiently cooked and is eaten
by man, the cysticercus is freed in the alimentary canal, the papilla
becomes everted, the scolex and neck project from the side of the bladder,
and the latter is destroyed.

5. This scolex attaches itself to the wall of the intestine, begins to
develop proglottids, and thus a new tapeworm is produced.

204. Behavior of Parasites.—The life history of these parasites shows
a changing behavior in the passage from one larval form to another and
from one host to another. Only changing chemical and contact reactions
can explain the entrance of a miracidium into the body of the snail and
the leaving of it by the cerearia. Changing physiological states undoubt-
edly accompany this changing behavior.

205. Practical Aspects.—A knowledge of the life histories of such
parasites as those that have been considered is evidently of great value,
since it dictates the character of the control measures which must be
taken. It is evident, for example, that if the fluke infection is discovered
in a flock of sheep, all infected animals should be removed from the flock.
It is also evident that to avoid infection, a flock should be removed to a
pasture which contains no standing water and, if possible, to one which
has never had sheep upon it before. Destruction of snails by copper
sulphate is an aid in control of the parasite. In man the liver fluke may
produce no serious symptoms, but in some cases it has produced fatal
results. It has been removed from superficial abscesses. In the sheep
heavy infection causes serious functional disturbances and often death,
though the parasite may pass out of the host and spontaneous recovery
ensue.

Man is subject to infection by tapeworms acquired from pork, fish,
and, less frequently, other animals, but the beef tapeworm is the most
common human tapeworm in this country. Most human tapeworm
infection can be avoided by measures that will insure careful meat inspec-
tion and the consumption of no meat that is not well cooked.

CHAPTER XXXIII
PHYLUM NEMATHELMINTHES

The representatives of the phylum Nemathelminthes (ném a thél
min’ théz; G., nematos, thread, and helminthos, worm) are called, collec-
tively, roundworms or threadworms. Some of them are free-living;
others are found in plant tissues, where they may be the cause of plant
diseases; and still others are parasites in
other animals. Generally speaking, the
nemathelminths are small and even micro-
scopic, but a small number reach a larger
size and a few may even be several inches in
length.

206. Structure of an Ascaris.—An ascaris
may be taken as a type of the phylum.
Logically a free-living type would be pref-
erable, but the size of an ascaris and the
ease with which it may be secured render it
the most practical one. The genus Ascaris
includes roundworms of about the size and
with the general proportions of an earthworm
but possessing no metamerism (Fig. 88). A
common species, known as the eelworm or
the pig ascaris, Ascaris lumbricoides Linnaeus,
is found in the intestines of pigs and human
beings. The ascaris found in man and in
pigs are morphologically identical but physi-
Linnaeus. A. male. X14. B, Ologicallydifferent. Because of certain phys-
the anterior end of the body. C, iological differences, some authorities prefer
end view of the anterior end, ; ‘ =
aiiowing thrediiivs, UD) posterior Lue mame Ascaris suum for the pig ascaris and
end. (B to D from Leuckart, Ascaris lumbricoides for the ascaris of man.
Ge cat Menschen”) Mag The mouth is at the anterior end of the

body and an anal opening near the posterior
end. When opened, the animal is seen to possess a relatively thin body
wall surrounding a central cavity, through which runs the alimentary
canal (Fig. 89). This canal consists of a short muscular sucking pharynx,
a long nonmuscular intestine, and a short rectum. The excretory system
consists of two longitudinal canals, one of which runs along each side of
the body, the two opening to the outside by a single pore on the ventral

178

PHYLUM NEMATHELMINTHES 179

surface near the mouth. There are no flame cells, but four large branched
excretory cells are present in the anterior part of the body. A circum-
pharyngeal nerve ring, containing nerve cells, surrounds the pharynx and
is connected with two larger nerve trunks, one dorsal and the other
ventral, and one or two smaller trunks on each side. ‘The body cavity
is not strictly comparable to the coelom of higher forms, since it lies
between the entoderm of the alimentary canal and the mesoderm which
forms the muscular layers of the body wall (Fig. 89), whereas a typical
coelom is completely invested by mesoderm. Large cells, containing
enormous vacuoles, have been described as present in the body cavity.

Dorsal sire

Dorsal nerve cord

arse ae
ey aia tween
Rese On

merve cord

Fic. 89.—Semidiagrammatic cross section of an ascaris. (Based upon Leuckart, wall
chart.) Processes of the muscle cells are seen running across to the dorsal or ventral nerve
cords.

207. Characteristics and Advances.—The nemathelminths are bilater-
ally symmetrical and triploblastic. The greatest advance is seen in the
development of an alimentary canal to replace the gastrovascular cavity,
the disadvantages of which are easily made apparent. An animal which
has a gastrovascular cavity, the one opening into which serves as both
mouth and anus, is manifestly at a great disadvantage when it comes to
taking in food and passing out waste. Attention has been called to the
fact that a hydra which has fed will not again take food until the food it
already has is digested and the waste matter is passed out. Should an
animal with such a digestive cavity take additional food, the mixing of
food at various stages of digestion would inevitably occur, which would
be clearly a disadvantage. Furthermore, although the digested food

180 METAZOAN PHYLA

seems to be effectively distributed by the much-branched gastrovascular
cavity of the turbellarian, the increasing development of the mesoderm
makes such distribution correspondingly more difficult. In contrast to
this the alimentary canal, open at the two ends and running the length
of the body, makes it possible for the animal to feed continuously, taking
food at the mouth and passing it gradually through the intestine. Here
each successive increment is kept separate, at least to a considerable
degree, from the food taken earlier and later, and the feces from each are
egested in due time. An alimentary canal, however, will be most effec-
tive if it is a straight unbranched tube, and it seems to be a significant
fact that when we come upon such an
enteron it is at once divested of all
branches. In the absence of this
means of distributing the digested
Hs food some other means must exist,
Process leading to rerv€ snd this is furnished by the body
ae aS ;

cavity.

208. Classification.—The
phylum Nemathelminthes may be
considered as including three classes:

1. Nematoda (ném ato’ da; G.,
| nematos, thread, and ezdos, form).—
Contractile part Either parasitic or free-living.
BGA 2. Gordiacea (gor dia’ shé a; L.,

gordius, referring to a complicated
knot).—Parasitic in the larval stages
Fra. 90.—A muscle cell from an ascaris. and free-living and aquatic as adults.
These cells run longitudinally and are oe Acanthocephala (a kan tho séf’-

shown in cross section in Fig. 89. At the _..
right are shown two sections of the same @ la; Ge akantha, thorn, and kephale,

cell to show how the appearances seen }egd),—All parasitic
in Fig. 89 are produced. (The cell from ). I sated °
Leuckart, ‘‘Parasiten des Menschen.) The 209. Free-living Nematodes.—

contractile part of the el nated bY An inconeeivably large number of

minute free-living threadworms exist
in the soil, in sand, mud, and debris from standing and running water,
and in the sea. They are thus adapted to a great variety of habitats,
are very resistant to drying and freezing, and are disseminated in numer-
ous ways. The number of species is now believed to be very large, but
to a great extent they are undescribed. At the present time these free-
living nematodes are often called nemas (Fig. 91).

210. Metabolism.—The food of roundworms is mostly fluid, being
either blood or other juices from the animal host; the juices of plants; or
water containing microorganisms or organic matter in solution if the
animal is free-living. This liquid food is pumped into the alimentary
canal by the pharynx, is digested in the intestine, and is freely passed by

PHYLUM NEMATHELMINTHES 181

absorption through the thin wall of the intestine into the body cavity.
By means of this cavity it is distributed throughout the body. Within
tissues it is passed from cell to cell. Egestion takes place through the
anal opening and elimination is effected by the excretory system.

211. Reproduction.—The nemathelminths are all diecious, and ferti-
lization is internal. After fertilization those eggs which develop within
the body, as in the species of T'richinella, are lodged in the uterus, where
they hatch, the female bringing forth living young. The shell is often
heavy and very resistant to chemi-

cals which would injure the organ- Mouth
ism. The larva, which at first may Oneal os
be free and may remain so in free- ee’ — Fsophagus

living nematodes, enters, in the
case of all parasitic forms, into
another animal which may be
either an intermediate or a final
host, or into some plant.

212. Life History of the Pig
Ascaris.—The parasitic nematodes
possess some very interesting and
remarkable life histories. One of
these is that of the ascaris found
in the pig. The adult pigs are
immune from infection by this
parasite, which, if it is found in a
mature pig, must have entered it
when the host was young. The
eggs of the parasite, after being

Nerve cells

passed out with the feces and
mixed with the soil in the hog
lot, are taken up by the young pigs
as soon as they begin to root about;

Sperv7

tolls
Fig. 91.—Monhystera sentiens Cobb, a
free-living nematode. Side view of female.

Probably the most widely spread nematode
genus, found in fresh water, in the sea, and in

or if infested soil is caked upon
the body of the mother, the eggs
may be taken in when suckling.
The eggs pass into the intestine of the young pig and by the destruction
of the shells the larvae are freed. The larvae then leave the intestine
by puncturing the wall and pass by way of the portal system of blood
vessels to the heart. From the heart they follow the pulmonary artery
to the lungs, make their way through the walls of the lung cavities into
the air spaces in the lungs, and then, by following the free surfaces
of the air passages to the pharynx, reach the alimentary canal. Follow-
ing this canal they return to the intestine to complete their growth,
mature, and reproduce.

soil. (From Cobb, in Ward and Whipple’s
‘““Presh-water Biology,” by the courtesy of John
Wiley & Sons, Inc.) X 94.

182 METAZOAN PHYLA

This journey through the body is injurious to the young pig, retarding
its growth. ‘The passage of the worm through the lungs causes serious
pulmonary disturbances, and young pigs harboring many parasites remain
poorly nourished, weak, and unprofitable to the grower. Infection is
avoided by carefully cleaning the body of the mother before the time for
farrowing, by placing her at this time in a perfectly clean pen with clean
straw, and by removing both her and the young pigs to clean pasture
just as soon as they can be taken out
of the farrowing pen. It is also not
advisable to use the same hog lot year
after year.

As already noted, the ascaris occur-
ring in the intestine of man is morpho-
logically identical with the pig ascaris.
Some physiological difference exists,
however, in that eggs from the human
ascaris will not complete their develop-
ment in pigs and eggs from the pig
ascaris will not reach maturity in man.
The larval migration, however, occurs
in cross infections.

213. American Hookworm.—
Another nematode that makes an
interesting journey through the body
of the host, which is man, is the hook-
worm, Necator americanus (Stiles). In
our southern states this worm (Fig. 92)
formerly affected a large population,
estimated to number two millions,

: : known as “poor whites,” who were
a TOP RR, te noteworthy for their shiftlessness,
‘Tropical Diseases,” after Paclencia, by although they were the descendants of
the courtesy of Cassell & Company.) very ood immigrant stoclaueiteds
people lived in cabins, frequently with no other floor than the bare earth,
were in the habit of going barefooted, and were unsanitary in the disposal
of fecal waste. The eggs of the hookworm produced in the bodies of the
persons afflicted with this parasite are passed out with the feces and
deposited on moist earth. The larvae which hatch from these eggs
moult twice before they become infective. Then when the skin of another
person, most frequently that on the foot or the hand, comes in con-
tact with this infested earth, the larvae enter the body by boring through
the epidermis and thus reaching the lymphatics or capillaries immedi-
ately under it. They may enter the body by means of water or food
but this is not the usual mode of infection. From the point of entrance

Mouth —®
Fsophagus—f 4)

ary
cy
Co)

PHYLUM NEMATHELMINTHES 183

the larvae pass either by the veins alone or by lymphatic vessels and
veins to the heart and from there, still following the stream of venous
blood, through the pulmonary artery to the lungs. Leaving the blood,
they pierce the walls of the lung cavities, enter those cavities and follow
the air passages to the pharynx, from which, by way of the alimentary
canal, they go to the intestine. Here they mature and another generation
is produced.

In the passage through the lungs injury is done which predisposes
the host to lung diseases. While in the intestine the worms feed upon the
blood of the host. This blood is obtained by puncturing the intestinal
wall. When this puncture is made, a poison is introduced which pre-
vents the coagulation of the blood, and so the person loses blood from
hemorrhage in addition to that which is taken
by the parasite. Anemia and weakness result
which incapacitate the person for any effective
effort. By preventive measures, such as
putting floors in the houses, requiring the
people to wear shoes, providing sanitary
means for the disposal of fecal waste, and by
appropriate treatment of the patients, the
disease has been considerably reduced in the
United States. Hookworm disease is a very
serious problem in many tropical countries. BL

214. Trichinella—A third parasitic nema-
tode is that known as Trichinella spiralis containing encysted larvae of
(Owen), which is the cause of a disease known apc tees.
as trichinosis in rats, pigs, and human beings. The host acquires
this parasite by eating the meat of another animal in whose flesh are
the encysted larvae in an advanced stage of development. When these
are taken into the alimentary canal they are freed from the cyst and
escape into the intestine, where they live upon the food in the canal and
become mature in as short a period as, perhaps, two days. In the body
of the female, eggs are produced, and fertilization, development, and
hatching take place, the worm being viviparous. The female burrows
into the wall of the intestine and deposits the young larvae in the lymph
spaces of the villi (Fig. 11). The larvae follow the lymphatics and blood
vessels to the voluntary muscles in various parts of the body where they
encyst (Fig. 93). After a rapid development in the cysts, they are ready
to be transferred to another host. If this transfer does not occur within
a few months, lime becomes deposited in the walls of the cysts. Although
the larvae may remain alive for some years, they eventually become calci-
fied and die. The length of life of the adults is usually only a few weeks.

Since both rats and pigs eat the dead carcasses of other animals,
the parasite, when present, is likely to be passed from one animal to

184 METAZOAN PHYLA

another and to give rise to generation after generation. Rats may
become infected by eating infected rats or infected scraps of pork in
garbage. While pigs may eat an infected rat, they more commonly
become infected from garbage containing scraps of pork. It has been
shown that grain-fed pigs are less likely to be infected than garbage-fed
pigs. Man comes into this cycle of parasitism by eating inadequately
cooked pork and thus taking in the larvae, which pass the rest of their
life history within his body. The disease is very serious, heavy infections
being often fatal.

The disease (trichinosis or trichiniasis) is difficult to diagnose, and
particularly so, since light infections produce mild symptoms. Most
cases pass undiscovered. Recent examinations
of cadavers reveal that infection with this para-
site is very common throughout the United
States, probably 12 to 15 per cent of the people
containing at least a few larvae in their muscles.
About 4 to 5 per cent of garbage-fed hogs are
infected. Preventive measures include inspec-
tion of meat (federally inspected pork is made
safe by prolonged refrigeration), destruction of
infected meat, cooking of garbage or burning of
pork scraps, and, above all, the thorough cook-
ing of pork. Even infected meat, of course, is
perfectly safe when well cooked.

215. Filaria——The nematodes known as
filariae include a number of species which affect
both man and domestic animals, particularly
in the tropics. The most injurious type is
cael Cae crane Wuchereria bancrofti (Cobbold), which is a
(Myriophyllum) in which is threadworm living in the lymphatic system of
Soe ha of Gordius. tan. The larvae of this parasite are carried

about in the blood, retreating to the center
of the body in the daytime but at night migrating to the peripheral
blood vessels in the skin. At times the adults seem to obstruct the pas-
sage of the lymph and in some cases may lead to an excessive growth
of tissue usually of the limbs resulting in a disease known as elephantiasis.
At night, when the larvae are active and are in the peripheral vessels, they
are sucked up by mosquitoes. They continue their development for a
time in the mosquito’s body and when the mosquito bites another per-
son are transmitted to him. Elephantiasis is a very serious disease,
particularly in the South Sea Islands.

216. Hairworms.—A worm which is popularly known as the horsehair
snake, and which is believed by many ignorant of zoology to be produced
in water from the hairs of horses, is by most authors placed in this

PHYLUM NEMATHELMINTHES 185

phylum. The class to which this worm belongs, known as the Gordiacea,
and of which the type genus is Gordius, includes several forms, some
occurring in fresh water and others being marine. The egg is laid in
the water and from it hatches a larva with a large proboscis and hooks
at the anterior end. Using these in boring, a larva has been known to
force its way into the body of an aquatic insect. Normally, however,
the larva encysts on submerged vegetation and is ingested by its host.
If the water level is lowered, these cysts will survive drying and may be
eaten by a terrestrial host, such as a grasshopper. Full-grown larvae
have been found in beetles, grasshoppers, and crickets, and larvae in
different stages of development in spiders, earthworms, snails, and even
fishes. The fully developed larva escapes from its insect host into water,
where it matures and where it may be found, often coiled about among
the leaves of aquatic vegetation (Fig. 94).

Probosers

Brain Testes

Frostate glands

Probosc/s
sheath

lnvertor 11UsCIes ketractor muscles

Fig. 95.—Echinorhynchus ranae (Schrank), showing general organization of a young
male. (From VanCleave, ‘Invertebrate Zoology,” by the courtesy of McGraw-Hill Book
Company, Inc.)

217. Spiny-headed Worms.— Another class of worms which may also
be placed in Nemathelminthes is that of the spiny-headed worms, or
Acanthocephala. Such a worm has a protrusible proboscis covered with
hooks at the anterior end of the body (Fig. 95). By means of the pro-
boscis it becomes attached to the intestinal wall of the host, which is
always a vertebrate. The worm lacks an alimentary canal and absorbs
digested food in the intestine of the animal in which it lives. All classes
of vertebrates are parasitized by these worms, but they occur more com-
monly in the fishes, in turtles, and in birds like the herons and bitterns.
The intermediate hosts are Crustacea or insects. A spiny-headed worm
which infects the pigs in the southern parts of this country has as its inter-
mediate host the larva of the June beetle, known as the white grub,
which is rooted up and eaten by the pig.

218. Economic Importance.—It is clear that some of the parasitic
nematodes are of great economic importance on account of the produe-
tion of disease both in man and in domestic animals. Among the thread-
worms are also some which are injurious to cultivated plants. Others
are distinctly beneficial, feeding upon the injurious forms or destroying
various injurious microorganisms.

CHAPTER XXXIV

OTHER UNSEGMENTED WORMS

From the time of Linnaeus in 1758 until the end of the last century a
diverse group called Vermes, or worms, was recognized as one of the
primary groups of the animal kingdom. It has now been broken up into
several groups, some of which are considered phyla. Included among
these are Platyhelminthes, Nemathelminthes, Annelida, and the phyla
considered in this chapter.

219. Phylum Nemertinea.—One of these phyla, by some classified
with Platyhelminthes, is Nemertinea (ném ér tin’@a; G., nemertes,

Chated

; y)! Cavada/ Cirrus

Fic. 96.—Cerebratulus, a nemertine. A, Cerebratulus lactews (Verrill), a common
species of the Atlantic Coast. The head seen from the ventral side. (From Pratt, ‘‘ Manual
of the Common Invertebrate Animals,” by permission.) Natural size. B, Cerebratulus sp.
from Naples, showing a side view of the head with proboscis everted. From a preserved
specimen. (From Sheldon, ‘‘Cambridge Natural History,” by the courtesy of The Macmillan
Company.) X 2.

unerring), which contains the bandworms. They are flat and bandlike
(Fig. 96), resembling somewhat in this respect the flatworms, but are
clearly distinguished by certain structural features, among which is the
presence of a long proboscis which lies in a sheath within the body, dorsal
to the alimentary canal. This proboscis may be protruded anteriorly
and used as an organ of touch and also as a protective and defensive
organ. Another characteristic is the development of the blood-vascular
system, which is seen here for the first time and which consists of a median
dorsal and two lateral trunks. These animals also possess an alimentary
canal with mouth and anus. They are generally considered as possessing
186

OTHER UNSEGMENTED WORMS 187

no coelom. The nervous system consists of two ganglia, one on each
side of the proboscis at the anterior end of the body connected by com-
missures dorsal and ventral to the proboscis, and two nerve trunks run-
ning backward along the sides of the body. Many of them have eyespots,
and some possess eyes with a sort of lens, pigment, and retina.

A few species are found in moist earth and fresh water, but most
of them are marine, being found coiled up under stones and other objects
on the beach at low tide or crawling over the sand as the water recedes
with the falling of the tide. Many nemertines are brightly colored.
They are exceedingly soft-bodied and when lifted have not sufficient

Bear z

Meserichyré
//

_ £ctodernial

“\ “Ectoderma/
InvaglHatior oh vagiHatior7
Vertrolateral

Jobe

fsophagus

Fic. 97.—The pilidium larva. Median section, showing the inner surface of the
ventrolateral lobe on the farther side. This lobe is hollow, containing a prolongation of the
cleavage cavity. (Compiled from several sources.)
consistency to support their own weight. As one raises them by one
end they continue to stretch until finally they may even break in two.
One genus, Malacobdella, is parasitic in a European marine bivalve
mollusk.

The bandworms feed on other animals, both dead and living. They
move by means of cilia which cover the surface of the body, by constric-
tions of the body wall, by vertical undulations of the body, or by using
the proboscis as an organ of attachment. They secrete a great deal of
mucus which may of itself become firm and produce a protective tube,
or they may make a tube by sticking together particles of sand. They
possess great powers of regeneration, which might be expected consider-
ing the readiness with which the body is broken. They also show autot-
omy, which is the ability of an animal itself to break its body into
pieces.

The larval nemertine, known as a pilidium, is a ciliated swimming
larva, somewhat conical in form, with downwardly projecting lateral
lobes and a long tuft of cilia at the apex (Fig. 97).

188 METAZOAN PHYLA

920. Phylum Chaetognatha.—The group Chaetognatha (ké tég’-
nith 4; G., chaite, horse’s mane, and gnathos, jaw) has been variously
placed in the animal kingdom; by some it is considered allied to the
nemathelminths and by others to the annelids. By still others it has been
ranked as a phylum. It is a small group containing the arrow worms,
the most familiar of which belong to the genus Sagitta (Fig. 98). These
are small, straight-bodied, and exceedingly transparent worms with a
true coelom, an alimentary canal, and the body divided into three parts,
which do not seem to be metameres. There are supraesophageal and
ventral ganglia, two eyespots, and other sensory structures. Arrow
worms have neither vascular nor excretory systems. They swim about
on the surface of the sea, being present at times in very large numbers.

921. Phylum Rotifera.—The phylum Rotifera (ro tif’ ér a; L., rota,
wheel, and ferre, to bear) includes small, aquatic animals, many of them

fir7 Ovieluct Fira Vaging

Coudal
al
Mouth Ventral s “Seminal
ganglion Ovary Anus vesicle

Fic. 98.—Sagitta hexaptera D’Orbigny. Ventral view. (From Lang, ‘“Text-book of Com-
parative Anatomy,”’ after O. Hertwig.) X 20.
not larger than protozoans. They are often encountered abundantly in
waters in which Protozoa also abound. Rotifers can be recognized by
the possession on the anterior end of cilia which in some cases are grouped
in circles. Such circular groups, owing to waves of motion which pass
rhythmically around the circle of cilia, produce when in action the effect
of revolving wheels (Fig. 99). Moreover, if the internal structure is
examined it at once reveals a complexity belonging not only to a metazoan
animal but also to one which is triploblastic and bilaterally symmetrical
and which possesses organs and systems. Owing to the presence of the
rings of cilia on the head, these forms have often been called Trochel-
minthes (trdk hél min’ théz; G., trochos, wheel, and helminthos, worm).
The bodies of rotifers are divided into three regions, which are a head,
a trunk, and a foot, the last forming the pointed posterior end. The
foot is frequently bifurcated and may assist in locomotion by serving to
kick the animal along (Fig. 100). It possesses a cement gland, the secre-
tion of which affords a means of attachment in the fixed forms. The
body is covered by a cuticula. The cilia are the principal means of
locomotion and also serve to create a current of water which brings food
to the mouth. ‘There is a large body cavity which is like that of Nema-
thelminthes and therefore not strictly comparable with the coelom of
higher forms. It contains the alimentary canal, and in its walls are the
gonads and flame cells. The alimentary canal opens into a cloaca near

OTHER UNSEGMENTED WORMS 189

the posterior end of the body. The term cloaca is applied to any cavity
into which open both the alimentary canal and ducts belonging to the
excretory and reproductive systems, and which in turn opens externally.
A ganglion lies against the wall of the body dorsal to the pharynx and a
second one may be present on the ventral side. Sense organs are also
present in the form of antennae, or feelers, and eyespots. The food of
rotifers consists of protozoans and other small organisms.

Chated disk porcler of cha

SyA\ SS YOZ ~
pa f wae Brov17
ae, MovtA oo Dorsal teeler
nik ia oy = Eye spor
fi ¥x\ Mastax e € .
ae Ut ae Re
dt 4 Stomach aie \eq- flame cell
uty ¥ {2 dt coricla
Excretory
Yolk glands ccrer
Ovary
Ovicuc? 7—I/ntestire
Contractile
eee Cloaca
Foor Cloacal qpermng

Fig. 99. Fic. 100.

Fic. 99.—Philodina roseola Ehrenberg. Not a typical rotifer, but one of the first types
studied by microscopists; the ciliated discs on the head of the organism suggested the
name rotifer. It both creeps andswims. A common American species. (From Jennings,
in Ward and Whipple, ‘‘Fresh-water Biology,” after Weber, by courtesy of John Wiley &
Sons, Inc.) X 300.

Fie. 100.—Diagram of a rotifer in section to show internal structure. The specimen

is a female. (From Parker and Haswell, ‘‘ Text-book of Zoology,” by the courtesy of The
Macmillan Company.)

Rotifers are diecious and also polymorphic. The males are usually
smaller than the females and frequently exhibit degeneration. This is a
condition involving simplification of structure and loss of organs. It is
exhibited by parasites, as in the platyhelminths, but it also occurs apart
from parasitism. The eggs are carried by the oviduct into the cloaca
and thus reach the outside. Rotifers produce three types of eggs—
female-producing summer eggs, smaller male-producing summer eggs,
and female-producing winter eggs. Both types of summer eggs have

190 METAZOAN PHYLA

thin shells and develop parthenogenetically. The winter eggs have thick
shells, and development follows fertilization. The winter eggs require
a considerable degree of acidity of the water to soften the shells so that
they may hatch, and if this does not occur they are capable of living
for many years and of still developing when placed under suitable

conditions.

A marked peculiarity of certain rotifers is their power to undergo

C Dy,

Fic. 101.—The rotifer Hyda-
tina senta Ehrenberg. A, mature
female. B,small degenerate male
showing the large sperm sac but
lacking a digestive system. C,
parthenogenetic egg covered with
a thin membrane and developing
within a few hours into a female.
D, smaller parthenogenetic egg
covered with a thin membrane
and developing within a few
hours into a degenerate male
which is sexually mature at time
of hatching. EH, fertilized egg
covered with a thick horny shell;
it will survive desiccation and the
extreme temperatures of 100°C.
or —180°C. Some have remained
viable in stale culture water for
22 years. If unfertilized, this
egg would have formed a small
parthenogenetic egg which would
have developed into a male.
(From Whitney, Journal of Experi-
mental Zoology.)

sea mats.

drying and when again under favorable con-
ditions to resume life activities. This ability
and their small size contribute to the ease
with which they may be dispersed by the
wind. ‘This ease of dispersal by wind, added
to the possibility of their being carried on the
feet of water birds, has resulted in a distribu-
tion which is of more cosmopolitan character
than that exhibited by any other group of
animals.

222. Phylum Bryozoa.—The animals
included in the phylum Bryozoa (bri 6 20’ 4;
G., bryon, moss, and zoon, animal) are known
in a general way as moss animals and sea mats.
They are colonial forms and their manner of
growth reminds one of the colonial hydroids,
but their structure distinguishes them very
clearly. Like the hydroids they have plant-
like characteristics which often cause them to
be included in collections of dried seaweeds
brought as souvenirs from a seaside trip.
The bryozoans are mostly marine, though a
few live in fresh water. They are all very
small, but a colony consisting of a large num-
ber of individuals may attain considerable
size, projecting in some cases several inches
from the surface to which it is attached.
Some of these colonies show a treelike man-
ner of growth and suggest the name sea moss
(Fig. 102). Other colonies form matlike
masses and are quite appropriately termed

They may be free, like the fronds of seaweeds, or encrust the

surface of stones or other objects.
The individual animal lives in a cup-shaped or tubular chitinous

shell, known as a zooectum, which is open at the outer end (Fig. 103).
The surface of the body lines the inner wall of the shell and the animal is
capable of withdrawing itself into a body cavity which in these forms is a
true coelom. The chitinous shells of the individuals form together a

OTHER UNSEGMENTED WORMS

support for the colony. Sometimes lime is added.

191

Because of their

superficial resemblance to corals such forms are known as coralline

bryozoans. A mouth at the outer end is sur-
rounded by a crown of ciliated tentacles termed
the lophophore, which exhibits the form of a horse-
shoe when it is expanded. The U-shaped alimentary
canal opens by an anus situated near the mouth
and either within or without the lophophore.
There are neither circulatory nor excretory organs.
The nervous system consists of a central ganglion
between the mouth and anus. Reproduction is both
sexual and asexual, the latter taking place by bud-
ding. These animals are either monecious or
diecious. The eggs are fertilized in the coelom and
develop in a modified portion of the body cavity
called an ooectum which serves as a brood pouch.
The larva is in many respects like a trochophore.
Certain species produce individuals of a peculiar
type called avicularia.
of strong jaws, which probably are used in defense.

A Lophophore
} N y / UN g
lus a ’ \ ‘ |
Zooeciurm i ‘ Hy oes
Avicularium, Aa ios
JAWS (& Anus

Fig. 102.—A colony
of marine bryozoans,
Bugula turbinata Alder.
A British species; an
American species,
Bugula turrita (Desor),
is somewhat similar.
(From Harmer, ‘‘Cam-
bridge Natural History,”
by the courtesy of The
Macmillan Company.)
Natural size.

These are highly modified forms possessing a pair

Esophagus

Aviculariurn,

Muscle 70 fe
body wall . SAWS OPE
Ovary
Muscle to
Stomach \ body wall
furyculus Stomach
Testes Oa
forucusys
Ooectur7 Testes

Fic. 103.—Bugula avicularia (Pallas), a European species.

is also included in the figure.

somewhat modified.) Much magnified.

Two zooids are shown, the
one at the left entire, that at the right turned 90 degrees to the first and sectioned.
avicularia are shown, one with the jaws closed and the other with them open.
(Redrawn from Parker and Haswell, ‘‘Text-book of Zoology,”

Two
An ooecium

192 METAZOAN PHYLA

The fresh-water bryozoans (Fig. 104) form moss-like colonies attached
to plants, sticks, or stones, usually near the surface of quiet water,
though they have been found in Swiss lakes at a depth of over 40

i} iY
sen
x Ye Lophophore

ff }
fsophagus h

’ Fouricul 7 (
Stafoblast

Fira. 104.—Fresh-water bryozoans. A, colony of Plumatella sp., growing on a frag-
ment of a small branch of a tree. About natural size. B, several zooids of a colony of
Plumatella repens (Linnaeus), a European species. (From Allman, ‘‘ Monograph of Fresh-
water Polyzoa.”’) Highly magnified.

fathoms. Some have the power of movement, the whole colony very
slowly crawling along onits base. These fresh-water forms have developed
a type of winter egg known as a statoblast (Fig. 105), which is produced in

Fic. 105. Fic. 106.

Fic. 105.—Statoblasts of two genera of fresh-water bryozoans. A, Plumatella. B,
Cristatella. Much magnified.

Fic. 106.—Two specimens of Terebratella transversa (Sowerby), a brachiopod common
on the Pacific Coast. Natural size.
the fall, inclosed in a chitinous shell, and may either fall to the bottom of
the body of water in which it is or float. Freezing does not interfere with
its development in the spring but rather seems to stimulate it.

An enormous number of fossil forms have been described and some

of these are very similar to species now living.

OTHER UNSEGMENTED WORMS 193

223. Phylum Brachiopoda.—The animals included in Brachiopoda
(brik i 6p’ 6 da; G., brachion, arm, and podos, foot) resemble certain
mollusks in that they possess a bivalve calcareous shell (Fig. 106). For
that reason they have in the past been generally considered as belonging
to the field of conchology, the science which deals with the mollusks.
They have also been frequently grouped with the Bryozoa in a phylum
called Molluscoidea. The brachiopods, however, differ from the bivalve
mollusks in that the two valves of the shell are dorsoventral and not
lateral and that the internal structure is more wormlike than mollusk-
like. They are often called lamp shells because of the resemblance of
the shell, when viewed from the side, to an antique lamp. The ventral

Marttle

soLe & PD ecrive
Lophophore One a BES
Dorsa/ a Stomach
va/ve Heart
Morgndl hifi
se. =

Mari tle

Fic. 107.—Semidiagrammatic longitudinal section of a brachiopod, Magellania
lenticularis. (From VanCleave, ‘‘Invertebrate Zoology,” after Parker and Haswell, by the
courtesy of the McGraw-Hill Book Company, Inc.)

valve is larger than the dorsal one and at the margin where the two
articulate extends beyond the other, forming a beak. The tip of this
beak is pierced by an opening, or foramen, through which is passed a
fleshy peduncle which permanently attaches the animal to some object.

Brachiopods also possess a lophophore (Fig. 107), which consists of
two coiled arms bearing many ciliated tentacles. The function of this
lophophore, as in the Bryozoa, is to collect food and draw it into the
mouth. A true coelom is present. The animal possesses a heart and
blood vessels.

The brachiopods are all marine and have lived in the seas since very
ancient times. In past ages they have been more abundant than at
present, but many of them have come down to us practically unchanged.
One, Lingula, lives in the seas today and exhibits the same characteristics
as it did in the Silurian period, which, according to different estimates,
was anywhere from 25,000,000 to 300,000,000 years ago (Fig. 371).

CHAPTER XXXV

STARFISH
A TYPE OF THE PHYLUM ECHINODERMATA

Starfishes are generally distributed along all marine coasts but are
much more abundant where the shore is rocky than where it is composed
of sand or mud, for they can make little headway on a bottom which is
soft and yielding. They are also found clinging to piers, piling, and other
solid objects in the water which offer firm attachment. Along the shores

Positiors
OF QHUS

Fic. 108.—Aboral surface of a starfish, Asterias vulgaris Verrill. From a preserved
specimen. X 14.

of the oceans, where conditions are very favorable and where starfishes
become exceedingly abundant, the rocks at low tides are often closely
sprinkled with them. In certain places a bushel basket may be filled
with those which have gathered under a large boulder, where they are
protected from the direct rays of the sun and from excessive drying.
They usually remain quiet in the daytime. When clinging they adhere
closely and it sometimes requires considerable force to dislodge them, the
tube feet being torn in the effort.
194

STARFISH 195

224. External Appearance.—A typical starfish is an animal consisting
of a disc from which arise five rays. The bases of these rays occupy the
whole circumference of the disc, but they taper to blunt points at their
tips. The upper, aboral surface (Fig. 108) is covered with spines, around
the base of which are grouped very minute organs known as pedicellariae.
When examined under the microscope a pedicellaria (Fig. 110 A) is
seen to possess two jaws which differ somewhat in different types. These
structures serve to rid the surface of the body of foreign objects. The
disc and rays may be considered as divided by imaginary lines into sec-

Margira/
goes

Kea vey ve AA Nah

WN eylgtuhtat tala!

Pesecety

Ambulacral
groove

goed Na eae ee NUN
LIEGE Lor
Nw

Fig. 109.—Oral surface of a starfish, Asterias vulgaris Verrill. From a preserved
specimen in which the water-vascular system had been injected before preservation so that
the tube feet are extended. One ray turned as if the animal were beginning an effort to
turn over. X 4.

tions, the lines running from the center of the disc to the tips of the rays
being radii, and those running from the same point to the apices of the
angles between the rays being interradiz. On one of these interradii
and sharply distinguished by its smooth appearance is a little circular area
which is called the madreporite. In some cases it is possible to distinguish
a very minute anal opening close to the center of the disc.

On the lower or oral surface (Fig. 109), and in the center of the disc,
is the mouth, which is surrounded by a soft membrane, the perioral
membrane, or peristome, bounded on the outside by a ring of calcareous
plates or ossicles. From this ring an ambulacral groove (Fig. 110 A)
runs out the middle of each ray to the tip, and, the width of this groove
being less than the width of the ray, a relatively narrow strip is left
along each side. The margin of this groove is furnished with longer

196 METAZOAN PHYLA

spines, known as marginal spines. ‘These project more or less over the
groove, which is nearly filled with tube feet.

225. Skeleton and Musculature.—The skeleton of the starfish con-
sists of a large number of calcareous ossicles bound together by connec-
tive tissue. These ossicles are regularly arranged about the mouth and
in the ambulacral groove, where they form flat plates lying vertically

Perrbrarnchial space

Ectoderr7
‘2 _— Spire
g Pyloric Caecura

Tube root ye es ae ge oy Spee spe

Ambulacra!
~ Pores

Antbhilacral Adarnbulacral
pla te B plave

Fic. 110.—Ray of a starfish. A, diagrammatic cross section to show especially the
arrangement of the ambulacral plates and the tube feet. B, ambulacral and adambu-
lacral plates of a dried and cleaned specimen, showing the pores for the tube feet. XX 3.

face to face on either side of the mid-ambulacral line and nearly at right
angles to it (Fig. 110 B). The plates in the ambulacral groove are also
inclined, so as to give the groove the form of a trough when looked at
from below. These ossicles, the fibrous tissue between them, and the
spines, as well as the pedicellariae, are covered with a soft epidermis.

The rays of the starfish are not rigid but may be moved about by
muscle fibers in the body wall. There are also longitudinal and trans-
verse muscle fibers in the ambulacral groove which make it possible for
the rays to be bent toward the aboral side and for the ambulacral groove
to be closed by the drawing inward of the margins.

STARFISH 197

226. Water-vascular System.—The water-vascular system, which is
peculiar to the echinoderms, is a system of canals filled with water, which
serves as a locomotor system (Fig. 111). The madreporite on the aboral
surface, when examined closely, is seen to be marked by fine radiating
grooves along which are numerous minute openings, making of the whole
plate a sieve. Through this water enters the system and is strained as
it does so. From the madreporite a stone canal, so called because of
lime in its walls, runs downward across the body cavity to a ring canal
which lies at the outer margin of
the perioral membrane. In the
stone canal just below the madre-

_Medreport te

Store cara!

porite, as well as on the inner sur- Katiah
° ° ONT

face of the madreporite itself, are ae

cilia, the movement of which forces WESTOE.

water into the canal. From the
ring canal a radial canal runs out
each ray, lying at the bottom of
the ambulacral groove. From this
radial canal arise lateral canals
which lead to the tubes connecting

F2;
the ampullae and the tube feet. cono/ Z
pee D

Ritg cartal

The ampullae are sacs lying inside

the wall of the ray; the tube feet as Z2
lie outside it and in the ambulacral FA
groove. The tube which connects Ampulla
each ampulla with its tube foot TUBE TOF

runs through an ambulacral pore Fic. 111.—Diagram to illustrate the
(Fig. 110 A), formed by the separa- arrangement of parts in the water-vascular
tion of adjacent ossicles awinese system. a, tubefoot and ampulla, enlarged.
pores are in two rows on each side corresponding to the two rows of tube
feet and the lateral canals are thus alternately longer and shorter. In
the walls of the ampullae are circularly arranged muscle fibers which by
their constriction, working on the principle of a bulb syringe, lessen the
capacity of these ampullae. The tube feet are hollow organs cylindrical
in form, in many cases with a sucker at the outer end, working on the
principle of a vacuum cup. The lateral walls of these organs contain
muscle fibers.

The manner in which locomotion is accomplished is as follows: When
the animal is about to move in a given direction, the ray—or rays—point-
ing in that direction is raised and the tube feet are elongated as water is
forced into them by the contraction of the ampullae. When these tube
feet have been extended as far as possible, which in a starfish a foot in
diameter may be as much as two inches, their outer ends are brought in
close apposition to the object upon which the starfish is resting and the

Sucker

198 METAZOAN PHYLA

suckers take hold firmly. The muscles of the tube feet now begin to
contract, forcing the water out of them and back into the ampullae. As
these tube feet shorten, those in other parts of the body relax their hold
and the animal is drawn forward by just the length of the contracting
tube feet. Now the tube feet in the other parts of the body again take
hold, after which the ampullae of the anterior rays are once more brought
into play. Water is forced into the tube feet which are in advance,
causing them automatically to release their hold and allowing them to
be again extended and attached in a new place. Thus the animal
literally pulls itself along a tube foot’s length each time a tube foot acts.
But since all the tube feet do not act in unison, progression is a steady
forward movement.

227. Internal Organs.—The starfish has a true coelom, which is very
large and reaches to every part of the body. The space in the water-
vascular system represents a portion of the coelom cut off from the

Rectal Avwus

A sree ne~- Sasa
NOSIS
PELL Ae

Stomach Mouth

lnterradial septurt Radial caral
Fig. 112.—Section of a starfish along one radius and the opposite interradius. The parts

in the median line of the ambulacral groove are not shown.
rest. The coelom is everywhere lined with a peritoneum, or lining mem-
brane, and is filled with coelomic fluid.

The digestive system of the starfish is an alimentary canal (Fig. 112),
shortened by the flattening of the body and considerably modified. It
is divided into a very short esophagus, a large thin-walled stomach, a
pyloric sac, and a very slender rectum ending at the anal opening, which
is small and non-functional. From the pyloric sac a tube extends into
each ray and divides into two branches. ach of these branches connects
laterally with numerous glandular pouches called pyloric, or hepatic,
caeca. These caeca nearly fill each ray. Pouches connected with the
rectum are known as rectal caeca. The number of these varies in different
species. The stomach seems to secrete only mucus, but the pyloric sac,
as well as the glands in the pyloric caeca, forms digestive enzymes which
act on proteins, fats, and carbohydrates.

Starfishes are diecious. The ovaries and testes are much branched
organs lying at the bases of the rays, one on each side of each interradius.
The sex cells are passed out through a number of ducts which open on
each interradial plate.

STARFISH 199

228. Feeding and Metabolism.—As the starfish moves about over a
surface it secures animals which are unable to run away, such as oysters,
mussels, barnacles, clams, snails, and tube-dwelling worms, or those
which are surprised and captured before they can escape, such as crus-
taceans and even small fish. When an object is captured which can be
passed through the mouth into the stomach, it is done and digestion
takes place within the body, after which the indigestible part is thrown
out through the mouth. If, however, a starfish finds itself over such an
animal as an oyster or mussel, which is firmly attached and protected
by a shell, it has recourse to a novel mode of circumventing its prey.
Tube feet are attached to the two valves of the shell and then a steady
pull is exerted which tends to draw the valves apart. This pull may be

Ne
Fic. 113.—Asterias feeding on a clam. The valves of the clam are pulled apart by the
tube feet and the stomach of the starfish is then everted about the soft parts of the clam.
The stomach juices of the starfish may aid in causing the valves of the clams to open.
(From H. L. Wieman, ‘‘General Zoology’’ (1938), by the courtesy of McGraw-Hill Book
Company, Inc.)

resisted by the mollusk for some time, but sooner or later the muscles of
the victim relax. The stomach of the starfish is then everted through
the mouth and immediately inserted into the crack between the two
valves. Though the mollusk may for a time again attempt to close
this crack, the starfish ultimately succeeds in inserting enough of its
stomach so that it can be wrapped about the body of the mollusk, which
is then digested within its own shell. When digestion is complete the
starfish draws the stomach back into its body by means of retractor
muscles and moves on to find other prey. It is stated by MacBride
that if the mollusk is not firmly attached, the starfish will pick it up
between its rays; on one occasion a starfish which was confined was
observed to walk about all day carrying with it a mussel which it was
unable to open.

200 METAZOAN PHYLA

When the food is digested it is absorbed through the walls of the
alimentary canal into the coelom and is thus distributed to all parts of
the body. Elimination probably takes place in part through the rectal
eaeca but also in another and very curious manner. Ameboid cells,
known as amebocytes, lying in the coelomic fluid, pick up particles of
waste and make their way out through the walls of the dermobranchiae—
literally, skin gills—thus getting outside the body with the waste, and
not returning. The dermobranchiae (Fig. 110 A) are formed by out-
pocketings of the coelomic wall which thrust through the skeletal pores
and protrude on the outside beneath the epidermis. They are filled with

Arntervor

Grcurmoral
cthated Lard

Fosterior

A B

Fig. 114.—Bipinnaria larva of Asterias vulgaris. (From Field, in Quar. Jour. Microsc.
Sci., vol. 34.) A, ventral view of a larva five weeks old, to show the bilateral symmetry
The body bears a number of lobes, and two bands of cilia, one preoral, the other postoral.
The latter is not visible where it passes over upon the dorsal surface, but actually is a
continuous band. The internal organs are seen through the partly transparent body.
x 52. B, latera view of a three-weeks-old larva, enlarged to the same size as A. X 63.
Because of the difference in age there is lack of agreement in certain details, but it is hoped
the two views will enable the reader to visualize the larva.
coelomic fluid, and their function is chiefly respiratory. When the animal
is exposed by the ebbing of the tide they are retracted within the body
wall and remain so until the animal is again covered by the return of the
tide. The effect of these dermobranchiae, when fully extended, is more
or less completely to cover the surface of the body with a soft tissue
through which one who touches the animal can feel the firmer wall of
the body. The amebocytes are produced in structures attached to the
ring canal of the water-vascular system, known as polian vesicles (Fig.
111) and Tiedemann’s bodies.

229. Nervous System and Behavior.—The nervous system is less

highly developed than in the phyla previously studied, there being no

STARFISH 201

central ganglion. The system consists of a nerve ring encircling the
perioral membrane, radial nerve cords lying at the bottoms of the ambula-
cral grooves and reaching to the tips of the rays, nerves on the dorsal
surface of each ray which converge toward the center of the aboral disc,
and scattered nerve cells and sense cells lying among the cells of the
epidermis and distributed above the nerve cords. The principal sense
organs are the pigment spots, one at the tip of each ray, below a so-called
tentacle (Fig. 112). The pigment spots are light-perceiving and the
tentacles tactile organs. The tube
feet and the pedicellariae are very
sensitive to touch.

Starfishes as a rule are not very
active in the daytime, but at night
they move about in search of food.
They respond to such stimuli as con-
tact, light, temperature, and chemi-
cals. Jennings carried out some
experiments which indicated the
ability of a starfish to form a habit.
When a starfish is placed upon its
aboral surface it draws two or three
of its rays back under its body,
attaches the tube feet to the sub-
stratum, and turns itself over. In
most starfishes there is a tendency
regularly to use certain rays, and
these were determined for those
experimented upon. By restraining
these rays, Jennings succeeded in
developing in individuals the use of Menils neon Grae poe
others. One animal was trained in of a group of starfishes noted for their
180 lessons, ten on each of eighteen Texeperative ability to C; thive spec
days, so to use certain rays; after animal from one ray. In such a case five
an interval of seven days and when #s2%e reaenerated making six altogether,
left to its own initiative it was still disk is uninjured only the rays lost are
using them. This type of action regenerated, asin D. Natural size.
acquired as a result of the repetition of an act is known as a habit.

230. Reproduction.—Both sperm cells and egg cells are set free in
the water, where fertilization takes place. The eggs undergo total and
equal cleavage, after which follows a typical embryogeny, including the
development of a hollow blastula, a gastrula formed by invagination,
and a triploblastic enbryo. A larva known as a bipinnaria (Fig. 114)
is produced which is bilaterally symmetrical but which gradually meta-
morphoses into the radially symmetrical adult.

WHEEL,

i538

f
rest

la
rt
<

ie
See
vee
Ey
Bt

202 METAZOAN PHYLA

231. Regeneration and Autotomy.—The starfish has a considerable
power of regeneration. If the disc is deprived of all its rays it will
regenerate them all, and a single ray with only a portion of the dise will
regenerate the whole animal (Fig. 115).

The starfish also possesses the power of autotomy. Ordinarily the
part dropped off is regenerated. This ability serves as a safeguard to
the animal which, if it finds itself caught by one or more rays, can simply
drop them off and make its escape.

232. Economic Importance.—Starfishes are of economic importance
only as they are enemies of oyster fishermen or as they destroy clams and
other marine animals which serve as human food. Oysters live adherent
to solid objects lying upon the bottom of the sea in areas known as
oyster beds. Starfishes come upon these oyster beds and destroy
the oysters in the manner already described (Sec. 228). Owners of oyster
beds formerly were in the habit of using drags made of ffayed rope ends
which they hauled over the beds behind a vessel and in which the star-
fishes became entangled. The drag with its starfishes was then hauled
upon the deck. The starfishes were chopped to pieces and thrown back
into the water, but since the pieces were capable of regeneration this
simply multiplied the number of animals. It is now the practice to
carry them to shore and deposit them above high water mark where they
are left to die. They may then be used as a fertilizer. The amount of
damage done by starfishes may be considerable if they are not actively
combated. A single one placed in a dish containing some clams of good
size was observed to devour over 50 of these in six days.

CHAPTER XXXVI

PHYLUM ECHINODERMATA

Echinoderms are sharply distinguished from all other animals by
several characteristics. One of these is a secondary radial symmetry
which does not entirely mask a primitive bilateral condition. If a line
is drawn through the interradius of a starfish on which the madreporite
lies and is continued along the radius of the opposite ray, the animal will
be seen to be divided into two similar halves. The two rays adjacent to
the madreporite have been called the b/viwm and the three nonadjacent
rays the triviwm. Other characteristics show retrogression, and still
others specialization.

233. Retrogression.—Retrogression may be defined as the possession
by an animal of a lower grade of organization than that which has been
attained by its ancestors—in other words, the animal has gone backward
in its development. This is not to be confused with degeneration, which
is simply the loss of characteristics. The cestodes, for instance, show a
loss of the alimentary canal due to degeneration. The echinoderms also
show a change in the alimentary canal, but instead of disappearing it
tends to return to a condition seen in animals with a gastrovascular
cavity. Retrogression should not be confused, on the other hand, with
the simplicity which belongs to animals lower in the evolutionary scale.
This mistake was for a long time made by zoologists in grouping echino-
derms with the coelenterates under the term Radiata, because they were
seen to possess radialsymmetry. Evidences of the fact that echinoderms
are much higher in the scale of animal life than are coelenterates are seen
not only in many details of the adult structure but also in the fact that
the larvae show indications of an advanced condition.

It is this evidence from the larva which leads to the view that echino-
derms illustrate retrogression, which is shown in the following ways: (1)
The larva shows bilateral symmetry, while the adult exhibits a secondary
radial symmetry; (2) the former possesses an alimentary canal, but in
starfishes the anal opening nearly or quite disappears and the enteron
structurally and functionally tends to return to the condition of a gastro-
vascular cavity; (3) the nervous system in the larva promises a develop-
ment higher than any type studied up to this time, but in the adult it
acquires a character not greatly in advance of the coelenterates, possess-
ing little evidence of centralization.

203

204 METAZOAN PHYLA

234. Specializations.—Specialization or adaptation is the develop-
ment of structures which fit an animal to perform certain particular
functions or to meet certain peculiar conditions in the environment.
The echinoderms show some of the most marked examples of specializa-
tion to be found anywhere in the animal kingdom. Among these are (1)
the entire water-vascular system, (2) the spines and plates which form
the exoskeleton, (3) the pedicellariae, (4) the dermobranchiae, and (5) the
amebocytes.

Fie. 116.—The 20-rayed sunflower star, Pycnopodia helianthoides (Brandt). Twenty
to twenty-four rays are produced, usually in even numbers, although an occasional specimen
is found with an odd number. Very young individuals may have as few as sixrays. This
species is one of the largest of starfishes, often attaining a diameter of 2 or more feet, and is
common from California to Alaska. (Photograph by O. Wade.)

235. Classification.—The phylum Echinodermata (é ki no dér’ ma ta;
G., echinos, hedgehog, and dermatos, pertaining to skin) is divided into
five classes:

1. Asteroidea (as tér oi’ dé a; G., aster, star, and eidos, form).—The
starfishes.

2. Ophiuroidea (6 fiti roi’ dé a; G., ophis, serpent, oura, tail, and
evdos, form).—The brittle stars and serpent stars.

3. Echinoidea (&k i noi ‘dé a; G., echinos, hedgehog, and eidos, form).—
The sea urchins and sand dollars.

4. Holothurioidea (hdl 6 thi ri oi’ dé a; G., holothourion, water polyp,
and ezdos, form).—The sea cucumbers.

PHYLUM ECHINODERMATA 205

5. Crinoidea (kri noi’ dé a; F., krinon, lily, and eidos, form).—The
feather stars and sea lilies.

236. Asteroidea.—The general characteristics of this class are illus-
trated by the starfish. The bases of the rays take up the entire circum-
ference of the disc and thus are not definitely marked off from it. The
number of rays varies in different species from 5, which is the most usual
number, to more than 40. Though usually an odd number, it is not

sop

a
—o
Fe

Sy =
Bri.
My
3

- “an
Ke, Dey
Reet on

A
iS, 8
. a

ye 4S

tg &

> =}

gr . 3 Ys A

Reese § 3
a m aM
a3 q
A A =
< 3
§s =
Sana R=
7 PS:
NNN apy §
Se 3 a =

S SR tie 5

¥ i. 4
S| e iS
; 5

way
att A
PE tir fe

Fic. 117.—The long-armed brittle or serpent star, Amphiodia occidentalis (Lyman),
occurs along the Pacific Coast from California to Alaska. It may voluntarily part with
one or more arms—this is autotomy—and regenerate new ones readily. (Drawn by

Robert Allen Wolcott.)
invariably such, since there are forms in which the number is regularly
six. In some the dise is small and the rays are long and slender; in
others the dise is large and the rays short and broad. This shortening
and broadening of the rays may go so far as to produce pentagonal types.
Starfishes are rather generally distributed, being absent only from the
polar regions.

237. Ophiuroidea.—This class differs from the preceding in that it
possesses slender rays sharply marked off from the disc and in that the

206 METAZOAN PHYLA

ambulacral grooves are closed by the ingrowth of the ventral plates.
Owing to the slenderness of the rays none of the viscera extends into them
and they are exceedingly flexible and capable of very rapid movement.
A madreporite is found on the ventral surface, but the tube feet have
lost their ampullae and sucking dises and protrude from the sides of
the rays as tactile structures, and also from the surface of the disc,
adjacent to the mouth, where they serve to test the food and pass it into
the mouth (Fig. 117). The types known as brittle stars and serpent stars
are found under stones on the beach at low tide. When the tide is in
they wander more or less about the bottom, having somewhat the same
feeding habits as the starfishes, but cannot eat objects of any considerable
size. Brittle stars are capable of rapid locomotion, but serpent stars are
even more active, the rays writhing about like the tails of so many snakes

Spire

Fic. 118.—A sea urchin, Strongylocentrotus drébachiensis (Miller). From a pre-
served specimen. The spines have been stripped from the right half; they and the tube
feet show on the left. 34.
when the animal is strongly stimulated. The basket stars are character-
ized by complexly branched rays ending in tendril-like tips. They are
found mostly in water of considerable depth, clinging to masses of sea-
weed. Owing to the slenderness of the rays of ophiuroids, they are more
likely to be broken than are those of ordinary starfishes. Autotomy is
also more frequent, while regeneration is relatively rapid and complete.

238. Echinoidea.—Echinoidea, or sea urchins (Fig. 118), are am-
mals which have lost their rays and possess a skeleton made up of rows
of plates running from the oral to the aboral surface. These plates are
divided into five pairs of ambulacral rows, between which are an equal
number of pairs of interambulacral rows. The ambulacral rows are
perforated for the exit of tube feet and correspond to those in the ambu-
lacral groove of the starfish, while the interambulacral rows would corre-
spond to the interradial plates of the starfish (Fig. 119). One may
conceive of a starfish being transformed into a sea urchin by an increase
in the vertical diameter of the body and a shortening of the rays until
they disappear. With the disappearance of the rays the ambulacral and
the interambulacral bands run up around the side of the body and ter-

PHYLUM ECHINODERMATA 207

minate in ocular and genital plates, respectively, leaving only the periproct
as aboral surface. The mouth of the starfish is simply an opening in the
center of a soft perioral membrane; in the sea urchins, however, it is
provided with five converging teeth. These are set in a complicated
skeletal box pentagonal in shape Pages

and known as an Aristotle’s lantern Madreporite
(Fig. 120). This is made up of nu-
merous ossicles, lies within the body,
and contains muscles which move the
teeth. The food of sea urchins con-
sists of algae, which they remove from
the surfaces of rocks with their teeth.

Respiration in sea urchins usually
takes place by ten branched pouches
arranged in acircle around the mouth.
The tube feet are also said to be
respiratory. The latter may be yi MEIN ae rare
exceedingly long if the spines on the Aferambulacra/
surface of the animal are long, since Plates
they reach beyond the spines. The
tube feet are used both in loco-
motion and in holding to surrounding
objects. In locomotion the spines are
used to prevent the pull of the tube
feet from rolling the animal over and
also as levers to help pry the animal
onward. The pedicellariae of sea
urchins are on a stalk and usually
have three jaws.

Sea urchins differ in the length
and number of the spines. The cake B
urchins and sand dollars are exceed- Fig. 119.—Dried shell of a sea urchin
: . _ of the genus Arbacia. Shows the arrange-
ingly flat forms with numerous very ment of plates. ‘A, “they aborall suxtsee

small and short spines;they arefound 8B, the oral surface, with the perioral
d b h wav Ay membrane torn loose for about two-thirds
on sandy beaches, burying €M- of its attachment. Dried pedicellariae

selves just under the surface of the are still attached to this membrane.
sand as the tide goes out but mov- “#t™! size-
ing about on the sand after the tide has returned.

239. Holothurioidea.—The sea cucumbers, which make up this class,
differ from other echinoderms in the fact that they are greatly elongated
along the oral-aboral axis, the mouth being at one end and surrounded
by branched tentacles, while the anal opening is at the opposite end of
the body (Fig. 121). The body wall is muscular and possesses few and

Ocular Plate

A Avbulacral
Plates

208 METAZOAN PHYLA

small calcareous plates. The madreporite is internal. . Tube feet are
present and serve as organs for clinging and for locomotion.

One type of sea cucumber is represented by those which conceal
themselves in the crevices between rocks and which have the tube feet
all around the body in five double rows. Some of the tube feet adjacent
to the mouth, as well as the tentacles, are used in procuring food. A
cloaca is present in a typical sea cucumber and contains the openings of
two long branched tubes, the respiratory trees (Fig. 122). Respiration
occurs in these as well as through the cloacal wall and the walls of the
tentacles and tube feet. The respiratory trees also serve as excretory

lLytestirie Storvach

Aristotles
lariter’?

Ampulla Govrrad

Fig. 120.—Internal structure of a sea urchin. (From Delage and Hérouard, “ Traité de
Zoologie Concréte,”’ after Milne Edwards.) The oral wall of the shell has been removed and
the contents of the body are viewed from the oral pole with the Aristotle’s lantern and
esophagus turned to the left.

organs. The madreporite takes water in from the coelomic cavity.
Other sea cucumbers possess tube feet on only one side of the body and
travel about on that side, looking like huge caterpillars. Still others
burrow in the mud like earthworms and have no tube feet at all; they
seek their food at the surface of the mud and secure small living plants
and animals by means of their tentacles. The last named are the most
primitive of echinoderms.

The holothurians exhibit a remarkable form of autotomy and regenera-
tion. When irritated the whole alimentary canal and the respiratory
trees may be thrown out through the mouth, there being developed from
the lower branches of the latter a mass of white tough threads in which a

PHYLUM ECHINODERMATA 209

possible enemy may become entangled. These structures, however, are
soon regenerated.

240. Crinoidea.—The sea lilies, which were exceedingly abundant in
the seas ages ago, are echinoderms which, typically, are attached by the
aboral surface to a stalk that rises from the bottom and frequently
possesses many rootlike branches (Fig. 123). The oral surface is upper-
most and the disc is surrounded by more or less complexly branched rays

Vi Oral teritacles

y \ fi if €:
king cared Be 2 Gerital opering

.\ Goriad

Poligr BS
vesicles Y

i Se “ WV a FE: Z é NY
‘ A / baie a = a P
e 2Qn g Py Intestive
) ) a b Connecting | HIP: \
at ~2N blood vesse/ N omnes ae 0rsa.
Woy ~ < 2 ST: blood vesse/
ZB Zz —— L ¥ Kat :
oe -, ie |) ones
Ft:
= KE 5 Vimar tl PAK :
SoG) ve Zee Ventral
Ss fae Orcular. = =\e blood vesse/
. . 4 t1USC/CS Sel
Dm ay we cS E:
Br x TRE:
= a ——= ; 3 LAW —F:
ARY VS Long/tudiizal \MEK
Sey 4 6 muscle band \KX
Dp P > f tA
ALY “RLY Respirator
‘A (= fo wate A , "ree a
ASB sf Covieriar7
sy ae 3 aX AIDS
ZA aed Radial rruscles
a
WE} be
Yio tey
Bre. 12a Img, IPR.

Fig. 121.—A sea cucumber, Cucumaria planct Brandt, from the Mediterranean.

From a preserved specimen. X 2%.
Fig. 122.—Diagram of the internal anatomy of a sea cucumber representing the animal

laid open and the wall of the body turned to each side.

bearing smaller pinnules arranged like the barbs on a feather. The
tube feet are tentacle-like and without ampullae. Crinoids are found
mostly at moderate depths, but a few belong to the deep-sea fauna.
They may be free-swimming, when they are known as feather stars
(Fig. 124).

241. Reproduction.—The reproduction of all echinoderms is similar
to that of the starfish. They all develop a bilaterally symmetrical
swimming larva, and all undergo metamorphosis. The larvae of the

210 METAZOAN PHYLA

different classes resemble each other in a general way, but each is quite
distinct. That of a starfish is known as a bipinnaria or brachiolaria, that
of a brittle star as an ophiopluteus, that of a sea urchin as an echino-
pluteus, and that of a sea cucumber as an auricularia.

242. Behavior.—In the echinoderms the nervous system does not
seem to control the muscles as does a centralized system in other animals
but simply maintains a certain tonus, a condition accompanied by readi-
ness to respond to stimuli. The response itself
is, generally speaking, direct. Pedicellariae react
to the presence of an object in contact with
the skin near them by seizing it in their jaws
and either holding it or so moving as to carry
it away from the point of contact. In this
way they serve to keep the surface clean, especially
over the dermobranchiae. A strong stimulus
results in locomotor impulses being carried to
! the tube feet through the nervous system, which
in this way functions in coordination. Habit

j formation has been referred to in the preceding

j chapter.
243. Color.—The echinoderms when living are
ff very strikingly colored, but color may mean
| nothing in the discrimination of species, some

forms quite regularly exhibiting a_ particular
color but others showing marked variations.
Almost all colors are represented in the group,
various shades of red and orange being common.

ASPDAIVINER ES
( ita, There are also varying tints from cream to almost
e ANI white, and innumerable shades of buff, brown,
oS SEW green, blue, and purple, some being almost black.
Ne Me MY When a group of sea cucumbers are seen on the
Sn See, bottom with their tentacles spread and possessing

; _ varied and brilliant hues, they appear as striking
Fig. 123.—Rhizocri- :

nus lofotensis Sars. (From 28 @ bed of variously colored flowers.

Bather, in Lankester, 244. Occurrence and Economic Importance.

ites Be rece Because of the possession of skeletons by echi-
a ee ee A.andC. noderms their parts have been preserved in
iy i abundance in rocks from very early times, and
many limestone strata are filled with such fossils. Among these
are spines and plates, sections of the stems of crinoids, and in some
cases the crinoid body, very nearly perfect. At the present time
echinoderms are widely distributed and abundant in all seas. The
phylum is one of the few which has always been exclusively marine.

PHYLUM ECHINODERMATA 211

Of this phylum only starfishes and sea cucumbers are of much eco-
As has been previously indicated, the starfishes
Sea cucumbers are used as food among the

nomic importance.
are enemies of the oyster.

Ces

Es
i

9
=|
Q

i)
”—~Aboral
teritacles
Natural size. The

Maadreporite
From a preserved specimen.

animal is in a breeding condition as shown by the dilatation of the base of the pinnules.

Fic. 124.—A feather star, Antedon sp.
islands and along the shores of the South Pacific Ocean and in China,
the animals being dried and passing as articles of commerce under the

names of béche-de-mer and trepang.

CHAPTER XXXVII

FRESH-WATER MUSSEL
A TYPE OF THE PHYLUM MOLLUSCA

Fresh-water mussels are found in ponds, lakes, or streams. In ponds
or lakes, where the water is quiet, they lie nearly buried in the mud at the
bottom, moving about from time to time and leaving a furrow to mark
the path which they have followed. In running streams they are found
most abundantly where the water runs rapidly and where plenty of food
is brought to them. In such locations they are sedentary, remaining
wedged in between the stones and protected from the force of the current

Umbo Hinge ligament

SOSA FET

\3 siphor

Fig. 125—Anodonta grandis Say. From a preserved specimen from Nebraska.
x 4. A typical fresh-water mussel. The lines of growth are numerous, but only the
most pronounced are annual lines. This specimen was probably in its sixth year.
which would otherwise sweep them down into quiet pools where they
might be buried in the mud. Because of this danger of being buried by
the deposition of mud they are not often found in the broad, deep estuaries
of such a river as the Mississippi or in wide, shallow rivers which carry
large amounts of sand and detritus, such as the Missouri, the Platte, and
the Kansas.

245. External Appearance.—A typical mussel is oval when viewed
from the side, the anterior end being rounded and the posterior pointed
(Fig. 125). The thickness is greatest at a point above and behind the
middle of the body and near the dorsal side; from this point it diminishes
in all directions but more gradually toward the ventral side, the margin
forming a sharp edge anteriorly, posteriorly, and ventrally. A cross
section has the outline of a conventional heart (Fig. 127). The animal is

212

FRESH-WATER MUSSEL 213

covered by a bivalve shell, the valves being lateral in position. When
buried in the mud only a small part of the posterior end is left free, and
at this point may be seen projecting from between the two valves the
open ends of the s¢phons, by means of which water is circulated through
the animal, the current entering through the ventral siphon and leaving
through the dorsal one. When the animal is moving, a muscular foot
is projected ventrally and anteriorly between the two valves. This foot
ends in a blunt point where its anterior and ventral margins meet, and this
point is capable of being extended to form a sort of hook by means of
which the animal pulls itself along.

246. Shell.—The two valves of the shell are fastened together along
the dorsal side by a hinge ligament which is elastic and which tends to

Hittge tooth

Attachment area
of posterior retractor
muscles

Attachrnent
area of

posterior
\ adductor
\ 7scles

Hinge hgament Umbo

Attachrrient area
of anterior 4
adducror 7
wuscles ff

Fig. 126.—Anodonta grandis Say. Showing inner surface of a right-hand valve. X 4.

draw the valves together dorsally and to cause them to gape ventrally.
Close to this ligament, on either side and nearer the anterior than the
posterior end, is a point known as the wmbo, which represents the oldest
portion of the shell. On the outer surface of the shell (Fig. 125) may be
seen a great number of concentric lines arranged around the umbo as a
center and gradually increasing in distance from the umbo until the
margin of the shell is reached. They represent lines of growth, several of
which may be formed during one year, though the annual lines are some-
what more prominent than the intermediate ones. In older animals the
shell at the umbo is very often eroded. Elsewhere it is covered by a skin-
like horny layer called the periostracum, which gives color to the shell.
The periostracum extends a little way beyond the margin of the shell
except at the hinge ligament.

The inner surface of each valve (Fig. 126) often possesses elongated
sharp ridges and toothlike projections which hold the valves from slipping
out of position. These are known as hinge teeth. They vary greatly in
degree of development in different types and may be absent. There are
roughened areas serving for the attachment of the anterior and posterior
adductor muscles, which run from one valve to the other and hold the two

214 METAZOAN PHYLA

together. A little way from the ventral margin and parallel to it is seen a
small groove, known as the pallial line, which marks the attachment of
the muscle layer of the mantle, or pallium.

Hinge lagamernt

Intestir7é

ea cavity
Suprabran- ff Auricle
chia/
charvrbers

Ce \ RY Iotestie

NMartle

Fic. 127.— Diagrammatic cross section of a fresh-water mussel. (From Parker and
Haswell, ‘‘ Text-Book of Zoology,”’ after Howes, by the courtesy of The Macmillan Company.)

When viewed in cross section the shell is seen to exhibit several layers:
(1) a horny layer or periostracum, sometimes called the epidermis; (2)

Opening Pericardia/

Verrtricle
of uretér caviry

Posterior retracfor muscle

Posterior
adductor muscle

Fsophagus

gerltal

Anterior duct

adductor

Visceral ganghor
HIUSCIE

Anal opening

Labial polp
Ce erebropleural
GHIGIONI

Fig. 128.—Diagram of the internal anatomy of a fresh-water mussel. (Compiled from
various sources.) ‘The mantle and gills on the near side are not shown, and the body is indi-
cated as having had a part of the wall cut away. The stomach, liver, gonad, pericardium,
and kidney are shown in section.

a series of layers of carbonate of lime, together known as the prismatic
layer; (3) a layer of nacre or mother-of-pearl, also carbonate of lime,

FRESH-WATER MUSSEL 215

made up of many thin lamellae. The nacre is thickest at the hinge
ligament and becomes gradually thinner toward the margin. The shell
is constantly being added to at the margin and thus increased in surface
area, and at the same time it is constantly increasing in thickness by
deposition from within.

247. Internal Anatomy.—Inclosed in the shell is an animal which is
very soft and which when removed from the shell becomes to a consider-
able degree shapeless. It may be described as made up of a body mass
and foot, a mantle, and two pairs of gills (Fig. 127). The relation of
mantle and gills to the body mass is similar to that which a man’s clothes
would have to his body if his coat, corresponding to the mantle, were
grown to his back, and he had on two vests, unbuttoned in front and
attached to the sides of his body, corresponding to the gills.

248. Body Mass and Foot.—The body mass, or visceral mass, is a soft
mass filling the upper part of the space between the two valves and is
continuous externally with the mantle. Ventrally it becomes narrowed
and hangs down in the mantle cavity (Fig. 127), its ventral muscular
margin forming the foot. In this body mass are contained various
organs, including those of the digestive, circulatory, excretory, and
reproductive systems (Fig. 128).

249. Mantle.—The mantle lines the inner surfaces of the valves of the
shell but it extends a short distance beyond their edges and forms, with
the periostracum, a soft margin. This mantle secretes the carbonate of
lime which is continually added to the edge of the shell and to its inner
surface. The space inclosed by the two halves of the mantle is known
as the mantle cavity.

250. Gills.—The gills, which with the mantle carry on respiration, are
platelike and are in pairs on each side of the body mass, to which they
are attached. Each gill is composed of two lamellae joined all around the
margin and also connected by a number of cross partitions known as
interlamellar junctions, which divide the inclosed space into a large number
of compartments known as water tubes. Each lamella is a sort of mesh-
work made up of vertical ridges, known as gill filaments, connected by
horizontal bars. Some of these meshes possess openings which permit
water to pass through into the water tubes. The gill filaments are
strengthened by chitinous rods. The water tubes open above each gill
into a passageway known as a suprabranchial chamber. The base of the
inner lamella of the inner gill is attached to the body mass from the
anterior margin backward for a distance which varies in different types of
mussels. The attachment is short in those known as anodontas and
long in unios. Where the lamella is free there is left a narrow slitlike
passage which leads from the mantle cavity directly into the inner supra-
branchial chamber on each side. Since these two inner lamellae meet
behind the body mass and continue onward posteriorly, the two inner

216 METAZOAN PHYLA

suprabranchial chambers unite, and the passage between the two inner
plates of the inner gills and the body mass is really narrowly U-shaped,
inclosing the body mass behind. The outer suprabranchial chambers
enter the chamber formed by the union of the inner suprabranchial
chambers and in the wall of the common passage is the anal opening.
From the anus onward the passage may be called a cloaca; it opens to the
outside through the dorsal siphon. The water which enters through the
ventral siphon fills the mantle cavity (Fig. 129), bathes the gills, enters
the water tubes, passes up them to the suprabranchial chambers, and by
means of the cloaca and dorsal siphon escapes from the body. A current
is maintained by cilia which cover the walls of these passages.

251. Digestive System and Metabolism.—The mouth is an opening
on the dorsal wall of the mantle cavity toward the anterior end. On

Qvenihg of Suprabranchial
gerttal duct chambers
and ureter GlogGa
Qveri1tg of
1 Water qubes ) /S Dorsal
Ey, HS siphor7
ZL
EE Eg
Verttra/
siphor

Lobia/
Paps
Mantle cavity Outer Gill

Fig. 129.—Diagram to illustrate the circulation of water through a fresh-water mussel.
The suprabranchial chambers are shown as if cut open. The current is maintained by cilia
on all the wall surface of the mantle cavity, aided by movements of the gills and labial
palps. The labial palps also direct the food particles into the mouth.

each side of it is a pair of triangular labial palps (Fig. 128). Cilia on the
surface of these palps help carry the food into the mouth. From the
mouth a short esophagus leads to a broadly dilated stomach, which receives
the secretion of the liver. From the stomach a narrow coiled intestine
leads to the cloaca, through which the feces obtain exit from the body.

The food of fresh-water mussels is made up of particles of organic
matter, including microscopic plants and animals, brought in through the
ventral siphon. These food particles are strained out as the water passes
through the gills and are carried toward the mouth by cilia, the labial
palps collecting and directing them into the mouth opening.

252. Circulatory System.—The circulatory system includes a heart
made up of a ventricle and two auricles. It lies in a pericardial cavity
in the dorsal part of the body mass (Fig. 128), and a portion of the intes-
tine passes through the ventricle. From the ventricle the blood is dis-

FRESH-WATER MUSSEL 217

tributed over the body. It is collected again in a vena cava lying below the
pericardium and is passed through the kidneys to the gills and back to
the auricles.

253. Excretory System.—The organs of elimination are two U-shaped
kidneys (Fig. 128) lying just below the pericardial cavity, one on each side
of the vena cava. From each arises a thin-walled tube or ureter which
opens on the lateral surface of the body mass toward the anterior end and
at the level of the inner suprabranchial chamber. If the inner lamella of
the inner gill is attached beyond the point where it opens, the opening is
directly into that chamber.

254. Musculature.—The muscles include, among others, the adduc-
tors which serve to close the shell (Fig. 128) and the protractors and
retractors which cause the protrusion and withdrawal of the foot.

255. Nervous System.—The central nervous system is represented
by several centers, or ganglia, scattered throughout the body in pairs.
Among these are the cerebropleural ganglia near the mouth, the pedal
ganglia in the foot, and the visceral ganglia below the posterior adductor
muscles (Fig. 128). The two cerebropleural ganglia are connected by a
commissure and each one by connectives with the other two ganglia on the
same side. From the ganglia nerves lead to various parts of the body.
There are few sense organs. An organ of equilibrium lies in the foot a
short distance posterior to the pedal ganglia and below them. It consists
of a cavity known as a statocyst, containing a mass of lime called a stato-
lith, the movement of which stimulates the sensory cells. On the surface
over each visceral ganglion is a sheet of epithelial cells which appear to be
sensory and form an organ known as an osphradium, the function of which
is uncertain. It may be used in testing the water in the mantle cavity.
Tactile cells are abundant on the edges of the siphons and along the mar-
gins of the mantle. The margins of the siphons also seem to be sensitive
to light.

_ 256. Behavior.—Since the unios move about but little and may even
remain for a long period of time in exactly the same place, behavior is
confined mostly to the opening and closing of the siphons, which are
stimulated both by light and by contact. The stimulation of the osphra-
dium by injurious substances dissolved in the water may cause the valves
to close. The sense of equilibrium would function in enabling the animal
to assume an upright position in case conditions forced it to take up a new
location. The anodontas move about freely and when turned upon one
side soon right themselves.

257. Reproduction.—In mussels the sexes are separate. The
reproductive organs are situated in the body mass just above the foot, and
the vasa deferentia in the male, or the oviducts in the female, open just
in front of the opening of the ureters. The sperm cells when passed out
through the vas deferens into the suprabranchial chamber escape from the

218 METAZOAN PHYLA

body through the dorsal siphon and are carried by the water to another
individual. The egg cells do not leave the body but, owing to a reversal
of the direction of motion of the cilia on the walls of the suprabranchial
chambers, are carried into the gills and reach the marsupia. Here they
are fertilized by sperm cells from another mussel, which have entered
through the ventral siphon. A marswpium in this animal is a portion of a
gill modified to serve as a place for the development of the eggs.

Larva/
thread

Foor

Fig. 130.—Diagram to illustrate the life history of a fresh-water mussel. A to EH are
of Unio complanatus (Dillwyn); F and G, of Lampsilis ligamentina (Lamarck). (B to E are
from Lillie, Journal of Morphology, vol. 10, and F and G from Lefevre and Curtis, Bull. Bureau
of Fisheries, vol. 30.) A, the adult. XX about 14. B, the egg. X55. C, Two-cell
stage, showing unequal cleavage. X 55. D, section of the gastrula. 122. E,
glochidium. X92. F, head of rock bass with operculum cut away to show glochidia
attached to the gills. G, young mussel one week after leaving the fish. Much magnified.
Several lines of growth have been formed.

The eggs undergo total and unequal cleavage. Blastula and gastrula
stages are passed through, and a larval form known as a glochidium is
developed (Fig. 130). This larva has a shell consisting of two valves
closed by an adductor muscle. A long larval thread extends from the
body of the larva between the gaping valves of the shell. It is, however,
not present in all species. Fertilization usually occurs in a unio in spring
or early summer, and by the middle of August or the first part of Septem-
ber the glochidia have become sufficiently mature to be freed from the

FRESH-WATER MUSSEL 219

marsupium and to be carried out of the dorsal siphon by the currents of
water flowing through the body. Since the unios usually live in running
water, their glochidia are swept along by the current until they are carried
into the mouths of fish with the water which the latter use in respiration.
As they pass the gills of the fish they attach themselves to the gill fila-
ments, clinging by the valves of the shell and also, perhaps, attached by
the larval thread, which penetrates the tissues. The closure of the valves
seems to be due to chemical stimulation by salts escaping from the tissues
of the fish. On the gills of the fish the larvae live a parasitic existence

—=

undulata (Barnes). From the Grand River,
Michigan. For comparison with the anodonta type, see Figs. 125 and 126. This is one
of the species most sought for button making. X 26,

until development is completed and they are ready to metamorphose into
small adults. After this has occurred they escape from the fish, fall to
the bottom, and begin an independent life.

258. Other Fresh-water Mussels.—A large number of species of
mussels, belonging to several genera, may usually be found in any portion
of this country. They differ markedly in size and shape and in the details
of structure. They may be all grouped under two types—the stream
type, known in a general way as wnios, and the quiet-water type, known
as anodontas. The former (Fig. 131), usually found in running water,
are generally stationary, have thick shells with prominent hinge teeth,
and the glochidia develop mostly in the gills of fishes, though they may
also be attached to external surfaces. The anodonta type (Figs. 125 and
126), on the other hand, includes forms that live in still water, moving

220 METAZOAN PHYLA

freely about in search of food, have thin shells with the hinge teeth
greatly reduced or absent, and the larvae develop on the fins, opercula,
or margins of the mouths of fishes, as well as on the gills.

An anodonta living in quiet water and not subject to rough treat-
ment needs neither a thick shell nor well-developed hinge teeth to safe-
guard itself against injury. Not being able to count upon its food being
brought to it, it wanders freely, especially at night, in search of the food
it needs, usually remaining quite stationary during the daytime. When
the glochidia from these forms are released from the parent, they fall to
the bottom and lie there until stimulated by the contact of some part
of a fish coming to rest on the bottom or until carried into the mouth of
a fish in its breathing. They then seize the surface of the fish thus pre-
sented and become attached by the teeth in the margin of the shell and
by the larval thread in the same manner as do the glochidia of the
unios. Here the closure of the valves seems to result from a contact
stimulus. In anodontas the eggs are usually fertilized between the middle
of July and the middle of August, and the glochidia are discharged the
next spring or early summer. When these glochidia attach themselves
to the surface of a fish, the skin of the fish grows around them and they
become known as blackheads.

Not all unios occur in rapidly running streams, and when one lives
under conditions described as proper for the anodontas it exhibits to a
considerable degree both the structure and the habits of an anodonta,
as well as a similar manner of reproduction. In a Minnesota lake,
surrounded by sandy beaches, unios of the species Lampsilis luteola
(Lamarck) have been observed to come in from the deeper water during
quiet nights and to migrate freely about in the shallow water alongshore,
returning to the deeper water with the coming of the dawn and leaving a
tortuous furrow as evidence of their nocturnal wandering.

One result of the parasitic period in the life of the mussel is a far more
rapid dispersal of the species than if that depended entirely upon the
locomotor ability of the mussel itself or even upon the current, since
transportation by fish permits the spreading of mussels from the lower
portion of a stream toward its source.

CHAPTER XXXVIII

MOLLUSKS IN GENERAL

Mollusca is a phylum more numerous in known species than any
other except Arthropoda. The mollusks are exceedingly varied in form
and structure and represent a great difference in grade of organization
between the lowest and highest. Indeed, the whole phylum may be
considered as representing a long line of descent from wormlike ancestors.
They have several features in common which distinguish them from all
other animals. One of these is the presence of a mantle, which varies
in form in different classes. Since the shell is secreted by the mantle,
its form varies accordingly. The mantle cavity, which is the respiratory
cavity, also varies in form and in the character of the respiratory organs
contained in it. Another characteristic structure of mollusks is the
ventral muscular foot, which is sometimes absent, as in the oyster. This
foot is typically not an appendage, since in most cases it is merely a
portion of the muscular wall of the body modified to form a smooth sur-
face which serves in locomotion. In the Cephalopoda, however, the
foot is modified in such a manner as to produce several long mobile
tentacles provided with suckers, which are truly appendages.

259. Classification.—The phylum Mollusca (mol lis’ ka; L., mol-
luscus, soft) is divided into five classes according to the characters of
foot, mantle, shell, and respiratory organs. The classes are as follows:

1. Amphineura (am fi nti’ ra; G., amphi, on both sides, and neuron,
nerve).—The chitons.

2. Gastropoda (gas trép’ 6 da; G., gastros, stomach, and podos, foot).—
The snails.

3. Scaphopoda (ski fp’ 6 da; G., skaphe, boat, and podos, foot).—
The tusk shells.

4. Pelecypoda (pél é sip’ 6 da; G., pelekys, hatchet, and podos, foot).—
The bivalve mollusks.

5. Cephalopoda (séf al Sp’ 6 da; G., kephale, head, and podos, foot).—
The squids, octopuses, and nautiluses.

260. Amphineura.—The Amphineura are clearly the most primitive
of the mollusks, reminding one in some respects of the worms. They
have a body which is flattened and elongated. In some of these forms
there is no shell, but the dorsal surface is covered by a soft mantle con-
taining many small limy spicules (Fig. 132 D). In this case the foot

lies in a groove on the ventral surface. In other forms known as chitons
221

222 METAZOAN PHYLA

the mantle is more or less completely covered with a series of eight
overlapping calcareous plates which together make up the shell (Fig.
132 A), and the broad flat foot occupies the greater part of the ventral
surface. Between the margin of the foot and the edge of the mantle on
each side is a row of marginal gill filaments (Fig. 132 B). The nervous
system includes four longitudinal nerve cords, united in the chitons to
an anterior nerve ring (Fig. 132 C) and in other types to cerebral ganglia.
As a whole the system is distinetly wormlike.

The chitons are all marine and live firmly attached to rocks and other
solid objects alongshore between tide marks and also just below the
low tide mark. They move with great slowness and adhere so closely
to rocks that they are torn loose only by the exercise of considerable

Skeletas Sublingual Mouth Gis
A GWIDMA =

Posterior
COMMPIISSUFC

A Cc

Fig. 132.—Amphineura. A, upper surface of a chiton, Ischnochiton sp. Natural size.
B, lower surface of the same chiton. C, diagram of the nervous system of achiton. (From
Cooke, ‘‘Cambridge Natural History,’ after Hubrecht.) D, a primitive, wormlike species,
Chaetoderma nitidulum Loven. (Also from Cooke, ‘‘Cambridge Natural History.”) X 234.
Found in the North Atlantic Ocean at considerable depths. Possesses no foot, and has
limy spicules in the skin. (C and D by the courtesy of The Macmillan Company.)

force. The more wormlike and shell-less forms are found on coral polyps
and hydroids at depths of 50 fathoms or more.

261. Gastropoda.—The gastropods possess an elongated flat foot
making up the entire ventral surface of the body except for a short por-
tion at the anterior end. They also possess a well-developed head and
may have a shell, which is often spirally coiled and therefore asymmetri-
cal. This class includes a number of very distinct types.

The type which is most commonly thought of as representative is a
snail with a spirally coiled shell. In such an animal the body consists
of a head, a more or less distinct neck, a foot, and a visceral mass which
is developed into a sort of hump on the dorsal side of the body. The
head bears two pairs of fleshy tentacles, a relatively short pair anteriorly
which are olfactory in function, and a longer pair posteriorly which bear
the eyes at the tip (Fig. 133). These tentacles are hollow and capable of
being inverted like the finger of a glove when the tip is pulled down inside.

MOLLUSKS IN GENERAL 223

When completely inverted the tentacles may not be seen at all, but as
eversion takes place they become longer and longer until finally in the
case of those bearing the eyes these appear like small beads at the tips.
These eyes can perceive light but do not afford vision, as do those of the
highest mollusks. The mouth contains a chitinous plate covered with
teeth which is known as a radula. It serves as a grater and by a rasping
action can remove the epidermis from leaves or a growth of algae from
the surface of submerged objects.

The visceral mass bears a mantle which secretes a shell, but since this
mass is not of sufficient consistency to hold the shell in a constant position
in the median line, its weight carries it to one side. Growth is also
faster toward the outside of the shell. Consequently, as the margin is
extended by growth it tends constantly to change its direction of inclina-

Mantle cavity or
pulnorary chamber

S Olfactory
vk terrtacle

lntesti7é

Qverurig tito
pulmonary Ahovaber

Fig. 133.—European edible snail, Helix pomatia Linnaeus, shown in an extended
condition. (Modified from Schmeil, ‘‘Text-book of Zoology,’ by the courtesy of A. and C.
Black, and of Quelle and Meyer.) Somewhat diagrammatic, the positions of the alimen-
tary canal and mantle cavity being indicated by dotted lines.
tion and gradually develops a spiral form. Under the mantle is a
mantle cavity the wall of which is kept moist by a secretion and serves in
respiration.

The number of ganglia in the gastropods is much greater than that in
the mussel, and they lie closer together in the body (Fig. 154). The
sense organs include, besides eyes and olfactory organs, a pair of organs
of equilibrium similar in structure to those of the mussel, one lying on
each side of the head near the supraesophageal ganglion.

In locomotion the body is protruded from the shell, the foot is
extended, and a slime gland behind the mouth secretes a mass of slime
which passes back under the foot and forms a pathway upon which the
animal glides, its course being marked by the track of slime left behind.
Some snails can move with a moderate degree of rapidity, Baker ascribing
to them a speed of two inches per minute.

Snails are either diecious or monecious. In case they are monecious,
self-fertilization does not usually occur, but in copulation there is recip-
rocal fertilization.

224 METAZOAN PHYLA

Some snails are terrestrial, in which case they breathe by taking air
into the mantle cavity, the surface of which is kept moist by a mucous
secretion. Other snails are pulmonate and inhabit fresh water, in which
case it is necessary for them to come to the surface of the water at inter-
vals to fill the mantle cavity with air. In the case of others, which live
at all times submerged, water is taken into the mantle cavity, where
respiration may take place through the walls of the cavity or by means of
gills. The majority of snails are marine. Many of them reach a con-
siderable size and develop shells possessing great beauty of form and
color. A snail shell may be made up of many coils in one plane, or it may

3 - Gug Lior

7 » A

Viscera/ gariglhor?

Fic. 134..-Somewhat diagrammatic representation of the central portion of the nervous
system of Helix pomatia. (From Lang, ‘‘ Text-book of Comparative A natomy.’’)

be extended into a spire. On the other hand, the mouth of the shell may
be greatly expanded and the coiling be reduced to one turn or even a part
of a turn, asin the abalone. There may be no coils at all, as in the case of
the limpets, which have symmetrical conical shells. Still another type of
snail is one fitted for pelagic life. This has the foot broadly expanded and
of the shape of a pair of wings, giving to the animals the names of butter-
fly snails or sea angels. Some snails secrete an operculum which serves as
a lid to the opening of the shell, being so placed on the body that when the
animal is in the shell the operculum just fills the aperture.

Another type of snail includes the naked snails or slugs, which may be
without any shell at all or may have only a small chitinous dise occupying
the same location on the body as the shell and embedded in the mantle.
These are in part terrestrial forms and are often seen under objects lying
on the ground in fields or gardens. There are also naked marine snails,

MOLLUSKS IN GENERAL 225

often called sea slugs or nudibranchs, usually found in beds of seaweeds,
which exhibit varied forms and a wide range of colors.

262. Scaphopoda.—This class contains a limited number of mollusks
which are found at moderate depths in the sea, buried in the mud at the
bottom. They have a mantle which is tubular in form and which secretes
a tubular shell open at both ends and larger at one end than at the other.
This shell is curved like the tusk of an elephant, so these are sometimes
called tusk shells (Fig. 135). The foot protrudes through the larger end
and is used for boring in the mud.

263. Pelecypoda.—This class, also known as the Lamellibranchiata,
is represented in both fresh and salt water and includes forms which
are similar to the fresh-water mussels already described.
They possess no head and have a bivalve shell. Among
the various types of pelecypods are mussels, scallops,
oysters, clams, and the shipworm. Many of the marine
forms in this group are permanently attached to firm
objects on the bottom or along shore, from which objects
they can be dislodged only with great difficulty. This
attachment may be by horny threads, forming a byssus, as
in the marine mussels, or may be due to the disappearance
of a part of the under valve of the shell and the firm
adherence of the animal’s body to the substratum through
the hole so formed, as in the case of some oyster-like forms.
Such forms do not move about in the adult stage, and the
foot may be undeveloped. Scallops have numerous eyes
along the margins of the mantle.

In many bivalves a blind sae or caecum connected
with the intestine secretes a rodlike body, or crystalline A tusk shell,

; : Dentalium preti-
style, the function of which is unknown. een Soweba

264. Cephalopoda.—The cephalopods make up a from Puget

: : ; C Sound. Natural
group which in many respects is very much higher than ;;,..
any other class of mollusks. They are all marine. They
have the dorsoventral diameter of the body greatly increased and
the anteroposterior diameter greatly reduced. They may even become
so flattened anteroposteriorly that ends and surfaces change places.
What appears to be the anterior or head end when the animal
is in its normal position is really the ventral surface, and the
apparent posterior or tail end is the dorsal surface; the real anter-
ior end is the upper surface and the real posterior end the lower
surface (Fig. 136). In addition to this marked change in the axes the
foot is concentrated about the ‘‘head” and has become divided into a
number of tentacles situated in a ring about the mouth. These tentacles
have a great many sucking discs. The relationship of mantle and shell
to the body differs in different types of cephalopods.

226 METAZOAN PHYLA

In the case of both the squid and cuttlefish, the mantle incloses a
cavity on the lower surface and ends just behind the head in a free margin,
or collar. From under this collar projects a s¢phon, out of which water
can be forced by the contraction of the mantle, driving the animal back-
ward throughthe water. Theshellinthe
cuttlefish is a horny and limy plate con-
cealed under the skin of the upper surface
of the body; it is formed by a pocket of
the mantle. In the squid it is only
horny. There are two fins, one on each
side of the body toward the tail end,
which may be used for forward locomo-
tion and as a means of directing the
course of the animal. When swimming
forward the tentacles are pressed
together, are extended in front, and are
used for steering. The siphon may also
determine the direction of backward
motion by being pointed either toward
or away from the head as the water is
forced through it. There are two power-
ful chitinous jaws in the squid resem-
bling an inverted parrot’s beak, and a
radula is present. There are also two
gills in the mantle cavity.

The nervous system consists of sev-
eral pairs of ganglia brought together in
close proximity in the head. The sen-
A sory organs include two very highly

Fic. 136.—Common squid of the developed eyes, two organs of equilib-
Atlantic coast, Loligo pealei Lesueur. P 3 3
From a preserved specimen. X14. Tium, or statocysts, and one which is
A, lateral view. 8B, diagram to probably an olfactory organ. The eyes
show position of certain internal 2 s
structures. The body is placed (Fig. 137) are large and superficially
so as to bring out the position of the similar to those of the vertebrates but
morphological axes; its normal posi- Se ‘
tion in the water is at an angle of When critically compared with them are
90 degrees to this, with the anterior found to be fundamentally different in
surface above (Sec. 264). i

structure. The resemblance is a case of
analogy. The eyes of cephalopods are capable of distinct vision—that
is, they have the power to form a picture.

Squids possess a gland known as the ink sac that secretes-a black
coloring matter which, when spread in the water, produces a cloud like
a smoke screen behind which the animal makes its escape from an enemy.

Another type of cephalopod is the octopus, or devilfish (Fig. 138),
which has no shell but possesses a large bulbous body. The mantle

Dorsal

Fir

Avterior

MOLLUSKS IN GENERAL 227

cavity and siphon resemble those of the squid and are used in a similar
way. Large octopuses are dangerous antagonists and are feared by

Correa (transp,)

Barn or ous ssosee

SENN
<<

Qoric verve

Qotic nerve

Fig. 137.—Eyes of mollusks. A, eye spot of Patella, a gastropod, possessing only an area
of pigmented epithelium, or retina, and optic nerve. 8B, the eye of Murex, another gastro-
pod, showing a well-developed eye with lens, retina, and optic nerve. (From Cooke,
“Cambridge Natural History,” after Hilger, by the courtesy of The Macmillan Company.)
C, diagrammatic section of the eye of a cuttle fish, Sepia sp., showing many parts analo-
gous to the eye of a vertebrate. (From Jordan, “ Allgemeine Vergleichende Physiologie der
Tiere,’ after Hensen, by the courtesy of Walter de Gruyter & Company.) All figures
much magnified.

Fig. 138.—An octopus, Polypus bimaculatus (Verrill). A small specimen drawn from
life. (From Johnson and Snook, ‘Seashore Animals of the Pacific Coast,’”’ by the courtesy
of The Macmillan Company.)

divers, for if they get hold of a person under water struggling merely
causes them to cling the tighter, and release can be secured only by chop-
ping the tentacles from the body.

228 METAZOAN PHYLA

A third cephalopod type is the chambered, or pearly, nautilus (Fig.
139), which lives on the bottom of the sea near certain islands in the
south Pacific Ocean. ‘The shell is coiled in one plane and is composed of
a series of compartments, each representing a chamber which has in the

Jerttacles Dorsa/ lobe
of rprartle

Shell nuscle Mortt/e betweerr chambers

Fic. 139.—A female chambered nautilus. (From Hertwig and Kingsley, ‘Manual of
Zoology,” after Ludwig and Leunis, by the courtesy of Henry Holt & Company.) The shell
is bisected, but the animal is not.

past been occupied by the animal, but which has been deserted as it
and the shell have grown larger. The animal lives in the outermost
compartment. The partitions are concave toward the mouth of the
shell. These compartments are stated to be filled with gas, which gives
buoyancy to counteract the weight of
the shell. Through the center of
them and piercing the successive
partitions clear back to the end of
the coil is a tube which lodges a
cylindrical mass of tissue known as
the stphuncle.

Another type of cephalopod is
the paper nautilus (Fig. 140). By
means of glands in certain tentacles
the female secretes a small and deli-

Fic. 140.—Paper nautilus, Argo- eately coiled shell which is really a
nauta argo Linnaeus. Female, swim- .
ming. (From Claus and Sedgwick, basket for the eggs, and which, because
FHestsot of Zeon” byte ont of its method of formation, is not

‘ , “ homologous with the shells of other
mollusks. The animal swims in this shell as if in a boat, propelling itself
by means of the tentacles.

Among the cephalopods are the largest of the invertebrates. The
giant squid reaches a total length of over 50 feet, including the tentacles,

MOLLUSKS IN GENERAL 229

and the outspread tentacles of the largest octopus possess a span of 30
feet. Among living types the shelled ones are relatively small, but some
fossil forms with shells reached a large size. The shells of the straight-
shelled Silurian types exhibit a length of 15 to 18 feet, and those of the
coiled ammonoids of the Jurassic, which resembled the living pearly
nautilus, a diameter of from 4 to 6 feet (Sec. 586 and Fig. 372).

265. Metabolism.—The food of mollusks is exceedingly varied. The
pelecypods take any minute particles of organic matter found in the
water, while the snails and their allies scrape the epidermis from living
or dead plants or gather algae from the surfaces on which they grow.
On the other hand, the cephalopods, powerful and active forms as they
are, are carnivorous and will feed upon any animal which they can over-
come. Squids destroy a great many fish, and an octopus will undertake
to capture anything which it can grasp with its tentacles. One large
marine gastropod, Sycotypus, lives in shallow water and feeds on other
mollusks, while another, known as the oyster drill, Urosalpinz, feeds
on oysters and other bivalves, boring a hole through the shell of the victim
with its radula and eating out the soft parts.

266. Behavior.—The behavior of the most simple types of mollusks
is correspondingly simple, but that of the cephalopods is very complex.
One striking fact about mollusks is that in the case of the lowest class,
the Amphineura, which is wormlike, there is a nervous system similar in
some respects to that of the flatworms. In other classes there are numer-
ous scattered ganglia. These in the cephalopods are grouped together
in the head. The close proximity of these ganglia offers an opportunity
for quick communication between them and effective coordination, pro-
duces a real brain, and has resulted in developing a very efficient
organism, to which some zoologists have attributed the possession of
intelligence.

The sense which most mollusks depend upon is that of smell. With
few exceptions they seem to be sensitive to light, and it has already been
shown that the cephalopods have excellent vision.

267. Reproduction and Regeneration.—Asexual reproduction never
occurs in Mollusca, but the animals may be either monecious or diecious.
Though some of them produce only a few eggs, others produce large
numbers. It is stated, for example, that 9,000,000 eggs may be laid by
a single oyster. Some snails, and also some small fresh-water bivalve
mollusks, are viviparous.

Molluscan eggs are holoblastic but undergo unequal cleavage. Devel-
opment typically includes a trochophore stage but this is not represented
in the development of land and fresh-water mollusks. The cephalopods
have a larva which develops within the egg. The trochophore larva
(Fig. 141) is top-shaped with a ring of cilia about the margin of the
expanded upper portion and with an eyespot at the apex of the body.

230 METAZOAN PHYLA

The mouth opens near the ring of cilia and the anal opening is below at
the tip of the body.

Since a trochophore larva appears in the development of bryozoans
and annelids, as well as mollusks, and since it resembles in a certain
degree some rotifers, the suggestion has been made that these groups
may have all descended from a common ancestral form called a trocho-
zoon. There are many strong arguments against this view as well as
others for it.

Regeneration of lost parts and repair of injuries are general among the
mollusks, but they do not include replacement of ganglia, the loss of which
causes death.

Ve/ur7

Seung

ava! cells

; Fic. 141.—Trochophore of Patella, one of the limpets. A, viewed from the ventral
side. B, median section of a slightly older trochophore. (From Korschelt and Heider,
“* Tezt-book of Embryology.’’)

268. Economic Importance.—Mollusks are of economic importance
in a number of ways. Their greatest value is for human food. The
report of the U. 8. Bureau of Fisheries of 1937 shows that the eastern
catch for oyster alone in 1936 amounted to over $7,000,000. Among the
bivalve mollusks, oysters, clams, scallops, and some other forms are
the chief articles of diet. - These animals appear on the market in both the
fresh and canned condition, as well as in the form of soup and chowder.
The canned oysters for 1936 amounted to $181,301, while that for clam
chowder was $271,767. Snails are much eaten in some countries of
Europe and to a certain degree in this country. The arms of squids are
also cut off and used as food. The shells of many of the mollusks are
for one reason or another articles of commerce. The marine pearl-shell
buttons for 1936 were valued at over $4,669,000 while that for novelties
was about $825,000. The pearl buttons from fresh-water mussel shells
for the same year amounted to over $4,621,000, while the crushed-shell

MOLLUSKS IN GENERAL 231

Apia!
/47 Tvblhe

Siphors

Fie. 142.—Shipworm, Teredo navalis Linnaeus. A, section of a block of wood showing
parts of several empty tunnels and one containing an animal. The whole is divided, and
only the two ends shown to allow it to come within the limits of the figure. (From Cooke,
“Cambridge Natural History,’ after Mébius, by the courtesy of The Macmillan Company.)
Natural size. B, the shell, mounted in position at the end of atunnel, lateral view. (From
Miller, Univ. of California Pub. in Zool., vol. 26, by the courtesy of the University of Cali-
fornia Press.) X 5%.

232 *METAZOAN PHYLA

product was $88,000. In the past Ohio, Illinois, and Iowa have furnished
our chief supply of fresh-water mussels, but the supply of shells is rapidly
diminishing, and the future prosperity of the industry will rest on the

Fr¢. 143.—Mollusea of considerable economic importance. A, shell of the edible
oyster, Ostrea virginiana. B, shell of a fresh-water clam, Lampsilis sp., from which pearl
buttons have been cut. (, shell of a snail from which many cameos are made by cutting
the design in one layer and allowing it to remain affixed to the underlying layer of a different
color. D, shell of a Japanese oyster, Meleagrina sp., showing a “ cultured pearl.”’

suecess of experiments in artificial propagation now being conducted.
Fresh-water mussel shells purchased by manufacturing plants in 1936
amounted to 58,484,000 pounds. These were bought from 18 states in

MOLLUSKS IN GENERAL 233

the Mississippi River Valley and Great Lakes region. Shells of mollusks
are crushed and sold as ‘“‘grit’’ to be fed to fowls, or used in liming the
soil, while cuttlefish bone is given to captive songbirds as a source of
lime. Pearls are also a product of mollusks, being developed in the shells
of a number of bivalve forms. Whenever any irritating particle gets in
between the mantle and the shell, a nacreous covering called mother-of-
pearl is secreted around the particle, thus forming the pearl. Parasitic
worms or infective organisms may also cause the formation of pearls.

Some mollusks are injurious. In addition to those which attack
oysters and other bivalves of value may be mentioned the shipworm,
Teredo navalis Linnaeus. This is a bivalve with an elongate wormlike
body (Fig. 142) ending posteriorly in two long slender siphons which
reach the outer end of the tube in which the animal lives. At the anterior
end are two small valves, not hinged but separate and movable. With
the sharp anterior edges of these valves the animal burrows into the
wood of ships and piling, sometimes to a depth of 4 feet, its siphons being
extended into the water for the purpose of feeding and breathing. It
has long been disputed whether or not the animal eats the wood which
it removes and the particles of which are passed back through the body
and out of the siphon, but it now seems definitely proved that it does
and that it also gets food in small particles from the water as do other
bivalves. The tube which serves to lodge and protect the soft body is
smallest at the outer end, largest at the inner, and becomes lined with
nacre secreted by the mantle. Some snails secrete an acid which dissolves
limestone rock and thus they excavate cavities in which they live.

The defenceless condition of most mollusks makes them the prey of
many other animals, including all classes of vertebrates. They are also
the intermediate hosts of many parasitic worms.

CHAPTER XXXIX

EARTHWORM
A TYPE OF THE PHYLUM ANNELIDA

Earthworms are very generally distributed, being absent only from
regions where the soil is nearly pure sand and deficient in humus or
from mountain regions where the soil is scanty and also poor. Very
heavy soils with much clay in them are not favorable, and earthworms
are unable to live in soils that are strongly acid. During the daytime
they remain in their burrows in the ground, but at night they may come
out and move about on the surface. A species common in Europe and
America, Lumbricus terrestris Linnaeus, may serve as a type.

269. External Characteristics The most prominent characteristic
of the earthworm is the division of its body into metameres to the number
of 175 in fully mature individuals, not including a half metamere at the
anterior end known as a prostomium (Fig. 144). The metameres, which
may be numbered XX XI or XXXII to XX XVII are, in mature worms,
swollen and whitish, the skin containing glands which take part in repro-
duction. These metameres form a region known as the clitellwm.

The surface of the worm is covered by a thin transparent cuticula
marked by fine striae which produce iridescence. The cuticula is pierced
by bristle-like setae, which are present in all but the anterior two or
three metameres and the last. These setae are very small chitinous rods
similar in form to an old-style s (f) and serve as organs of locomotion.
There are four pairs to a metamere, one pair on each side just below the
middle and the other on each side midway between the first pair and the
ventral median line.

The mouth is a crescent-shaped opening on the anterior side of the
first metamere, overhung by the prostomium. The anal opening is a
slit on the posterior side of the last metamere. Minute dorsal pores
located in the mid-dorsal line at the anterior margin of each metamere
behind the eighth or ninth are openings from the coelom directly to the
outside. Other openings on the surface will be referred to in connection
with the organs of elimination and reproduction.

270. Internal Structure.—The body wall of the earthworm surrounds
(Fig. 145) a cavity which is called the coelom; this is separated into com-
partments by cross partitions, or septa, which mark the boundaries of
the metameres. These partitions are more or less incomplete or even
absent between certain metameres at the anterior end of the body. From

234

EARTHWORM 235

one end of the animal to the other the alimentary canal extends, a
tube within a tube. It is held in place by the partitions, the coelomic
spaces thus becoming ringlike. In the ventral portions of metameres
IX to XV are the reproductive organs, while above the alimentary
canal is the dorsal blood vessel and below
it are the ventral blood vessel and nerve cord.
Each metamere, except the first three and the
last, also contains a pair of nephridia. The
coelomic spaces are lined by a delicate epi-
thelium known as the peritoneum. They are
filled with the coelomic fluid, which is colorless
and which, when the worm contracts, can flow
from one space to another through a small
opening in each septum above the ventral
nerve cord.

271. Alimentary Canal and Metabolism.—
The alimentary canal is divided into a greater
number of regions than in any type previously
discussed and these are more highly special-
ized (Fig. 146). These regions include a buccal
cavity; a muscular pharynx; a narrow esophagus;
a thin-walled ciliated crop, or proventriculus;
a thick-walled muscular gizzard; and a thin-
walled zntestzne, which begins at metamere XIX.

The food of earthworms consists of organic
matter in the soil and of living or decaying
leaves: The worms will also eat bits from the
bodies of dead animals when they can secure
them. The organic matter in the soil may be
gathered at any time, but the fragments of
leaves are secured from the surface of the
ground at night. Taken in at the mouth by a
sucking action of the pharynx and mixed with

[Opening of
ni] vas deferens

Arus

Fie. 144.—An earth-
worm, Lumbricus terrestris
Linnaeus. Ventral side of

a secretion from salivary glands in the pharyn-
geal wall, this food is passed to the crop.
From here it goes into the gizzard where it is
ground and is then passed on into the intestine
where digestion and absorption take place.

the more anterior and the
most posterior segments.
From a preserved specimen.
X 113 in length and about
2 in diameter. Segments
numbered in roman.

Feces are egested at the anal opening. They appear in the form of cast-

ings outside the burrows.
act on proteins, fats, and carbohydrates.

Enzymes are produced in the intestine which

272. Circulatory System.—The circulatory system of the earthworm
is a complicated system of blood vessels through which the blood is
forced by a sort of peristaltic action of the muscle fibers in the walls of

236 METAZOAN PHYLA

the vessels. Peristalsis is the passage of a series of rhythmic contrac-
tions along a vessel which advances the contents of that vessel in the
same direction as that in which the waves of contraction progress. In
metameres VII to XI are five pairs of dilated vessels, often called hearts
(Fig. 146), which connect the dorsal to the ventral blood vessels. They
are really dilated vessels exhibiting powerful peristaltic movements.
Valves in the dorsal vessel and hearts prevent back flow. The blood
which is circulated in these vessels consists of a liquid plasma in which
float colorless ameboid cells called corpuscles. In the plasma is dissolved
a red coloring substance known as hemoglobin which aids in the inspiration
and transportation of oxygen by combining with it, forming oryhemo-
Wa EES Coticula

Hypoderr71/s
Circular muscle

lntestivial
blood vessel!

Yyphlosole

first loop of

nephridiur7t me i
Tf Chlora -
I: ogue
ie 22H
Third Joop of
nephridiura \) =
\ sai —/n tes tire
Second Joop 07 KK NG \ KN =
nephtidiar \ <7 \ Oe abies “
= — Dy 7] 2 ne tinal
Nephridiopore es ae ae BS githeliurr
Pos (tran oF S f : 4 Ny eee A
intersegrmental Sass foe phe = Seta
seprun = _\ Coe/om
Nerve Subinfestinal
blood vesse.
NS NEADS ISS Giarrt fibers

Vertra/
verve cora's Svborevral blood vessel

Frc. 145.—Diagrammatic cross section of an earthworm at about the middle of the
body. (From Marshall and Hurst, ‘‘Practical Zoology,” by the courtesy of G. P. Putnam’s
Sons.) The nephrostome of the segment in front is shown for the sake of completeness.

globin. Much more oxygen can be contained and transported in com-
bination than if it were free. This combination is formed when oxygen
is taken into the body and is only temporary, being broken up again in
the tissues, thus liberating free oxygen for their use. Respiration takes
place through the whole surface of the body.

273. Excretory System.—The excretory system consists of a number
of organs called nephridia (Fig. 147), a pair of which is present in every
metamere but the first three and last. Each of these possesses a ciliated
funnel called a nephrostome located in the posterior part of the coelom
of one metamere and opening into a thin ciliated tube passing through
the septum into the coelom of the next metamere. Here the tube
becomes complexly looped and ultimately opens by a nephridiopore
between the two double rows of setae. Cilia on the nephrostome create

EARTHWORM 237
Prostormium
Mouth
Nerves
Buccal cavity

Suprapharyrgeal

m— op
Ky gs ganghor
= Pharyhx

> Hearts

=
Esophageal
xI- | ) (Mad
=a C8 Calcite
x f Weene (7) glands
maa corms
XxV—- Crop
a Bh
el
EN: oY Gizziner
= Z Intestive
(aay ease)
I} — Dorsal blood
vesse/

Fig. 146.—Diagrammatic representation of the anterior part of the alimentary canal
of an earthworm. Based upon a preserved specimen. The blood vessels are shown in

solid black. Segments numbered in roman.
Nephridiopore
lntersegrerttal

, —Nephrostorne

' Collecting
region

; Fic. 147.— Diagrammatic view of the nephridium of an earthworm. In the body this
is invested with soft connective tissue and the loops of the tubule are crowded together.

Highly magnified.

238 METAZOAN PHYLA

a current which draws solid waste particles from the coelomic fluid into
the nephridial tube and cilia in the tube pass these onward and out of ©
the body. Glands in the coiled tube also take nitrogenous waste matter
from the blood and add it to the liquid in the nephridium, which results
in its elimination.

274. Musculature and Locomotion.—The muscles in the body wall
of the earthworm are arranged in two layers. An outer layer of circular
muscle fibers just below the skin forms a continuous sheet about the body.
An inner layer of longitudinal muscles is arranged in several bands—
two dorsal, one ventral, and a lateral one on each side between the double
rows of setae (Fig. 145).

Suprapharyrgeal Phar yrx
garglhor7

cavity Subpharyngeal

Circurnpharyngeal IVF!" WCegmental
COMMECTIVE . ganghor

Fic. 148.—Anterior portion of the nervous system of an earthworm. Lateral view.
(From Hess, in Jour. Morphology, vol. 40.) Segments numbered in roman.

Locomotion is carried on by extension, contraction, and flexion of
the body due to the muscle layers and also by the use of the setae.
Muscles attached to the latter structures serve to retract them within
their sheaths in the skin or, when they are protruded, to move them
either forward or backward. Accordingly, acting as so many little
levers, the setae serve to propel the body in either direction within the
burrow.

275. Nervous System.—The nervous system in the earthworm con-
sists of segmentally arranged, paired ganglia and a nerve trunk, which
together form a central nervous system, and of peripheral nerves which
distribute fibers to various parts of the body. The central nervous
system begins anteriorly in a pair of swprapharyngeal ganglia (Fig. 148)
fused into a bilobed mass which is located in the third metamere and
above the front end of the pharynx. From these ganglia two nerve
trunks known as circumpharyngeal connectives pass around the pharynx,
one on each side, to unite below in the same metamere, forming a ventral
nerve cord. Upon this is a bilobed dilatation in the fourth metamere
formed by the subpharyngeal ganglia. The ventral nerve cord continues

EARTHWORM 239

backward to the posterior end of the body showing a ganglion in each
metamere, each ganglion being really a fused pair. Nerves from each
ganglion supply the structures in that metamere.

276. Behavior.—The earthworm possesses no special sense organs,
but it responds to several stimuli. The response to a mechanical stimulus
is positive when the stimulus is not too strong and is continuous but
negative when it is of contrary character. This leads the worms to
remain at rest when in their burrows and to seek the greatest amount
of contact with surrounding objects when moving about on the surface.
They will move from their places in their burrows when the ground is
struck a hard blow and, if they are close to the surface, will come out of
them. It is this type of reaction which leads earthworms to leave their
burrows during a heavy rain, when the earth is pounded by the falling
raindrops, and to crawl upon the surface where they are soon beaten into
insensibility and even killed. This explains why it is that after such a
rain dead worms are frequently found on the surface of the soil, while
others are crawling about seeking again to enter their burrows. Darwin
showed that they did not react to sounds but would respond to sound
vibrations as mechanical stimuli. This result was secured when a flower
pot containing soil in which were some earthworms was placed upon a
piano, thus providing a means by which the sound vibrations could be
communicated to the pot and to the soil.

EKarthworms respond positively to chemicals which indicate the pres-
ence of food and negatively to harmful substances. This reaction occurs
not only when the substance comes in contact with the body but also
when a substance with a pungent odor is still a little distance away.
The reaction to moisture is always positive.

Earthworms are sensitive to light in varying degree in different parts
of the body, the anterior end of the body being most sensitive, the
posterior part next, and the middle portion least. Slight differences in
light intensity are detected, and the worms will seek a region of faint
illumination in preference to one where the light is strong. The effect
of these responses to strong and weak light tends to keep the worms in
their burrows during the daytime but leads them to emerge at night.

The earthworm exhibits various physiological states which are deter-
mined not only by the state of metabolism within the body but also by
previous stimulation. After being repeatedly and violently stimulated
the nervous system is put into such a condition that even slight stimula-
tion causes a response out of all proportion to the strength of the stimulus.

EKarthworms remain near the surface of the soil only when it is moist,
and as the soil dries they gradually retire farther into the ground, remain-
ing below the upper limit of moisture. This occurs regularly in the latter
part of the summer when the ground becomes dry. They return to the
surface again in the spring when the ground is full of moisture. At

240 METAZOAN PHYLA

Lincoln, Nebraska, their burrows have been followed to a depth of more
than 18 feet from the surface. In winter they usually remain below
frost line.

277. Reproductive System and Reproduction.—Both sets of sex
organs (Fig. 149) are present in each individual earthworm, this animal
being monecious. The male organs include three pairs of seminal
vesicles, the first two of which open into a common central reservoir in
the tenth metamere and the third into a similar reservoir in the eleventh
metamere. In each reservoir is found a pair of testes. From each
reservoir vasa efferentia lead into a common vas deferens on each side,
the outer opening of which is in metamere XV. The sperm cells are
produced in the testes, matured in the seminal vesicles, and passed out
through the vasa efferentia and vasa deferentia.

Nerve cord

ves/cles

=e Seryria/

Ovary
Oviduct

Vas deferens

Fic. 149.—Reproductive organs of an earthworm. (From Wieman, ‘‘General Zoology,”
by the courtesy of McGraw-Hill Book Company, Inc.)

The female organs consist of a pair of ovaries in metamere XIII.
The egg cells are set free in the coelom and on each side are collected by
a ciliated funnel which leads into an oviduct in metamere XIV; this opens
to the outside on the ventral surface of this metamere. In addition
there are two pairs of seminal receptacles in the ninth and tenth meta-
meres, which open to the outside in the grooves between the ninth and
tenth, and the tenth and eleventh, metameres.

Self-fertilization does not take place in the earthworm, but sperm
cells are transferred from one individual to another by a process of
copulation. Two worms come together with their ventral surfaces in
contact and with their anterior ends pointed in opposite directions (Fig.
150 A), placing themselves so that metameres IX, X, and XI of one
worm are opposite the clitellum of the other. They are held together
by two slime tubes formed from mucus secreted by the clitellar and other
skin glands, each tube extending from metameres IX to XV of one worm

EARTHWORM 241

and through the length of the clitellum of the other. Sperm cells are
passed out through the vasa deferentia of one worm and flow in two
canals formed by apposing grooves on the ventral surfaces of the two
animals (Fig. 150 B), to the openings of the seminal receptacles of the
other, into which they enter. The same thing occurs on the part of the
other animal, and thus a reciprocal exchange of sperm cells takes place.
The worms then separate, each having its seminal receptacles filled with
sperm cells from the other worm, ready to be used in fertilization when
egg laying occurs. At that time a cocoon (Fig. 150 D) is secreted about
the clitellum and then slipped forward over the head of the worm. As
it passes the openings of the oviducts several egg cells are passed into it;
and as it passes the openings of the seminal receptacles, numerous sperm

Openings of

Opening of
seminal receptacles

vas deferens

rT (AC
(a

Fic. 150.—Reproduction in the earthworm. (From Curtis and Guthrie, ‘‘ Text-book of
General Zoology,’’ by the courtesy of John Wiley & Sons, Inc.) A, two worms enclosed in
bands of mucus. 8B, transverse section showing the seminal grooves. C,a sketch to show
the path of the seminal fluid from the openings of the vasa deferentia on one worm to the
openings of the seminal receptacles on the other. D, the cocoon.

cells. There is also added an albuminous secretion from skin glands
which serves to nourish the developing embryos. As the cocoon leaves
the worm the two ends come together and thus is formed an inclosed
cavity, within which fertilization takes place. One worm will form many
such cocoons.

The eggs are holoblastic but undergo unequal cleavage; a hollow
blastula is formed, and later a gastrula by invagination. All of the egg
cells which are contained in the cocoon do not develop into embryos,
most of them appearing to serve as nurse cells. Those which do develop
produce small worms very similar to the adult, which escape from the
cocoon in about two or three weeks. When the young worm first becomes
free it lacks a clitellum, which appears later as the worm acquires sexual
maturity.

Asexual reproduction does not occur in the earthworm.

242 METAZOAN PHYLA

278. Regeneration.—Earthworms have a considerable power of
regeneration and exhibit it in a manner which suggests two axial gradients.
One of these has a maximum near the anterior end of the worm and fades
out rapidly in the metameres which lie behind (Fig. 151). The other
has a maximum at the posterior end and fades out toward the anterior
end of the body. A new head can be regenerated at the anterior end of
a posterior fragment only as far back as the fifteenth to the eighteenth
metamere; beyond the fifteenth the newly formed head is not perfect.
A new tail cannot be developed on an anterior fragment farther forward
than the twelfth metamere; back of the twelfth a posterior piece may
regenerate an anterior tail, but such a two-tailed worm, of course, cannot
survive. This existence of two gradients seems to be correlated with
the greater sensitiveness of the body at the two ends than in the middle.

<e
Fig. 151.—Diagram to show the possibilities of regeneration in the earthworm. Regen-
eration of a head end is perfect back to segment XV, imperfect as far as XVIII; regenera-

XII
moo he,
XY XVIII
tion of a tail end occurs back of XII.

Experiments in grafting earthworms have been tried by’ suturing
fragments together and then permitting them to unite. In this way
several pieces may be combined to make a very long worm, a short
anterior and a short posterior piece may be united to form a very short
worm, two tail pieces may be united to form a worm with a tail at each
end, or two tail portions may be grafted to an anterior piece to form a
double-tailed worm. Of course such abnormal individuals cannot long
survive.

279. Economic Importance.—Earthworms are exceedingly important
economically because of their influence in increasing soil fertility. By
opening up the ground and permitting access of air they help to freshen
it, and by bringing earth from below to the surface they serve to develop
a thicker layer of humus. In passing the soil through their bodies
nitrogenous waste is added to it in such form that it can be utilized by
plants. The honeycombing of the soil also permits moisture to penetrate
it more rapidly. Charles Darwin, in his book on “The Formation of
Vegetable Mould through the Action of Worms, with Observations on
Their Habits,’ brought together the results of forty years of observation
and a wealth of facts bearing upon this subject. Rarely does the
work of earthworms cause injury; the loosening of soil in the walls of
irrigation ditches, however, has given trouble in some parts of the West.

CHAPTER XL

REFLEX ACTION

Reflex action has already been referred to in Chap. XXX; it exists
in animals of the grade of flatworms and those higher in organization.
The structure in the earthworm is so easily correlated with the elements
in this type of action that it is usually studied in detail in this connection.

280. Nervous Functions.—The functions of the nervous system are
the reception of stimuli, the conduction of nervous impulses, and the
stimulation of other cells of the body (Sec. 126). A nervous stimulus
is any outside influence exerted upon any part of a nerve cell and produc-
ing an effect. This effect transmitted through the cell or any of its
branches is termed a nervous zmpulse, and when this impulse is com-
municated as a stimulus to any other cell through an appropriate point
of contact, it starts an impulse in the other cell if it is a nerve cell or
causes it to act if it is a muscle or gland cell.

The unit of structure in the nervous system is the neuron, which may
be defined as a nerve cell including all of its branches, or fibers. In
development the branches grow out from the cells to their ultimate point
of distribution. A neuron which receives a stimulus is known as a
receptor or sensory neuron, and one which conducts the impulse from the
receptor to the acting cell or cells is known as a motor neuron. The cells
which act and which are not neurons but muscle or gland cells are termed
effector cells, or simply effectors. Neurons generally show polarity,
which is the ability to transmit impulses more readily in one direction
than in another. ‘This is related to their position with reference to other
neurons, to sense cells, or to effectors.

The impulse is passed from a fiber of one neuron to that of another
at one or more points where the fibers or their branches come in intimate
contact. It is believed that they do not become structurally continuous.
Such a contact area is known as a synapse. An impulse is communicated
to the effector cell usually through some form of nerve ending, such as
a motor end plate, in which the fiber comes in intimate contact with the
cell.

All fibers which transmit impulses toward the cell body of the neuron
are termed dendrites, of which there may be several; the one which trans-
mits an impulse in the opposite direction is termed the azon, or the axis
cylinder fiber. The path by which impulses pass from the periphery to
a nerve center is termed the afferent path, while that by which impulses

243

244 METAZOAN PHYLA

are carried from the center to the effector cells is known as the efferent
path.

281. Reflex Acts.—In a typical and ideally simple reflex act a receptor
neuron situated in the hypodermis is stimulated from without (Fig. 152),
the stimulus being received directly by the cell body or by a short den-
drite. An afferent impulse follows the axon of this cell to a synapse in a
central ganglion where it is passed to the dendrite of a motor neuron.
This sends out an efferent impulse along its axon to a muscle cell, the
effector, causing contraction and resulting in a movement appropriate
to the stimulation which originated the act. This entire mechanism is

Giant fibers

ventral ganglion
Synapse
Afferent
nerve frber

Motor neuron

Efferent nerve
Fiber

Effectors Ventral LOG Fai aal,
muscle

Fic. 152.—Diagram illustrating reflex action in an earthworm. (From Parker, in Popular
Sct. Mon., vol. 75, after Retzius.)

called a reflex arc. Actually such a simple reflex does not occur, since
in any action several receptors are stimulated at the same time and several
effectors participate in the action. Also one or more connective neurons
are usually involved, and they form chains of neurons through which
conduction takes place. Since each of these connective neurons is in
communication with others, spreading of the impulse also occurs.

In the earthworm not only may several connective neurons in the same
ganglion be involved in a reflex act, but it is also possible for impulses to
pass from one ganglion to another. This transmission of an impulse from
one metamere to another is due to connective neurons the axons of which
are known as association fibers. The association fibers are contained in
three tracts known as giant fibers—though they are really bundles of fibers
—which lie in the dorsal part of the ventral nerve cord. These fibers put
the cell of which they are a part into communication with cells in other
ganglia in front of the ganglion in which this cell is located or behind it.
In this way very strong stimuli may affect the entire body of the earth-
worm, causing it to act as a whole.

282. Anterior Ganglia.—The two anterior pairs of ganglia of an earth-
worm are larger than the others, and to the first of these, or the supra-

REFLEX ACTION 245

pharyngeal ganglia, is often given the term brain (Fig. 148). However,
these ganglia apparently do not function in any way different from the
other ganglia of the body. Their size is due in part to the greater area
from which they receive afferent impulses and to which they distribute
efferent impulses and in part to the fact that they innervate the most
sensitive part of the body. LHither an increase in the area served and
the number of structures involved or an increased sensitiveness of the
parts means an increase in the number of neurons in any nerve center
and therefore an increase in size. The ganglia in question do not possess
two attributes which are associated with the brains of higher animals.
They do not exercise that dominance over the functions of the body
generally that a true brain should and may be removed and regenerated
with the rest of the anterior end of an earthworm. It is better, therefore,
to avoid the use of the word brain in connection with this type and with
other invertebrates with a nervous system of this character.

CHAPTER XLI

ANNELIDS IN GENERAL

The most striking advance shown by annelids is the appearance of
metamerism. A forecast of this may be considered as shown in the
transverse groovings on the body of some of the Nemathelminthes, but
in that case no internal segmentation corresponded to this external
indication. Metamerism in annelids involves not only the division of
the body wall into a series of sections but also a metameric arrangement
of internal structures, shown most strikingly by the excretory and nervous
systems. The irregularly distributed groups of flame cells opening to

oral
tentacles

Eye spots

ate a

Dil
Y Ppaspth: BU iit
+

Ganglion

Protonephri-
jurn

Fic. 153.—Polygordius sp., showing two stages in the development of the trochophore
larva (A, B) and the adult (C). (From Wieman, ‘‘General Zoology,’ after Hatschek and
Fraipont, by the courtesy of McGraw-Hill Book Company, Inc.) A and B much magnified,
C X about 2.
the outside through one or a few openings of a common large duct, seen
in some previous phyla, are replaced in members of this phylum by a series
of excretory organs arranged in pairs, one pair to a metamere and each
complete in itself. The nervous system, also, instead of showing a
general tendency toward the accumulation of cells in a few ganglia or
nerve tracts, shows the development of metamerically arranged ganglia
connected by a ventral nerve cord. An earthworm is really composed of

246

ANNELIDS IN GENERAL 247

a series of nervous units. These may act individually but are so related
in a central nervous system as to be capable of acting in concert. This
makes possible both unity and variety of action. Differ-

ent metameres may carry on differing activities, while in

case of necessity all may be brought into play in an action phir Si
involving the body asa whole. Still another advance is ae
seen in the greater degree of specialization exhibited by 2
the digestive system, which is here divided into a larger ace

number of regions than heretofore, each region having

: ‘ =5

a special function to perform. E==
283. Classification.—The phylum Annelida (i nél’i BSS
ct

da; L., anellus, a little ring, and G., ezdos, form) is
divided into four classes:

1. Archiannelida (ar ki & nél’ i da; G., archi-, first, +
annelida).—Primitive annelids which possess neither
setae nor parapodia.

2. Chaetopoda (ké tp’ 6 da; G., chazte, horse’s mane,
and podos, foot)—Annelids with setae, and in one of A
the two subclasses with parapodia, which are fleshy :
lateral outgrowths of the body wall. <=

3. Hirudinea (hiroo din’ éa; L., hirudo, leech).— ec ews
Annelids possessing neither setae nor parapodia but ey
having suckers, which are an adaptation to parasitic oe
life.

4. Gephyrea (jéfiré’a; G., gephyra, bridge).—A
small and heterogeneous group, apparently more
appropriately placed in Annelida than in any other
phylum.

284. Archiannelida.—A type of this class is Poly-
gordius, a small worm about an inch and a half long,
living in the sand of the seashore (Fig. 153). It is only
indistinctly metameric externally, but internally it
shows clear metamerism and a structure somewhat
resembling that of the earthworm. The prostomium
bears a pair of fleshy tentacles which are both sensory _ Fic. 154,-—A

: i sandworm, Nerets
and respiratory. The other members of this class are yirens Sars. From
also small in size, simple in structure, and are marine. ® specimen. The
F é parapodia bear foli-
One type, Dinophilus, has eyespots, moves by means Of aceous lobes which
cilia, and has other characters reminiscent of the 4” conspicuous in

5 the figure. xX %.
planarians.

285. Chaetopoda.—The chaetopods are divided into two subclasses,
one being Polychaeta, the species of which are mostly marine. The
type of this subclass usually studied is Nerezs, the sandworm (Fig. 154).

os
\\WNt
gel

So aS
dee
Ws

AWS
ves

a
vy

“

. SeSueysver:
flay 4st ad AVY: :
aan

so,
¢

¥
ii
Je

248 METAZOAN PHYLA

This is an abundant form living in burrows in the sand or mud of
the seashore. These burrows sometimes reach a depth of two feet
and the sand forming their walls is held together by mucus. Although
similar in many ways to the earthworm, the sandworm shows many
striking differences. The anterior metameres (Fig. 155) are distinct
from the rest and are recognized as forming a head, divided into two
parts, prostomium and peristomium. The prostomium possesses a pair
of feeling organs or palpi, a pair of tentacles, and two pairs of eyes.
The peristomium contains the mouth, with a pair of chitinous jaws,
and bears four additional pairs of tentacles. The tentacles are organs
of touch, the palpi probably of taste and smell, and the eyes of light
perception. Along the sides of the body are fleshy projections, or
parapodia, a pair to each metamere except those of the head, each
parapodium bearing clusters of several setae. ‘These parapodia also are
abundantly supplied with blood ves-
sels and serve in respiration. The

Peristornal : : :
tentacles Pale sexes are separate in Nereis, which
is generally the case in all the poly-
EX Prostomium chaets.
with four eyes Certain polychaets show a tend-
: ency to the division of the body into
Be setae in portions which have been likened

oS di.
Parspow" + the thorax and abdomen of arthro-

pods. Some of the polychaets reach
a large size, attaining a length of

Fig. 155.—Anterior end of a sandworm three feet. These frequently con-
(Nereis) with prostomium and peristomium struct tubes, various in nature, in
aso Ear So daa ae which they live. The tube may be
courtesy of McGraw-Hill Book Company, limy, cylindrical, and attached to
ine) rocks, over the surface of which it
follows a very irregular course, gradually growing in length and increas-
ing in diameter as the worm grows. In other cases it is made of grains
of sand cemented together, and in still other cases it is chitinous. In
these tube-dwelling forms the parapodia frequently become much reduced
in size. In worms known as sabellids the palpi become greatly developed,
complexly branched, frequently feather-like, and serve as organs of respi-
ration (Fig. 156). These so-called gills are often vividly colored and
when expanded are objects of great beauty. Polychaet worms exhibit
all colors, and many are luminescent.

The other subclass of the chaetopods is Oligochaeta, most of the
members of which are either terrestrial or fresh-water forms. They
lack parapodia and tentacles, and the setae are single, though they may
be near together in pairs. This group includes not only the earthworms

ANNELIDS IN GENERAL 249

but also many other forms of varied structure. Some species are very
large, one found in Java being said to attain a length of several feet with
a correspondingly great diameter. Some are able to climb trees.
Among the fresh-water oligochaets are a large number of small and
relatively simple forms with not more than a pair of setae to each met-
amere, in some cases less. These are, in general, very transparent and

Operculum

Gills ~~ ------- SR

Tube...

Fie. 156.—A colony of sabellid worms, Serpula vermicularis Linnaeus, showing the mass
of tubes and the expanded tentacles. (From Benham, ‘‘Cambridge Natural History,’’ after
Cuvier, by the courtesy of The Macmillan Company.) The operculum closes the tube after
the worm has withdrawn into it. Natural size.

without color, though sometimes, as in Aeolosoma, they contain brightly
colored oil globules scattered through the body. Some of them live in
aquatic vegetation, crawling about by movements of the setae combined
with the undulations of the body. Others live in tubes in the mud at
the bottom of ponds or other bodies of water, and frequently in colonies
containing large numbers of individuals. They extend themselves from
the mouth of these tubes, waving their bodies backward and forward,
but retreat into the tubes when strongly stimulated. A colony of such
worms in activity presents an animated appearance.

250 METAZOAN PHYLA

286. Hirudinea.—The class Hirudinea contains the leeches, which
differ from other annelids in that they possess two suckers, one inclosing
the mouth and the other being ventral to the anal opening. They also
have a smaller number of metameres than other annelids. There appears
to be a greater number of these, however, than really exist, because each
one is marked by several transverse grooves (Fig. 157).

Anterior
Sucker

JSaws
Mouth

Opening of
vas deferens _

iit

Opening of = fea
ouiauer. 4
a

Nephridio- {
P ore (Tc

Sense
papillae
10th diverticulum
of crop
Stornach
Intestine
Rectum
: Anus

Posterior
sucker

Fic. 157.—The medicinal leech, Hirudo medicinalis Linnaeus. (From Borradaile,
‘‘ Manual of Elementary Zoology,’ by the courtesy of Oxford University Press.) A, the whole
animal, ventral surface. The segments are shown by Roman numerals. B, the anterior
end, dorsal surface. C, the posterior end, dorsal surface. JD, the digestive system. A,
natural size; B and C slightly enlarged.

The suckers are used as organs of attachment when the animal is at
rest and also as organs of locomotion when the animal moves about upon
a firm substratum, the movement being similar to that of a measuring
worm. Certain leeches are able to swim freely through the water, the
body performing vertical undulations.

The mouths of some leeches, such as the medicinal leech (Fig. 157),
are provided with jaws armed with chitinous teeth. When such a leech
attaches itself to another animal for the purpose of securing blood these

ANNELIDS IN GENERAL 251

teeth make incisions in the skin thus permitting the blood and lymph
to flow freely. At the same time salivary secretions are introduced
into the wound which prevent coagulation of the blood. The blood is
sucked up by the action of a muscular pharynx and passed into a crop,
which is very long and provided with lateral pouches that give it great
capacity. It has been stated that a medicinal leech can take in three
times its own weight of blood and that this supply will last it for nine
months. The blood which is stored in the crop is from time to time passed
on into the stomach, where it is digested; from here it goes into the intes-
tine for absorption.

There are other leeches which have a protrusible proboscis in place
of jaws, and still others which lack both jaws and proboscis. Some of
these are not parasitic but are predatory species which feed upon worms,
snails, and insect larvae. Snails, fish, and turtles are among the forms
most commonly parasitized, but mammals entering the water also become
victims. There are land leeches, occurring chiefly in the tropics, which
are serious pests. One species of European land leech feeds upon
earthworms.

Leeches are hermaphroditic, but, as in the case of the earthworm,
cross-fertilization takes place. The sperm cells are collected in bundles
called spermatophores which are passed from one worm into the body of
another. The eggs of many leeches are deposited in chitinous cocoons
that are attached to the surface of hard objects in the water where they
form brown, elliptical, moderately convex objects. Some leeches are
Ovoviviparous. Others carry their eggs attached to the ventral surface of
their bodies, and when the young are hatched they remain with the
parent for a time, attached by the posterior sucker.

287. Gephyrea.—The affinities of this group are uncertain but it has
more resemblance to the annelids than to any other phylum recognized
in this text. Its members are all marine (Fig. 158). Some show traces
of metamerism while others do not, but they may have lost this feature
through degeneration.

288. Metabolism.—The food of some annelids is made up entirely of
small animals; that of others includes both microscopic plants and ani-
mals; that of still others consists of any organic detritus; while that of
certain leeches is only blood and lymph. The alimentary canal of all
annelids shows a high degree of specialization. Circulation is carried on
by means of coelomic cavities and blood vessels. Excretion takes place
into both the coelomic fluid and the blood, and elimination is accom-
plished by means of nephridia. These in some cases have no opening
into the coelom and so eliminate only liquid waste. Respiration always
takes place through the body surface but may be more or less limited to
certain areas, as to the parapodia or the tentacles about the head.

252 METAZOAN PHYLA

289. Behavior.—The behavior of the earthworm has been described.
That of other types seems to be similar, subject to the possibilities and
limitations depending upon different modes of living and in some cases
to life in a fixed tube. Many annelids have eyespots or eyes. An
eyespot is simply an area on the surface of the body made up of cells
with light-absorbing pigment. By the addition of other parts such an

Fig. 158.—Three types of Gephyrea. A, an echiurid, Echiurus pallasii Guerin, found
along the northern Atlantic coasts, of both this continent and Europe, living in sandy and
muddy beaches. South in North America to Maine. (From Delage and Hérouard,
““Traité de Zoologie Concréte.”) X%. B, a sipunculid, Dendrostoma alutaceum Grube.
Found among coral reefs near shore from Cape Hatteras to the West Indies. (From
Gerould, in Proc. U.S. Nat. Mus., vol. 44.) xX 4. OC, a priapulid, Priapulus caudatus
Lamarck. Found in shallow water along the sand and mud beaches of the Arctic ocean.
(From Shipley, ‘‘Cambridge Natural History,” by the courtesy of The Macmillan Company.)
Natural size.

area becomes an eye. The eye of the sandworm is globular and is com-
posed of a cup of pigmented cells, the inner ends of which form rods; a
central gelatinous mass, the lens; and a thin cuticular area on the surface,
the cornea. The rods, lens, and cornea are all transparent and transmit
light. Some leeches have simple or compound eyes; others which possess
no eyes, palpi, or tentacles, are exceedingly sensitive to touch and are
apparently attracted to their prey by vibrations coming to them through
the water. It is possible that they are also stimulated by secretions from
the bodies of the animals on which they feed, also brought through the
water.

ANNELIDS IN GENERAL 253

290. Reproduction.—Reproduction has been referred to in the case
of the earthworm and of the leech. Sexual reproduction in other forms
is similar. There is, however, a type of asexual reproduction by means
of transverse fission which sometimes occurs in annelids, the result of
which is to produce a colony of several individuals moving about together
(Fig. 159). In Aeolosoma this is the ordinary mode of multiplication,
and sexual reproduction seems to be rare. Budding is also exhibited by
certain Chaetopoda. In some of them lateral buds form on certain
segments, while in others the budding is restricted to the undersides of
the last two segments. In most
cases the buds eventually break
away from the old worm and become
new individuals.

The sex cells of most polychaets
are reproduced from the epithelium
lining the coelom and either pass
into the water through the nephridia
or escape through ruptures in the
body wall. In connection with this
liberation of the sex cells, the palolo
worm (Fig. 160) of the Southern
Pacifie Ocean exhibits a phenome-
non known as swarming. This oc-
curs regularly every year beginning
on the first day of the last quarter of
the October-November moon and Fic. 159.—A chain of individuals
continues for two or three days. EGR apa etre US A)
The worms which lie in burrows at “ Manual of Zoology,” after Milne-Edwards,
the sea bottom have their whole {ys erty of Hewy Holt & Company
posterior portions modified for the the one at the end of the chain being the
production and retention of the Sex ey ine gene ena a of pro-
cells. When the day for swarming
arrives the reproductive portions break away and rise to the surface in
such numbers that the surface of the sea is covered with the writhing
mass. In a short while the sea becomes milky with the millions of
liberated egg and sperm cells. The anterior portions of the worms
remain at the bottom to regenerate another reproductive portion for the
following year. <A similar palolo worm in the West Indies region swarms
in the third quarter of the June—July moon.

In one family of the polychaets the sexual zooid does not at once
separate; both it and the asexual one multiply by transverse fission and
this continues until a chain of as many as 16 individuals is produced, the
anterior ends of each of which are asexual and the posterior sexual. All
of the individuals in a chain are of the same sex.

Ta
a

254 METAZOAN PHYLA

The larva of most of the marine annelids is a trochophore (Fig. 153 A
and B). This larva does not occur in fresh-water forms and of course is
never seen in the terrestrial types.

291. Occurrence and Economic Importance.—The distribution of
various annelids, especially the marine forms, is very general, though
characteristic species are found in each region. Economically the earth-
worm has been shown to be generally a very beneficial type. The

Asexual
Zoold

Fic. 160.—A palolo worm, Leodice viridis (Gray), showing asexual and sexual zooids.
(From VanCleave, ‘‘ Invertebrate Zoology,” after Woodworth, by the courtesy of McGraw-Hill
Book Company, Inc.)

leeches, which live a parasitic life, should be considered injurious, enemies
of both man and the domestic animals which serve him. The medicinal
leech, however, has in the past been of service to physicians and played
such a part in medicine in the sixteenth century as to earn for the phy- -
sician himself the appellation of leech. Most other annelids are of little
importance, though some are used as fish bait. In the South Seas the
palolo worm (Fig. 160) is gathered by the natives when it swarms in the
breeding season in October and November and is used as food, being
considered a great delicacy.

CHAPTER XLII

CRAYFISH
A TYPE OF THE PHYLUM ARTHROPODA

The crayfish is a fresh-water form abundant throughout this country
and represented by many species. Some of these prefer quiet waters
and others running streams. There are also cave dwellers among them.
Crayfishes usually spend the day in hiding under rocks or logs or other
objects at the bottom of the pond or stream in which they live, though
they may be tempted to come out by food which comes near their place
of concealment. At night they become active, leaving their holes and
wandering freely in search of food. They are often captured in large
numbers in traps and may easily be caught by hand on a hook baited
with a piece of meat. The meat is grasped by the pincers and held with
such tenacity that a quick jerk will bring the animal out of the water.
In books they are called crayfishes, but the names more commonly
applied in this country are crawfishes or crawdads.

292. External Characteristics.—Crayfishes are metameric, the met-
ameres being grouped into two regions, the cephalothorax and the abdomen
(Fig. 161). The surface of the body is covered with an exoskeleton
composed of chitin mingled with lime salts. On the dorsal surface of
the cephalothorax the skeleton forms a continuous shell known as the
carapace. A transverse cervical groove marks the division between parts
corresponding to the head and thorax. A median forward extension of
the carapace beyond the eyes is called the rostrum.

The cephalothorax includes 13 metameres, 5 representing the head
and 8 the thorax, to each of which characteristic appendages are attached
ventrolaterally. The first metamere bears a pair of antennules, and the
second a pair of very long, many-jointed antennae, or feelers. On the
third metamere is a pair of mandibles, and on the fourth and fifth met-
ameres are two pairs of mazillae. <A portion of the second maxilla is
modified to form a scooplike plate known as the scaphognathite, or bailer.

Of the thoracic metameres, the sixth to the eighth bear mazillipeds,
and the ninth to the thirteenth, walking legs. In the first three pairs of
legs the segment next to the last is prolonged so that its tip is even with
that of the last, and the two together form a pincer. In the first pair this
pincer is very powerful, is called a chela, and the leg, a cheliped.

Of the six abdominal metameres, the first five bear appendages
which are quite typical, except that the first two pairs are somewhat

255

256 METAZOAN PHYLA

modified in the male for use in reproduction (Fig. 162). The abdominal
appendages are known as swimmerets, although they have no function
in connection with swimming. The appendages of the last abdominal
segment are broad and flat and are called uropods. With the telson,
which is a projection of the segment backward in the median line, they

Antenna

Pereropods
Walking Legs)

Telson
Fig. 161.—Cambarus diogenes Girard, one of the commonest and most widely distrib-
uted crayfishes of the United States east of the Rocky Mountains. [From Hagen, Mem.
Mus. Comp. Zool., Harvard Univ., vol. 2 (under the name C. obesus Hagen).] xX %

form a broad, flat, paddle-like structure used in one form of locomotion.
All of these appendages are reducible to a common plan and result from
the modification of a typical biramous, or two-branched, appendage.

In addition to these appendages there are two stalked eyes, attached
to the first segment on each side of the base of the rostrum, which can
be extended and withdrawn and also pointed in different directions.

CRAYFISH 257

Antennule
Be <> 4 Antenna
oe Vay Opening of
oes 4 Baad & ‘green gland
j NY Third
i} ree A maxilliped

OU ae

Perejopods

Pleopods

rt Endopodite
Telson Anus
Fig. 162.—Under side of Cambarus virilis Hagen, a common species in the central states.
From a male specimen from Wisconsin. X 2.
Heart
Pericardiurm
Liver. \psahonX, Vas deferens
Sternal artery
Intestine Gill
chamber
Muscle Gill

thoraci¢e artery
Vertra/ rerve
cord

Fic. 163.—Diagrammatice cross section through a crayfish (Cambarus) in the posterior part
of the cephalothorax, to show the gill chambers and gills.

258 METAZOAN PHYLA

There are two gill chambers, one on each side of the body, lying
between the lateral wall of the body and a broad plate extending ven-
trally from each side of the dorsal surface. Each gill chamber is open in
front, forming a channel in which lies the scaphognathite, or bailer, of the
second maxilla. It is also open by a narrow slit along the ventral side
and in the lower part of the posterior end. In this chamber, in the cray-
fishes of the eastern states, which belong to the genus Cambarus, are two
rows of gills. The outer row is attached to the first joints of the append-
ages from the second maxilliped to the fourth walking leg (Fig. 163).
The inner row of gills, double except in the case of the first one, arises

Supraesophageal
Digestive ganglion
stomach
Testis att Antennule
Vas deferens '°9""S\ Gringing Antenna

Heart Tormac. ese
e Dorsal ab-
dominal

Intestin

Vag SNA bsaphagus
C( Mouth

a iT \\| \
Dé: K Y & \ VY x Mandible
Venthat ‘ Y v SSA
nerve cord .
an

—
AN
A \

Fic. 164.—Partly diagrammatic longitudinal section of the European crayfish, Astacus
fluviatilis Fabricius. (From Borradaile and Potts, ‘‘The Invertebrata,’ after Shipley and
MacBride, by the courtesy of The Macmillan Company.) The individual is a male and the
first two swimmerets are modified to form copulatory appendages.

Opening of
vas deferens

from the membrane attaching the same appendages to the wall of the
body. In the crayfishes of the Pacific coast, which belong to the genus
Astacus, there is also a third row attached to the wall of the body itself.
In respiration a current of water is maintained in the gill chambers pro-
duced by movements of the swimmerets, which direct water into the
posterior end of the chamber, while it is being bailed or scooped out from
the anterior end by the scaphognathite.

293. Internal Structure.—The various systems in the crayfish are
well developed (Fig. 164). Some of the systems, like the muscular and
nervous systems, are still metameric in their arrangement, but others
show much condensation. The coelom is greatly reduced in capacity and
becomes divided into separate cavities, including those containing the
reproductive organs and those about the green glands, which are excretory

CRAYFISH 259

organs. Other spaces around the alimentary canal which contain blood
and form what is known as a hemocoel are not truly coelomic.

The alimentary canal consists of a buccal cavity, esophagus, stomach,
and intestine. The stomach is divided into two portions, one grinding
and the other digestive in function. Between

the two portions is a strainer composed of hairlike So b Supraesoph-
setae which permits the food to pass only when Aya 97208! 99°F

. = j i
it has been ground into very fine particles. AD Cireumesopt

The circulatory system includes the heart, ageal con-

seven arteries leading to various parts of the ade
: ¢ : Esophagus

body, and spaces in the tissues called sinuses
which communicate with a large space around the Snel
heart known as the pericardial sinus. Valves are Fag IL
present in the arteries and also guard the open-
ings from the pericardial sinus into the heart.

The excretory organs of the crayfish are a
pair of bodies known as green glands situated in
the ventral part of the head in front of the i
esophagus, the ducts from which open through =<
papillae on the basal segments of the antennae. ic} Asis

The crayfish possesses a well-developed mus- me
cular system, the muscles being attached to the |
various portions of the exoskeleton. 4

The nervous system (Fig. 165) is in many é pesoilioess
respects similar to that of the earthworm, includ- ex cord
ing a supraesophageal ganglion; circwmesophageal iy
connectives; a subesophageal ganglion, representing ~fy
a fusion of the ganglia of the metameres from III y
to VII; and a ventral gangliated nerve cord with =! -
ganglia in each segment posterior to the seventh. = ~
This condensation of metameric ganglia in the Se
subesophageal ganglion promotes coordination of I
all of the appendages used in connection with the +

process of food taking. The largest sense organs

of the crayfish consist of a pair of eyes and a pair

of statocysts. Tactile organs also are well devel- ‘Fic. 165.—The central

oped in different parts of the body, particularly eke ce Cnr ae

upon such parts as the chelae of the walking legs, Lang, “ Text-book of Com-

the mouth parts, the ventral surface of the abdo- an aint By es

men, and the edge of the telson. There also seems

to be a general distribution of organs for the perception of chemical stimuli.
Crayfishes are diecious. The vasa deferentia open on the median

side of the base of each last walking leg. The openings of the oviducts

are at the base of each third walking leg.

260 METAZOAN PHYLA

294. Eyes and Vision.—The crayfish possesses compound eyes.
Each eye is hemispherical in form and is covered by a transparent
cornea which represents a modified portion of the cuticula (Fig. 166).
The cornea is divided into rectangular facets, each one of which is the
outer end of a rodlike unit known as an ommatidium. These ommatidia
—of which there are approximately 2500—are radially arranged rods
tapering toward the base, which causes their axes to converge toward
a common center. From the ommatidia lead nerve fibers which, together,
make up the optic nerve. When an ommatidium is observed carefully
it is seen to consist of a corneal facet at the outer end (Fig. 167) beneath
which are corneagen cells which secreted it. Inward from these are four
elongated cells, »trellae, which form a crystalline cone near their distal
ends. The distal and proximal ends of the vitrellae are enveloped by
several retinular cells, sensory in function, from which nerve fibers extend
inward to optic ganglia. The rhabdom,

0. cH ; é
U7 ad a refractive body secreted by the proxi-
S< T>> Orreg SS C = =
Ke a5 mal retinular cells, lies in the axis of the
f NYS ‘di miss d. Th
A Kp GAZ ommatidium at 1ts ner end. e cor-
f S YUE Basement Sys , i
— YA neal facet, vitrellae, crystalline cone, and
Y

rhabdom are all transparent. The re-
tinular cells cover the surface of the
ommatidium and contain pigment. In
bright light this pigment is distributed

Optic nerve throughout the length of the omma-
Fic. 166.—Diagrammatic representa- tidium; in dim light, however, it con-

Hon} ol theryefore cra eer tracts toward the respective ends leaving
much of the surface without this dark covering.

A compound eye sees just as many little images as there are omma-
tidia, and since these images together make up the whole of the picture
received by the animal, the picture has been termed a mosaic image.
There is, however, some overlapping of the separate images. The pro-
duction of a separate image for each ommatidium results from the fact
that each of the ommatidia is long and slender, and since the pigment
along the sides absorbs all the rays which it receives, only those rays
reach the bottom and stimulate the retinula which are practically in line
with the axis of the ommatidium. In dim light the withdrawal of pig-
ment from the wall of the ommatidium permits all of the rays entering
it to be reflected inward, increasing the amount of light falling upon
the retinula and thus giving a stronger stimulation. This probably
does not result in a clear image but enables the animal to distinguish
between light and dark. A compound eye has a great disadvantage
when compared with such an eye as the vertebrate eye in that the animal
cannot focus with it, thus limiting the distance to which vision is possible.
It has, however, the great advantage that it more readily perceives move-

va
i { a
y Wh | j

CRAYFISH 261

ment in the visual field, since any motion almost inevitably results in a
stimulus being withdrawn from one retinula and applied to another.
295. Statocyst.—At the distal end of the basal segment of each
antennule is a sac, lined with chitin, which is continuous with the chitin-
ous covering over the surface of the body; this is a statocyst. On its
walls are sensory hairs and in its cavity are grains of sand or other hard

Corneagen ce/t

Cap of come
ce//

i
STS

Distal retinular
pigment? ce//

eens 5")

i
@
4,

nn 5 Fee = Ta

Crysfalline cone

Serer

yaks Sisk shehe
‘
Dre
pe ee
<=

PEEL ew eT PE

brane

RYE SIAN PE OY Ae GN

Proximal! portion
of cone ce//

Proxima] retinular
| Pigment? ce/]

Fie. 167.—Longitudinal sections through two ommatidia, which show the different
arrangement of pigment. A, in dim light. B, in bright light. (After Parker.)
objects known as statoliths. While the animal is in a normal position
there is no movement of these statoliths, and though they are in contact
with certain of the sensory hairs no stimulation is received. When,
however, the position of the animal is changed, their movement causes
them to come in contact with other hairs and this acts as a stimulus.
There results a sensation which causes the animal to respond in such a
way as to maintain its equilibrium. When the cuticula over the surface
of the body is shed, that which lines the statocyst is lost with the rest,

262 METAZOAN PHYLA

and other statoliths must be placed by the animal in the statocyst before
it can again function.

296. Feeding Habits.—The food of the crayfish consists mostly of the
flesh of dead animals lying at the bottom of the body of water in which
it lives, bits of which it tears off with its large chelae. Living animals
which the crayfish can grasp and hold with its chelae may also serve as
food, such animals including snails, tadpoles, insects, and even small
fish. The food is held by the maxillae and maxillipeds and chewed by the
mandibles. Crayfishes readily devour one another when in captivity.
They feed at night but are most active at dusk and dawn.

297. Behavior.—When the bottom of a lake or stream is observed
through clear water, there may very frequently be seen beside a stone
or other object a slight depression leading to a burrow under the object
and presenting a very clean appearance. This appearance, which
gives one the impression of every particle of debris having been swept
away, is due to the presence of a crayfish in the burrow and the constant
current of water maintained by the animal in its breathing. Sometimes
the antennae may be seen projecting from the opening. The animal is
more or less in contact with the walls of the burrow. It faces the open-
ing, ready to receive any stimuli which may come and to emerge quickly
to seize any food which is presented. In this position the swimmerets
are waving forward and backward, and the bailer is working actively in
the opening at the anterior end of the gill chamber, resulting in the main-
tenance of a current forward through the chamber. The legs are often
moved quietly backward and forward, serving to wave the gills back and
forth and thus aid in respiration. Some crayfish live in burrows in the
ground which reach down to the water level. When a pond or stream
dries up, the crayfish digs its burrow deeper and the earth excavated by
the animal is brought to the entrance and built up into a characteristic
mud chimney, which may be capped over with mud.

When attacked the crayfish defends itself with its chelae and resists
being dragged from the burrow, but if the object under which it is hidden
is raised, the animal is ready to dart away in the turbid cloud which is
spread through the water and so escape. If food comes near enough to
the opening of the burrow that its presence is detected, the animal
emerges, walking by means of its walking legs, seizes the food, and
immediately backs into the burrow again.

The crayfish is able to walk in any direction. It can also dart back-
ward, the movement being the result of an extension of the abdomen and
a spreading of the telson and the uropods, followed by a sudden flexion
of that part of the body. The resistance of the water drives the animal
rapidly backward. Since this action carries the animal only a short
distance, it is often repeated, and thus the crayfish makes a series of
backward darts.

CRAYFISH 263

Crayfishes respond positively to contact stimuli and seek to place
themselves in such a position that as much of the body as possible is in
contact with a firm surface. Chemical substances dissolved in the water
also act as stimuli. Food not only causes a movement of the animal
toward it but also excites chewing movements, and if meat juices are
added to the water, vigorous movements of such a type result. Chemicals
which are not normal to the water in which the crayfish is living may cause
it to rub its legs together or to scratch the surface of its body with them.
If the chemicals are in considerable concentration, however, the animal
may endeavor to escape entirely from the stimulus.

Simple experiments have led to the general opinion that the behavior
of the crayfish is in part instinctive and in part habitual. An instinct is
an action involving an inherited association of reflexes all tending toward
a certain end. A habit is an action of similar character, but it is acquired
during the lifetime of the individual by the continued repetition of a
particular action.

Fig. 168.—A female crayfish with eggs attached to her swimmerets. (From Andrews, in
Am. Natur., vol. 38.)

298. Reproduction.—Pairing of the sexes may take place either in
the spring or in the fall. If at the former time, the young become well-
developed before winter; if at the latter, the eggs may not be laid until
the following spring. The author, however, has observed a female
crayfish with very recently hatched young as late as the latter part of
October in an extremely warm fall season. During pairing the sperm
cells are transferred by the first two pairs of swimmerets of the male
from the opening of the vasa deferentia to the seminal receptacle of
the female. The seminal receptacles are cavities inclosed in folds of the
cuticula between the fourth and fifth pairs of walking legs. There the
sperm cells remain until the eggs are matured. At this time the latter
are passed out of the oviducts, which open at the bases of the third pair
of legs, and backward in a groove between the bases of the legs of the
two sides of the body, receiving sperm cells and being fertilized on the
way. The eggs finally become attached to the swimmerets by a gluelike
secretion, masses of them appearing like so many bunches of grapes
(Fig. 168). They remain attached during development, their aeration
being assisted by movements of the swimmerets.

Cleavage is superficial and the embryo develops from a thickening
of the blastoderm on one side. Limb buds appear, which correspond to

264 METAZOAN PHYLA

the different appendages; metameres are formed; and the embryo gradu-
ally assumes the characteristics of the adult. Hatching takes place in
from five to eight weeks, but the larvae remain clinging to the swimmerets
of the mother for about four weeks longer. During this time they grow,
shedding the exoskeleton at intervals but undergoing no metamorphosis.
The process of shedding, which is an adjustment to permit growth, is
known as molting, or ecdysis. This occurs seven or more times during
the summer. It is said that the life of the crayfish covers a span of from
three to four years. They reproduce annually after the second year.

299. Regeneration and Autotomy.—Crayfishes have the power of
restoring lost appendages, and under normal conditions the same sort
of appendage is restored as that which was lost. Under experimental
or abnormal conditions, however, an abnormal appendage may replace
the lost one. Crayfishes also have the power of autotomy, breaking off
a walking leg at a point near the base known as the breaking point.
This enables a crayfish to escape if its leg is grasped by an enemy or
closed upon by the valves of a mussel buried in the bottom over which
the crayfish is walking. The structure of the leg is modified at the
breaking point to make autotomy easier, but the action is under the
control of the individual.

300. Economic Importance.—In many localities crayfishes serve as
food, but in most parts of this country they are used only as fish bait.
They are an agency in the destruction of decaying animal bodies in the
water and from this standpoint are beneficial. Since they make holes
in dams and levees they may cause serious damage. The cotton growers
of the South also suffer because the crayfish eats young cotton plants.
Especially in the clay lands of Alabama and Mississippi do they interfere
with the raising of both corn and cotton.

CHAPTER XLIII

CRUSTACEA

Crustacea (kris ta’ shé a; L., crusta, a hard shell), to which the
crayfish belongs, is a class included in the phylum Arthropoda (dr thrép’
6 da; G., arthron, joint, and podos, foot). The animals of this class are
distinguished from the other arthropods by the fact that they carry on
respiration by means of gills, though some of them have become adapted
to terrestrial life. Many of the aquatic forms are found in fresh water,
but most of them are marine. Wherever found the individuals are
very numerous and frequently occur in vast numbers. The body is
usually divided into three regions, which are head, thorax, and abdomen;
in some cases, as in the crayfish, the first two divisions may be united to
form a cephalothorax. The head usually consists of five united met-
ameres and bears two pairs of antennae; a pair of mandibles, or jaws; and
two pairs of maxillae. The number of metameres in the thorax and
abdomen varies in different types. The thorax possesses a number of seg-
mented appendages, usually locomotor, while the abdomen may bear
appendages with other uses. The appendages exhibit homology, being
biramous and constructed on the same plan, but are specialized, each in
a manner fitting it for the function it performs. Not all crustaceans are
brightly colored, but among the marine shrimps are some of the most
brilliantly colored of animals.

301. Malacostraca.—The many forms of Crustacea may be divided
between two subclasses. The first of these, or Malacostraca (mal a
kos’ tra ka; G., malakos, soft, and ostrakon, a hard shell), includes types
which in general are of large size, the largest being the largest of the
arthropods and so large as to be conspicuous among invertebrates
generally.

The decapods, which agree in having five pairs of walking legs, include
crayfishes, lobsters, crabs, and shrimps. The more familiar of the crabs,
or those which may be termed typical, differ from crayfishes and lobsters
in the breadth of the cephalothorax, which frequently is broader than
long, and in the fact that the abdomen is brought forward under the
cephalothorax and closely applied to it (Fig. 169). The legs in the shore
crabs are particularly short and in consequence of the breadth of the
body are separated by a considerable interval. This explains the peculiar
sidling gait of the animal in rapid locomotion. Shore crabs occur in
abundance on the beach between tide marks, crawling under the rocks
and other objects for safety when the tide goes out. Other larger crabs,
including the edible ones, find it less easy to hide and follow the water
out with the tide. A soft-shelled crab is one which is caught in the process

265

266 METAZOAN PHYLA

of molting after it has shed its old shell and before the new one has
hardened. Among the various types of crabs isthe hermit crab (Fig. 365),
which possesses a soft abdomen and lives in the empty shells of snails.
Sometimes the sea bottom along the shore will be covered with what at

Antennule

Antenna

Fic. 169.—The blue or edible crab of the Atlantic coast, Callinectes sapidus Rathbun.
From preserved specimens. A, upper surface. X 14. B, under surface of female to
show breadth of abdominal metameres between which and the thorax the eggs are carried,
attached to the swimmerets. X14. C, under surface of body of male to show the
narrowness of abdominal metameres. XX ]4.

first glance appear to be unnaturally active snails, but which on examina-
tion prove to be snail shells containing young hermit crabs. Sometimes
these snail shells also bear other animals, such as sponges, hydroids, and
sea anemones. Some crabs, known as spider crabs, have very long legs,
which give them considerable speed in locomotion. A Japanese spider
crab, the largest known crustacean, is said to reach a measurement of 11
feet from tip to tip of the outstretched legs.

The zsopods are common in the sea and in bodies of fresh water and
are in part terrestrial. They are flattened dorsoventrally and lack a
carapace. All the legs are similar in structure except the posterior pair
and, in the male, the anterior pair. The terrestrial forms, commonly
known as sow bugs (Fig. 170) and pill bugs, live under stones, boards
and other objects upon the ground and are also found in damp cellars
and in greenhouses, where the air is moist.

CRUSTACEA 267

The amphipods are a third group of Malacostraca distinguished from
the two preceding groups by being laterally compressed and by the
absence of a carapace. The first thoracic segment may be fused to the
head and there is usually no sharp distinction between thorax and
abdomen. The first five pairs of thoracic appendages are used in feeding,
and the last three pairs aid in crawling. Of the six pairs of abdominal
appendages, the first three serve in swimming and directing water toward

Fic. 170. Fie. 171.

Fig. 170.—An isopod, Oniscus asellus Linnaeus. (From Paulmier, in Bull. 91, N. Y.
State Mus., by the courtesy of the New York State Museum.) XX 3.

Fic. 171.—An amphipod, Hyalella dentata (Say). (From Paulmier, in Bull. 91, N. Y.
State Mus., after Smith, by the courtesy of the New York State Museum.) XX 6.

appendages. They are found in all waters, a common fresh-water form,
Hyalella (Fig. 171), being one of the most generally distributed of all
North American animals. Amphipods are also found on the beach
between tide marks, where, because of their power of jumping, they are
termed beach fleas.

302. Entomostraca.—Entomostraca (én t6 més’ tra ka; G., entomos,
cut in pieces, and ostrakon, a hard shell) are, generally speaking, of small
size but they occur in numbers that can hardly be realized. It has been
estimated that on the average each cubic meter of water in the small
Wisconsin lakes contains about forty thousand individuals. Cladocerans
have been observed in a small alkaline lake in Cherry County, Nebraska,
in such numbers that the whole lake, when seen from a distance, was of
a red color. Entirely around the shore was a windrow of these animals,
cast up by the water, a foot wide and from an inch to two inches in depth.
A wide-mouthed bottle filled by one dipping from the water of the lake
at the shore was about half filled with the organisms after preservation of
the material and on settling. The group (Fig. 172) includes Cladocera
(kla dés’ ér a; G., klados, sprout, and keras, horn), also known as water
fleas; and Copepoda (k6 pép’ 6 da; G., kope, oar, and podas, foot), some
of which are parasitic on fish, being called fish lice. A third order is
Ostracoda (6s tra ko’ da; G., ostrakodes, having a shell), which are
inclosed in bivalve shells and look like miniature mollusks.

268 METAZOAN PHYLA

Among the Entomostraca parthenogenetic reproduction is very
common. During the spring and summer, in our fresh-water ponds
and lakes, only the females of the common water fleas are to be found,
and generation after generation of female individuals come from thin-
shelled summer eggs which develop parthenogenetically in the brood
pouch of the mother. In the autumn males appear, and thick-shelled
winter eggs are produced, which are fertilized and live over the winter,
to hatch and produce females the following spring. Parthenogenetic
reproduction represents a rapid method of reproduction, which is neces-
sary if these animals are to maintain themselves, since they form a very

Fic. 172.—Three types of entomostracans. A, a copepod, Cyclops sp. (From Bronn,
“‘ Klassen und Ordnungen des Tierreichs,”’ after Claus.) Female witheggs. B, an ostracod,
Cypris sp. (Modified from Thomson, ‘‘Outlines of Zoology.) C, a cladoceran, Daphnia
pulex (de Geer), a very common and widely spread species. (Compiled from several figures.)
The figure represents a female with six eggs in the brood pouch. All figures highly magni-
fied.

large element in the food of fishes and other animals. They are thus
indirectly of economic importance.

The barnacles, which form an order of Entomostraca known as Cir-
ripedia (sir i pé’ di a; L., cirrus, curl, and pedis, foot), are Crustacea that
have become fixed and inclosed in a calcareous shell of several pieces.
The larvae are free-swimming but soon fasten themselves, back down,
upon a firm surface, grow shells, and remain for the rest of their lifetimes
attached (Fig. 173). If above the level of low tide barnacles close their
shells when the tide goes out but when the tide returns reopen them and
begin to collect food. This is brought to the mouth by the currents of
water created by the movements of the legs. They occur sometimes in
great numbers, completely covering the surfaces of rocks; younger
generations, graded in size, are attached to the surfaces of the older
generations so that 25 or 30 may exist within the area of only one square

CRUSTACEA 269

inch. A square meter of rock area observed in Puget Sound was
estimated to bear 41,500 individual barnacles, and since the rocks every-
where were covered with them, the total in the one locality must have
been enormous. A colony of such barnacles presents a very animated
spectacle when all of the individuals are kicking their legs at the same
time.

303. Behavior.—The behavior of the crayfish has been described,
and that of forms which are like it in structure is similar. Brief references
to behavior have also been made as it is exhibited by barnacles, but
nothing has been stated with regard to that of swimming forms, par-

Fic. 173.—Barnacles. A, Balanus hameri Darwin. The shell of the animal is closed
up, concealing the occupant. B, Balanus tintinnabulum Linnaeus, showing the internal
anatomy of the animal, also with the shell closed. (From Bronn, ‘‘Klassen und Ordnungen
des Tierreichs,’’ after Charles Darwin.) Natural size.

ticularly entomostracans. These are exceedingly active, swimming
or darting here and there and seeking that light intensity to which they
are best adapted. Owing to the fact that some are adjusted to bright
light and others to dim, there is a vertical movement in bodies of water
of considerable depth which brings to the surface in the daytime certain
forms which are found at deeper levels at night. Others which remain
at these deeper levels in the daytime come to the surface at night. In
addition to light stimuli, crustaceans respond to contact and to chemical
stimulation.

304. Reproduction.—Most crustaceans are diecious, though the
barnacles are not. The eggs are centrolecithal, undergo superficial
cleavage, and from them are produced larvae, which, as in the crayfish,
may be miniatures of the adults, or which, as in the shrimp, may be
quite different and pass through several larval stages, thus undergoing
complicated metamorphoses. The young of crabs have very prominent
eyes and for this reason have received the name of megalops. Many
crustaceans carry their eggs about attached to abdominal appendages
and in some cases contained in a brood pouch. The larvae may also be
so carried for a time.

270 METAZOAN PHYLA

305. Economic Importance.—A large number of crustaceans have
been used as food, especially lobsters, shrimps, and crabs, and, in some
localities, crayfish. Reese states with reference to lobsters that in
Canada alone 100,000,000 have been caught in a single year. He also
says that the total catch in the United States in 1892 was about 23,250,000
pounds; in 1905 it was about 11,750,000 pounds, which sold for more
money than the catch of 1892. The catch in 1936 amounted to nearly
13,158,000 pounds. No more recent figures are available. The supply
has been seriously depleted and efforts are being made to replenish it by
the artificial rearing of young lobsters, which are liberated at places
favorable for their growth. The crab-fishing industry centers about
Chesapeake Bay, but there has been a serious diminution in the supply.
The Gulf States furnish most of the shrimps marketed in this country
and they are the most important one element in the fisheries of those
states. In 1931 the value of canned shrimps and crabs, and by-products
of these, amounted to over $4,000,000; and in 1936 the amount was over
$5,000,000. These figures are from the U. 8. Fish Commission; they
do not include the value of the animals used in a fresh state.

306. Biogenesis——More than a century ago Von Baer directed
attention to the fact that there was a resemblance between the early
stages in the life of higher Metazoa and the adults of lower forms. With
the general acceptance of the concept of evolution a very natural expla-
nation of the fact was to assume that this resemblance was due to ances-
try, the lower forms having ceased to develop after reaching the condition
in which they now are, and the higher forms having continued to develop
but indicating in their early stages the characters of their ancestors.
This conception has been termed biogenesis and formulated in the
biogenetic law. Biogenesis is to be contrasted with abiogenesis, a term
synonymous with spontaneous generation, the idea of which is that each
individual form was the result of a separate creative act and when
created had the characteristics it now possesses. Strong arguments
for the biogenetic theory have been derived from the Crustacea.

The shrimp, Penaeus (Fig. 175), is a good example of the application
of the biogenetic law. This organism hatches as a larva known as a
nauplius (Fig. 174), exhibiting three pairs of appendages and a frontal
eye and resembling the larvae of Crustacea generally, including those of
the simpler forms. From the nauplius is produced the protozoaea, which
has 6 pairs of appendages and rudiments of segments. From this in
turn is derived the zoaea, which possesses 8 pairs of appendages, with 6
more developing, and has a distinct cephalothorax and abdomen. The
zoaea changes to a mysis, with 13 pairs of appendages on the cephalo-
thorax. Finally, from the mysis is produced the adult shrimp, which
has 19 pairs of appendages. Many crustaceans pass through a nauplius
stage, which may represent an ancestral type now extinct. The proto-

CRUSTACEA 271

zoaea and the zoaea correspond also to no living forms, but the mysis
resembles very closely crustaceans belonging to the genus Mysis, which is

an ancient type still living.

Fic. 174.—The developmental stages in the life history of a shrimp, Penaeus sp.
A, nauplius stage. B, protozoaea stage. C, zoaea stage. D, mysis stage. Highly
magnified. (From Lang, ‘‘ Text-book of Comparative Anatomy,” after Fritz Miiller.)

Fig. 175.—An adult shrimp, Penaeus semisulcatus. X14. (From Huzley, ‘The Study of

Zoology,’ after de Haan.)

Because of this correspondence, which seems to show in the higher
forms a succession of stages recapitulating ancestral conditions, the
biogenetic law has also been termed the law of recapitulation. It has

272 METAZOAN PHYLA

been expressed in the phrase “ontogeny recapitulates phylogeny,”
ontogeny being defined as the development of the individual, completed
within a single lifetime, and phylogeny as the development of the race,
covering, perhaps, ages of time and unnumbered generations. The
biogenetic law has also been considered by some authors as illustrated by
a correspondence between the egg cell and a single-celled animal; the
blastula and a colonial protozoan; the gastrula and a supposed gastrula-
like ancestor of the Metazoa called a gastraea; and the triploblastic embryo
on the one hand and a triploblastic animal on the other.

This conception has been a fruitful one in its influence upon zoological
progress, since it has directed attention to the broader principles under-
lying embryological development. However, it has also been criticized
very severely because its proponents have applied it in too sweeping a
manner and without due consideration of the fact that the animal king-
dom represents many lines of descent and that resemblances may be the
result not of common ancestry but of similar adaptations arrived at
independently and adjusting unrelated forms to similar environments.
It is interesting to note, in this connection, that while the shrimp possesses
all those free-living larval types and thus shows a complicated meta-
morphosis, the crayfish, in the same group, which carries its young
about on its swimmerets, shows no metamorphosis at all.

CHAPTER XLIV

ONYCHOPHORA AND MYRIAPODA

Onychophora (6ni kéf’ 6 ra; G., onychos, claw, and phoros, bearing)
is a class of the phylum Arthropoda which in a natural classification
should come first, since it is not only the simplest of the arthropods but
has a pronounced resemblance to the annelids, suggesting a derivation of
arthropods from annelid-like ancestors.

307. Onychophora.—The typical genus of this class is Peripatus,
which contains numerous species reported from widely separated local-
ities in Australia, New Zealand, Tasmania, New Britain, the Malay
Archipelago, South America, Mexico, West Indies, and Africa. It is a
wormlike form with a soft skin covered by papillae, each papilla bearing

Fig. 176.—Peripatus; an individual shown entangling a cockroach in sticky threads
formed by a secretion ejected from papillae on its head. (From Pearse, ‘‘General Zoology,”
by the courtesy of Henry Holt & Company.)

a spine. Metamerism is not marked externally, but there is a series of
short, fleshy legs in pairs, each ending in two claws. There is also an
oral papilla on each side of the mouth and a pair of simple eyes.

Peripatus shows a number of annelid-like characteristics. The skin
is thin and not so heavily chitinized as in the arthropods generally, there
are paired and segmentally arranged nephridia in all but the first two
metameres, and there is amarked resemblance to the annelids in the general
arrangement of internal organs. In other ways, however, it seems to be
truly an arthropod, since it has tracheae (Sec. 313), appendages modified
to form jaws, and body cavities which are hemocoelic. It differs, how-
ever, both from other arthropods and from annelids in having a single
pair of jaws, in the texture of the skin, and in the simplicity of the
metamerism.

The species of Onychophora live in crevices in rocks, under stones,
and in the dark recesses of rotting logs, where they move slowly about
from place to place, always avoiding the light. When disturbed a very

273

274 METAZOAN PHYLA

sticky slime is ejected from the oral papillae. This indicates that this
is a weapon of defense, although it ordinarily serves in the capture of
small insects and other animals used as food (Fig. 176). Owing to the
velvety texture of the skin and its rich coloring, Onychophora are

described by Sedgwick as animals ‘‘of striking

Nek f° beauty.”

i ri ein 308. Myriapoda.— Myriapoda (mir i dip’ dda;
FRSA G., myrios, ten thousand, and podos, foot) is a
ap third class distinguished particularly by four
LL IT I characteristics: (1) The metameres are many,

Lett all metameres back of the head are alike in
Ey ts\ appearance, and there are numerous pairs of
povasa similar running legs; (2) the head has a pair of
pa a antennae, a pair of mandibles, and one or two
petit pairs of maxillae; (8) the animal breathes by
C=lGiaN tracheae opening to the outside by metamerically
ATTEN \ arranged pores; and (4) the excretory organs are
(ALE PY malpighian tubules, like those present in insects,
ame Gi =. opening into the posterior end of the intestine.
ZAGER’ The body of all myriapods is elongated, some-
f JIL TT times nearly cylindrical and at other times
( 2S || PAX dorsoventrally flattened, rarely being com-
| aa ) pressed from side to side. Myriapods are widely
( apse distributed and flourish under a variety of condi-
Ee BO) \ tions. The class is divided into several orders,
U Ne two of which are larger and better known than
é/ \\ the rest, one including the centipedes and the

H ‘ other the millipedes.

ee 309. Centipedes.—The centipedes, or hun-
ene Deana agot dred-legged worms (Fig. 177), have a body
pendra sp. From a dried which is, generally speaking, flattened dorsoven-
Seer aa rae trally and which may consist of from as few as
15 to as many as 175 or even 200 metameres. Each of these metameres,
except the last two and the one just behind the head, bears a pair of
legs. The one next to the head has a pair of poison claws, or maxillipeds,
by means of which the animal kills the other small animals which it uses
for food.

Centipedes are active, rapidly moving myriapods living under the
bark of logs and objects lying upon the ground. They are predaceous,
catching and devouring any living animals which they are able to over-
come. In tropical countries they reach a considerable size, sometimes
a foot in length, and the bite of such a centipede is often painful, although
not ordinarily dangerous to human life. A form known as the house
centipede, Scutigera, is common in the southern, western and central

ONYCHOPHORA AND MYRIAPODA 275

United States in houses. It is slightly compressed laterally, has long
legs, and runs with great speed. It is a beneficial form, since it preys
upon various noxious insects, such as cockroaches and bedbugs, which
live around houses.

bee x

Fig. 178.—The fone. nes hovee aehiioede: Scutigera forceps. When a few of its
numerous legs are seized and held the centipede instantly detaches its body from these
legs and flees to safety on its remaining legs. In about a month it will regenerate new legs
to replace those that were lost. If, however, it is irritated sufficiently, it will detach all
of its legs, and then accelerating its regenerative processes will produce a full set of new
legs in about two weeks. Ranges from southern United States as far north as New York
and Nebraska. (From Marlatt, Farmer’s Bulletin, No. 627, U. S. Dept. Agriculture, 1914.)

310. Millipedes.—The millipedes, or thousand-legged worms (Fig.
179), differ from the centipedes in several ways. The body is sub-
cylindrical rather than flattened. The legs are very short, generally two
pairs to a segment, and the animal tends to react by rolling up into a flat
coil instead of by running away. ‘There is a pair of mandibles and one
of maxillae, and either simple or compound eyes. ‘The millipedes also

ey) TG (

se
—— SS

a = ——- = SS = —=——

a a Ze PS SS ——

Fig. 179.—A millipede, Spirobolus sp., from South Carolina. From a preserved specimen.
Natural size.

live in dark, moist places but feed principally upon plant food and

therefore are likely to be injurious, whereas the centipedes are likely to

be beneficial.

311. Reproduction in Myriapods.—In all myriapods the sexes are
separate. Some millipedes are known to lay large numbers of eggs in
cells excavated in the ground, which are later sealed up, but in the
case of the centipedes the eggs are laid singly in the damp earth. The
larva when hatched has only a few metameres and a few legs. The larvae
of the millipedes have only three pairs of legs, in this respect resembling
insects. As the animal grows, new metameres, each with a pair of legs,
are added just in front of the posterior one. Thus the total number of
metameres in the body is an indication of the age of the individual.

CHAPTER XLV
CLASS INSECTA

The fourth class of Arthropoda is Insecta (in sék’ ta; L., insectum,
having been cut into). The number of described species of insects is
enormous; some of the more recent figures place it between 600,000 and
700,000, about 75 per cent of all known living animals. It is also certain
that a very large number have not yet been described. In the number of
known species the class far surpasses all other animal groups combined,
and the number of individuals is correspondingly large. Insects are
represented everywhere on the land surface of the earth, except at the
poles and at the glaciated summits of the highest mountains. They
are also numerous in fresh water but are almost entirely absent from the

Compoundeye legmen
Prothorax

2 ty
eee saw ant
SEN Uanen. ENEENE

AEC E Meg se

Fig. 180.—A locust, Schistocerca americana Drury, which may serve as a typical insect.
(From Lutz, ‘‘Fieldbook of Insects,’ by permission.) Natural size.

oceans, though one type of true bug is known which occurs on the sur-
face of the sea even at a considerable distance from land. ‘The largest
insects are certain beetles, the bodies of which reach a length of more
than 6 inches, and certain moths, the wing spread of which may be as
great as 10 inches. On the other hand, the most minute insects known
are no longer than 0.01 inch.

312. External Characteristics.— Insects agree in having three divisions
of the body—head, thorax, and abdomen (Figs. 180 and 181). The
metameres represented in the head are so fused as to make it difficult to
determine the exact number, but the full number is considered to be six.
The thorax contains three, termed in order, prothorax, mesothorax, and
metathorax. 'The first is freely movable, but the two others are fused.
There is much variation in regard to the number of metameres present
in the abdomen, where the posterior ones are variously modified, but the
full number is considered to be 11.

The head bears a pair of antennae, several mouth parts, and a pair
of compound eyes (Figs. 181 and 182). The antennae usually consist of

276

CLASS INSECTA 277

many segments but vary greatly in their length and still more in the
details of form and structure. They bear a variety of sense organs which

p Antenna

S\ PEE ee

Labiurn

Mana'b/e

Maxilla

Jarsus

Fic. 181.—Diagram of a locust with parts of the body separated to show metameres and
other structures. Abdominal metameres numbered in roman numerals. (Modified from
several previous authors.)

are tactile, olfactory, or auditory infunction. The difference in structure
of those of the male and female frequently serves to distinguish the sexes.
The mouth parts of insects are
of two distinctly different types,
one fitted for biting, the other for BD compound eye
sucking. The former are referred '
to as mandibulate, the latter as
suctorial. Mandibulate insects
(Fig. 182) possess an upper lip, or
labrum, and a lower lip, or labrum.
Between these and meeting in the
median line are two strong jaws,
or mandibles, and behind the
mandibles is a pair of mazzllae.
Both the maxillae and the labium
bear jointed organs known as
palpi. The mandibles are used
for chewing and work transversely,
the labrum and labium preventing
dietleadpeMctia the -foodtatromime ice ean rien te, Pena a
between them. The maxillae help Girard, from Nebraska. X 31%. Illustrates
to feed the food into the man- Pitite mouth parts.
dibles, while the palpi are sensory, sending impulses into the nerv-
ous system which determine the activity of the other mouth

Ocelli

Lab/jal palpus

278 METAZOAN PHYLA

parts. In suctorial insects (Fig. 183) a proboscis is present which
can be thrust into the tissues of plants or other animals or which
may be used in merely taking liquid from a surface. This proboscis
is not developed from the same parts in different insects. In the bee
it is formed by the maxillae and the labial palpi; in the mosquito, by
the labrum and epipharynx; in the butterflies and moths, by the maxillae;
and in other insects, in still different fashions. The adults of some
insects have only rudimentary mouth parts and are incapable of feeding;
this is true, for instance, of our large native silkworm moths. It is inter-
esting to note that many insects that have sucking mouth parts in their
adult stage have the chewing type in their larval stage. This is inter-
preted by most authorities to indicate that the chewing type is the more
primitive.

Compound
eee

Antennae

Labrum

Mandrble
axillary ;
palpus Manadibles
Max///ae
Labiunr
Hy popharynx

A &—Souton

Fig. 183.—Suctorial mouth parts. A, honeybee. Head viewed from in front. (From
Herms, ‘‘ Medical and Veterinary Entomology,” by the courtesy of The Macmillan Company.)
B, head of a female mosquito, viewed from side with mouth parts separated. (From
Matheson, ‘‘ Handbook of the Mosquitoes of North America,” by permission of the publisher:
Charles C. Thomas.) Both greatly enlarged.

Insects generally have a pair of compound eyes, although these differ
greatly in size in different types or may be absent. Some of them also
possess simple eyes, or ocelli, placed between and in front of the compound
eyes (Figs. 182 and 183).

Each of the metameres of the thorax bears a pair of legs and each of
the two posterior ones, as a rule, a pair of wings. The legs of insects
are variously modified and used in a great variety of ways (Fig. 184).
Running insects generally possess long and slender legs, the three pairs
being equally well-developed. Jumping insects like the locusts and

CLASS INSECTA 279

crickets have extremely long hind legs, and the joint known as the femur
is very large. Insects living in the water may have hind legs modified
for swimming by being broadened and paddle-like and having their
area increased by hairs along the margins; or in the case of other aquatic
insects, the middle pair of legs is elongated and used like a pair of oars.
Some burrowing insects have the forelegs modified for digging; in the
mole cricket they have a curious resemblance to the forefeet of the mole.
In the mantids, and also to a lesser degree in the walking sticks, the fore-
legs are increased in size and fitted for grasping prey. Many insects

F \ Pulvillus

Claw

Fig. 184.—Modifications of legs in insects. A, praying mantis, Stagmomantis carolina
(Linnaeus), to show forelegs modified for holding prey. Natural size. B, water boatman,
Coriza sp., with hind legs used as oars. X 2. C, part of under surface of a water beetle,
Dytiscus sp., to show hind leg modified as a paddle. X14. D, part of under surface of a
ground beetle, Calosoma scrutator Fabricius, to show hind leg fitted for running. X 14.
E, fore part of the body of a mole cricket, Gryllotalpa hexadactyla Perty, to show digging
forelegs. About natural size. F, foot of a house fly to show claws and pulvilli, or pads
of hairs, together with longer hairs, all for clinging. Highly magnified. (A to E from
specimens; F from Kellogg, ‘‘ American Insects,” by the courtesy of Henry Holt & Company.)

cling to walls and ceilings by means of pads of hairs at the tips of the legs.
When these are pressed against a surface the air is forced out and the pad
holds by suction.

The great majority of insects have wings, though there are wingless
types in a great many of the orders. The wings represent outfoldings
of the wall of the body in which the two sides of the fold have come into
contact and have grown together. Along certain lines, however, they
have failed to unite, and in these places hollow rods are developed.
These are called veins, or nervures; they serve to strengthen the wing and
divide its surface into areas called cells. While wings are growing and, in

280 METAZOAN PHYLA

the case of insects with complete metamorphosis, while they are being
expanded after emergence from the pupa, tracheae extend into the
veins. Around the tracheae are spaces which are extensions of the
hemocoel and which convey blood. In growing wings there are also
living tissues between the two sheets of surface cuticula. When the
wing becomes mature and fully expanded, however, these tissues cease
to be living; respiration and circulation in the wing stop; and the append-
age becomes a hard, dry structure, which is moved as a whole by muscles
within the body.

There are a great many modifica-
tions of wings shown in several of the
accompanying figures. Sometimes
they are soft and membranous, at
other times heavily chitinized and
very rigid. Examples of the latter
type are the anterior wings of the
beetles, known as elytra, and of the
locusts, known as tegmina; in both
cases these wings serve as protecting
sheaths for the folded posterior pair
of membranous flight wings when
the latter are not in use. In the flies
the hind wings are reduced to minute
threadlike rods tipped with knobs
and known as halteres, or balancers
(Hig2 85).

Fic. 185.—A crane fly, showing the Insects generally possess tracheae,
can ragar nt eating a Pe or breathing tubes, which open to
after Weed, by permission.) About natu- the outside by laterally placed
ral size: spiracles (Fig. 181). Spiracles are
elliptical openings each guarded by two flaps, which may be closed
to prevent the entrance of dust. The number of spiracles varies in
different types, though there is only one pair to a metamere. The
number of metameres which may have them is 11, including the pro-
thorax, the mesothorax, the metathorax, and the first 8 metameres in the
abdomen. They are always lacking, however, in one or more of these
metameres, and the maximum number present is 10 pairs.

The abdomens of insects never bear true legs but may possess bristle-
like or springing appendages as in some primitive insects (Fig. 189) and in
aquatic forms have tracheal gills along the sides or at the posterior
end of the body. These gills may be threadlike or leaflike or may be
much branched. In them is a network of tracheae between the cavities
of which and the water there is an interchange of gases.

The terminal metameres of the abdomen are often greatly modified,
being reduced in size and forming parts which enter into various types

CLASS INSECTA 281

of apparatus used in copulation, egg laying, or stinging. This results ina
lessening of the apparent number of metameres.

313. Internal Structures.—The body cavities of an insect are not
truly coelomic but are parts of a hemocoel. A heart lies under the dorsal
wall of the abdomen and blood circulates through these hemocoelic
spaces. The circulation is not so important as in most animals, however,
since it plays practically no part in respiration. When the body of an

Si
BO a
7 IS

\
p
q
3
\2 a

Subesophageal
ganglia

Thoracic
ganglia

Fic. 186.—A honeybee dissected to show the digestive, nervous, and tracheal systems.
(From Leuckart wall chart.) Illustrates the digestive system of a suctorial insect.
insect is opened, many white glistening tubes are seen. These are the
tracheae. ‘They are held open by rings of chitin, branch repeatedly, and
the finer branches reach all parts of the body. In insects of active
flight the tracheae are dilated in certain places and form azr sacs (Fig.
186). By means of this system of tubes oxygen is conveyed directly
to the tissues of the body and carbon dioxide is carried away. Although
the blood contains both oxygen and carbon dioxide it is only in the
amount that any tissue would have. In some cases aquatic forms do
not possess gills, but water is taken into the posterior end of the alimen-
tary canal, the wall of which is lined with papillae supplied with tracheal

tubes.

282 METAZOAN PHYLA

The alimentary canal is modified according to the character of the
food. A mandibulate insect (Fig. 187) usually possesses an esophagus,
which may be dilated posteriorly to form a crop; a muscular gizzard, or
proventriculus, which grinds the food and also strains it; a digestive
stomach or ventriculus, which receives the secretion from a number of
gastric glands, or caeca; and an intestine, which receives the digested
food and into which also open the tubular organs of elimination, the
malpighian tubules. Suctorial insects do not have a gizzard, but in place
of it they have a muscular pharynx
which acts as a pump and a sac for
the storage of juices (Fig. 186).
“ntenna The nervous system of insects is
similar in general plan to that of the
earthworm, but there are two ventral

a geal
Yt go Mandible

Ea Esophagus

(3) Crop nerve cords (Fig. 186). The two gan-
e Proventriculus glia of each pair are usually fused and
ViTTi Ventriculus communicate by commissures. In

the lower insects there is a pair of
ganglia to each segment, but in

tubules the higher forms the number is

\. LJ ZF reduced. In the latter forms the
ae A C2 thoracic ganglia are increased in size
© ad be A\ K and the supraesophageal and _ sub-
Cy @) Hind gut esophageal ganglia are not only
RoC increased in size but tend to be

Co {TE S brought together by the shortening of

= wy p> t h e circumesophageal connectives.

ex Yi Aa This results in such a degree of
Se glands centralization and cephalization that

Fic. 187.—Digestive system of a these anterior ganglia may properly
beetle, Carabus auratus Linnaeus, as an 4 ‘
example of a mandibulate insect. be ealled a brain (Figs. 186 and 188).
(From Lang, ‘Text-book of Comparative Their removal interferes with the
Anatomy,’ after Dufour.) : ,

coordination of movements and
results in death, though this is not immediate.

314. Senses of Insects.—Insects possess a great variety of sense
organs and several may serve for the reception of the same general
type of stimulus. The compound eyes and ocelli are both organs of
sight, although the exact function of the latter is not well understood.
The compound eye of an insect is similar to that of the crayfish (Fig. 167).
The sense of smell is highly developed in insects though olfactory organs
are known to exist only on the antennae. On the palpi and about the
mouth are organs of taste. Tactile organs are located on the antennae
and elsewhere about the body. That insects have a sense of hearing is
indicated by the variety of sounds they produce. Sometimes these are

CLASS INSECTA 283

the result of rubbing rough surfaces together. The locust rubs the
femora of the hind legs against the outer surfaces of the tegmina or
produces the crackling sound heard in flight by rubbing the front edges
of the hind wings against the tegmina. The shrilling of a male cicada is
due to the vibration of a tense membrane drawn across a sound chamber
on the first abdominal segment above the hind leg; its vibration is con-
trolled by special muscles. The buzzing of many insects is due to rapid
vibration of the wings. The auditory organ of a locust is a pit, or
tympanum, on the first abdominal metamere, closed by a thin kidney-
shaped tympanic membrane (Fig. 181). Organs believed to be auditory

NSA
sh

Fic. 188.—Nervous systems of three insects to illustrate condensation and cephaliza-
tion. A,atermite, one of the lowest insects. B,awater beetle. C, a fly, the highest type.
(From VanCleave, ‘‘ Invertebrate Zoology,’ A after Lespés, B and C after Blanchard, by the
courtesy of McGraw-Hill Book Company, Inc.)

also exist on the antennae of many insects and on the legs of katydids.
A few insects, especially the larvae and adults of certain beetles known
respectively as glowworms and fireflies, are luminescent. The emission
of light, like the utterance of sounds and the production of odors, probably
serves to bring the sexes together.

315. Reproduction.— Insects are always diecious. The eggs, which are
fertilized internally, undergo superficial cleavage and develop much as do
those of the crayfish. From the egg hatches a larva which differs in char-
acter in the various groups. Most of the insects exhibit a metamorphosis.
Only a few do not do so, and these are termed ametabolous (Fig. 189) and
are grouped together as Ametabola (4m @ tab’ 6 la; G., ametabolos,
unchangeable). Most of them pass through several stages, but the num-
ber of stages varies. In all insects the larval form is the period during
which growth takes place, no insect growing after it has become adult.

284 METAZOAN PHYLA

In many cases the larva bears a considerable degree of resemblance
to the mature insect, being hatched, however, with a relatively large
head and small thorax and abdomen and frequently with no more than
rudiments of wings. As successive molts occur the proportions of the
body gradually change and the wings increase in size. The stages which
follow the successive molts are called instars. The last molt transforms
the larva into an adult in which the regions of the body have acquired the
adult size and proportions and wings have become of full size and func-
tional. ‘Thus in this type of insects there is no stage corresponding to the
pupa, and the metamorphosis is termed incomplete. Incomplete metamor-
phosis (Fig. 190) occurs, generally speaking, in the lower insects, and those
which have it are called Hemimetabola
(hém i mé tab’ 6 la; G., hemi, half, and
metabole, change) or Heterometabola
(hét ér 6 mé tab’ 6 la; G., heteros, different,
and metabole, change), the latter because
metamorphosis is varied in character in
different types. Usually the larvae of
insects with incomplete metamorphosis
are termed nymphs, and sometimes those
of such of these as are aquatic, nazads.
There is a more pronounced change when a
naiad, which carries on aquatic respiration,
becomes an air-breathing adult than when
the nymph of a terrestrial form changes
into the adult insect.

SO es ene ee A third group of insects, according to
Campodea_ staphylinus Westwood. development, is Holometabola (hd 16 mé-
The ‘simplest diving insect, and an +41’ 6 la; G., holos, whole, andemerapale
example of Ametabola. (From , ’ * y z
Kellogg, ‘‘ American Insects,” by the change). ‘These pass through a complete
eee of Henry Holt & Company.) metamorphosis, which includes both a

larval and a pupal stage (Fig. 191).
The larva of a butterfly or a moth is called a caterpillar; that
of a beetle, a grub; that of a fly, a maggot; and those of other
groups have still other names. During the larval period organs
may develop which are peculiar to that stage in the life history
of the animal. At the close of the larval period a pupa, or chrysalis,
is formed, which is covered with a hard shell. During the pupal stage
the greater part of the organs of the larva undergo degeneration, the
organs of the adult developing in their stead. However, the nervous
system is not thus “‘scrapped,’’ nor are the reproductive organs. Most
pupae are inactive, but some are able to move about by flexion and exten-
sion of the body or by using the spines on the movable metameres as
levers.

CLASS INSECTA 285

Fic. 190.—Life history of a hemimetabolous insect, a locust. A, oviposition. B, egg
mass in the ground. (From Walton, Farmers’ Bull. 747, U. S. Dept. Agr., A after Webster.)
C to H, stages in the development of the red-legged locust, Melanoplus femur-rubrum
De Geer. C, just hatched; D, after first molt; Z, after second molt, showing beginning of
wing pads; F, after third molt; G, after fourth molt; H, adult. C to G enlarged, H, natural
size. (From Kellogg, ‘‘ American Insects,” after Emerton, by the courtesy of Henry Holt &
Company.)

286 METAZOAN PHYLA

Some insects exhibit in their life history even more than the four
stages—egg, larva, pupa, and adult. In some beetles there are two types
of larvae, one with legs and caterpillar-like, the other without them and
maggot-like, and one of these types follows the other. This condition
is known as hypermetamorphosis. In other cases a subimago stage
precedes the adult, or imago stage, the change from subimago to imago
involving a molt and a modification of certain details of structure.

Fic. 191.—Life history of a holometabolous insect, a butterfly, Danaus archippus
Fabricius. a, egg. 6, larva. c, pupa. d, adult. Natural size. (a toc from Jordan and
Kellogg, ‘‘ Animal Life,”’ by the courtesy of D. Appleton and Company; d, from a specimen.)

316. Autotomy.—Some insects possess the power of autotomy, but
regeneration is known to occur regularly only in the walking sticks.
An example of autotomy is presented by the locust; if held by the hind
leg and pressure is made on the femur, it will break off this leg. Termites
and ants have deciduous wings, which are shed after swarming.

317. Injuries Due to Insects.—Insects affect man injuriously in
a great many ways. They may annoy him by their presence or by
their bites or stings. The insect may be poisonous and its attack may
cause effects which are painful though rarely of themselves serious.
Insects may also be the means of transmitting disease-producing organ-

CLASS INSECTA 287

isms. They may injure man by affecting in similar ways his domestic
animals. Insects may destroy grain, fruit, and vegetable crops or
ravage forests, and they are the cause of serious damage to a great
variety of manufactured products. The annual losses due to insect
pests in the United States have been estimated in recent years at close to
$1,000,000,000.

318. Benefits from Insects.—To offset these injuries some benefits
are to be credited to insects. Perhaps the most prominent benefit is
the destruction of injurious insects by certain others which thus become
beneficial. Some insects themselves are made use of by man, examples
being blister beetles, used in medicine, and the cochineal insect, which
is a source of coloring matter. The products of insect work, such as
the silk of the silkworm and the honey and wax of bees, may also be
utilized. Many insects are extremely useful in bringing about the fertili-
zation of fruits, vegetables, and flowers. A few insects are used by uncivi-
lized races as food. A very curious use of ants by the Indians in South
America, described by Bruner, is in a sort of surgical procedure where they
serve in place of sutures in holding the margins of wounds together. The
ants used are large and have powerful jaws. Bringing the margins of the
wound together with one hand, the “surgeon” holds the ant with the other
and permits it to bite through the two margins. When the jaws are
locked, the body is torn from the head and the locked jaws remain in posi-
tion until healing has closed the wound. Then they are removed. In
this country and in Europe, fly maggots have recently been made use of in
surgery. Introduced into wounds in which there is a considerable amount
of dead and decaying tissue, they remove this, thus paving the way for the
repair of the wounds by the regeneration of healthy tissue.

One relation of insects, which is important in the preservation of the
normal checks and balances which regulate all animal life, is that they
form a very large part of the food of other animals, particularly birds.
Their abundance under normal conditions provides an adequate food sup-
ply for such animals, many of which themselves are of great value. Some
of these economic relationships will appear in the discussions which follow.

319. Injurious Types.—Some of the more common and familiar of the
injurious insect types may be briefly reviewed. The food of the termites
(Fig. 192) consists of dead wood; they sometimes attack piled lumber, and
in the tropics they are a scourge because of their attacks on dwellings,
furniture, and all other articles made of wood. In the Eastern states their
ravages, however, are chiefly observed in the tunneling of dead logs in
forests. Termites shun the light except when a new brood of winged
individuals swarms, mating occurs, and new colonies are established. If
lumber is removed from contact with the ground and placed upon cement
blocks or stones, making it necessary for the termites to come to the light
to reach it, the wood will not be attacked, as it would be if placed upon

288 METAZOAN PHYLA

wooden blocks or directly upon the ground. It has long been a source
of speculation as to how these insects can digest the cellulose of the
wood, which few other animals are able to do. It has recently been
discovered that it is because of the presence of symbiotic intestinal
Protozoa which change the cellulose so that the termites can use it.
In the absence of this protozoan the termite is helpless and dies, and the
protozoan seems not to flourish outside the alimentary canal of the
termite.

Being vegetable feeders and likely to attack growing crops, locusts
have affected man since the very beginning of agriculture. Migrations

Fig. 192.—A termite, Reticulotermes flavipes Kollar. A, winged male. B, larva.
C, dealated queen. D, soldier. , queen of a tropical African species. (A to D, from
Bruner, ‘‘ Study of Entomology,” after Riley; E, from Kellogg, ‘‘ American Insects,” after
Nassonow, by the courtesy of Henry Holt & Company.) A to D, X 4; E, natural size.

of locusts from arid regions into those under cultivation have frequently
been recorded in history, accompanied by marked economic results, in
some cases even causing large numbers of people to move from the region
thus affected. Locusts belong to the order Orthoptera (6r thdp’ tér a;
G., orthos, straight, and pteron, wing), which also includes cockroaches,
mantids, walking sticks, katydids, and crickets.

Many different types of insects are known as lice. What are called
biting lice are found on birds and mammals. They do not bite their
hosts but eat feathers, hairs, and epidermal scales. Their presence
on the animal, however, occasions irritation, and it is to allay this
irritation that birds are in the habit of dusting themselves. The sucking
lice possess probosces which can puncture the skin of their hosts, which
are birds and mammals. They attack domestic poultry but come
upon them only at night, hiding in crevices in the poultry houses during
the day.

CLASS INSECTA 289

The term bug in its proper sense is applied only to the Hemiptera
(hé mip’ tér a; G., hemi, half, and pteron, wing). Among terrestrial
bugs are bedbugs, chinch bugs, squash bugs, and a variety of others,
which are all injurious for one reason or another. Chinch bugs, which
have been very destructive to small-grain crops in the Mississippi valley,
have not been successfully controlled by means of a contagious disease

Fig. 193.—The corn-root aphis, Anuraphis maidiradicis (Forbes), the eggs and larvae
of which are cared for by the brown ant, Lasius niger var. americanus (Emery). A, the
winged form. 8B, the wingless form. Both much enlarged. C, diagram to illustrate the
care of the adults by the ants during the winter and their placing them on the roots of
the young corn plants in the spring. (From Davis, Farmers’ Bull. 891, U. S. Dept. Agr.)

spread among them by releasing in the fields artificially infected bugs.
Aquatic forms include the water boatmen, back swimmers, and water
striders, which skate about on the surface of water upheld by the surface
film.

The order Homoptera (hd mop’ tér 4; G., homopieros, having similar
wings) is related to Hemiptera. In it are the plant lice, or aphids. They
are small but they exist in enormous numbers, since many partheno-

290 METAZOAN PHYLA

genetic female generations occur during the summer. In the fall males
are produced and fertilized eggs are laid which hatch out in the spring,
starting the first of another series of parthenogenetic female generations.
Most plant lice (Fig. 193) are wingless, though there are winged females.
They are often very injurious to plants, puncturing the leaf or stem and
sucking the sap. Many of these excrete a sweet substance known as
honeydew which is eaten by ants and other insects. The bodies of others
are covered with a white, cotton-like substance, also an excretion.

In the order Homoptera belong also the cicadas or harvest flies.
Among these are the longest-lived of insects, the seventeen-year cicada
living as a nymph for seventeen years before emerging as an adult.
These insects lay their eggs in slits made by the female in the twigs of

Fic. 194.— Work of an engraver, or scolytid, beetle, in a box elder limb. The female
beetle bores through the bark and then makes tunnels between the bark and wood. She
deposits eggs from time to time and the larvae hatched from them begin lateral tunnels
which increase in size as the larvae grow. When fully grown the larvae change to pupae
at the end of the burrows they have formed and when the beetles emerge they eat through
the bark and make their escape. In the specimen from which the figure was drawn is
evidence that when the female encountered a knot, she backed up and started a tunnel at
an angle which caused her to avoid the obstacle. Natural size.

trees. When hatched the nymphs drop to the ground, burrow into it,
and live in chambers on the roots, feeding upon the sap.

The order Coleoptera (k6l @ 6p’ tér a; G., koleopteros, sheath-winged),
which includes the beetles, contains many destructive forms. Among
these are borers in the trunks and limbs of trees (Fig. 194) and stems of
other plants; and the leaf beetles, which destroy foliage. The potato
beetle is a leaf beetle. Weevils attack seeds and fruits, carpet beetles
injure rugs and carpets, and wireworms damage the roots of plants.
Both the larvae and imagos of beetles cause damage.

The order Lepidoptera (lép i dép’ tér 4; G., lepidos, scale, and pteron,
wing) includes butterflies and moths, which are destructive only in the
larval stage. The larvae of some are borers in the stems of plants and
those of others eat the foliage. Among those affecting trees are the larvae
of the tussock and gypsy moths, the webworm, and the cankerworm.
The army worm, the cotton worm, and the corn bollworm injure field
crops. The codling moth attacks the fruit of the apple (Fig. 195). The
clothes moth is injurious to clothing and other articles made of wool, to
furs, and to feathers, and the grain moth destroys stored grain. Most

CLASS INSECTA 291

WN) \ Wh \
i Ny }
tl

i it
AME
vi ,

\
Mii,

a im

eh
ait OY aL

VOL HPL
{)

Fig. 195.—Codling moth, Carpocapsa pomonella Linnaeus. (From Farmers’ Bull. 171,
by Simpson, and 283, by Scott and Quaintance, U. S. Dept. Agr.) A, larva, working in an
apple. B, pupa. C, pupa in cocoon. D, adult. A, B, and D, X 14;C, x. The
moth lays eggs in the eyes at the blossom ends of young apples and the larvae which hatch
from them burrow into the apple, where they pass the larval period. After the apple falls,
the larva leaves it, seeks a sheltered crevice, such as a crack in the bark, or under an object
on the ground, and forms a silken cocoon, within which it changes to a pupa.

Fic. 196.—Plant galls due to insects. A, galls on oak caused by a gall wasp, Dryo-
phanta tanata Gill. B, a gall on a goldenrod stem caused by a gall fly, Zurosta solidaginis
Fitch. C, a gall developed on the end of a willow shoot and caused by a gall fly, Rhabdo-
phaga strobiloides Walsh. D, a so-called oak apple, caused by a gall wasp, Amphibolips
confluens Harris. E, a blackberry gall, caused by a gall wasp, Diastrophus nebulosus Osten
Sacken. F, a rose gall caused by a gall fly, Rhodites rosae (Linnaeus). All about two-
thirds natural size. (A to E from Metcalf and Flint, ‘‘Fundamentals of Insect Life,’’ after
Felt, by the courtesy of McGraw-Hill Book Company, Inc.; F from Comstock, ‘‘ Manual of the
Study of Insects,” by the courtesy of Comstock Publishing Company.)

292 METAZOAN PHYLA

of the moths, and some of the butterflies, form cocoons as protective
coverings for the pupae, and the same is true of some other insects,
including beetles. A cocoon may be constructed by the cementing
together by the larva of the hairs of its own body or objects of various
kinds, including grains of sand or bits of vegetation. It may also be
made of silk spun by the larva from a secretion formed by silk glands.
The old larval skin is used by some insects as a pupa case and is known
as a puparium.

Common or Culex Malarial or Anopheles Yellow Fever or Aedes

Fie. 197.—Comparison of three important kinds of mosquitoes, Culex, Anopheles
(Sec. 115), and Aedes, showing eggs, larvae, pupae, and adults. ™& 2or3. (From Metcalf
and Flint, ‘Destructive and Useful Insects,’’ after Pieper and Beauchamp, by the courtesy of
Scott, Foresman and Company-) Aedes transmits yellow fever. The horizontal lines
indicate water level; the stages of each type are in a vertical column.

The gall wasps, belonging to the order Hymenoptera (hi mén 6p’ tér a;
G., hymenopteros, membrane-winged), and the gallflies, belonging to
the Diptera, are small and rarely noticed, but the results of their work are
frequently conspicuous (Fig. 196). They possess an ovipositor with

CLASS INSECTA 293

which they pierce plant tissues and deposit eggs. Either poison injected
at the time of the laying of the egg or irritation arising from the growth
of the larva causes a hypertrophy, or overgrowth, of the plant tissues
and results in the production of conspicuous swellings on the stems or
leaves. ‘These growths are galls. Galls are also produced in other ways,
and the study dealing with them is a subject by itself known as cecidiology.
The Diptera (dip’ tér a; G., dipteros, two-winged), or flies, have only
the forewings developed and functional. Among them are the mos-
quitoes, which are annoying because of their bites but which are more
important because of the part they play in the transmission of diseases.
Mosquitoes (Fig. 197) lay their eggs on the surface of the water, the larvae
feeding upon organic matter contained in it. In this way they help
to clear up the water of stagnant pools. The larvae, or wrigglers, have
an air tube at the posterior end
of the body through which they
can obtain air for breathing. At
frequent intervals they wriggle
their way to the surface and thrust
this tube through the surface film
to take in oxygen and throw off
carbon dioxide. The oxygen lasts Fic. 198.—House fly, Musca domestica
them for a time, but its exhaustion Linnaeus. A, larva. B, pupa. C, adult.
makes another trip to the top ae ee ai ARE ZEIT oe
necessary. The pupa spends much
more of its time at the surface, breathing through two tubes at
the anterior end. When the adult emerges it rests upon the
surface of the water until its wings are expanded and dried, and then it
flies away. Only the females, which may be recognized by their simple
antennae, suck blood. The males, which have feathered antennae, are
not known to feed at all. In the absence of blood the females make use
of nectar or juices of plants, and it is possible that the males may at times
use the same food. The larvae of mosquitoes may be destroyed by
coating the surface of the water with oil, which gets into the breathing
tubes and kills them by suffocation. The numbers of mosquitoes may
also be reduced by draining off water standing in barrels, cans, or other
receptacles, which offers opportunity for the development of the larvae.
The house fly (Fig. 198), which also belongs in the Diptera, is an
abundant insect and exceedingly dangerous, since it carries disease germs
upon its feet and its habit of alighting upon moist masses of all kinds
results in its transferring them to human food. Germs are also deposited
on food by defecation and regurgitation. It is thus known to transmit
typhoid fever and tuberculosis. The larva feeds in manure and other
filth. The insect may be controlled by covering the receptacles for such
material and by the use of poisons and traps. It should be kept out of
houses and away from food by the use of screens.

294 METAZOAN PHYLA

Fleas make up another insect order. Since they are suctorial and
attack man, they are also transmitters of human diseases. The rat flea
seems to be the agent in the transmission of bubonic plague from rats to
man.

320. Combating Injurious Insects.—Injurious insects may be com-
bated in many ways. Mandibulate insects can be destroyed by placing
poisoned food where they will get it or by spraying poisons in solution on
the leaves or fruit of plants which they eat. This method can be applied
in the case of cockroaches, crickets, earwigs, ants, beetles, and lepidop-
terous larvae. Suctorial insects may be sprayed with oily substances
which close the spiracles and cause suffocation, or if they are in a room
where they can be reached with poisonous gases, they may be destroyed
by fumigation. Insect powders usually produce their effects by being

Fig. 199.—Australian lady-beetle, Rodolia cardinalis Mulsant, and the scale insect on
which it feeds, Icerya purchasi Maskell. A, the adult beetle. B, the pupa. C, larvae
feeding on the scale insect. D, atwig of range, showing the scale lice, and larvae and adults
of the beetle. Ato C, X 4; D, natural size. (From Marlatt, Year-book, U. S. Dept. Agr.,
for 1896.)

breathed into the tracheal tubes. In some cases insect pests may be
secured in numbers by jarring the plants and collecting them in appro-
priate hoppers, after which they may be killed in the manner most
convenient. Insects may be trapped in a number of ways and may be
combated by the spreading of infectious diseases among them. One most
important factor in insect control, however, is their destruction by birds,
and to this end insectivorous birds should be zealously protected.

321. Beneficial Insects.—Both the larvae and the adults of the order
Odonata (6 do na’ ta; G., odontos, tooth), which includes the dragon flies
and damsel flies, are carnivorous, the adults being beneficial because of
their destruction of mosquitoes, which form a large part of their food.

Some beetles are beneficial because they attack injurious insects.
Among these are the tiger beetles and ground beetles, which live upon the
ground and destroy cutworms and other noxious insects which they can

CLASS INSECTA 295

capture. Ladybirds, or lady beetles, are very beneficial because of their
destruction of plant lice and scale insects, both the larvae and adults
being eaten by them. The orange growers of California, when made
desperate by the ravages of a cottony scale insect which threatened ruin
to their orange groves, were able to overcome the pest by the help of
ladybirds imported from Australia (Fig. 199). Here also are to be con-
sidered the burying and the carrion beetles, which bury or destroy dead
animals and organic matter which might otherwise prove offensive.

The order Neuroptera (ni rép’ tér a; G., newron, nerve, and pteron,
wing) includes the lace-winged flies, sometimes called aphis lions because
the larvae, as well as the adults, feed on plant lice. Their eggs are laid

Fic. 200.2Ant lion, Myrmeleon sp. A, larva, X 2. B, pit, with concealed larva,
x 244. C, adult, slightly enlarged. (A and C from Kellogg, ‘‘ American Insects,’ by the
courtesy of Henry Holt & Company; B, original.)
on plants and mounted on a long stalk, which insures immunity from the
attacks of enemies. The order also includes the ant lions, the larvae of
which live in the ground, concealed at the bottom of conical pits (Fig. 200).
They feed upon insects which when running over the ground fall into
these pits. The victim slides to the bottom of the pit, where it is seized
by the ant-lion larva and sucked dry, after which the body is thrown clear
of the pit by a jerk of the head of the captor. The pits are excavated
by the throwing out of the grains of earth in a similar fashion and natur-
ally are found only in loose, sandy soil. They are also placed under
overhanging ledges of rock or under vegetation where they are protected
from rain. Frequently many occur within a small area. The pits
increase in size with the growth of the larva until they reach a diameter
of from two to three inches.

296 METAZOAN PHYLA

Some Lepidoptera are useful because their visits to flowers result in
cross-pollination. Chalcid flies and ichneumon flies, which belong to
the Hymenoptera, are beneficial because they attack the developmental
stages or the adults of many injurious insects. The egg is laid on or in
the host the soft tissues of which the developing larva destroys. The
larva of one ichneumon fly attacks the borers in shade trees, and the fly
is often seen laying its eggs in the trunks and larger limbs of such trees
(Fig. 201). It is 144 inches long and has an ovipositor 6 inches long,
with which it drills a hole through the wood into the burrow of the boring
larva where it deposits an egg. Many seeing this insect at work hold
it responsible for the injury to the tree
which is caused by the concealed borer.

322. Social Insects.—Social
instincts are exhibited by a consider-
able number of insects, especially by
termites and some beetles. Some
lepidopterous larvae make communal
webs. In the Hymenoptera, however,
and especially among the ants, bees,
and wasps, the most striking instances
of insect societies occur.

Among social insects the honeybee
(Fig. 202) stands out not only as a
type but also as that invertebrate

ere a aS ae Sa which has been most intimately
iM cauriianc Funater (Bapncia)s ae associated with mankind and has
positing its eggs in the tunnels of reached the highest degree of domes-
mee: ae Asa ee ap ne tication. For ages before man learned
size. (From Graham, ‘‘Principles of how to manufacture sugar he depended
A eae ee for his sweets very largely upon the
The figure shows the characteristic bees, and honey was an important
attitude of the insect when drilling. elemental annette

A swarm, or society, of honeybees includes three distinct types—
workers, drones, and queen. In a swarm of 60,000 individuals there are
perhaps 200 drones and but one queen, all the rest being workers. Of
the three types the queen is the largest, is distinguished by the greater
length of her abdomen, and lives for many years. She lays all of the
eggs from which are produced the rest of the members of the swarm.
The workers are infertile females which do not normally lay eggs but
perform all the labor of the society. They live for only a few weeks.
The drones are the males and are produced from unfertilized eggs. They
perform no service in the hive but exist only for the purpose of fertilizing
the eggs. They are relatively broader than either queens or workers,
intermediate in size between them, and may be recognized by their very

CLASS INSECTA 297

large eyes. The activities of the workers comprise the gathering of
nectar from flowers, the manufacture of honey from this nectar, the
collecting of pollen and gum, the building of the comb, the care of the
young, the cleaning and ventilating of the hive, and the guarding of it
from enemies.

Me =
35 ene c

} Wi... 5
2= GA Ee

Yigg
Fig. 202.—The honeybee, Apis mellifica Linnaeus. A, worker. B, queen. C, drone
D, portion of comb showing queen cells. H,egg. F,younglarva. G,oldlarva. 4H, pupa.
A to C, somewhat enlarged; D, natural size; EH to H, much enlarged. (From Phillips,
Farmers’ Bull. 447, U. S. Dept. Agr.) In D a part of the cells are capped.

The pollen is gathered from flowers and is used as food by all members
of the society, but particularly by the growing larvae. In the form in
which it is fed it is known as beebread. For the first three days of their
lives all of the larvae are fed the same kind of food, which is a bee milk
composed of digested honey and digested pollen. After this time the

298 METAZOAN PHYLA

food is different for the different types of larvae. Those which will
develop into workers are given by the nurse bees undigested honey
mixed with digested pollen, and those which will develop into drones are
fed undigested honey and undigested pollen. The queen larvae, however,
are fed upon a rich albuminous bee milk composed of digested honey
and digested pollen, mixed with a glandular secretion, the whole being
known as royal jelly.

The number of bees in a hive increases rapidly during the spring,
since the queen lays from 1000 to 1200 eggs per day and new workers
are continually being produced. A few new queens are also developed
and a small number of drones. When the number in the hive becomes
too great for its capacity, swarming occurs, as a result of which a new
society is started elsewhere. If the present swarm is too weak to allow
swarming with safety, as rapidly as new queens develop they and the
old queen are permitted to have access to one another, whereupon one
stings the other to death, since a queen will tolerate no rival in her hive.
If, however, the swarm is strong enough, then the worker bees keep the
two queens apart and the old queen with a portion of the swarm leaves
to establish a new swarm elsewhere, while the new queen becomes the
queen of the parent swarm. The new queen soon leaves the hive on her
marriage flight, during which she mates with a drone. She then returns
to the hive, not again to leave it until, perchance, she is in turn forced
to lead a new swarm away, resigning her place to a still younger queen.
During her marriage flight she receives into the seminal receptacle of
her body all of the sperm cells of which she will make use as long as she
lives in fertilizing the eggs she lays. A strong swarm will produce many
queens; Comstock states that ‘‘one morning we found the lifeless bodies
of 15 young queens cast forth from a single hive—a monument to the
powers of the surviving Amazon in triumphant possession within.” At
the close of the season the drones are killed off or driven out by the
workers to die, and no new drones are produced until the following spring.
If at any time the swarm is without a queen, the workers are able, by
proper care and feeding, to develop a new queen from an egg or young
larva, which, in the ordinary course of events, would have produced a
worker.

Among wild bees are found a great many different kinds, showing
every gradation between those which live a solitary life and those types
which approach the honeybee in the degree of specialization and the
variety of activities carried on. Among bees which lead a solitary life
there is no distinction between queen and worker, since the queen herself
provisions the cells in which she lays her eggs. In the case of bumblebees
only the females live through the winter, and in the spring each queen
bumblebee has to select a place for her nest, lay her eggs, feed the young,
and thus develop a society before she can assume the prerogatives of a

CLASS INSECTA 299

queen and be cared for by the workers. In some bumblebee societies
there are workers of different sizes, as well as queens and drones. The

Fic. 203.—Polymorphism as illustrated in an ant, Pheidole instabilis Emery. a,
soldier. 6 to e, intermediate types between soldier and worker. f, typical worker. 9;
dealated female. h, male. Much enlarged. (From Wheeler, ‘‘ Ants,” by the courtesy of
Columbia University Press.)

bumblebee performs a very important service in cross-fertilizing the red
clover, which in many parts of this country is an important hay crop but

300 METAZOAN PHYLA

which will not develop fertile seeds unless there are bumblebees in the
region to carry the pollen from one flower to another.

Wasps and hornets may be distinguished from bees by the possession
of jaws instead of suctorial probosces, by the body being smooth and not
hairy, and by the legs being never modified for the carrying of pollen.
They provision their cells with other animals which they collect and
paralyze by stinging before placing them in the cells. Sometimes these
cells are provisioned and then sealed, while in other cases the young are
fed by the parents as in the case of the bees. An example of the latter
are the bald-faced hornets that build paper nests from particles of
weathered wood (Fig. 204). The solitary wasps mine in the earth,

Fic. 204.—Large paper nest, 10 by 8 inches, of the bald-faced hornet, Vespa maculata,
made from weathered particles of wood mixed with saliva. There is a legend that man
gained the idea of making paper out of wood pulp from observations of these nests. Nest
collected in Vermont.

excavate cavities in wood, or build mud nests. They take no care of
their young. The wasps which build the mud nests of many cells, which
are familiar in most parts of this country, provision these cells either
with the caterpillars of butterflies or moths or with spiders.

Another insect type which lives in societies and shows specialization
and polymorphism is the ant (Fig. 203). There are many species of
ants, and they form what has been recognized as a dominant type,
dominance being shown in the following ways: (1) by the vast number
of individuals which exist; (2) by the variety of structures they display,
there being a very large number of species; (3) by their wide distribu-
tion, which includes the entire globe; (4) by their longevity; and (5)
by their manifold relationships to plants and to other animals. Though
the males live only for several months, the lives of the workers may

CLASS INSECTA 301

cover a span of five or six years, and the queens may survive for as many
as fifteen years. Among ants 27 different types of individuals have been
recognized, including 7 types of males, an equal number of types of fertile
females, and 13 types of infertile females or workers. Not all of these
exist in the same society, but many of them may. Among the workers
are food gatherers, nurses, soldiers, and other types. This division of
labor is accompanied by great differences in size and structure, fitting
the different types of workers for the particular duties they have to
perform.

CHAPTER XLVI

CLASS ARACHNIDA

The spiders and allied forms make up the fifth class of Arthropoda,
known as Arachnida (a rik’ ni da; G., arachne, spider, and ezdos, form).
They are distinguished from most of the preceding classes by the fact
that the head and thorax are grown together forming a cephalothorax.
They have four pairs of walking legs, no antennae, and no true mandibles.

323. External Structure of Spiders.—Spiders have a compact body
and a large and more or less globular abdomen, without any trace of

Opening to book lung
Opening fo reproauctive organs
Abdomen

Fic. 205.—Under side of a spider, Araneus sericatus Clerck. Enlarged. (From
Linville, Kelly, and Van Cleave, ‘‘ Text-book in General Zoology,” after Emerton, by permission
of the authors.)

metamerism. The abdomen is separated from the cephalothorax by a deep
constriction which leaves a slender peduncle connecting the two.

The mouth parts consist of a pair of jaws, known as chelicerae, and a
pair of pedipalpi. Each chelicera bears a terminal claw, at the tip of which
opens the duct from a poison gland (Fig. 206). The pedipalp7 are leglike in
appearance but their function is rather that of palpi (Fig. 205). Dorsally
on the front of the head are the simple eyes, of which most spiders have
four pairs, though there may be only one, two, or three pairs (Fig. 206).

302

CLASS ARACHNIDA 303

Certain cave spiders are blind. The legs often have pads of hairs between
their terminal claws which enable the animal to cling to walls and ceilings.
At the posterior end of the abdomen are the spinning organs, con-
sisting of two or three pairs of spznnerets (Fig. 205), which are finger-like
appendages, sometimes jointed. At the tip of each of these spinnerets
open many small spinning tubes, from which
the silk is spun. The silk is secreted in glands
within the body and passes out in a liquid
condition, hardening as soon as it comes in
contact with the air. ‘The anal opening lies
just posterior to these spinnerets, and just in
front of them may be a single spzracle. On
the ventral surface of the abdomen anteriorly
are three openings. In the median line is
the genital opening, protected in the female
by a plate known as the epigynum; and on BAe ECR AL ONT 1
each side, a slit placed transversely, which is head of Lycosa carolinensis
the opening into an air sac. The spiracle eee Te ed eure
may be farther forward than stated above,
and there may be two just behind the genital opening.
324. Internal Structures.—The alimentary canal includes a narrow
esophagus, a sucking stomach, a digestive stomach, and an intestine
Intestine with

EL IGP
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, A / ae With Ey} oes

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BOS a =
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Book /ung loinds Anus
; Chelicera gered ie Wrerers
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recepracle ‘a

Fic. 207.—Median section of a female spider, diagrammatic, to show internal structure.
(From Comstock, ‘‘ The Spider Book’’; copyright, 1912, by Doubleday, Doran & Company,
Inc.) The nervous system in the cephalothorax is stippled, as is also the reproductive
system in the abdomen. The heart is crosslined and the blood vessels in black; the poison
gland and duct are also black. The malpighian tubules are finely crosslined and the spin-
ning glands and ducts shaded. The alimentary canal and diverticula are white.

(Fig. 207). Malpighian tubules empty into the intestine near the
posterior end. The circulatory system consists of a heart, lying in a
pericardial cavity situated dorsally in the abdomen; and of arteries,

304 METAZOAN PHYLA

sinuses, and veins. In the tissues the blood flows through irregular
spaces which lack continuous walls; it is, therefore, not a closed system.

The respiratory organs are two air sacs from the anterior walls of
which arise from 15 to 20 leaflike folds supplied with blood capillaries.
These folds are sometimes called collectively a lung book, and the whole
apparatus is generally termed a book lung. There are tracheae in the
abdomen but the tracheal system is not extensive and does not play a
large part in respiration.

In addition to the malpighian tubules, which the spiders share with
insects and myriapods, they also possess in many cases two coxal glands
in the cephalothorax, which are homologous with the green glands of the
crayfish.

The nervous system of the spider consists of a bilobed ganglion above
the esophagus and a subesophageal ganglionic mass, with nerves running
to various parts of the body (Fig. 207). This represents a condensation
in the nervous system greater than that met with in any other type which
has been studied and goes along with a high degree of coordination of
activities.

325. Metabolism.—The food of spiders consists of juices from their
prey, which are drawn into the sucking stomach by the contraction of
muscles attached to the wall of the cephalothorax. When a spider feeds
upon an insect, the latter is well crushed between the bases of the che-
licerae and is then held against the mouth while the sucking process is
going on. From the sucking stomach the food passes onward through
the alimentary canal; it is acted upon by digestive fluids secreted by the
digestive stomach and also by a secretion from the so-called liver, which
is a large digestive gland surrounding the alimentary canal in the abdo-
men, the secretion of which resembles the pancreatic secretion of higher
forms. Elimination takes place through the malpighian tubules, the
coxal glands being generally small and frequently quite degenerate.

326. Reproduction.—The sexes of spiders are always separate, and
fertilization is internal. The male transfers the sperm cells from his
genital opening to the seminal receptacle of the female by means of his
pedipalpi, which in this sex are very large and modified in many and
curious ways characteristic of different species. The eggs are laid in a
silken cocoon which may be attached to some object or carried about by
the female (Fig. 208). The young leave the cocoon soon after hatching.
They undergo no metamorphosis.

327. Spinning Activities ——The characteristic feature in the life of
all spiders is the production of silk, which may be used in the construc-
tion of a hiding place or home for the spider, and which also serves in
the construction of a web for the capture of prey. Small silken threads
are used in spinning the silk of attachment discs which serve to fasten
larger threads in place, for making a swathe of silk to be thrown about the

CLASS ARACHNIDA 305

struggling prey, and sometimes in making a broad, wavy band across
the center of the web. Larger threads are used in the construction of the
web itself.

The spiders make use of two kinds of silk, one of which is dry and
inelastic, the other viscid and elastic. In the orb web, which may be
taken as a type, the framework is composed of threads made up of the
first kind of silk, while the spiral threads which pass around the web
from one radiating thread to the next are made of silk of the second kind.
If examined with a lens this viscid and elastic silk will be found to have
numerous beadlike masses of sticky material which help to hold the prey
when it touches the web. It is supposed that these two kinds of silk
are spun from different spinnerets.

Fic. 208.—Wolf spider, Lycosa helluo. A, female carrying a sac of eggs. B, a few
weeks later; the eggs have hatched and the spiderlings are being carried upon the back of
the mother. (Photographed and contributed by Edson H. Fichter.)

Silk is used not only to line a nest or form a web but also to fashion
the cocoon. Some spiders spin an anchor line by means of which they
may return to a certain point from which they have leaped or fallen.
Others make bridges of silk, spinning threads off into the air until they
become attached to some object on the farther side of a space; the lines
are then drawn taut by the spider. Still other spiders make use of silk
in the construction of balloons, spinning a loose mass of threads which
are sufficient to buoy the animal up and enable it to be carried along by
the wind. These aeronautic or flying spiders have been known to travel
many miles in this fashion.

328. Behavior.—Spiders can see but a very short distance, apparently
distinctly only within a radius of four or five inches. They do not seem
to use other senses, except the one of touch, but that sense is exceedingly
delicate, especially on the pedipalpi and on the terminal joints of the
legs. Spiders act largely from instinct but they form some habits and

306 METAZOAN PHYLA

have many activities which are as justly considered intelligent as are
those of many insects.

329. Economic Importance.—Spiders are of little economic impor-
tance other than the service which they render in the destruction of
injurious insects, but this is more or less offset by their destruction of
beneficial ones. Many spiders, particularly those of the tropics, are
feared as being poisonous. The larger ones might be able to inject by
their bite a sufficient amount of poison to cause marked effects, though
rarely, if ever, are they fatal to man. In this country the only spider
whose bite is serious is the greasy-looking black Latrodectus mactans, the
black widow, with a very large globular abdomen on the lower side

\ Pedi -
“\ palpus
pal
~ fe &

*, = Ppa

Chelicerae

Poison —¥ BN
claw a

SS ay re le Metasoma
Fria. 209.—A Tropical American scorpion, Centrurus sp. A, dorsal view of entire animal.
B, under side of body. Natural size.
of which are some yellow or reddish spots. It is usually found under
objects lying upon the ground but may also inhabit dark outbuildings.
330. Scorpions.—Another type of arachnid is the scorpion (Fig. 209),
the body of which is clearly metameric. It is divided into a prosoma or
cephalothorax; a mesosoma, which is made up of the broadened anterior
abdominal metameres; and a metasoma, which includes the slender
posterior metameres of the abdomen. The metasoma forms a tail which,
in an attitude of attack or defense, is carried above the rest of the body
so that the posterior end is directed forward. This terminates in a
sharply pointed, clawlike appendage, which is not a metamere, and at the
point of which opens the duct from a poison gland. ‘There are two
median simple eyes on the upper side of the cephalothorax and three
lateral ones on each side. The pedipalpi are very large and curiously like

CLASS ARACHNIDA 307

the chelipeds of the decapod crustacea. There are four pairs of lung
books opening by spiracles on the under surface of the abdominal meta-
meres from the third to the sixth.

Scorpions live mostly in tropical and subtropical regions and are
nocturnal in their habits. Their food consists of spiders and large

Fic. 210.—Mites. A, an itch mite, Sarcoptes scabiei DeGeer. Male from above.
Greatly enlarged. B, Texas cattle fever tick, Margaropus annulatus (Say). Male. ™X 16.
C, chigger, Trombicula trritans (Riley). Larva. X75. D, adult of chigger. Greatly
magnified. (A, C, and D from Ewing. ‘‘ Manual of External Parasites,’ by permission of
the publisher: Charles C. Thomas; C after Oudemans; B from Salmon and Stiles, ‘‘ Cattle Ticks
of the United States,’’ Bur. An. Indus., U. S. Dept. Agr.)

insects which they seize with the pincers of their palpi and sting to death.
The sting also serves as a weapon of defense. It is, however, impossible
for the scorpion to sting itself to death, as it has often been said to do.
The sting rarely, if ever, proves fatal to man.

331. Mites.—Another group of arachnids contains the mites (Fig.
210), which in turn include the ticks. These are very small arachnids

308 METAZOAN PHYLA

without external signs of metamerism and without division into cephalo-
thorax and abdomen. Mites breathe through tracheal tubes. The
cephalothoracic appendages are similar to those of spiders, although the
pedipalpi are not so large. The abdomen shows neither the slits into
the air sacs nor the spinnerets at the posterior end. Nevertheless, some
mites can spin silken threads from openings on the ventral side of the
abdomen, in some cases near the anus.

Among mites are many which are of decided economic importance.
The ticks are parasitic upon various animals and, since they pass from
one individual to another, are capable of transmitting diseases. In the
case of the cattle tick, which carries the organism causing Texas cattle
fever, the parasite is taken up by a parent mite and introduced into
another host by the bite of the young of that mite, transmission thus
involving two generations. This disease 1s said at one time to have

Fic. 211.—A pseudoscorpion. (From Comstock, ““The Spider Book’; copyright, 1912, by
the courtesy of Doubleday, Doran & Company, Inc.) X about 9.

caused an annual loss of $100,000,000, but its frequency has now been
reduced by preventive measures.

Other parasitic mites are the itch mites; the follicle mites, which live
in the sweat glands and hair follicles of man and some domestic animals;
and the scab mites, which produce scabies. The animals generally known
in this country as chiggers are the larvae of red velvet mites.

Mites that infest plants include the ‘‘red spider,” which attacks house
plants; many that produce galls; and some that cause diseases of leaves,
such as the one which causes the pear-leaf blister. Still other mites are
found in food products, such as cheese, sugar, and preserved meats.

332. Other Arachnids.—Among other arachnids are the well known
harvestmen, or daddy longlegs, which are like small-bodied and very
long-legged spiders, but which have no constriction between the thorax
and the metameric abdomen.

Another type is represented by the pseudoscorpions (Fig. 211).
These have pedipalpi much like those of the scorpions but are very much

CLASS ARACHNIDA 309

smaller and lack the long tail. They are found throughout this country,
but, being of small size, they do not often excite notice.

Still another type which belongs here is the king crab, or horseshoe
crab (Fig. 212), which is a marine animal found along the Atlantic coast
from Maine to Yucatan. It differs from other arachnids in that it lacks
malpighian tubules and possesses book gills. Book gills are similar to
book lungs but the leaves lie exposed on the ventral side of the abdomen,

Cephalothorax

Book
g! //s

Abdomen

Fic. 212.—King crab, Limulus Basil ais (UES) A, dorsal view. 8B, ventral.
and oxygen is taken from the water. The king crab, together with a
few mites, represents all of the arachnids which are marine. It is noc-
turnal, wandering along the shore in shallow water and feeding upon any
animal which it can overpower.

Relatives of king crabs and perhaps their ancestors are the trilobites
(Fig. 373). They were present in the earliest known fauna, that of the
Cambrian age, and reached their maximum of size, number, and variety
in the Silurian (Fig. 371). The largest were nearly two feet long. They
disappeared at the end of the Carboniferous, and related types resembling
modern crustaceans and king crabs appeared.

CHAPTER XLVII

ARTHROPODS IN GENERAL

From what has already been said in regard to the different groups
included in this phylum the general characteristics have become evident
and need only to be briefly reviewed.

333. Characteristics and Advances.—The animals belonging to
Arthropoda are metameric (Fig. 213), and all exhibit more or less of a
tendency for the different metameres to be grouped into three regions,
namely, head, thorax, and abdomen. In Onychophora and Myriapoda
the latter two are not evident; in the spiders and in many Crustacea the
head and thorax are combined into a cephalothorax; in insects, however,
the three regions are distinct. Another characteristic of the group is
that typically each metamere bears a pair of jointed appendages, though

pamenp ReOseerermn
Supraesophagea
A a i: see ed: Blood vessel Alimentary canal

ence mame ereeetaee es

By HN
Ch ae Pee eZ eB RHEE
Mouth 4 Sd) ~ .

POS RS

Subesop Bi
ae. pe

Nerve cord

Fig. 213.—Diagrammatic representation of the structure of an arthropod. (From Schmeil,
“‘ Text-book of Zoology,” by the courtesy of A. and C. Black, and Quelle and Meyer.)

in many groups a greater or less number of these appendages are absent.
The cuticula often becomes highly chitinized and in the Crustacea is
also hardened by the addition of lime salts. The nervous system resem-
bles that of the annelids in having metamerically arranged ganglia, but
in the higher forms it shows a pronounced tendency toward fusion of the
ganglia and to cephalization, or the localization of control in the head
region. The increase in the number and variety of appendages, which
makes for variety in action, as well as the higher development of both
central nervous system and of sense organs, is the chief advance shown
by the phylum. There is also a high degree of specialization in the ali-
mentary canal, which is divided into distinct regions and modified in
accordance with the character of the food.

334. Classification.—The classes of this phylum have been discussed
but may be systematically presented as follows:

310

ARTHROPODS IN GENERAL 311

1. Crustacea. Mostly aquatic, possessing gills, green glands, and two
pairs of antennae.

2. Onychophora. Primitive, annelid-like, with nephridia, tracheae,
and one pair of antennae.

3. Myriapoda. Body wormlike, either flattened dorsoventrally
(centipedes) or subcylindrical (millipedes) ; possess tracheae, Malpighian
tubules, and one pair of antennae.

4. Insecta. Body divided into head, thorax, and abdomen; have
tracheae, Malpighian tubules, three pairs of legs, and one pair of antennae.

5. Arachnida. Body usually divided into cephalothorax and abdo-
men; have no antennae and no mandibles; possess four pairs of walking
legs; respire by book gills, book lungs, or tracheae; have coxal glands and
Malpighian tubules.

335. Behavior.—The most striking thing in the behavior of arthro-
pods is the great development of instincts. These frequently become so
complex and adjust the animal so perfectly to the conditions of its exist-
ence that to most people they seem to imply the exercise of intelligence.
In addition to instinct most zoologists recognize a primitive form of
intelligence in the higher insects and in the spiders. However, the readi-
ness with which some higher crustaceans modify their behavior would
seem to indicate that if intelligence is attributed to the two groups men-
tioned, it should also be recognized in them.

CHAPTER XLVIII

PHYLUM CHORDATA

The last of the phyla, and from the standpoint of efficiency the
highest, is Chordata (kér da’ ta; G., chorde, cord, referring to the noto-
chord). In addition to the vertebrates, which are, generally speaking,
the largest, most conspicuous, and best known animals, the phylum
includes several forms of which only zoologists are aware and which,
therefore, have no common name. Some of these lower chordates
are very unlike the vertebrates and reveal their chordate character
only after close study.

336. Characteristics —The chordates possess three ‘characteristics
which set them apart from all other animals, these being the possession
of (1) a notochord, (2) pharyngeal slits, and (3) a tubular nervous
system dorsally situated in the body. Of these structures the notochord
and the pharyngeal slits remain throughout life in only the lower chor-
dates, being in the higher ones replaced by other structures. The
nervous system, however, persists with the same general character
throughout, in the highest chordates reaching a very high degree of
development and functional activity.

The notochord first appears, both ontogenetically and phylogenetically,
as a rod of cells. It is derived from a longitudinal outfolding of the
dorsal wall of the archenteron which becomes pinched off and lies at
the dorsal side of the alimentary canal (Fig. 214). It is, therefore,
entodermal. In somewhat higher types it becomes converted into a
more rigid, though still flexible, rod which runs the length of the body,
serving to stiffen it. In the vertebrates the notochord develops in
the embryo, but in the adult there is formed about it a series of bones
which together make up a vertebral column. This is the axis of an
internal skeleton, to which is added a large number of other bones
providing support and giving attachment to muscles. As the vertebral
column becomes more highly developed the notochord practically
disappears. Flexibility in the vertebral column is secured either by
articulation of the separate bones, or vertebrae, or by the compression
of dises of fibrocartilage between them.

The pharyngeal slits are a series of passages on each side of the body
toward the anterior end leading from the cavity of the pharynx to the
outer surface of the body. They are found in all chordates except in the
higher vertebrates. In their walls are developed networks of blood

312

PHYLUM CHORDATA 313

vessels making it possible for respiration to take place. In the higher
vertebrates, however, these slits, which start to develop during embryonic
life, either become closed or never are open, and the respiratory func-
tion is assumed by lungs. The latter are developed as ventral out-
pocketings of the pharynx somewhat posterior to the pharyngeal slits.
Because of their formation from evaginations of the wall of the pharynx
which unite with invaginations of the body wall, the pharyngeal slits are
lined with entoderm internally and ectoderm externally.

The dorsal tubular nervous system develops from a strip of ectodermal
cells lying in the median line on the dorsal side of the body. This strip
of cells sinks inward, forms a groove, and then, by the meeting of the
side walls above, becomes a tube,

Medullary
called the medullary tube (Fig. 214). tube
This development of acentral nerv- ../,,, =
ous system by invagination is pecul- Sag.ct Eee ei

iar to the chordates. Typically
this tube, when first formed in the
embryo, opens at the anterior end
to the outside and at the posterior
end turns ventrally around the end
of the notochord to become con-

Splanchnic
mesoderm ~te

Alimentary

tinuous with the posterior end of
the archenteron. Later the open-
ing to the outside is closed and
the connection with the archenteron
is severed, thereby producing a
closed neural tube the cavity of
which is known as the neurocoel.

canal

Fic. 214.—Somewhat diagrammatic rep-
resentation of a section through the body of
an amphioxus larva at a stage later than any
shown in Fig. 51, with which it may be
compared. The ectoderm is white, the
entoderm lined, and the mesoderm stippled.

In the vertebrates the anterior end

of this neural tube becomes dilated and forms the brain, while the
remaining part becomes inclosed in the vertebral, or spinal, column and
forms the spinal cord.

Another characteristic of the chordates is the gradual obscuring
of the metameric condition in the body wall by the fusion of the meta-
meres and the shifting of superficial muscles. Throughout the phylum,
however, the deeper structures remain metamerically arranged.

337. Advances Shown by the Chordates.—In the characteristics just
enumerated the chordates show advances over all lower phyla, and these
advances are the foundation upon which their supremacy in the animal
kingdom rests. An internal skeleton does not give so great a leverage
to muscles as does the exoskeleton of the arthropods, but this mechanical
disadvantage is more than balanced by the freedom of movement which
is allowed. An organism incased in an exoskeleton can never be so
mobile, and on the whole can never become so efficient, as can one the
body and appendages of which possess an internal skeleton. The

314 METAZOAN PHYLA

pharyngeal slits offer a mode of respiration more effective than that
allowed by any type of respiratory organ possessed by lower forms
because the slits are interposed directly in the path of the circulation
and all of the blood in the body passes through them. In tracing the
development of the earlier phyla it has been evident that their nervous
systems have advanced in the degree to which they have been centralized,
thus bringing about more effective coordination. The development of
a continuous tubular nervous system makes possible the most intimate
association of ganglionic masses and thus the greatest degree of interaction
between them. A continuous tubular nervous system also offers more
surface for the escape of fibers than do separate metamerically arranged
ganglia, thus permitting an increase in the number of nerve cells. The
obscuring of the metamerism is in the interest of unified action of
the body. The arthropods develop the possibilities of metamerism to the
maximum degree. The chordates retain the advantages of the metameric
plan but relinquish it to the degree desirable to secure the greatest
unity in the operation of the body as a whole. Other advances possessed
by the vertebrates but not shared by the lower chordates remain to be
discussed later.

338. Classification.—The chordates are usually separated into
four subphyla, as follows:

1. Hemichordata (hém i kor da’ ta; G., hemz, half, and chorde, cord),
or Adelochorda (id @ 16 kér’ da; G., adelos, concealed, and chorde, cord).
Includes two or three types of marine wormlike animals, which are
all small, some very minute.

2. Urochordata (a rd kor da’ ta; G., oura, tail, and chorde, cord).
Includes the tunicates, also marine, which illustrate extreme degener-
ation and, in a sense, retrogression.

3. Cephalochordata (séf 416 kér da’ ta; G., kephale, head, and chorde,
cord).—Includes a marine type known as the amphioxus which has a
somewhat fishlike form.

4. Vertebrata (vér té bra’ ta; L., vertebra, a joint).—Includes fish,
amphibia, reptiles, birds, and mammals.

CHAPTER XLIX
LOWER CHORDATES

The chordates belonging to the first three subphyla represent three
very diverse types. ‘The hemichordates were for a long time considered
worms and the tunicates were formerly put in a phylum by themselves

Proboscis

Se he ae

Fia. 215.—Dolichoglossus kowalevskit (A. Agassiz), found on the Atlantic coast. An
adult male. X 2. (From Bateson, Quart. Jour. Mic., Sci., n. s., vol. 25.) Dolichoglossus
is by many considered only a subgenus of Balanoglossus.

and included among the invertebrates. The resemblance of the amphi-
oxus to the vertebrates has, however, long been recognized.

339. Hemichordata.—A member of the order Balanoglossida may be
taken as a type of this subphylum (Fig. 215). These are wormlike
animals which burrow in muddy areas along the seashore, passing the
mud through their bodies and taking out the organic matter in the same
fashion as the earthworm takes organic matter from the soil. Different
species range in length from an inch to 4 feet, and some of them are
brightly colored. The body consists of three portions, an anterior
proboscis, a ringlike collar, and a metameric trunk similar in many ways
to the body of an annelid. The mouth opening is ventral and just in

315

316 METAZOAN PHYLA

front of the collar. There is a dorsal pouch at the anterior end of the
alimentary canal which originates like a notochord as a dorsal outpocket-
ing of the archenteron and which runs forward into the proboscis. Its

Ee pical sense organ

Pore entering.
proboscis cavity

Oo Apa CHiared
ring

Fic. 216.—Tornaria larva, viewed from the side. (From Thomson, ‘‘ Outlines of Zoology,”
after Spengel.)

function is to stiffen the proboscis, which is an aid in burrowing. The
claims of Balanoglossida to inclusion in the phylum Chordata rest upon
the fact that this structure is consid-
ered a notochord, that the animals
ie eee possess a large number of pharyngeal
urine! . ; :
slits, and that in the region of the collar
a portion of the dorsal nerve cord seems
to be definitely tubular. The dorsal
nerve cord is also formed by a process
of invagination. This type is mone-
cious. The larva, called a tornaria
(Fig. 216), is free-swimming and resem-
bles the larvae of the echinoderms.
340. Urochordata.—This subphylum
includes the tunicates, which may be
either free-swimming or fixed forms.
The typical tunicates, also called ascidi-
ans, are fixed forms in the adult stage
(Fig. 217) and are inclosed in a tunic
Fic. 217-—Pyura aurantium composed of animal cellulose the com-
(Pallas), a sessile ascidian from position of which is very similar to that
Labrador. xX 24. (From VanName, 2
Proc. Boston Soc. Nat. Hist., vol. 34.) of plant cellulose. It is the presence
of this tunic which suggests the name
tunicate. When one of these animals is stimulated, the body is strongly
contracted and the water is thrown out through the mouth with consider-

Oral furne!l

LOWER CHORDATES 317

able force. This reaction suggests another name, sea squirts, that has
been applied to such animals.

Inside the body wall of one of these ascidians and surrounding the
large pharynx proper is an atrial cavity (Fig. 218). The body always has
two openings, one known as the oral funnel, which is the mouth opening,
and the other as the atrial funnel, or atriopore, which opens into the atrial
cavity. Water enters the oral funnel, passes into a pharynx in the walls
of which are numerous pharyngeal slits, through these into the atrial
cavity, and out through the atriopore. As the water passes through
the pharyngeal slits, respiration occurs and food is strained out. On
the side of the pharynx which corresponds to the ventral surface is a
ciliated groove called the endostyle. A sticky mucous secretion produced

<<
fsophagus

Fig. 218.—Anatomy of a typical ascidian. (From Newman, “ Vertebrate Zoology,” after
Hertwig.)

in this endostyle is continually being passed onward to the intestine and
serves to convey food particles to that portion of the alimentary canal,
where digestion and absorption take place. The anus is situated near the
atriopore and the water which passes out through the atriopore carries
with it the feces. The heart is a pulsating tube, lying ventral to the
stomach, which drives the blood first one way and then the other by
alternate series of beats opposite in effect. The adult animal possesses
one of the characteristics of a chordate in having pharyngeal slits, but it
has no notochord and the nervous system consists only of a ganglion
embedded in the body wall between the two funnels and associated with
a subneural gland or hypophysis.

A study of the development of a tunicate reveals all of the chordate
characters. The tunicates are all monecious, but cross-fertilization is the
rule, fertilization taking place outside the body. From the egg is pro-

318 METAZOAN PHYLA

duced a larva somewhat like a frog tadpole in form. It possesses a
typical notochord which is largely confined to the tail and which extends
only as far forward as the pharynx. The pharynx has but a small num-
ber of slits. There is a dorsal neural tube, which is dilated at the anterior
end into a simple brain. At the anterior end of its body the larva pos-
sesses adhesive papillae, a pair of eyes, and otocysts which are comparable
to the otocysts, or primitive ear cavities, of vertebrates. When the organ-
ism, after a short, active, free-swimming life, is ready to metamorphose,
it attaches itself to a solid object by the adhesive papillae (Fig. 219);
the tail, notochord, and most of the neural tube disappear, and the brain
degenerates into a simple ganglion. The sense organs are lost. While
the tail is disappearing, the mouth is gradually changing its location
from the point of attachment to the free end of the body, moving up
along the dorsal side. This results in a great increase in the extent of
the ventral surface, and since there is a corresponding decrease in the

sna
ry

QRS

six 5 5 5: E cy . =
aa Bessponens BABE os my; / Adhes/ve
SSS Se « papillae
[aralatale celts ae se {> paid - Y ESS |

Leribrarichial
vesicle
Fic. 219.—Section through the anterior part of the body of a tunicate larva, after

fixation. From the right side. (From Delage and Hérouard, ‘‘ Traité de Zoologie Concréte,”
after Kowalevsky.)

dorsal surface, the anal opening and the mouth are brought near to one
another, causing the alimentary canal to become U-shaped. ‘The atrial
cavity develops from invaginations of the ectoderm on the two sides of
the body. These invaginations (peribranchial vesicles) come to open
into each other and to surround the whole pharynx; the external opening
of the cavity becomes the atrial funnel. The outer wall of the atrial
cavity is the mantle or body wall proper. Finally the cellulose tunic is
secreted by the ectoderm of the mantle and the animal has ceased to have
any resemblance to the active tadpole-like larva, which bore the promise
of a highly developed adult. The number of pharyngeal slits increases
and becomes multiplied by the appearance of partitions, so that the wall
of the pharynx becomes quite sievelike.

It is thus clear that in the metamorphosis of the tunicate a very pro-
nounced degeneration has taken place; indeed, the fixed adult upon
superficial examination seems little higher than a sponge. Some fixed
tunicates which reproduce by budding form colonies having a common
tunic. These colonies may be spread over the surface of rocks and have
somewhat the appearance of a piece of pinkish fat pork.

LOWER CHORDATES 319

Free-swimming tunicates, either single or colonial, live in the surface
waters of the sea. They move about by forcing water out of the pos-
terior atrial opening or openings. Some colonial types form chains of

Fin and fin-rays

Pharnyngeal Gonad ‘
Slits

A
Buccal Metapleural folds opening

tentacles

Fie. 220.—An amphioxus. A, seen from the side; B, from below. Some details of internal
anatomy indicated by dotted lines. From aspecimen. X 24.

individuals produced from a parent by budding, omitting the free-
swimming larval stage.

The tunicates are the only chordates which exist in attached colonies.
There is an alternation of generations in the life cycle of some, a sexual

Ventral
nerve

Muscle
Aorta

Coelon7

Gil] Pharynx
arch

Gill

Slit Liver

Fic. 221.—Cross section through the gill region of amphioxus. (From Hertwig and Kings-
ley, ‘‘ Manual of Zoology,” after Lankester and Bovert.)

generation, in which develops a larva that metamorphoses into a free-
swimming adult, being followed by an asexual generation, which repro-
duces only by budding. Thus these animals show retrogression in their

320 METAZOAN PHYLA

manner of reproduction, returning to asexual reproduction and metagene-
sis, which are found elsewhere only among the lower metazoan phyla.
They also show a type of colonial life which is more characteristic of the
lower phyla. Another striking phenomenon is the marked degeneration
which takes place during metamorphosis.

341. Cephalochordata.—The type of this subphylum is the amphi-
oxus, often called the lancelet (Fig. 220). It is a small marine animal
reaching a length of but two or three inches and pointed at both ends.
It is found on the sandy beaches of tropical and subtropical regions of
the world.

Certra/ cara/

of verve cord Dorsal ti rays
| 4 eee fie Cord" Pigmmert
a Sor ae
Ay oy Notochora
a Hust : il ll tt ae — ita Uinta uD

Oral tentacles

Ora/ ee
Fi OOF oF Velar
cae atrium — tenfacles Oral Hing
YET Atriun
Phar yhx

Fig. 222.—Median longitudinal section of an amphioxus. (From Borradaile, ‘‘ Manual of
Zoology,’ by the courtesy of Oxford University Press.)

The mouth of the amphioxus opens on the ventral surface of the body
near the anterior end, while the anal opening is on the same surface
nearly at the posterior end. A fin runs in the median line of the body
from the anterior end backward, around the tail, and forward on the
ventral surface, passing to one side of the anus, and ends at the atriopore.
The atriopore is an opening at a point about one-third of the length of
the body from the posterior end. From this point two lateral, ventral
folds, known as metapleural folds (Fig. 221), run forward to the region of
the mouth. The atriopore is the posterior opening of an atrial cavity
which surrounds the large pharynx, except dorsally, and into which
open the pharyngeal slits. Water passes from the mouth to the pharynx,
through these slits into the atrial cavity, and thence out through the
atriopore. Food is secured in the same manner as in the tunicates, a
ventral endostyle functioning exactly as in those forms.

Running dorsally from one end of the body to the other is the noto-
chord, a slender rod tapering at both ends and composed of vacuolated

LOWER CHORDATES 321

cells. Stiffness is added to this rod by the cells being filled with liquid.
On the dorsal side of the notochord is the tubular nerve cord which pos-
sesses a small central canal and which is dilated at the anterior end to
form a rudimentary brain, known as a cerebral vesicle (Fig. 222). At the
anterior end of the nerve cord is an eyespot. The only sense other than
light perception known to be possessed by the amphioxus is that of touch.

The circulatory system, which does not include a heart, is not more
complicated than that of the earthworm. As in vertebrates, however,
the course of the blood is the reverse of that in the earthworm, being
forward in a ventral vessel and backward in a dorsal one. Elimination
is carried on by a nephridial system resembling that of some of the annelid
worms.

Amphioxus lives buried in the sand with only the anterior end exposed.
The current of water rising from the atriopore serves to create a sort of
tube in which the body lies. In this position the oral hood and buccal
tentacles are opened wide to gather all the food particles possible. At
times the animals come out and swim around in the water, particularly
at night and during the breeding season. When they bury themselves in
the sand, they enter head first and burrow very rapidly.

Amphioxus is diecious. In the evenings of the breeding season, which
is in early summer, eggs and sperms are set free at the surface of the sea,
and there fertilization takes place. Cleavage is total and equal (Fig.
51), and a gastrula is produced by invagination. ‘The mesoderm develops
from entodermal pouches. A free-swimming larva is produced which
gradually grows into an adult, at which time it begins to bury itself in
the sand. There is, therefore, no metamorphosis.

342. Economic Value.—None of the lower chordates is of much
economic value except as laboratory material. However, in Science for
July 27, 1923, Light tells of the use of the amphioxus for food in China
and speaks of ‘‘a total of hundreds of tons of amphioxus taken during the
year’’ off the southern coast of that country.

CHAPTER L

SUBPHYLUM VERTEBRATA

The last subphylum, Vertebrata, is distinguished from the other
subphyla of Chordata by several characteristics, though it shares with
them all the characteristics that belong to the chordates as a whole.
Instead of a type being selected to illustrate this subphylum, a general
description will be given applicable to vertebrates generally.

343. Distinguishing Characteristics——Some of the distinguishing
characteristics of the vertebrates are as follows: (1) The notochord is
more or less completely replaced by a vertebral column, which is made up
of a series of separate bones called vertebrae. (2) The vertebral column
and other supporting structures form an internal skeleton, or endoskeleton.
(3) As arule two pairs of appendages, either fins or limbs, are developed.
(4) All vertebrates possess a heart, ventrally situated, with at least two
chambers. (5) In the blood are red blood corpuscles which contain hemo-
globin; white blood corpuscles also exist, as they do in the blood of the
higher invertebrates, but the hemoglobin was in those types dissolved in
the plasma of the blood. (6) All vertebrates possess a brain which is
divided into five parts known as vesicles, each in its primitive form con-
taining a cavity. (7) They possess a more or less distinct head, in which
are situated several organs of special sense. (8) A large coelom is present
which is almost entirely filled with the organs of the digestive, respiratory,
excretory, and reproductive systems; the small space not occupied by
these systems contains a serous, or watery, fluid. (9) The vertebrates
are also alike generally in that they possess a posterior prolongation of
the body behind the anal opening, forming a tazl.

344. Body Plan.—Generally speaking, the vertebrate body is divided
into three parts—head, trunk, and tail. A neck, which is simply a con-
stricted region between the head and trunk, may be present, though
this is not marked in the lower vertebrates or those fitted for aquatic
life. In the terrestrial types, one of the two pairs of appendages is
situated in the thoracic region, which is the anterior portion of the trunk,
while the other pair is situated in the pelvic region, which is the posterior
portion just in front of the tail. These positions are dictated by mechan-
ical necessity when the limbs are used for both support and locomotion.
In aquatic types where the body is buoyed up by the water this arrange-
ment is frequently modified. The anterior appendages appear in
various animals as pectoral fins, forelegs, arms, or wings; the posterior
ones are either pelvic fins, hind legs, or the only legs.

322

SUBPHYLUM VERTEBRATA 323

In all the vertebrates below the mammals the coelom is divided into
a pericardial cavity containing the heart and a general body cavity. In
the mammals it includes a pericardial cavity, a thoracic cavity containing

Pancreas Mesonephros

Cloaca
Ureter

Urinary

bladder

Oviduct

Fic. 223.—Diagrammatic longitudinal section of the female of a lower vertebrate. (Re-
drawn, with modifications, from Hegner, ‘‘ College Zoology,” after Wiedersheim.)

Stormach Spleen

the lungs, and an abdominal cavity filled with the organs of the digestive,
excretory, and reproductive systems.

The general arrangement of organs within the body may best be
understood by the aid of diagrams (Figs. 223 and 224) showing ideal

Dorsal fin

Fin ray

centrum Spinal cord
of verfebr
Dorsal aorta o Inter muscular
rib
Cardinal esonephros
yeln
Subperitoneal
TIb

Mesonephric
Auct
Lateral vein

Mesentery Intestine Peri ftoneurt

Fic. 224.—Diagrammatic cross section of a lower vertebrate. (From Parker and Haswell,
““Text-book of Zoology.’’)

longitudinal and cross sections, neither of which corresponds precisely to
the structure of any one vertebrate but both of which bring out general
relationships illustrated more or less completely in all.

324 METAZOAN PHYLA

345. Skin.— Vertebrates are covered with a skin, or integument,
consisting of two parts—an outer epidermis, derived from the ectoderm
and consisting of stratified pavement epithelium; and an inner dermis,
mesodermal in origin and consisting largely of connective tissue (Fig.
225). The epidermis has only a few nerve endings, which are in the
deeper layers, and no blood vessels. The deepest layer is made up of
living cells which regularly undergo division and replace the dead,
horny, scale-like cells which are continually being lost from the surface.
These deep cells are supplied with blood from a dermal capillary network
lying just below the epidermis. From the epidermis are derived certain

Hair Hair PES
follicle j ie a ee
Ml ending weat duct
epidermis = = —=

W~Tactile
corpuscle

Erector
muscle of the

hair

Derniis

Sweat
gland

Blood
vessel

Fat

Blood vessel “Werve

Fig. 225.—Somewhat diagrammatic section of the mammalian skin.

external skeletal structures such as horny scales, feathers, hairs, nails,
hoofs, and claws and the enamel of the teeth.

The dermis, or corium, contains nerve endings, and specialized groups
of epidermal cells forming tactile organs. It also contains numerous
lands, which are epidermal in origin, such as the mucous glands of fishes
and amphibia and the sweat, oil, and milk glands of mammals. Bones
are sometimes developed in the dermis. From the dermal layer are also
derived the dentine and the cementum of the teeth, and bony scales of fish.

346. Skeleton.—The skeleton of vertebrates is divided into an ezo-
skeleton and an endoskeleton. The former includes many dermal cartilages
and bones, to which may be added the epidermal skeletal structures; and
the latter, all the deeper-lying skeletal parts. The endoskeleton is made
up of arial and appendicular portions (Fig. 226). The former includes
the skull, the vertebral column, the ribs, and in the higher forms a

SUBPHYLUM VERTEBRATA 325

sternum; the latter is composed of the girdles and the limbs. The girdles
are parts of the skeleton which more or less completely surround the
trunk and to which the skeletons of the limbs are articulated.

The skeleton in the lowest vertebrates may be completely mem-
branous; in those next higher in structure it is made up largely of car-
tilage; while in those still higher it includes both cartilage and bone.

Ade: is Cranium
| ty Orbit
Ser Mandible
Cervical ef
vertebrae ss a UES
Scapula sae ae
Sternun Say Humerus
Thoracic =)
vertebrae ;
. = Rib
Lumbar i
vertebrae Bese] BS
Sacrum AA ‘ ee
: : Radius
Ulna
Yeas
Metacarpals 4 F / : \ Hand
i \
‘M MN 7
Tibia
Metatarsals Ss

Fig. 226.—Human skeleton.

Although some cartilage always remains, the proportion of bone increases
in the highest forms. In the embryological development of a vertebrate
the same order is to be observed, the skeleton passing through mem-
branous, cartilaginous, and bony stages. This is an illustration of the
biogenetic law. When the bones develop they either replace cartilage,
in which case they are known as cartilage bones, or they are formed in
fibrous tissue, frequently around cartilage, when they are known as
membrane bones.

326 METAZOAN PHYLA

Bone tissue consists of an intercellular matrix of salts of lime which
masks the cellular elements. Under the microscope, however, the cells,
called bone corpuscles, appear, lodged in spaces called lacunae. Small
canals called canaliculi run from one lacuna to another and contain the
branching processes of the cells. In the living animal the bones are sur-
rounded, except on articular surfaces, by a fibrous covering, or periosteum,
firmly anchored by fibers which penetrate the bone itself; the articular sur-
faces are covered with cartilage. Blood vessels and nerves enter the
mass and are distributed through it, reaching, in many bones, a central
cavity containing fatty, yellow marrow. The marrow, of two kinds,
red and yellow, is a form of connective tissue. The red marrow is com-
posed largely of cells, from which are produced the new red blood cor-
puscles needed continually to replace those worn out by use.

The skull may be divided into two parts, a cranial part, or cranium,
made up of the bones which surround the brain, and a visceral part,
which includes the bones developed about the mouth and nasal chambers.
To the latter are added other bones which lie in lower forms about the
gill slits and which in the higher forms are more or less intimately associ-
ated with the skull.

The vertebral column consists of a series of individual bones known as
vertebrae. It extends through the neck, trunk, and tail and is divided
into four regions. The cervical vertebrae are in the neck; the thoracic
vertebrae in the anterior part of the trunk; the sacral vertebrae in the
posterior part of the trunk where, with the pelvic girdle, they form the
pelvis; and the caudal vertebrae in the tail. Ribs may be developed in
connection with all the regions of the vertebral column, though they are
most constant and reach their greatest development in the thoracic
region, where in the highest forms they, together with the sternum, form
a thoracic basket inclosing the lungs. Several ribless vertebrae lying
between the thoracic and sacral regions, corresponding to what is known
in the human body as the small of the back, are in the mammals recog-
nized as a fifth region and called lwmbar vertebrae.

The girdle to which is attached the anterior pair of appendages is
known as the pectoral girdle. The girdle of the posterior appendages is
known as the pelvic girdle, and with the sacral vertebrae forms the pelvis.
The bones forming the skeletons of the two limbs correspond, and this
correspondence may be brought out by reference to diagrams (Fig. 227).
The homology between the limbs also includes blood vessels, muscles,
and nerves.

347. Muscular System.—The voluntary muscles of vertebrates are
usually attached to some part of the skeleton and are known collectively
as the flesh of the animal. Their attachment is frequently by means of a
tendon, which is a dense mass of parallel connective tissue fibers continu-
ous on the one hand with the muscle sheaths and on the other intermin-

SUBPHYLUM VERTEBRATA 327

gled with the fibers of the periosteum. In some cases striated muscles
have ceased to be under the control of the will and thus in a sense
have become involuntary. Such are some of the muscles used in swallow-
ing. The involuntary, or non-striated, muscles are found mostly in the
walls of the alimentary canal and the blood vessels. The cardiac muscles
are in the wall of the heart. Both involuntary and cardiac muscles are
usually not recognized as part of the muscular system but as parts of the
systems to which the organs containing them belong.

348. Digestive System.—The digestive systems of different verte-
brates present many modifications. In general the system may be

Scapu/a

LTliur

Glenoia

cavity Pelvis Acetabulum

Clavicle ]

Coracoid
Ischium
Radius

U/na
OQ
C5O
APAS5G Tarsus
Ce L800 } carpus ) Fibula
Ss Pe ea LTE
S 9 Acar pa a
SES oD) ony)
& C7 -Phalanges OCB
aoe EE
SSeS
——
— Phalanges
Fig. 227. Diagram to illustrate the homology between the skeletons of the fore and hind
limbs.

divided into an alimentary canal, beginning at the mouth and ending at
the anus, and accessory organs, including glands connected to the ali-
mentary canal by ducts. The regions of the alimentary canal are, in
order, mouth, pharynx, esophagus, stomach, and small and large intes-
tines (Fig. 228).

The mouth cavity, also called the buccal cavity, usually contains jaws
bearing teeth for the mastication of the food and a tongue which is used
in handling the food and may assist in securing it. In connection with
the mouth may be several pairs of salivary glands the secretion of which,
the saliva, contains an enzyme, ptyalin. This in an alkaline medium con-
verts starches into sugars. The saliva also contains the secretion of
mucous glands lying in the walls of the mouth; it serves to moisten the
food and make its swallowing easier.

328 METAZOAN PHYLA

The pharynx in the lower vertebrates is respiratory in function, since
in its walls lie the gill slits. In the higher forms it serves as a common
passageway for the food from the mouth into the esophagus and for the
air from the nasal chamber into the windpipe, or trachea.

LISS
el R
res

ADS
QUA TTY cnas

Fig. 228.—Alimentary canal of man. (From Sobotta, ‘‘ Deskriptive Anatomie,” by the
courtesy of J. F. Lehmann’s Verlag.)

The esophagus may be simply a passageway for food on its way to the
stomach, or as in the case of many birds, it may be dilated to form a
crop for the storage of food.

The stomach is an organ in which the food is reduced to liquid form
and in which a certain amount of digestion takes place. In the walls of
the stomach are glands which secrete pepsin and rennin and also hydro-
chloric acid, giving to the contents an acid reaction. The rennin, in this
acid medium, serves to coagulate the protein of milk, separating it as the

SUBPHYLUM VERTEBRATA 329

curd, while the pepsin partially digests proteins, changing them to pep-
tones. A sphincter muscle called the pyloric valve controls the passage of
the food from the stomach into the small intestine and prevents the
passing of any food which is not quite liquid. Thus as the food is gradu-
ally reduced to liquid form in the stomach it is passed in a series of spurts
or jets into the intestine.

The small intestine is digestive and absorptive in function. Into it
empty the ducts of two glands, the liver and the pancreas. In the secre-
tion of the pancreas are several enzymes. These are (1) trypsin, which
completes the conversion of proteins into peptones and also changes the
peptones into amino acids, which can be absorbed; (2) amylopsin, which
forms soluble sugar from starches; and (3) steapsin, which changes fats
into soluble fatty acids and glycerin, both of which are capable of being
absorbed. The wall of the intestine also contains glands which secrete
enzymes capable of changing nonabsorbable sugars into those which are
absorbable. All of these enzymes act in an alkaline medium. The
secretions poured into the intestine serve to neutralize the hydrochloric
acid of the stomach and to render the food alkaline.

In the small intestine the digested food is absorbed into lymphatics
and blood vessels which lie immediately under the lining epithelium.
The absorbing surface of the intestine is increased by its length, usually
much greater than that of the body, and also by the formation of folds
and of very numerous minute finger-like projections called villi (Fig. 11).

The liver is the largest single organ in the body and has a variety of
functions. In it is secreted the bile, which is stored in the gall bladder
and passed into the intestine as needed. The bile assists in breaking up
the fats and preparing them for absorption and is also antiseptic in its
action. In addition to this the liver is a great storage organ where carbo-
hydrates are accumulated in the form of glycogen, so that though the
animal may be unable for some time to secure more, it has a supply
sufficient to maintain muscular activity. In the liver protein wastes are
broken up into urea, which is then carried by the blood to the kidneys
where it is eliminated.

The large intestine contains mucous glands the secretions of which
lubricate the passage. In the anterior part of it digestion and absorption
are completed. In the posterior part, called the rectum, the undigestible
residue is accumulated. Sphincter muscles control its passage from the
body. In some vertebrates the anal opening is not upon the surface but
upon the wall of a cloaca, into which also open the ducts from the
excretory and reproductive systems. The cloaca in turn opens to the
outside.

349. Respiratory System.—In the lower forms of vertebrates respira-
tion occurs through the walls of the gill slits or through gills, which are
branched projections from the walls of the slits. In terrestrial verte-

330 METAZOAN PHYLA

brates, however, is generally found a pair of lungs (Fig. 228), together
with the windpipe, or trachea, and a voice box, or larynx. The lungs
originate as outpocketings from the ventral side of the pharynx and are
lined, as is the pharynx, with entodermal epithelium.

350. Circulatory System.—The blood in vertebrates consists of a
fluid plasma in which float white corpuscles, red corpuscles, and blood
platelets. The red corpuscles contain hemoglobin, which unites with
oxygen to form oxyhemoglobin. In this form the blood carries a much
greater amount of oxygen than could be carried if it were only in solution
(Sec. 272). The carbon dioxide is carried in the plasma in combination
as sodium carbonate. The white corpuscles, or leucocytes, are ameboid
in character and are able to ingest and digest foreign particles, including
unicellular organisms. These corpuscles escape from the blood through
the walls of the capillaries and wander about in the looser tissues. They
serve to rid the body of deleterious substances or may help to protect it
from disease-producing germs. ‘The blood platelets produce a substance
active in bringing about the coagulation of the blood. It is believed that
they are formed from white blood corpuscles. The fluid part of the blood,
or plasma, contains in solution proteins which may be caused to coagu-
late, or clot. Clotting occurs whenever an opening is produced in the wall
of a blood vessel and the clot blocks the opening, preventing excessive
hemorrhage. When the clot is removed from the plasma, the fluid
remaining is termed serum.

The circulatory system of vertebrates includes a central organ—the
heart—which receives the blood returned from the body or, in certain
cases, also from the lungs and sends it out again either over the body or
to the lungs. In addition there is a closed system of vessels, including
arteries, capillaries, and veins. It is through the walls of the capillaries
that the interchange of gases and of food and waste takes place.

The circulatory system also includes the spleen and the lymphaties.
The spleen (Fig. 228) is an organ in which old and useless red blood
corpuscles are broken down and the products passed into the blood.
The lymphatic vessels serve as an accessory return circulation, picking
up the plasma which has leaked from the blood vessels all over the body,
together with white corpuscles which may be added to it, and pouring
both back into the veins a short distance from the heart. ‘This fluid
containing plasma and white corpuscles is called lymph. Into the lym-
phatic system are also absorbed the products of fat digestion. At inter-
vals along the lymphatic vessels are masses of tissue known as lymph
nodes, or lymph glands, in which are formed the white blood corpus-
cles and in which foreign particles and infectious organisms are removed
from the lymph before it is added to the blood.

351. Excretory System.—The excretory system consists of the
kidneys, the ducts leading from them, and in some cases a bladder.

301

SUBPHYLUM VERTEBRATA

The kidneys serve for the elimination of liquid waste which contains the

end products of metabolism. The ducts connected directly to the

kidneys are known as ureters; they meet to form the urethra.

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they meet, a bladder may be developed in which the urine is stored before

being passed out of the body.

There are three types of kidneys in the vertebrates, all similar in
plan, which involves a series of nephridial tubules metamerically arranged

332 METAZOAN PHYLA

(Fig. 229). If these tubules belong to metameres located far forward
in the body, the organ is called a pronephros, or head kidney; such
tubules open freely into the coelom by a ciliated funnel and take the
waste from it (Fig. 230). The vascular organ which passes the excretions
from the blood into the coelom is known as a glomus. If the metameres
represented are farther back, the organ is called a mesonephros; each

Cavity

P oe os, myo-
ronePpnric Orme
Gibule
— G/ormus
Pronephric
duct
Coelom
A
Mesonephric
tubule

Glomervsus

Glomerulus

Bownnans
eee
Coe/omn c

Fic. 230.— Diagrams to show relations of kidney tubules. A, pronephros; B, mesonephros;
C, metanephros.

mesonephric tubule, while still opening into the coelom, takes the
excretions from a knot of capillaries known as a glomerulus. The third
type, or metanephros, originates still farther posteriorly; the metanephric
tubules do not communicate with the coelom, but each ends in a cuplike
cavity inclosing a glomerulus. These appear to form both a phylogenetic
and an ontogenetic series. The pronephros is functional only in the
lowest group of vertebrates; although it appears early in the embryological
development of all higher forms, only vestiges remain in the adults
of the highest. The mesonephros is functional up to and including the

SUBPHYLUM VERTEBRATA 3393

Amphibia; it appears in early stages in the development of still higher
forms and also becomes vestigial in the adults of these forms. The
metanephros is the functional kidney of the reptiles, birds, and mammals.
This, therefore, is another illustration of the biogenetic law.

352. Nervous System.—The nervous system of a vertebrate consists
of a brain and a spinal cord, together forming the central nervous system;
and of nerves, ganglia, and sense organs, forming the peripheral nervous
system. The nerves which lead to and from the brain or spinal cord are
called respectively cranial or spinal nerves and form the cerebrospinal
system. They receive afferent impulses from various sense organs
which they carry to the appropriate center and efferent impulses which
they conduct from nerve centers to active organs, such as muscles and

Spinal Sensor
White cord nerve Dorsal root
t7 bers

a? (effector)

Ventral root
Gray matter Motor nerve
Ther

Fig. 231.—Diagrammatic cross section of spinal cord and diagram of a spinal nerve. To
show reflex paths; should be compared with Fig. 152.

glands (Fig. 231). Peripheral ganglia connected with these and situated
in various parts of the body serve as local centers for the control of
certain localized activities. Ganglia of that character are similar in
function to the central ganglia of animals in the lower phyla.

There is also a portion of the peripheral nervous system, including
both ganglia and nerves, which is to a considerable degree detached
from the rest and which carries on its functions mostly without any
interference from the central nervous system. For this reason it is
known as the autonomic nervous system. The nerve fibers belonging
to the cerebrospinal system run from origin to destination without
branching; this serves to keep the impulses separate and distinct, as is
necessary in all voluntary action. Those of the autonomic system,
however, branch freely, causing the effect of any stimulus to be radiated
in all directions. The system is thus very widely affected by stimuli,
which suggests the term sympathetic, also applied to it. This system
controls the involuntary muscles of the body. Its nerves run to and
from the various regions of the alimentary canal and through the cere-
brospinal nerves fibers from it reach all of the blood vessels throughout

334 METAZOAN PHYLA

the body, where they control the involuntary muscles in the walls of
these vessels. The largest ganglion in this system, the semzlunar ganglion,
is situated in the upper part of the abdominal cavity. It gives off
nerves to the liver, stomach, and other neighboring organs.

The brain, or encephalon, originates in the embryo as a dilated portion
of the neural tube, divided at first into three parts known respectively
as the forebrain, mid-brain, and hind-brain. It becomes divided later
into five parts, the forebrain and the hind-brain each being again divided
into two. From the anterior part of the forebrain, or telencephalon,
are developed the cerebral hemispheres, which are prolonged anteriorly
into a pair of olfactory lobes. The other part of the forebrain lies below
or behind the cerebral hemispheres and is known as the ’tween-brain, or
diencephalon. From the mid-brain, or mesencephalon, are developed
the optic lobes. From the two parts of the hind-brain come the cere-
bellum, or metencephalon, and the medulla, also called medulla oblongata,
the myelencephalon. In the wall of the brain, the gray matter, which
contains nerve cells, is on the outside, and the white matter, which is
made up of bundles of nerve fibers, is on the inside.

In the spinal cord the relationship of gray matter and white matter
is reversed (Fig. 231). The gray matter, the original medullary tube
developed in the embryo, is within, and the fibers from the brain which
reach the surface in the medulla, together with fibers derived from the
cells of the spinal cord itself, form a sheath of white matter on the surface.
Each spinal nerve arises by two roots, dorsal and ventral. The dorsal
root is made up of afferent fibers and bears a ganglion in which are the
cell bodies connected with these fibers; the ventral root contains the
efferent fibers.

In the earthworm the receptor or sensory neurons are located on the
surface; in the chordates they lie in the ganglia of the dorsal roots of
the spinal nerves. In the latter epithelial sense cells, forming receptors,
receive the stimuli and start impulses in the sensory fibers, which are
the dendrites of the sensory neurons. These impulses pass through the
dorsal root ganglia and by the axons of the sensory neurons into the cord,
where through synapses they are delivered to the connective and motor
neurons (Fig. 231). Part of the cranial nerves correspond to the dorsal,
or sensory, roots of spinal nerves; others, to ventral, or motor, roots;
and still others, to the two united.

353. Sense Organs.—Vertebrates possess a number of highly devel-
oped sense organs, or receptors. Some of these are stimulated by con-
tact; among them are various types of cutaneous sense organs, including
receptors for pain and temperature, as well as lateral line organs, and
also organs of hearing. Others, such as organs of taste and smell,
respond to chemical stimulation; and eyes serve for the reception of light.
There are also internal receptors which function mostly in muscular

SUBPHYLUM VERTEBRATA 335

control and in the directing of the activities of the alimentary canal.
The organs of equilibrium are an example of internal receptors and will be
considered in connection with the ear.

Frontal sinus . Olfactory Ethrmoidal
Upper turbinate bone ce/| cells

Olfactory
Olfacror,
| Pereeane LU aRTaRE

~ Sphenordal
DBs / SITIOS

Anterior ey. A y WV

oe i Nerve Palate Nasa/
LEAN Ve Palate Opening of Posterior WM
Middle Bustachiap anes Tieers septum

and /ower
turbinates A tube Cc B

Fig. 232.—Olfactory organ in man. A, view of side wall of the nasal cavity to show
distribution of olfactory epithelium. Probes are passed through the passages leading to
the frontal sinus, the sphenoidal sinus, and the antrum in the cheek bone, all of which may
be infected from the nose. Olfactory membrane stippled. 8B, vertical section, made on
line ab in Fig. A, showing the nasal cavity of one side, with the nasal septum. A probe
is passed through the passage to the antrum. Olfactory membrane black. C, section of
a portion of the olfactory mucous membrane; highly magnified.

A variety of tactile organs is found in various vertebrates and on
different parts of the body, but they share a common plan. Nerve end-
ings occur between epithelial cells, and the impulses are produced by the

mechanical stimulation due to pressure upon these delicate endings.
The lateral line organs, which are found along

the sides of the bodies and about the heads of SUPROET HIG Gell
some aquatic vertebrates, have for a long time She as fee”
puzzled zoologists and have had attributed to

them various functions. Recent investigations
by Parker, however, seem to show that these are
stimulated by vibrations in the water of too
great wave length and too little frequency to
cause sensations of sound and that they give to
the animal information concerning movements

; : A ; : N\ Verve
in the water which are important in the securing trbers
of food, the avoidance of enemies,andadjustment = y,,. 233.Human taste
to currents. bud, somewhat diagram-

The most important organs of smell (Fig. ae ARE ene A

232) consist of sheets of sensory epithelial cells situated in the nasal pas-
sages. Substances carried through the air in the form of fine particles
fall upon this sensory epithelium and are dissolved in the fluid secreted
on its surface. The cells are stimulated chemically, and the result is the

336 METAZOAN PHYLA

sensation which is called smell. Small particles entering the mouth
through the air and being dissolved in the saliva or entering it in solution
can affect in a similar way groups of cells known as taste buds (Fig. 233)
on the surface of the tongue, soft palate, or pharynx and cause a sensation
which is recognized as taste. From what has been said it is evident that
the senses of smell and taste are allied and that one may both smell and
taste a substance at the same time. In some aquatic animals taste buds
also occur on the outer surface of the head and especially on the soft
barbels of such fish as the catfish and bullhead.

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Pinna Malleus Fenesra OVE
Semicircu/ar
Utriculus A cand/s ieee
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auditory ernbrare i
gee Closing hese Ce UStachian tube

ba “mpanic

tha rotu.
embrarié ! WL

7Tyrmpanurm

Fic. 234.—Diagrammatic section of the humanear. Cavities unshaded. The cochlea
is shown made up of three coils, each divided by a continuous membrane which is not
continued to the tip. In this way the cavity of the cochlea is divided into two parts, the
scala vestibuli, which opens into the sacculus, and the scala tympani, the end of which is
shut off from the tympanum by a membrane closing the fenestra rotunda, also called
fenestra cochlearis. The two scalae communicate at the tip of the cochlea.

354. Ear.—The ear of vertebrates may consist of three parts (Fig.
234). These are the inner ear, which is present in all forms and which
contains the essential organs of hearing and equilibrium; the middle ear,
which is found only in the Amphibia and in higher classes of vertebrates;
and the outer ear, which is confined to the reptiles, birds, and mammals.
The outer ear is well developed only in the mammals, in which is usually
added a broadly expanded pinna—the visible part and that ordinarily
called the ear. The human ear, as an example of the most highly devel-
oped auditory organ, will be described.

The inner ear is inclosed in the temporal bone, which forms a part of
the side wall of the cranium. It consists of cavities and canals, sur-
rounded by a fibrous membrane, which together form the membranous
labyrinth. These in turn are fitted into a system of bony spaces forming
the bony labyrinth. Lymph fills the membranous labyrinth and occupies
the space between it and the bony labyrinth. The membranous labyrinth

SUBPHYLUM VERTEBRATA 337

includes one chamber known as the sacculus, in connection with which is
the organ of hearing, and another known as the utriculus, in connection
with which is the organ of equilibrium. The two cavities—sacculus and
utriculus—communicate with one another through the endolymphatic
duct, and each has a potential opening into the middle ear.

The sacculus is the ventrally situated chamber. In connection with
it is a spirally coiled canal known as the cochlea (Fig. 235). In the canal
of the cochlea is a sheet of sensory cells known as the organ of Corti, which
is the essential organ of hearing. Sound waves stimulating these cells
mechanically give rise to impulses which, transmitted by the auditory
nerve, are interpreted by the brain as sensations of sound. In connec-
tion with the utriculus, which is the more dorsal chamber, are three semi-
circular canals lying in three planes. At the end of each of these canals

Scala
vestibuli

Fic. 235.—Section of the cochlea showing the two scalae, which communicate at the
tip of the coils, and the essential organ of hearing, the organ of Corti, lying in a part of the
scala vestibuli separated from the rest by a thin membrane, called the vestibular mem-
brane, or membrane of Reissner. The mass of bone surrounding the cochlea is shown cut
away from the rest of the temporal bone, in which the whole auditory organ is contained.

is a dilatation known as an ampulla, in which are sensory hairs which,
when the body is moved, are stimulated by waves of movement in the
lymph and give a sense of position or of equilibrium. The opening from
the utriculus into the middle ear, known as the fenestra ovalis, is closed
by the innermost of three bones known as the stapes, while the opening
from the sacculus into the middle ear, the fenestra rotunda, is closed by a
thin membrane.

The middle ear is known as the tympanum. It is a cavity filled with
air across which sound waves are conducted by a chain of three bones
called, from without inward, malleus, incus, and stapes. The first two of
these are not present in amphibians, reptiles, and birds, instead there is one
bone, the columella. This cavity is in communication through the ewsta-
chian tube with the cavity of the pharynx and thus opens to the outside.

The outer ear is separated from the middle ear by a tympanic mem-
brane. Sound waves entering the outer ear set this membrane into vibra-

338 METAZOAN PHYLA

tion; these vibrations are transmitted by the bones of the middle ear to
the fluid which fills the inner ear and by this fluid to the sensory cells in
the cochlea.

The withdrawal of the inner ear from the surface and its lodgment in
a cavity in the skull are clearly in the interest of greater protection to an
organ which has acquired a more delicate adjustment in the higher forms.
The chain of ossicles forms a very sensitive sound-conducting apparatus.
The connection of the middle ear with the pharynx adjusts the pressure
in the middle ear to changing air pressures outside the body. As the
pressure of the surrounding air rises or falls, the tympanic membrane
would be forced inward or outward if it were not that through the eusta-
chian tube air enters or leaves the middle ear, thus equalizing the pres-
sures on the two sides of the membrane. Pressures are equalized as
between the middle and inner ear by the elasticity of the membrane
closing the opening in the wall of the sacculus. When the pressure in the
middle ear rises, this membrane is bent inward, and the pressure within
the inner ear is correspondingly increased; when the pressure in the mid-
dle ear falls, the membrane is bent outward, and the pressure in the inner
ear is correspondingly diminished.

355. Eye.—The eye is the organ of sight and consists of a variety
of structures which contribute to this function. The eyeball, which is
the organ of sight proper, is somewhat like a roughly spherical camera
(Fig. 236). Its wall is composed of three layers. These are the outer,
or sclerotic, layer, which gives support; the middle, or choroid, layer,
which is vascular; and the inner, or retinal, layer, which is sensory.
Light is admitted through a transparent cornea, which is the anterior
portion of the sclerotic coat. The amount of light falling upon the sen-
sitive retina is regulated by a circular curtain, the zris, a part of the
choroid layer; the central opening in the iris is the pupil. Just behind
the iris is the lens, by means of which the rays of light, which have to a
degree been brought together by the convex cornea, are focused on the
retina. When light stimuli fall on the retina, they give rise to impulses
which are conveyed to the brain and there produce the sensation of sight.
Behind the retina is a pigment layer the function of which is not known.
The cavity of the eye is divided into two chambers. The outer chamber,
in front of the lens, is filled with watery aqueous humor; and the inner
chamber, behind the lens, is filled with the jelly-like vitreous body. The
outer chamber is again divided into the anterior chamber in front of the
iris and the posterior chamber behind it. The structure of the eye can
best be understood by reference to a diagram (Fig. 236).

Accommodation, which is the adjustment of the eye to far and near
vision, is limited in the lower vertebrates but is well-developed in the
higher ones. It may be described as it occurs in man. It should first
be stated that the lens is elastic and tends of itself to become thicker

SUBPHYLUM VERTEBRATA 339

or more convex. It is inclosed in a thin fibrous capsule which is attached
all around by a suspensory ligament to a body known as the czlzary body,
from which also the iris extends and which contains a muscle known
as the ciliary muscle, in the form of a complete ring. All of these belong
to the choroid layer. The tendency of the eyeball to maintain a globular
form causes a tension on the lens capsule which serves to flatten the
lens and to focus the eye for distance. When it is desired to focus on a
near object, the ciliary muscle contracts. This pulls the margin of the
suspensory ligament inward or toward the pupil, releases the tension
of the capsule, and permits the lens to thicken or become more convex.
Thus focusing on a distant object is not a muscular act, but focusing
on a near object is. As an individual grows older this elasticity of the

<== ww _ sclerotic layer
SQ
Posterior Ciliary muscle Qe choroid layer
chamber Ciliary body Retinal layer

Suspensory
ligament

Fovea
centralis

Inner
chamber

Cornea

Pupil = Optic

lens Anterior nerve
: X <3 margin o

Anterior RS retina

Fic. 236.—Somewhat diagrammatic section of the human eye.

lens becomes lcss and less, and thus, while old people with normal eyes
find no serious difficulty in seeing things at a distance, they have to put
on glasses to assist the eye in focusing on near objects and to enable
them to read. .

Accessory organs of sight are the lids, which protect the eye; the
lachrymal glands, which secrete a fluid that bathes the free surface of
the eyeball and prevents it from drying; and several muscles, which
move the eyeball and point it in various directions.

356. Reproductive System.—All vertebrates are diecious, except
the lowest group, the hagfishes. In the male the essential organs are the
testes and the vas deferens; in the female they are the ovary and
the oviduct. There may be present in the female, along the oviduct,
glands which add albumen, shell, and pigment to the eggs, while in the
male sex there may also be accessory organs of copulation. In most

340 METAZOAN PHYLA

lower vertebrates fertilization takes place in the water, both egg cells
and sperm cells being passed out of the body of the parents. In terrestrial
vertebrates, however, internal fertilization is the rule. Internal fertiliza-
tion is usually accompanied by egg laying, and the young develop outside
the body. Thus these forms are oviparous. In all groups of vertebrates
except birds, however, occur those animals which are viviparous; and
the mammals, almost without exception, possess that character.

357. Advances Shown by Vertebrates.—The advances shown by the
chordates, as enumerated in Chap. XLVIII, give to them advantages
over all other animals, and in the highest vertebrates there result the
most efficient types of animal life this earth has yet known. As a
group the vertebrates are the largest and most powerful of animal types;
they are the most complex in structure and at the same time their
functions are most perfectly coordinated; and their sense organs, if
not the most varied, are at least the most effective in furnishing to their
possessors knowledge of their environments. Far ahead of any other
system in the extent to which its development has been carried is the
nervous system the functioning of which is such as to lift the higher
vertebrates much above all other animals.

358. Classification. Vertebrates may be divided into seven classes
as follows:

1. Cyclostomata (si kl6 std’ ma ta; G., kyklos, circle, and stomatos,
mouth).—Roundmouths, including hagfishes and lampreys.

2. Elasmobranchii (@ 14s mo briin’ kii; G., elasmos, metal plate,
and L., branchia, gill) —Cartilaginous fishes, including sharks, skates,
and rays.

3. Pisces (pis’sés; L., pisces, fishes)—Bony fishes, including all
ganoids, teleosts, and lungfishes.

4. Amphibia (im fib’i a; G., amphibios, leading a double life).—
Salamanders, frogs, and toads.

5. Reptilia (rép til/ia; L., reptilis, creeping).—Lizards, snakes,
turtles, and crocodiles.

6. Aves (a’ véz; L., aves, birds).—Birds.

7. Mammalia (ma ma’ li 4; L., mammalis, of the breast).—Mammals.

CHAPTER LI
CLASS CYCLOSTOMATA

The cyclostomes are sometimes separated from the other classes
of vertebrates to form a group higher in rank than a class and known
by the same name, Cyclostomata, or the name Agnatha. Such a group is
coordinate with Gnathostomata (nath 6 std’ ma ta; G., gnathos, jaw, and
stomatos, mouth), which includes all the other classes. The lack of jaws

Fic. 237.—Cyclostomes. A, California hagfish, Polistotrema stouti (Lockington), with
12 gill slits. xX 14. (Redrawn from Chidester, ‘‘ Zoology,” after Dean.) B, sea lamprey,
Petromyzon marinus Linnaeus, with seven gill slits. x 14. From preserved specimen.

and teeth, which are replaced by a suctorial mouth, is the most prominent
characteristic of the cyclostomes. They also have an elongated, eel-like
body with a leathery skin devoid of scales, are without paired appendages,
possess a fibrous skeleton to which in one type is added some cartilage,
have a single olfactory pit, usually an unpaired gonad, and a variable
number of gill slits. They have no air bladder, oviducts, or cloaca.
While free-living when young and feeding upon small particles in the
water, they are as adults generally parasitic on other fishes.

359. Classification——The cyclostomes are divided into two sub-
classes. The first of these, Myxinoidea (mik si noi’ dé 4; G., myzinos,
slime fish, and ezdos, form), includes the hagfishes; the second, Petro-
myzontia (pét rd mi z6n’ tia; G., petra, stone, and myzontos, sucked in),
contains forms known popularly as lampreys, or sometimes as lamprey
eels. The latter name, however, is not correct since the eels, properly
speaking, are bony fishes.

360. Myxinoids.—The myxinoids, or hagfishes (Fig. 237A), are
noteworthy because of the enormous amount of slime they secrete when
captured and confined in a small space, one large specimen being said to
produce enough to filla bucket. Hagfishes do not lead a strictly parasitic

341

342 METAZOAN PHYLA

life, since they commonly enter the gills or mouths of dead fishes and
remain in the body, feeding upon it, until they have completely destroyed
all but the skin and skeleton. They also may attack living fish, if these
are disabled. Marked characteristics are the possession of four pairs
of tentacles around the mouth, the presence of but one semicircular
canal in the inner ear, a variable number of gill slits, and a pronephros, as
well as a mesonephros, in the adult. There is no metamorphosis in the
hagfishes. They are hermaphroditic.

361. Lampreys.—The mouth of the lampreys (Fig. 237B) is provided
with an oral funnel armed with chitinous teeth (Fig. 238). Cartilages
are present in the skull, about the notochord, around the gills, and in
connection with the fins, but the notochord remains in full development.
The number of gill slits is usually seven. The functional kidney in the
adult is a mesonephros. The ureter opens into a urinogenital sinus,

: Dorsal aorta
Nofochord Spinal ; Nasa! opening
aes Brajn Olfactory sac
: Dorsal cartilages

Annular cartilage

Buccal
cavity with
chitinous teeth

Se
= Mouth
V
Internal nr Annular
openingsof Esophagus cor: age cartilage
gill slits

Fic. 238.—Median longitudinal section of the anterior Da of the body of an adult sea
lamprey. From aspecimen. X 34.

which also receives the sex cells liberated in the body cavity from the
gonad. ‘The brain is primitive, in many respects resembles that of the
embryos of higher forms, and has a very small cerebellum (Fig. 239).
The spinal cord is much flattened dorsoventrally. There are two semi-
circular canals in the ear.

Lampreys live in both fresh and salt water and are active and pre-
daceous, attacking fish much larger than themselves. They attach
themselves to their prey by means of the sucker-like oral funnel, which
is prevented from slipping by the chitinous teeth it contains. The
flesh of the fish is then lacerated with the sharp chitinous end of the
tongue so that blood and lymph can be sucked from the body. When
the body of the fish has been drained of its fluids or the lamprey is filled,
the latter loosens its hold, and the former goes away with an open sore
which usually becomes infected and results in its death. Lampreys
are, therefore, serious enemies of fish.

The larva, called an ammocoetes, has an oral hood something like
that of amphioxus, a median eye, an endostyle, also resembling that of

CLASS CYCLOSTOMATA 343

amphioxus, and undergoes a metamorphosis when it becomes adult.
In the adult the endostyle becomes the thyroid gland.

362. Relationship of the Cyclostomes.—The characteristics which
have been given for these animals show distinctly adaptation to a parasitic
mode of life. However, the relatively large number of gill slits, the pres-
ence of a pronephros in the adult hagfish, and the condition of the brain

Pineal

Fic. 239.—Brain of lamprey. <A, dorsal view; B, lateral view. (From a Ziegler model,
after Wiedersheim.) The roots of the cranial nerves are marked by roman numerals.

all point to the fact that the cyclostomes are more primitive than other
vertebrates. Some of the amphioxus-like characteristics of the larval
lamprey seem to show the inheritance of characteristics possessed by a
common ancestor of both cephalochordates and vertebrates.

363. Economic Importance.—The hagfish has never been an article
of food, but the flesh of the lamprey is sometimes eaten both in Europe
and in America. Both of these animals, however, are to be considered
as economically injurious.

CHAPTER LII
CLASS ELASMOBRANCHII

The elasmobranchs are very different from the cyclostomes and
in many respects resemble the bony fishes. They have paired fins,
jaws, fishlike gill arches, and gills. There is a well-developed carti-
laginous skeleton. They differ, however, from the bony fishes in the
following respects: (1) There is no bone in the skeleton; (2) the paired
fins do not have fin rays; (3) they possess a peculiar type of scale known

Fic. 240.—Dogegfish shark, Squalus acanthias Linnaeus. X 409. (From Jordan, ‘‘Guide
to the Study of Fishes,’ by the courtesy of D. Appleton & Company.)

as the placoid scale; (4) the openings of the gill slits are exposed; (5)

they have a spiral valve; and (6) there is no swim bladder. A dogfish

shark is usually selected as the type of Elasmobranchi.

364. Dogfish Sharks.—The dogfish sharks belonging to the genera
Squalus and Acanthias are abundant in both the north Atlantic and
north Pacific oceans. Their bodies are fusiform, or spindle-shaped
(Fig. 240); they do not reach a large size, being not more than four or
five feet in length. They possess two dorsal fins, each of which in
Squalus has a spine in front of it, a caudal fin, and pectoral and pelvic
fins. The caudal fin is of a type known as heterocercal—that is, its
dorsal lobe is larger than the ventral lobe. A portion of the pelvic
fins is modified in the male sex forming organs known as claspers, which
are used in copulation.

The mouth is a transverse slit on the ventral side of the head, and
the jaws are armed with very sharp teeth the points of which are directed
backward. Behind the teeth in use are numerous rows of other teeth,
lying against the inner surface of the jaw, which are ready to be brought
forward and replace teeth which may be lost. On each side are five
gill slits. Behind each eye is an opening known as a spiracle, which
develops like a gill slit but which in the adult is modified and is no longer
functional as such. By means of the spiracle, however, water may be
taken in for respiration when the mouth is full of food.

344

CLASS ELASMOBRANCHII 345

The body of the dogfish is covered with placoid scales (Fig. 250),
each of which is composed of a sharp toothlike portion projecting through
the skin and attached to a plate lying in the dermis. They neither
overlap nor touch one another. The points of these scales are directed

Fic. 241.—Spiral valve of a ray. The arrow shows the direction in which the food
passes. (From Wieman, ‘‘General Zoology,” modified from Mayer, by the courtesy of McGraw-
Hill Book Company, Inc.)

backward. They thus offer no resistance to the hand if the animal is
stroked from the head to the tail, but when an attempt is made to stroke it
firmly in the opposite direction they effectively resist the movement.
The stomach is U-shaped and is marked off from the intestine by
a constriction containing a sphincter muscle. The characteristic feature
of the digestive system of the dogfish is the presence in the intestine

Precava/ vein Dorsal -Cardinal vein

: aorta , , Rerral
SiNMUS VETIOSUS Coehac artery Mesenteric Rena/ ortal
arte rf at
Efferent POWCr CaS neny — ariery |} vein
° Ki
Carotid eee Gonad “arey Caudal
arrery artery

Jugular
vein

wih

FirstgiI! slit yy

Afferent
branchial artery Hepati Intestine

Lateral ves

ventral aorta Cae), Spleen cae

Conus Hepatic\ Hepatic portal vein “Iliac
arteriosus Vertricle : vein “Stomach vein

Subclavian

vein Subclavian artery

Fic. 242.—Diagrammatic lateral view of the circulatory system of a dogfish shark.
Vessels carrying oxygenated, or arterial, blood in black, those carrying venous blood, light.
(From Woodruff, “‘ Animal Biology,” by the courtesy of The Macmillan Company.)

of a spiral fold, the spiral valve (Fig. 241), which projects inward from
the wall and which, by interfering with the direct passage of the food,
increases the time during which it is in the intestine and therefore the
time allowed for absorption.

346 METAZOAN PHYLA

The circulation of the dogfish is typical of fishes generally (Fig. 242).
The blood passes from the ventricle forward in a ventral aorta and by
afferent branchial arteries to a capillary network in the gills, where it is
oxygenated and where the carbon dioxide is given off. It is collected
again by efferent branchial arteries which carry it to the dorsal aorta,

Olfactory bulb

Olfactory tract

Cerebrum

Diencephalon Pineal body

\— Optic lobe
TY

Cerebellum

Medu//a

Fic. 243.—Brain of a European dogfish shark, Scylliwm canicula Cuvier. Dorsal
view. (From a Ziegler model, after Wiedersheim.) The roots of the cranial nerves are
marked by roman numerals.

by which, in turn, it is distributed throughout the body. Returning
from the peripheral vessels by the veins the blood is carried back to the
heart, that from the tail passing through a renal-portal system in the
kidney, that from the alimentary tract through a hepatic-portal system
in the liver. A portal system is a capillary system interposed in the
course of a vein. All of the venous blood is received by the sinus venosus,
from which it passes into the auricle, or atrium, and then into the ven-

CLASS ELASMOBRANCHII 347

tricle. A dilated portion of the ventral aorta just in front of the ventricle
is known as the bulbus arteriosus, or conus arteriosus.

The dogfish brain (Fig. 243) is much more highly developed than
that of the cyclostomes, and three regions stand out prominently, these
being the olfactory lobes, the optic lobes, and the cerebellum. The
size of these three lobes is connected, respectively, with the high develop-
ment of the sense of smell, the considerable development of the sense
of sight, and the very delicate sense of equilibrium. The olfactory
sac is correspondingly large and the eyes well-developed. Only the
inner ear is present. It lies in the auditory capsule and consists of a
utriculus with three semicircular canals and a small simple sacculus.
There are lateral line organs and mucous canals on the head, the latter
lodging organs similar in structure to the lateral line organs and which
probably have a similar function. The kidney is a mesonephros.

Fertilization is internal and the eggs develop in a portion of the
oviduct known as the uterus. The dogfish shark is viviparous, but
when born the “‘pups”’ still possess a yolk sae.

365. Other Sharks.—A great variety of other sharks exist, many
of which, unlike the dogfish shark, lay eggs. Such eggs are always
provided with horny shells which protect them from injury after they
are laid. Whale sharks are interesting because they grow to be the
largest fish known, being said to reach sometimes a length of 50 feet.
Fossil remains indicate the former existence of types still larger.

366. Skates and Rays.—Skates and rays are distinguished from
sharks by the fact that the sides of the body are greatly extended, the
whole animal being flattened dorsoventrally. The skates (Fig. 244),
viewed from the upper side, have somewhat the shape of a kite with a
sharp tail. The electric rays are more nearly circular in shape and have
paired electric organs on the dorsal surface which are capable of giving
a powerful electric shock. These electric organs are pillars of modified
muscle cells, and the energy produced is turned into a difference in
electrical potential instead of being expended in movement. ‘The
difference in potential between the two ends of such a pillar of cells is
considerable, and the shock correspondingly severe. The sting rays
have long flexible tails with spines which are capable of producing
very serious wounds. Another interesting type is the sawfish. The
body of the sawfish, which may attain a length of 20 feet, is not broadened
like that of skates and rays, to which it is related, and the snout is
greatly prolonged, reaching a length of half the rest of the body. The
snout is armed on each side with a row of sharp teeth, which make it a
very formidable weapon capable of cutting a gash in the body of even
such an animal as the whale.

367. Extinct Elasmobranchs.—There are many extinct elasmo-
branchs, some of which are obviously more primitive than any now living

348 METAZOAN PHYLA

and also more primitive than any other known fishes, either living or
extinct. They had a terminal mouth instead of a ventral one. The
eggs were probably fertilized externally, since the male had no claspers.
In them also the notochord persisted as a continuous rod, the vertebral
column being little more developed than that of the cyclostomes. Their
characteristics seem to point to a primitive fish ancestor which had an

Fic. 244.—Common skate, Raja erinacea Mitchill. X 4%. (Redrawn from Jordan
‘Guide to the Study of Fishes,’’ by the courtesy of D. Appleton & Company.)

elongated spindle-shaped body, a mouth armed with dermal teeth,
a greater number of gill slits than in any existing forms, paired fins,
and a diphycercal tail fin made up of a single lobe.

368. Economic Facts.—Sharks have always been feared by man and
undoubtedly at times they will attack human beings in the water. They
are, however, less likely to do so than is generally supposed. One of
the dogfish sharks found on the Atlantic coast is an important enemy
of the lobster, and a second devours large numbers of valuable food
fish. It is stated that the spined dogfish shark, Squalus acanthias

CLASS ELASMOBRANCHII 349

Linnaeus, also does serious damage by destroying nets, devouring
captured fish, stealing bait, and driving away or destroying schools
of squids used as bait. The damage done by these sharks to Massa-
chusetts fishermen alone has been estimated at not less than $400,000
per year. Sharks, however, have been made use of in various ways.
From them is secured an oil; the flesh which remains after the removal
of the oil is sold as a fertilizer; the skin has been tanned; and the flesh
itself has been canned and sold commercially under the name of grayfish.

CHAPTER LIII

CLASS PISCES

The bony fishes, which form the third class of vertebrates, Pisces,
include a large assemblage of types of an exceedingly varied character.
They possess the following characteristics: (1) The skeleton is more
or less bony, resulting from replacement in different degrees of the
primitive cartilaginous skeleton. (2) The gills on each side are covered
and protected by a fold, or operculum, supported by dermal bones.
(3) The pelvic girdle is usually small or absent. (4) The fins are sup-
ported by fin rays. (5) The dermis may contain scales of different types,
but in no case are they placoid. (6) Most of the bony fishes have an
air bladder. (7) The brain includes a small cerebrum, very small
olfactory lobes, and well-developed optic lobes and cerebellum. Prac-
tically all of these characteristics present a contrast to those of the
elasmobranchs.

369. Classification The class Pisces may be divided into two
subclasses—Teleostomi (tél é 5s’ to mi; G., teleos, complete, and stoma,
mouth), or bony fishes proper, and Dipnoi (dip’ nd i; G., dipnoos, with
two breathing apertures), or lungfishes. There are four divisions of
Teleostomi: (1) Crossopterygii (krd sdp térij/ii; G., krossoz, fringe,
and pterygion, fin), or lobe-finned ganoids; (2) Chondrostei (kon dros’ té 1;
G., chondros, cartilage, and osteon, bone), or cartilaginous ganoids;
(3) Holostei (hdl 5s’ ti; G., holos, whole, and osteon, bone), or bony
ganoids; and (4) Teleostei (tél é ds’ té i; G., teleos, complete, and osteon,
bone), which includes the common bony fishes and far exceeds in number
of species all of the other divisions combined.

370. Crossopterygii—The lobe-finned ganoids were very abundant
in the Devonian period, twenty million years or more ago. Because
fishes were the dominant form of animal life, this period is known as
the age of fishes. The Crossopterygii, however, are now represented
by only two types, both found in Africa. There are evidences that
this group is ancestral not only to all higher fishes but also to the terres-
trial vertebrates. A fact which indicates the last relationship is the
existence of a larva which is very similar to the tadpoles of Amphibia.
Both the larva and the adult use the pectoral fins as supporting append-
ages. The air bladder in Polypterus (Fig. 245), one of the two living
types, is used not only as a hydrostatic organ but also as an accessory
respiratory organ, being connected by a primitive trachea with the
pharynx and used as a lung.

350

CLASS PISCES 30l

371. Chondrostei—In the Chondrostei, or cartilaginous ganoids,
the pectoral fins are not used to support the body, and the tadpole-like
larva has been considerably modified. The skeleton is largely car-

<< dla beg
» SWAN) oe

Fig. 245.—A crossopterygian. Polypterus senegalus Cuvier. a, the adult. X 4.
b, the larva. X 224. (a from Bridge, ‘‘Cambridge Natural History,” b after Budgett, by
the courtesy of The Macmillan Company.) ‘The latter figure does not show the fact that
the gills are alternately long and short. The arrow and line in Fig. a point to the position
of the left spiracle.

tilaginous, but the cartilage is overlaid with dermal bones. The sturgeons
(Fig. 246) and spoonbills are the living representatives of this group.

Fig. 246.—Lake sturgeon, Acipenser rubicundusaLeSueur. X ‘4g. (From Jordan, ‘Guide
to the Study of Fishes,’’ by the courtesy of D. Appleton & Company.)

372. Holostei—The Holostei, or bony ganoids, have a completely
ossified skeleton. In some the scales form a complete armor. Examples

Fic. 247.—Alligator gar, Lepisosteus tristoechus (Block and Schneider). X Mo. (From
Jordan, ‘‘Guide to the Study of Fishes,’’ by the courtesy of D. Appleton & Company.)

are the bowfins, and the gars (Fig. 247). The development of the bow-
fin betrays primitive characteristics. The eggs are nearly holoblastic,
and the early development is rather more like that of a lobe-finned

352 METAZOAN PHYLA

ganoid or an amphibian than like that of a bony fish. Gars use the air
bladder as an accessory lung.

373. Teleostei.—The teleosts are a large and relatively modern
group, presenting now a variety and a number of species greater than at
any previous time. In this division come the most of our food fishes
(Fig. 248).

374. Dipnoi.—The lungfishes have for a long time been noteworthy
among fishes because of the belief that they represented the forms
from which amphibians and higher vertebrates arose. More recent
investigations, however, seem to throw doubt upon this belief and to
indicate that the lungfishes are degenerate descendants of the lobe-
finned ganoids, which they resemble. When the bodies of water in

Dorsal fins

Y LPP.
ABE Lateral
ie beds GY YY,
i OY yee
rary. iz aR tse ure Ke ZB y E- Py

Opercu/um
Ay 5, sa es, Ys

A see Ufo:
"eye Gen ite
Bir
ARO AE SAN

tui
GEER
,
r

Pectoral
f7n

Branchiostegal
Trays
Fic. 248.—Common perch, Perca flavescens (Mitchill). X14. Labeled to show features
of a typical fish. (From Forbes and Richardson, ‘‘Fishes of Illinois.’’)

which they live dry up and become foul, lungfishes possess the ability
to come to the surface and take air into their air bladder, which serves
functionally as a lung. Others live in marshes and when these dry
up the fish becomes dormant in the mud at the bottom, coiled in a burrow
lined by a capsule of hardened mucus secreted by the glands of the
skin. Within this capsule the lungfish remains, surrounded by a slimy
mucus and breathing through an air hole the margin of which is turned
inward to form a tube which is inserted in the mouth of the fish. During
this time the air bladder is used constantly as a lung.

The lungfishes are represented by several types, one in Australia,
another in Africa (Fig. 249), and a third in Paraguay. Not only do
they have a tadpole-like larva similar to that of the Crossopterygii on
the one hand and the Amphibia on the other, but they also have a more
primitive type of embryogeny than that of any other living fish, the egg
being holoblastic and gastrulation taking place by invagination.

CLASS PISCES 353

375. Body Form.—The form of a primitive or typical fish is that
of a spindle, broadest in front of the middle. This is also the shape
of a submarine torpedo and is that shape which enables a body to cleave

Fic. 249.—African lungfish, Protopterus annectens Owen. X lg. (From Packard,
“Zoology,” after Boas, by the courtesy of Henry Holt & Company.)

the water with the least amount of retardation from resistance in front,
friction laterally, or suction behind. The fins, being thin in the plane
of movement, offer little interference.

a
C D

Fig. 250.—Scales of fishes. A, cycloid scale of northern pike, Esox lucius Linnaeus.
x 8. B, ctenoid scale of common perch, Perca flavescens (Mitchill). x 9. C, placoid
scales of dogfish shark, Squalus acanthias Linnaeus: a, surface view of a number; b, view of
dorsal surface of one; c, side view of one, isolated, the surface level being indicated by a line;
d, side view of one in position. a, X 30;b,c, andd, xX 60. D, ganoid scales of Lepisosteus
osseus (Linnaeus). X 2.

Those fish which are built for speed or live in swift currents approxi-
mate the typical spindle shape; those which live in quiet waters tend
to become flattened from side to side; and those which are adapted
for life close to the bottom become flattened dorsoventrally. A slender,

354 METAZOAN PHYLA

eel-like body permits the animal so shaped to explore crevices. Many
fishes, however, show modifications, often very curious, that fit into none
of these general types.

376. Scales.—Most fishes possess a soft epidermis, below which are
dermal scales. These are composed of bone and appear in several
different forms (Fig. 250). Placoid scales are characteristic of elas-
mobranchs, and the types known as cycloid and ctenoid are found in the
bony fishes. Scales of the two latter types overlap like shingles on a
roof. Cycloid scales are elliptical in shape, are marked by concentric
lines, and are found more frequently in the lower teleosts. In the
ctenoid type the portion not overlapped by adjacent scales is covered
with small, toothlike points. A fourth type of scale, the ganozd scale,
has a hard external enamel-like covering of ganoin which is produced
by the dermis. (True enamel is a product of the epidermis.) Ganoid
scales may overlap, but in some cases they are rhombic in form and are
arranged like tiles, meeting but not overlapping. Insome fishes thescales
are small and completely hidden in the skin, or they may, as in the cat-
fishes, be entirely absent; in other cases, as in the trunk fishes, they form
a complete bony box, the fins being articulated into openings in this box.

377. Fins.—The swimming appendages vary in number and precise
location, but they are always of two kinds: (1) unpaired median fins,
which include the dorsal, caudal, and anal fins; and (2) paired lateral
fins, which include the pectoral and pelvic, or ventral, fins. The principal
use of the fins is in locomotion, but various modifications for other
purposes occur. By some fish they are used in walking; by others,
such as the flying fish, they are used as gliding planes; or they may be
modified to form sucking discs. The pelvic, and rarely the caudal,
fins may be absent.

378. Locomotion.—In a fish the caudal fin is the principal locomotor
organ, the paired fins being held closely against the side of the body
in rapid movement and the other median fins being spread to maintain
the vertical position. The body is relatively rigid anteriorly but toward
the base of the tail it is very flexible. In rapid swimming the tail is
carried from side to side in such a manner as to trace a figure 8, a path
of motion similar to that of an oar in the sculling of a boat. The course
taken by the fish is directed upward or downward and to either side by
modification in the strength of the strokes of the caudal fin. In quiet
maneuvering the paired lateral fins come into service, being used like
oars. When the body is at rest they also serve to maintain equilibrium.
If both paired fins are removed, a fish turns completely over with the
ventral side upward, and removal of those of one side causes the fish
to lie upon that side.

379. Air Bladder.—A characteristic organ in bony fishes is the
air bladder, or swim bladder, which lies against the dorsal wall of the

CLASS PISCES 355

coelom and which is a hydrostatic organ. It arises as an outgrowth
from the anterior portion of the alimentary canal, and in the ganoids,
dipnoi, and in some teleosts an open duct still connects the two. The
organ tends to be divided into two chambers. The anterior chamber
contains in its wall a so-called red gland which takes oxygen from the
blood and passes it into the bladder; the posterior chamber is thin-
walled and permits reabsorption of gases by the blood. In this way
the amount of gases in the air bladder may be controlled. If more gases
are passed into the air bladder, the specific gravity of the fish is lessened
and it rises in the water. If, on the contrary, gases are removed, the
specific gravity is increased and the fish sinks. A fish, therefore, is

A B Cc

Fic. 251.—Diagrams to illustrate various types of caudal fins in fishes. A, diphycercal
type (dipnoan, Protopterus). B, heterocercal type (cartilaginous ganoid, sturgeon).
C, homocercal type (bony ganoid, Lepisosteus). D. homocercal type (teleost, salmon).
E, homocercal type (higher teleosts). These figures show a progressive series. F, a
heterocercal type representing a secondary modification for a particular purpose (teleost,
flying fish, Cypselurus).

able to maintain its position in the water without muscular effort and
quietly to rise or sink in this manner. Some species, particularly bottom
forms, have no air bladder.

380. Forms of Tails.—The caudal fins of fishes differ in shape, these
differences being correlated with the habits of the fish (Fig. 251). The
primitive type of tail is that which is evenly rounded dorsoventrally
and consequently termed diphycercal, or protocercal. This form is not
exhibited by many living fishes but is more common among extinct types.
The heterocercal type in which the caudal fin is divided into two lobes,
one lobe being larger than the other, has been described for the shark.
This type of tail, when the dorsal lobe is the larger, gives greater strength
in swimming to strokes that would serve to direct the fish toward the

356 METAZOAN PHYLA

bottom, and it is, therefore, possessed very largely by those forms
which are bottom feeders. The presence of a tail of this type in the
shark is correlated with the ventral position of the mouth and the
fact that the animal turns over when it seizes an object on the sur-
face. The larger ventral lobe of the flying fish enables it to attain a
maximum speed as it leaves the water. A third type is the homocercal
type, in which case the two lobes, dorsal and ventral, are about equally
well developed. This type of tail is common in the large majority of bony
fishes.

381. Colors of Fishes.—While most of our common fishes do not
possess bright colors, some of them are at times very brightly colored.
Especially is this true of the males of certain minnows during the breeding
season. Still more brilliant, however, are certain fishes in tropical waters,
particularly those found on the coral reefs, the colors of which are not
exceeded in variety or intensity by any other group of animals. The
colors of fishes are due to pigments developed in certain dermal cells known
as chromatophores. The main pigments are red, orange, yellow, and
black, but various tints are produced by varying combinations of these
pigments. These colors are modified by the reflection of light from the
scales, which contain crystals of guanin. Blue is such a structure color,
and by combinations between it and the pigments a variety of shades
of green is produced. The reflection of light from the irregular surface
of the scales produces iridescence. White is the result of an absence of
pigment. All of these factors together serve to produce the brilliancy
and variety of colors which fish present.

The colors of fish can be modified by the contraction and expansion
of the chromatophores, which have an ameboid character (Fig. 265).
By means of changes of the chromatophores not only are spots rendered
less brilliant or more so but also the general tone of coloration is made
to vary considerably. These changes are appropriate to the environment
of the fish and may serve to produce a protective coloration.

382. Internal Anatomy.—The internal skeleton of fishes includes
a vertebral column composed of simple vertebrae, the body of each
being hourglass-shaped, or amphicoelous. The skull contains a large
number of parts and is mostly bony but still contains some cartilage.
The pelvic girdle is absent, the ventral fins being attached to a flat
bone which is not recognized as representing the pelvis. On the whole
the bones which form the skeleton are slender and not very firm. They
do not help to support the weight of the body, which is buoyed up by
the water, but are only for muscle attachment.

The digestive system consists of a mouth; a pharynx, the walls of
which are pierced by four pairs of gill slits; a short esophagus; a stomach;
and an intestine. Teeth are found on the roof of the mouth as well
as on both jaws. Three tubular outpocketings, called pyloric caeca,

CLASS PISCES 357

open into the intestine and serve to increase its capacity, taking the
place of the spiral valve of the dogfish shark.

The circulatory system includes a heart which lies in a sac called the
pericardium lying below the pharynx. The pericardial cavity represents
a portion of the coelom.

Olfactory lobes

Roof of cerebrum, part
between removed

Cerebrum

Pineal body

Optic /obe

Cerebellum

Fig. 252.—Dorsal view of the brain of a teleost fish, a salmon. (From a Ziegler model,
after Wiedersheim.) The cerebellum conceals the medulla in this view. The roots of the
cranial nerves are marked by roman numerals.

The respiratory system consists of four pairs of gills supported by
an equal number of bony arches. Each gill bears a double row of gill
filaments abundantly supplied with capillaries. The gills are protected
externally by the opercula. Internally they are safeguarded by gill
rakers from injuries which might be caused by solid particles carried into
the pharynx with the water. The gill rakers are projections from the
gill arches and are supported by spinelike bones.

The excretory system consists of paired kidneys, lying just below
the backbone in the coelomic cavity, which are mesonephroi. From

358 METAZOAN PHYLA

them ureters carry the excretions to a urinary bladder which opens
to the outside through a urinogenital sinus located posterior to the anus.
The brain consists of small cerebral hemispheres, small olfactory
lobes, large optic lobes, a large cerebellum, and a medulla (Fig. 252).
383. Food of Fishes.—The food of fishes is highly varied, consisting of
aquatic vegetation; of all the smaller forms of animal life found in the
water, such as insects, crustaceans, mollusks, and worms; and in some

a Oral valve

Mouth opening

Branchiostegal

membrane Gills

Mouth

Fic. 253.—Diagrams to illustrate the mode of breathing in teleosts. (From Dahlgren,
Zool. Bull., vol. 2.) A, the passage of water into the mouth; B, its passage out through the
gills; C, front view of the mouth of a sun fish, Eupomotis gibbosus (Linnaeus). The anterior
part of the mouth cavity is shown in vertical section in A and B, the posterior part in
horizontal section. The large arrows indicate both the movement of the water and the
pressure exerted by it; the smaller arrows the direction of movement of the walls of the
mouth.
cases of larger animals, including not only other fishes but amphibians
and higher vertebrates which accidentally get into the water. Some are
distinctly predaceous and others more decidedly herbivorous; still others
gather mud and debris from the bottom, straining out the living and
dead organisms which it contains. All fish are very voracious. The
ultimate source of the food of most fishes is the plankton (Sec. 539) which
the water contains and which, though not often serving directly as fish

food, provides food for the multitude of organisms upon which fish feed.

CLASS PISCES 359

The teeth of fishes are not used in mastication but in holding the prey.
They are, however, sometimes modified as crushing organs, especially
in those forms which eat large numbers of mollusks. Teeth that are
lost are generally soon replaced.

384. Respiration—The mechanism of respiration in the fish involves
the use of the mouth, the opening of which is guarded by fleshy valves;
the passages through the gill slits; and chambers outside the gill slits
and under the opercula, which open by the slits behind the opercula.
The chambers of the two sides communicate below and the exit from
them is closed externally by a mucous membrane called a branchiostegal
membrane. While the mouth is held open (Fig. 253) the walls of these
cavities are dilated by the action of certain muscles and water rushes in
through the mouth opening, being prevented from entering through the
gills by the closing of the branchiostegal membrane. Then, pressure
being applied to the water in these cavities by other muscles which
contract the walls, the oral valves are forced shut, the branchiostegal
membrane opens, and the water escapes between the gills. While the
gills are thus bathed with the water which passes them, respiration takes
place. This whole operation is continually repeated. The mouth does
not need to close in breathing, though its opening is usually seen to
become alternately larger and smaller as breathing continues. Thus
the apparatus acts something like a force pump the chamber of which
is guarded by valves in such a manner as to permit water to pass only
in one direction. A fish can be smothered by preventing the closure
of either the oral valves or the branchiostegal membrane. If a stringer
is passed through the mouth and the gill slits, breathing is interfered
with and the fish is soon killed.

385. Senses of Fish.—A fish possesses two olfactory sacs, one on
each side of the head and each opening to the outside through two
external apertures. Taste organs occur in the mucous membrane of
the mouth. The senses of smell and taste are, at best, not well-developed.
The sense of touch is better developed, however, especially around the
mouth and on barbels on the head.

The lateral line system consists usually of a continuous tube on each
side of the body just below the surface, lodging on its inner wall the
lateral line organs and opening to the outside by pores passing through
the tips of the scales lying in this line. Sometimes there are two or
three lateral lines.

The eyes possess no eyelids, the cornea is flattened, and the lens is
almost spherical. The pupil is very large to allow free admission of
light, because the water absorbs so much of the light that even at a
moderate depth it is greatly reduced. When at rest the eye is focused
at a distance of about 15 inches and adjustment to more distant vision
is afforded by the movement of the lens. In other words fishes are near-

360 METAZOAN PHYLA

sighted. They can see clearly enough, however, to catch very active
prey, and some are even known to capture insects above the water.
Others, like trout, evidently detect movement very quickly even at a
distance of many feet from the water in which they are. Anableps, a
surface minnow, has its eye divided horizontally into two parts (Fig. 254),
one part for seeing below the surface of the water and the other for
seeing above it. It feeds on insects flying over the water and is found
in the streams of tropical America.

The ear consists of a membranous labyrinth which is lodged in a
cavity in the side wall of the skull. This cavity does not form a bony
labyrinth. Sound waves to be perceived must be transmitted through
the outer wall of the skull and through the tissues forming the wall of the
body outside it. In all fishes the ear is mainly an organ of equilibrium.

386. Behavior.—Fishes, generally speaking, lead very active lives;
the predatory ones, especially, are constantly on the alert for food.
In our lakes and ponds the smaller fishes retreat to deeper water and

Fic. 254.—Anableps dovit Gill, a Tropical American fresh-water surface fish, the eyes
of which are divided by a partition into two parts, one for seeing outside of the water, the
other in it. X 144. (From Jordan, ‘Guide to the Study of Fishes,” by the courtesy of
D. Appleton & Company.)

to the protecting cover of vegetation during the day, but at night they
approach both the surface and the shore in search of food. This is
accompanied by a similar movement on the part of the larger predatory
fishes which prey upon them. In captivity, and when given an abun-
dance of food, fishes may at times be seen to rest, buoyed up by the water
or lying against the bottom, and it is probable that in a natural state
they spend brief intervals at ease. Bottom feeders and herbivorous
forms go more quietly at the task of securing food than do the predatory
types, which dart rapidly at anything that appears like prey. This
instinct to snap at a moving object is the cause of the readiness with
which predatory fishes take an artificial bait. Some fish, known as gobies,
live in holes in the mud of beaches, where they may be dug out at low
tide; others are able to attach themselves to rocks by a sucker, or they
may hide in cracks and crevices. Some show a definite preference for
a certain locality, and a predaceous fish may regularly frequent a
particular station from which it watches for prey. Nevertheless fish
migrate to avoid adverse conditions such as seasonal changes, to
search for food, or to find proper conditions for reproduction. Practically

CLASS PISCES 361

all of the actions of fish are dictated by instinct, though they also form
habits. They possess little intelligence.

387. Reproduction.—The sexes of fish are separate. Fertilization
may take place by the male depositing the seminal fluid, or mzlt, which
contains the sperm cells, over the eggs, called roe, at the time of laying;

Protoplasa Blastomeres Blastoderm
and Nucleus

Yolk sac

Fic. 255.—Stages in the development of a salmon. A, the egg before development
begins. B, four-celled stage. C, the blastoderm. D, blastoderm elongated in the direc-
tion of the longitudinal axis. EH, the blastoderm with thickened margin, and the beginning
of the neural groove. F, the yolk nearly covered by the growth of the margin of the
blastoderm and the embryo elongating. G, the embryo raised up on the top of the egg,
the yolk now being enclosed by the yolk sac. 4H, the animal much older, having developed
the external appearance of a fish, but with the yolk sac still attached below. (Mostly from

Parker and Haswell, ‘‘ Text-book of Zoology,” after Henneguy, by the courtesy of The Mac-
millan Company.)

or it may occur a short time after the milt and roe are passed out into the
water. The female is usually the larger sex, in some cases her length
exceeding that of the male several times. Most of the eggs of fishes are
relatively small and surrounded by a protective covering which may be
adhesive and by which they may be attached to each other and to solid
objects. They may be laid separately, or they may be deposited in
groups. If laid separately, as in the case of some marine types, very
large numbers of eggs are devoured by other fish and by other animals.

362 METAZOAN PHYLA

If laid in masses, they are less likely to be eaten. The young fish, or
fry, are constantly exposed to destruction. Though enormous numbers
of eggs are produced, amounting to many millions for each individual
during the breeding season, only a relatively small number produce
young that reach maturity. Some fish are viviparous, both fertilization
and development being internal and the young being born with the
characteristics of the adult. The young of oviparous forms often differ
in appearance from the parents, and sometimes the change from larval to
adult characters is so pronounced as to amount to a metamorphosis.

The egg is telolecithal, and discoidal cleavage takes place (Fig. 255).
The division of the protoplasmic area at the top, called the germinal
disc, results in the formation of a germinal area, or blastoderm, which
at first forms a disc on the upper side of the egg. As development
proceeds, the blastoderm spreads out, gradually grows around the egg,
and comes to inclose the yolk completely. While the germinal disc is
still confined to the upper side of the egg, a thickening appears at the
margin of this disc, which is more marked at the point that will form
the head of the embryo. From this point a raised strip of tissue runs
backward in the median line. This strip is marked lengthwise by
two lateral ridges bounding a median groove which is the medullary,
or neural, groove. The embryo, which thus develops on the upper side
of the egg, rises higher and higher above the surface, the yolk being
at the same time gradually used up as food. The neural groove becomes
a neural tube by the meeting of the lateral margins and so the central
nervous system arises. The head becomes free at the anterior end of
the embryo and a tail forms at the opposite end. As the length increases,
various structures characterizing the fully developed fish make their
appearance, and the young animal hatches with the underside of the
body distended by the yolk still remaining. The yolk is soon used
up, but by that time the fish is sufficiently dev eloped to secure
its own food, and it begins to eat the more minute forms of life in the
water, increasing the size and variety of its food as it grows larger.
It is evident from this description that the entire egg ultimately con-
tributes to the body of the fish.

During the breeding season temporary mating in some cases occurs
and the pair may cooperate in the construction of a nest and the care
of the eggs and young. The males of some marine forms have pouches
for the carrying of eggs during development. This indicates the wide
diversity in behavior during reproductive activity.

388. Ages of Fish.—Some fish, such as the ice fish of China, live only
a year. Such short-lived fish fave a definite size which they may attain.
Others, like salmon, live only a few years and usually die at the first
reproductive period, after spawning. Still others live for a long time,
it is believed for from twenty to thirty, and perhaps even sixty, years

CLASS PISCES 363

and continue to grow as long as they live. The rate of growth, however,
gradually diminishes as they get to an advanced age.

389. Deep-sea Fishes.—There are many fishes which have become
adjusted to the conditions of life at great depths in the ocean and have
become highly modified in several directions (Fig. 256). Some of these
have become exceedingly slender, thus reducing the bulk of the body.

(}

See

Fic. 256.—Deep-sea fishes. A, Photostomias guernei Collett; length 114 inches, taken
at a depth of 3500 feet. B, Idiacanthus ferox Giinther; 8 inches, 16,500 feet. C, Gastro-
stomus bairdii Gill and Ryder; 18 inches, 2300 to 8800 feet. D, Cryptopsaras couesii Gill;
214 inches, 10,000 feet. (From Lull, ‘Organic Evolution,” after Schuchert, ‘‘ Historical
Geology,’’ by the courtesy of The Macmillan Company.)

Another modification which is very general in these forms is the develop-
ment of enormous mouths, as if to make the most of the opportunities
presented for the securing of food. Sometimes the eyes are enlarged to
gather all the light possible, though in other cases they are small and
apparently useless. Luminescent organs are frequently developed.
Extraordinary shapes are assumed, and in one very marked type the
male has become minute, lives parasitically attached to the surface
of the female, and receives food passed from the blood vessels of the
female into its own, the two sets of vessels being in contact.

364 METAZOAN PHYLA

390. Remarkable Fishes.—Among the bony fishes are a number
remarkable for one reason or another. The smallest species, and also
the smallest of the vertebrates, is a Philippine goby, which does not
reach 1/4 inch in length; the largest of those which possess the usual form
attain a length of 15 to 20 feet; but one fish which is elongated, flat, and
bandlike in shape and is known as the oarfish, or the king of the herrings,
attains a length of 25 feet. The oarfish may be the basis of some stories
of sea serpents. The climbing perch of southeastern Asia climbs out
of the water on the roots and trunks of low trees for the purpose of
capturing food. The anterior spine of the dorsal fin of an angler fish is
greatly elongated and can be directed forward. At the end of this
spine and in a forward position of the spine in front of the mouth hangs
a fleshy, brightly colored bulb which in some species bears an interesting
resemblance to an animal upon which the fish feeds. In one type the
bulb at the tip is luminescent. This fish with the luminescent bulb was
described by Aristotle in the fourth century B.c. Ever since his time
these fish have been described as using the bulb as a lure to bring other
fishes within reach of its mouth, and to this these fish owe their name.
Some recent authorities, however, have doubted the validity of the
assumptions upon which rests this ancient zoological tradition.

391. Economic Relations.—F ish are of great economic importance,
having from time immemorial been an important element in the food of
man. Not only are they themselves eaten as food, but the eggs of
certain ones, particularly Russian species of sturgeon, are eaten as
caviar. The flesh of some marine forms, however, is poisonous and
cannot be safely eaten. Some of the best known game fish are not
good for food, the tarpon, for example, being one of the most famous
and yet ordinarily not being eaten.

Some fish are capable of inflicting a poisonous wound by means
of dorsal spines or a spiny operculum. Others are harmful because of
their destruction of the eggs, the young, or even the adults of food fish
and other valuable animals.

Fish are more often cultivated in the Old World than in the New,
both for food and as pets. The Japanese have produced many curious
artificial varieties of the goldfish, originally a native of China. From
some fish is secured guanin, which in suspension in water is known as
pearl essence and which is used in the manufacture of artificial pearls.

CHAPTER LIV

TERRESTRIAL VERTEBRATES

The fourth class of the vertebrates, Amphibia, exhibits a transition
from life in water, which has been characteristic of all vertebrates pre-
viously studied, to life in air. Nevertheless, the amphibians do not
show a complete emancipation from aquatic life. Some of them remain

Mucous
gland (a)

Mucous
gland (c)

Pigment cell

Fic. 257.—Somewhat diagrammatic section of the skin of a frog to show stages in
activity of mucous glands. Gland a, the section of which does not go through the duct,
is not active; gland 6} is beginning to form mucus in the epithelial cells which line it; and
gland c is actively secreting.

gill breathers through life and are confined to bodies of water. The
majority pass through their earlier stages in the water, then acquire
lungs and become air-breathing and terrestrial. A very few have by
the acquirement of special adaptations become quite independent of
water for their reproduction, but even these require some moisture
and cannot live under arid conditions.

392. Changes Incident to the Acquirement of a Terrestrial Mode
of Life.—The first change involves the acquisition of an abundance
of mucous glands in the skin to keep it from drying (Fig. 257). A fish
possesses glands in its soft epidermis, the secretions of which protect
it from contact with the water and from the entrance of infectious
organisms, but these are inadequate to prevent rapid drying when the
animal is exposed to the air. Even the least adapted amphibian can
remain out of water a much longer time than can a fish. The soft
skin which salamanders and frogs possess is in toads replaced by a

dry, hard skin, which adjusts them to relatively drier situations.
365

366 METAZOAN PHYLA

A moist skin may, under certain conditions and in certain types of
amphibians, serve also for respiration. Generally speaking, how-

»
a
Gill arches

Pulnionary
arrery
Cc
‘ Pulmonary
Dorsal aorta Caretidartery artery
Ez
(00 ((—2asr <
7
Lit
Lung
Capillary ventral E
system aorta

Subclav-|!
jan \3

‘
{7
Wer
A

} /
/ \
fi 7
7 NS Aorta

/ Pulmonary Ss
arfery

oa) oO |
\ | fore leg
Sy j \

F G H
Fig. 258.—Diagrams showing steps in the changes in the branchial arches accompany-
ing the development of lung breathing. A, the primitive or embryonic condition; B and D,
the condition in fishes, D being a lateral view; C and E, tailed amphibian, FH being again
a lateral view; F, areptile (lizard);G,a bird; H,amammal. The corresponding arches are
numbered in roman numerals. Vessels which have disappeared are indicated by dashes.
(Based upon Wiedersheim and Parker, ‘‘ Elements of the Comparative Anatomy of Verte-

brates.’’)

ever, terrestrial amphibians have acquired lungs, which are paired
sacs developed from the ventral side of the pharynx, by means of which
they take oxygen directly from the air. This is a second adaptive

TERRESTRIAL VERTEBRATES 367

change. Gills are still possessed by the larvae but these are lost during
metamorphosis.

The third adaptation, which is connected with the development of
the lungs, involves changes in the circulatory system which result in
a double circulation, the blood being sent from the heart to the lungs
for aeration, returned again to the heart, and then sent out over the
body. This change includes a remodeling of the branchial circulation
possessed by fishes which is still retained by larval amphibians (Fig. 258).
Of the four pairs of branchial arches characteristic of the fish, the first
of each side becomes in lunged amphibians the basal portion of a common
carotid artery, which supplies the corresponding side of the head; the
Propterygium ee

A Horny rays
Bony rays Mefapterygium B

Pectoral arch
LLM Ulna

MXss

Cc Ut; bo,

SS =
Three parts Humerus :

of ee mit Radius Metaprerygium

Fic. 259.—Diagrams to illustrate the theoretical change of a fin into a limb. A,
pectoral fin of Ceratodus, an extinct dipnoan, representing a primitive fin. B, pectoral fin
of Polypterus, a lobe-finned ganoid, or crossopterygian, now living. C, D, E, hypothetical
changes from A to B. F, pectoral fin of a pro-amphibian, a hypothetical form intermediate
between Polypterus and an amphibian. G, condition in amphibian. (A, B, F, and G from
Wiedersheim, ‘‘ Vergleichenden Anatomie der Wirbeltiere’’; C to E, from Kingsley, ‘‘ Outlines
of Comparative Anatomy of Vertebrates,”’ by the courtesy of P. Blakiston’s Son & Co.)

second becomes an aortic arch, through which the blood passes to the
trunk and tail; the third disappears; and the fourth, in part, becomes
the basal portion of the pulmonary artery leading to the lung. A
cutaneous branch of this fourth arch also becomes developed in an
amphibian when it breathes through the skin. The heart becomes three-
chambered, the single auricle of the fish being divided in the amphibian
into a right auricle which receives the blood from over the body and a
left auricle which receives the blood returned from the lungs. The
ventricle remains single and serves to send out blood both to the systemic
and to the pulmonary circulation, but its walls become thrown into
folds which prevent a complete mixing of the arterial and venous blood.

A fourth change is incident to the use of the limbs to support the
body and to serve as locomotor appendages on land. Paddle-like

368 METAZOAN PHYLA

fins become replaced by jointed limbs (Fig. 246) which form a system of
levers, these limbs being divided into three portions—upper limb,
lower limb, and a third portion which in the forelimb becomes the carpus
and the forefoot, and in the hind limb the tarsus and hind foot. A tail
fin is present in some amphibians, but this fin is simply a fold of the skin
without a fin skeleton. In order better to support the weight of the
body, the limb skeletons become rather firmly attached to the axial
skeleton and the skeleton as a whole becomes to a greater extent bony
and distinctly more rigid.

A fifth adaptation in the amphibians for terrestrial life is seen in the
eyes, which become supplied with lids for protection and with lachrymal
glands to moisten the eyeball and prevent it from drying. The lens
also becomes more flattened and capable of more distant vision.

Since sound waves are transmitted less perfectly in the air than
in the water the ear also shows adaptation. A middle ear is formed
which in many amphibians is closed externally by a tympanic membrane
and across which sound is transmitted by means of a bony rod called the
columella. The columella is articulated with a second bone, the stapes,
set into the opening in the wall of the sacculus. <A eustachian tube is
developed connecting the middle ear with the pharynx. This tube and
the cavity of the middle ear together represent a modified pharyngeal slit
and correspond to the spiracular canal of the elasmobranchs.

393. Origin of Terrestrial Adaptations.—Although, as has been
noted before, certain fishes do at times leave the water it is only for a
brief interval that they do so and they show no changes adapting them
to life in the air. Other fishes such as the lungfishes have acquired
adaptations which enable them to breathe air when the water becomes
very foul or during periods when they remain dormant in the mud left
by the drying up of bodies of water. Recent investigations show that
the African lungfish must come up for air at intervals or suffocate.

For the origin of the amphibians one must go back to the Crossop-
terygii, which were noted as possessing larvae similar to amphibian
tadpoles and which used the forelimb as a means of support while resting
on the bottom. The Devonian period (Fig. 371) was the age of fishes,
at which time they were the highest animal types living. It is thought
that during the latter part of this epoch there were seasons of warmth
and heavy rainfall followed by more and more prolonged periods of
drouth. Under these conditions it is believed that from the lobe-
finned ganoids arose different types of animals showing adaptations
to terrestrial life and that the amphibians represent a successful type
which has persisted to the present day. For a long time, the earliest
trace of amphibians known was the footprint of a three-toed animal
found in Pennsylvania in rocks of the upper Devonian period. Quite
recently, however, definite remains of very primitive amphibians from

TERRESTRIAL VERTEBRATES 369

the same Devonian period have been discovered in Greenland. Previous
to these no remains were known earlier than the Carboniferous period.
As to the manner in which feet developed out of fins, it has been suggested
that the fin rays might have been reduced in size and number and the
basal bones of the fin rearranged (Fig. 259.) Thus an appendage was
formed which came to possess five digits, the typical number in the
amphibian limb.

CHAPTER LV

CLASS AMPHIBIA

The name Amphibia indicates that these animals live at different
times in their life history in two environments—water and air. They
generally possess a soft skin which is kept moist by an abundant mucous
secretion. No exoskeletal structures are developed in any living forms,
except in the Apoda, but some extinct amphibians possessed a more
or less complete dermal armor made up of bony plates. Typically they
possess four jointed legs. Two nostrils are present which open directly
into the anterior part of the mouth cavity. Lungs appear in amphibians
developing as outpocketings from the ventral side of the pharynx.
These have thin elastic walls the superficial area of which is increased
by folds; the recesses between these folds are known as alveoli. There
are renal-portal and hepatic-portal systems. The kidney is a meso-
nephros and the urinary ducts open into a cloaca. The eyes are usually
supplied with lids. The middle ear appears, and in many species of
frogs a flat, circular tympanic membrane is to be seen behind each
eye.

394. Classification.—The class Amphibia is usually divided into
three orders:

1. Urodela (a ri dé’ la; G., owra, tail, and delos, visible).—Amphibians
with tails, including salamanders and newts.

2. Salientia (si lf én’ shi a; L., saldentis, leaping).—Tailless amphib-
ians, including frogs and toads.

3. Apoda (Ap’ 6 da; G., a, without, and podos, foot).—The cecilians,
which are legless types.

395. Urodela.—The tailed Amphibia are the typical forms. They
retain a tail throughout life; possess limbs of a primitive character set at
right angles to the body, the fore- and hind limbs being approximately
equal in size; and have teeth in both jaws. These forms show a gradual
transition from aquatic to terrestrial life and may be divided into two
groups. Those which retain their gills and are aquatic throughout life
are known collectively as perennibranchs; those which lose their gills
upon becoming adult and assume a terrestrial mode of life are known
as caducibranchs.

Among the perennibranchs living in eastern United States is the
mud puppy, Necturus, which has three pairs of fringed external gills

and a gill cleft behind each (Fig. 260A). As in all of the perennibranchs
370

CLASS AMPHIBIA 371

the eyes are without lids. These animals live a rather sluggish existence
on the muddy beds of lakes and rivers. They are most active at night,
when they wander about in search of food. The most primitive of the
perennibranchs and perhaps the most primitive salamander is the hell-
bender, Cryptobranchus, of the eastern states, which is a large species
reaching a length of nearly 2 feet. It has no external gills and the gill
clefts are vestigial. The hellbender, like the mud puppy, is a very

Fig. 260.—Urodeles. A, mud puppy, Necturus maculosus (Rafinesque), from Ohio.
x 14. B, axolotl larva of Ambystoma tigrinum (Green). X 24. C, tiger salamander,
Ambystoma tigrinum (Green). The axolotl and the salamander from Nebraska. X %
A and B from preserved specimens, C from a living one.

voracious bottom form. A similar amphibian, found in Japan, is the
largest of all living types, exceeding 5 feet in length.

One of the commonest caducibranchs found in eastern United States
is the large tiger salamander, Ambystoma tigrinum (Green). This
animal (Fig. 260C) deposits its eggs singly in ponds in the spring. The
young salamander passes through a tadpole stage and metamorphoses
into a terrestrial form which lives in damp situations under stones or
logs and which frequently finds its way into cellars. The common newt
found in eastern United States is a small form, Desmognathus, which
lives under logs and stones. The female lays her eggs in a hole in the

372 METAZOAN PHYLA

moist earth and coils her body about them. When hatched the larvae
are nearly mature. An alpine newt which occurs in mountain lakes in
Europe brings forth its young alive, the tadpole stage being undergone
and metamorphosis taking place in the uterus of the mother.

396. Salientia.—The tailless Amphibia are, generally speaking,
divided into two types, those without a tongue and those with one.
Among those without a tongue is the curious Surinam toad. This is an
aquatic toad with very large hind feet and a short, broad head. During
pairing the oviduct is protruded through the cloaca and passed forward
between the back of the female and the abdomen of the male. As the

Fic. 261.—A common toad, Bufo woodhousii (Girard), of the family of true toads,
Bufonidae. Toads use temporary pools in early spring in which to deposit their eggs.
The young tadpoles are very small when hatched and remain in the tadpole stage only a
few weeks, at the end of which they metamorphose into small toads. These may require
4to 5 years to grow to mature size. Warts on man cannot be contracted by handling toads.
(Photographed and contributed by George E. Hudson.)
eggs are passed from the oviduct, they are fertilized and spread over
the back of the female, to the surface of which they become firmly
adherent. Gradually they sink into pockets in the skin, each pocket
having a sort of lid. In these pockets the young develop until they
are prepared for independent life. The male of the European obstetrical
frog carries strings of eggs on his hind legs and releases the tadpoles in
water when they are ready to hatch.

The tongued forms include both frogs and toads. There is no sharp
distinction between the two but usually a soft-skinned, partly aquatic
type is known as a frog (Fig. 268G), and a harder-skinned, more terrestrial
one as a toad (Fig. 261). The true toads also have no teeth on either jaw.

CLASS AMPHIBIA 373

They have a harsh, warty skin, ridges on the head, and a kidney-shaped
raised area behind the head on each side known as the parotoid gland (Fig.
264). When disturbed, toads frequently pass water from the bladder,
and the superstition is widely spread to the effect that the handling of
a toad will cause warts. They are nocturnal in habits, feeding upon
insects, worms, and snails. Their skins contain glands which produce
noxious secretions and they are therefore rarely eaten by other animals.

The frogs have a body which is somewhat spindle-shaped, pointed
anteriorly, and rounded posteriorly. The forelegs are weak and the
toes only slightly webbed; but the hind ones are long and strongly
muscled, with long, fully webbed toes, fitting them for leaping and
swimming. On superficial examination the male of common frogs
may be distinguished from the female by the greater thickness of the

SSRURRRASREVAPE”, S

Ts
b g ity
bN Pith bp . SOS" in
Gh on bry
> Ass y t
5 é

Fic. 262.—Common eastern tree Fig. 263.—Asiatic cecilian, Ich-

toad, Hyla versicolor LeConte. Male, thyophis sp., with eggs. (Modified
from Staten Island, New York, from Thomson, ‘‘ Outlines of Zoology,”’
<x 24. (Redrawn from Dickerson. after P. and F. Sarasin.)

‘“Frog Book.’’)

inmost digit of the forefoot. The metamerism of the body wall is
greatly obscured, this being due in part to the shortness and compactness
of the body and in part to the development of muscles connecting the
limbs to the trunk.

Just in front of each gonad is a yellowish fat body which in the frog
consists of a series of finger-like lobes; it seems to be a fat-storage organ.
Above the anterior end of the cloaca is the spleen in which worn-out
red blood corpuscles are destroyed and in which white corpuscles are
formed. There are also several glands falling under the general designa-
tion of ductless glands, the secretions of which are known as internal
secretions. Examples of such are the thyroid glands, one of which is
situated ventrally on each side in the region of the throat. Others are
the thymus glands, which lie below and behind each tympanum, and
adrenal bodies, one on the central side of each kidney.

The tree frogs, or tree toads (Fig. 262), possess dilated adhesive
discs upon the toes. Among these types is an interesting tree frog

374 METAZOAN PHYLA

found in Brazil which makes a nest for its eggs and young at the bottom
of a pond, building a mud wall about it; another frog found in Venezuela
carries its eggs and young in a shallow pouch on its back until the latter
are almost ready for metamorphosis; and still another found in Java has
ereatly enlarged fect, the toes being connected by webs, making it pos-
sible for the animal to glide or sail through the
air for a considerable distance. A curious little
tree frog found on the island of Martinique glues
its eggs to a leaf, where they form a foamy mass.
In this case development is practically completed
in the egg and there is no aquatic larval period.
Thus some of these forms have practically eman-
cipated themselves from any need of an aquatic
environment, though they must still live in a
moist locality.

397. Apoda.—The Apoda, or cecilians, some-
times called blindworms, are generally distributed
in tropical and subtropical countries. They pos-
sess neither girdles nor limbs but have concealed
dermal scales. They burrow in the earth some-
what as does an earthworm and are not unlike an
earthworm in general appearance. The anal
opening is almost at the posterior end, there being
merely a rudiment of a tail. The eyes are also
rudimentary and practically functionless, but the
animal possesses a protrusible tentacle-like organ
lying in a groove between the eyes and nose by

Fie. 264.—Showing the
manner in which a toad

takes an insect. The
tongue is extruded in an ex-

ceedingly rapid movement
and is inverted in the
action; the insect adheres
to its sticky dorsal surface,
which is underneath, and
is drawn back into the

means of which it feels its way about. In one
type found in southern and southeastern Asia the
female lays her eggs in masses in a shallow hole
near the water and coils herself about them

mouth. (Modified from

Dickerson, “Frog Book.) (Hig. 263). The larval stage is passed in the

egg, the larva possessing three pairs of external
gills which are lost when it hatches. This larva swims about in the
water for a while, coming to the surface for air, but at length the
gill clefts close, the tail fin is lost, and the animal becomes terrestrial,
leading a burrowing life. Some types of Apoda are viviparous. The
whole group is to be looked upon as the result of a very pronounced
degeneration.

398. Food.—The food of frogs and toads is composed of any living
animals which they can secure, particularly worms and insects. These
are captured by means of the protrusible tongue, which can be extended
considerably beyond the margin of the mouth and which is covered by a
sticky secretion (Fig. 264). Some animals are also grasped by the jaws

CLASS AMPHIBIA 3790

but the teeth are not used in mastication, the food being swallowed whole.
Salamanders, on the other hand, have much better developed teeth, which
they use in biting and tearing flesh. They not only feed on worms,
crustaceans, insects, and mollusks but also eat fish and other amphibians
and will tear pieces from the bodies of dead animals in the water. They
are distinctly cannibalistic.

399. Color Changes in Amphibia.—The skin of amphibians contains
color-bearing cells, or chromatophores, which in the case of many forms,
particularly the frogs, are ameboid and enable the animal to modify its
color (Fig. 265). There are also cells containing granules of guanin
which change these colors by refraction of light. The conditions here
are similar, therefore, to those existing in fish. Color changes occur as a
result of direct stimulation of the chromatophores by light, temperature,
or moisture in the environment and in response to stimuli received from
the nervous system.

Fia. 265.—Diagram to illustrate modification of color by ameboid chromatophores.
A, pseudopodia fully extended; B, partially extended; C, contracted. In A the color
present in deeper layers of the skin, represented by crosslining, is obscured, and in C it
prevails.

400. Nervous System and Sense Organs.—The brain of the frog
includes two large olfactory lobes which are united in the median line
and two cerebral hemispheres which are relatively larger than those
possessed by any forms lower than the amphibians (Fig. 266). There
are also two well-developed optic lobes and a medulla. The cerebellum,
however, is so reduced that it can hardly be distinguished; it is a trans-
verse mass dorsally located at the anterior end of the medulla. The
spinal cord is short, corresponding to the shortness of the body. Like
the brain it is inclosed in two membranes—a firm outer protective dura
mater and a more delicate inner vascular pia mater. The cerebrospinal
system includes 10 pairs of cranial nerves and also 10 pairs of spinal
nerves.

The principal sense organs of the frog are the eyes, the auditory
organs, and the olfactory organs. In addition to the upper and lower
eyelids there is a third, called the nictitating membrane, which is fused
with the lower one. The lens is large and nearly spherical and there

376 METAZOAN PHYLA

is little power of accommodation. The auditory organ has been in a
general way described in the preceding topic. There are three semi-
circular canals. The olfactory epithelium lines cavities just within
each of the nostrils.

401. Behavior.—Endeavors have been made to determine the
function of the different regions of the frog’s brain by the removal of

Olfactory lobe

Cerebrum

Pineal
Stalk

Optic
Jobe

Cerebellum

Medullq

Fic. 266.—Brain of European frog, Rana esculenta Linnaeus, viewed from above.
(From a Ziegler model, after Wiedersheim.) The roots of the cranial nerves are marked by
roman numerals.
one after another in the living frog. It has been found that removal
of the cerebral hemispheres, together with the olfactory lobes, seems
to have little effect. When the mid-brain is removed the frog loses
its power of spontaneous movement, which is clearly connected with
the loss of sight. Also the spinal cord becomes more irritable, which
shows that the destruction of the mid-brain and the loss of sight have
removed a control which was necessary to nervous equilibrium. The

CLASS AMPHIBIA 377

very small size of the cerebellum seems to show that it has no important
function. After all of the brain is removed except the medulla the
animal still continues to breathe, will snap at food brought in contact
with its jaws, is able to leap, swim, and right itself when placed upon
its back. Destruction of the medulla, however, results in death. This
clearly shows that the vital centers are lodged in the medulla and that
the actions of the frog are very largely reflex in character.

Since it is true that the activities of the frog are mainly reflex it is
also clear that they are governed largely by instinct. The fact that
the removal of the anterior part of the brain in front of the medulla has
so little effect upon its activities indicates plainly the low grade of
intelligence possessed by the animal. The roof of the cerebrum in all
forms up to Amphibia has been epithelial and without nerve cells. In
Amphibia it contains nerve cells, but these are inside and are covered by
fibers and are not organized into a cortex.

The frog responds directly to many external stimult. It is sensitive
to light, the whole skin being affected. The animal is said to exhibit a
negative phototropism since it avoids bright light and, when exposed
to it, faces it, that being the position in which the smallest amount of
light will be received by the skin of the body as a whole. Frogs are
also stimulated by contact and tend to crawl under objects and into
crevices. Both of these responses are modified by temperature. Natu-
rally frogs avoid a degree of heat which would cause their skin to become
dry.

Frogs can form simple habits, although they do so very slowly.
Yerkes found that after about a hundred trials a frog was able to traverse
the proper path in a simple labyrinth of passages. Some intelligence
may have been involved in this behavior, but it was, apparently, largely
the formation of a habit.

402. Reproduction and Development.—All Amphibia are diecious.
The eggs are set free in the body cavity of the female and are collected
by the open ends of coiled oviducts. They are accumulated in thin-
walled distensible portions of the oviducts known as uteri. The glands
of the oviducts secrete the gelatinous coating of the eggs. Certain
facts have been stated in regard to the reproduction of particular forms,
but the development of the frog will be given in detail as typical of the
class.

Frogs deposit their eggs in water in the spring. While in the body
of the female the eggs are surrounded with a layer of transparent jelly
which is thin, but as soon as the eggs are brought in contact with the
water this jelly absorbs water and swells, becoming thick and serving
as a protective covering. During egg laying the male clasps the body of
the female by his forelegs and fertilizes the eggs by depositing sperm
cells upon them as they are passed out of the cloaca. The upper pole

378 METAZOAN PHYLA

of the egg, called the animal pole, is dark in color, and the lower, or
vegetal pole, is light because of the massing of the yolk in that portion.
The eggs are holoblastic but, owing to the amount of yolk, undergo
unequal cleavage (Fig. 267). The upper and smaller cleavage cells
are known as micromeres, the lower and larger ones are the macromeres.
A blastula cavity, or blastocoel, is formed, and gastrulation takes place
by epibole, a fold of micromeres growing around and inclosing the
macromeres, leaving the yolk visible only through the blastopore. This

al slits

Mouth ;
invagination Archenteron

I J Yo/k K

Fic. 267.—Early stages in the development of a frog. A, egg cell, before cleavage.
B, two-cell stage, and C, four-cell. D, blastula, and £, section of it. F, beginning gas-
trulation by epibole, and G, the process more advanced. H, stage showing the yolk plug,
and J, somewhat later. J, stage with pharyngeal arches and slits; K, median section of
same stage. From specimens and Ziegler models.

visible portion of the yolk is termed the yolk plug. Soon after gastrula-
tion there is developed a groove, called the medullary groove, running
dorsally from the blastopore forward toward what will become the
anterior end of the larva. The blastopore gradually becomes obliterated
by the contraction of its margins. The embryo now becomes elongated
and the head and tail become free.

Later, and after the embryo has become better developed, a swelling
appears on each side near the anterior end of the body. Below each
swelling is developing a gill arch, and in front of it a depression which
moves toward the ventral side of the body and unites with that of the
other side to form a ventral sucker. Above the ventral sucker an invagina-

CLASS AMPHIBIA 379

tion called the stomodewm marks the beginning of the mouth, while
toward the posterior end of the body, below the tail, which is developing
backward, is formed another invagination, the proctodeum, which will
become the cloacal opening. The medullary groove is converted into a
medullary tube by the meeting of the ridges on each side of it. This
tube in turn develops into the central nervous system. Eyes appear
on each side of the head, and ezternal gills are formed which project
outward from the branchial arches. At the same time muscle segments

Oral sucker

Fic. 268.—Later stages in the development of the frog. A, embryo at time of hatching.
B, tadpoles clinging to vegetation after hatching. C, stage showing external gills. D,
gills covered by an operculum, the branchial chamber opening to the outside by the spiracle;
hind legs appearing. EE, hind legs well developed. F, late stage in metamorphosis; legs
all present, and tail nearly gone. G, the adult leopard frog, Rana pipiens Schreber. From
models, and preserved and living (Fig. G@) specimens.

ANN

are seen developing under the skin on each side of the body and tail.
The yolk is massed in the ventral portion of the body causing it to be
much swollen.

While still within its albuminous envelope, the embryo moves about
inside this envelope by means of cilia on the epidermis, but, upon hatching
(Fig. 268), the cilia disappear and the animal swims by the movement
of its tail. The two pairs of external gills become long and branched.
For a few days after hatching, the larva spends most of its time clinging
to objects in the water by its ventral sucker and lives upon the yolk

380 METAZOAN PHYLA

still contained in the archenteron. About the time the yolk is used up
a connection appears between the cavity of the stomodeum and that of
the anterior end of the archenteron and another between the cavity
of the proctodeum and that of the posterior end of the archenteron,
which thus becomes converted into an alimentary canal. The animal
now begins to swim, feeds on algae and other vegetable matter, and is
known as a tadpole. At this time the external gills begin to shorten,
and internal gills, of which there are four pairs, are being formed. A
fold grows around the body just behind the head, covering the gill
slits on both sides and producing a chamber known as a branchial pouch.
The branchial pouch opens on the left side by a circular opening called
the spiracle. By this time the external gills have ceased to function
and the internal gills serve in respiration, water being passed through
the mouth, on through the gill slits, into the branchial pouch, and out
through the spiracle.

Of the two pairs of limbs the hind pair appear first. Later the fore-
limb on the left side emerges through the spiracle, while the one on the
right side breaks through the wall of the branchial pouch. The tail
diminishes in size, being in part absorbed by cells in the body and in
part inclosed by the body, until it is no longer apparent from the outside.
The internal gills are also absorbed and lungs develop to function as
respiratory organs, after which the gill slits close and the branchial
pouch disappears. During the time between the giving up of branchial
respiration and the functioning of the lungs the skin becomes very
vascular and respiration is carried on through it. Thus metamorphosis
takes place gradually and changes the tadpole into a frog differing from
the adult only in size.

403. Neoteny and Pedogenesis.—Pedogenesis has already been
defined as the production of young by an immature animal. In the
tailed amphibians cases are known in which the larval characters are
retained until after sexual maturity. These animals may either be
looked upon as adults which, having not metamorphosed, retain certain
larval characteristics, or they may be considered as being larvae in
which the reproductive organs are precociously developed. This
prolongation of larval characteristics into advanced age has been termed
neoteny, and reproduction by these animals may be termed pedogenesis.
A classical example of these is seen in the larvae of species of Ambystoma.
Under certain circumstances these salamanders do not metamorphose
but retain their gills and their aquatic life and yet become sexually
mature. In this condition they are known as azolotls (Fig. 260B).
It has been found possible under experimental conditions to control
metamorphosis and to produce the axolotl type at will. In nature these
are particularly abundant in alkaline lakes and ponds throughout the
semiarid regions of the West, and south into Mexico.

CLASS AMPHIBIA 381

404. Regeneration.—Amphibians possess greater powers of regenera-
tion than any other vertebrates. Limbs and tails of larvae when cut
off readily regenerate. This of course is an advantage when mutilation
occurs as the result of seizure by enemies.

405. Hibernation.—The power of hibernation is sometimes considered
as an additional adaptation to terrestrial life. During the winter and
frequently during seasons when bodies of water become dry, amphibians
will bury themselves in the mud at the bottom and remain there in a
dormant condition until spring or until the water is restored. During
this period of dormancy the lungs are not used in breathing, and respira-
tion must take place through the skin. The temperature of the hiber-
nating animal remains slightly above that of the earth about it, only a
small amount of physiological activity is maintained, and the organism
lives on food stored in its body. In some tropical countries amphibians
exhibit a similar dormant condition during the heat of the summer.
This phenomenon is known as estzvation.

406. Economic Importance.—Amphibians are almost without excep-
tion beneficial, and some, particularly the toads, are of considerable
importance as destroyers of noxious insects. Frogs are used as food,
the hind legs only being eaten, and frog farms are now being operated
in Wisconsin, California, and a number of other states. Frogs are
also used very extensively in laboratory experimentation and as fish
bait.

CHAPTER LVI

REPTILES AND BIRDS

The next two classes, which include reptiles and birds, have so
many features in common that it has been suggested that they form a
single class, the Sauropsida. This view, however, has not been generally
accepted. These classes differ from Amphibia by characteristics which
show a more decided adaptation to terrestrial life and which completely
emancipate the animals included in them from an aquatic environment.
Although there are in each class types that have returned to aquatic
life, they do not again regain the characteristics which belong to aquatic
vertebrates as such.

407. Structural Characteristics—Among the structural character-
istics which the reptiles and birds possess in common and which separate
them from the amphibians are: (1) They possess but one occipital con-
dyle for articulation of the cranium with the vertebral column; the
amphibians have two. A condyle is a rounded projection with an
articulating surface. (2) A complete thoracic basket is formed by the
ribs, which meet a sternum, or breastbone, in the ventral median
line. (3) Respiration is carried on throughout life by lungs, and though
branchial arches appear early in embryonic life, their development ceases
before gill slits are formed. (4) The kidney is a metanephros. (5) The
eggs are meroblastic and not holoblastic. (6) Embryonic membranes
known as the amnion and allantois are developed during embryonic life.

Reptiles and birds also differ from mammals in the following ways: (1)
The latter have two occipital condyles. (2) In mammals the lower jaw
articulates directly with the cranium. In other vertebrates with bony
skulls there is a quadrate bone interposed in this articulation. The
quadrate bone is thought to have disappeared from its original position
in this articulation by becoming one of the three bones incorporated in
the middle ear of mammals. (3) Mammalian development shows
characteristic modifications, adjusting the young to development within
the body of the mother.

408. Embryonic Modifications.—The eggs of fishes and of amphibians,
which are laid in the water and buoyed up by it, are usually protected
only by a gelatinous covering. The aquatic environment prevents them
from drying and they do not suffer from the effects of mechanical con-
tacts since they move freely in the water in which they are suspended.
When, however, as in the case of the reptiles and birds, the eggs are
deposited outside water, they need protective envelopes to prevent

382

REPTILES AND BIRDS 383

them from drying and also need to be safeguarded from mechanical
injuries. Certain coverings of the egg meet the former need, and an
amniotic sac the latter.

409. Egg.—Three coverings are added to the egg cells as adaptations
to development in a non-aquatic environment (Fig. 269). These are
(1) a layer of albumen, which provides protection against drying and
mechanical injury and also serves as food for the embryo; (2) an egg
membrane, which in some cases becomes thick and leathery and to which
may be added lime; and (3) a shell, present in many reptiles and normally
in all birds, composed entirely of lime. The albumen, the membrane,
and the shell are all secreted by glands lying along the course of the
oviduct in the order in which the envelopes which they form are added.

Vite/line

Pen aeraie Germinal area

Whife Yellow yolk

Fic. 269.—Diagrammatic section of a hen’s egg.

410. Amnion.—Since the eggs of reptiles and birds are meroblastic,
discoidal cleavage occurs and a sheet of cells, the blastoderm, is formed.
From a part of the blastoderm is developed the embryo, and the rest
of it grows around and completely envelops the yolk. The blastoderm
splits, forming two layers, ectoderm outside and entoderm inside, next
to the yolk. Between these appears a third layer, the mesoderm, and
this also splits into two layers, one of which, the somatic layer, lies next
to the ectoderm and with it forms the somatopleure, the other, the
splanchnic layer, next to the entoderm and with it forms the splanch-
nopleure. Between these two mesodermal layers is the coelom (Fig.
270 B). The amnion is a fold of the blastoderm outside the area forming
the embryo and is composed of two layers (Fig. 270 C), ectoderm and
somatic mesoderm, or somatopleure. As this fold grows up around the
embryo it meets above and incloses a sac, known as the amniotic sac,
which surrounds the embryo and which becomes filled with a watery
liquid known as the amniotic fluid (Fig. 270 D). The embryo is free
to move in this sac and thus is protected from jar, though the eggs may

384 METAZOAN PHYLA

be rolled about or be subjected to blows from without. The outer wall
of the amniotic fold is continuous with a fold of the somatopleure which
grows down around the yolk sac. As the splanchnopleure also extends
down around the yolk sac a space is formed between the somatopleure
and splanchnopleure, the wall of which is mesoderm and which is known
as the extra-embryonic coelom (Fig. 270 D).

Nerve cord Nerve cord

Notochord

fold Embryo fold
A
Extra-embryonic Sis a
coe/orm Se
Amn ° é catia ‘
GALA ty ; shell Uff (LUT ITLGD
pee sor? ps OLo7 Or Sees
~*~ bs Q:0° C SOS
eae gee 5
ss Pe La ge Fa
—=\— Extra-embryonic
Of coe/on?

Amniotic
cavity

(SITTOMBSD G,
Yl pgracaceonseita

Allanfois

Fic. 270.—Diagrams of the development of a bird’s egg. A, cross section of an
amphibian embryo for comparison with B, which is a cross section of an avian embryo
at an early stage. (C, D, and E, stages in the development of amnion and allantois in the
bird, shown in longitudinal section. Ectoderm is shown in C, D, and E by a solid line,
entoderm by dashes, mesoderm in mass by crosslines, and somatic mesoderm and splanchnic
mesoderm by dots.

411. Allantois.—Since the embryo needs oxygen, a means must be
provided for respiration. This is afforded by the allantovs, which is an
outpocketing of the enteron posterior to its connection with the yolk
sac, the wall of which consists therefore of entoderm and splanchnic
mesoderm. This outpocketing projects into the extra-embryonic
coelom (Fig. 270 D) and as it develops it expands mushroomlike against
the outer wall of that cavity (Fig. 270 #). The somatopleure and allan-
tois together form what has often been termed a chorion, which is spread

REPTILES AND BIRDS 385

out over the inner surface of the egg membrane. The egg membrane
is in turn in close contact with the shell. Into this chorion extend the
allantoic blood vessels, which form a rich capillary network (Fig. 271).
Since the shell is in all cases sufficiently porous to permit of the passage
of air, respiration is carried on through it. This explains why it is
fatal to the developing embryo if the shell is covered by any material
which closes the pores and makes it impervious to the passage of gases.

Gea Dassen Vi ae
MMILOTIC are ae (24 (efe]
cavi See vessels
ty ESSA
Beets WSR
Sah,

Yolk circulatron

Fic. 271.— Diagram of a stage in the development of a bird’s egg, later than shown in

Fig. 256, and indicating the circulation; arteries unshaded, veins black. Arrows show

direction of blood flow. (From Wilder, ‘‘ History of the Human Body,’’ by the courtesy of
Henry Holt & Company.)

When the young animal hatches from the egg, the connections of
the amnion and allantois with the body are broken, and these are left
behind in the empty shell. Though not much tissue is sacrificed, this
presents a contrast to the condition seen in previous types, in which
all of the egg cell became part of the body of the animal to which it
gave rise.

412. Body Coverings.—The birds and reptiles are distinguished
particularly by the body coverings. Reptiles possess horny epidermal
scales which usually overlap, and birds are covered by feathers which are
similar in their origin and in their general mode of development to scales
and which are, therefore, looked upon as modifications of them.

CHAPTER LVII
CLASS REPTILIA

All reptiles possess three regions of the body—head, trunk, and tail.
There is generally a sufficient constriction behind the head to form a
neck. The typical body covering is composed of overlapping epidermal
scales which form a hard coat of mail, protecting the body against drying
and also against injury from ordinary mechanical contacts. Well-
developed eyes are present, protected by lids in all forms except the
snakes; two nostrils are situated toward the end of the snout; and behind

Right Right — Left .

carotid aortic
arfery\,

WG
iS a

eee
_— Pulmonary ye

Ya artery
: Left aortic
ATC. L e ae
eft Right auricle

ricle auricle

ae
auricle Gpenines
Fror
auricles
Ventricle Aorta

Fic. 272.—Reptilian hearts. A, heart and associated blood vessels of snapping turtle,
viewed from in front. From a specimen, but somewhat diagrammatic. 3B, similar repre-
sentation of crocodile’s heart. (From Hertwig and Kingsley, ‘‘ Manual of Zoology,’’ by
the courtesy of Henry Holt & Company.) Neither figure shows the venae cavae or the
pulmonary veins. In both figures the vessels carrying arterial blood are unshaded; those
carrying venous blood are black; and those carrying mixed blood are crosslined. The heart
of the crocodile is cut open to show the chambers; the direction of blood flow is shown by
arrows and the connection between the two aortic arches is also shown.

each eye is an ear opening except in the snakes. The mouth is terminal
and the jaws may bear teeth, as in most reptiles, or may be furnished with
a horny beak, as in the turtles. A cloacal opening marks the posterior
end of the trunk region proper and the beginning of the tail. Four limbs
are generally present, lacking only in snakes and a few lizards.

413. Classification.—The class Reptilia is divided into four orders:

1. Squamata (skwi ma’ ta; L., squamatus, scaly).—Chameleons,

lizards, and snakes.
386

CLASS REPTILIA 387

2. Rhynchocephalia (rin ko sé fa’ lia; G., rhynchos, snout, and
kephale, head).—One living type, a lizard-like animal found only in New
Zealand.

Olfactory lobes

Cerebrurn

Optic lobe

Cerebellum

Medu/la

LE P
Fic. 273.—Brain of alligator, Alligator mississippiensis (Daudin), viewed from above.

(From a Ziegler model, after Wiedersheim.) The roots of the cranial nerves are marked by
roman numerals.

3. Crocodilia (krék 6 dil’ia; G., krokodeilos, crocodile).—Crocodiles
and alligators.

388 METAZOAN PHYLA

4. Testudinata (tés ti di na’ ta; L., testudinatus, like a tortoise).—
Turtles and tortoises.

414. Internal Structure——The heart of a reptile consists of two
auricles and a double ventricle (Fig. 272A), the latter being divided by a
septum which, however, is perforated, except in Crocodilia. The blood
from the veins enters the right auricle, passes into the right ventricle
and thence to the lungs. From the lungs it is returned to the left
auricle, goes to the left ventricle, and out through the two aortic arches
to the arteries. The blood in the two ventricles mingles to a certain
extent, and so mixed blood is sent out over the body. In the Crocodilia
(Fig. 272 B), where the ventricles are quite separate and the left aortic
arch as well as the pulmonary artery arises from the right ventricle, a
communication between the two aortic arches permits mixing of the

Fic. 274.—Six-lined race runner, Cnemidophorus sexlineatus (Linnaeus). A common
member of the group of sand lizards which has an extensive distribution occurring in the
United States, Mexico, Central and South America. They are fast runners and can dart
away from their pursuers very swiftly; this is responsible for the name race runners com-
monly applied to them. When seized by the tail by an enemy, they can detach their tail
and escape. Detachment of an organ when stimulated is known as autotomy. (Photo-
graphed and contributed by George E. Hudson.)

arterial and venous blood. Renal-portal and hepatic-portal systems are
both present, the latter being better developed than in the amphibians.

The lungs of reptiles are rendered more complex than those of the
amphibians by repeated divisions of the bronchi and an increase in the
number of the alveoli. This increases considerably the surface through
which respiration is carried on.

The brains of reptiles (Fig. 273) show an advance over those of the
amphibians in the better development of the cerebral hemispheres and
of the cerebellum. The greatest advance, however, is in the appearance
of a cerebral cortex. Here, as a result of the multiplication of the nerve
cells and their regular arrangement, the roof of the cerebrum is divided
into an outer gray layer and an inner white one. The cells in the gray
matter are arranged in distinct groups or areas corresponding to the par-
ticular activities which they control. Such a brain roof is called a cortex.

CLASS REPTILIA 389

In the reptiles the organs of sight and hearing are generally well
developed, as are also to a lesser degree those of taste and smell, while the
skin over various parts of the body is very sensitive to touch. There
is usually a middle ear, with a tympanic membrane, a eustachian tube,
and a columella. In the chameleons and snakes the tympanic membrane
is absent; in the turtles it is on the surface of the body; and in the lizards
and crocodiles it is at the bottom of a pit, which may be considered the
beginning of an outer ear.

415. Squamata.—This order is characterized by a typical scaly
covering which is shed periodically. In the case of snakes it is cast off

\
Ss ss
\

Fie. 275.—Common chameleon of southern Europe, Chamaeleo vulgaris Daudin. (Based
upon figure in Brehm, ‘‘ Thierleben.”) X %.

complete and at one time, but in the lizards it is stripped off in shreds

during a period of several days. Three groups of Squamata are usually

recognized—chameleons, lizards, and snakes.

416. Chameleons.—Some lizards are erroneously called chameleons
because of the ease with which they change color. This term, properly
speaking, should be restricted to certain Old World reptiles which possess
several pronounced characteristics (Fig. 275). Among such character-
istics connected with arboreal life may be mentioned a lateral com-
pression of the body; the possession of a long prehensile tail which is not
easily broken and which if lost cannot be regenerated; and the existence
of long, slender limbs with digits so arranged that two are opposable
to the three others, making the foot effective in grasping. In each group
of digits, they are united nearly to their tips. The eyelids are united,
except for a small central opening. The eyes are capable of being moved

390 METAZOAN PHYLA

independently and it is stated that the peculiar structure of the lids
makes it possible for the animal to locate objects very exactly. The

((

Seute ) %s

V
i)

Cat

((

4egeaee

Ear opening

Fic. 276.—Heads of a bull snake, Pitwophis sayi (Schlegel), and of a lizard, Plestiodon
septentrionalis Baird. A, head of the snake, fromthe side. B,frombelow. C, head of the
lizard, from the side. D, from below. A and B, X 24; C and D, X 2.

tongue, which is club-shaped and abundantly provided with a sticky
secretion, can be protruded to a distance of six inches or more. ‘This
enables the chemeleon to use its
tongue for capturing insects, which
constitute its entire diet. Cha-
meleons are generally oviparous,
although a few produce living young.
They are famed for their power of
changing color, which is due to the
presence of chromatophores in the
skin, affected both by outside stimuli
and by stimulation by the nervous
system.

417. Lizards.—The lizards are
the most typical of the reptiles.
The limbs are well-developed and
modified for running, climbing, or
digging. Rarely limbs are absent,
in which case the animal has very

much the appearance of a snake.

Eig oti Wall gecko,» Taremoa. This is tmuemonithe, so-called glass
mauritanicus (Linnaeus), of southern
Europe. (From Brehm, “ Thierleben.”) snakes of Europe and America, which
x %- are really lizards, and of some bur-
rowing lizards, known as worm lizards, found in southern United States.
Legless lizards may be distinguished from snakes (Fig. 276) by the

ay =

CLASS REPTILIA 391

presence of movable eyelids and an external ear opening, both of which
snakes lack, and by having small overlapping scales on the ventral side of
the body instead of the transverse scutes which snakes: possess. The
tails of lizards are generally long, easily broken off or separated into pieces,
and with equal ease more or less completely regenerated, though the regen-
erated appendage does not possess vertebrae. Lizards are in most cases

- mancremmerrmecen I

4
|
|
|

b — Pe Sie ann a Soa ae Coen ins Skee nn wie fi sapere ee
Fic. 278.—Common horned lizard, Phrynosoma douglassti hernandezi (Girard), of the
Great Plains and Rocky Mountain region represents a genus of lizards, often incorrectly
called horned ‘“‘toads”’ because of their broad, flat, toadlike bodies. The head and body
are armed to a varying degree with spines; their range is the western United States and
Mexico. (Photographed and contributed by George E. Hudson.)

oviparous and the eggs are protected by a thin shell with little lime in it.
They feed largely on insects, worms, and other small animals and some
are to a considerable degree vegetable feeders.

Geckos (Fig. 277) are lizards which inhabit all warmer regions, are
nocturnal, and have toes fitted for suction, thus enabling them to run over
trees and rocks and even over the walls and ceilings of buildings. The
flying dragon, (Fig. 279) found in southeastern Asia and the East Indies,
has the ribs extended beyond the sides of the body and covered by a thin

392 METAZOAN PHYLA

membranous fold of the skin, forming a sort of gliding plane. In tropical
America are large lizards known as iguanas which are a favorite article

Fic. 279.—The flying dragon, Draco volans. From a preserved specimen. X 4.

in the native diet. The horned toads (Fig. 278) of the West are lizards.
The only poisonous lizard known is the Gila monster found from south-

a a Se oleae Late

oi aS Ss enctat ne

Fic. 280.—Common garter snake, Eutaenia sp., swallowing a toad which has inflated
itself as a ‘‘preventive measure.’’ Frogs, toads, and earthworms are the principal food.
The striped snakes, called the garter snakes, are the most common snakes of North America.
(Photographed, copyrighted, and contributed by Gayle Pickwell.)

western United States to Central America. The largest lizards in the
world are the monitors of the Lesser Sunda Islands in the Malay Archi-
pelago which reach a length of 15 feet.

CLASS REPTILIA 393

418. Snakes.—Snakes differ from lizards and chameleons in the
structure of the lower jaws; in the absence of both free limbs and

Fic. 281.—Skeleton of a bull snake, Pitwophis sayi Schlegel, showing all the ribs reaching
the same level. The broad ventral scutes manipulated by muscles in connection with the
ribs enable the snake to move about. No sternum is present. (Prepared, photographed,
and contributed by George E. Hudson.)

girdles, though in a few forms a trace of the pelvic girdle is present;
and in the absence of a urinary bladder.
Owing to the fact that the lower jaws
on the two sides are but very loosely con-
nected in the median line and that they
are also loosely attached to the quadrate
bone, which is in turn loosely attached
to the skull, the mouth may be greatly
expanded. Since there is no sternum and
the ribs are not attached ventrally, the
throat and body are capable of great dis-
tension. Thus snakes, which always swal-
low their food whole, are able to ingest iT
objects actually much greater in diameter i id
than the head or body of the snake itself. }
During the process of swallowing, the teeth,
which point backward, are used to hold the
prey and those of the two sides are brought
into use alternately. While the teeth of the
jaws on one side are holding the prey, those - :
of the jaws on the other side are loosened , F's: ?52.-—A constricting snake,
2 : ‘ Boa sp., clinging to a post and
and that side of the mouth is carried forward maintaining its position for more
over the object being swallowed, after which than an hour with about a third of
those teeth are again set in. Then the Se Latent ae: 2
teeth on the first side are loosened and that side of the mouth is carried
still farther ahead and those teeth in turn set in. Thus by working the

394 METAZOAN PHYLA

two sides of the mouth alternately, the animal gradually forces the
object down into its esophagus, through which it is passed by peristaltic
movements to the stomach. Since during the process of swallowing food
the passage of air through the mouth would be interfered with, the glottis
is carried far forward in the floor of the mouth, opening just behind the
lower teeth. The cartilages of the trachea prevent it from being closed,
and breathing is in this way permitted while the food is being swallowed.

Over the sides and back of the body of a snake the scales overlap
like shingles on a roof. On the ventral surface, however, there is a series
of broad scales known as scutes, each one of which runs from one side of
the body to the other and which overlap like the weatherboards on a

Fic. 283.—Eggs of the bull snake, Pituophis sayi (Schlegel), which are buried in the
ground and incubated and hatched by the heat from the sun. (From J. E. Guthrie, The
Snakes of Iowa, Bulletin #239, Iowa State College of Agriculture, 1926.)

house. The posterior margin of a scute can be projected, and as its
rough edge is pressed against the surface on which the snake is, it serves
as an organ of prehension. As waves of contraction pass from the head
backward, while the scutes are used in clinging, a slow forward gliding
results, which is the normal mode of locomotion. ‘This is usually accom-
panied by lateral convolutions. The latter mode of locomotion is also
used on a smooth surface where the scutes do not gain a hold. When the
snake is alarmed, it attempts to make more rapid headway by alternately
throwing the body into coils and then straightening it again. Swimming
is also accomplished by lateral convolutions of the body.

The whole epidermal covering is shed at one time and several times
during a year. After a new horny covering is formed under the old one,
the latter is freed along the margins of the Jaws and the animal literally

CLASS REPTILIA 395

crawls out of its old skin, which becomes turned inside out in the process.
When snakes are about to shed, the old skin becomes dull and opaque.
The eyelids are fused over the eyes, which are covered with transparent
scales, and since at this time these scales also become opaque, the snake
is partially blind. After shedding, the new covering is bright and the
eyes are once more perfectly clear.

Snakes possess very good vision. As they have no tympanic mem-
brane, their sense of hearing is not highly developed. Their sense of

Fic. 284.—Timber rattlesnake, Crotalus horridus Linnaeus, of the eastern United
States. Range from Vermont to Florida and west to the Missouri river. Referred to by

”?

Ditmars as ‘‘one of the most beautiful of the North American rattlesnakes. A very

poisonous snake. (Photographed and contributed by George E. Hudson.)

smell is good, but that of taste poor. The tongue is slender, deeply
forked, and lodged in a sac in the floor of the mouth. When the jaws
themselves are closed, the tongue may be protruded through an opening
formed by notches in the two jaws (Fig. 276). The tongue is used as an
organ of touch and with it the snake tests objects. Contrary to popular
opinion it can inflict no wound.

The majority of snakes lay eggs but a few are viviparous. The
idea is prevalent that snakes can swallow their young when danger
threatens but this is not supported by scientific observation.

396 METAZOAN PHYLA

Snakes are more abundant in the tropics than elsewhere and are
frequently absent from islands, though found on the adjacent continents.
Some snakes live mostly in and about fresh water; some live in salt
water; others are subterranean, burrowing in the ground; while still others
are expert tree climbers. The largest of snakes are a python found in
Burma, which reaches a length of over 30 feet, and an anaconda of the
region of the Amazon in South America, which may approach 40 feet
in length.

419. Venomous Snakes.—The sea snakes are very poisonous. They -
are tropical forms found in the Indian Ocean, the adjacent parts of the
Pacific, and along the western coast of South America. They belong

nes —- —— —— Saar nee SaEaEEEErneEo — —

Fig. 285.—A water moccasin or cottonmouth, Agkistrodon piscivorus, living in the
lagoons and sluggish streams of the southeast U. S. A very poisonous snake. (From
Ditmars, ‘‘ Snakes of the World,” by courtesy of The Macmillan Company.)

to the same general group as the coral snakes and the cobras, all having
tubular fangs in the anterior part of the upper jaw, in connection with
which are poison glands which secrete a very effective venom. The
cobras are found in India, China, the Malay Archipelago, and Africa.
These snakes are capable of expanding the anterior part of the body
and of raising the head and that part of the body well off the ground.
In this attitude they strike. India is outstanding for the number of
species of venomous snake living in its area and many people die there
from snake bites each year.

Among the snakes called vipers are the pit vipers, represented in
the New World by the many species of rattlesnakes (Fig. 284), the water
moccasin (Fig. 285), and the copperhead. In all of these the young are
brought forth alive. The water moccasin, found in southern United
States, is one of the most poisonous of all snakes. The copperhead
(Fig. 286), found farther north, is also very venomous. Rattlesnakes

CLASS REPTILIA 397

possess a series of hollow epidermal buttons linked together and forming
a rattle attached to the tip of the tail, which is itself flattened, hardened,

Fig. 286.—A copperhead, Agkistro
southward to Florida and westward into Texas and the Great Plains as far north as southern
Nebraska. A very poisonous snake. (From Ditmars, ‘‘ Snakes of the World,” by courtesy
of The Macmillan Company.)

and constricted to receive the anterior margin of the basal rattle. A new
button is added to the series of rattles every time the skin is shed.

DEE 4

E
a 3

Fic. 287.—Coral snake; harlequin snake, Micrurus fulvius Linnaeus, of southeastern
United States. A poisonous snake. (From Ditmars, ‘‘ Snakes of the World,” by courtesy of
The Macmillan Company.)

Since this may occur several times a year, and since rattles may be lost,
the number possessed by an individual is no precise indication of the age
of the snake.

398 METAZOAN PHYLA

The pit vipers (Fig. 288) have long, curved fangs near the anterior
end of the upper jaw. When the mouth is closed they lie against its
upper wall, but when it is opened widely they are raised and stand nearly
at right angles to this surface. In this position when the snake strikes,
they are driven straight forward into the body of the animal struck;
at the same moment poison is injected into the wound from the poison

Palatine

Fig. 288.—Skull of rattlesnake, Crotalus confluentes Say. From a young specimen from
Montana. A, entire skull, seen from the side and below. 8B, lateral view, with jaws of
only one side shown. The erection of the fang is caused by the thrust of the ectopterygoid
against the movable maxilla. When the mouth is closed the ventral end of the quadrate,
to which the lower jaw is articulated, is carried backward, the palatine and pterygoids are
brought up toward the floor of the cranium, and the fangs lie against the roof of the mouth.
But when the mouth is opened the articulation of the quadrate and lower jaw is brought
forward, causing the palatine and pterygoids also to be carried downward and producing a
forward movement of the ectopterygoid, which in turn erects the fang. In C and D is
shown a mechanism which would work in the same fashion.

glands at the bases of the fangs. Snakes use their poison fangs both in
securing prey and in defending themselves from enemies. When fangs
are lost they are replaced by other fangs which lie concealed behind the
functional ones and come up one at a time to take the place of the lost
fangs.

420. Rhynchocephalia.—The only representative of this order is
the tuatara of New Zealand, Sphenodon punctatum (Gray). It was
formerly found throughout New Zealand but is now restricted to some

CLASS REPTILIA 399

small neighboring islands and is threatened with extinction. It is
lizard-like (Fig. 289), about two feet in length, lives in burrows, is
nocturnal in its habits, and feeds on any other animal it can secure.
Among other structural features which it possesses is a more highly
developed pineal eye than is possessed by any other animal. ‘This is an
eye developed from a median dorsal outgrowth of the diencephalon;
it is rudimentary in all living vertebrates, being most highly developed
in lizards, and is often called the pineal gland or pineal body. It is

Fic. 289.—Tuatara, Sphenodon punctatum (Gray), of New Zealand, an example of the

Rhynchocephalia. X about 14. Compiled from several sources.
believed to have functioned as an eye in types now extinct and in living
forms it has been looked upon as a gland of internal secretion. Although
this animal has some characteristics which belong to fossil reptiles and
has long been looked upon as the most primitive of existing reptiles,
it now seems probable that it is not an ancestral form from which other
lizards have sprung but rather a highly modified type which has persisted
from earlier times.

Fic. 290.—North American alligator, Alligator mississippiensis (Daudin), an example of
the Crocodilia. X about 4g. From several sources.

421. Crocodilia.—This order contains some of the largest living
reptiles, the gavials of southern Asia and the caymans of South and
Central America reaching a length of 20 feet or more and a Philippine
crocodile having been captured measuring 29 feet in length. A crocodile
and an alligator (Fig. 290) are found in southern United States. Cro-
codilia are lizard-like in form but the scales do not overlap, being set
into the thick, leathery skin, broadened, and sometimes raised to form
ridges. The snout is long and the nostrils are at its tip. Since the eyes

400 METAZOAN PHYLA

project from the top of the head, the animal can lie just under the
surface of the water with only its eyes and its nostrils exposed. The
eyes are covered with lids and both nostrils and ears possess valves which
may be closed when the animal is under water.

There are several noteworthy details connected with the internal
anatomy. The teeth are conical, set in sockets, and are capable of
being shed at intervals and replaced. The tongue is flat and cannot

Llium Ischium
Fig. 291.—Skeleton of a European turtle, Cistudo lutaria (Marsili). From a mounted
preparation. A, the carapace with the internal skeleton. 3B, the plastron, removed.
The limb skeletons are inside the ribs. X 34.

be protruded, but it may be lowered and carried backward to prevent
water from entering the esophagus if the mouth is opened while the
animal is submerged. Lateral folds meet and form a palate that separates
the nasal chamber from the mouth and the nasal chamber becomes
divided by a median septum. The lungs are in a pleural cavity sepa-
rated from the rest of the body cavity by a diaphragm analogous to that
of the mammals.

422. Testudinata.—The turtles and tortoises, which names are
used interchangeably, show the greatest departure from the typical
reptilian form. The body is inclosed in a shell (Fig. 291) consisting

CLASS REPTILIA 401

of a dorsal carapace and a ventral plastron. Into this shell may be
drawn the head and neck, limbs, and tail, when the animal is threatened
with danger. Although turtles breathe by means of lungs, they may
remain under water for a considerable time before needing to come
to the surface for air. Since the shell prevents the lungs from being
expanded and contracted, air is pumped into them by movements of the
neck and feet. Some aquatic turtles also possess thin-walled sacs
on each side of the cloaca which may be alternately emptied and filled
with water and through the vascular walls of which respiration may take
place while the animal is submerged.

— . - Hoe

Fic. 292.—Common snapping turtle, Chelydra serpentina (Linnaeus). Abundant over
an extensive area in the New World, from southern Canada to Ecuador. It is among the
largest American fresh-water turtles, grows to a weight of 30 to 40 pounds and is a bold
and aggressive turtle. (Photographed and contributed by George E. Hudson.)

All turtles are oviparous. Their eggs, which are nearly spherical,
are covered with a hard, white shell and deposited in nests in the ground.

America is the richest of all regions in its turtles and tortoises. Giant
tortoises found on islands off the west coast of South America and in
the Indian Ocean reach a weight of more than 300 pounds and probably
attain an age of over four hundred years. They are relics of past ages
and owe their survival to the isolation of the islands on which they live.
The largest turtle known is a marine leathery turtle which reaches a
weight of 1000 pounds or more.

423. Economic Importance.—Reptiles are economically either injuri-
ous or beneficial. Most snakes, being nonvenomous and destroying
injurious insects and mammals, are distinctly beneficial, though some
do injury by destroying birds and their eggs and young. Other snakes
are dangerous to man because of their venom. Generally speaking,
lizards are beneficial because of their insect-eating habits; one or two

402 METAZOAN PHYLA

have been noted as being sources of food; and on the other hand one
type, the Gila monster, is poisonous. Crocodiles and alligators are
feared in the countries in which they live because of their attacks upon
persons going into the water, but they are valuable for the skins which
they furnish. None of the turtles is injurious, and some are useful as
food, being more often used in this manner than are any other reptiles.
Certain tortoises yield tortoise shell which is widely used commercially,
although it is now largely replaced by an artificial product.

CHAPTER LVIII

CLASS AVES

Although birds have many points in common with reptiles, living
birds are easily separated from living reptiles by certain pronounced
characteristics, the most prominent of which is the possession of feathers.
Living birds are all bipeds, the only reptiles sharing this characteristic
being the extinct dinosaurs from which the birds were probably derived.
Another characteristic of living birds is the absence of teeth, their jaws
being covered by a horny beak; this characteristic, however, is shared
with the turtles. The caudal vertebrae are greatly reduced in number,
and all but a few of the anterior ones, which remain free, are fused into
a single bone known as the pygostyle, or plowshare bone; this charac-
teristic is shared with extinct reptiles. Birds also possess forelimbs
modified to form wings; there were flying reptiles, now extinct, but their
wings were not constructed upon the same plan as those of birds, nor
are they regarded as the ancestors of birds. It appears, then, that in
spite of the similarities in fundamental structure and mode of develop-
ment between birds and reptiles, both living and extinct types can be
clearly distinguished.

424. External Characteristics.—The body of a typical bird is spindle-
shaped, being fitted by form for rapid movement through the air. It
is divided into four regions—head, neck, trunk, and tail. The neck is
long and flexible, correlated with the modification of the forelimb for
flight. The latter fact makes it necessary for the bird to use its beak
in the carrying on of certain activities connected with the securing of
food and nest building which in other animals belong to the forelimbs.

The forelimb is relatively slender, possesses muscles nearly to the
tip, and is covered by feathers. Along the posterior margin are attuched
a series of long flight feathers which make up most of the surface of the
wing. When the bird is at rest the wing is folded against the side of
the body and occupies little space, but when it is outstretched the
distance from one wing tip to the other usually exceeds—in some cases
very greatly—the length of the body.

The functions of the hind limb are mainly those of support and
locomotion on the ground or in the water, it being used in perching, walk-
ing, running, climbing, or swimming. It terminates in a foot typically
made up of four toes, three anterior and one posterior, these ending in
curved claws. The thigh and more or less of the next joint are muscular

403

404 METAZOAN PHYLA

and are covered with feathers; the rest of that joint, called the shank,
and the toes are naked and covered by horny epidermal scales. The
bare portion of the leg is not muscular, containing only tendons passing
from muscles in the upper part of the leg to the toes (Fig. 302). In
some birds, as in the owls, however, feathers may even extend over the
toes themselves. Birds walk only on their toes (Fig. 293).

Humerus Fermur
Patella
Radius Frbula
Ulna
Tibiotarsus

OD
Phak
pp Tit eo Tarsormeratarsus
J Metacarpals
( Ze il
nN\ Il aah 2 First metatarsal
\ ae yf! Pha/anges

7
I Iv
A Phalanges B

Fig. 293.—Leg and wing bones of a pigeon to show the homology between the two and
the modification of each. A, wing. B, leg. From a specimen.

The fleshy part of the tail is small and is concealed by the feathers
of the trunk. It bears a number of long tail feathers which when spread
often present a considerable area. The tail serves as a rudder and an
accessory organ of flight, though it is also used in balancing and in some
birds in the support of the body when the bird is clinging to a vertical
surface.

425. Feathers.—A typical feather is composed of a quill set into
the skin and a continuation of the quill, the shaft, which bears slender
barbs (Fig. 294). The barbs in turn bear still smaller and slenderer
lateral projections known as barbules. When the barbs lie parallel
and the barbules are hooked together so as to make of the whole a

CLASS AVES 405

practically continuous surface, the structure is known as a vane. When

the barbs are not systematically arranged and form only a confused,
fluffy mass, this is known as down.

B D
Fic. 294.—Types of feathers. A, flight feather. B, contour feather. C, down feather.
D, filoplume. From specimens. X 46.

Four types of feathers may be distinguished: (1) The typical feather,
known as a contour feather, is downy near its base and toward the tip
has a vane. The vanes overlap and provide a smooth outer plumage
surface, while the down forms a heat-conserving layer under it. (2)
Flight feathers differ from contour feathers in that they have practically
no down and in that the vanes are greatly lengthened and stiffened.
The flight feathers are set along the posterior margins of the forelimbs

406 METAZOAN PHYLA

and along the lateral margins of the tail. (8) Down feathers, consisting
entirely of down, are inserted into the skin between the contour feathers
and serve to increase the thickness of the down layer. (4) Floplumes,
or hair feathers, are feathers reduced to a slender, hairlike shaft with
few or no barbs and remain when the bird is divested of the rest of its
feathers.

The feathers are not distributed at random over the surface of the
bird’s body but are gathered together in certain tracts known as feather
tracts; the form and arrangement of these vary among different species
of birds, and thus they assist in determining relationships between them
and aid in classification.

426. Internal Structure.—Throughout the internal anatomy of a
bird are seen modifications which fit it for flight. The bird is of all
animals that one most thoroughly and effectively modified for a particular
type of locomotion.

The skeleton of the trunk is rigid, this rigidity being attained by
fusion of the head bones and of the body vertebrae, by fusion of the ribs
with the vertebral column and the sternum, and by an additional bracing
at the sides due to processes of bone called wncinate processes passing
from one rib backward over the next. The flexible neck and a joint
at the base of the tail provide the only movement in the axial skeleton.
The sternum in all flying birds has a pronounced crest or keel for the
attachment of flight muscles. All of the bones of the skeleton are
slender but very firm and in some cases are hollow. Lightness in bones
is secured by a minimum of actual bone and strength by a general
application in their internal structure of the engineering principles
involved in the use of thin plates at right angles to one another as in an
I-beam.

The muscles of the back are greatly reduced, while those of the
breast are correspondingly developed, since they are the ones most
used in flight. The muscles of the hind limbs are also well-developed.

In many types of birds there is an enlargement at the lower end
of the esophagus forming a crop (Fig. 295). This is most highly developed
in seed-eating birds, less so in insect eaters, and is practically absent in
fish-eating birds. The stomach consists of two portions—an anterior
glandular part called the proventriculus, which secretes the gastric
juices, and a posterior muscular gizzard, which is used in grinding the
food. Birds swallow pebbles and other hard objects which contribute
to the grinding, the gizzard having a horny lining which protects its
wall during this process. The alimentary canal opens into a cloaca.

The heart of a bird is relatively large and is composed of two entirely
distinct ventricles and two thin-walled auricles. The systemic and
pulmonary circulations are entirely separate. The one aortic arch
corresponds to the right aortic arch of reptiles.

CLASS AVES 407

The lungs of birds are not capable of dilation since the thoracic
skeleton forms a rigid framework. Breathing is allowed by the presence
of a large number of azr sacs (Fig. 295) lying among the muscles and
about the viscera in various parts of the body and communicating with
the bronchi of the lungs. Air is drawn into the windpipe, through the
lungs, and on into the air sacs by the action of the muscles of the thorax

Cere Ear opening

Auricle

Ventricle

eT
Intestine

Fia. 295.— Dissection of a pigeon, Columba livia Linnaeus. (Based upon a Pichler chart, by
the courtesy of Martinus Nijhoff.)

and abdomen. Thus the air in the lungs is practically entirely changed
with each respiration, which results in a very perfect aeration of the
blood and is responsible for the high temperature of the body. Birds
maintain temperatures varying from 40.5°C. (105°F.) to as high as
46°C. (115°F.). While flying the movements of the wings contribute
to respiration by compressing and dilating the air sacs, and thus the
bird breathes more easily when in flight than at other times. The posses-
sion of air sacs is also shared with many reptiles, especially the lizards,

408 METAZOAN PHYLA

among which, however, they are not developed to the degree seen in
birds.

The kidneys have a reduced renal-portal system. ‘There is no
urinary bladder and the urinary secretion passes directly out with
the feces. The female bird has but a single ovary, the right one dis-
appearing during development.

The brain of the bird is relatively short and broad (Fig. 296). The
cerebral hemispheres are large, but the olfactory lobes are very small,
indicating a poor development of the sense of smell. The large optic
lobes correspond to the great development of the power of sight, and
the very large and much convoluted cerebellum indicates the delicate

Cerebellun7

Cerebrurm

> See arr
\iviw V CAU/ IA

0/ yee
I oe Optic lobe

Optic
tract Hypoph Sts

Fic. 296.—Brain of pigeon, viewed from the side. (From a Ziegler model, after Wieders-
heim.) The roots of the cranial nerves are marked by roman numerals. The hypophysis
is also called the pituitary body.

sense of equilibrium and the great power of muscular coordination
belonging to birds.

Although the sense of taste is present it is not well-developed. The
ear is more complex than that of reptiles and the sense of hearing rela-
tively acute. The eyeball is large, is much elongated, and the cornea
is very convex. Bony plates are developed in the sclerotic coat. There
is also a peculiar structure known as the pecten, suspended in the vitreous
body. The pecten is highly vascular, pigmented, and fan-shaped.
Its function is not known, although it may have something to do with
the nutrition of the eyeball—or, possibly, it assists in accommodation,
which in birds is remarkably well-developed. Birds of prey, which
plunge at high speed from great heights upon their quarry on the ground
or which follow it through the branches of a woodland, need the maxi-

CLASS AVES 409

mum of quickness and accuracy in accommodation. Birds possess a
nictitating membrane, which is drawn across the eyeball from the inner
angle of the eye outward.

427. Classification.—The classification of birds is a question about
which there has been much difference of opinion. All classifications
agree, however, in distinguishing between the ancient, reptile-like,
fossil birds forming the subclass Archaeornithes (ir ké ér’ ni théz; G.,

Fic. 297.—Fossil remains of Archaeopteryx siemenst Dames, representing a specimen in the
Berlin Museum. (From Steinmann-Déderlein, ‘‘ Elemente der Palaeontologie.”’) XX 24.

archaios, ancient, and ornithes, birds) and represented by a single genus,
Archaeopteryx, and a second subclass, the Neornithes (né ér’ ni théz; G..,
neos, new, and ornithes, birds), or modern birds, containing only a few
extinct forms. Archaeopteryx (Fig. 297) was a bird about the size of
the common crow, had teeth, three free fingers on the wing, flight feathers
on the legs, and a long, lizard-like tail (Fig. 298). The caudal vertebrae
were separate and the tail had flight feathers on both sides for its full
length. The largest living bird is the African ostrich, which is 8 feet in
height, but the aepyornis, a bird which lived in Madagascar until about
five centuries ago, attained a height of 10 feet. On the other hand, a

410 METAZOAN PHYLA

hummingbird found in Central America is only 114 inches long from
the base of the bill to the base of the tail.

428. Origin of Birds and of Flight——A former theory of the origin
of birds was that they were derived from the flying reptiles, or ptero-
dactyls. These reptiles, however, do not resemble birds structurally
in the degree that some of the bipedal dinosaurs (Fig. 299) do. At the
present time, therefore, the latter are usually looked upon as the ancestors
of birds.

Fra. 298.—One conception of the appearance of Archaeopteryx macrura Owen, based
upon aspecimen in the British Museum. (From Wieman, ‘‘General Zoology,” after Romanes,
by the courtesy of McGraw-Hill Book Company, Inc.) X \%.

Different theories of the origin of flight have been proposed but
the most probable theory seems to be the one which traces the develop-
ment of wings to the broadening of the limbs and tail due to the increase
in length of scales and their modification to form feathers. Apparently
early birds had flight feathers both on their fore- and on their hind limbs
and on both sides of the tail. Such a bird was capable of gliding through
the air from a tree or elevated point, perhaps to a considerable distance.
Gradually the forelimbs developed into wings, the feathers disappeared
from the hind limbs, and the tail shortened and became modified into
such a tail as birds possess today.

429. Flight—In sustained flying the wings strike downward and
forward and the bird rides over the air, which serves to buoy it up.

CLASS AVES 411

The tip of the wing traces a path in the air which is characterized by
long downward and forward strokes, alternating with shorter upward
and backward strokes. This action is modified in other modes of flight.
After making several strokes, some birds hold their wings motionless
and glide for a considerable distance before again making several more.
Thus gliding is a second form of flight. In some cases before a high
wind a bird will partly flex the wings and permit itself to be carried by
the wind. This is a form of flight known as flex gliding. Another
modification of flight is known as soaring, characterized by the bird,
usually at a high elevation, describing great circles without any move-

Fig. 299.—Restoration of a bipedal dinosaur, Ornithomimus. (Redrawn from Barbour,
‘* Reptiles and Amphibians.’’)

ments of the wings whatever. As it describes these circles it gradually
works along with the wind. There is no doubt that soaring is usually
due to the bird taking advantage of the upward rush of currents of
air, though it may be that the bird can soar by taking advantage of a
wind blowing horizontally. Still another form of flight is known as
hovering, in which the bird remains poised in the air before a flower or
above an object upon the ground, the tip of its wings apparently describ-
ing a figure eight.

430. The Bird as a Flying Animal.—aA bird flies on the principle
of an airplane, or heavier-than-air machine, rather than on that of
a balloon, or lighter-than-air machine. Such a machine requires light-
ness and rigidity, which are secured by the character of the bird’s skeleton.

412 METAZOAN PHYLA

Air sacs do not lighten the bird’s body to any appreciable extent when
the bird is in the air, as is often stated, though they do lighten it when
the bird is swimming. Another requirement of an airplane is the posses-
sion of planes for support; in a bird the wings and tail furnish such planes,
giving a broad surface for support and also being capable of adjustment

Z
Z
B-
%
g

Fig. 300.—Beaks of birds. From mounted specimens. A, generalizéd beak of ring-
necked pheasant. X14. B, straining beak of canvasback duck. X 24. (C, spearing
beak of bittern. X15. D, probing beak of greater yellowlegs. X lg. E, beak of
brown pelican. X lé. F, chisel-like beak of hairy woodpecker. X 28. G, carnivorous
beak of Swainson hawk. X 24. 4H, insectivorous beak of nighthawk. X 24. IT,
insectivorous beak of myrtle warbler. X 24. J, graminivorous beak of black-headed
grosbeak. X 24. K, beak of red crossbill. xX 3.

to variations in the direction and strength of the wind. A third necessity
in such a machine is the development of a large amount of sustained
power; this is secured by the great size of the flight muscles and by the
very effective aeration of the blood, which results in rapid and continuous
oxidation. The large size of the heart and the relatively great capacity
of the blood vessels also contribute to the same end. Still other char-
acteristics which contribute to the considerable and constant production
of energy are the effectiveness of the processes of digestion and elimina-

CLASS AVES 413

tion. Steering and balancing are done by adjustments of the tail and
wings. The feather covering insulates the body perfectly, prevents loss
of heat and consequent chilling, and enables the bird to maintain under
all conditions a practically constant temperature. In order to secure
heat regulation and at the same time to guard against too high a tempera-

Fig. 301.—Feet of birds. From mounted specimens. Showing the inner side of the
foot, except J. A, wading foot of greater yellowlegs. X14. B, totipalmate foot of
cormorant. X 24. C, swimming foot of blue-winged teal duck. X 24. D, generalized
foot of ring-necked pheasant. X 2%. E, perching foot of yellowthroat. About natural
size. F, raptorial foot of Swainson hawk. X 24. G, syndactyl foot of kingfisher. xX %
H, lobate foot of coot. xX %. JI, clinging foot of flicker. X 24. J, running foot of
ostrich. X 45.
ture, the bird perspires into the air sacs. Thus during flight, when a
great deal of heat is produced, the temperature is regulated by a very
perfect internal cooling device. There are no skin glands in the bird
except the oil gland at the base of the tail, the secretion of which is used
in oiling and dressing the plumage.

431. Modifications of Birds.—Birds show less variation than any
other class of vertebrates, but within narrow limits they exhibit a large

414

number of modifications.

Jruscles

Fie. 302.—Mechanism of perch-
ing in birds. Preparation from a
crow. The flexor muscles are shown
ending in tendons which pass behind
the joint between the tibiotarsus
and the tarsometatarsus and under
a bony arch near the upper end of the
tarsometatarsus. Running under a
ligament at the bases of the toes
they are distributed to the individual
digits. Because of this structure,
when a bird flexes its legs and sits
upon the perch, the toes grasp the
perch with a very powerful grip.

like.

METAZOAN PHYLA

These involve modifications of the beak

(Fig. 300) connected with the kind of food
and the manner of securing it. Examples
of such are the flat, straining beaks of
ducks; the powerful and sharply pointed
spearing beaks of herons; the long, slender,
probing beaks of snipes and sandpipers;
the chisel-like beaks of the woodpeckers;
the stout, hooked beaks of gulls, hawks,
and owls; the small, slender, and sharply
pointed beaks of the insectivorous birds;
and the relatively larger, heavier beaks of
the grain-eating song birds. The feet are
also modified (Fig. 301) in accordance with
the character of the environment and the
manner of locomotion. These modifica-
tions are illustrated by the lobed feet of
diving birds; the webbed feet of swimming
birds; the long legs and long toes of wad-
ing birds; the possession of two toes in
front and two behind by climbing birds;
and the long, slender toes and curved claws
of perching birds (Fig. 302). Other
modifications involve the tail, which is
practically absent in some diving and run-
ning birds and most highly developed in
birds whose powers of flight are greatest.
The wings are also variously developed in
proportion to the power of flight and are
much reduced in all running birds and in
some diving birds which do not fly at all.
In the diving birds the feet are carried
back to the posterior end of the body
where they serve effectively as propellers
in underwater swimming.

432. Plumage.—The plumage of birds
varies considerably, fitting them for vari-
ous modes of living. Among the flightless
birds are many which live in desert regions
and the feathers of which are slender and
do not overlap. The wings of penguins
are covered with feathers which are scale-

There are also numerous curious modifications of the feathers
on the head, wings, and tail.

Especially conspicuous among birds

CLASS AVES 415

Fic. 303.—The barn-owl, Aluco pratinocola (Bonaparte). The owls are noted for their
noiseless flight which enables them to approach their prey without being detected. This
one showing a defensive attitude when ‘‘cornered.’”’ (Photographed, copyrighted, and
contributed by Gayle Pickwell.)

Fic. 304.—A ptarmigan, Lagopus sp., in its winter plumage which renders it incon-
spicuous on snow. In the summer its feathers have a mixture of brownish white and a
small amount of black. The feet and legs are feathered, which not only protects them
against the low temperatures but may aid them in traveling on the surface of soft snow.
(Photographed from a specimen in the University of Nebraska State Museum.)

416 METAZOAN PHYLA

whose wing feathers are highly developed are the birds of paradise;
among those which have a highly modified tail are the peafowls, lyre
birds, pheasants, and turkeys.

The colors of birds are in part due to pigment; such colors appear
the same when seen under any condition. Other colors are produced by
a combination of pigment colors with interference colors resulting from
reflection and refraction of light, caused in part by ridges and furrows

Fic. 305.—The parrot, Psittacus sp., sitting on the hand of man and conversing with him
concerning the daily menu.

on the surface of the feather and in part by the internal feather structure.
These colors are metallic and changeable.

Birds’ feathers are shed and replaced at intervals, the process being
known as molting. Some birds molt but once each year—early in the
fall—while others also undergo a partial or complete molt in the spring.
The spring molt is usually accompanied by the development of a highly
colored breeding plumage. Some changes in the color of birds are due
not to molting but to the wearing off of the feather tips, which are of a
different color from the rest of the feathers.

CLASS AVES 417

433. Songs.—At the lower end of the trachea, or windpipe, where
it branches into the two bronchi leading to the lungs, birds possess
an organ known as a syrinz, which is the organ of voice. Both the
trachea and the bronchi are held open by cartilaginous rings. In the
syrinx these rings are variously modified and give attachment to stretched
membranes the vibration of which produces tones.

434. Migration of Birds~—Among the most remarkable of the
phenomena connected with bird life is migration. Many animals
migrate but none to such distances and with such regularity as the birds.
Their power of flight makes it possible for them to cover great distances
in relatively short periods of time and their highly developed cerebellum,

Fig. 306.—A precocial young, less than two days old, of a killdeer, Oxyechus v. vociferus
Linnaeus, ‘‘freezing’’ at the approach of an enemy and thus escaping detection by remain-

ing motionless and blending with its background. (Photographed, copyrighted, and contri-
buted by Gayle Pickwell.)

nee

combined with their dependence upon flight, has endowed them with
an ability to find their way which exceeds that possessed by any other
animal and is difficult for man to comprehend. Not all birds migrate,
and every gradation may be found between those which do not and those
which cover thousands of miles in their migrations.

The greatest migration recorded for birds is that of the Arctic tern.
Since the breeding range of the Arctic tern extends south to Labrador not
all of the individuals of this species make a journey of the maximum
length, but those do which nest far north on the shores of the Arctic
Ocean, in a region of permanent ice and snow. As soon as the young of
these birds are able to fly they start upon their southward journey, and
moving at the rate of about 150 miles a day they cover in ten weeks a
distance of over 10,000 miles. After spending the southern summer in

418 METAZOAN PHYLA

the Antarctic, far removed from the northern winter, they return again
through the same distance to spend the northern summer at their breed-
ing grounds in the Arctic. Like all birds they nest in only one region and

Fic. 307.—Eggs of the largest bird, an ostrich, Struthio camelus, and the smallest bird, a
humming bird, Archilochus colubris Linnaeus, showing the relative size.

have only one breeding season. As a result of their migrations, Arctic

terns cover annually a distance of more than 20,000 miles. Another bird

which takes a long journey is the golden plover, which nests in the barren

grounds of the northern part of British America and winters in southern

of. i tis Se: ae ee

Fie. 308.—Altricial young birds. <A, a blind, naked, weak and helpless 26-hour old
woodpecker. (Photo. by Ralph S. Palmer, U. S. Nat. Mus. Bull. 174, Arthur C. Bent.)
B, a scantily feathered, blind but stronger 3-day old domestic pigeon. (Photo. by Edson H.
Fichter.)

Brazil and Argentina. Its journey involves a round trip of more than
16,000 miles. This trip is taken also by a few other shore birds.

How birds find their way has never been satisfactorily determined.
This faculty seems to rest upon a very accurate sense of location which
enables them to fly practically in a straight line from one point to another,
even though darkness or fog may hide any features that would guide them.

CLASS AVES 419

435. Reproduction.—Birds may mate either for the rearing of a
single brood or for life. The nesting locality seems to be in most cases
chosen by the male, though it is the female which determines the precise
location of the nest. One or both members of a pair take part in its
construction, which varies greatly among different types of birds. Some
birds make no nest but lay their eggs directly upon the ground (Fig. 309).
Others make crude nests by scratching out hollows and then utilizing
a few pebbles, twigs, or bits of grass. At the other extreme in nest
building are the beautifully woven pendant nests of orioles, especially
of certain tropical ones, which in many cases are several feet in length.
Some birds do not rear their own young but deposit their eggs in the

p % $ i . a . = 9"
£ ea : of eee) tm iy .
Fie. 309.—‘‘ Nest”’ of a Texas nighthawk, Chordeiles acutipennis texensi Lawrence.
Blotched eggs laid on coarse gravel with no prepared nest. The markings on the eggs render

them inconspicuous on the gravel. (Photographed, copyrighted, and contributed by Gayle
Pickwell.)

nests of others who become foster parents. This is true of the European
cuckoo and of some American cowbirds. In these species mating of
one bird with a single one of the opposite sex does not take place, for
each female mates with many males, a phenomenon known as polyandry.
On the other hand, as in the case of ostriches and of some fowls, one male
may mate with several females, and this is known as polygyny.

Birds’ eggs vary greatly in size, color, and number. The size of the
egg bears a general relationship to the size of the bird, while the number
is greatest in those birds whose nests are most exposed to destruction.
The color of eggs is related somewhat to the place where they are laid.
Those laid in cavities are usually white, or white with reddish spots;
those upon the ground are usually streaked or mottled in such a way as to
resemble the surroundings (Fig. 309); and those in nests in trees and bushes
are frequently blue or bluish white with markings of various patterns.

420 METAZOAN PHYLA

The time of incubation varies somewhat with the size of the bird,
the cowbird having an incubation period of only about ten days; the
ordinary song bird, about two weeks; fowls, three weeks; ducks, geese,
and swans, from four to five weeks; the ostriches, from seven to eight
and a half weeks; and the Australian emu, ten or eleven weeks. Usually
the duty of incubation is assumed mainly by the female, and in other
cases the two share in it. In a few instances, however, the male does
all of the incubating. The last is true in the case of the American ostrich
and also in the case of a small sandpiper-like bird known as the phalarope.
The female of the African ostrich participates very little in incubation.

S ty
=e KA Tas : f EE cet at BLS . a ePa Ss.

Fic. 310.—A simple nest of the mourning dove, Zenaidura macroura Linnaeus, made of
loose straws and stems on the ground. (Photographed, copyrighted, and contributed by
Gayle Pickwell.)

Two types of young birds are recognized, depending upon the degree
of development at the time of hatching. Someare known as precocial (Fig.
306) and form a group called nidifugae (nest-fleeing). They are covered
with down at the time of hatching, have their eyes open, and run or
swim as soon as their plumage is dried. Examples of this type of bird are
the fowls, ducks, geese, and water birds generally. An extreme case
is that of the brush turkeys of Australia and the East Indies. Neither
parent incubates; the eggs are hatched by the heat of the sun, and when
the young birds leave the eggs they are in full plumage and able to take
care of themselves. Others are known as altricial (Fig. 308) and form the
group nidicolae (nest-living). The young of these birds are naked and
blind when hatched but gradually acquire the down plumage, which is
later replaced by a mature plumage before they are able to leave the
nest. Our ordinary song birds all belong to this group.

CLASS AVES 421

ee es Bh a Se = sa Aneel if

Fig. 311.—A more complex nest of the robin, Planesticus m. migratorius Linnaeus, made
of dried grasses and fibers of a rope and cemented together by a thick layer of mud. Lined
with fine grasses and hair and built in a tree. (Photographed, copyrighted, and contributed
by Gayle Pickwell.)

Fic. 312.—The hanging nest of a California bush-tit, Psaltriparus minimus californicus
Ridgeway, made of fine gray moss and oak blossoms, is one of the most elaborate of all
North American birds’ nests. (Photographed, copyrighted, and contributed by Gayle Pickwell.)

422 METAZOAN PHYLA

Fie. 313.—The passenger pigeon, Metopistes migratorius (Linnaeus), which was very
abundant in the United States a century ago but has been exterminated by man who has
used it for food. (Photographed from a specimen in the University of Nebraska State Museum.)

ee,

’
St

|
ae:

Fria. 314.—The dodo, Didus ineptus, formerly lived on the Island of Mauritius in the
Indian Ocean. It was slaughtered ‘‘for fresh meat’? and exterminated by sailors during
the seventeenth century. (Photographed from a reconstruction, University of Nebraska
State Museum.)

CLASS AVES 423

436. Economic Importance.—Birds are economically of great impor-
tance because of their use in various ways and also because of the service
they render as destroyers of injurious animals, particularly insects.

Both the flesh of birds and their eggs are used as food. The feathers
of many species serve for adornment in a variety of ways and are also
used in the manufacture of down quilts and pillows. On islands off
the coast of Chile, where little rain falls and which are resorted to by sea
birds for breeding, the feces accumulate in enormous quantities. These
deposits, known as guano, are used as a source of fertilizers.

There are now large accumulations of data showing the great value
of birds as insect destroyers or as destroyers of other injurious animals.
Many hawks, owls, and other birds of prey, which are killed because
of their occasional depredations in the poultry yard and their attacks
upon game birds, should be considered beneficial because of the number
of field mice, ground squirrels, and other injurious mammals which they
destroy. It may be urged against birds that they destroy beneficial
insects as well as injurious ones, so in the case of every bird it becomes a
matter of striking a balance between the injuries done and the good
accomplished. However, when such balance sheets are made up for
birds, cases are very rare indeed in which a credit is not shown in favor
of the bird.

The problem of bird protection resolves itself into a matter of the
destruction of the very few relatively injurious types; the strict con-
servation of all which are of value for their service in the destruction of
insect pests; and the restriction of the killing of game birds to such a
degree as to permit the greatest number of persons to profit by hunting
and at the same time prevent the destruction of the stock upon which
the existence of future generations depends.

Several birds have been domesticated by man, some for many cen-
turies. Many of the numerous cultivated varieties of the common
domestic fowl were probably derived from a jungle fowl of India. The
domesticated pigeons are descended from a wild blue rock pigeon ranging
from Europe to Central Asia, and here again a great variety of cultivated
types have been developed. There are also to be included among
domesticated birds the geese; ducks, most of which have come from the
wild mallard; turkeys, which were natives of North America; and pea-
fowls, which originally were found in Oriental countries. Many birds
have been cultivated for their ability as singers, a conspicuous example
being the canary; and others, such as parrots, parrakeets, and love birds,
for their plumage.

CHAPTER LIX
CLASS MAMMALIA

The last and highest class of vertebrates is Mammalia. ‘The mammals
present without any question the dominant forms of animal life on the
earth today, being supreme over land and sea. Not even man, however,
has yet successfully questioned the dominance of birds in the air. Few
people are familiar with the term mammals, some
using the term animals in the same sense and others

M Cuticle the word beasts. ei

A 437. External Characteristics— Mammals are dis-
A 2 ig Cortex tinguished by the possession of hair, in connection
A es NZ with which they have developed sebaceous, or oil,
‘ Al ZN y glands. They also possess sweat glands and mam-
a4 ie My mary glands and in some cases scent glands. Lips

INN

NUNN
————

Seen UNO

Medulla_ and cheeks are found in all except the whales, and
there is a fleshy and cartilaginous lobe about the
Cuticular external opening of the ear known as a pinna (Fig.
scale 234). The eyes are protected by lids, the upper of
ine which is the movable one, in contrast with the birds,
a anes in which the lower lid is movable. The facial portion
of the skull usually projects to form a muzzle, or snout.
a ib ae Typically, mammals possess four feet with five toes
gram ofalongitudinal on each foot. Both the feet and toes are modified in a
omy ener ae
main parts. (From 438. Hair.—Hairs are lifeless epidermal struc-
Rees American tures arising from a living bulb which incloses a
dermal pulp (Fig. 225). Since their development
is initiated by an outgrowing of the dermis, hairs are not strictly homolo-
gous with the feathers and scales of birds and reptiles. The latter origi-
nate in a thickening of the epidermis. Sometimes hairs are replaced
by overlapping dermal scales, while in the case of the armadillo the body
is invested by an armor or carapace of bony dermal plates (Fig. 332).
On some aquatic mammals the hair covering is reduced to a few bristle-
like hairs on the upper lip.

439. Internal Structure.—The skulls of mammals (Fig. 317) are
compact and have fewer bones than those of the reptiles, although the
various parts are not fused so completely as in the birds. Of the bones
which in reptiles made up the lower jaw, part may have united with the
424

Sesrtt

WOON

NUNANNENN

RSBSE

Toe

TOONS

CLASS MAMMALIA 425

dentary to form a single bone which articulates directly with the skull,
and one has passed into the service of the ear (Sec. 407). The lower
jaws of the two sides are sometimes fused to form one mandible. Carti-
laginous discs separate the bodies of the successive vertebrae. The ribs
articulate both with the vertebrae and with the sternum and thus the
expansion and contraction of the thoracic cavity are made possible.

Fic. 316.—Hair, magnified, showing the outer covering of the thin, horny, transparent,
plates of various forms. A, golden mole. B, red bat. OC, beaver. D, woodchuck. £,
mink. F, skunk. G, cat. H, horse (Percheron). (Redrawn by Paul T. Gilbert from
Hausman, “‘ Natural History,’ and the Sci. Mon.)

The teeth of mammals generally are represented by two sets, a milk
dentition and a permanent dentition. The teeth are set in sockets
in the jaws and are generally differentiated into several types, known
as incisors, canines, and molars. The character of the dentition shows
a specialization corresponding to the character of the food and the
manner of securing it. Each tooth consists largely of dentine (Fig. 318),

426 METAZOAN PHYLA

which is bony in character and derived from the dermis. The dentine
is covered over the crown with a layer of enamel, which is derived from
certain epithelial cells, and around the root by cementum, deposited
by the dermis after the dentine has been formed. In the center of the
tooth is a pulp cavity of soft connective tissue provided with blood vessels
and nerves.

Fre. 317.—Types of mammalian skulls, showing character of dentition. From speci-
mens. In each case the anterior part of the skull is shown from the side and one-half of
the same part from below, with the lower jaw removed. A, European hedgehog, an
insectivore. X 24. B, coyote, or prairie wolf, a carnivore. X 1g. C, beaver, a rodent.
x 24. D, sheep, an ungulate. xX 4.

The alimentary canal shows several distinguishing characteristics.
The mouth cavity is separated from the nasal chambers by a hard palate,
which is a shelf of bone covered by soft tissues (Fig. 232). This is
supplemented posteriorly by a fleshy soft palate. The passage from the
mouth into the pharynx is known as the fauces, on each side of which
lie the tonsils. The latter are masses of lymphoid tissue and their

CLASS MAMMALIA 427

significance is not definitely known. The opening from the pharynx
into the windpipe is the glottis. In connection with this opening is a
fleshy and cartilaginous structure called the epzglottzs. At the junction
of the small and large intestines the latter is prolonged into a blind sac
called the caecum, which is enlarged in herbivorous mammals, where its
purpose seems to be to increase the capacity of the intestine. The
contracted tip of this caecum is known in higher primates including man,
as the vermiform appendix. Excepting in the egg-laying monotremes,
there is no cloaca, the anal opening being
at the surface of the body.

The lungs are contained in coelomic Phi
spaces called pleural cavities, which are the Ay Wins Enarme!
lateral portions of the thoracic cavity, the ane
middle part of which is the pericardial
cavity. The thoracic cavity is separated
from the abdominal cavity by a thin, mus-
cular diaphragm, which is convex anteriorly
and concave posteriorly. The thoracic
cavity is expanded in breathing by raising
the ribs and flattening the diaphragm.
Thus air is drawn into the lungs. By the
relaxation of the muscles of the ribs and
diaphragm and the resulting contraction of
the thoracic cavity, air is forced out. At
the upper end of the windpipe, or trachea,
lies a larynx, or voice box.

Mammals are warm-blooded animals
possessing a well-developed heat-regulatory M
mechanism. The heart is four-chambered /~— Blood vesse/
and.the systemic and pulmonary circula- F1- 318. Canine tooth of'a mam-

; : mal, diagrammatic.

tions are entirely separate (Fig. 319).

There is a single aortic arch which turns to the left. A hepatic-portal
system is present but there is no renal-portal system.

The brain of mammals (Fig. 320) shows a high development of the
cerebrum and cerebellum, the latter always being convoluted but the
former showing convolutions only in the higher forms. The size of
the cerebellum of mammals is equal to that of birds, while the cerebrum
far exceeds in its development that of any other vertebrate type. The
olfactory lobes are well-developed and each of the two optic lobes is
divided, making four altogether.

The eye is without a pecten, which was present in birds, and the
nictitating membrane is reduced in size, varying in its degree of develop-
ment among the different groups. <A spiral coiling of the cochlea and
the possession of a chain of three bones in the middle ear, together with

Mm Pulp cavity

Dentine

428

METAZOAN PHYLA

the pinna, characterize the ear of the mammals (Fig. 234). These

bones are the malleus, incus, and stapes.

Of these the malleus corresponds

to the articular bone in the lower jaw of reptiles and birds and the incus

y
Carotid artery
Vugular vein
/ Aortic arch
Pulmonary Subclavian
artery and artery an
vei \ 4 law, vern

™

“~.

a ‘

= OS
fa Se Zart m
et f x" ie tj
So ie Ne
ggusa0aee™,|

Hepatic "2S
ortfal ~Z
system ; z
Vena eee ge
Ne.

Fig. 319.—Diagram of mammalian circulation.

Mesenteric
vessels fo
/ntesrtiré

Vessels carrying venous blood, black;

arterial blood, white.

to the quadrate, the stapes having been present from the amphibians

onward. The sense of hearing is very acute.

Mammals possess a

highly developed sense of smell, the sensory olfactory membrane being
spread over the upper and lateral walls of the nasal chambers. The

CLASS MAMMALIA 429

sense of taste is also well-developed, taste buds being collected in certain
areas on the tongue.
440. Classification Mammalia may be subdivided as follows:

CLASS MAMMALIA

Subclass I. Prototheria (pro t6 thé’ri a; G., protos, first, and therion, mammal).—
Egg-laying mammals.

Order 1. Monotremata.

Subclass II. Eutheria (i thé’ ri 4; G., ew, true, and therion, mammal).—Viviparous
forms, including all the rest of the mammals.

Division I. Dipewputa (di dél’ fi a; G., dis, double, and delphys, uterus).—Form
no true placenta and carry the young in a pouch after birth.

Order 2. Marsupialia.

Division II. Monopenpnta (mon 6 dél’ fi 4; G., monos, single, and delphys,
uterus).—Placental mammals, which form a true placenta by means of
which the young are nourished before birth and which never carry the
young in a pouch.

Section I. Unguiculata (tn gwik t 18’ ta; L., ungutculatus, provided with
claws).—Clawed mammals.

Orders 3 to 10. Include Insectivora, Chiroptera, Carnivora, Rodentia, and
Edentata.

Section II. Primates (as a zoological group name, pri ma’ téz; L., primatis,
one of the first in rank).—Contains mammals with nails on the fingers
and toes.

Order 11. Primates.

Section III. Ungulata (tn gi 1a’ ta; L., ungulatus, provided with hoofs).—
Hoofed mammals.

Orders 12 to 16. Include, besides the typical hoofed forms, the Sirenia.

Section lV. Cetacea (sé ta’ shé a; G., cetos, whale) —Mammals without limbs.

Orders 17 to 18. Include whales and other similar types.

In this arrangement those mammals which are most primitive are
placed first and those which are most highly modified last. This brings
the primates and man in the middle of the series since they are neither
the most primitive nor the most highly modified.

441. Origin of Mammals.—There are many resemblances between
mammals and certain primitive reptiles known as cynodonts, the fossil
remains of which have been found in South Africa. These resemblances
point so clearly to relationship between the two groups that the cynodonts
are now generally considered the ancestors of the mammals.

442. Monotremes.—The Monotremata (mdn 6 trém’ a ta; G., monos,
single, and trema, hole) are found in Australia, New Guinea, and Tas-
mania. They are mammals which lay eggs resembling the eggs of
reptiles and birds in being abundantly supplied with yolk and albumen
and covered by a shell. Monotremes also resemble those animals in
having a cloaca. In the case of the duckbills (Fig. 321) the egg is
deposited in a nest constructed at the end of a subterranean tunnel
in the bank of a river, the nest being above the water level and the

430 METAZOAN PHYLA

outer end of the tunnel opening under the water. The egg is not incu-
bated but it soon hatches and the young is fed from milk produced by
milk glands. However, there are no teats, the milk being passed into
two grooves on the ventral side of the mother’s body. In nursing, the
mother lies upon her back and the young animal laps the milk from the
milk grooves. The spiny anteater, Hchidna, possesses a temporary
pouch in which the eggs are incubated.

Olfactory
bulb

Cerebrurm

Fic. 320.—Brain of European rabbit, Lepus cuniculus Linnaeus, seen from the side.
(From a Ziegler model, after Wiedersheim.) The roots of the cranial nerves are marked by
roman numerals.

443. Marsupials.—In the case of the Marsupialia (mir sii pi a’ li a;
G., marsypion, pouch) the egg has a thin membrane and only a little
albumen. It is retained in the uterus, the young being nourished through
the embryonic membranes, which are in contact with the uterine wall.
Rarely a primitive allantoic placenta is developed. The young is born
in an exceedingly immature condition and makes its way or is transferred
by the mother to a brood pouch, or marsupium, on the ventral surface
of her abdomen. Here it attaches itself to a teat, remaining so attached

CLASS MAMMALIA 431

until it has become sufficiently developed to move about and resume
its attachment when it wishes to feed.

"Wily , Wy
py, Y

“= Le. (a
= Sl if a A D>»
i A yl 8 Zi

Fig. 321.—The Australian duckbill, Ornithorhynchus anatinus (Shaw). (Drawn from
Lydekker, ‘‘ Wild Life of the World,” vol. II.) xX Y%.

The marsupials were distributed ages ago over both Europe and
North America, but now they are confined to Australia and neighboring

Fia. 322.—Opossum, Didelphis virginiana Kerr. Common in the United States east
of the Rocky Mountain region. Feeds on fruit, eggs, insects, frogs, small birds, and
mammals. The young are born in a very immature stage and are carried in a skin pouch
on the abdomen of the mother. Said to feign death when in danger, hence the term playing
possum. The opossums are the only present-day representatives of the Marsupialia in
North America. (Photographed froma specimen inthe University of Nebraska State Museum.)

islands, South America, and the southern portion of North America.
In Australia, where they are the only native mammals, the marsupials

432 METAZOAN PHYLA

Pith
Gael
YA t%!'

ij a Ih}

yy i

Cy hy

y’

Fig. 323.—An Australian marsupial, the red kangaroo, Macropus rufus (Desmarest).
(Drawn from Lydekker, ‘‘ Wild Life of the World,” vol. II.) Attains a total length of 9 feet.

Fic. 324.—A mother red bat, Nycteris borealis (Miller), and her hairless young. The
wings are the thin expansive membranes between the extremely long fingers of the modified
fore limbs; they continue from these along the sides of the body to the hind limbs and to the
tail. (Photographed, copyrighted, and contributed by Gayle Pickwoell.)

CLASS MAMMALIA 433

have adapted themselves to various modes of living, some of them having
become carnivorous, and others molelike, shrewlike, or rodent-like.

Fic. 325.—A mongoose, Herpestes mungo, ready to attack the deadly cobra, Naja
iripudians, of India. The mongoose is so extremely agile that it dodges the strike of a
snake. After the snake becomes slightly fatigued by continuous striking, it is seized and
killed by the mongoose. (Photographed from specimens in the University of Nebraska State
Museum.)

The phalangers are flying marsupials similar to the flying squirrels.
The kangaroo has very large hind legs and a large tail, the former being

Fig. 326.—Racecoon, Procyon lotor (Linnaeus). A carnivore which enjoys a great
variety of food, from fruit, nuts, and corn to insects, fish, frogs, reptiles, eggs, birds, and
small mammals. Raccoons prefer the vicinity of water, always where there are trees or
brush, favor hollow trees or logs for their homes, and are nocturnal. They have an inter-
esting habit of washing their food when possible before eating it; however, they will not
refuse to eat it unwashed as is sometimes stated. Raccoons of various species are found in
both Americas. (Photographed froma specimen in the University of Nebraska State Museum.)

used for leaping while the small forelegs serve merely for grasping food
and handling the young (Fig. 323).

434 METAZOAN PHYLA

444, Unguiculata.—Insectivora (in sék tiv’ 6 ra; L., insectum, insect,
and vorare, to devour) includes the hedgehogs, moles, and shrews. They

a - —

Fic. 327.—Hyenas, Crocuta maculata, of South Africa. They are nocturnal, scavenger-
feeding animals that live in small packs in the open country. (Photographed from speci-
mens in the University of Nebraska State Museum.)

are probably the most primitive of placental mammals and are found
everywhere except in Australia and a large part of South America.

ae - . f ee
a

& “
i

Fic. 328.—Lion and lioness, Felis leo, of Africa are among the largest and most powerful
of the eat family. (Photographed from specimens in the University of Nebraska State
Museum.)

They are all insect eaters and are, generally speaking, beneficial.
The order Chiroptera (ki rép’ tér 4; G., cheiros, hand, and pteron,
wing) includes the bats. These are the only true flying mammals, the

CLASS MAMMALIA 435

wings being formed by membranes stretched between the greatly elon-
gated digits of the forelimbs and between those and the hind lmbs

LR
“,
j
\
\

AY ie
1. ZS
c aly. Y ly

Uy’
WN
Yr ry /! ax

i if
ERE

‘Eat rea 9)

Cis lie

\
,

Fig. 329.—Feet of carnivores. A, plantigrade foot (foreleg) of a bear. B, digitigrade
foot (foreleg) of a cat. (From Schmeil, ‘‘ Text-book of Zoology,’ by the courtesy of A. and
C. Black, and of Quelle and Meyer.)
and tail. They do not use their wings as gliding planes as do other
flying mammals but fly with them in somewhat the same manner as

Fig. 330.—The front feet in even-toed ungulates. <A, pig. B, deer. C, camel.
(From Schmeil, ‘‘ Text-book of Zoology,’ by courtesy of A. and C. Black, and of Quelle and
Meyer.)
do birds, though the flight of the bat is weaker and less direct. Bats
are largely insectivorous and nocturnal in habits and are very widely

distributed (Fig. 324).

436 METAZOAN PHYLA

Carnivora (kir niv’ 6 ra; L., carnivorus, flesh-eating) are flesh-eating
mammals characterized generally by their large size and predatory
habits and by the fact that their incisor teeth are small while the canines

Fig. 331.—Porcupine, quill-pig, Hrethizon dorsatum (Linnaeus). An example of the
arboreal porcupines which are limited to the New World. They are large clumsy animals
with long, sharp spines which are modified hairs in their pelage. These quills are protective
and defensive structures; they dislodge easily, almost at a mere touch, and adhere ten-
aciously to anything they penetrate because of the barbed tips; quills cannot be thrown by
the animal—this is a mistaken belief. (Photographed from a specimen in the University of
Nebraska State Museum.)

are highly developed and the molars are of a cutting type. This order
is divided into two great suborders. The first, Fissipedia (fis i pé’ di a;

L., fissus, cleft, and pedis, foot), is made up of the terrestrial carnivores,
whose feet are divided into toes armed with well-developed, curved

Fic. 332.—Nine-banded armadillo, Dasypus novemcinctus Linnaeus, found from Texas
and southern New Mexico to Argentina. From a mounted specimen. X /4.
claws (Fig. 329), and includes the cats, hyenas, dogs, wolves, foxes,
raccoons, badgers, weasels, minks, skunks, otters, and bears. A large
number of these are valuable for their fur. The second suborder,
Pinnipedia (pin i pe’ di a; L., pinna, feather, and pedis, foot), is made
up of marine mammals in which both the fore- and hind limbs are modified

CLASS MAMMALIA 437

to form finlike flippers. It includes such gregarious forms as the seals,
walruses, and sea lions, which are often found collected in rookeries on
islands, particularly in the Arctic regions. The fur seal is very important
commercially.

The order Rodentia (13 dén’ shi 4; L., rodentis, gnawing) contains
mostly small animals characterized by the absence of canine teeth and
the great development of the incisors, which are used in gnawing. The
incisors continue to grow throughout life and the wearing away of the
soft dentine behind leaves the hard ,-
enamel at the front of the tooth con-
stantly extended beyond the rest as a
sharp, cutting edge. This order in-
cludes the hares, rabbits, squirrels, rats,
mice, porcupines, and beavers; it is
very rich in species, these numbering
about one-third of all of the species of
mammals.

Edentata (@ dén ta’ ta; L., edenta-
tus, rendered toothless) includes the
highly modified and decidedly ar-
chaic sloths, armadillos, and ant bears.
They are found mostly in South
America, although some species occur
as far north as Texas. Ants form a
large part of their diet. In spite of !
the name Edentata, ant bears alone 4 ;
are toothless; the others possess teeth, ~
but these lack enamel and are absent -
in the front part of the jaws. The Fig. 333.—Lesser anteater, Taman-
sloths are interesting because they are @%¢ ‘tradactyla. A partly arboreal

bie sa . edentate, with prehensile tail, long
distinctly arboreal animals, having a snout, small mouth, long and wormlike
habit of hanging from the underside of tongue, and no teeth. It eats mainly
3 = _ termites, and inhabits forests of tropical
branches and following them in this America. (Photographed from a speci-
position mlocomotion (Hig 334).90 when 2 Meee) Uniterady op Nebraska. State
: 3 Museum.)
armadillos (Fig. 332) have a well-
developed dermal skeleton consisting of bony plates covered with horny
scales in which hairs are embedded. When danger threatens, the animal
curls itself up in its shell and thus protects itself.

445. Primates.—The lemurs, monkeys, apes, and man are primates.
Of these the lemurs are the most primitive and least manlike, having
many resemblances to the clawed mammals. In appearance they seem
to be intermediate between squirrels and monkeys. They are found
mostly in Madagascar but also in Africa and the Malay Archipelago.
The monkeys are divided into two types. The New World monkeys

438 METAZOAN PHYLA

bo
Fic. 334.—Three-toed sloth, Bradypus tridactylus. A member of the hairy edentates,
found in the forests of tropical America, it is entirely arboreal and adapted to hanging, back
downward, from branches of trees by its long, curved claws, and as an adjustment to this
habit it can rotate its head 180 degrees. In the damp forests sloths commonly have green
algae growing on their hair. This condition, together with their exceedingly slow move-
ments, renders them inconspicuous among the foliage. The hair on the legs and underside
of the body forms a water-shedding device by being directed downward. Sloths have
short jaws with teeth, the tail is short or absent. The three-toed sloth has nine cervical
vertebrae, whereas the two-toed form has but six. (Photographed from a specimen in the
University of Nebraska State Museum.)

4
|

Psalteriury7 F Esophagus

Reticulup
Aborrasur1. Rurver7

Fic. 335.—A ruminant stomach. The arrows show the direction of movement of the
food. The balls of cud pass down the esophagus and into the rumen; then into the reticu-
lum and back to the mouth; on the second swallowing the food enters a small passage in
the upper part of the reticulum formed by the apposition of folds, or valves, and is directed
into the psalterium. From several sources.

CLASS MAMMALIA 439

are distinguished by a broad nasal septum, a small nonopposable thumb,
and a long prehensile tail, which is used like a fifth hand. The Old
World monkeys, on the other hand, have a narrow nasal septum with
the nostrils directed downward, an opposable thumb, and a shorter
tail, which is nonprehensile and in many cases rudimentary. The great
toe is opposable in both New and Old World monkeys. Among the Old
World monkeys are the baboons, mandrills, macaques, and anthropoid
apes. Man is most closely related to the last-named.

EL od Sa aA i Sy So eas 2 eee |

Fig. 336.—Musk ox, Ovibos moschatus (Zimmerman), of the Arctic barrens of North
America. The males secrete musk in the breeding season; hence, the name. They live in
herds, and at times of low temperatures or attacks by wolves they ‘‘huddle together” to
conserve their body heat or to protect themselves from the wolves. (Photographed from a
specimen in the University of Nebraska State Museum.)

446. Ungulata—The Artiodactyla (ir ti 6 dak’ tila; G., artzos, even,
and dactylos, digit), or even-toed ungulates (Fig. 330), include the swine,
hippopotamuses, camels, deer, antelopes, cattle, sheep, and goats.
They are animals with a hoof on each toe and are mostly two-toed,
although the hippopotamus has four toes. Many of these ungulates
are mud-loving and are restricted to the vicinity of water, while others,
like the camel, are adapted to desert life. The latter adaptation involves
particularly two features—the possession of cells connected with the
stomach in which they carry a reserve water supply, thus enabling
them to go for long periods of time without needing to secure more;

440 METAZOAN PHYLA

Fic. 337.—A male American bison, Bison bison (Linnaeus). Buffalo occurred in vast
herds in the central United States less than a century ago and were slaughtered for their
skins and meat. At present there are a few thousand generally protected in large national
parks. (Photographed from a specimen in the University of Nebraska State Museum.)

a Bae ae

Fria. 338.—Cape buffalo, Syncerus caffer, of South Africa. Among the most powerful
and courageous of the ruminants. When enraged it has been known to attack and kill
large carnivores. (Photographed from a specimen in the University of Nebraska State
Museum.)

CLASS MAMMALIA 441

and the presence of fatty humps containing a store of food which may
be drawn upon when they are forced to fast. Some of the ungulates
have the stomach divided into chambers and the cattle, which are
ruminants and chew their cud, have four chambers, known as rumen,
reticulum, psalterium, or omasum, and ge
abomasum (Fig. 335). The food when
eaten is swallowed in the form of balls,
which are accumulated in the rumen.
Later, one by one, these are brought
back into the mouth, thoroughly masti-
cated, and swallowed again, passing ©
into the reticulum and on through the
other chambers, being digested in the
meantime.

The odd-toed ungulates, or Peris-
sodactyla (pé ris 6 dak’ ti la; G., peris- —
sos, odd, and dactylos, digit), include
those forms in which the middle digit
of both the fore- and hind limbs is
highly developed and carries most of
the body weight. Aside from the
horses the order includes tapirs and
rhinoceroses.

Proboscidea (prd bé sid’ é a; G.,
pro, in front, and boskein, to feed)
contains the elephants, which are the
largest terrestrial mammals and which
live to a great age, sometimes as much
as two hundred years. The order Si-
renia (si ré’ ni a; G., sezren, a sea We
nymph) includes the manatees and Fic. 339.—A male giraffe, Giraffa
Eivcorten/ se vidi re apni eae neritic (a eee
orous animals with several character- in nature elsewhere. They have an
istics betraying a relationship to the arent ees Se aan
elephants. The manatees are found in long, there are only seven vertebrae in
the fresh waters along the coasts of CS Ba oa a ee
southern North America, northern the University of Nebraska State
South America, and Africa; the du- ee)
gongs inhabit Oriental and Australian waters.

447. Cetacea.—In this section are the whalebone whales and the
toothed whales, the latter group including the sperm-whales, killer whales,
porpoises, and dolphins. They have a single or double median nostril
near the crown of the head and, when they rise to the surface, expire
forcibly, throwing a great column of steam into the air. This act is

442 METAZOAN PHYLA

Fre. 340.—African elephants, Loxodonta africana, have heads which slope back more
and have much larger ears than the Asiatic species. They are also more savage and diffi-
cult to domesticate. Elephants possess very little hair and have the nose and upper lip
prolonged into a proboscis. The upper incisors are developed into long tusks. (Photo-
graphed from grouped specimens in the University of Nebraska State M useum.)

ave

Fria. 341.—The two-horned rhinoceros, Rhinoceros bicornis. A three-weeks-old young
one and an adult male accompanied by a small flock of rhinoceros birds, Buphaga africana.
These birds eat the external parasites on the skin of the rhinoceros and warn their host of
approaching hunters by uttering a loud harsh note. (Photographed from specimens in the
University of Nebraska State Museum.)

CLASS MAMMALIA 443

called blowing. They have to come frequently to the surface for the
purpose of obtaining air, and each time they come they “blow.” Among
the whales are the largest animals that have ever lived, reaching a
maximum length of 100 feet. The teeth of the whalebone whales are
rudimentary and functionless and are replaced by whalebone, or baleen.
Whalebone is a horny material developed from the epidermal lining

Fic. 342.—Ground squirrels, showing hibernating position in nest in the ground. In
this state the functional processes are greatly reduced, the body is cold to the touch, and
the animal is helpless. Upper, gray ground squirrel, Citellus franklinii (Sabine). Lower,
Golden-manteled ground squirrel, C. lateralis (Say). (From Wade, Journal of Mammalogy.)

of the mouth, which is arranged as a series of curtain-like plates to form
a sieve. These whales feed on relatively small animals which occur in
large numbers and which they strain from the water with this sieve.
448. Hibernation.—Hibernation has previously been noted as an
adaptation to terrestrial life and was briefly discussed in connection
with Amphibia (Sec. 405). Among the reptiles, turtles bury themselves
during winter in the mud of the shores and bottoms of the bodies of water

444 METAZOAN PHYLA

Trophoderm D

Amniotic

Blastocoel
(yolk sac)

Re
EES ee
Arnrioric
cavity

Extra-embry-
orric coelorn

Fic. 343.—Diagrammatic representation of the stages in mammalian development.
A, egg cell, in section. B, two-celled stage. C, morula. D, section of morula. £,
blastula, in section. F, development of entoderm. G, formation of amniotic cavity, and
separation of embryonic disc into entoderm and ectoderm. H, development of mesoderm
and extra-embryonic coelom. J, formation of body stalk and beginning of placenta.
J, cross section of same stage as J. K, embryo in amniotic cavity, the increase of size of
which is about to obliterate the extra-embryonic coelom. In H to K, ectoderm is white,
entoderm crosslined, and mesoderm stippled.

CLASS MAMMALIA 445

in which they live, and lizards and snakes hide in crevices in rock ledges
or crawl into holes in the ground. No birds hibernate. While many
mammals remain active in the winter, protected by their heat-conserving
covering of fur and subcutaneous fat and find a sufficient supply of food to
meet their needs, others hibernate during all or a part of that season.

In preparation for hibernation a mammal becomes very fat, storing
up a supply of heat-producing food. During true hibernation the
temperature of the animal falls, frequently to within a few degrees of
freezing; respiration becomes very slow and shallow; the heart beats
slowly, and the circulation is sluggish; in fact all metabolism is carried
on at a very slow rate, and the temperature-regulating mechanism is
temporarily suspended. In other words the organism becomes for
the time cold-blooded. The muscles are rigid and the animal is uncon-
scious. The mammals of this country that may undergo true hibernation
include jumping mice and pocket mice, woodchucks, ground squirrels
(Fig. 342), chipmunks, and bats.

Some mammals, such as the skunks, badgers, and raccoons, do not
undergo a true hibernation but only spend the time in their winter
quarters in prolonged and profound sleep. Still others carry on various
activities during that time. It is while in their winter dens that female
bears bring forth their very small and partially developed young.

449. Reproduction.—In all mammals fertilization is internal, the
sperm cells being introduced into the oviduct by copulation. The
egg cell of the mammal is small and possesses only a limited amount
of yolk. The development of the monotremes is essentially like that
of reptiles and birds. The mammals of the next group, the marsupials,
retain the embryo for a certain period of time within the uterus, though
it does not become attached by a placenta such as is found in all remaining
mammals.

Cleavage is apparently total and approximately equal. The egg cell
thus appears to be holoblastic although there are details in the develop-
ment which seem to indicate that it has been modified from a meroblastic
type. The egg cell divides first into two and then into four cells, equal
in size and normally arranged (Fig. 3438). As cleavage continues,
however, the cells shift about and finally a structure is formed which
consists of an outer layer of cells, called the trophoderm, and an inner
cell mass. Gradually the trophoderm and the inner cell mass become
separated by a cavity filled with fluid and corresponding to a blastula
cavity, but the two remain in contact at one pole of the vesicle. The
trophoderm becomes attached to the wall of the uterine cavity and from
it finger-like projections or papillae grow into the uterine mucosa; these
serve to anchor it firmly to the wall of the uterus.

From the inner cell mass is developed not only the entire embryo
but also the amnion, chorion (in part), allantois, and yolk sac. In what

446 METAZOAN PHYLA

may be called a typical mammalian embryogeny the inner cell mass
becomes separated into two portions. In the upper portion develops an
amniotic cavity, while in the lower portion is formed an archenteron.
The amniotic cavity is lined with ectoderm, the archenteron with ento-
derm, and from the cells between is formed the mesoderm. From the
archenteron a yolk sac develops, which, however, does not contain yolk.
Now the development of the embryo proceeds in much the same manner
as in the development of a meroblastic egg cell. The entoderm together

Placenta

f embryonic

coelom

Amniotic
cavi iy

Fia. 344.—Diagram of the embryonic membranes and circulation of amammal. Stage
between J and K in Fig. 343. For comparison with Fig. 271. Arteries crosslined, veins
black. Arrows show direction of blood flow. (From Wilder, ‘‘ History of the Human
Body,” by the courtesy of Henry Holt & Company.)
with the splanchnic mesoderm forms the wall of the yolk sac, which is
connected with the enteron of the embryo by a slender yolk stalk. The
ectoderm and somatic mesoderm grow around the wall of the blastodermic
vesicle forming an amnion, which unites with the trophoderm to produce
a chorion. 'The chorion of mammals is, therefore, not homologous with
that of reptiles and birds (Sec. 411). Branching processes, larger and
more complex than those which attached the trophoderm, extend from
the chorion into the uterine tissues, which become thick and congested,
the two masses together forming the placenta. Between the two layers
of the mesoderm is the extra-embryonic coelom.

By the development of the amniotic cavity and the extra-embryonic
coelom a considerable space is produced between the body of the embryo

CLASS MAMMALIA 447

and the placenta, the two being connected only by a body stalk, which
represents the original connection between the trophoderm and the
inner cell mass. In some cases the allantois grows into this body stalk
and assists in the formation of the placenta. Such a placenta, accord-
ingly, is termed an allantoic placenta. If the allantois remains rudimen-
tary and does not enter the placenta, the latter is called a chorionic
placenta; in this case the mesodermal layer of the chorion becomes
much thickened and very vascular. Whether the placenta is allantoic
or chorionic the blood vessels of the mother come into intimate contact
with those of the embryo, though the two sets of vessels never actually
communicate. As a result of this condition a free interchange of sub-
stances in solution occurs. From the maternal blood oxygen and food
are passed into the fetal circulation, while carbon dioxide and waste
substances pass in the opposite direction. In the case of the human
embryo and of other forms the amniotic cavity becomes very large, the
extra-embryonic coelom becoming correspondingly reduced, or even
eliminated, and the embryo comes to lie suspended in the amniotic cavity
by the body stalk, which is called the wmbilical cord. This amniotic
cavity is, of course, filled with amniotic fluid. When the young animal
is born, the umbilical cord is either ruptured or is severed soon afterward.
The placenta, which is called the afterbirth and often forms a mass of
considerable size, is immediately passed out. Since in the birth of a
human infant the umbilical cord is not ruptured, it has to be cut, the
end attached to the infant being tied to prevent hemorrhage.

The process described above is subject to a variety of modifications
in different groups of mammals. Sometimes the amnion is formed as a
circular fold of ectoderm and mesoderm somewhat as in birds, the
margins of the fold coming together to form the amniotic cavity. The
placenta takes a variety of shapes in different mammalian types. Birth
takes place in different groups with the young in various stages of develop-
ment. In such animals as cattle and horses the young at birth are
well-developed, have their eyes open, and are soon able to walk and run,
needing the care of the mother only at the time of feeding. The young
of various carnivores and rodents, however, are born naked, blind, and
helpless and have to be cared for during a considerable period of time.
The human child, while not blind at birth, is nevertheless quite helpless
and demands parental care longer than the young of any other animal.

450. Economic Importance.—Mammals are economically important
for many reasons. Among them are animals which for ages have served
man for food, both their flesh and milk being used. Their hides have
furnished leather and fur for the manufacture of clothing and for a
variety of other purposes. Horses, asses, camels, cattle, and other
mammals have been used as beasts of burden and have assisted man in
his labor of cultivating the soil. Of all mammals perhaps the horse

448 METAZOAN PHYLA

has played the largest part in the development of human civilization.
Mammals have also been the pets and associates of man since early in
his history. Many mammalian products have been important articles
of commerce, including musk, which is the product of certain glands
of ruminants; ivory, which is supplied mainly by the tusks of walruses
and elephants; oil, which has been secured from the fat of sperm whales;
and ambergris, a product of the intestinal canal of whales used as a
base in the manufacture of fine perfumes. Formerly whalebone was
an important article of commerce but its value has diminished in recent
years.

Not only are many mammals valuable to man, but the group also
includes some which are decidedly injurious. Among these are rodents,
which destroy crops in the fields or commit ravages about houses and
outbuildings. The rat is injurious not only for this reason but because
it is also a carrier and distributor of the germs of disease. Various small
mammals are now known to harbor the bubonic plague, tularemia, and
Rocky Mountain spotted fever. In some countries, particularly tropical
ones, wild mammals are a menace to the lives of people, and everywhere
carnivorous mammals are a constant threat to the safety of cattle and
other domesticated animals.

CHAPTER LX
ANTHROPOID APES AND MAN

Excluding entirely from our estimate of man any thought of a spiritual
nature or an ethical culture, he is physically an animal, although the
mental development of civilized man (Fig. 347) so far exceeds that
of any other animal as to make apparently a great gap between him and
all animals below him. When one compares the higher apes, especially
when they have been affected by human teaching, and the uncivilized

Fia. 345.—The chimpanzee, Pan sp., inhabits the jungles and forests of tropical Africa.
More arboreal than the gorilla and in common with the other Simiidae, it has opposable
thumbs and toes. Its gripping strength is about three times that of man. (Photographed
from a specimen in the University of Nebraska State Museum.)

human races, the gap does not appear so wide, although it still remains.
However, the evidence furnished by geology as to the physical character
and intellectual development of earlier races of mankind enables us to
close the gap entirely. For this reason it is possible to discuss man
and the higher apes in the same connection.

451. Manlike Apes.—The anthropoid apes include four genera repre-
sented by living species, these four containing, respectively, the gibbons,
orang-utans, chimpanzées (Fig. 345), and gorillas (Fig. 346). All these

449

450 METAZOAN PHYLA

animals are tailless, all assume a semi-erect position, and all have oppos-
able thumbs and great toes. With the exception of the gorilla they are
preeminently arboreal. As compared with man the anthropoid apes
have stronger Jaws and teeth; they have a relatively low cranial capacity;
the structure of the mouth is not such as to admit of articulate speech;
the arms are long and, together with the scapulas, or shoulder blades, are
developed in accordance with their use as organs of locomotion in trees;

Fig. 346.—Male gorilla, Gorilla gorilla, reared in captivity. Gorillas are the most
powerful of all the primates. They are chiefly terrestrial, usually quadrupedal, but able
to stand erect. Their skin is black—the only ‘‘negro”’ ape. They inhabit heavily forested
country of tropical West Africa; now protected in sanctuary by Belgian government.
(Photographed by permission of Zoological Society of Philadelphia, by Hartman, from the
Science News Letter, May 20, 1939.)

the feet as well as the hands are grasping appendages; and they cannot
assume a fully erect posture.

The gibbons, the least manlike of the great apes, are strictly arboreal,
whereas the gorillas, regarded as the most manlike in bodily form, are
preeminently terrestrial in habit. From this it would appear that there
has been a gradual tendency to change from arboreal life to life on the
ground.

A noteworthy characteristic of these apes is the specialization of the
two pairs of limbs for entirely different modes of locomotion. The
arms, adapted for grasping and for swinging from limb to limb, serve
as locomotor organs in the trees, while the legs, though still showing

ANTHROPOID APES AND MAN 451

some adjustment to tree life, as in the opposable character of the great
toe, are more fitted for locomotion on the ground.

452. Erect Position.—In an animal going upon all fours the vertebral
column forms an arch and the greatest degree of flexibility is at the base
of the neck and at the base of the tail. With the assumption of the
erect position there come changes which are shown somewhat by the
semi-erect apes but which are exhibited completely only by man. One
of these changes is the appearance of curvatures in the spine needful

Lateral Fissure
Cerebrum —___
GG Central sulcus

Fic. 347.—Human brain from the side. From a preserved specimen. The roots of the
cranial nerves are indicated by roman numerals.
in the balancing of the erect body. Thus the human spine comes to
have a backward curvature in the thoracic and sacral regions and a
forward curvature in the neck and lumbar regions. A second change
is in the increasing development of the bodies of the vertebrae at the
lower end of the trunk where the weight is transmitted to the legs.
With the complete emancipation of the forelimbs from any part in locomo-
tion and their specialization for other purposes, the pectoral girdle
becomes lightened and less firmly connected to the axis of the body.
On the other hand the pelvis remains strongly developed, is closely
attached at right angles to the body axis, and is broadened for the support
of the viscera. The leg bones also become straighter and the great toe
ceases to be opposable. The foot becomes arched, which is an adaptation

452 METAZOAN PHYLA

to permit it to take up the jar due to contact with the ground, which
would otherwise be transmitted upward to the body.

453. Evidences in Man of Former Arboreal Life.—There are, how-
ever, still evidences in man of a former arboreal condition, these being
more evident in the child than in the adult. In the young child there
are indications of opposability on the part of the great toe, and the
position of the legs also reminds one of these appendages in the apes.
Attention was called by Darwin to the grasping instinct of the child.
Very young children show a tendency to grasp things in their hands and
have a surprising strength in their arms, being able to support their
weight by clinging to an object which they can grasp.

Ale Ze

B Cc

Fic. 348.—Races of men. A, the ape man, Pithecanthropus. B, the Neanderthal man.
C, the Cro-Magnon man. (Pen sketches in New York State Museum Handbook 9, from
original busts in the American Museum of Natural History, by McGregor, by the courtesy of
the American Museum and the New York State Museum at Albany.)

454. Intermediate Forms.—The remains of several primates have
been found in Europe and Asia which seem to have characteristics of
both man and apes but which are more clearly apelike. Evidences
have recently been discovered in South Africa of an extinct type which
also seems to be more apelike than manlike. In no case, however, are
the parts of the body which have been found sufficient to justify a very
accurate conception of the character of the animal of whose body they
formed apart. In the island of Java parts of the skeleton of a prehistoric
type have been found belonging to the genus Pithecanthropus, which has
been called the Java ape man (Fig. 348A). This type is extremely low
in the human scale according to one view, whereas another view looks
upon it as being more like the apes than like man. A recently discovered
Asiatic type, bearing many similarities to the one from Java, but with
greater skull capacity, is the Peking man, Sinanthropus pekinensis.
These remains, found in a limestone cave near Peiping, China, include
one almost perfect brain case.

ANTHROPOID APES AND MAN 453

455. Fossil Men.—There are several extinct races whose fossilized
bones seem clearly to indicate their human character and which, there-
fore, have all been placed in the genus Homo. ‘The first of these is
known as the Heidelberg man, since the bones were found near Heidel-
berg, Germany. In some ways the parts which have been found are
quite different from those of modern man but the teeth are distinctly
human. A second type is the Piltdown man, named for the locality in
England near which the bones have been secured. This race combined
both primitive and advanced characteristics. The jaw is apelike but
the cranium is human in form, lacking the prominent ridges found on the
skull of the ape. With these remains have been found crudely shaped
flints which indicate a primitive culture. A third primitive race has
been found, known as the Neanderthal man (Fig. 348B). This race
lived later than did the Piltdown man but in the character of the skull
has a greater resemblance to the apes than has the latter. Also there
is less curvature of the spine than in modern man, a fact which would
seem to indicate a less erect position. The arms were powerful, the
hands large, and the thigh bone was much curved. The Neanderthal
man was a cave dweller and the bones of men of this race have been
found in Europe accompanied by worked flints, bones of animals, and
indications of the use of fire.

None of these races has been referred to the same species as modern
man, Homo sapiens Linnaeus. The character of the skeletons of the
Cro-Magnon race (Fig. 348C) has, however, led to its inclusion in this
modern species, since physically and mentally it seems to have been
the equal of primitive existing races of mankind. The Cro-Magnons,
who probably lived in Europe not more than 20,000 years ago, are
believed to have produced drawings and paintings now existing on the
walls of European caverns. They worked flint and bone and seemed to
have possessed weapons such as the spear and harpoon. The brain was
large in comparison with the earlier races and compares favorably with
that of modern man. The Cro-Magnons have been generally looked
upon as the forerunners of the present human race.

456. Present-day Man.—Today man has spread throughout the
world, having adjusted himself to practically every climatic condition.
Three primary types have been generally distinguished: (1) The Negroid
type is characterized by kinky hair; dark skin; a broad, flat nose; thick
lips; prominent eyes; and large teeth. (2) The Mongolian type has
coarse, straight hair; yellowish skin; a broad face with prominent cheek
bones; a small nose; narrow, sunken eyes; and teeth of moderate size.
(3) The Caucasian type has fine, soft, straight hair; light-colored skin;
a well-developed beard; a prominent, narrow nose; small teeth; and is
without prominent cheek bones.

454 METAZOAN PHYLA

The Negroid races are native in Africa, Australia, and the adjacent
islands. The homes of the Mongolian races are in northern and central
Asia, northern and eastern Europe, the Arctic regions (the Eskimos),
the East Indies (the Malays), many Pacific islands, and America (the
American Indian). The original homes of the Caucasian races included
southern and western Europe, northern Africa, and southern and south-
western Asia, but they have become the most widespread of all and are
now distributed over most of the surface of the earth.

PART V
GENERAL CONSIDERATIONS

CHAPTER LXI

ANIMAL ORGANISMS

In the chapters which have preceded the phyla of the animal king-
dom have been reviewed. In connection with some of these phyla
general phenomena were described and certain principles presented.
When the animal kingdom is viewed as a whole, however, many con-
ceptions are at once suggested which are related to the more general
divisions of zoology. In the chapters which follow these will be briefly
discussed.

457. The Organism.—The word organism has been previously used
but its meaning has nowhere been clearly stated. When reference is
made to such a type as a colonial hydroid or a tapeworm (Fig. 83), and
particularly to the Portuguese man-of-war (Fig. 68), the word animal
is somewhat equivocal. What is apparently an animal is really an
assemblage of many animals. Also during the regeneration of a frag-
ment from which develops a complete organism it is a matter of opinion
as to when the term animal should be first applied. Likewise, during the
development of an egg cell it is a matter of judgment as to when the
use of the word animal becomes proper. Since it is clear that the applica-
tion of the word animal is attended with a considerable degree of uncer-
tainty, it seems desirable to employ another term which may be so
defined as to be capable of application to any living thing. No word
seems more appropriate than organism.

458. Definition.—In the sense indicated above an organism may be
defined as a mass of living matter capable of maintaining individual exist-
ence, and in which all parts contribute more or less to the activities of
the whole. This definition covers the one-celled and many-celled ani-
mals, the colony, the regenerating fragment, and the individual animal
in any stage of development from the egg to the adult. Referring to an
organism as a mass of protoplasm need not cause confusion with the defi-
nition of a cell, for it should be remembered that if a cell carries on con-
tinued independent existence, it is also considered an organism. On the
other hand a many-celled animal is a mass of protoplasm divided into
cells. In the case of symbiotic forms, whether or not the term organism
could be applied to the two taken together would depend upon the
degree of cooperation. Thus the green hydra is an organism, though
it is composed of an associated plant and animal. In the case of the
white ant and a symbiotic protozoan which lives in its alimentary canal,

457

458 GENERAL CONSIDERATIONS

the degree of dependence of one upon the other is not sufficient to counter-
balance the essential individuality of each and the two are considered as
separate organisms.

459. Income and Outgo.—An organism is continually carrying on
metabolic activity. When this reaches a low level and the animal is
incapable of immediate action, it is said to be dormant; and when
metabolism ceases entirely, death has occurred. It was pointed out
long ago that life was essentially the result of combustion under com-
plicated circumstances, and Mach has referred suggestively to a living
organism as that which is able to ‘‘keep itself going, produce its own
combustion temperature, bring neighboring bodies up to that tempera-
ture and thereby drag them into the process, assimilate and grow, expand
and propagate itself.”’

Metabolism involves the addition to the organism of matter and
energy and also the giving off of both. The income and outgo of the
organism may be expressed as follows:

Income.

1. Material income.
a. Food.
b. Oxygen.
2. Energy income.
a. Chemical energy, contained in the food.
b. Light energy, received naturally from the sun.
c. Heat energy, received from the sun and the earth

Outgo.

1. Material outgo.
a. Solid wastes, egested.
b. Liquid wastes, eliminated.
c. Gaseous wastes, expired.
2. Energy outgo.
a. Mechanical energy, evident in muscular activity.
b. Heat, produced as a result of chemical changes in the body.
c. A small amount of electrical energy, freed during these same changes.
d. A smal] amount of light in organisms which are luminescent.

If the income exceeds the outgo, then the difference represents growth
in substance and an increase of the potential energy contained in the
body. If the reverse is true, both the size of the body and the energy
contained in it diminish. Theoretically it should be possible, taking
into account the condition of the organism at the beginning and end of a
given period, to reach a perfect balance between income and outgo for
that period. Some of the most recent and most successful experi-
ments have approached such a balance within a fraction of one part in a
thousand.

460. Differentiation.—It has been seen that differentiation, accom-
panied by division of labor, presents itself in organisms in several ways

ANIMAL ORGANISMS 459

(Sees. 112, 116, 117, 139, 177). One is the differentiation which exists
within the individual cell; a second is that of cells in the many-celled
organism, which results in the formation of tissues; a third is that of the
individuals in a colonial organism, as in the Portuguese man-of-war.
Division of labor may also appear in a community, as in social bees and
ants, accompanied by polymorphism. Differentiation within individuals
exists throughout the animal kingdom, but it is carried to the highest
degree in the highest organisms.

461. Integration.—Parallel with differentiation occurs a process
known as integration. Differentiation appears as the result of differences
which develop in the parts of an organism. Integration is brought about
by such an intimate coordination between these various parts as to
result in unity of action and tends to increase the efficiency of the organ-
ism as a whole. Integration is shown in the association of tissues to
form organs and of organs to form systems and in the coordination of all
parts in the activities of the entire organism. Just as in the highest
animals differentiation reaches its highest expression, so in the same
animals integration is developed to the greatest degree. Two factors
concerned in coordination are centralization in the nervous system and
chemical control in other systems of the body.

462. Centralization.—Centralization, which involves the development
of a central nervous system, has been traced through the different phyla
and has been seen to culminate in the highly centralized nervous system
of the vertebrates. In the higher organisms has been added cephalization,
which is the setting apart of a head region containing the brain and many
of the organs of special sense (Chap. X X XI and Sees. 266, 275, 282, 333,
337, 352).

463. Chemical Control.—The chemical control of one part of the
body over another is exercised through the production of substances
which may be included under the general term of hormones. A hormone
may be defined as a substance of chemical nature produced in one part
of the body which, when carried to another part, serves to stimulate it to
activity. In this broad sense the term covers all internal secretions.
The glands which produce these are often called the endocrine glands.
The discovery of hormones is comparatively recent and our knowledge
of them is far from complete. Nevertheless, certain facts may be
definitely stated.

An example of a hormone is presented by the gastric secretin. The
stomach is called into action by the stimulation of the vagus nerve.
This effect soon wears off but the cells are caused to continue functioning
by the gastric secretin which is formed by the cells themselves and passed
into the blood. Similar substances are produced by the pancreas and
liver. The ovary of a pregnant mammal produces a hormone which,
carried through the blood to the mammary gland, stimulates the latter

460 GENERAL CONSIDERATIONS

and brings it into functional activity just at the time when the young
animal requires food.

Not only are hormones agents by which coordination between dif-
ferent parts of the body may be secured and integration be brought about,
but undoubtedly they are also active agents in differentiation. They
stimulate the development of the characteristics which distinguish
individuals, the result of their combined activities determining size and
bodily configuration. Hormones produced in the sex organs stimulate
the development of structures characteristic of either one sex or the
other; those from the testis are active in the production in various parts
of the body of characteristics that belong to the male; and those from
the ovary, in the production of characteristics that belong to the female.

464. Individuality—As a result of the processes associated with
differentiation and integration organisms acquire the differences that
characterize species, varieties, and races and also that which we call
individuality, by which each individual is distinguished from others of its
kind. It is probably true that no two individual organisms are ever
precisely alike, though they may resemble each other very closely. This
individuality is maintained throughout the life of the organism, even
though in the process of metabolism the exact composition of the body is
constantly changing. From this point of view an analogy has been
drawn between an organism and a whirlpool in astream. At no two suc-
cessive moments of time is the whirlpool composed of the same material;
water constantly enters it and constantly leaves, and yet the appearance
of the whirlpool remains essentially the same. Aliving animal is continu-
ally taking in food and in that way bringing matter into its organization
and also continually throwing off waste, yet it constantly maintains its
individual character. It has been said that the body changes once in
every seven years. This isa statement which is not exactly true, though
it suggests a truth. Some materials in the body, such as bone, remain
the same throughout all or a large part of the individual’s life, while in
other structures of the body, as in the cells of the skin, replacement is
continually taking place.

465. Life Cycle in Birds and Mammals.—The life cycle of the highest
vertebrates, including man, may be divided into three distinct periods,
each of which is characterized by certain physical appearances and
functional relationships. These three periods are adolescence, maturity,
and senescence (Sec. 70). The period of adolescence is the time during
which the organism increases in size; metabolism is active and the chief
energy of the body is directed toward the production of an organism with
the adult stature, possessing the complete equipment of organs and
endowed with the energy necessary to carry on life most effectively.
In most cases the reproductive function does not become active until
this period of the life cycle is well advanced. The period of maturity

ANIMAL ORGANISMS 461

is characterized by the maintenance of a generally uniform size, an
approximate balance between income and outgo, and the regular carrying
on of all activities, including reproduction. This period is usually not
set off sharply from either of the two others. Fora time in the early part
of the period some growth still occurs, though it gradually ceases, and
toward the end some wasting occurs, but it does not become prominent.
In the period of senescence, however, the katabolic processes exceed the
anabolic, the body shrinks, the energy production falls, and the repro-
ductive powers lessen or are lost altogether. These senescent changes
appear gradually, become slowly accelerated, and culminate in the
natural death of the organism.

Throughout the life cycle changes occur in the degree of activity
of the various parts of the body which modify the factors concerned in
the production of a balance exhibited by the body as a whole. Certain
endocrine glands are active during youth. During maturity the activity
of these glands gradually lessens and the hormones produced by them
exert a lessened effect, while other glands reach full activity during this
period. In the same way the period of senescence shows a change in the
relative activity of different parts of the body and in the hormone pro-
duction of various organs. Thus the successive periods in the normal
life cycle follow each other as the result of a perfectly natural succession
of changes, and the body as a whole participates in them.

466. Other Life Cycles.—The life cycles of different types of animals
vary greatly. Those of the malarial organism (Sec. 115), the sheep liver
fluke (Sec. 202), the beef tapeworm (Sec. 203), several parasitic round-
worms (Secs. 212 to 217), the fresh-water mussel (Sec. 257), the crayfish
(Sec. 298), and the mosquito (Sec. 319) have been described at greater
or less length, and certain aspects of the life cycle of many other forms
have been mentioned. In protozoans there is a period of growth, but
in many cases the cycle ends when full size is attained, the animal divid-
ing by binary fission; under ideal conditions death does not occur.
Many invertebrates, such as the insects, come to maturity, reproduce,
and die. In other invertebrates and in many lower vertebrates growth
never ceases but merely slows up as the animal ages; there is no period of
senility. Generally speaking, few animals ever live to what could be
called an advanced age, since as their powers diminish, those which have
so far survived the struggle for existence fall prey to other more vigorous
animals. The fierceness of the struggle for existence stands as a con-
stant threat to the completion of a normal life cycle.

467. Practical Considerations.—Certain practical considerations
applicable to man appear as a result of the study of the lifecycle. Adoles-
cence is a period of plasticity, of rapid change and ready adjustment;
senescence is accompanied by stability, slow change, and the inability of
the organism to adapt itself easily or perfectly to changed conditions.

462 GENERAL CONSIDERATIONS

During adolescence the adverse effects of departures from the normal are
minimized or overcome; during senescence such effects tend to remain and
to be accentuated. The curve of a normal life cycle possesses a certain
symmetry; its span varies and is a part of the inheritance of the individual
(Fig. 14). Normal living throughout life conduces to the fullest realiza-
tion of this inheritance; departures from such living tend to shorten the
cycle. Theoretically, senescence should be a period as normal and accom-
panied by as perfect health as is either of the previous periods.

468. Organismal Concept.—The concept of the organism as a unit
may be contrasted with the cell concept. The cell is the morphological
unit, some organisms existing as single cells, others asmany. In the first
case all functions necessary to life are performed by the one cell; in the
second these functions are apportioned to the different cells that make
up the whole, and the individual cells become dependent upon their
association for continued existence. Applying the organismal concept
to a protozoan, it is seen to be comparable not to any one cell of a meta-
zoan but to the whole metazoan body. It is, from a physiological stand-
point, correspondingly complex.

The many-celled organism has possibilities greater than the sum
total of the possibilities possessed by the cells taken one by one or even
in groups. These larger activities are the result of interaction between
cells. Thus such an organism is comparable to a community made up of
individuals, having peculiar powers and activities which belong to it
as an organized whole and capacities which it possesses by virtue of its
organization.

CHAPTER LXII

STRUCTURE OF ORGANISMS

In the simplest types of one-celled organisms there is little distinction
between the different parts of the cytoplasm, all being freely interchange-
able and all being able to carry on the same activities. In the higher
Protozoa, owing to intracellular differentiation, certain portions of the
cytoplasm become set aside for the performance of particular functions
(Fig. 34). Since they are thus analogous to the organs of higher animals,
they have been designated by such names as cell organs and organelles.

469. Grades of Organization.—Differentiation has already been
considered in Chaps. XIX, XX, and LXI, in which attention was called
to the fact that tissues are formed as a result of intercellular differentia-
tion. These become associated together to form organs, and organs
become related to each other to form systems. Four types of tissues
are generally recognized, and eight or nine systems of organs. It
was also seen in Chap. XXX that this organization was first fully
shown in the flatworms; in higher animals it has been found to reach
a high degree of complexity. Generally speaking, an organism is said
to be low in organization when its structure is comparatively simple and
high in proportion to its complexity. Accordingly the various phyla
show various grades of organization.

470. Germ Layers and Tissues.—In the development of a metazoan,
as traced in Chap. XXV, it has been seen that differentiation first
results in the formation of three germ layers. It has also been noted
that as the organism develops, these germ layers each give rise to certain
definitive tissues, which are mature tissues as contrasted with those found
in embryos or young animals. The classification of tissues depends,
however, not upon the germ layers from which they are derived but upon
the character which they possess. Thus epithelia may come from any
one of the three germ layers (Sec. 148); most muscle cells are mesodermal
in origin but in certain cases they are derived from ectoderm or entoderm;
and the supporting framework of nerve tissue, called in general neuroglia
and ectodermal in origin, has the character and function of a connective
tissue, a type of tissue which is usually mesodermal.

471. Relationship of Cells in Metazoans.— While the cells in a meta-
zoan may be considered structural units, they should not be thought of as
independent of one another. As a matter of fact many of the processes
which result in the formation of cells do not go on to a complete separa-

463

464 GENERAL CONSIDERATIONS

tion of the cells formed. Thus there may be found multinucleated cells,
such as the striated muscular fibers. There are also living meshworks
made up of cells which have only incompletely separated and remain in
structural continuity. A tissue of the latter type is called a syncytiwm
(Fig. 349), or, in the case of nervous tissue, a nerve net (Fig. 60). There
are protoplasmic bridges which connect the epidermal cells of vertebrates,
while in other cases fibrils extend from one cell into an adjacent cell and
serve to coordinate the activities of the two. In no metazoan do the
individual cells all live as they would if they were alone, but each is
affected more or less by the proximity of the others. This is part of the
organismal concept. Nevertheless, cells furnish convenient units on
which to base our conceptions of morphology.

472. Organs and Systems.—In Chap. XXI the different systems
were enumerated and brief references were made to some of the organs

Fia. 349.—Semidiagrammatic representation of a syncytium, as seen in mesenchyme.

included within each. As each phylum has been taken up in turn, many
facts have been given in regard to the development of various systems in
it. Here, however, the phylogenetic development of these different
systems will be reviewed but only in general terms. It is desirable to
begin the discussion of each system with those structures which in lower
types, in which the system does not exist, perform the corresponding
functions.

473. Tegumentary System.—In the lower protozoans, it has been
seen that no cell wall, properly speaking, exists. Many of these forms,
however, secrete shells of one kind or another which serve for protection.
In the higher protozoans there is a surface layer, or pellicle, which is
really a wall secreted by the cell for the purpose of protection.

The epithelial cells of metazoans always have cell walls. In sponges
the body is covered by a single layer of flat pavement cells called a dermal
layer. In coelenterates and ctenophores there is no definite epithelium,
but the body is covered by an ectoderm. In some cases, as in colonial
hydroids, this ectoderm secretes a perisarc, and in the case of the corals

STRUCTURE OF ORGANISMS 465

and related forms it deposits lime which is built up around the polyp.
In flatworms a single-layered epithelium is found which in free-living
forms is ciliated and attached to a basement membrane. In the adults of
the parasitic flatworms the epithelium secretes a firm cuticula. In
nemertines and in echinoderms, except the ophiuroids and crinoids, there
is a simple ciliated epithelium.

In the mollusks the skin is made up of an epidermis composed of
epithelial cells. This may secrete a cuticula. Under the epidermis is a
connective tissue dermis which produces a limy shell. The shell is con-
tinually being extended at the margin as it grows and also thickened by
addition from within. Brachiopods and bryozoans are similar to mol-
lusks in this respect.

In the remaining non-chordate phyla the epidermis secretes a cuticula
over the surface, which may be thin or thick and either flexible or not.
The living cells beneath this cuticula are usually termed the hypodermis
(Fig. 350). In the arthropods this cuticula contains a large amount of
chitin, is thick, and tends to be quite rigid; in the crustacea it is still
further hardened by the addition of lime.

ead culg

SSS SS SSeS 5555

RAMA AERRGRER

Connective tissue
Fic. 350.—Diagrammatic section of an insect’s skin.

The tegumentary system reaches its greatest development in the
chordates, in which the skin, or cuticle, is made up of two layers, an
epidermis which is ectodermal in origin, and a dermis, or corium, which
is mesodermal and developed from mesenchyme. In the lower chordates
and in the aquatic vertebrates it remains relatively simple. In the fishes
scales are developed in the dermis, but in living amphibians they occur
only in the form of scattered plates in the skin of the backs of a few
exotic toads, and of rings of scales in cecilians. In the terrestrial
vertebrates the epidermis becomes many-layered and from it are devel-
oped scales, hairs, and feathers; the dermis becomes thicker and also
contains many structures, as indicated in Chap. L.

474. Skeletal System.—The skeleton, strictly speaking, is meso-
dermal in origin and develops in mesenchyme. In the vertebrates the
skeletal parts produced in the mesenchyme of the dermis make up the
exoskeleton, which is therefore tegumentary in relationship. To this are
usually added the epidermal hard parts. The endoskeleton is formed in
the mesenchyme lying deeper in the body.

A hard endoskeleton is present in only a small number of invertebrate
types, though it is seen in some protozoans, where either a calcareous or
siliceous framework may be developed in the animal, and in the sponges,

466 GENERAL CONSIDERATIONS

where spicules form a skeletal support for the body. In coelenterates
and ctenophores the mesoglea furnishes some support. In other inverte-
brates generally the only internal support is formed by fibrous connective
tissues, though in the case of cephalopods and arthropods cartilage is devel-
oped around the central nervous system. The chitinous shell of the
squid, though developed from a tegumentary pocket, is actually internal.
In the case of the cuttlefish lime is added to the shell, which becomes bony.
The Aristotle’s lantern of the sea urchin is an internal calcareous skeleton.

In chordates an internal skeleton is well-developed, represented first
by a notochord, which later becomes replaced by a vertebral column,
to which many other parts are added. In the lowest vertebrates
this endoskeleton is membranous with only a little cartilage in some cases;
in elasmobranchs it is cartilaginous; and from the bony fishes onward,
more or less bony.

The axial skeleton presents a biogenetic series in that in the embryog-
enies of the highest forms there appears a continuous notochord sur-
rounded by membranous sheaths, corresponding to the condition in the
hag; this is replaced by a cartilaginous, segmented vertebral column,
which corresponds to the condition in sharks. This in turn gives way to
bony vertebrae, each consisting of several elements as in the bony
fishes, amphibians, and reptiles; and in the final stage, seen in the adults
of birds and mammals, each vertebra consists of a single bone.

In the absence of endoskeletons, exoskeletons are often highly devel-
oped among the invertebrates. They are in nearly all cases epithelial
in origin and, therefore, belong to the tegumentary system. In the
echinoderms, however, the plates are developed from the connective
tissue below the epithelium and are, in the strict sense of the word,
skeletal.

475. Digestive System.—The only structure concerned with digestion
in protozoans and sponges is the food vacuole. In the coelenterates,
ctenophores, and flatworms, however, digestion takes place in a gastro-
vascular cavity. Roundworms and all invertebrates above them possess
an alimentary canal, which may be divided into three parts known re-
spectively as the foregut, mid-gut, and hind-gut. The first of these is
derived from the stomodeum, which is an infolding of the body wall that
meets the anterior end of the archenteron and through which that cavity
comes to open anteriorly. The foregut is lined with ectoderm. The
hind-gut is derived in a similar fashion from an infolding that meets the
posterior end of the archenteron, and is called the proctodewm. This
region of the alimentary canal is also an ectodermal infolding. The
mid-gut, which represents the archenteron, includes all of the rest of the
alimentary canal and is lined with entoderm. The external opening of
the stomodeum is the mouth opening; that of the proctodeum, the anal
opening. There is considerable difference in the extent of the alimentary

STRUCTURE OF ORGANISMS 467

canal comprised in these three regions. In some theforegut and hind-gut
are relatively short and the mid-gut long, but in arthropods the mid-gut
is only a short region between the stomach and intestine. In chordates
the mid-gut is very long, the stomodeum giving rise to the mouth and the
proctodeum to the posterior part of the rectum.

In the phyla possessing an alimentary canal has been observed the
gradual appearance of specialization, shown by the setting aside and
modification of particular regions for the performance of certain functions.
For more effective functioning, various types of glands secreting different
digestive enzymes have appeared. The absorptive surface has been
increased by an increase in the length of the canal, by the formation of
folds and villi, and by the production of blind sacs called caeca the cavities
of which open into the lumen of the canal. A correspondence can gen-
erally be traced between the length of the canal and the character of the
food, carnivorous forms having a short alimentary canal and herbivorous
ones a longer and more capacious one. In the vertebrates this specializa-
tion of the canal reaches its highest development; the conditions there
have been outlined in Chap. L.

476. Glands.—One of the functions of epithelial cells is the produc-
tion of secretions or excretions. Epithelia of which this is the most
prominent function are termed glandular. The secretion may be watery,
in which case it is termed serous; or it may be thicker and contain
mucin, when it is called mucous. The secretion may serve to moisten
the surface of the body and prevent drying, as does that of the glands of
the amphibian skin; it may prevent contact with water and the entrance
of infectious organism, which is true of that of the skin glands of the
fishes; or it may assist in temperature regulation, as in the case of perspi-
ration in mammals. Other functions of such secretions are to lubricate
internal surfaces or to aid in digestion, as in the cases of many glands
connected with the digestive system.

One gland cell may function alone or many such cells may be grouped
to form a gland. Glands are usually removed from the surface, and the
secretion which is poured into the cavity, or lumen, of the gland reaches
the surface through a tubular duct. In the case of oil glands and milk
glands, the secretion is not passed out of the gland cell leaving the latter
intact, as in most glands, but the cells themselves are broken down and
passed into the lumen and out through the duct as part of the secre-
tion, new cells appearing to take their places. Glands are classified,
according to their shape, as tubular or alveolar and, according to the
number of parts into which they are divided, as simple or compound
(Fig. 351).

477. Respiratory System.—The respiratory system is one of the
latest systems to appear. In protozoans respiration takes place through
the surface of the cell and expiration is supposed to be aided, at least, by

468 GENERAL CONSIDERATIONS

the contractile vacuole. Respiration takes place through the general
body surface in all metazoan forms up to and including the annelids,
except in the brachiopods and bryozoans. In the case of the last two
groups, as also in many of the annelids, respiration occurs through the
surface of certain projecting tentacles. In the echinoderms a form of
skin gill is developed, as described in Chap. XXXV. In mollusks gzlls
are developed which are outfoldings of the surface of the body and into
which is projected a network of blood vessels. In a few mollusks,
however, gills are absent, and respiration takes place through the wall
of a hollow sac, or lung, and also through the surface of the mantle. In
crustaceans external gills are found; in the insects, a system of tracheal
tubes; and in the arachnids, book lungs, or book gills. In all of these

Fic. 351.—Diagrams showing types of glands. A, simple acinous gland. 8B, simple
tubular gland. C, simple tubular gland showing coiling. D, compound acinous gland.
E, compound tubular gland.

various types the blood is a medium of transport for the gases within the
body, except in insects, where the circulatory system is poorly developed
and where the gases are distributed by the complicated tracheal system.

In the lower chordates respiration takes place through the walls of
pharyngeal clefts; and in the lower vertebrates, up to and including part
of the amphibians, this is still the case, gills of different types being
developed on the walls of those clefts. In the adults of most amphibians,
however, it has been seen that the gills are replaced by lungs, while in
higher vertebrates lungs are the only functional respiratory organs,
reaching in the highest forms the greatest complexity and the largest
amount of surface exposed (Fig. 352).

478. Circulatory System.—In the protozoans circulation takes place
within the cell; in the lower Metazoa, up to and including the Platy-
helminthes, from cell to cell. In the sponges it is aided by ameboid

STRUCTURE OF ORGANISMS 469

cells; and in the other phyla, by canals or branches of the gastrovascular
cavity. The circulatory system when it does appear is formed in the
mesenchyme, both the cells of the walls of the vessels and the blood
corpuscles being modified mesenchymal cells appropriately arranged.
The vessels become lined with endothelium, a type of epithelium also
derived from the mesenchyme, and to this is added connective tissue
and nonstriated muscles, making up the wall of the vessel.

No true circulatory system appears in the nemathelminths or rotifers,
but a blood-vascular system is found in nemertines, bryozoans, brachio-
pods, echinoderms, and higher forms. In the primitive stages of the
blood-vascular system as it appears in the lower animals pulsations
occur throughout the system. In the higher forms, however, the pulsa-

Trachea

Cc SSA Posterior
A 7ubules D E

Fic. 352.—Diagrams illustrating the increase in the amount of surface exposed to the
air in different types of lungs. A, lung of Necturus, without alveoli. B, lung of a frog,
with simple alveoli. C, lung of a lizard, showing increasing complexity. D, lung of a
bird, seen from the inner side, showing the bronchus entering anteriorly and the passage
to the air sacs posteriorly; the passages in the lungs are seen to form a continuous system
of tubules, none of which ends blindly. £, lung of a mammal showing the branching of the
bronchi. fF, a portion of the bronchi of a mammal, to show the ending in alveoli. (D from
Locy and Larsell, Amer. Jour. Anatomy, vol. 20.)

tions become limited to certain structures called hearts, which are dilated
chambers provided with a larger amount of muscular tissue than exists
in vessels generally. A single heart is found in all arthropods, in mol-
lusks, and in chordates with the exception of Hemichordata. In verte-
brates there has been seen a gradual increase in the number of chambers
in the heart.

There are two types of circulatory systems, one known as the closed
type, where the blood is confined within a closed system of vessels; and
the other the open type, where the blood circulates through sinuses
which are hemocoelic. Nemertines and annelids have a closed system;
in mollusks and arthropods the system is open; and in echinoderms and
chordates it is again closed.

479. Excretory System.—In protozoans the only eliminative struc-
ture is the contractile vacuole. Sponges, coelenterates, and ctenophores

470 GENERAL CONSIDERATIONS

have no specialized cells for elimination, but in flatworms there is a simple
type of excretory organ known as a protonephridium, represented by the
flame cell of the planarian. Similar structures are present in nemathel-
minths, nemertines, and rotifers. In higher invertebrates a structure
known as a metanephridium, representing a collection of protonephridia,
is the excretory organ. In annelids these structures, which have been
described as nephridia, are represented in nearly every metamere.
Nephridia are also present in brachiopods, bryozoans, and mollusks,
though there is only a limited number of pairs. In echinoderms excre-
tory cells, the amebocytes, carry on elimination, while in myriapods,
insects, and arachnids malpighian tubules, and in the crustaceans green
glands, perform this function.

Nephridia or excretory cells also carry on elimination in the lower
chordates but in vertebrates there is a higher type of structure known as a
kidney, or nephros. This contains tubules originating in a certain num-
ber of body segments and is called, according to its position in the body,
a pronephros, mesonephros, or metanephros (Sec. 351, Fig. 229). The
pronephros is functional only in the myxinoids, while a mesonephros
is the functional kidney in lampreys and in all forms up to and including
amphibians. This is replaced by a metanephros in reptiles, birds, and
mammals. The pronephros and mesonephros, however, appear in the
embryonic stages of these higher forms and vestiges of them remain
even in the adults.

480. Reproductive System.—In the least differentiated protozoans
the individuals are all alike and reproduction is purely asexual. In
the higher protozoans reproductive cells, which are essentially sexual
organisms, are distinguishable as macrogametes and microgametes,
and reproduction becomes, therefore, sexual. In the lower metazoans
asexual reproduction still occurs; nevertheless, even in the lowest of the
Metazoa reproduction by means of specialized sex cells becomes a
prominent method of multiplication, and in higher forms it becomes
the only type. In urochordates, however, asexual reproduction again
appears.

In coelenterates the sex cells are derived from interstitial cells, and the
mass of cells in which they are produced is either the testzs or the ovary,
though here these are not, strictly speaking, organs. In the flatworms
not only do the testes and ovaries become organs but a whole system of
accessory organs arises and in these animals the reproductive system is the
most highly developed of all systems. In higher forms the number and
variety of accessory organs vary considerably with the type of habitat
and the conditions under which reproduction takes place.

In many of the lower Metazoa a monecious condition prevails, testes
and ovaries being found in the same individual, though as a rule self-
fertilization is avoided by structural conditions which prevent it or by

STRUCTURE OF ORGANISMS 471

differences in the time of maturation of the two kinds of sex cells. In the
higher metazoans the diectous condition is prevalent and is usually
accompanied by a certain amount of sexual dimorphism. This involves
differences between the two sexes and these in part are of such a nature
that each sex is able more effectively to perform its part in reproduction.
Dimorphism is one aspect of the general phenomenon of polymorphism.
In some cases the male becomes degenerate and, rarely, even parasitic
upon the body of the female. In still other cases parthenogenesis prevails;
this may give way to sexual reproduction at times, but cases are known
among rotifers where it is the only form of reproduction and males are
not known to exist.

481. Muscular System.—The first contractile structure met with was
the myoneme of the protozoan cell. In the sponges certain cells are set
aside as neuromuscular cells in which both irritability and responsiveness
are more highly developed than in other types of cells. In the coelen-
terates contractile fibers occur which are processes of epitheliomuscular
cells, and in some forms contractile cells themselves are found. A more
advanced condition is seen in the etenophores in which muscle cells
derived from the mesenchyme take the place of contractile fibrils asso-
ciated with ectodermal cells or muscle cells derived from such cells.
Muscle cells have already been distinguished as involuntary, or non-
striated, and voluntary, or striated. Nonstriated fibers are of very wide
distribution and are the only type present in the lower Metazoa; striated
fibers are developed in mollusks, arthropods, and chordates.

In ctenophores, flatworms, and rotifers muscle fibers exist as isolated
fibers or are associated in bands or sheets. In the higher forms, the
voluntary muscles are in the form of definite organs known as muscles.
In the highest form of such an organ as it occurs in the higher vertebrates
may be distinguished a belly, or body, of the muscle and tendons at either
one or both ends (Fig. 43). The attachment from which the muscle
exerts its pull is called the orzgin; the attachment pulled upon, the znser-
tion. In the most highly developed muscles of the vertebrates the
muscle is not only enveloped in a connective tissue sheath but it may be
separated into bundles, each with its own sheath, each fiber also being
surrounded with connective tissue. In such a muscle the connective
tissue sheaths are continuous from one end to the other and are brought
together to form the tendons. Thus, while they do not interfere with the
contraction of the muscle fibers themselves, they resist overstretching of
the muscle and in that way serve to protect the soft, contractile fibers
which they surround.

482. Nervous System.—In the more highly differentiated protozoan
cells strands of cytoplasm are found (Fig. 34) which conduct the effects
of stimulation, and there may be even structures analogous to nerve
centers present. The sponges possess neuromuscular cells. In the

472 GENERAL CONSIDERATIONS

coelenterates nerve cells are met for the first time, and these exhibit more
or less variety.

Generally speaking, in the coelenterates are found certain ectodermal
or entodermal cells which receive stimuli and which may be called sense
cells or receptor cells. These are connected to nerve
cells lying deeper in the ectoderm or entoderm, which
with their connecting and conducting fibers form a
nerve net. This net is structurally continuous and
is intimately connected to the contractile fibers.
There is a tendency for these nerve cells to concen-
trate in one ring about the hypostome and in another
about the basal dise (Fig. 353). In the flatworms
this tendency toward centralization results in the
development of central ganglia below the eyespots and
a ganglionic cord on each side, and central and periph-
eral portions may be distinguished in a nervous system
(Fig. 77C). This also leads to a type of reflex action.

A typical reflex act has been described in connec-

353.— A

Fig.

young hydra show-
ing the nerve net,
which tends to be

condensed in one
zone about the base
and another about
the hypostome.
(From Hadzi, Arbei-
ten aus der Zoolog.-
Institut der Univer-
sitait Wien, vol. 17.)

tion with the earthworm (Sec. 281). This results
from the development of a synaptic nervous system.
The unit of such a nervous system is the neuron, or
nerve cell, and neurons are related to each other
through synapses in which their fibers come into
intimate contact. At first, reception of stimuli is by
neurons lying upon the surface and called receptor
neurons. As the nervous system becomes more highly

developed the neurons are withdrawn from the surface and certain epithe-
lial cells receive the stimulus and may be associated in groups in sense
organs or receptors. "These modified epithelial cells receive the stimulus
and pass it over to the dendrites of the sensory neurons, which are lodged
in ganglia within the body. In the vertebrates such neurons are found in
the dorsal root ganglia of the spinal and cranial nerves (Fig. 231). In all
cases, however, neurons are ectodermal in origin and are developed upon
the surface early in embryonic life, later to sink deeper in the body and
reach their ultimate location.

The types of nervous systems may be named as follows:

1. Reticular Nervous System.—This has also been called a diffuse
nervous system. In its primitive form such a system consists of a nerve
net; this is found only in the coelenterates. In some coelenterates,
ctenophores, and echinoderms ganglion cells are added. In the flat-
worms and the phyla which follow them ganglia are present. Nerve nets
persist in all higher forms and are found even in the walls of the blood
vessels and alimentary canal in vertebrates.

STRUCTURE OF ORGANISMS 473

2. Ganglionic Synaptic Nervous System.—Nervous systems of this
type possess neurons the cell bodies of which are in ganglia. In the
mollusks these ganglia are more or less scattered, but in the annelids
and arthropods they are arranged segmentally along a double ventral
nerve cord (Figs. 165 and 188).

3. Tubular Synaptic Nervous System.—This type of nervous system,
characterized by a dorsal central nervous system which is tubular in
shape, is characteristic of chordates and has been described in connection
with them (Sec. 336).

In the tracing of the nervous systems of the various phyla two
phenomena have been noted, centralization and cephalization. These
have been referred to in Chap. LXI.

As epithelial cells become modified to form receptors, taking the
place of receptor neurons which in lower types lie upon the surface of
the body, they come to be specialized so that each group receives only
a certain kind of stimulus. A great variety of accessory structures are
added to the organs which increase their effectiveness. The sense organs
of vertebrates, especially such organs as the eye and ear, become in this
way exceedingly complex.

Those receptors which receive stimuli from without are known as
exteroceptors. Such receptors are those associated with the chemical
senses of taste and smell; those receiving contact stimuli, giving rise to
sensations of touch, pressure, and pain; those receiving temperature
stimuli, giving rise to sensations of heat and cold; and those stimulated
by sound vibrations and by light. There are others which are known as
interoceptors, stimulated by conditions within the digestive system and
giving rise to sensations such as hunger, nausea, and visceral pain.
There is also a third group, known as proprioceptors, which are stimulated
by vital processes within the organism itself. Included in these are the
pressure receptors of muscles, tendons, ligaments, and other internal
organs, pain spots, and organs of position which give to the animal a
sense of equilibrium.

483. Convergence and Divergence.—To the field of morphology
belong the phenomena of analogy and homology, which were early
defined. Homology may exist between organs or parts metamerically
arranged in the same body; between those on the opposite sides of a
bilaterally symmetrical animal; between the antimeres of a radially
symmetrical animal; or between corresponding organs and parts of dif-
ferent individuals. Analogy involves no correspondence in manner of
origin and only such correspondence in position as is mechanically neces-
sary to the performance of a function. An illustration of the latter is
seen in the case of wings, which have to possess a certain position with
reference to the center of gravity of the organism, or of a locomotor fin,
which has to be at the posterior end of the body. Phenomena which are,

474 GENERAL CONSIDERATIONS

in a way, related to homology and analogy are those of convergence and
divergence. As a result of convergence, which occurs when dissimilar
animals become adjusted to a common environment, animals very unlike
in ancestry and different in structure have a close superficial resemblance

Fia. 354.—Figures to illustrate convergence. A, a mackerel-shark. (Based upon
Jordan, “Guide to the Study of Fishes.’’) B, restoration of a fossil aquatic reptile, Ichthyo-
saurus. (Based upon Schuchert, ‘‘ Historical Geology.’’) C, a dolphin. (Based wpon
Brehm, ‘‘ Thierleben.’’)

to each other, while as a result of divergence related animals, because of
adaptation to dissimilar environments, become quite unlike. Con-
vergence is well illustrated by aquatic types belonging to different classes
of vertebrates, which are similar in the shape of their bodies and the
character of their locomotor organs (Fig. 354). Divergence is shown very
strikingly by mammals, which, derived from a cursorial ancestor, have
become modified for fossorial, arboreal, volant, and aquatic life.

CHAPTER LXIII

DEVELOPMENT OF THE ORGANISM
EMBRYOLOGY

In Chap. X the subject of reproduction in animals was first presented
and it was there stated that while in the most primitive of Protozoa repro-
duction occurs by simple division of the single cell which makes up the
organism, in the higher types of Protozoa and in Metazoa certain cells
known as gametes function as reproductive cells. The latter was spoken
of as sexual reproduction. In Protozoa, however, a zygote so formed is a
one-celled animal and so the process of reproduction does not involve the
formation of an embryo. In the many-celled animals the zygote gives
rise to the metazoan body only after repeated cell division, and the
structures of the adult are gradually formed. During this time differen-
tiation and integration occur and the organism passes through a series
of developmental stages during which it is known as an embryo. The
study of such stages provides the subject matter of embryology.

484. Germ Cells.—Strictly speaking, embryogeny begins with the
zygote, but since many of the phenomena concerned with early embryonic
development are directly traceable to those which attend the production
and maturation of the germ cells, the development of these cells is usually
taken as the starting point in this field. In Chap. XXIII has been con-
sidered the development of the germ cells, or gametogenesis, which
includes both spermatogenesis and oogenesis.

485. Origin of Germ Cells.—The difference between the germ cells and
somatic cells in animals may usually be detected early in the life of the
individual. In the case of the arrow worm, Sagitta, the egg cell possesses
a so-called x-body. In the 4-cell stage this body still remains in one of
the cells and this continues until the 64-cell stage. The cell which then
contains this z-body may be identified as the primordial germ cell; from
the other cells are developed the somatic cells of the body. In Ascaris
the primordial germ cell may be recognized as one of the cells in the
four-cell stage. In certain insects the primordial germ cells have been
recognized very early by their large size and peculiar structure. In
vertebrates the primordial germ cells are not recognizable until consider-
ably later, but still early in embryonic life, cells lodged among those lining
the digestive tract may be seen to migrate to a place in the wall of the
coelom, where they accumulate and form the beginning of a reproductive
organ. These cells are recognized as the primordial germ cells.

475

476 GENERAL CONSIDERATIONS

486. Maturation of the Germ Cells.—In the maturation of a sperm
cell is involved its metamorphosis into a mature sperm cell in which a
head, middle piece, and tail are recognizable. Egg cells exhibit no
metamorphosis but merely accumulate a supply of yolk. This has been
described in connection with the origin of the sex cells in Chap. XXIII.
The steps there given are true of animals generally. There is, however,
one modification known as tetrad formation to which no previous reference
has been made. In synapsis it is sometimes true that not two chromo-
somes are seen but that a group of four appears. This has been shown
to be due to a precocious division of the two chromosomes of a synaptic
pair. In this case the four are known as a tetrad; and its division into
two bodies, each composed of two chromosomes, one pair derived from
one parent and one from the other, produces a dyad. This is equivalent
to a mitosis, and chromosome reduction occurs in some cases when the
two dyads become separated in the first maturation division or in other
cases when the parts of each dyad which have separated in the first
maturation division separate from each other in the second maturation
division.

487. Egg.—In the maturation of the egg cell is usually produced a
protective cell membrane which, with the egg cell, forms the egg. In the
higher vertebrates a variety of protective coverings may also be added,
until in the case of the reptiles and birds an egg becomes composed of
added material which may exceed in bulk the egg cell itself. In the case
of the eggs of many invertebrates which possess a shell, particularly those
of the arthropods, there is an opening in the shell known as a micropyle
through which the sperm cell enters in fertilization. In the case of
vertebrates, however, the protective envelopes which are added prevent
the entrance of a sperm cell, so fertilization occurs before their addition.
In the case of reptiles and birds, indeed, so much time elapses between
fertilization and the laying of the egg that the egg, when it is laid, contains
an embryo and not an egg cell.

488. Fertilization.—The time and place of fertilization vary in differ-
ent animals. The egg may be fertilized while still within the body of the
female, in which case fertilization may take place at any point along
the oviduct; or it may not occur until after the egg is laid. In some
cases the sperm cell enters the cytoplasm of the egg cell before maturation
is complete, in which case the union of the two pronuclei (Sec. 141) is
delayed until the formation of the second polar body. Sometimes,
indeed, the entrance of the sperm cell is necessary to bring about the
maturation of the egg cell.

489. Cleavage.—Soon after fertilization—in some cases within a few
minutes—cleavage occurs and the development of an embryo is begun.
In Chap. XXV attention was called to differences in the distribution of
the yolk in homolecithal, telolecithal, and centrolecithal eggs. This

DEVELOPMENT OF THE ORGANISM 477

yolk distribution also affects the cleavage. Homolecithal eggs are holo-
blastic and if there is only a small amount of yolk and it is quite evenly
distributed, cleave totally and equally; if there is more yolk and it is not
evenly distributed, they cleave totally but unequally. Telolecithal eggs
in some cases, as in that of the frog, are still holoblastic and show total
and unequal cleavage; in other cases the excessive amount of yolk causes
them to be meroblastic and the cleavage to be discoidal. Centrolecithal
eggs are also meroblastic, but they cleave superficially. Generally
speaking, the lower metazoans and the lower chordates exhibit total and
equal cleavage. Many of the higher worms and mollusks, as well as the
lamprey and frog among the vertebrates, possess total and unequal
cleavage. The arthropods exhibit superficial cleavage, and the fishes
and higher vertebrates generally show discoidal cleavage. In the mam-
mals, however, pronounced and characteristic modifications in develop-
ment occur. In eggs exhibiting total cleavage the blastomeres which
are formed remain entirely distinct and each possesses a complete wall.
In diseoidal cleavage, however, the cells adjacent to the yolk remain
for a considerable time without a wall on the side next the yolk. In
superficial cleavage the first blastomeres formed have no cell wall next
the yolk and even after they have migrated to the surface the cell wall
next the yolk remains for some time incomplete.

490. Blastula.—In embryos produced by total and equal cleavage the
first appearance of a blastocoel, or segmentation cavity, is the central
cleft between the cells in the eight-cell stage, which is open to the outside
by crevices between adjacent cells. As the cells multiply and press
upon one another these crevices are closed, the central cleft becomes a
cavity entirely surrounded by one or more layers of cells forming the
blastoderm, and the embryo is a blastula. Thus there is no morula
stage. In telolecithal eggs this segmentation cavity lies toward one side
of the embryo and is produced by a splitting apart of the cells which
earlier formed a solid morula.

491. Gastrulation.—In Chap. X XV were also noted certain modifica-
tions in the manner of gastrulation. In homolecithal eggs gastrulation
takes place by znvagination, but in some telolecithal and in centrolecithal
eggs it is the result of a splitting apart of the cells, or delamination.
In other telolecithal eggs gastrulation is accompanied by overgrowth, or
epibole, as a result of which the cells at the animal pole grow around and
envelop the vegetal pole of the egg, ultimately hiding it from view. This
has been described in the development of the frog’s egg (Sec. 402).

492. Mesoderm Formation.—In the chapter already referred to
(Chap. X XV) were also described the formation of the germ layers and
the fate of the embryonic cavities. In the sponges and in some coelen-
terates the wandering cells form a middle layer, which, for reasons
previously stated (Secs. 150 and 185), is not considered to be a mesoderm.

478 GENERAL CONSIDERATIONS

In the ctenophores for the first time a distinct mesoderm lying between
the ectoderm and entoderm is encountered, although it is composed of
few cells. From the flatworms onward, however, the mesoderm makes
up a large part of the mass of the body and as a result of differentiation
forms a variety of tissues.

The mesoderm includes both mesothelium and mesenchyme (Sec. 146).
The mesenchyme is derived from ectodermal or entodermal cells which
are freed in the blastocoel and form a loose meshwork; this appeared for
the first time in the ctenophores and became well developed in the plana-
rian. Cavities may appear in the mesenchyme; when these form spaces
between the viscera and appear like portions of a body cavity but
contain blood, they form what is known as a hemocoel. In some forms
the mesothelium is derived by a process of delamination from the ento-
derm; in others, however, it is produced by outpocketings of the wall
of the archenteron which become cut off from that cavity and form
what are known as mesodermal pouches. These pouches are metameri-
cally arranged in pairs. Each pouch extends upward and downward
and becomes divided into three portions known respectively as the
epimere dorsally, mesomere laterally, and hypomere ventrally. The
hypomere is divided into the somatic and splanchic layers, the space
between these two layers being the true coelom.

493. Tissue Formation and Organogeny.—F rom the three germ layers
develop all the tissues of the mature animal. Organogeny has been
defined as the development of organs by the association of tissues and
leads to the development of systems. Organogeny takes place in a
variety of ways in the different phyla, and references in various places
earlier in this text, especially in the preceding chapter, have indicated
certain details in the development of the organs and organ systems.

494. Postembryonic Development.—In all the lower animals as long
as the embryo remains within the egg it is spoken of simply as an embryo.
In these forms when the organism escapes from the egg it is extremely
simple, and a considerable degree of growth and development is neces-
sary before it becomes mature. During this period it is known as a
larva. In higher forms, however, the organism is much more complex
when it is freed from the egg and may be quite similar to the adult, in
which case it is simply recognized as young. In birds the young animal
within the egg is given the same name as after it has hatched; an example
is the chick. The young within the body of the mammal is called an
embryo until about one-third of the time during which it is retained in
the uterus has elapsed, after which it is called a fetus.

If pronounced changes take place during larval life, the phenomenon
is known as metamorphosis. This has been noted in the discussion of the
biogenetic law and in connection with the development of several types
including the sheep liver fluke (Sec. 202), echinoderms (Sec. 241),

DEVELOPMENT OF THE ORGANISM 479

insects (Sec. 315), tunicates (Sec. 340), and amphibians (Sec. 402).
Metamorphosis may be varied in degree, but the terms complete and
incomplete are applied only to metamorphosis in the insects, depending
upon whether development does or does not include a pupal stage.
Sometimes more stages than those in complete metamorphosis are
observed; this phenomenon is called hypermetamorphosis. Larval life
may be accompanied by the development of characteristic larval organs
which later disappear and are lacking in the adult. Such adaptations
usually characterize individual types, are termed cenogenetic, and are to
be distinguished from larval characteristics which are ancestral and
phylogenetic in character.

495. Potential Immortality of Germ Cells.—Germ cells possess a
potential immortality, since any germ cell has the capacity under proper
conditions to take part in the production of another individual, and this
may continue for an indefinite number of generations. Nevertheless,
they perish in enormous numbers, since many eggs are never fertilized
and a greater number of sperm cells never find an egg cell with which to
unite. In contrast to germ cells, somatic cells present no possibility of
life beyond the lifetime of the individual of which they are a part. The
distinction between germ cells and somatic cells, or, more exactly, between
germ plasm and somatoplasm, was emphasized by Weismann (Sec. 118).
He also stressed the independence of the germ cells and likened the body,
or soma, to a vehicle for the nourishment and transmission of germ cells.
The hereditary units, which determine the possibilities open to the ani-
mal, are passed from generation to generation in the germ cells, while
in the various types of somatic cell, under the environmental conditions
which surround each, are realized and manifested such of these possi-
bilities as, taken together, equip the individual with its characteristic
features.

CHAPTER LXIV
ENERGY CHANGES IN ORGANISMS

From what has been stated previously it is clear that a living organism
may be looked upon as representing a store of potential energy, while
the organization which it possesses may be regarded as a system for the
transformation of energy. During the carrying on of life activities
energy is constantly being changed from potential to kinetic form.
Kinetic energy is dissipated and thus the body will run down unless an
additional amount of potential energy is continually being supplied to it.
That additional amount is secured from the food for the most part, the
small amount of kinetic energy received by the animal organism directly
from the light and heat of the sun and the heat of the earth being quite
insufficient to maintain life.

496.. Chemical Changes in the Body.—For the most part the chemical
changes which occur in the body are of the nature of oxidations. These
occur in all tissues but to a greater degree in the more active ones. The
food taken in is, after digestion, circulated to various parts of the body
and built into the organization of the living cells. Oxygen also enters
the body and is circulated so as to reach every cell. Then, within the
cell, and under the influence of enzymes known as oxidases, the oxygen
is caused to unite with the food, which has now become part of the
protoplasm. Later, when the complex compound thus produced is
broken down and simpler compounds are formed, kinetic energy is
liberated. These simpler compounds, now waste products of the cells
which produced them, have to be carried from these cells to some point
where they can be eliminated from the body. It is probable that no food
is oxidized in the body until after it has been added to the living matter.
In some cases, as we have seen, the waste may temporarily serve some
purpose in the body, in which case it is known as a secretion. Among
such substances are the hormones and those which themselves stimulate
other chemical changes, as, for instance, the digestive enzymes.

497. Organism Compared to a Fire.—Since life involves constant
oxidation, it was pointed out nearly two centuries ago by the French
chemist, Lavoisier, that life might be considered combustion taking
place under certain peculiar and complicated circumstances. Fire is
usually thought of as combustion, or oxidation, taking place rapidly
with the appearance of flame, but slow burning involves exactly the
same chemical changes and leads to the same result. The correspond-

480

ENERGY CHANGES IN ORGANISMS 481

ence between the slow burning which takes place in the body and any
burning which takes place outside it seems to be very perfect when one
considers several facts. Food and fuel are both converted into simpler
chemical substances, and during this conversion a certain amount of
potential energy is liberated as free energy, chiefly as heat and light. In
both cases carbon dioxide and water are formed, and in both cases the
unburned residue remains, forming the ashes of the fire or the feces passed
out of the body. Chemical changes taking place outside the body liberate
the same amount of energy as when these changes take place within the
body, but the conditions within the body determine the speed of libera-
tion and the manner in which the energy is expended.

498. Organism Compared to an Engine.—In certain respects the
comparison between the body and the fire is inadequate. The fire
liberates kinetic energy but this kinetic energy can be controlled and
directed only in case there is added to the fire some form of apparatus by
means of which this control is possible. Such an apparatus is an engine.
The body may, therefore, be compared to the fire plus the engine which
utilizes the energy and directs its expenditure. As a result of this
control the organism is capable of doing a large amount of effective
mechanical work.

499. Organism More Than a Machine.—An organism, though it may
be compared to an engine, is, however, far more than any inorganic
machine for the following reasons:

1. It possesses control from within. It is true that living organisms
are constantly stimulated and caused to act by various forces in the
environment, but it also seems to be true that the very organization of
living matter, unstable as it is and prone to change, makes possible
activities which are initiated from within the organism without any
immediate stimulus from the outside.

2. It exhibits a degree of harmonious activity between structures
which exceeds that shown by any nonliving assemblage of parts.

3. It has the capacity to regenerate itself. Changes taking place in
inorganic masses lead to disintegration and are permanent unless the
masses are again acted upon by an outside agent.

4. It has the power of reproduction, which implies the development of
new individuals like the parent.

5. It possesses individuality, and this individuality is capable of
being transmitted from parent to offspring through many generations.

500. Individuality.— Individuality, as has been indicated in a previous
chapter (Sec. 464), is universal among animals. In reference to man it is
what we call personality. Its source is in minute differences in the precise
character of the organization and, therefore, in the exact functioning
of the organism. It involves distinctions which we recognize as existing

482 GENERAL CONSIDERATIONS

between individuals and not between species. Such individual differ-
ences may be the material basis of evolution.

501. Rhythmicity.—All animal activities seem to be rhythmic in
character. The nature of the organization seems to result in a gradual
acceleration of metabolism to a maximum and then a recession, followed
by another acceleration and recession. Regular periods of rest and sleep
are very general phenomena among animals. The rhythms which
accompany the function of reproduction are particularly evident, but
other functions of the body exhibit rhythms which are less pronounced.

502. Uses of Foods.—As has been previously stated (Sec. 45),
food is necessary to replace waste, provide material for growth, and
furnish energy to the organism. It follows then that the food must be
made up of the same substances of which the body is composed or else
of substances readily transformable into them. The following sum-
marizes the present views on the uses of food in the body of a higher
vertebrate.

1. The protein of food replaces the protein of the body which is used
up.

2. All of the protein is used as it is secured, none of it being stored.
An excess is burned and the products of combustion eliminated. In
this process a certain amount of heat is developed, but proteins are not an
economical source of heat.

3. Carbohydrates are mainly used in the body as sources of muscular
energy, being incorporated in the organization of muscles and then
broken down during muscular activity.

4. The oxidation of carbohydrates also produces heat and more
than does the oxidation of protein but not so much as is obtainable by the
oxidation of fats.

5. Fats are the most economical sources of heat in the body.

6. The body maintains at all times in the liver a large store of carbo-
hydrates, from which a constant supply is furnished to the blood for use
by the muscles.

7. An excess of both carbohydrates and fats is changed to fat and
stored as such. The fat taken into the body is not always stored in the
same form as the fat which is taken in but is broken down in the process
of digestion and recombined to form the various fats characteristic of the
particular organism.

8. Salts are necessary elements in food because they serve to facilitate
chemical changes, create conditions which determine the solubility of
various substances, and, through the part they play in osmosis, partici-
pate in the transfer of water from one part of the body to another.

9. Water is important as a solvent, as a carrier of substances in solu-
tion, and as a regulator and distributor of heat. The last function is
possible because of the high specific heat which water possesses.

ENERGY CHANGES IN ORGANISMS 483

10. Vitamins are essential elements in food, serving in some way to
make possible the incorporation of foods into the living organization.
In the absence of vitamins perfectly appropriate food may be brought to
cells which need it, but these cells are unable to make use of it. Asa
result the body may be starved though it may contain an abundance of
food.

It remains to be noted that the specific character of many substances
affects their availability as food; for instance, vegetable fats are less
readily assimilated and less serviceable in the animal organism than are
animal fats.

503. Planes of Metabolism.—All animals do not live on the same
metabolic plane, some animals demanding a high protein content in their
food, others a high carbohydrate content. Not only is this true of differ-
ent types, but the individual organism may at any time adjust itself to
a metabolic plane different from that upon which it has previously lived
and, it may be, different from that of other animals of its kind. This,
of course, is true within certain limits, which, however, seem rather wide.
Different types of animals are capable of adjusting themselves to great
differences in the amount of water they require. An aquatic organism
may demand constant immersion; on the other hand, a desert type
may find all its needs satisfied by the dew which forms during the night or
the little moisture which comes in the form of rain. The clothes moth
can go through generations in a dry closet without apparently needing
any moisture from without. It is believed that any animal can manu-
facture a certain amount of water in its body as a result of oxidation
processes. In the case of the clothes moth this metabolic water is appar-
ently sufficient in amount to meet the needs of the organism.

504. Body Heat.—‘Since all active organisms are constantly carrying
on chemical changes which mainly involve oxidation, all produce heat,
though the amount of heat produced varies greatly with the size and
character of the organism. The most minute organisms produce an
amount of heat so small as to be beyond the registering power of the
most delicate instrument; larger organisms, howéver, produce a con-
siderably greater amount. Many animals which singly produce an
amount that is imperceptible to us may in large groups yield enough to
produce a very distinct sensation. That is true, for example, of ants in
an ant hill or bees in a hive. The amount of heat produced is determined
by the plane of metabolism and the character of the food, as well as the
degree of activity of the organism. The higher animals with their
more efficient respiratory organs are capable of producing a relatively
larger amount of heat than are the lower forms with their less efficient
systems.

505. Heat Regulation.—Since the lower animals have no means of
regulating the heat of their bodies, it is radiated as fast as it is produced.

484 GENERAL CONSIDERATIONS

Warm-blooded animals, however, possess methods of heat regulation
which involve both structures and functions. Structures tending to
regulate the heat produced by the body are body coverings, such as
feathers and fur.

Functionally the body regulates heat through the nervous system
by controlling the activities of the various parts, particularly the activity
of the circulatory system. By increasing the speed of circulation oxygen
may be more liberally supplied to the various parts of the body, and the
amount of heat developed correspondingly increased. The opposite
is true of a slowing up of the circulation. By changes in the caliber of
the blood vessels, the distribution of heat in the body may also be
modified; the blood may either be accumulated toward the center of the
body, where the heat is conserved, or be carried to the periphery, where
the heat is allowed to radiate freely.

Another factor in heat regulation is evaporation from the surface
of the body. In many mammals the sweat glands provide a means by
which the body surface may be cooled when the temperature becomes too
high. Water, because of its high specific heat, absorbs a large amount
of heat in passing from a liquid to a gaseous state. This heat has to
come from the media with which the water is in contact, and so the
evaporation of perspiration cools the surface of the body.

506. Warm-blooded and Cold-blooded Animals.—<As a result of the
varying amounts of heat produced by different animals and the varying
efficiency of heat regulation, there is great diversity in the temperature
maintained in the bodies of different forms. In the case of all but birds
and mammals, however, this temperature approximates the temperature
of the air or water in which the animal lives, being sometimes a little
higher and sometimes a little lower but never departing far from it. In
the case of birds and mammals, because of the amount of heat they
produce, the presence of a heat-conserving body covering, and a heat
regulatory mechanism, the organism is able to maintain a temperature
independent of that of the surrounding medium and approximating a
constant under all conditions. What this constant temperature shall be
is determined by the individual type and varies considerably as between
different species. Animals which can maintain a constant temperature
are termed homoiothermous, though popularly they are known as warm-
blooded; animals unable to do so are termed potkilothermous, or cold-
blooded. In the case of mammals that go into true hibernation, which
involves a state of lethargy and practical cessation of all activity, a warm-
blooded animal may relinquish temporarily its power of heat regulation
and become actually cold-blooded. Thus an active ground squirrel,
which at different seasons and under different conditions possesses rectal
temperatures of 30° to 39°C. (86° to 102°F.), may in hibernation exhibit
a temperature of 5°C. (41°F.) or even less. The average temperature of

ENERGY CHANGES IN ORGANISMS 485

mammals is about 39°C. (102°F.) but that of the monotremes is only 25°C.
(77°F.). The temperature of the monotremes is also more variable than
in other mammals. Birds average about 5°C. (9°F.) higher than
mammals.

507. Temperature of the Human Body.—The normal body tempera-
ture of man is 37°C. (98.6°F.), taken in the mouth or arm pit. The
temperature of a healthy individual, however, varies somewhat during
the day, being highest at the end of a day of activity and lowest at the
close of a night of rest. It also varies in different parts of the body,
in the liver rising to nearly 42°C. (107°F.) and in muscles varying from
normal to 40.5°C. (105°F.), depending upon the degree of activity.
In the skin the temperature approximates that of the surrounding air;
thus at the one extreme it may approach freezing, while at the other it
may rise to 34°C. (93°F.), though normally not higher, even on a hot
day.

Below an air temperature of 60°F. the heat of our bodies is main-
tained by increasing the rate of heat production and by heat-conserving
clothing. Between 60° and 70°F. regulation on the part of the body is
accomplished largely by changes in the caliber of the peripheral blood
vessels and control of radiation. From 70° to 98.6°F. the temperature is
regulated by evaporation from the surface, combined with flushing of
the skin. Above the normal body temperature maximum radiation and
abundant perspiration are the most effective means of control, supple-
mented by modification of the diet and reduction of muscular activity.

From this discussion it is clear that excessively high summer tempera-
tures, those from 100° to 105°F. or more, place a heavy burden on the
heat regulatory mechanism of the body and that the strain increases as
the temperature rises. It is also evident why excessive temperatures are
more easily borne in dry climates or in dry weather, with rapid evapora-
tion, than in humid climates or when there is a high degree of humidity
in the air, tending to decrease evaporation.

CHAPTER LXV

FUNCTIONS OF ANIMAL ORGANISMS
GENERAL PHYSIOLOGY

The energy changes discussed in the preceding chapter have been
seen to be intimately associated with the metabolic activities of the
organism. These in turn cause or accompany the exercise of the various
functions. It is logical, then, to consider next certain facts in regard to
metabolism and to review the general physiology of animals.

508. Chemical Cycles.—In the carrying on of metabolism an animal
takes certain foods into its body, breaks them down in dissimilation
into simpler waste substances, and then eliminates these wastes, adding
them to the air, the water, or the soil. These wastes, together with the
bodies of dead organisms, are reduced to very simple substances by
decomposition due to the action of bacteria. Plants take these sub-
stances, or in some cases the animal wastes, from the air, water, and soil
and build up again from them the foods which the animal needs. Thus
the elements which these various substances contain may be followed
through cycles, partly related to the inorganic environment, partly to the
life of plants, and partly to the life of animals. In that portion of
the cycle occurring in plants and animals the element is involved in the
chemical changes included in metabolism. Of the elements found in
animal organisms certain ones which are of peculiar interest or importance
may be cited as examples.

The carbon cycle (Fig. 355) may be begun with carbon dioxide which
the plant takes from the air or water and by photosynthetic processes
builds into proteins, fats, and carbohydrates. The animal uses these as
foods in building up protoplasm and as sources of heat and muscular
energy. Carbon dioxide is expired into the air. The carbon compounds
in other waste products and in the bodies of dead animals are decomposed,
yielding carbon dioxide, which is added to the air, to be used once more
by plants. Of all chemical elements carbon is the one which enters into
the greatest number and variety of combinations, such combinations
being particularly characteristic of living things. Organic chemistry,
and particularly that portion of it known as biochemistry, is the chemis-
try of carbon compounds.

Atmospheric nztrogen can be utilized only by nitrogen-fixing bacteria
(Fig. 356). When thus fixed, however, the nitrogen is in the form of
compounds with other elements which may be used by any plant and be

486

FUNCTIONS OF ANIMAL ORGANISMS 487

Dissimilation

Tissues
Assimilation
Digestion
. tes
Animal Waste Ji
GOs & rganism ays
— Elimination
Wess — Expiration
Proteins, Fats,
Carbohydrates in Bacterial
HRS Decay
Photosynthesis
In Plants

CO,
Air & Water
Fic. 355.— Diagrams to illustrate the carbon cycle in organisms. The steps above
the line of dashes occur in the animal organism, those below outside of it and in part in
plants.

Dissimilation
Tissues
Assimilation
Digestion Nitrogenous
Wastes(Urea)
Animal
Food Organism De. nee
ay Z
tes ioe we
Be se __ Elimination
Photosynthesis ye sited Bacterial Decay
In Plants Nitrogen — Denitrifying
Miates | NS) WBacteria = Aminorta
Nitti
Bacterial Action in Ee eens

Soil & Water
Nitrites

Fig. 356.— Diagram to illustrate the nitrogen cycle related to organisms and their activities.
The steps above the line of dashes take place in animal organisms.

488 GENERAL CONSIDERATIONS

built into vegetable proteins. These vegetable proteins may be used
by the animal organism which returns its nitrogenous wastes more or
less directly to the soil and water, to be again utilized by plants. In
contrast to carbon, nitrogen is one of the most inert of elements while
in the animal organism.

Animals secure oxygen either directly from the air or from water in
which it is in solution. After it is used in oxidation processes the organ-
ism returns the oxygen to the environment, largely in combination with
carbon as carbon dioxide and with hydrogen as water. Carbon dioxide
and water are in turn taken in by green plants which use the carbon and
hydrogen and free the oxygen.

Phosphorus is an important element in animal cells, particularly
in the nerve cells of higher forms. It occurs in the soil and water in the
form of phosphoric oxides. Taken up by plants and built into certain
proteins it is utilized by the animal, to be later returned either to the
soil or to the water.

509. Water.—The constant need of every animal for water requires no
demonstration. Water is important because of its various rdles in the
metabolic cycle. It gives to living matter the proper consistency. It is
the vehicle by which foodstuffs are brought to the cells of the body and
by which wastes are removed; no substances enter or leave the cell
except in the form of aqueous solutions. It enters into digestive proc-
esses involving hydration, such as the change of starch to sugar. These
processes, however, are very limited when compared with the oxidations
which occur in metabolism. Protoplasm contains a large amount of
water, which makes up two-thirds of the human body by weight. The
amount varies in different animals and in such an extreme case as jelly-
fishes reaches 96 per cent.

For all aquatic organisms the water which bathes them is their
environment. All natural waters contain salts in solution and the
exact composition changes with a variety of factors, such as evaporation,
rainfall, decomposition processes, and so on. Not only must an aquatic
animal be adapted by the chemical composition of its body to the waterin
which it lives, but it must be capable of continual adjustment to these
changes in composition. If an animal cannot so adjust itself, its only
recourse is to encyst and await the return of favorable conditions or
produce eggs or spores which can withstand the adverse conditions.
Similar phenomena occur in response to extreme temperatures. To the
cells in the metazoan body the fluids of the body are a comparable
environment.

510. Digestion and Absorption.—Digestion in protozoans is intra-
cellular and occurs in food vacuoles. In the ameboid forms these food
vacuoles may be formed anywhere at the surface, but in more complex
types a gullet admits the food to the body and the vacuoles are formed

FUNCTIONS OF ANIMAL ORGANISMS 489

at the end of it. In the sponges, too, digestion is intracellular. In
coelenterates and ctenophores digestion is begun in the gastrovascular
cavity and is, therefore, partly extracellular, but it is completed intracel-
lularly ; the same thing is true of flatworms, except in the case of cestodes,
where a digestive system is entirely lacking. From the roundworms
onward digestion takes place in the alimentary canal, is extracellular,
and is caused by a variety of enzymes.

A number of the most important digestive enzymes have heen
been mentioned in connection with the different phyla. In the proto-
zoans proteins form almost the exclusive food, though under certain
conditions these organisms can utilize fats and starches. In passing
gradually to the higher phyla the number of enzymes increases and the
variety of foods which can be digested and absorbed also becomes
greatly increased.

In mammals the stomach is largely mechanical in its function,
though it also carries on digestion. It serves in many cases to hold a
considerable supply of food, which is gradually reduced to liquid form,
being changed to the consistency of thick pea soup, in which state it is
known as chyme. The hydrochloric acid of the gastric juice serves to
convert the pepsin-containing secretions of the gland cells to active
pepsin, to create proper conditions for the activity of these secretions,
and also to control the action of the sphincter muscle which guards the
pylorus, the opening into the intestine, and which is normally in a state
of tonic, or continuous, contraction. The acid inhibits the sphincter
muscle, which dilates and permits a portion of the chyme to pass, after
which the sphincter again closes and remains closed until the chyme in
the intestine has become alkaline and that of the stomach has reached
the necessary degree of acidity. Then the pylorus is again opened and
another portion of the chyme is passed on.

In the intestine the chyme is slowly passed along by peristalsis.
During the time that the chyme is in this part of the alimentary canal
the amino acids and sugars are absorbed into the blood; the fatty acids
and glycerin, into the lymphatics; and water, into both. The fatty
acids and glycerin are recombined in the process of absorption, appearing
in the lymphatics in the form of fats, changing the lymph to what is
known as chyle.

It may be added here that the amino acids are taken up by the various
cells of the body and are either used in growth or are immediately oxi-
dized; the fats are also taken up by the cells and oxidized, releasing energy
in the form of heat, or are stored; while the sugars are stored in the
liver, to be passed into the blood and used, especially by the muscles, as
needed.

511. Circulation.—Circulation takes place in protozoans only by
currents within the cytoplasm. In sponges it is effected through the

490 GENERAL CONSIDERATIONS

middle layer, assisted by the wandering cells or by the passage of material
from cell to cell. In coelenterates, ctenophores, and flatworms the
only circulation is that of food carried in the branches of the gastrovas-
cular cavity and also passed from one cell to another. In the higher forms
circulation is carried on by means of a blood-vascular system.

The circulating medium of lower animals up to and including the
echinoderms is watery in character; in the annelids it is a hemolymph
containing protein in solution; but in the mollusks, arthropods, and
chordates it is blood, consisting of fluid plasma and corpuscles. In
the blood of invertebrates and the lower chordates the cells in it are
generally ameboid and are similar to the white corpuscles in the verte-
brates. In the latter red corpuscles are also present. Hemoglobin,
which is the main carrier of oxygen in the blood, is dissolved in the
plasma of forms which do not have red blood corpuscles. In the mollusks
and crustaceans there is another similar substance present known as
hemocyanin. Instead of iron, which is an essential constituent of hemo-
globin, copper is contained in hemocyanin. Some poisons produce their
effects by dissolving the red blood corpuscles, a phenomenon known as
hemolysis; this destroys most of the oxygen-carrying power of the blood.

The circulatory system is the transport system of the body, carrying
food and oxygen, wastes of all kinds, and internal secretions from one
point to another. The loss of the watery circulatory medium in lower
animals is not serious, since replacement is easy, but injury to the blood
vessels involving loss of blood is very serious to the higher types, in which
replacement takes considerable time. They have, however, a safeguard
in the coagulation of the blood. In the lowest animals which possess
corpuscles this consists only of a massing together of the corpuscles at
the point of injury, but later coagulation includes the formation of a
clot which closes the injured vessels and stops the hemorrhage. The
clot is composed of fibrin formed from fibrinogen, which is a protein in
solution in the plasma. It is produced by the action of thrombin, which
is in turn derived from the cellular elements in the blood. The precise
nature of thrombin and the exact method of its origin are not known.

The pressure necessary to maintain the circulation in higher animals
is due to the dilation of the arteries by blood forced into them by the
heart, the elasticity and muscle tonus of the arterial walls, and the
peripheral resistance due to the narrow capillaries. By tonus is meant
that the muscle fibers are constantly slightly stretched and tense, which
causes them to react more quickly when stimulated. This condition
is partly nervous in origin.

512. Respiration.—Respiration is necessary to furnish the oxygen that
the animal organism needs and to rid the body of waste carbon dioxide.
The absorption of gases into the body and their passage outward both
take place in obedience to the laws of diffusion of gases. Among

FUNCTIONS OF ANIMAL ORGANISMS 491

these is the law of partial pressures, which is that in a mixture of gases
each gas exerts a pressure proportionate to the amount present and inde-
pendently of the pressure exerted by any other gas which may be mixed
with it. Since the oxygen pressure is higher in the air or water about the
animal than in the body itself, oxygen enters the organism; and since the
reverse is true in regard to carbon dioxide, this leaves the organism.
Oxygen enters the animal body, as has already been noted, in a variety
of ways. It may enter through the general body surface, as in the lower
invertebrates; through respiratory papillae, or skin gills, and also through
respiratory trees in the echinoderms; through gills in the higher aquatic
invertebrates generally; through the tracheae in the insects; and through
the lungs of terrestrial vertebrates.

The fact that most of the oxygen is transported by the blood in com-
bination with hemoglobin as oxyhemoglobin has previously been noted
(Secs. 272 and 350). It is also true that the greater part of the carbon
dioxide is carried in combination with sodium oxide in the plasma as
sodium carbonate.

It has been discovered that a large amount of carbon dioxide in the
blood causes the oxyhemoglobin to break down more rapidly and thus
liberate more oxygen in the capillaries for the use of tissues; while when
the oxygen reaches a maximum, as in the lungs, the sodium carbonate is
broken down more rapidly and carbon dioxide is set free at an increased
rate, which hastens its expiration into the lung alveoli.

Under conditions where no oxygen is available many animals have
been found to survive for a considerable length of time, apparently
being adjusted to small oxygen consumption and being able to produce
the necessary free oxygen from their own bodies. In the absence of
oxygen about it an earthworm has been known to live one day; a plan-
arian, from one to two days; leeches, four days; and an ascaris, from
four to six days.

513. Secretion.—The secretions of various animal bodies are very
numerous, are produced by a great variety of organs, and perform a
large number of functions, both mechanical and chemical. Some secré-
tions assume a solid form and furnish protection, connect structures, and
give support to bodies; examples are bone, cartilage, connective-tissue
fibers, chitin, and spongin. Others which become solid, such as silk
and wax, are used by the animal in various ways, as in the spinning of
cocoons by silkworms and the making of comb by bees. Watery secre-
tions such as tears serve to moisten surfaces and prevent drying; in
aquatic forms, as mucous skin secretions of fishes and amphibians, they
prevent drying and infection by the spores which produce disease; or, as
perspiration, they assist in temperature regulation. Some, such as
digestive secretions, contain enzymes, and others, such as mammary
secretions, provide nourishment for the young. The secretions of plant

492 GENERAL CONSIDERATIONS

lice are used as food by ants. Still other secretions contain scents which
attract individuals of the opposite sex, repel enemies, and identify
related individuals. Finally should be named the internal secretions, or
hormones.

All animals produce secretions as the products of metabolism. These
are in many cases accumulated in the cells as droplets or granules and are
passed out when the cell receives the appropriate stimulus. Ferments
may be thus stored in an inactive form, when they are called zymogen,
and then be made active by some other substance; pepsinogen, for
example, which is formed in the cells of gastric glands, is changed to active
pepsin by the hydrochloric acid of the gastric Juice.

514. Internal Secretions.—Among the metabolic activities of animals
have been noted the production of a number of internal secretions, or
hormones, which concern some of the most vital activities of the organism.
It is presumable that these occur in a great variety of animals and are
most numerous in the higher forms. They have been most studied in
man and the higher vertebrates.

Among the organs which produce internal secretions in man (Fig.
357) is the thymus gland. This gland, which is situated anteriorly in the
upper part of the thorax, produces a hormone which retards both
development and differentiation. When the gland is overdeveloped the
condition leads to a prolongation of immaturity and lessens the size of the
organism. The thyroid gland, which usually consists of two lobes, one
on each side of the trachea in the neck region, has an effect the opposite
of that of the thymus. Its activity promotes growth and differentiation,
resulting in large size and the early maturing of the organism. The
secretion of the thyroid gland also stimulates the metabolic processes in
the body and increases energy transformations. The secretion of the
adrenal organs, above the kidneys and in contact with them, increases
blood pressure and stimulates the body to more intense activity in cases
of emergency. The pituitary body, lying at the base of the brain, is made
up of two parts, the secretions from both influencing growth and differ-
entiation. The secretion of the anterior portion increases particularly
the growth of connective tissues and muscle, sensitizes the brain cells,
stimulates the intellectual activities, and develops the memory. The
secretion of the posterior part of the pituitary gland increases vascular
tonus and magnifies the emotions. The parathyrozds are glands situated
at the sides of the thyroid or in some cases are included in it. The secre-
tion of these glands increases calcium metabolism and has a regulating
effect upon the irritability of the cells of the body. ‘The pancreas is not
only a gland secreting several digestive ferments but within it are masses
of cells known as islets of Langerhans, the secretion of which controls
carbohydrate metabolism in the body. The failure of these cells to
- function leads to a disease known as diabetes.

FUNCTIONS OF ANIMAL ORGANISMS 493

In addition to producing the sex cells the gonads are also organs of
internal secretion, such secretions being produced by what are known as
interstitial cells lying in the framework of the gland and not belonging
to the germinal epithelium. The secretions of both the testis and the
ovary influence the development of the respective sex characteristics;
those of the testis affect sexual behavior in the male; and those of the
ovary control the rhythm of sexual activity and are active in bringing

Islets of Langer-
hans in paricreas

Ovary

Fic. 357.—Diagram to indicate the location of the more prominent ductless glands in the
human body. A, male. B, female.

about the attachment of the fertilized ovum to the wall of the maternal
uterus.

Other glands produce internal secretions which affect basal metabolism.
This term is applied to the metabolic processes that continue all the time
in an animal, although it may be carrying on no digestive or voluntary
muscular activity; they are involved in the circulation, maintenance of
body temperature, and other vital processes.

515. Excretion and Elimination.—In the process of excretion liquid
wastes are passed from all the cells of the body. The elimination of these
wastes takes place directly from the cells which form them only in the
lowest organisms; in higher organisms, as has been shown, organs and
systems are set aside for this purpose.

494 GENERAL CONSIDERATIONS

In the protozoans excretion takes place in part into the contractile
vacuole, whence the material is eliminated. In part, however, these
wastes escape from the surface of the cell, in which case excretion and
elimination are the same. Many protozoans also accumulate the wastes
from metabolism in solid crystalline form in the protoplasm. The
material that forms these crystals may be dissolved again and eliminated
through the contractile vacuole or the wall of the cell. In the sponges
and coelenterates excretion and elimination occur at the same time from
the cells. In the echinoderms elimination is through membranes on the
surface of the body or lining cavities within the body. There is also the
curious form of elimination carried on by the amebocytes. In echino-
derms, too, some excretions are stored as granules and crystals.

A variety of conditions has been seen in the worms. In the lower
forms, such as the planarians, excretion takes place into the watery lymph
between the cells and elimination, through the flame cells. In the higher
worms, where a coelom is present, excretion takes place into the coelomic
fluid and elimination is by means of nephridia, which are very varied in
form and exhibit many degrees of complexity. There are also groups of
cells in the coelomic epithelium which take up wastes, escape into the
coelom, and then disintegrate, the wastes being passed out through the
nephridial tubes. In the mollusks there are both nephridia, known as
pericardial glands, and the cells formed from the coelomic epithelium.
In the crustaceans are modified nephridia, forming what are known as
antennary, or green, glands, opening on the basal segments of the antennae,
and in some cases shell glands, opening on the bases of the second maxil-
lae; elimination also occurs through the skin, the lining of the intestine,
and the liver. In the insects the excretory organs are the malpighian
tubules, which may be considered as being modified nephridia. In the
vertebrates, however, three types of kidneys are found—pronephros,
mesonephros, and metanephros—which have been previously described
(Sec. 351).

516. Motor Functions.— Organisms can in most cases secure food only
by moving about in search of it. Other conditions also necessitate loco-
motion, and so, even though many animals are sessile and some almost
motionless, movement generally is a prominent feature in animal life.
Movement has been seen to be of three general types—ameboid; ciliary,
or flagellar; and muscular.

Ameboid movement, which has been noted in the rhizopods, results
from a difference in consistency in different parts of the protoplasmic
mass. The movements of cilia and flagella, which are similar, are the
result of contractions of masses known as basal granules to which the
cilia or flagella are attached. Since these granules are in pairs on opposite
sides of the base of the cilium or flagellum, movement is produced in only
two directions. In metazoans similar granules are connected to an inter-

FUNCTIONS OF ANIMAL ORGANISMS 495

cellular network of fibers which brings about coordinated movement
among numbers of cilia or flagella. The reversal of ciliary movement is
not understood. The existence of cilia in the early stages of the larvae of
metazoans is of common occurrence and cilia are even retained as the
locomotor organs in the adults of ctenophores, turbellarians, nemertines,
and rotifers. In metazoans generally, however, since the increased size
of the body makes ameboid movement impossible and ciliary movement
ineffective, locomotion comes to be accomplished by movements of the
whole body or by means of appendages developed for the purpose. At
no time during life in nematodes and arthropods are locomotor cilia
developed.

Muscular movement is the direct result of oxidation processes in the
muscles. The oxidation, which takes place in a muscle after it has acted,
is, however, a recovery process tending to build the organ up ready for the
next contraction. This process involves the synthesis from carbohy-
drates of a complex and unstable compound which when the muscle is
stimulated breaks down into simpler compounds. ‘The result is to disturb
the equilibrium in the cell to such a degree that there is a flow of cyto-
plasm and the contraction results. The protein part of the cell is not
affected.

The contraction of a striated muscle fiber results from the sending in
to the muscle of a number of stimuli close together. Such a contraction
is termed tetanic, and its continuation gives rise to a condition known
as tetanus. What is called tonic contraction and the condition already
referred to as tonus (Sees. 242 and 511) is most marked in nonstriated
muscles. No chemical changes take place in the muscle during tonus and
therefore such a condition can be maintained without using up the
resources of the cells. It is present particularly in the nonstriated muscle
cells in the walls of the blood vessels and the alimentary canal. Fatigue
of muscles is due in part to the accumulation of waste matters within the
cell. A distinct nervous element is also involved.

There are in higher vertebrates both red and white striated muscles
which seem to differ in several respects. 'The white muscles are more
irritable and contract more quickly, while the red ones are less irritable,
show a greater degree of contraction in tetanus, and maintain contraction
longer.

517. Nervous Activities.—The basis of all nervous activity is the
irritability and conductivity which characterize living matter as such.
As has been noted in connection with the ameba, stimuli are varied in
kind and the response due to each has received its own particular desig-
nation. The response may be positive or negative, maximal or minimal,
or it may be given to an optimum degree of a stimulus. Among the
Protozoa the functions of irritability and contractility are not separated,
and the same cell both receives and responds to a stimulus. This is true

496 GENERAL CONSIDERATIONS

of the neuromuscular cells in the sponges, which are thus zndependent
effectors. There are also muscle cells in higher animals which are inde-
pendent effectors, responding directly to a stimulus. Such are the cells
of the circular muscles of the iris the response of which is due to the
presence of a light-sensitive pigment in the cells.

In the coelenterates have been noted simple receptor-effector mecha-
nisms and the presence of a nerve net. The activity of these structures
leads to responses which are nonspecific—that is, they do not differ with
different stimuli. The responses follow no definite path, and the struc-
tures are autonomic—that is, independent, functioning when severed
from the rest of the body. Such a mechanism does not show polarity,
which is the property of transmitting impulses only in one direction, this
being a property of a synaptic system made up of neurons.

The next step in the phylogenetic development of the nervous system
is the development of ganglia, which results in the appearance of the
receptor-adjustor-effector mechanism, or the reflex are. Owing to the
polarity which exists in neurons the character of any reflex act is predict-
able. At the same time there appear what are known as conditioned
reflexes the result of which depends not only upon the appropriate stimulus
but also upon a cerebral element, higher centers in the central nervous
system either favoring or inhibiting the carrying out of the reflex act. At
first the ganglia in different parts of the body are to a large degree inde-
pendent of one another and only at certain times is their activity coordi-
nated. As the nervous system becomes more highly developed in higher
forms centralization appears, and then cephalization. Centralization and
cephalization reach their highest development in the vertebrates, cul-
minating in man.

Although centralization and cephalization have been carried to the
highest extent in man, there still remain scattered over the human body
a large number of centers controlling small groups of organs and govern-
ing certain acts which may take place independently of the will. Many
of these acts, however, can with sufficient warning be controlled. Such
centers, which are most numerous in the medulla, govern respiration,
steady the beating of the heart, and control mastication, swallowing,
sucking, the reflex secretion of the saliva and digestive juices, vomiting,
coughing, sneezing, and winking. Numerous other similar centers are
scattered up and down the cord and exist in outlying ganglia in the cere-
bral, spinal, and sympathetic nervous systems.

Sense organs, generally speaking, are known as receptors, but sensa-
tions are functions not of sense organs but of the central nervous system.
The appropriate receptor, for instance, receives a chemical stimulus and
sends an impulse to one or the other of two centers in the brain, which
when stimulated gives rise to the sensation of taste in the case of one or
of smell in the case of the other. .

CHAPTER LXVI

BEHAVIOR OF ANIMAL ORGANISMS

Behavior, as already defined, is the sum total of an animal’s move-
ments. Reference has been made to various modes of behavior in con-
nection with different phyla, but it is desirable to review the whole
subject in one chapter. Before beginning the discussion, however, it
may be said that no other subject in zoology invites so much speculation
as does this and in no other is such a variety of opinions held.

518. Memory.—Memory is due to the persistence of some modifica-
tion in a nerve cell resulting from its activity. Just what is the nature of
such a modification is not known. In the lowest animal organisms
the effect of a stimulus is exceedingly transitory, passing away in a short
time. In a noncentralized nervous system, also, it does not persist for
any great length of time, although it remains very much longer than in
any one-celled organism. One of the characters of a centralized nervous
system is a longer memory, and the development of the brain permits
memory to last even throughout the lifetime of along-lived animal. How-
ever, even in animals which possess a brain, the effects of stimulation
may not remain long. What makes the difference is not exactly known,
but one thing is clear and that is that the effect of a very marked stimulus
remains for a greater length of time than that of one which is inconsider-
able. It is also true that frequent repetition of the stimulus increases the
duration of the effect. Association of ideas, too, tends to prolong mem-
ory. From what has been said it is evident that memory is the outgrowth
of a property which, like irritability, conductivity, and contractility,
belongs to all living matter, though it is only slightly developed in undif-
ferentiated protoplasm. It plays a steadily increasing rdéle in the dif-
ferent modes of behavior from the lowest to the highest.

519. Types of Animal Behavior.—The various types of animal behav-
ior can be clearly interpreted only when studied in the light of the struc-
ture involved. When so studied six general modes of action may be
recognized: direct response, simple reflex action, instinct, habit, intelli-
gent behavior, and reasoning. ‘These will be taken up in turn.

520. Direct Response.—This term has generally been applied to
the type of behavior which involves action by the same cell that receives
the stimulus. It is the mode of action of protozoans and sponges and is
seen in the action of the cnidoblasts of the coelenterates, which are some-
times called independent effectors. All movement in the coelenterates,

497

498 GENERAL CONSIDERATIONS

generally speaking, is the result of direct response, since though the
receptor and effector elements are distinct they are in direct connection.
Coordination is through a nerve net and some localization of responses
exists. In direct response there is to a considerable degree a correspond-
ence between the strength of the stimulus and the vigor of the response,
though this is subject to the effect of different physiological states—as,
indeed, are all modes of behavior. Direct response is modifiable, the
modifications depending upon the physiological state, antecedent stimula-
tion, and attendant environmental conditions.

521. Simple Reflexes.—Simple reflexes may not be different in the
general character of the action from a direct response but the mechanism

Hip ane

al
1 ASE SPAN PETIA NL VOT eee

i Outer dense layer
E Middle, /oose
/ayer

Inner dense

E Jayer
ae Chrysalis
Cast-off /arval
Skin

Fia. 358.—Cocoon of the silkworm, Samia cecropia (Linnaeus). A, external appearance.
B, section to show construction. From a specimen.

involved is different, since receptors, adjustors, and effectors are all

involved. The results differ in that the responses are distinctly more

localized, and this adds definiteness and variety to the actions. This

mode of behavior is first developed among the flatworms.

522. Instincts.—Instincts can best be defined as made up of associated
and coordinated reflexes. It is clear from this definition that it is difficult
to draw a sharp line between the simplest instinct and a simple reflex act,
but in the most complex of instincts we have a very characteristic type
of action.

The nature of an instinct may be well illustrated by a description of
one which 1s very complicated, such as the spinning of a cocoon by a
cecropia silkworm (Fig. 358). If one keeps a caterpillar of this species in
a box, giving it fresh food daily, it is a very docile prisoner, eating vora-
ciously and, so long as fresh food is regularly supplied, seeming to have

BEHAVIOR OF ANIMAL ORGANISMS 499

no desire to escape. As the time approaches for the spinning of the
cocoon, however, there comes a day when the larva refuses to eat. It
is now restless, traveling repeatedly around the box as if seeking some
avenue of escape. At this period in the life history the caterpillars of
many butterflies and moths show a change in color. The restlessness as
well as the change in color indicate a change in physiological state. As
if in response to this change the larva soon betrays a tendency to place
itself in contact with as much surface as possible. It enters a mass of
leaves or, in the absence of them, seeks a place where it can have a maxi-
mum of contact, as in the corner formed by three sides of the box. It
then begins to spin, drawing the leaves close about its body and holding
them in place by silken threads. ‘This spinning continues until a layer of
silk is formed which hides the larva from sight. Now the description
must be finished from inferences drawn from the finished cocoon. When
this outside layer is of a certain thickness something causes a change in
the physiological condition of the organism, and, acting like a machine in
which a lever has been operated, it suddenly ceases to add to the layer
formed and begins to spin looser silk. Soon, however, another shift
takes place and the caterpillar stops spinning this loose silk and begins to
spin another dense layer of silk within. In this second dense layer the
insect apparently uses all of its remaining supply of silk. This inner
layer of the cocoon is then coated on the inside with a secretion which
when it hardens makes the surface appear as if it had been shellacked.
Its task done the caterpillar sheds its skin and becomes a chrysalis, pro-
tected by its cocoon until the time comes for it to emerge as a moth.

Examination of the cocoon brings to light several remarkable features.
The outer layer of the cocoon seems to be a weather-resisting layer,
though it is not waterproof; the loose layer seems to function in a way asa
heat-conserving layer; while the inner dense coat appears as a second
protective layer which is waterproofed within. Furthermore, the cater-
pillar has made provision for emerging, since at the end of the finished
cocoon through which the moth will emerge each layer is loosely woven
to provide an easy avenue of escape.

The spinning of the cecropia cocoon illustrates several of the salient
facts in regard to instincts. (1) The instinct is manifested when a certain
physiological state appears. (2) It is initiated in response to an outside
stimulus. (3) It is an action involving many different reflex activities,
all of which contribute to the one end. (4) It involves changes in action
due to some internal change in the nervous system or to a changed physio-
logical state in some part of the organism. (5) It involves time and space
relations. (6) The capacity to exhibit the instinct is inherent and the
instinct itself is clearly inherited.

Other facts about instincts are brought out by the spinning of this
cocoon. One is the stereotyped character of the whole action. All

500 GENERAL CONSIDERATIONS

cocoons of this particular species are similar; they bear the stamp of the
species just as clearly as does the adult insect. To one acquainted with
silkworm cocoons that of each species is characteristic of the species to
which it belongs. The caterpillar needs no teaching. The results are
perfectly adjusted to the needs of the case. This adaptation of particular
instincts to the particular conditions involved is so perfect that it has
been likened to the relationship between the key and the lock which it
fits. Yet it is clear that there can be no forethought or anticipation of
results on the part of the animal. To many who are not students of
animal behavior this perfect adjustment seems to prove the presence
of intelligence.

Other striking instincts are concerned with the securing of food and
with mating, nest-building, and other reproductive activities, especially
of the arthropods.

From what has been stated it is clear that instincts are inherited and
this inheritance seems to involve a certain structure in the nervous
system. This structure has sometimes been referred to as an action
pattern. It can be said that any particular animal inherits a certain
action pattern which, when it is brought into play under proper condi-
tions and by the appropriate stimulus, leads to results which are always
the same for any individual belonging to the same species as the animal in
question. Instincts as such are, therefore, not subject to modification
unless they are involved in the process of evolution, for they have evolved
in the same way as have the structural characteristics of the species and
the other functions which depend upon these structures.

523. Habits.— Reference has been made to the development of habits
in the starfish (Sec. 229) and in the frog (Sec. 892). To a certain degree
a habit resembles an instinct and it is this resemblance which gave rise to
the former impression that instincts are inherited habits and has sug-
gested the idea that habits are lapsed instincts. The two, however, may
be sharply separated in certain ways: (1) Habits are individual and not
specific. (2) They are formed during the lifetime of the individual which
possesses them. (3) They are not transmissible to the next generation.

A habit is acquired as a result of repeated action. ‘This repetition
may have to occur only a moderate number of times or it may need a
considerable number. Nevertheless it is clearly the result of an action
repeated many times under the same conditions, which has had such an
effect upon the nervous system that, with the aid of memory, the action
is again repeated when similar conditions arise. A habit may be gradu-
ally developed and gradually modified. It has some of the characteristics
of an instinct in that it needs a certain stimulus to bring it out and that it
frequently fits very perfectly certain conditions. At times habits will
simulate instincts very closely but the two may be differentiated when
other individuals of the same species are brought into comparison. The

BEHAVIOR OF ANIMAL ORGANISMS 501

individuality of the habit will then become apparent. Habits exhibited
with instincts make it seem as if the instincts were subject to modification.
However, the same test of comparison with other individuals may be
applied to separate the habitual from the instinctive element when the
two are combined.

524. Learning.—The fact that habit involves repetition and in this
sense learning has led to its confusion with intelligence, but learning by
repetition without an appreciation of cause and effect is not at all the
same as the learning which accompanies intelligent activity.

525. Intelligence.—Intelligence on the part of an animal is the
capacity to profit by previous experience. It is distinguishable from
habit by requiring few if any repetitions for its development and by
its free modifiability. Habits, though capable of modification, are
modified slowly and by repetition, in the same manner as that by which
they are formed. An intelligent animal, as opposed to one controlled
by habit, changes its behavior quickly, adjusting it to the results of past
experiences.

Intelligence is usually considered as involving (1) associative memory,
(2) consciousness, and (8) ability to exhibit emotions and to feel pain and
pleasure.

By associative memory is meant the ability to connect previous experi-
ences with the results of such experiences—in other words, to appreciate
cause and effect—and to profit by that ability. An insect acting only
from instinct will persistently try to reach a certain opening even though
beaten back time after time. An animal, like a dog, when guided by its
intelligence, will, if beaten back, retire and endeavor to find another
means of escape. Consciousness in the sense in which it is here used
implies this awareness of cause and effect.

Emotions are somewhat difficult to define because different theories
are held as to their nature. They are produced under certain conditions
in the brain of intelligent animals, but they seem to be affected by condi-
tions in other parts of the body. Especially is the production of certain
emotions stimulated by the presence of particular hormones in the blood.
The physiological state of the body predisposes the organism to anger or
fear. Emotional conditions often have a controlling effect in the sus-
ceptibility of the body to pain, for an intelligent animal under great emo-
tional excitement is insensible to injuries which under normal conditions
would cause severe pain. This has often been noted in human experience.
Three of the most primitive emotions are anger, fear, and love. ‘These
have their counterparts in instinctive activity, which frequently gives rise
to confusion. We speak of the ‘‘angry bee” when undoubtedly the bee
is actuated only by an instinct which leads it to defend itself or its home.
An insect frequently flies as if trying to escape under the influence of
fear, when it is only the instinct which is aroused by the sight of move-

502 GENERAL CONSIDERATIONS

ment. A spider defending its egg cocoon is actuated by an instinct and
not by love.

Pain and pleasure do not seem to be felt by animals which have little
or no intelligence. An injured dog will cry out in pain, loses its appetite,
and in some cases seems to suffer as much from an injury as does a human
being. Fish, however, after having been caught on a hook and then
liberated with their mouths severely mutilated have been known to bite
again immediately, as if feeling no pain from the wounds. Numbers of
cases might be cited of similar insensibility to pain on the part of nonintel-
ligent or slightly intelligent animals. In the same way pleasure seems to
be associated with intelligence.

The only way to determine whether or not an animal is intelligent is
to place it under experimental conditions. Then if it shows an ability
freely to modify its behavior and this modification is in such a direction
as to imply an appreciation of cause and effect, we may term the behavior
intelligent. It is clear that intelligent behavior can never be stereotyped,
and it is equally clear that intelligent acts cannot be inherited, although
the quality of intelligence is.

526. Reasoning.—Theoretically it may be possible to draw a distine-
tion between reasoning and intelligence, but practically such a distinction
is difficult to apply. Perhaps the most obvious difference is that reason-
ing involves the ability on the part of an animal to form an abstract
conception and be guided in action by it. An animal which after having
had a certain experience successfully meets the same conditions when they
occur again does not thereby evidence a power to reason; but if he so
adjusts the results of a previous experience as to meet conditions some-
what different, this would seem to imply an ability to reason to the
degree that differences exist between the two experiences. Reason
enables an animal to meet conditions which it has never met before by
the perception of analogies between them and conditions connected with
previous experiences. Many stories are told of dogs, horses, and other
mammals which seem to imply the ability to reason on their part. The
capacity to dream may be taken as implying the ability to form abstract
conceptions and as an evidence of the power to reason. Numerous
instances of dreaming on the part of domestic animals have been recorded.
Reasoning does not seem to have ever been attributed to animals other
than the higher mammals, and in no other mammal is it so highly devel-
oped as it isin man. There is a tremendous gap between the reasoning
capacity of the savage and that of the highly civilized man, but even the
savage seems to be raised so far above other mammals by his capacity to
reason that man as a type has been characterized as the reasoning
animal.

527. Combinations of Modes of Behavior.—It is clear from what has
been said that the actions of an animal may be the result of a combina-

BEHAVIOR OF ANIMAL ORGANISMS 503

tion of different types of behavior. A bird building a nest obeys a specific
instinct and the nest is stamped by characteristics shared by all the nests
of the species to which the bird belongs. At the same time, however, the
bird may exhibit individual peculiarities in its construction which are the
result of habit and may meet conditions that arise during its construc-
tion in such a manner as clearly to indicate the possession of some intel-
ligence. Many published observations upon animals are confusing or
inconclusive because to what degree different types of activity have
entered into the act or the series of acts described has not been made
apparent.

528. Behavior of Lower and of Higher Animals.—It is evident that
the behavior of the lowest organisms is dictated by responses to stimuli
received from the environment and it is equally clear that as the different
phyla pass in review different modes of behavior appear. It is also true
that as higher animals have acquired other modes of behavior they have
not entirely laid aside those modes of behavior possessed by forms lower
than they and that the progression from phylum to phylum up to the
highest is a record of accumulation. Any higher animal may show all
of the modes of behavior which the animals below it possess, as well,
it may be, as a characteristic mode which sets it off as different from those
which have preceded. Man, as the highest animal, exhibits all of the
different modes. He shows direct response in the iris of the eye and
simple reflexes in various parts of the body and exhibits a considerable
number of instincts, which have by some been termed innate tendencies.
He is also capable of acquiring habits, of using intelligence, and of exer-
cising reason. A general principle is that higher modes of behavior are
dominant over lower ones. An animal carrying out an instinct will fail to
exhibit direct responses which have no relation to the instinct; intelligence
enables an animal to control an instinct; and in man reason may
dominate all.

529. Mind and Consciousness.—These terms are very often used but
with varied significance, the application depending upon the point of
view of the person using them. Mind and consciousness of a sort have
by some persons been attributed even to inorganic matter and by others
to plants as well as animals. Within the animal kingdom the line has
been drawn at many levels. As has been seen, consciousness of a certain
kind is associated with intelligence but it is clearly a question whether or
not this is the beginning of consciousness. If one thinks of both con-
sciousness and mind as attributes of life, then the conception of both as
having been gradually evolved is a logical one. From that point of view
both would culminate in man. Asa matter of fact, when we compare the
consciousness of the savage, who perhaps conceives of nothing beyond the
mountain-inclosed valley which is his home and who entertains only

504 GENERAL CONSIDERATIONS

vague speculations as to physical phenomena, with the consciousness of
the highly civilized individual, which not only encompasses the earth
but reaches out to the limits of the universe, a gap exists which is greater
than that between the savage and the animals below him. The con-
sciousness of each individual human being expands with added knowledge
and experience, and the same thing may be said of the mind.

CHAPTER LXVII

ANIMAL ORGANISMS IN RELATION TO THEIR
ENVIRONMENT

ECOLOGY

That field of zoology which deals with the relations of organisms to
their environment is called ecology. In other words, it is the study of the
animal in its home. The field is not new, for it is practically equivalent
to what has long been known as the natural history of animals, but the
name is new and the exact methods of modern ecology are of very recent
development.

530. Facts of Ecology.— Observation reveals a varied distribution of
animal forms. In one locality with a certain character are found certain
animals, while in another of a different character there is quite a different
group. In fact there is no species of animal which is uniformly distrib-
uted. Not only is this true but it is a matter of common knowledge that
the animals which are active at night are not those which are active in
the daytime. ‘The species of animals about us change from one season to
another; birds come and go, and insects appear at their appropriate times.
One who observes life from year to year finds differences both in the
species to be found and in their relative numbers. The animals found
associated in a certain type of locality, however, are usually associated
together in any other locality of the same character, and so definite types
of animal communities can be recognized. The study of the causes of
these phenomena lies within the field of ecology.

531. Relations of Animals to Plants.—Early in this text (Sec. 69) the
dependence of animals upon plants was emphasized. Directly or indi-
rectly plants are the basis of all animal food and the capacity of any
given region to support a large animal population depends upon the
amount of plant food available. Plants, however, serve not only as food
for animal organisms but also for shelter and concealment. For these
reasons the plant life has a very important influence—and in many cases
even a controlling one—over the animal life of a given area. It is dif-
ficult to study intelligently either plant or animal ecology without also
studying the other, and for this reason the term biota is commonly
employed in reference to all of the living organisms of a given area,
including both plants and animals. Owing to the fact that few plants
possess the power of locomotion, plant ecology presents fewer difficulties
than does animal ecology and has not only preceded animal ecology in

505

506 GENERAL CONSIDERATIONS

time but has excelled it in the development of methods and in the definite-
ness and certainty of its conclusions.

532. Physiological Life Histories.—The life histories of animals have
been referred to many times but always with particular reference to the
structural changes which the animal passes through in its development.
The life history of an animal may be viewed from another aspect, and
that is as controlled by its physiological reactions. This aspect of an
animal’s life history belongs to ecology. Shelford has enumerated five
types of physiological life histories. One is that in which the annual
eycle and the life-history cycle agree and in which the life history occupies
but one year. A second is that in which the development of the animal
occupies two or more years and the adults are produced at such intervals.
Usually broods of insects appear every year, but in some cases many
years may elapse between broods in any particular locality. In the
seventeen-year cicada, seventeen years intervene between the appearance
of the adults of one generation and those of the next. In a third type
the adult lives over a number of years and reproduces a number of times.
This is generally true of higher forms. A fourth type includes those
animals in which there are a number of generations in each year. The
fifth and last type includes those which reproduce continuously and either
at a uniform rate because of uniform conditions or at different rates under
varying conditions. Included in this group are certain plankton organ-
isms which live where conditions are nearly uniform throughout the
year.

533. Habitat.—The particular locality in which any one species of
animal is found is known as its habitat. Some animals are capable of
occupying several habitats, affording, perhaps, a variety of conditions;
others are very narrowly restricted in their choice. Perhaps the most
narrowly restricted of all animals are parasites that can live only in a
certain part of the body of a particular species of animal. There are,
however, many free-living forms which can live only under a very precise
set of conditions which are rarely found. Generally speaking, aquatic
animals cannot adjust themselves to life outside the water, although
there are those which make brief excursions outside their aquatic habitat
or which can remain living for some time when deprived of water. Exam-
ples of such types are the animals living above low tide or the many
animals of ponds and rivers which have to endure periods when the body
of water in which they live becomes dry.

The inability of a plant to move, generally speaking, forces it either to
live or to perish in the habitat in which the seed germinates. It is thus
possible for botanists to study the reactions between plants in any given
habitat and to define very exactly the composition of plant communities.
However, the mobility of animals and the different reactions they display
at different times in their physiological life histories present difficulties

ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 507

in the precise definition of animal communities which zoologists have
hitherto been unable to overcome. The seashore affords examples of
rather definite and stable animal communities, but such communities are
to a lesser degree characteristic of fresh water and are rarely found in
terrestrial environments.

534. Ecological Factors.—The factors in the environment which
affect animals most strongly may be enumerated under two heads:
physical and biotic.

Among the physical factors are the presence and composition of water,
temperature, light, and molar agents, such as wind and currents. The
chemical composition of water involves salts which are more or less
ionized (Sec. 18). This results in an increase in alkalinity or acidity. A
measure of this ionization is the acidity corresponding to the number of
hydrogen ions present in a given unit volume of a solution, indicated by
the symbol pH (potential hydrogen). A pH concentration indicated by
7 corresponds to neutrality. Any concentration indicated by a larger
number implies alkalinity; by a smaller number, acidity; and the amount
of departure measures the degree of either alkalinity or acidity. It
has been found in the case of certain organisms that the pH concentration
of water is a very important factor, but in the case of others it seems
to have little or no effect. The pH can be determined not only for bodies
of water but also for soils containing water and for body fluids.

The biotic factors in the environment relate to plants and to animals
of either the same or other species and affect the organism in such ways
as through food supply, competition, mutual help, as in animal com-
munities, and the relations of the sexes.

535. Reactions of the Animal.—In response to these various factors
of the environment, reactions take place within the body of the animal
which find expression in form, size, and color; in the physiological adjust-
ments which the animal makes; in its behavior; in its mode of reproduc-
tion; and in its length of life. Ecology should concern itself fully as much
with the reactions within the animal itself as with the conditions of the
environment in which it lives and to which it reacts, though up to the
present time investigations have dealt largely with the latter aspect.

536. Communities.— As has been stated above, animals present them-
selves in communities which may vary greatly in extent. A pond or
lake is in a sense a great community of aquatic organisms, but within
this environment are many lesser environments such as the shore, beds of
aquatic vegetation, the surface, and the deeper portions. Each of these
smaller parts of the whole has its peculiar animal types. The character
of the vegetation, differing in different areas, may directly affect the
character of the community of animals found in each. In bodies of
water of any considerable depth, as well as in the vegetation in the case
of terrestrial forms, there is the phenomenon of stratification. Certain

508 GENERAL CONSIDERATIONS

animals in forests are restricted to the ground; others, to the lower
vegetation; others, to shrubs; while still others reach the tops of the
tallest trees. These may be considered as separate communities. Thus
the number of possible community relations becomes great and an animal
may belong to a small community unit and at the same time be a member
of several larger units differing in numbers and areas of distribution.
The precise limitation of such communities, however, is made difficult
by the freedom with which animals migrate from one place to another,
since some animals may be members of different communities at different
times of the day or year, under different weather conditions, or during the
different periods of their lifetimes.

In any given animal community there is a food cycle or a food chain.
We find a starting point in such a chain or cycle in the plant life upon
which herbivorous animals feed. These in turn are devoured by car-
nivorous types, and through the metabolism of the animal or its ultimate
death the materials of the body are returned to the environment, again to
be used by plants. Itis rarely, however, that food chains can be expressed
in such simple terms. Thus in a pond the decaying organic matter is
utilized by bacteria. These in turn are eaten by certain small protozoans
which form the food of larger and more complex ones. ‘The protozoans
may be eaten by rotifers, crustaceans, and other animals, which in turn
form the food of aquatic insects. The smaller fish feed upon these insects
as well as upon other forms, and they in turn are eaten by the larger
predatory fish. From these fish the food cycle may, within the body of
water, return to the stage of decomposition and decay; but since these
larger fishes are eaten by a variety of animals outside the water, the
cycle may not be completed within the aquatic environment.

In nature a balance is often developed within a given environment
which results in a very stable condition. Such a balance, however, does
not long remain. Changes ensue which cause it to be disturbed, and so
constant readjustments are necessary. Readjustment may result in a
balance at a new level, but usually such changes are progressive, and thus
the successive states of balance are not permanent but merely steps in a
process which forms an orderly sequence and leads to a definite end.

537. Succession.—The series of readjustments which have just been
referred to result in what has been termed biotic succession, which is a
term applied to the progressive changes in the composition of a fauna
and flora that ensue because of modifications in the environment. For
instance, if an area of land is denuded of all vegetation certain pioneering
plants will appear first in the process of restoration. These in turn will
give place to others, and this will continue through a series of vegetation
changes. The process ultimately leads to the establishment of a per-
manent grassland or forest. With the vegetation changes which have
occurred there are corresponding changes in the character of the animal

ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 509

life, especially involving the forms most dependent upon vegetation, and
thus there follows a succession of animals represented by more or less
clearly defined stages and ending in a practical balance.

The glacial lakes of the northern part of this country have exhibited a
definite series of changes which may be likened to the periods in the life
of an organism and permit us to speak of any particular one as a young

ecropia Moth
Swamp Sparrow

nails
Turtles

Flower-seeking Insects
Bittern

GS
Muskrats

Smal! Bivalve Molluscs

ked-winged Blackbird
Diving Beetles

Alder Flycatcher
Garter Snakes
Meadow Mice

Yellowthroat
Swimming Birds

Marsh Wren
Water Striders

tS 4

Flowering Herbs, | Marsh \Floating| Open
Sphagnum Moss, Grasses |Bunches| Water
Tamarack & Willow of Grass

a
C

teh bv ALT

cee

Fic. 359.—Diagram to illustrate succession in a bog lake which is advanced in the
process of being filled in. The grading in the shading under the open water indicates the
loose, flocculent deposit which at the top offers no resistance to penetration by a pole or
other object, but which becomes more and more dense as the bottom is approached. The
animals in each zone are represented by only a few types selected to show variety.

lake, a lake of middle age, or an old lake. At the beginning of this
process the lake has clean sand and gravel bottom and shores. Later
plants appear and by their growth, death, and decomposition, and by
the washing of soil into the lake, there are gradually developed deposits
of mud and vegetable mold on the bottom or in quiet places along the
shore. Sand and gravel are then left exposed only on beaches where
combined wind and ice action prevent the accumulation of either vegeta-
tion or mud. In time, however, there is built out from the shore a mass
of vegetation which produces a bog or marsh; at the same time the lake

510 GENERAL CONSIDERATIONS

is filling up from the bottom. As the shores become firm the bog and
marsh plants give way to shrubs and trees, and ultimately a forest may
cover the area earlier occupied by the lake. In some cases grasses may
become so firmly established that the forest is unable to enter, and then
the lake is represented by a grassy area of flat prairie surrounded by
forest. With all these changes it is inevitable that there should be a
corresponding succession in the types of animal communities (Fig. 359).
Corresponding periods may be observed in any permanent body of water.

In addition to such succession as has been referred to and which
involves long periods of time, there are annual or seasonal changes, which
take place at intervals during the year and are more or less dependent
upon the season. In the tropics where conditions are very uniform there
are no marked seasonal changes. In the polar regions, too, where condi-
tions change with relative abruptness from winter to summer and back
to winter again, there are no marked seasonal gradations. In the tem-
perate zone, however, where the four seasons are of more nearly equal
length and the transition from one to another is gradual, definite and
orderly seasonal changes take place.

538. Rhythms.—Somewhat similar to succession are the rhythms
that present themselves in animal communities, as a result of which many
of the animals belonging to that community never meet. Among these
rhythms is the night and day rhythm, as a result of which the animals
which are active during the night form a group distinct from those active
during the day. This is most strongly marked in deserts, where the
conditions between night and day are most different. There is no night
fauna in the polar regions because there the night is too cold to permit
any activity. Night faunas are, on the other hand, exceedingly rich in
the tropics where the heat of the day enforces quiet on animals and the
night is the time of maximum activity. Other rhythms are those seen
along the seashore and connected with tidal currents and those, in the
ease of terrestrial animals, involving dry and wet weather.

It has recently been recognized that the abundance of many animals
varies over a period of years in a rhythmic manner, and these rhythms
seem to be related to predatory enemies and disease producing organisms.
As an animal increases in abundance its enemies also increase; when its
numbers reach a maximum these enemies gather in maximum numbers,
epidemics of disease develop, and the animal may be so reduced in num-
bers as to become searce. This results in a reduction of the enemy forms,
which affords the animal the opportunity again to increase. However,
many years must pass before it can regain its former numbers. Then the
process is repeated.

539. Marine Faunas.—When one takes up the subject of faunas he
finds himself on a border line between ecology and zoogeography. The
modifications and the adjustments which marine animals show to varying

ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 511

environments within the sea are, properly speaking, ecological phe-
nomena. A discussion of marine faunas (Fig. 360) in detail or of the
animal communities of the sea is impossible here but a few statements
may be made. The conditions met by animals living between high and
low tide marks vary so that these animals find life very difficult and have
to make many adjustments. Shifting beaches are unsuitable for many
forms which are abundant on permanent rocky shores. The fauna of
mud flats consists largely of burrowers. Pelagic forms show adaptations
making it possible for them to float and swim, and these affect the form
of the body, its structure, and the development of special organs. Such
animals, particularly if they are plankton forms, are characterized by
transparency and delicacy of color. The bottom forms of the sea exhibit

Shore Fauna
“9 Intertidal Fauna

Plankton °
Nekfon i

Fic. 360.—Diagram to show distribution of animal life in the seas. The light is shown
fading out with increasing depth and ceasing entirely at 900 meters.

many adjustments, including the presence of stalks to raise the sessile
forms above the mud, the development of long legs on the part of walking
forms, the production of few and large eggs, and other adjustments
permitting the parent to carry the young about and to protect them.
Even such deep-sea animals as echinoderms carry their offspring about
during development and until they are able to shift for themselves.

The fauna of the sea in general has been divided into (1) the littoral
fauna, found along the shores, (2) the pelagic fauna, occupying the upper
levels away from the shores, and (3) the abyssal fauna, living in the depths
of the sea. The pelagic fauna is divided into the nekton, which includes
all the larger forms, that are able to control their own movements, and
the plankton, which includes the smaller forms, that are at the mercy of the
currents and the movements of the waves. The bottom forms in the
depths of the sea are called, collectively, the benthos.

512 GENERAL CONSIDERATIONS

540. Fresh-water Animals.—Fresh-water animals show a variety of
adaptations to permit swimming and respiration in water and to meet
periods of drying and freezing. Animals in rapidly flowing streams
develop organs for attachment, including suckers and hooks on their
legs, while the larvae are often protected by cases in which they live.

Lakes of considerable depth show in the winter a circulation involving
all of the levels, but in the summer a body of cold, stagnant water
remains below while above it is a surface stratum of water acted upon
by wind and currents which produce a constant circulation (Fig. 361).
The plane separating these two bodies is known as a thermocline. It
is established in the spring and destroyed again in the fall when the
surface stratum falls to a temperature Jess than that of the water below,
in consequence of which an overturning and a thorough mingling of the
two strata take place. The conditions below the thermocline, particu-

above the water show the direction of the wind; those in the lake above the thermocline
show the circulation currents in the water. Below the thermocline the water is stagnant.

larly the low temperature and the small amount of oxygen, make possible
the existence of only a limited fauna.

541. Terrestrial Faunas.—Owing to the great variety of conditions
which exist, terrestrial faunas are more varied than those of either the
sea or fresh water. Such faunas include subterranean forms which are
influenced by temperature, moisture, degree of aeration, and the chemical
composition of the soil. Certain types of animals are restricted to the
surface of open ground or the floor of the forest. Still others live at
various vegetational levels depending upon the character of the vegeta-
tion. Finally, there are the aerial types, which are capable of flight.
In terrestrial faunas should be included the cave faunas, among the mem-
bers of which there is a tendency toward the loss of eyes, color and organs
of flight, while their senses of touch and hearing become very acute.
Animals of desert faunas are active mostly at night, so as to escape the
heat of the day; frequently develop the power of rapid locomotion; show
numerous cases of adaptive coloration; pass off dry excretions so as to
conserve water; and generally exhibit a low rate of reproduction.

ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 513

542. Mimicry and Protective Resemblance.— What is essentially an
ecological phenomenon, since it involves adjustment to the environment,

Cc D H
Fic. 362.—Mimicry. Several cases of resemblance between animals of different groups,
one of which is said to mimic the other. A, a clear-winged moth which resembles a wasp,
B. C,a beetle with very short elytra which resembles a wasp, D. E, a fly which resembles
awasp,F. G,aspider which resembles an ant, H.

is that of color and form in animals as related to surrounding objects.
Concealing coloration is a color possessed by the animal which makes

Fic. 363.—Protective resemblance. A, the leaf butterfly of India, Kallima sp., which
when resting upon a twig resembles a dead leaf. B, a lepidopterous larva, which assumes
a resting attitude in which it resembles a twig. C,aleaf insect of South America, Phyllium
sp., which belongs to the walking sticks, and which resembles in form and color an assem-
blage of leaves. (A and C from specimens; B redrawn from Jordan, Kellogg, and Heath,
‘Animal Studies,”’ by the courtesy of D. Appleton & Company.) All X %.

it invisible against the background of the environment. It is evident
that concealing coloration would be useful both to a nonpredatory animal

514 GENERAL CONSIDERATIONS

seeking to escape its enemies and to a predatory animal seeking to
approach its prey. Warning coloration is believed to be of value to the
animal because of the immunity it affords from the attacks of enemies.
Recognition colors are those possessed by an animal which enable it to
be recognized by others of its race or by the other sex. Mzmicry (Fig.
362) involves both color and form but the resemblance is to some other
animal and not to features in the environment, which is termed pro-
tective resemblance (Fig. 363). Many edible insects are believed to mimic
those which are inedible and in that way secure immunity from the
attacks of enemies. Caution must be used, however, in the citation of
cases of mimicry and protective resemblance and in the drawing of con-
clusions as to the significance of the resemblance.

CHAPTER LXVIII

ANIMAL ORGANISMS IN HEALTH AND DISEASE

A very practical application of animal biology is in the development
of an understanding of health and disease. The principles involved are
of general application to all animals, but since we are more concerned with
health and disease in man and more is known of human diseases than of
those of other animals, any discussion of this subject will inevitably have
a strong human emphasis.

543. Definitions.—If an organism is in perfect adjustment to its
external environment, and in case it is a metazoan, if the cells which com-
pose the body are perfectly adjusted to the conditions within it, then
theoretically the organism will be carrying on the activities of life with the
maximum degree of ease and effectiveness. Such a condition could be
referred to as one of zdeal health. If, on the other hand, there is any
departure from that condition so as to interfere with the carrying on of
such activities even in slight degree, the condition might be termed one of
disease. Perfect adjustment, however, either external or internal, is
rarely if ever encountered. To all organisms life involves a constant
struggle to reach an adjustment sufficient to avoid a serious interference
with the performance of bodily functions. Therefore, for practical
purposes health is defined as the existence of such a condition of the organ-
ism as permits it to carry on all functions in a normal fashion, though it
may be not to a maximum of effectiveness. Disease, on the contrary,
is the existence of a condition which interferes with such normal functional
activity. With reference to ourselves, we ordinarily overlook little
troubles in various parts of the body, such as mild headaches, slight dis-
turbances of digestion, and other small ills which appear to us of little
consequence, and consider ourselves well in spite of them.

544. Health in a Protozoan.—The protozoan, a frequent environment
of which is the water in which it lives, needs for healthful living food
of the right kind and in the necessary amount. It also needs to be
perfectly adjusted to the environment about it so that there will be no
interference with the interchange of material between itself and the water,
including respiration and the prompt elimination of waste. Given these
conditions the presumption is that normal metabolic activity will be
carried on and that the protozoan will continue to live a healthful life.
Most one-celled organisms also need a certain amount of light, have an

515

516 GENERAL CONSIDERATIONS

optimum temperature, and if not aquatic must have a certain amount of
moisture, all of which conditions should be added to those which make for
healthful living.

545. Comparison of Protozoan and Metazoan Cells.—The cells in the
body of a metazoan are related to the body fluids in the same manner as
are the one-celled organisms to an aqueous environment. Though
there are cells on the surface which are not surrounded by these fluids
they must be related in some way to them in case they are to remain
living. The deeper cells of the human epidermis, for instance, are in
contact with blood vessels and with lymph, but as they are carried toward
the surface by the multiplication of cells below them, they lose this
contact, become dead, change in form and composition, and are finally
cast off. The surface cells of other animals are protected by a cuticula
which they secrete, by slime, or in some other fashion. Within the body
the health of the individual cell rests on much the same conditions as
does the health of the one-celled organism. Each cell must be provided
with the proper kinds of food and in the proper amounts, must be freely
supplied with oxygen, and must have its waste quickly removed. Each
cell must also have the proper environment maintained, including an
appropriate temperature, especially in the case of warm-blooded animals.
Light is a factor in the health of most animals, as are also a great variety
of internal secretions, especially in the higher forms. Since the health
of a metazoan is necessarily the resultant of the health of the different
cells which make it up, anything that interferes with the health of any
of the cells of the body produces a condition of disease, though it may be
local in character.

546. Conditions of Health—From what has just been stated it is
clear that four conditions are necessary for the maintenance of health:

1. Proper kinds and amounts of food.

2. Maintenance of normal metabolic activity.

3. Prompt and complete elimination of waste.

4. A proper physical environment.

547. Causes of Disease.—The causes of disease are not alone the
converse of the conditions of health, although this is true of the first
cause here enumerated. These causes may be named as follows:

1. Wrong Living Conditions—These conditions involve food, air,
sleep, exercise, metabolism, elimination, and internal secretions.

2. Inheritance.—In certain cases a disease may be acquired before
birth; this is true of syphilis. Such acquisition is not true inheritance.
In other cases, the situation involves not the passing on of the disease
but the passing on of a weakened constitution which predisposes the
individual of the next generation to that disease; this is true in the case of
tuberculosis. Such susceptibilities may be truly inherited through the
germ plasm.

ANIMAL ORGANISMS IN HEALTH AND DISEASE 517

3. Traumatism.—In a popular sense traumatism is synonymous with
the term accident, but it is, properly speaking, a somewhat more inclusive
term, since it would include the results of any overactivity or strain
leading to abnormal conditions in the body.

4. Infective Organisms.—Infective organisms comprise both plant
and animal parasites which cause disease either by depriving the organism
of something which it needs or by the creation of wrong living conditions,
including the development of poisons, in the body of the host.

Some infectious diseases, such as scarlet fever, yellow fever, infantile
paralysis, and influenza, are caused by viruses. Viruses may also cause
diseases in other animals or in plants. It has been known for some time
that a virus is composed of particles too small to be seen with the micro-
scope. Recently, Dr. William Stanley of the Rockefeller Institute has
isolated the virus of tobacco mosaic disease and found it to be a protein
with a very large molecule. Although it can be crystallized and is
apparently nonliving under some conditions, when it is applied, much
diluted, to a living tobacco plant, it multiplies or increases rapidly as if
it were living. Viruses are thus most interesting biologically and
chemically since they seem to be intermediate between living and non-
living matter.

548. Effect of Individuality—The individual character of the organ-
ism has a decided effect upon the susceptibility to disease. Naturally
this individuality may be a matter of inheritance but it also may be an
acquired characteristic. Particularly is this seen in the way individuals
react to articles of food. Some persons are unable to eat certain foods,
as, for example, acid fruits and the yolk of eggs, although to most people
the same foods are entirely innocuous. This individuality also affects
the responses which the body gives to certain drugs, and the degree of
toleration shown by different individuals toward drugs must be taken
into consideration in the treatment of disease. Any such peculiar
and individual responses to food or drugs are often known by physicians.
as idiosyncrasy. It may be a form of allergy (Sec. 553).

549. Self-regulatory Tendency in the Body.—A one-celled organism
subjected to modifications of its environment changes in such a way
as to adjust itself to the altered environment or, if it cannot meet the
changes, protects itself against them by encystment. In the same way
the cells of a many-celled organism tend to adjust themselves to changed
conditions within the body or, if they cannot so adjust themselves, resort
to certain means of protection from those conditions. These means
sometimes involve such extreme measures as the dropping off of a portion
of the body, or autotomy. A self-regulatory tendency is very fortunate
for man as well as for other organisms, since through it an animal will
of itself tend to regain its health when subjected to conditions that cause
disease, even though no assistance is given from without.

518 GENERAL CONSIDERATIONS

550. Toxins and Antitoxins.—The living of one organism in the
body of another may impose certain hardships upon the latter. In
addition to its own wastes the body of the host must eliminate the wastes
produced by the other organism. Of course, if the relationship is one
of symbiosis the elimination of these wastes does not impose a hardship
sufficient to counterbalance the advantage accruing from the relation-
ship, but in the case of a parasite this may be a serious strain upon the
host. Whenever these wastes act as a poison in the body of the host
they are included under the general term of toxins. This term, however,
also includes all other poisons which may be introduced into the body,
whatever their source.

To any such poison, either elaborated within the body by some
parasitic organism or introduced into the body in any manner, the body
reacts by producing a substance which tends to neutralize the poison
and which is known as an antibody, or antitoxin. Such a substance is
produced by the body in response to the presence of any foreign chemical
substance and is part of the self-regulatory function of the body by
which it can adjust or defend itself. By neutralizing the toxins, the
antitoxins safeguard the body cells against injury and give time for the
body to eliminate the cause of the disturbance. Since the response to
each toxin is specific, a different antitoxin is produced for each one.

551. How the Body Fights Disease.—One method by which the body
fights disease is, as has already been indicated, by the production of
antitoxins. Another way is through the activity of the white blood
corpuscles, or leucocytes. A leucocyte is an ameboid cell which shows
a tendency to take into its body other organisms and other materials
in the same fashion as an ameba takes in bits of food. Normal body
cells are not attacked by the leucocytes, but cells in the body which
become abnormal or which are injured, or foreign cells of any kind, are
taken up by them and destroyed. When thus taking in other cells
they are termed phagocytes (literally, eaters of cells). Phagocytes are
active in the destruction of certain cells in the body when the absorption
of tissue is desirable. For example, they play a part in the absorption
of the tail of a tadpole when it changes into a frog. When injury results
in the death and destruction of cells in the body, the phagocytes attack
the dead and injured cells and by destroying them and clearing away
cellular debris pave the way for normal regeneration and the return of a
healthful condition. They are also active whenever disease-producing
organisms enter the body. Attracted to the place of entrance of these
infective organisms, apparently in response to the unusual chemical
stimuli due to the invaders, the phagocytes ingest and destroy them.
If the number of invading organisms is not great, the phagocytes may in
this way safeguard the body against the onset of disease. If, however,
the invading organisms are so numerous at the point of infection that the

ANIMAL ORGANISMS IN HEALTH AND DISEASE 519

leucocytes are unable to cope with them, then enough tissue may be
broken down to cause the formation of a pus cavity, or abscess, and the
leucocytes become pus cells.

552. ImmunityImmunity may be defined briefly as the absence
of susceptibility to disease. It may be of three kinds: natural, acquired,
and artificial.

Natural immunity is possessed by an animal because of the character
of its body. Many animals are naturally immune to certain diseases
to which others are susceptible. More or less immunity to some diseases
is possessed by certain human races; for example, the Jewish race is very
resistant to tuberculosis, while Negroes and the Irish are particularly
susceptible to it. There is also age immunity, adults being generally
free from so-called children’s diseases.

Acquired immunity is the immunity which an animal enjoys by virtue
of having had a disease and having built up such a power of resistance as
makes it immune to succeeding attacks. To many infectious diseases
the human body develops resistance by the formation of antitoxins at
the time of the attack and by their continued formation afterward. Thus
conditions in the body are made unsuitable for the development of the
disease organisms should they again gain admission. This acquired
immunity may last for only a certain time or it may persist throughout
life.

Artificial immunity is an immunity produced by artificial means;
there are several ways in which such immunity may be secured. (1) One
is by introducing into the body living but weakened cultures of the infec-
tive organism. A mild attack of the disease is produced which immunizes
the body against a serious attack, which would result from the entrance
of virulent organisms. An example of such an artificial immunity is that
resulting from vaccination for smallpox. The reaction to the vaccina-
tion is ordinarily not serious and results in immunity to the disease
itself. Immunity to rabies may also be produced in this way. (2)
Another method is by the introduction of virulent cultures in small doses
which the body can successfully withstand and as a result of which it
will build up an immunity to more serious infection. This mode of
securing artificial immunity has been practiced in the case of cholera and
bubonic plague. (3) Immunity against typhoid fever is secured by the
introduction into the body of extracts containing the dead bacteria.
Responding to the presence of these extracts, the body builds up the
appropriate antitoxin and thus safeguards itself against disease due to
the introduction of virulent organisms of the same kind. This is the
method now used in immunizing to plague and cholera. (4) Still another
way of securing artificial immunity is by the introduction of an antitoxin
developed in the body of another animal. The organism that causes
diphtheria in man, when grown in an artificial culture, will produce a

520 GENERAL CONSIDERATIONS

toxin. This may be introduced into the blood of a horse and the horse,
in response to its presence there, will manufacture an antitoxin. The
serum from the blood of the horse containing both the toxin and antitoxin
may then be injected into the body of a person and will not only confer
immunity but will tend to stop the disease if it has already been initiated.
The same type of procedure is followed in the case of scarlet fever, but in
tetanus the antitoxin alone is introduced.

Many other toxins as well as infective organisms may be combated
in the body by the development of an appropriate antitoxin, which when
injected protects the body from the effect of such agents.

553. Anaphylaxis and Allergy—Anaphylaxis may be defined as an
exaggerated irritability of the body with respect to some foreign
substance. It may follow a case of mild poisoning by the substance con-
cerned and is associated with the eating of a great many foods, particu-
larly proteins and sea foods. A person may have eaten such a food
freely and without evil effects until, under certain conditions, poison-
ing occurs. If thereafter, whenever the food is taken, the body shows a
pronounced reaction, a case of anaphylaxis exists. Allergy is a similar
exaggerated susceptibility to contact with dust, the pollen of plants, and
hairs or other particles from the bodies of animals. Hay fever is such a
condition. The word allergy is sometimes used in a more general sense
to include anaphylaxis and immunity, thus referring to any altered
response of the body to foreign substances of any kind.

554. Maintenance of Health in Human Beings.—Many of the condi-
tions which are necessary to maintain the body in a state of health may be
inferred from statements made earlier in this chapter. The field of
investigation which deals with the effect of conditions within the body
upon health is called hygiene; when conditions outside the body are
involved, it is spoken of as sanitation. Preventive medicine covers in
general both fields.

Always in considering the maintenance of health allowances must .
be made for the effect of routine. The body forms habits relating to
all procedures connected with hygiene and these have the same control
over the body, when once formed, as do all habits. In changing in any
way his mode of living a person has to consider the adjustment which the
body can make. This power of adjustment is great in youth but dimin-
ished rapidly in old age, when changes of any kind have to be gradually
brought about.

CHAPTER LXIX

RELATIONS BETWEEN ANIMAL ORGANISMS

From time to time, in reviewing the different phyla, relations between
organisms have become apparent, and some of these have been indicated
by name. It is, however, desirable to pass in review these relationships
in such a manner that the logical connection between them will appear.

555. Solitary Life.—Solitary life is a possible mode of living only
to those organisms which are able to play their part in reproduction with-
out relation to any other individual. Examples would be the simpler
protozoans, like the ameba, which reproduce by fission; and some meta-
zoans, Which reproduce by budding. It may also occur, as far as the indi-
vidual is concerned, in the case of some sexual organisms, like clams and
mussels, the males of which simply pass their sperm cells out into the
water, chance alone determining their entrance into the female and the
fertilization of the egg cells. Since, however, most animals, even though
they may live alone at other times, come together at the time of breeding,
a strictly solitary existence is a rare phenomenon.

556. Associations of Animals of the Same Species.— Associations of
this type have their logical beginnings in the relation of animals as mates
and also include families, colonies, and societies.

557. Mating.— Mating refers to the association of two individuals
for purposes of reproduction, one taking the part of the male and the
other that of the female. Such an association may be an exceedingly
temporary one, ending as soon as the eggs have been fertilized, or,
on the other hand, it may be a relationship which lasts throughout the
lifetimes of the individuals concerned. As a general principle animals
low in the scale of animal life exhibit a mating which is quite temporary,
but in ascending the scale the relationship is found to be gradually pro-
longed. Especially is this true of animals in which the care of the
parents is necessary for the successful rearing of the young. The mating
of one male with several females is termed polygyny; and of one female
with several males, polyandry.

558. Families.—If the offspring remain together and are accompanied
by the parents, the relation is that of a family. This may persist only
until the young are able to care for themselves, which is true of birds and
mammals generally, or, on the other hand, it may last longer and lead to
the next type of association.

521

522 GENERAL CONSIDERATIONS

559. Colonies.—When the parents and offspring remain in physical
continuity, as in colonial hydroids, many anthozoan polyps, bryozoans,
tunicates, and some other forms, what is known as a colony is produced.
In such a colony there is often division of labor and dependence of one
upon another. It may even result in the functioning of the whole as an
organism made up of many individuals.

560. Societies.—If the offspring do not remain in physical contact
but become separate individuals and yet these associate together, the
eroup is termed a society. A society may, as in the case of various worms
and barnacles, involve no dependence and no division of labor, but it is
also possible to have division of labor and polymorphism within a society,
such as in the case of antsand bees. Societies are not always the descend-
ants of a single pair but may include unrelated individuals of the same
species brought together by a social instinct.

561. Associations of Animals of Different Species.—Animals of
different species are also found associated together. This involves
relationships of various degrees of intimacy and with a varying distribu-
tion of benefits and injuries. The different terms which have been
applied have been used in such varied senses that what they mean in
any given place can be determined only by recourse to definitions or
inferences from the facts presented. For this reason the examples of
certain associations given here may be found elsewhere under dif-
ferent names. The terms used are aimed to bring out various degrees of
relationship.

562. Gregariousness.—Gregariousness is a term applied to the
tendency of animals to gather together in one place. If these are of the
same species, a society may result; but it may involve different species
and be due to the presence in that place of desirable conditions of exist-
ence, including food, shelter, moisture, and other environmental factors.
Such a relationship is exhibited when in a marsh are gathered together a
variety of marsh-loving organisms. It is also exhibited when birds of
many species gather on an island in the ocean where they find conditions
favorable for nesting and the rearing of young. In the highest animals,
including those which possess intelligence, eregariousness may be the
result of a desire for companionship, which also may be mingled with a
feeling of safety in the presence of numbers, even though the individuals
may be of different species. This safety may be a real factor if in the
gregarious assemblage there are individuals which by their sounding of
an alarm give warning of danger to the others.

563. Epizoic Associations.—The word epizoic implies the living of
one animal upon another, not as a parasite but as it might live on any
nonliving object. Colonial protozoans and hydroids which ordinarily
attach themselves to rocks and other objects in the water may live upon
the shells of mollusks, crustaceans, and other marine forms and thus

RELATIONS BETWEEN ANIMAL ORGANISMS 523

become epizoic, though the relationship means nothing to either
animal.

564. Commensalism.—Epizoic associations merge into a type of
association in which one organism benefits and the other is not injured.
If, for instance, the accumulation of colonial hydroids upon the surface
of a crustacean forms a covering sufficient to conceal the crustacean,
which thereby secures benefit from the presence of the hydroids, the
relationship may be considered one of commensalism. The word com-
mensalism, however, means, literally, eating at the same table and was
originally applied with the idea that one of the organisms secured food
by utilizing the bits which the other dropped. It refers to other relation-
ships than those concerned with food. The remora, or sucking fish
(Fig. 364), fastens itself to the body of a shark and thus secures trans-
portation. Certain small fish hide among the tentacles or within the
bodies of coelenterates and gain security from their enemies.

fic. 364.—A remora, Remora remora (Linnaeus), cosmopolitan in warm seas. From a
preserved specimen. xX 4,

565. Mutualism.—Such an association, however, as has been indi-
cated above under the name of commensalism merges into a third type
which may be termed mutualism and which involves association between
two animals of different species with benefits to each. Under this head-
ing come many associations which have often been called commensalism.
Such cases are, for instance, the association of a hermit crab and a sea
anemone (Fig. 365) or a sponge, either of the latter two being attached to
the shell which contains the former. In such a case the hermit crab
profits by being protected either by the nematocysts of the sea anemone
or by the inedibility of the sponge, and being a rather slovenly feeder it
allows bits of food to escape which are utilized by the associated animal.
Another similar association, described by Herodotus in the fifth century
B. C., is that between the crocodile of the Nile and a small plover-like
bird which enters the mouth of the reptile to pick leeches and insects of
different kinds from crevices in the skin and morsels of food from the
teeth. Mutualism not only merges into commensalism on the one hand,
but it also is rather arbitrarily distinguished from symbiosis on the other.

524 GENERAL CONSIDERATIONS

566. Symbiosis.—If symbiosis is to be clearly separated from mutu-
alism, the separation must be on the basis of maximum intimacy and
the vital nature of the association. It is essentially an extreme form
of mutualism. One case of symbiosis has been noted in the relationship
which exists between a hydra and a green alga (Sec. 169), the cells of the
alga living a symbiotic life in those of the animal organism and furnishing
oxygen to the hydra in return for its own food. Other similar cases
are known. Symbiosis is also shown by the termite and the protozoan
symbiont which lives in its intestine (See. 319). It was long a matter of
speculation as to how termites are able to digest the cellulose of the wood

Fic. 365.—Hermit crab in a snail shell, which also bears two sea anemones. Con-
sidered by some as an illustration of commensalism, but referred to here as one of mutualism.
From a preserved specimen. X 2%.

on which they feed, since other insects are not known to have this ability.
Cleveland has recently discovered that this is due to the presence in the
intestine of the termite of a protozoan which prepares the wood for
digestion and absorption by the insect. In the absence of the protozoan
the termite is unable to use this food. On the other hand, the protozoan
finds appropriate conditions for existence only in the intestine of the
termite, and thus the association is vital to both.

567. Parasitism.—The associations of organisms of different species
which have so far been defined all involve benefit to one or both but
injury to neither. If injury is done to one, then the association becomes
either one of parasitism or predatism.

Parasitism has already been defined as the association of two organisms
of different species in which one, termed the parasite, lives at the expense

RELATIONS BETWEEN ANIMAL ORGANISMS 525

of the other, called the host (Chap. XXXII). Parasitism might logically
be made also to include the relation of two individuals of the same species
when one lives at the expense of the other. Among worms are examples
of one sex being carried about and nourished by the other, usually the
male by the female. A similar phenomenon is presented in the case of
certain fish. If, however, this association of the sexes is considered
division of labor, then this is not true parasitism but the relationship of
mates. It has been suggested that a young animal living within the body

Fia. 366.—An extreme case of parasitism. A, semidiagrammatic representation of an
individual of Sacculina sp. in the body of a crab and projecting from its ventral surface.
B, nauplius larva of the parasite (compare with Fig. 174.A). The nauplius lives free in
the water and changes to a form known as a cypris; this attaches itself to a seta on the body
of a crab by its antennules and loses its thorax and abdomen with their appendages. The
rest of its body undergoes degeneration and becomes a mass of cells. From the antennules
rootlike filaments penetrate the body of the host and this mass of cells enters the body
cavity of the crab and becomes attached to the ventral side of its intestine. The fila-
ments of the parasite permeate the tissues of the host, and these tissues are in consequence
partly absorbed. Ultimately the parasite develops a sac-like body containing reproductive
organs and a ganglion, and this, pressing upon the skin of the ventral surface of the crab’s
abdomen, finally passes through the skin and shows itself as a tumor-like growth, shown in
A. Sacculina belongs to the Cirripedia, or barnacles, and is therefore a distant relative of
the crab which it parasitizes.

of the parent, especially if it receives nourishment directly from the
parent, as in the case of mammals, should be considered a parasite. The
nourishment of the young, however, seems to be one of the natural func-
tions of the parent, and the relationship for that reason ought not to be
considered one of parasitism. To extend the term parasitism to all these
cases is to limit its significance and impair its usefulness and it seems best
to limit its application to two animals of different species.

Internal parasites, such as intestinal worms and certain protozoans,
are called endoparasites, while those which live on the surface of the
host, such as fleas, mites, and lice, are called ectoparasites. Parasitism
also may be either temporary or permanent. Instances of temporary

526 GENERAL CONSIDERATIONS

parasitism are afforded by lice which resort to the body of the host for
but a brief time, as in the case of some lice which live in poultry houses
and go upon the poultry for only a short time at night to feed; other
examples are mosquitoes, sucking flies, leeches, and ticks, which are on
the host only long enough to fill themselves with blood. Permanent
parasites (Fig. 366) are such as the ascaris, which enters the host in
the form of an embryo within the egg and remains in that host throughout
life.

568. Predatism.—Parasitism may in the end lead to the death of the
host. This is not, however, to the interest of the parasite, which finds
ease and safety in the relationship. Predatory animals feed upon other
animals by killing and devouring them. Unlike parasites they are thus
more powerful and usually larger than their victims. Predatism is the
term applied to such a relationship. Most relationships among organ-
isms may exist between animal and plant organisms but predatism is
not applied to the eating of a plant by an animal.

CHAPTER LXX

DISTRIBUTION OF ANIMALS

ZOOGEOGRAPHY

The study of the geographical distribution of animals upon the surface
of the earth and of the factors which have brought about such distribu-
tion forms the subject matter of a field of zoology which has generally
been known as zoogeography. It is related on the one hand to paleon-
tology, since present-day distribution depends in part upon past dis-
tribution; and on the other hand to ecology, in that the environment
affects, and in many cases determines, the ability of individual animals
to maintain themselves in any given locality.

569. Present Distribution——The area occupied by any species of
animal is usually termed its range. Generally speaking, throughout
that area there must be, to a certain degree at least, similarity of condi-
tions in so far as they are determining conditions in the life of the animal.
Some animals, which are dependent apparently upon a very particular
set of conditions, are confined to a limited range; others, able to adjust
themselves to greater differences in conditions, possess a very extended
range. Indeed there are a few which are cosmopolitan, being distributed
practically throughout the world.

As a rule a particular species as well as related species occupy ranges
which are continuous; but this is not always true, for there are many
cases of discontinuous distribution when a given species or genus is repre-
sented in small areas far apart. This is rarely true of birds, because of
their power of flight, but it is known of many mollusks and insects. The
genus Peripatus offers an example of a type which has a very wide and at
the same time discontinuous distribution (Sec. 307).

570. Past Distribution.—It is clear from the evidence furnished by
the fossil remains of animals that their distribution over the earth has
been very different in the past from what it is at the present time. An
extreme instance of this fact is the occurrence of remains of animals
which are now confined to the tropics as far north as the northern United
States and the northern parts of Europe and Asia. Since the animals
living on the earth today are descendants of those of the past, the facts
of past distribution have a distinct bearing upon that of the present and
may be the clue to discontinuous distribution. In order to explain some
marked cases of such distribution, especially when it involves a number of
different types, the existence of former land masses serving as bridges and

527

528 GENERAL CONSIDERATIONS

paths of migration have been assumed. An example of such a land mass
is the hypothetical land named Lemuria, supposed formerly to have con-
nected India and Madagascar; this would account for the fact that there
are many types common to these two regions. The former existence of a
continent connected to both Australia and South America, known as
Antarctica, has also been suggested as a means by which related types
now found on those continents may at one time have been distributed
over a continuous area.

571. Place of Origin.—It is usually assumed that each species of
animal must have originated in one particular place on the surface of
the earth, from which locality it has been dispersed throughout its range.
It is also generally assumed that closely related species have had a com-
mon origin and have been produced by modification of the type during
its dispersal.

572. Dispersal of Animals.—Assuming a certain species or type of
animal to have originated in a certain place, its dispersal, if it is successful,
is inevitable; increasing numbers and the competition for food will of
themselves cause the individuals to spread over a constantly widening
area. Other factors which tend to cause animals to disperse are the
search for favorable places in which to rear their young and the safety
which isolation gives. Animals may, by changes in the environment
which make it untenable, be forced to move from their original home.
The following factors favor the wide dispersal of animals: (1) length of time
during which dispersal has taken place; (2) uniformity of climatic condi-
tions over a wide area; (3) continuity of habitat; (4) transportation by
water currents, floating objects, and wind; (5) attachment to the bodies
of other animals which move about; (6) human agencies. While a
criterion to be applied with great caution, there are some grounds for
the assumption that widely distributed types have had a long time dur-
ing which to acquire wide dispersal and are, therefore, older types than
those of a more restricted distribution. Most of the factors need no
explanation. It may be related, however, as an example of dispersal
by other animals, that on one occasion a blue-winged teal, shot near
Lincoln, Nebraska, was found to have in a mass of mud on one foot
five living crustaceans, Hyallela dentata (Say), two of which were females
with eggs. The introduction of foreign types of animals into any region
is being made constantly more easy by the development of international
systems of transportation and the freedom of commercial intercourse.
The dispersal of animals is sometimes called migration, though it should
be carefully distinguished from periodic migration (Sec. 575).

573. Factors Hindering Dispersal.—Many conditions act as barriers
to the dispersal of animals. (1) Geographic barriers, such as mountains,
large bodies of water, and deserts, put a check to the dispersal of many
types. Open areas are barriers to woodland forms, while the forest is a

DISTRIBUTION OF ANIMALS 029

barrier to forms adapted to life in the open. (2) Climatic conditions
often form barriers. ‘Though temperature, generally speaking, is not
a serious barrier to many animals it may be an indirect factor in their
distribution by limiting the growth of plants upon which they feed and
by shortening their active season to such a degree that they cannot pass
through their entire life history and thus reproduction cannot take place.
Low humidity may be a barrier limiting the distribution of animals which
cannot withstand the drying to which they are subjected. (3) Lack of
proper food often forms an insuperable barrier for animals which feed
upon certain types of vegetation and the area of distribution of which is
strictly limited by the distribution of such plants. (4) Lack of locomotor
ability is also a barrier to rapid dispersal, although over a long period of
time it may not prevent the spread of animals into all areas suitable for
their existence.

574. Modification of Types.—As animals have spread from their
point of origin and have met varying environmental conditions they have
become modified and adjusted to the conditions, and thus what was origi-
nally one form has been developed into a number of forms. These modi-
fications may indeed be such as to permit their possessors to surmount
barriers which otherwise would have prevented dispersal.

575. Periodic Migration.—Periodic migration may be defined as the
repeated movement of animals from one place to another, at more or
less regular intervals, participated in by all of a species or by all of those
occupying a certain area. This excludes the movement of large numbers
when it is not repeated, the gradual dispersal of species, and the chance
wandering of individuals. In this restricted sense it is found throughout
the animal kingdom, being confined to those organisms capable of loco-
motion and being exhibited by them in proportion to their power of
movement. Migration is often associated with the search for food or for
proper conditions for rearing young. In many animals it is the expres-
sion of an instinct, involving factors which initiate the movement and
others which direct it. Among the initiatory factors are hunger, tem-
perature, and light conditions; the functional activity of the reproductive
organs; and a desire for home surroundings. Among the directive fac-
tors are relative temperatures, wind direction, water currents, and a sense
of location. Reference has been made to the limited vertical migration
of certain crustaceans (Sec. 303), which is shared by a large number of
aquatic forms belonging to several phyla. The migration of birds has
also been discussed (Sec. 434).

Among the invertebrates the most extensive and best known migra-
tions are those of locusts, which have taken place in past times in Asia
Minor, southern Africa, Argentina, and the western United States.
These are due to lack of food in high barren regions and are directed
toward lower and more fertile areas. They occur at irregular intervals.

530 GENERAL CONSIDERATIONS

In the northern part of this country the monarch butterfly migrates
southward each fall and northward in the spring. Fish find in continuous
bodies of water little to impede their movement and also make extensive
migrations. Some fish live in the sea and migrate up rivers to lay their
eggs; examples are the salmons, in the case of which the temperature of
the water is probably a directing factor. Other fish, such as the eels,
which live in rivers, migrate to the sea to spawn. European eels migrate
to a region in the Atlantic Ocean southeast of the Bermudas. The
adults of both the salmons and the eels die after spawning, so an individual
makes but one round trip in its lifetime, but this may cover several

N Snow & Ice

SnowLine

Mosses & Lichens
LowHerbaceous Vegetation

m— Jree Line

Coniferous Forests

\
AAAS
=e

oe Deciduous Forests

ANS
Wd)
\Y
N \

\
\\

4 tf Tropical Forests

anti pe Esezealy,

SAN ht ch ee aa
of 2°
a ot u? ey fot
iC qu? ou @
ror oe! : fe Met
w

Fig. 367.—Diagram to show the correspondence between the vertical life zones met
in ascending a tropical mountain with a permanent ice cap and the horizontal zones encoun-
tered in traveling from the base of the mountain to the pole.

thousands of miles. Other fish living in rivers migrate to the headwaters
to spawn. Birds, with the ease of locomotion they possess, have the
longest migration routes known. Among mammals may be noted the
irregular migrations of the Scandinavian lemmings from the mountains to
the lowlands near the coast, migrations which have been recorded since
the beginning of historic times. Another example is the American bison,
which formerly migrated regularly in the spring from the winter pasturage
in western Texas and New Mexico to the summer pasturage and breeding
grounds in the Dakotas and Montana, returning southward in the fall.

576. Altitude.—Altitude has a marked effect on animal distribution
through the varying climatic conditions produced. Even in the tropics
high mountains may exhibit vertically a series of climates corresponding
to those found in passing from the tropics to the poles (Fig. 367). At the
base of such mountains is a tropical climate with only the seasonal

DISTRIBUTION OF ANIMALS 531

variations involved in the existence of wet and dry seasons. Higher
up is a zone which possesses a warm temperate climate, with summer and
winter, but involving at no season a cessation of life activity. At a
still higher level there is a cool, temperate climate with such a lowering
of temperature in the winter as necessitates dormancy on the part of a
number of living forms. The upper limit of this zone is the limit of tree
growth. Above the tree line is a frigid zone, extending to the limit of
all vegetation, and still above this an arctic zone. Thus vertical life
zones occur on the mountains corresponding to the horizontal life zones
at sea level; and just as seasonal changes occur in temperate regions far-
ther north, so will they occur in temperate zones on the mountain in the
tropics. Vertical migrations will also occur. In many cases particular
species found on the upper parts of mountains are also found farther
north where a similar climate exists. Such a fact is usually explained by
assuming that in the past has occurred such a climatic change, affecting
a widely distributed species, that the species has become extinct except
on higher mountains southward and in areas farther north with a cor-
responding climate. Thus in both cases it has persisted in a region where
the climate is suitable.

577. Oceanic Distribution.—The conditions at any particular loca-
tion in the ocean are much more uniform than are those in fresh water or
on land. This stability shows itself especially in temperature, salinity,
and in the gaseous content of the water. At points widely separated in
the ocean, however, there are marked differences in temperature and,
to a certain extent, in the other factors mentioned, which affect the dis-
tribution of life. Another factor concerned in marine distribution is
the existence of ocean currents, by means of which many species are
dispersed. Vertical distribution in the ocean is affected by a variety of
conditions, including pressure, which increases by 14 pounds to the square
inch for every increase of 10 meters in depth. At a depth of 3660 meters,
or about 12,000 feet, this is over 2!4 tons to each square inch of area.
This great pressure does not affect animals living at great depths, since
the pressure within their bodies is equal to that which is without, but it
limits vertical movements. Light decreases by absorption and ceases to
have any effect upon life at a depth of about 900 meters (3,000 feet).
It is stated that all of the heat due to the rays of the sun is lost below
about 275 meters (900 feet). In the Atlantic Ocean it has been found
that with a surface temperature of 20°C. (68°F.) the temperature at 500
fathoms is about 38°C. (39°F.) and that at 1000 fathoms it is little less,
decreasing very slowly to the bottom, where it is about freezing. The
highest sea temperature known is in the Persian Gulf, where there is a
surface temperature of 35°C. (95°F.), while in the polar seas surface
temperatures of approximately —3.3°C. (26°F.) have been recorded.
In the depths of the sea animals live under a condition of high pressure,

532 GENERAL CONSIDERATIONS

a temperature little above freezing, no light, and no movement of the
water sufficient to produce a current.

578. Island Faunas.—The fauna of an island depends upon whether
or not the island is what is known as a continental one, adjacent to a
continent, with which in the past it may have been in communication,

Hawaiian ;
Islands

ae os
me is .

Fic. 368.—The zoogeographical regions of the western hemisphere.

or an oceanic island, lying at a distance from any continent and with no
such past connection. Frequently the faunas of continental islands
are similar to those of the adjacent mainland, even the same amphibians
—which never occur in the ocean— being found, as well as mammals to
which even a narrow channel would be a barrier. Often characteristic
types are absent from such islands, as are snakes, for instance, from Ire-

DISTRIBUTION OF ANIMALS 533

land. Oceanic islands are characterized by a complete absence of mam-
mals and amphibians. On many such islands there is a decided tendency
toward the existence of wingless birds and insects. This has been
explained by the fact that flying forms would be likely to be swept away
by winds and thus destroyed.

0 120
Fig. 369.—The zoogeographical regions of the eastern hemisphere.

New Zealand is an island which is peculiar in possessing some of the
faunal characteristics of a continent. There are only two native mam-
mals, a bat and a rat, both of small size. The birds of New Zealand have
been in the past and are now very characteristic. Among them are the
gigantic moas, now extinct, and the curious A pteryz, or kiwi, now becom-
ing very scarce. Among the reptiles is the peculiar Sphenodon (Fig.
289). The amphibians are represented by a single species of frog.

534 GENERAL CONSIDERATIONS

579. Faunal Divisions of the Earth.—On the basis of the distribution
of animal types the world has been divided into a number of different
regions, those originally proposed by Sclater being most widely accepted
(Figs. 368 and 369). These involve only land distribution and are based
to a greater extent upon the distribution of the higher vertebrates than
upon that of any other group. According to this plan the earth is divided
into six regions. The first is the Australian, which includes Australia,
New Zealand, part of the East Indies, and the South Pacific islands.
The second, or Neotropical, includes South and Central America and
part of Mexico, with the West Indies. The third, or Ethiopian, includes
Africa south of the northern boundary of the Sahara, Arabia, and
Madagascar. The island of Madagascar is in many respects quite dif-
ferent from the rest of this region and is sometimes called the Malagasy
subregion. The fourth, or Oriental, includes Asia south of the Himalaya
Mountains and west to the Persian Gulf, southern China, and a large
part of the Malay Archipelago, including the Philippines, Borneo, and
Java. The fifth, or Palearctic, includes Europe, that part of Asia
north of the Himalaya Mountains, and Africa to the Sahara. The
sixth, or Nearctic, includes North America south into Mexico.

Of the different regions the Australian is the most distinct. Here are
found all the monotremes and most of the marsupials among the mam-
mals and such characteristic birds as the birds of paradise, the honey-
suckers, lyre birds, brush turkeys, cassowaries, emus, and, in New
Zealand, the kiwi. Here are also the Sphenodon, peculiar tortoises, and
the Australian lungfish.

In the Neotropical region are found the opossums, which are mar-
supials; many peculiar edentates, including sloths, armadillos, and ant-
eaters; the American monkeys, marmosets, and vampire bats. There
are also many peculiar birds, among the most remarkable of which are
the toucans, the hoatzin, curassows, guans, and the rhea, or American
ostrich. There are certain peculiar snakes, including the anacondas,
and also electric eels.

In the Ethiopian region are the gorilla, the chimpanzees, the broad-
nosed monkeys, the lion, the African elephant, rhinoceroses, hippopota-
muses, the zebras, and many antelopes but no bears or any deer. Among
the birds are the African ostrich, guinea fowls, and the secretary bird.
Characteristic types of lungfishes are found here. There are no cray-
fishes. In Madagascar are found some lemurs and the flying foxes; the
island lacks the rodents characteristic of Africa and once possessed a
gigantic extinct bird, the aepyornis, which is not known to have occurred
elsewhere.

In the Oriental region are found the orang-utan, gibbons, the ma-
caques, the tiger, peculiar lemurs, some antelopes, the Indian elephant,
the Malayan tapir, and rhinoceroses differing from those occurring in

DISTRIBUTION OF ANIMALS 535

Africa, as well as many characteristic birds, such as cuckoos, pheasants,
babbling thrushes, and broadbills.

The Palearctic and Nearctic regions have not such rich faunas as the
regions already mentioned and not so many characteristic forms. Com-
mon to these regions are deer, bisons, bears, wolves, beavers, and marmots.
There are remains of the mammoth in Siberia and the hairy rhinoceros in
Europe. Distinguishing the Palearctic region are certain wild sheep,
the ibex, the chamois, wild horses and asses and camels. The Nearctic
region, on the other hand, has a relative scarcity of hollow-horned
ruminants, which are represented by the bighorn, the American bison,
and the mountain goat and also possesses badgers, prairie dogs, ‘and
certain pouched rats.

There are some interesting analogies between regions in the exist-
ence of corresponding but unrelated forms. Such forms are the humming
birds, the greatest number of which are in South America with some
species extending into North America but which are entirely absent
from the Old World, where their place is taken by the sunbirds. Other
forms that illustrate such an analogy are the large-billed toucans of South
America, to which the hornbills in Africa and southern Asia correspond.

580. North American Life Zones.—This continent has been divided
by Merriam into regions and life zones based upon temperature, forming
bands crossing the country from east to west, and carried southward along
the mountains and northward along the central valleys by the effect
of altitude. In the western part of the United States, where great
differences of elevation, soil, and climate exist, there is great irregularity
in distribution. Three regions were recognized by Merriam—the boreal
region, which included most of Canada; the austral region, which included
roughly the United States and northern and central Mexico; and the
tropical region, which covered the southern tip of Florida, the West
Indies, and the coasts of Mexico north to about 25 degrees latitude.
This mapping of the continent has been severely criticized recently
because of too great dependence upon temperature and the failure to
recognize vegetation regions.

The vegetation regions of North America (Fig. 370) may be roughly
outlined as follows: South of the frozen arctic region is a treeless area
referred to in a general way as tundra. In the northern part of this
area the ground thaws to a depth of but a few inches in summer and the
surface supports a growth only of lichens and mosses. As one proceeds
southward the depth to the frost line increases and grasses, low her-
baceous vegetation, and even low shrubs appear. Finally the limit of
trees is reached and one enters the northern coniferous forest, which is
represented on the Pacific coast by western and northwest coniferous
forests, extending down into California and along the Rocky Mountains
into Arizona and New Mexico. Central United States in the east is

536 GENERAL CONSIDERATIONS

——
Ss

LC JAretic ; Tundra

EN Gussland

EE" Desert

£3 Deciduous Forest

(Ml) Western Coniferous Torest
MW Worth West Coniferous Forest
Southern Coniferous Forest
FEES Northern Coniferous Forest

Fic. 370.—A diagrammatic map of North America showing the vegetation regions
north of the tropical region. (Areas in the United States based upon Shantz and Zon,
‘Atlas of American Agriculture,’ Part I, Sec. E; those in British America upon Weaver and
Clements, ‘‘Plant Ecology”’; and those in Mexico upon both.) One line separates the prairie
(Pr) from the plains (P/), and a second the sagebrush area (S) from the southern desert
region (D). The existence of elevated areas presenting conditions corresponding to the
northern coniferous forest, the tundra, or the arctic zone in the Alleghenies and the western
mountains is indicated by areas of black.

DISTRIBUTION OF ANIMALS 537

occupied by a deciduous forest, and southern and eastern United States
by a coastal-plain forest, which includes both coniferous and deciduous
trees. West of the forest region is the region of the prairies and plains,
extending from Alberta to central Texas and the Great Basin; and the
southern desert region, which occupies the interior basin between the
Pacific coast states and the Rocky Mountains and extends southward
far into Mexico. Finally, there is a tropical and subtropical forest region,
which is the same as the tropical region of Merriam.

At present there is a strong tendency to bring the faunal regions into
agreement with the floral regions just outlined. Each of these regions
has characteristic animal types, some of which may be mentioned.
The prairies and plains comprise the ranges of the pronghorn, the bison,
several ground squirrels, prairie hares or jack rabbits, the prairie chicken,
and the burrowing owl. The deciduous forest harbors the Virginia
deer, the opossum, the gray fox, the fox squirrel, the cardinal, the Carolina
wren, and the yellow-breasted chat. The northern coniferous forest
has the moose, the snowshoe rabbit, the pine marten, the northern
jumping mouse, the three-toed woodpecker, and the spruce grouse. The
region of tundra and snow is the range of the musk ox and caribou and
in summer is the home of a host of water and shore birds.

CHAPTER LXXI

PAST DISTRIBUTION OF ANIMALS

PALEOZOOLOGY

As suggested in the previous chapter (Sec. 570) the present distribu-
tion of animals on the earth is determined in part by their past distribu-
tion. Those of the present are the descendants of those of the past.
Though a few living types are of relatively recent origin many have
existed for many millions of years. The study of the organisms which
have lived during the past ages is the field of paleontology, which may be
divided into paleobotany and paleozoology, depending upon whether
the organisms the remains of which are studied were plant or animal.

581. Fossils.—Fossils, which means literally objects dug up, are the
remains of plants or animals, or records of their presence, preserved in
the rocks or in soils. Aside from the fossils commonly found in rocks,
they include mammoths, related to present-day elephants, frozen in the
soils of northern Siberia; other mammals buried in peat bogs in different
parts of the earth; animals found submerged in a lake of asphalt in south-
ern California; and insects, scorpions, and spiders, preserved in amber,
a fossil resin from the shores of the Baltic Sea.

582. Stages in Fossilization.—A dead animal is soon eaten by other
animals or is destroyed by the processes of decay, even skeletal parts
being subject to destruction. If buried in the soil, however, particularly
if air cannot reach them, such remains are preserved for a much longer
time. Bog waters are antiseptic and in them decay takes place very
slowly. Animals are preserved for a long time in volcanic ashes or
deposits of fine wind-blown soils. If dead organisms are quickly buried
in the mud at the bottom. of bodies of water and especially at the bottom
of the sea, disintegration is exceedingly slow. Skeletal structures may be
preserved for a very long time, and even soft parts may remain long
enough to allow mineral matter to replace the organic matter and thus in
its arrangement reflect the structure of the organism. Later, as this
mud becomes consolidated and forms rock, owing to the pressure of
overlying strata and the cementing of the particles together through
precipitation of mineral substances from solution, all the organic matter
becomes replaced by mineral matter and the fossil is said to have become
petrified. In this condition it will last until the rock containing it is
broken up by changes in the earth’s crust, reduced again to dust by
erosion, or metamorphosed (Sec. 585) by the action of heat. Mud

538

PAST DISTRIBUTION OF ANIMALS 539

bearing animal tracks or the imprint of soft-bodied organisms may be
buried by other deposits and the evidences be preserved as fossils.

These facts make it evident why fossils of marine animals are most
abundant and why animals which are most likely to fall into rivers and

Eras

Periods

Length, millions
of years

Cenozoic

Quaternary:
Recent (20,000—
50,000 years)
Pleistocene, or Glacial

Dominant life

Age of man

Tertiary:
Pliocene
Miocene
Oligocene
Eocene

Age of mammals and
modern flowering
plants

Mesozoic.....

Cretaceous
Jurassic
Triassic

Paleozoic

Proterozoic...

Permian

Carboniferous:
Pennsylvanian
Mississippian

Age of reptiles and

medieval floras

Age of amphibians and
ancient floras

Devonian
Silurian

Age of fishes

Ordovician
Cambrian

Age of higher inverte-
brates

Keweenawan
Huronian

Age of primitive marine
invertebrates

Archeozoic....

300
(Total 1,000 +)

Age of unicellular life

Pregeologic

Fig. 371.—Geological time scale.

Geology.’’)

(Based upon data given in Schuchert’s ‘‘ Historical

be carried down to the sea and buried in mud are more likely to be
represented in fossils than other terrestrial forms. Since decay in very
dry climates is slow and animal remains may dry up before decaying,
such remains may easily be covered by wind-blown deposits and thus be
preserved, especially if the area is later covered by oceanic water, and the
deposits converted into rocks.

540 GENERAL CONSIDERATIONS

583. Geological Ages.—Since the same conditions have been many
times repeated in the history of the earth and the same minerals have
been present at all times, it is impossible to determine the age of a stratum
by its structure or composition. However, the study of fossils in strata
which remain in the same order as that in which they were deposited
shows that the forms of life have changed from time to time. Conse-
quently, by critical study of the fossils and comparison of strata in dif-
ferent parts of the earth, geologists are enabled to recognize those strata
belonging to the different ages and to relate them in a sequence which
corresponds to the order of their deposition. From the thickness of the
strata, in connection with their character, an estimate can be made as to
the duration of past ages. This estimate may be confirmed or in some
cases modified by the application of knowledge of chemical changes which
involve a time factor capable of precise calculation. The last data have
in general considerably increased the length of time formerly allotted to
the different ages by geologists. Estimates of the duration of these
ages still remain, however, very uncertain.

584. Geological Time Scale.—On the basis of the estimates referred
to in the preceding section, geologists have prepared a time scale, which,
as represented in North America, is shown in abbreviated form in Fig. 371.

585. Metamorphism.—The rocks of the Archean period are uni-
versally distributed over the earth, though to a great extent they are
covered by more recent formations. (This is not true of the distribution
of those of any other period.) They show evidences of sedimentary origin
but are everywhere modified by metamorphism. Metamorphism occurs
when stratified sedimentary rocks become buried deeply by overlying
strata and are subjected to the internal heat of the earth. The rocks of
the Huronian and Keweenawan periods are more decidedly sedimentary
in character, but they still show much metamorphism. Those of the
Cambrian and Ordovician are less generally metamorphosed; and the
later periods show no such effect. The result of this metamorphism was
to destroy all fossils and with them most of the evidences of the existence
of life. In the Archean, beds of carbon in the form of graphite, of iron
ore derived from carbonates, and of limestone of sedimentary origin are
all indications of the presence of living organisms in that period. The
few ill-preserved fossils found are all considered to be remains of algae.
In the Proterozoic are found fossil caleareous algae, bacteria, radiolarians,
sponges, fragments of crustaceans, and the tracks of marine annelids, but
these fossils are exceedingly rare and scattered. Owing to the general
metamorphism of rocks older than the Cambrian, therefore, geological
records of earlier life have been almost entirely obliterated. Conse-
quently, it is not surprising that in the oldest strata which contain fossils
in any considerable numbers, most of the invertebrate phyla are repre-
sented. The result is, however, that there are no evidences from pale-

PAST DISTRIBUTION OF ANIMALS 541

ozoology of the steps in the evolution of animal life on the earth until the
process was far advanced.

586. Animals of the Past.—Of the Protozoa, only fossilized foramini-
ferans and radiolarians are found, the latter in rocks of the Proterozoic
era and onward, the former in the Cambrian period and onward. Sponges
are also known from the Proterozoic. Hydrozoans have been abundant
since the Cambrian, when they were represented by a type known
as graptolites. Scyphozoans, represented by impressions and molds,
have existed since the Cambrian, as have also corals, the skeletons
of which are abundant and of great variety in the rocks of all periods since
that time. Brachiopods appeared early in the Cambrian, and bryozoans
are abundant from the Ordovician onward. There are over 6000 fossil
species of brachiopods, but only 160 are
known to be living now.

Starfishes and holothurians date from
the Cambrian, and brittle stars and sea
urchins from the Ordovician. Crinoid
remains represented by stalk sections
(Sec. 240) have been found in the Cam-
brian, and fragments of crinoids occur in
beds of crinoidal limestone from the
Ordovician to the Jurassic. Blastoids £&
and cystoids existed throughout the Zz
Paleozoic era; they resembled crinoids &
in many ways but are more primitive.

Of the mollusks, chitons and scaph-
opods have existed since the Ordovician;
and pelecypods, gastropods, and cepha-

: : h : Fic. 372.—An ammonoid from the
lopods since the Cambrian. Their fossil lower Jurassic period, resembling in

: ig. 132),
shells are found in abundance and are a cea pee Me

widely distributed. The nautiloids, rep- removed to bring into view the com-
resented today only by the chambered pee secs septa that disunguahed
nautilus, were abundant in the Silurian;

2500 species have been described. Twice as many species of ammonoids
(Fig. 372) are known; these were most abundant in the Mesozoic and are
now extinct. The earliest forms resembling squids and cuttlefishes
appeared in the Triassic.

Chaetopods are the only fossil annelids. They are found from the
Cambrian onward, though worm tracks have been found in rocks of the
Proterozoic era.

The earliest arthropods were branchiate and marine. ‘Trilobites
(Fig. 373) had their origin before the Cambrian, being abundant in the
oldest fossil-bearing strata. They were a dominant type in the Cambrian
period and disappeared before the end of the Paleozoic. Other crus-

542 GENERAL CONSIDERATIONS

taceans appeared during the Cambrian and Ordovician. Myriapods
appeared in the Devonian, scorpions in the Silurian, and spiders in the
Carboniferous. Eurypterids, which were scorpion-like but marine,

a
Zt TS SS
HH, Vil Wp
7
55 a
po ee
(S|
] a TE Raa, |
ly [—

Fig. 373.—A trilobite, restored, showing limbs and antennae. Dorsal surface. (From
LeConte, ‘‘ Elements of Geology.’’)

lived throughout the Paleozoic. They have considerable resemblance to
the king crabs, which appeared in the Triassic. Insects appeared first
in the Carboniferous, and the largest insects that have ever lived were in

edacaddsn, P ie ee a
o~

Fic. 374.— An ostracoderm, restored. (Based upon Schuchert, ‘‘ Historical Geology,’’ after
Koken.)

the Pennsylvanian, a dragon fly from the coal deposits of Belgium measur-
ing 29 inches across the wings.

Ostracoderms (Fig. 374), the first vertebrates, appeared in the Ordo-
vician, and only they and primitive sharks are known from the Silurian,
but ganoids and lungfishes existed in great variety in the Devonian,

PAST DISTRIBUTION OF ANIMALS 543

which is known as the age of fishes. The more modern fishes, the
teleosts, were not known until the Jurassic and are represented now by the
greatest number and diversity of species they have ever possessed. A
footprint from the Devonian is the earliest trace of an amphibian. There
were giant armored amphibians, the Stegocephala, in the Pennsylvanian
and Permian periods. These reached a maximum length of 15 to 20
feet in the Triassic, when they became extinct. The reptiles appeared
about the time of the amphibians but did not become dominant until
the Mesozoic, which was the age of reptiles; they reached their highest
development during the Jurassic. The aquatic and marine types
attained a length of 40 feet; a flying pterodactyl had a wing spread of 25
feet; and the gigantic dinosaurs reached 100 feet in length. These
giants suddenly became extinct in the Cretaceous.

Fic. 375.—Archidiskodon maibeni Barbour, one of the largest skeletons of a mammoth
on record—14 feet at the shoulder—in comparison with the skeleton of a recent Indian
elephant and with the two men standing under it. (Photographed from a specimen in the
University of Nebraska State Museum.)

Fossil remains of the earliest bird, Archaeopteryx (Fig. 297), have been
found in the Jurassic slates of Bavaria, and a number of other birds
are known from the Cretaceous. The earliest mammalian remains are
from the upper Triassic, and many types are known from the Creta-
ceous. Mammals have been dominant since the beginning of the
Cenozoic era. Human origins have already been discussed. Steps in
the development of other mammalian types will be referred to in the next
chapter, on evolution.

CHAPTER LXXII

EVOLUTION OF ANIMALS

Two conceptions of the universe in which we live have been held.
One was that it is a static universe created some 6000 years ago, with the
character it now has, and that it has remained in that condition ever
since. This, however, is contrary to facts easily secured by careful
observation and is untenable in the light of modern scientific knowledge.
Changes are now seen to be taking place everywhere and there is ample
evidence that this has always been the case. It is also clear that the
history of the earth has involved unnumbered ages. Animals and plants
no lenger appear to us as organisms created in the beginning with exactly
the same character they have today but as having the characters they now
possess in consequence of gradual changes which have come about
through the ages that have passed. So the second conception—that
the universe is ever changing and progressing—now prevails. This
progressive change is called evolution. Certain theories of the origin of
life were stated early in the text, but organic evolution, which is the
evolutionary conception applied to living things, concerns itself only with
the changes which have ensued in living things since life first appeared.

587. History of Evolution.—For the beginnings of the concept of
evolution it is necessary to go back to the time of the early Greek philoso-
phers. In striving to explain the nature of the universe the idea of evolu-
tion suggested itself to them, though they had no data by which to test it.
Anaximander (611-547 B.c.) presented the idea of an actual change in
living organisms, including a change from aquatic to terrestrial life.
He even included man in his theory. Empedocles (495-435 B.c.) has
been called the father of evolution. He believed both in spontaneous
generation and in the gradual development of different types of organisms.
In a crude way he also expressed ideas of competition between organisms
and of natural selection, or the survival of the most fit. Aristotle (884—
322 B.c.) did not accept the idea of the survival of the fittest but he did
believe in the development of organisms from a primordial living slime,
and he suggested a sequence of animal types forecasting a phylogenetic
series such as is accepted today. He also believed in heredity and recog-
nized evidences of relationship in rudimentary organs.

Throughout the Dark Ages no progress was made, but even during
this period there were theologians, including Augustine (353-430 a.p.)

544

EVOLUTION OF ANIMALS 545

and Thomas Aquinas (1225-1274), who upheld the evolutionary concep-
tion and expressed beliefs in the symbolic nature of the biblical story of
creation.

The first of the modern zoologists to entertain clearly the evolutionary
conception was Buffon (1701-1788) but he hesitated to urge his ideas.
He was the first to believe in the direct modification of organisms by their
environment. Erasmus Darwin (1731-1802), the grandfather of Charles
Darwin, recognized the fact of a struggle for existence and also accepted
the theory of the inheritance of acquired characters, believing that
forces within the organism responding to environmental changes formed
the basis for modifications in the organism. Lamarck (1744-1829)
made valuable contributions to the field of biology, including the pro-
posal of the term biology itself and the use of a tree of life to express
phylogenetic relationships among organisms. His most noteworthy
contribution was a definite theory of evolution based upon the use and
disuse of organs. He believed that necessity in the organism might give
rise to new organs and suggested that the use of any organ strengthens,
develops, and enlarges it, while a lack of use causes a progressive degen-
eration and ultimate disappearance. He also believed that these changes
were passed on by heredity—that is, he believed in the inheritance of
acquired characters.

The most famous name in this field is that of Charles Darwin (Fig.
376), who lived from 1809 to 1882. His preeminence is indicated by the
fact that popularly Darwinism has come to be looked upon as synonymous
with evolution, though at the present time many of the ideas which Dar-
win advocated are no longer accepted. Darwin’s great work was the
“Origin of Species,” published in 1859, in which he presented his theory
of natural selection. This same theory was arrived at independently by
Wallace (1822-1913) who, however, had not such a wealth of observa-
tional data to support it as had Darwin. For this reason, though he
joined with Darwin in first presenting the theory, he stood aside and
permitted Darwin to publish it alone. The publication of Darwin’s
work excited violent controversy, but since neither he nor Wallace was
fitted by disposition effectively to defend the theory in public, that task
fell to Huxley (1825-1895) who successfully championed the cause of
evolution.

Since the time of Darwin a flood of contributions has appeared which
involve a great many avenues of approach to the subject of evolution.
Weismann (1834-1914) supported Darwin’s conception of evolution by
emphasizing the distinction between germ plasm and somatoplasm and
the part played in inheritance by the germ cells. The discovery of chro-
mosomes and the development of the field of genetics have provided a
physical basis for evolutionary changes. Many of these modern theories
will be referred to in discussing the causes and method of evolution.

546 GENERAL CONSIDERATIONS

588. Evidences of Evolution.—Among the evidences which support
the evolutionary conception is the fact that variation is seen everywhere
among organisms and that many of these variations are clearly trans-
missible to succeeding generations. Other evidences are that it is pos-
sible under cultivation to modify animals in certain definite directions
which are advantageous, and that under experimental conditions in the
laboratory animals have been caused to undergo changes which have been
inherited.

The facts that all protoplasm is practically the same in character, that
metabolism is carried on by all living organisms, that such phenomena as

Fic. 376.—Charles Darwin, 1809-1882. (From Shull, ‘‘ Principles of Animal Biology.”
Photo by Leonard Darwin in University Magazine. By the courtesy of McGraw-Hill Book
Company, Inc.)

mitosis, gametogenesis, and embryogeny are phenomena which, in
general, always occur in a similar fashion are all further evidences of
relationship due to common origin.

Comparative anatomy furnishes numerous evidences of evolution,
many of which can be summed up under the general head of homology.
The existence of homologous structures, which is very prevalent among
animals, indicates relationship and a common ancestry. Some particular
examples are the homologies which exist in the series of vertebrate skulls
from fish to man, the uniformity of plan in the vertebrate limb, and the
common plan of structure which shows itself in the brains of vertebrates.
Another evidence derived from comparative anatomy is the existence in
certain animals of vestigial parts and organs (Fig. 377), together with the
fact that in other animals which from their general structure are appar-

EVOLUTION OF ANIMALS 547

ently either ancestral to those which possess the vestiges or are related
to such ancestors these parts and organs are fully developed. Among
the vestigial structures in man are the existence of supernumerary mam-
mary glands; the persistence of hair on the body; the presence in rare
cases of vestiges of a tail; the existence of a third eyelid, or nictitating
membrane; the presence of vestigial muscles, particularly in connection
with the ears; and the possession of a vermiform appendix. ‘The last, in

“Auricularts
superior muscle

Helicis
\ major muscle Ee
Z

\ Yip

Auricu/ar/s 3 \ A Z
Feri | ZO : y
Fe uecie | ns —_. Url cular’s
i anterior muscle B
Ss! Helicis minor
GA muscle
Tragicus
Carti/ age inuscle
Antitragicus
tnuscle
A
Pharyngeal Fore limb

S/its x

4-5 Sacro- , Curvarores
coccygea, coccygIs
mia : Tail / garhents D “7uscle

Fig. 377.— Vestigial structuresin man. A, the muscles of the ear, displayed by removal
of the superficial tissues. B, appendix, seen from behind. C, embryo, showing the tail.
D, abnormal persistence of tail muscles in adult, seen from behind. (Figs. B, C, and D
from Romanes, ‘‘Darwin and After Darwin,’ part I, by the courtesy of The Open Court
Publishing Company; Fig. A compiled from works on human anatomy.)

many mammals, is a functional part of the intestine, adding to its capac-
ity; especially is this true of herbivorous forms.

Many facts from comparative embryology also support the idea of
evolution. Prominent among them are the examples of the biogenetic
law to which references have been made previously. The embryos of
different classes of vertebrates resemble each other very closely in early
stages, and differences begin to appear at points which may be assumed
to be where ancestral lines have diverged (Fig. 378). Among the most
striking of such differences are those which mark the separation of ter-
restrial from aquatic vertebrates. Other evidences are the stages in the
evolution of the circulatory system, including the changes in the branchial

548 GENERAL CONSIDERATIONS

arches (Fig. 258), the gradual development of the vertebral column to
replace the primitive notochord, and the development of the different
types of excretory system (Fig. 229), all of which have been previously
noted.

Paleozoology offers very many evidences that animal life has gradually
changed. Apparently no rock strata exist which show traces of the

B Cc D E F G H

Fie. 378.—Parallel stages in the development of several vertebrates. A, fish; B,
salamander; C, turtle; D, chick; E, pig; F, calf; G, rabbit; H, man. In each series, a is an
early stage, showing the pharyngeal slits. 6, a later stage, in which the first two have
developed gills and the last six show the pharyngeal slits disappearing and limbs and tails
developing. c,astill later stage, in which the differences between the reptile and bird on the
one hand and the mammals on the other have become pronounced, marked resemblances
between those of each group persisting. (From Guyer, ‘‘ Animal Biology,” after Haeckel,
through Romanes.)

earliest life. This is due to metamorphism (Sec. 585). The older rocks
give only indirect evidence of the existence of life on this earth. The
earliest of the sedimentary rocks to contain an abundance of fossil types
belong to the Cambrian period. Since that time strata deposited, one
after another in order of time, show the appearance of many types which
have gradually increased in numbers and variety, have reached a climax,
then have declined, and finally have become extinct. Others have been
able to maintain themselves to the present. But there has been a steady

EVOLUTION OF ANIMALS 549

advance in the character of the highest forms from age to age. In the
oldest strata the highest types are invertebrates; from the Ordovician
through the Silurian, the fishes; from the Devonian to the Triassic, the
amphibians and reptiles; and from the Triassic and Jurassic to the present
time, the mammals and birds, the more modern forms being the highest.

Many facts of geographical distribution are explainable only on the
assumption of the truth of the evolutionary conception. The mar-
supials are evidently very primitive mammals, represented both in
Australia and in South America. Though fundamentally alike, the
marsupials of South America and Australia are nevertheless quite dis-
tinct. These facts are best explained by assuming a common origin of
these animals and later modification after separation.

589. Causes of Evolution.—The causes of evolution are not well-
understood at the present time. Neither the inheritance of acquired
characters nor the effect of use and disuse as direct causes of evolution
is accepted today. On the contrary, the causes are sought in certain
changes taking place in the chromosomes. Hereditary units in the
chromosomes, according to the present theories, must register every
transmissible modification. How they may be modified and thus cause
changes to occur is unknown; it is, however, possible that there is an
innate tendency for the genes to change, which results in evolution,
and it is also possible that hormones or other substances in the blood may
affect them. Recent work on the effect of X-rays upon the gametes
have shown that by their application structural modifications may be
brought about in the fruit fly, Drosophila, and that these changes are
heritable. Whether or not such rays are a factor in natural evolution
is yet to be determined.

The appearance of a characteristic is often referred to as if it were a
response to a need or as if it had appeared for the purpose of adapting
the animal to a condition. When used by modern writers, however, such
expressions should be recognized as figures of speech. Of course no struc-
ture appears because it is needed, neither is necessity a cause of evolu-
tion; but if a structure does appear and is advantageous to its possessor,
it contributes both to its own persistence and to the perpetuation of the
race.

590. Methods of Evolution.—Various theories have been put forth
as to the method by which evolution is brought about. The first modern
theory to be presented is that of natural selection, which was the one put
forward by Darwin and Wallace. This may be summarized as follows:
(1) All organisms produce a greater number of young than can survive,
(2) This results in competition for the necessities of life and a struggle
for existence. (3) All organisms tend to vary; some variations are
advantageous, others harmful, and still others of no moment. (4) Asa
result of the struggle for existence favorable variations would tend to be

550 GENERAL CONSIDERATIONS

preserved and harmful ones eliminated. (5) Such changes are passed
on from generation to generation and result in a gradual change in the
character of the species. (6) If upon being dispersed into other regions
individuals can escape the effect of the struggle for existence or exist
under altered conditions, they may develop characteristics different
from those they have previously possessed and this may result in the
production of a new species. Such species usually show definitely their
relationship to the parent species.
Natural selection seems to be with-
out doubt one means by which
changesmaybepassedon. Though
it is not the only method it is the
one which is supported by the most
extensive evidence from observa-
tion and may be supplemental to
any of the others.

A second method of evolution
is mutation. Mutation may be
defined as a sudden hereditary
change in the appearance of an
animal type; the term was originally
applied especially to those changes
which were striking, but any minute
hereditary change is a mutation.
Slight mutations are not clearly
distinguishable from the continuous

Fic. 379.—An albino crow, Corvus b. Variations which are assumed in the
brachyrhynchos Brehm, an example ofa muta- theory of natural selection. Sup-
ia Vireoatig Seance and in which | ort for the theory of mutation as a

method of evolution is seen in the
appearance in nature of so-called sports—new types which have suddenly
appeared and which have transmitted their characters to succeeding
generations. Many cases of mutation in the fruit fly are known, which
involve eye color, the shape and size of wings, body color, additional
bristles, and other less obvious characteristics. DeVries, who was the
author of the theory that evolution was due to mutation, based it upon
experimental work with evening primroses. The suggestion has been
made that he was dealing with hybrids and not with pure forms and that
thus the types which he produced did not, at least in part, represent true
mutations. Nevertheless this does not explain all of his results, and
numerous examples of mutation are now known in both plants and
animals (Fig. 379).

Another method of evolution was suggested by Lotsy. He believed
that since all animals are from the genetic standpoint impure, new types

EVOLUTION OF ANIMALS 551

may be developed constantly as a result of hybridization, not in the sense
of crossing two species but in the sense of crossing genetic characters.
Still another method is that known as orthogenesis. Paleontologists
have been very active in urging the importance of this as a method and
the evidence to support it comes largely from fossil types. According to
the theory of natural selection, when a characteristic becomes harmful
to an animal it should disappear, but there is paleontological evidence
to the effect that in the past many types that have specialized in certain
directions have gone on developing in that direction, even when over-
specialization has resulted in harm and has ultimately led to the extinc-
tion of the animal. An example of such a type is the saber-toothed tiger
(Fig. 380); its upper canines developed until they became exceedingly
effective both in the securing of prey and in defense, but they seem to

Fic. 380.—Saber-toothed tiger; restoration. (Redrawn from Scott, ‘‘ History of Land
Mammals in the Western Hemisphere,’ by the courtesy of The Macmillan Company.)

have gone on developing until they became a handicap and perhaps were
ultimately a factor in its extinction. Another animal often given as an
example of overspecialization is the Irish elk the antlers of which were
greatly developed; at first this was to the advantage of the animal but
later they reached such a size as to impede its progress in the forests and
place it at a disadvantage in escaping from enemies. To explain such
cases as this it has been suggested that the development of a character is
due to a hereditary tendency accompanying a progressive change in the
genes, which causes the animal to develop constantly in a certain direc-
tion. If the result is to make the animal more effective, natural selection
tends to perpetuate the type, but when overdevelopment and disadvan-
tage follow, the hereditary tendencies cannot be reversed and the result
is extinction.

591. Evolutionary Series.—Several evolutionary series exist, the best
known of which are those of the elephants, horses, and camels.

The earliest known elephant, known as Moeritherium, is found in the
upper Eocene deposits in northeastern Africa. This animal somewhat
resembled a hog in form, with a projecting snout, and was of only moder-
ate size, being between 3 and 4 feet in length. From Eocene time to the

552 GENERAL CONSIDERATIONS

Parelephas
Pleistocene

Stegodon
Upper Pliocene

Stegomastodon
Pliocere

Tetrabelodon
Pliocene & Miocene

_ Palaeomastodon
Oligocene - Upper Eocene

Moeritherium
Lower Eocene

—=
Ss Sa

¥ 7
X36
Fic. 381.—A series illustrating several types of extinct elephant-like mammals, and
showing on the left the skulls and hypothetical outlines of the heads and probosces, and
on the right the grinding surfaces of the corresponding last lower molar teeth. While
arranged in a series, beginning with the earliest known form, Moeritherium, the types above
probably do not represent a single line of descent; Parelephas is an extinct cousin of the
modern Elephas and not an ancestor. Moeritheriwm and Palaeomastodon are found only
in the Old World, the others in North America. The magnification is indicated for each
figure. (From several sources, in general after Lull.)

EVOLUTION OF ANIMALS 593

present many types have appeared (Fig. 381) which show, in general,
an increase in size, the production of a trunk, or proboscis, the develop-
ment of tusks, and changes in the molar teeth. Present-day elephants
are not the largest in the series, which culminated in a type 14 feet high;
the largest exact measurement recorded for a modern elephant is 11 feet,
though 13 feet has been reported. The trunk, or proboscis, representing
an elongation of the nose and upper lip, is at a maximum in the modern
types. It is a powerful but delicate prehensile organ used in gathering
food and in taking up water, which is then passed into the mouth. In
the evolution of the elephants tusks were developed from both the lower
and the upper jaws, but in the more recent types those in the lower jaw

D C Sf 153 BA a
Fic. 382.—The evolution of the horse during many millions of years showing an increase
in its size and a decrease in the number of its toes, from four to one. A, four-toed Hohippus
of the Eocene epoch. B, three-toed Mesohippus of the Oligocene epoch. C, Merychippus
of the Miocene epoch with its large central toe and smaller side toes. D, Equus of the
Recent epoch with only one visible toe. (Redrawn from Mavor, ‘‘General Biology.’’)

have been suppressed while those in the upper have been retained and
developed. The length of the tusks possessed by the modern elephants
is far exceeded, however, by the tusks of some of the extinct types. This
is perhaps an example of excessive specialization. The skull has become
very high, a fact which makes it easier for the animal to carry the weight
of the large tusks. The molar teeth have become greatly developed and
reduced in number, not more than eight being functional at the same time.
These teeth, as in all herbivorous animals, are fitted for grinding the food.

The most ancient horses, that from Europe known as Hyracotherium
and that from western North America as Eohippus, also belong to the
Kocene period. Eohippus (Fig. 382A) was an animal about the size of a
small dog, standing only about 12 inches high. The head was elongated,

554 GENERAL CONSIDERATIONS

the legs and neck only moderately long, and there were four digits on the
forelimbs and three on the hind ones. There was little resemblance
to a modern horse but the geological record fills in the gaps in the series.
In the intermediate forms, which lived on open plains, is shown a gradual
reduction in the number of toes leading to the one highly developed middle
toe which the modern horse possesses (Fig. 382). There has also been
an increase in size and particularly an increase in length of the limbs and
the neck, fitting the animal for speed and for reaching the ground when
grazing. The teeth have also become larger and better fitted for grinding
and the premolars have become like the molars.

The camels present a third series which parallels that of the horses
and apparently illustrates adaptation to similar conditions. The
Eocene Protylopus, which was about the size of a jack rabbit, had two
accessory toes as well as the two toes which exist in present-day types.
Its home was in North America. Evolution in this group has been
accompanied by reduction in the number of toes; increased length of
the limbs, particularly those parts of the limbs representing the hand and
foot; an increase in size of both the animal and its skull; and modification
of the teeth for grinding. Other adaptations which are prominent in
the modern camels are the development of water storage cavities in
connection with the stomach and the development of humps in which
fat 1s stored.

A fourth evolutionary series is that represented by the anthropoid
apes and man, but this has been discussed in a previous chapter (Chap.
LX).

CHAPTER LXXIII

INHERITANCE IN ORGANISMS

GENETICS

It is a fact old in human experience that resemblances appear between
parents and children, and it is to this fact that the adage “‘like father,
like son”’ refers. Unscientific man overlooked the differences since they
did not seem to him significant, but it has been learned that heredity
involves inheritance of both resemblances and differences. The science
which deals with inheritance is known as genetics.

592. Organisms from the Genetic Viewpoint.—An organism exhibits
a number of inherited physical characteristics involving size, form, color,
character of body covering, structure of internal parts, and so on. In
higher animals these become very numerous. It is found, however, that
organisms may inherit and may pass on potential characteristics which
do not appear in the individual. It has been found that some visible
characteristics correspond to one gene in inheritance, some represent
two, and still others are the resultant of three or more. Every somatic
cell of a metazoan possesses potentially all of the characteristics which
belong to the organism, but some cells display certain of these, other
cells others of them, and all combined comprise the characteristics of the
animal. These characteristics may be other than physical, such as
abilities, modes of behavior, and even particular instincts, but these are
all based upon structure which is inherited.

593. Determiners or Genes.—In Chap. XI it was noted that the
splitting of the chromosomes in cell division had been interpreted as
implying the equal division of units arranged in a longitudinal manner
along the chromosome thread, these units corresponding to the charac-
teristics of the organism. This interpretation has been supported by
many subsequent observations, and it is now accepted as a fact that the
split chromosome consists of two series of units, those of one daughter
chromosome being opposite those of the other, and each pair resulting
from the division of one unit in the chromosome of the parent cell. Thus
for each unit in one of the daughter chromosomes there is a similar unit
in the other. These hypothetical units are known as determiners, factors,
or genes, and these form the physical basis for the visible characteristics,
which are known as characters.

555

556 GENERAL CONSIDERATIONS

594. Behavior of Chromosomes in Maturation and Fertilization.—In
Chap. XXIII it was stated that synapsis occurred in the growth period
of both oogenesis and spermatogenesis. In the next chapter synapsis
was explained as being the temporary union of like chromosomes from
each of the two parents Fig. 391. It was followed by reduction.

In fertilization (Chap. XXIV) the entrance of the sperm nucleus into
the egg cell was described and the statement was made that even after
the two pronuclei united the maternal and paternal chromosomes retained
their individuality. In each succeeding cell generation in the individual
produced from the fertilized egg these maternal and paternal chromosomes
appear separate and distinct (Fig. 391). In the maturation of the sex
cells of this individual, however, synapsis and reduction again take place.

595. Effect of Chromosome Reduction.—In chromosome reduction
the two of each pair of chromosomes separate and go to opposite poles of
the meiotic spindle of either the spermatocyte or oocyte, as the case may
be. Since it is a matter of chance as to which of the two chromosomes
will go to either of the two poles, the resulting sperm cells or egg cells
may differ from each other in the assortment of maternal and paternal
chromosomes which they receive. During synapsis the two chromosomes
of each pair may twist about one another and fuse more or less so that
when separation occurs the two chromosomes which result may each
represent portions of both of the chromosomes which were united. Thus
it is apparent that different offspring from the same parents may inherit
different combinations of parental characteristics.

596. Allelomorphs.—In fertilization the zygote receives chromo-
somes from both parents; when the sex cells mature in the individual
which develops from this zygote the corresponding chromosomes from
the two parents unite in synapsis and hence are called synaptic mates.
Corresponding genes exist in such synaptic mates (Fig. 391). If genes
for any pair of characters are alike, the individual is said to be homo-
zygous for that pair of characters; if not, it,is said to be heterozygous. The
unlike genes of a heterozygous individual are known as allelomorphs. Of
course the presence of both cannot be shown fully in visible characters and
one will be evident and the other repressed or concealed. The one which
is shown is called dominant, the other recessive. In some cases, however,
there is an incomplete dominance as shown in the blue Andalusian fowl
(Fig. 387). The term genotype refers to the whole combination of
inherited genes which any individual possesses; phenotype, to the assem-
blage of characters which manifest themselves. If, for example, both
genes for hair color in a mammal were alike, the animal would be homo-
zygous and have the common color; if one gene was for red and the other
for black, the animal would be heterozygous for this pair of characters

INHERITANCE IN ORGANISMS 557

and might appear either black or red, depending upon which showed
itself. The one which appeared would be a phenotypic character.

597. Mendel.—The first scientific explanation of the manner in which
inherited characters are passed on was given by Gregor Johann Mendel
(Fig. 383), an Austrian peasant boy who became a monk and abbot in
the monastery at Briinn and who lived from 1822 to 1884. In the mon-
astery garden he experimented with the inheritance of characters in
garden peas and formulated laws of inheritance which have come to be
known by the term mendelism. He published the results of his work in

Fic. 383.—Gregor Johann Mendel, 1822-1884. (From Shull, ‘‘Principles of Animal
Biology,”’ after Report of the Royal Horticultural Society Conference on Genetics, 1906, and
by the courtesy of McGraw-Hill Book Company, Inc.)

1866, but, owing to the fact that his contribution was in an obscure
publication and that men’s minds were occupied at the time with the
subject of evolution and the work of Darwin, which had appeared seven
years before, Mendel’s contribution remained unknown to scientists until
the beginning of the present century. Mendel’s work was done before
chromosomes were discovered, but all that has been learned since has
served only to justify his conclusions.

598. Mendelism.—Mendelism involves particularly three principles
or laws which may be stated as follows:

1. There is in each individual a pair of hereditary units corresponding

to each character which the individual possesses (law of paired units).
These we now know as genes, one coming from each parent.

558 GENERAL CONSIDERATIONS

2. When the two hereditary units are unlike, the one which shows
its presence by a visible character may be called dominant and the other
recessive but in many cases there is incomplete dominance.

3. In breeding individuals heterozygous for any pair of characters,
three kinds of offspring will be produced—those which are homozygous
and possess the dominant character; those which are homozygous and
possess the recessive character; and those which are heterozygous and
show the dominant character or intermediate stages of the dominant and
recessive characters (law of segregation).

Homozygous Male Hornozygous Female

Primary
Oocyre

Primary
Spermatocyte

Sperm Cells - any ore £gqg Cell Polar Bodies
Fertilized Eqg (Aa)
Produces

Heterozygous Male or Heterozygous Females
Primary Primary
Spermatocyte Oocytes

Possible Combinations
Fic. 384.—Diagram showing the inheritance of unlike characters by a hybrid between two
homozygous but unlike parents and the results of interbreeding these hybrids.

599. Hybrids.—In Chap. XXII reference was made to cross-fertiliza-
tion and hybridization. A hybrid in the sense in which the term was there
used referred to the production of an offspring by two individuals belong-
ing to different species. It is evident from what has just been stated that
we may also use the term hybrid in reference to heterozygous individuals
belonging to the same species. An animal may be a hybrid for certain
characteristics and yet be pure-blooded so far as the species or variety is
concerned, the characteristics which it possesses being recognized as
varying within the species.

600. Distribution of Characteristics in Hybrids.—In crossing two
individuals homozygous for the same pair of characters it is evident that

INHERITANCE IN ORGANISMS 559

since the genes are similar all progeny will be alike and will resemble both
parents. If two parents, both homozygous but for different characters,
are crossed, the offspring will all be hybrid and will exhibit the character
which is dominant. In crossing these hybrid or heterozygous individuals
it is clear that different genes may be distributed to the different sex
cells which the individuals develop and that thus when these unite with
other cells, varying combinations of genes will occur. For instance,
considering only one pair of genes, if the dominant gene received from
one parent is indicated by A and the recessive gene received from the
other parent, by a, some of the sex cells will possess A and others a. In

an 2. oe

Black albino Black a/bino
BB y b BB bb
Gametes P == "(6) ——
Fy :aP x 9

Black
Bb

Gamerfes OF ? Fy

Gu
Gametes OF & F,

Fig. 385.—Diagram showing the results of crossing two guinea pigs differing by one
character and each homozygous for that character, and the checkerboard showing the
results in the F2 generation. (From sketches by D. D. Whitney.)

the union of sex cells two A’s may be brought together, or two a’s, or an
A may be united withana. The individuals with two A’s and those with
two a’s are homozygous, while those with A and a are heterozygous or
hybrid for this pair of characters and will show the dominant character.
These symbols being placed in circles corresponding to the cells, the facts
may be fitted into diagrams showing oogenesis, spermatogenesis, and
fertilization in successive generations as indicated in the accompanying
schemes (Fig. 584). From these schemes it is apparent how segregation,
is brought about by the chance union of similar genes and thereby produc-
ing the homozygous types from hybrid parents. In the discussion of these
successive generations it is usual to refer to the generation which forms
the starting point as the P (parental) generation; the first generation pro-

560 GENERAL CONSIDERATIONS

duced by the union of the sex cells from these parents is the F’; (first filial)
generation; the second is the F: (second filial) generation; and other
generations are labeled correspondingly.

601. Checkerboard Diagrams.—The possibilities in the breeding
of hybrids may also be indicated in a diagram resembling a checkerboard,
which shows the gene constitution of sperm cells and egg cells and the

Parents

ras

o of xXi& OE

Rough Black smooth albino Rough Black smooth albino

RR ~BB rrr bb RR _ BB rr bb
Gametes P —~€B) oe
F, < ieee Xx Bae °
Rough Black Rough Black
Rr Bb Rr Bb

Gametes OF £ F,

(RB) ugh Bi Rough Black
RR Bb

kough albino
Rr bb

Rough black
Rr Bb

RR bb

Rough Black
Rr Bb

Rough Black
Rr BB

smooth Black | smooth Black

Rough Black
Rr Bb

smooth Black \smooth albino
Fats Bb Gis bb

Rr bb

Fic. 386.—Diagram showing the results of crossing guinea pigs differing by two
characters, for each of which they are homozygous. The genotypic ratio in the F2 genera-
tion is 4:2:2:2:2:1:1:1:1; the phenotypic, 9:3:3:1. (From sketches by D. D. Whitney.)

resulting possible combinations in the offspring. The accompanying
diagram (Fig. 385) would illustrate what would happen if the sex cells
differed in reference to a single pair of characters, black and albino,
represented by B and b. It is clear that in this case the result could be
expressed by a ratio 1:2:1, in which the first 1 is an individual homozygous
for the dominant character; the second 1 is an individual homozygous for
the recessive character; and the 2 is for the two individuals which are
heterozygous or hybrid for these characters. This is the genotypic ratio;
the phenotypic ratio would be 3:1, since three individuals, including the

INHERITANCE IN ORGANISMS 561

homozygous and dominant and the two heterozygous individuals, would
show the dominant character, while the other, which is homozygous and
recessive, would exhibit the recessive character.

602. Multiple Hybrids.—So far we have considered individuals
heterozygous for only a single pair of characters. Such individuals are
termed monohybrids. Since every individual is a combination of large
numbers of characters, however, it is clear that an individual is likely
to be a hybrid not for one characteristic but for more. If hybrid for two,
the individual may be termed a dihybrid; if for three, a trihybrid; and if
for more, it is usually called a multiple hybrid. In these cases the variety
of sex cells produced would be greater and the number of possible com-
binations in the offspring would be very much greater. A checkerboard
diagram for a dihybrid cross in which the characters are represented by
Rk, r, B, and b—for rough and smooth, black and albino—would appear
as in the diagram (Fig. 386).

There would be four types of sperm cells and four types of egg cells.
The phenotypic ratio would be 9:3:3:1, in which 9 represents individuals
that show both dominant characters; 3 are dominant for the first pair
of characters but recessive for the second; 3 are recessive for the first
pair but dominant for the, second; and 1 is recessive for both pairs. The
genotypic ratio would include nine types of individuals as indicated by
the diagram.

A corresponding phenotypic ratio for a trihybrid cross in which
there are eight types of sperm cells and eight of egg cells would be
27:9:9:9:3:3:3:1. In this case 27 would show all three dominant. char-
acters and only 1 would show all three recessive characters, the others
being varying combinations of dominants and recessives. There are
here 27 different genotypes.

603. Actual Cases.—Certain cases may be given to illustrate the
actual results of crossings. It should be observed that since these
ratios are based on chance they may not hold good in individual cases
where small numbers are involved. The larger the numbers the closer
they are likely to be approached. Mendel crossed tall pea plants with
short pea plants, where tallness is a dominant character and shortness a
recessive one. The hybrids were all tall. He found that in breeding
these hybrids three-fourths of the offspring were tall and one-fourth short
but that of the three-fourths which were tall one-third were pure for this
character and two-thirds were hybrid. Mendel actually obtained in the
second filial generation from original parents, one of which was tall and
the other short, 787 tall plants and 277 short ones, when, if the results
had been mathematically exact, he should have had 798 tall and 266 short
ones. Other experiments have resulted in ratios more nearly mathemati-
cally exact, and the departure has not been considered sufficient to invali-
date the principle involved.

562 GENERAL CONSIDERATIONS

If two pairs of characters, one pair for curly hair and straight hair,
curly being dominant, and the other pair for dark color and light color,
dark being dominant, are represented in the breeding of hybrid indi-
viduals, then nine of the phenotypes will have dark, curly hair; three will
have dark, straight hair; three will have light, curly hair; and one will
have hair which is both light and straight.

The result when three pairs of characters are involved is illustrated
by the work of Castle on guinea pigs. When a short-haired, dark-
colored, and smooth-coated guinea pig is crossed with one which is long-
haired, white, and rough-coated, all of the Ff; generation will belong to
one phenotype and will be short-haired, dark-colored, and rough-coated,
since these are the dominant characters. When these individuals are
bred together, however, they will produce eight different phenotypes.
Twenty-seven will have hair which is short, dark, and rough; nine will
have hair which is short, white, and rough; nine, hair which is long, dark,
and rough; nine, hair which is short, dark, and smooth; three, hair which
is long, white, and rough; three, hair which is short, white, and smooth;
three, hair which is long, dark, and smooth; and one, hair which is long,
white, and smooth.

604. Breeding the Test for Characters.—In determining the genetic
constitution of an animal the test applied is that of breeding. If indi-
viduals bred generation after generation show only one character, then
they must be homozygous for that character; if they are hybrid, breeding
will betray the fact. ‘The genetic constitution of an animal the genotypic
character of which is unknown may be determined by breeding it with
other animals the genotypic character of which is known.

605. Variations in Inheritance.—The result of an enormous amount
of experimental work has shown that while there are certain characters
in plants and animals that behave exactly according to Mendel’s prin-
ciples, others do not. ‘This has been explained in many cases by assuming
that interactions occur between genes. It has also been found that genes
may change, causing the characters to vary. A heritable change appear-
ing in a line of descent which cannot be traced to any ancestor is known
as a mutation. A mutant is an animal which shows such a change. It
has recently been demonstrated that radiations, particularly X-rays, are
capable of bringing about gene mutations (Sec. 590).

In many cases both traits develop resulting in incomplete dominance
of either trait. In such cases the F; generation differs from both parents
and is more or less intermediate with respect to a certain pair of characters,
but in the Ff. generation there are individuals like both grandparents as
well as intermediates. This occurs in the case of plants called four-
o’clocks; plants with red flowers crossed with those with white flowers
produce hybrids with pink flowers, and when these pink hybrids are
crossed the result is a ratio of one red dominant, two pink hybrids, and

INHERITANCE IN ORGANISMS 563

one white recessive. Another case of incomplete dominance is that
of the blue Andalusian fowl (Fig. 387), which is a hybrid between a black
individual and an individual which is white splashed with black. When
two blue Andalusians are bred, there is a ratio of one splashed white, one
black, and two blue individuals.

Another variation is that due to the cumulative effect of multiple
genes. This is observed in mulattoes, whose skin color may vary from
very dark to very light. It also has been found that there are lethal

P Gen.

Fo Gen.

Fic. 387.—Incomplete dominance, as illustrated by the blue Andalusian fowl. The
P generation is splashed white and black, the Fi generation all blue, and the F2 generation,
one splashed white, two blue, and one black.

genes, which when present in pairs cause the death of the organism. In
some cases, as walnut combs in fowl, the character is believed to result
from the interaction of two genes, either kind of which, when in a homo-
zygous pair, produces a different comb pattern.

606. Breeding for Certain Characteristics.—In both animal and plant
breeding the end sought is the bringing together of desirable characters
and the development of stock which will breed true for those characters.
This involves the careful selection of breeding animals and the elimina-
tion of all progeny which are not homozygous for each pair of characters
desired. In this way, and by persistent inbreeding, a line can gradually
be developed which not only possesses the desired characters but also

564 GENERAL CONSIDERATIONS

has them in pure form or, in other words, is homozygous for them. The
final result is an artificial variety or strain which can be maintained by
the exercise of sufficient care. If only one character is sought which is
a pure mendelian character, this is not a long process, but since characters
may be in view which are the result of interaction between genes, consider-
able experimentation and judicious selection are often required to arrive
at the desired result.

607. Inbreeding and Crossbreeding.—Inbreeding is the breeding of
closely related individuals; crossbreeding, the breeding of those not so
related. There are genes in man which correspond to certain defects such
as deafness, certain types of insanity, feeble-mindedness, and a tendency
to excessive bleeding known as hemophilia. These defects are usually
recessive. Since they are usually recessive, when present and paired with
a normal character they do not appear; if not so paired the defect becomes
apparent. For this reason inbreeding is likely to bring out these defects
to a much greater degree than crossbreeding, in which the chances are
great that the other individual will be normal with respect to the defect
which the one may have. If there are no defects in the inheritance,
inbreeding has no ill effects; on the contrary if the inheritance contains
many valuable qualities the result may be to produce superior individuals.

Crossbreeding has been found in some cases to result in increased
vigor, called heterosis. That is said to be true in the breeding of mules.
This is, however, often accompanied by sterility.

608. Inheritance of Acquired Characters.—From what has been said
it is evident that since the characters of an animal are determined by
its inheritance, any somatic character acquired during the lifetime of the
individual cannot be passed on unless it is accompanied by a germinal
modification. Since zoologists define acquired characters as purely
somatic modifications, it is not possible for them as such to be passed on.
Whether or not these somatic modifications can so affect the germ cells
as to produce a germinal modification which can be transmitted is not
known. No convincing proof, however, has ever been presented of this
having occurred, and so.at the present time the possibility of the inher-
itance of acquired somatic characters in any fashion is not generally
accepted, though it is recognized by many as conceivable.

609. Inheritance of Disease and Abnormalities.—In Chap. LX VIII
reference was made to the inheritance of disease. Abnormalities, such
as extra digits, which are not an evidence of disease, are also heritable.
Congenital diseases may be due in certain cases to the actual passing on
of disease-producing organisms from parent to offspring; but it is also
clear that diseases and abnormalities due to defective genes may be
inherited. Some of these defective genes, as has been stated above, are
recessive, while others are dominant. Among the latter are those for dia-
betes insipidus, a disease characterized by an insatiable thirst and the

INHERITANCE IN ORGANISMS 565

production of an excessive amount of urine; hereditary night blindness;
congenital cataract; and the presence of extra digits or of short digits on
either hands or feet. Among the recessive defects, aside from feeble-
mindedness, are those of dwarfness and certain types of nerve degenera-
tion causing paralysis.

Fic. 388. Small eyes in four successive generations of the same family. I, the great-
grandmother. IJ, son of I. III, the daughter of II. IV, the daughter of III. (Photo-
graphs by Whitney.)

For every abnormal character inherited, there is its allelomorphic
normal character inherited, but many normal characters have no known
allelomorphic abnormal mates. In man only a few abnormalities are
inherited, whereas there are great numbers of normal traits inherited.
Among these are eye, hair, and skin color; size of eyes (Fig. 388); form
of hair, whether kinky, curly, wavy, or straight; number of fingers and

566 GENERAL CONSIDERATIONS

toes; length of fingers; tapering fingers (Fig. 389); ear lobes; few hairs on
the middle segment of the fingers; mathematical, musical, and artistic
abilities; probably memory, and reasoning ability; and many others.

Some characters seem very unimportant but are inherited as con-
stantly as our valuable characters: for instance, the ear lobes or lack of
ear lobes; the few hairs or their absence on the middle segment of the
fingers; ear pits; a white lock of hair; and numerous others.

610. Sex Determination.—In all that has hitherto been said of
chromosomes reference has been made only to what are called ordinary
chromosomes, or autosomes, which exist in pairs. ‘There are, however,

eee

FATHER

The fingers of mother are nontapering.
(Photographs by Whitney.)

Fig. 389.—Tapering fingers in father and son.

other chromosomes, which, because of their connection with the deter-
mination of sex, are known as sex chromosomes.

In the males of certain insects there are two sex chromosomes, recog-
nized as the z-chromosome and the y-chromosome, the latter often being
the smaller; but in the males of other animals there may be only one sex
chromosome and this an z-chromosome. In both cases in the female
there are two z-chromosomes. Experiments have shown that it is the
x-chromosome which in some way determines sex.

Since in chromosome reduction these sex chromosomes must go
to either one sex cell or the other (Fig. 390), it follows that in cases in
which the male has one x-chromosome, only half the sperm cells will
contain such a chromosome, while, since the female has two, all of the egg
cells will contain one. If a sperm cell which contains an z-chromosome
unites with the egg cell, then the zygote will contain two z-chromosomes
and from it will develop a female; if, on the other hand, the sperm cell

INHERITANCE IN ORGANISMS 567

without an z-chromosome unites with an egg cell, the zygote will contain
only one z-chromosome and will be male. In case the male possesses
both an z-chromosome and a y-chromosome, then the x-chromosome
goes one way and the y-chromosome the other, and the result is that

* @@aa
Two kinds of sperm ce//s ge Kind of egg cell
Produces 3
formula 3 =8+2x Formula 9 =8+#x
Primary \ Primary
Spermatocyte Oocytes
@R
oH!) @ @ ©
One kind ZA Two_kinds “of egg cells
ne kind of sperm ¢ wo
Pp. Ss
Produces
formula 3 = 8+x+t y Formula 9 = 8+t2x
Primary 4 Primary
Spermatocyte Oocytes
@)
Cc

lig On
Two kinds of sperm cells

Produces Ng 2}

This may be reversed as in the case of A and B
Fig. 390.—Diagrams illustrating sex determination in three different cases.

there are again two types of sperm cells. If the sperm cell containing the
x-chromosome unites with the egg cell also containing an x-chromosome,
a female is produced; but if the sperm cell containing the y-chromosome
unites with an egg cell, then a male is produced, with both an x-chromo-
some and a y-chromosome.

In other cases instead of there being two kinds of sperm cells there are
two kinds of egg cells. In fowls and in moths females have either one

568 GENERAL CONSIDERATIONS

z-chromosome or both an z-chromosome and a y-chromosome, while the
males have the two z-chromosomes. In this case it is the type of egg cell
which is fertilized that determines sex.

In man, and perhaps in all mammals, there are an z-chromosome and
a y-chromosome in the male and two z-chromosomes in the female. Man
also has 46 autosomes. The formulae for the two sexes may be written:

o=46+2+y; 2 =46+ 22. The sex cells are 23 + z or 23 + y.

611. Twins.—The production of twins may be mentioned in connec-
tion with sex. There are two types, known respectively as identical and
fraternal twins. Identical twins agree precisely in their characteristics
and are always of the same sex. For this reason it is assumed that they
are developed from a single zygote, each of the two sections of the early
embryo developing into a complete individual. In the case of the
nine-banded armadillo it has been found that the four young usually
produced are identical quadruplets resulting from the development of a
single zygote and possessing a common placenta. On the other hand

@o Oo: Paternal

ROD 6 (Driver ong: hl) je Kk -LM

Fic. 391.—Diagram of two homologous chromosomes, paternal and maternal, showing
that similar genes are placed at the same level and at definite locations in the length of the
chromosomes. Both genes of a pair may be dominant (black discs with capital letters)
or may be recessive (white discs with small letters), or one may be a dominant and the other
recessive.

fraternal twins, being produced from two zygotes, may differ in sex and in
other characteristics to the same extent that two offspring produced
by the same parents at different times may differ.

612. Sex-linked Characters.—The sex chromosomes may not only
determine the sex but also carry genes for many other characters. It thus
follows that these characters will be determined when sex is determined
and will be associated with one sex or the other. In the case of color
blindness in man the defect is carried in some of the 2-chromosomes
and is recessive, while normal vision, which is dominant, is carried in the
other a-chromosomes. The sex chromosome carrying the defect may
be indicated by x. A color-blind father (xy) and a normal mother
(xx) will have no color-blind children, since the zygotes will be either xy
or xt. In the F, generation, however, one-half the grandsons and one-
half the granddaughters are free from this defect and the other half of
the granddaughters carry the gene for color blindness as a recessive and
the other half of the grandsons are color-blind. A normal father with a
color-blind mother will have only color-blind sons while the daughters
will be normal. Also one-half the grandsons and one-half the grand-

INHERITANCE IN ORGANISMS 569

daughters will be color-blind. There are many such sex-linked char-
acters. These should not be confused with other characters which
distinguish one sex from the other, since the corresponding genes are |
carried in autosomes and the characters are developed under the influence
of hormones produced in either the ovary or the testis.

613. Linkage and Crossing Over.—lIt is evident that many charac-
ters not associated with sex may be linked in inheritance because of the
fact that genes are carried in the same chromosome (Fig. 391). In certain
cases, however, it is found that such linkage is broken. This can be
explained only by assuming that when two chromosomes unite in synapsis
there is fusion at certain points and that when they separate portions of
the two are exchanged. This is called crossing over. The existence of
such linked characters and of crossing over has been seized upon as a
means of determining the position of genes in the chromosomes. In
the fruit fly, Drosophila, where a number of such characters occur, it
has been found that by comparing cases of variation in linkage the chances
of such crossing over vary. It may be assumed that when genes are far
apart the chances are greater than when they are close together. As a
result of thousands of experiments by Morgan and his students it has
been found possible in the case of the fruit fly to locate the genes con-
cerned in the production of a large number of characters not only in par-
ticular chromosomes but also at particular points in those chromosomes.

614. Eugenics.—The application of the principles of genetics to the
production of human offspring with the aim of developing a superior
race has been strongly urged. This field of investigation and social
endeavor is known as eugenics. There are many individuals and organ-
izations seeking to develop an intelligent eugenic program, to secure
its adoption, and through it to improve the human race. The adoption
of such a program, however, can be brought about only as a result of
long-continued agitation and the gradual education of the public.

CHAPTER LXXIV

CLASSIFICATION OF ANIMALS

TAXONOMY

Taxonomy is that field of zoology which deals with the classification
and nomenclature of animals. It has often been referred to, especially in
the past, as systematic zoology. A number of matters properly belonging

Fig. 392.—Carolus Linnaeus, 1707-1778, at the age of forty. (From Shull, ‘‘ Principles
of Animal Biology,” by courtesy of the New York Botanical Garden, and of McGraw-Hill
Book Company, Inc.)

to this field were considered in Chap. XIV, and while they should be
reviewed in connection with this subject they will not be repeated here.

615. History.—The first known systematic arrangement of animals
is that of Aristotle, who divided the animal kingdom into the Enaema,
or blood-holding animals, which included forms with red blood; and the
Anaema, or bloodless animals, which included all the rest. Each of
these categories contained four groups, all of which were founded on
superficial characteristics. Following the revival of learning and the
era of discovery at the end of the Dark Ages, an enormous number of new
species of animals from all corners of theearth became known. Aristotle’s

570

CLASSIFICATION OF ANIMALS 571

system was inadequate and so there was great confusion in the classifica-
tion and naming of animals. A step forward, however, was taken by
Ray (1627-1705), who first fixed a definite conception of a species and
also used anatomical facts in the discrimination of the larger groups.
His work paved the way for that of Linnaeus (1707-1778). Linnaeus
(Fig. 392) believed in the fixity of species and his classification was dis-
tinctly artificial, but it is so simple in theory, so clear, and so plastic
that it has furnished the basis of all the work in this field which has been
done since. He divided animals into six classes (four of which were
vertebrate groups), 32 orders, and a large number of genera and species.
He also gave a brief characterization in Latin of each higher group and of
each species. While they were sufficient for the limited number of groups
and species recognized then, these characterizations are not sufficiently
detailed or exact in the light of our present knowledge; however, they
served as models for those who followed him. It is to be observed, more-
over, that his groups were of much higher rank than the groups given
the same names today, his classes being equivalent to present-day phyla,
his orders to what are now classes, and his genera to orders as now recog-
nized. While we use his generic names we have greatly limited the
extent of his genera. Linnaeus believed in the possibility of arranging
animals in a single series. To Lamarck (1744-1829) is to be given credit
for first recognizing the fact that there are different lines of descent in the
animal kingdom and for representing this in the form of what has since
been known as a genealogical, or phylogenetic, tree. The effect of the
general acceptance of the evolutionary conception was to cause a change
toward a more natural system of classification. In 1866, Haeckel pre-
sented the first really modern classification.

616. Species.—While to most people a species seems to be a very
definite conception, insuperable difficulties are immediately encountered
when an attempt is made to define it. Although many criteria have been
used in the definition of species, it is still a matter of opinion as to whether
a species is a natural entity or an artificial assemblage of individuals
and as to what terms shall be used in the definition of the word. An
assemblage of individuals which to one zoologist appears clearly separable
into a number of species may be considered by another as simply repre-
senting the variations possible in one. The extent of any taxonomic
eroup is to a considerable degree a matter of opinion. Generally speak-
ing, the differences between species are qualitative. If the differences
between two forms are only quantitative, such as differences in size and
shade of color, the usual conclusion is that they are only forms of one
species. Some species are very conservative, exhibiting little variation
throughout their range; others are radical, varying with every change in
the environment.

572 GENERAL CONSIDERATIONS

617. Polymorphism.—When the variation within a species is marked,
when the different forms are not connected by intermediate gradations,
and when they can be correlated with some other factor or condition, then
the species is called polymorphic and is said to exhibit polymorphism.
If there are but two types, dimorphic and dimorphism are usually the
terms used. Sexual dimorphism is present in the large majority of
animals, the male and female being recognizable by characteristic fea-
tures. What might be called sex-linked polymorphism is shown by some
butterflies, the males of which are all alike, the females being of different
types; in other animals the males may be polymorphic. Polymorphism
may also be geographical, climatic, seasonal, or social. Many widely
distributed animals are represented by so-called subspecies, varieties, or
geographical races, which are often also directly related to climatic condi-
tions and represent geographical and climatic polymorphism. Insects
which have several broods in a year may exhibit seasonal variation; and
ants and bees are examples of social polymorphism, accompanied by
division of labor. Some tropical butterflies show several color variations
which may all appear together at the same time and place.

618. Basis of Classification.—The basis of classification is, of course,
the resemblances and differences which exist between animal types.
It has been seen, however, that resemblances may be of two kinds, homol-
ogous and analogous. It has also been shown that resemblances may be
due to convergence, which, of course, means analogy, and that differences
may be due to degeneration and retrogression. All of these tend to
interfere with a recognition of natural relationships among animals.
The biogenetic law has a bearing here, and it is usual to consider evidence
from early stages in the life of an animal as very important in revealing
its real affinities. All of these things have to be carefully considered in
arriving at a truly natural classification.

619. Basis of Nomenclature.—Our nomenclature is based upon a
proposal made by Linnaeus that the name of each animal shall be com-
posed of two parts: the name of the genus and of the species to which it
belongs. This is what is known as binomial nomenclature. In recent
years there has been a tendency, especially in connection with certain
groups of animals, to add to these scientific names a third, or subspecific,
name, and even in some cases a fourth, or varietal, name. If this prac-
tice becomes general, our nomenclature will become either trinomial or
quadrinomial. At present it remains distinctly binomial. The method
of citation of a species was described in Chap. XIV.

620. Rules of Nomenclature.—It has been stated that Linnaeus
knew only a limited number of species. His work proved such a stimulus
to systematic zoology that the number of these rapidly multiplied. Con-
tributions were published in all parts of the world and in all civilized
languages, and the number of species became so great as to cause much

CLASSIFICATION OF ANIMALS 573

confusion as to the correct names of animals. As a consequence some-
what more than a half century ago a very strong demand arose for the
formulation of a system of rules by which confusion could be avoided.
Out of the agitation has developed an International Commission on
Zoological Nomenclature the work of which is represented both in a code
of rules and in a long series of decisions, involving particular cases, which
have been officially promulgated by the Commission.

According to these rules a family name is made by adding -idae to
the stem of the name of a genus selected as the type. Thus the type
genus of the cats is Felis and so the family name is Felidae. The name
of a subfamily is made in the same way by adding -inae. Whenever for
any reason the type genus is changed then the name of the family must
be changed accordingly. Generic names must be single words written
with a capital letter and specific names either a single or compound word
written with a small letter. The generic name is nominative singular in
form and the specific name is usually an adjective in grammatical
agreement with it. The author of a name is the first person who defi-
nitely attached that name to the particular species. There has, however,
been a considerable amount of confusion in literature in regard to who
first described a given species and also as to whether or not certain
species are actually distinct. This makes necessary a decision as to
priority and in connection with this matter of priority most of the deci-
sions of the International Commission have been rendered. The best
usage in the description of a new species is to designate a particular
specimen as a type and to state exactly where the type is to be found and
how it is to be recognized. The commission has not undertaken to
make any pronouncement in regard to the names of groups higher
than families, and, therefore, the names of the higher groups that any
particular writer adopts are evidences of individual opinion.

621. Phyla.—In earlier chapters of this text the various phyla have
been briefly reviewed and the characteristics and advances shown by each
noted. It may be well at this point to present a table (Fig. 393) com-
paring the characteristics of the different ones. However, two phyla,
the Protozoa and the Porifera, are omitted because their plan of structure
is so different from that of the rest that they cannot be brought directly
into comparison with them. Reading vertically down the columns of
this table will furnish an indication of the trends in development of partic-
ular characteristics, while reading horizontally will give briefly the
characteristics of the individual phylum.

622. Phylogenetic Tree.—Various diagrams representing a phylo-
genetic tree have been presented, reflecting different ideas of the inter-
relationships of animal groups. One of the defects of many of these
diagrams has been that in spite of the fact that the animal kingdom
represents not one line of descent but many, the diagrams have repre-

GENERAL CONSIDERATIONS

574

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575

CLASSIFICATION OF ANIMALS

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GENERAL CONSIDERATIONS

576
;
Vertebrata
Cephalochoraata
Arthropoda Urochordata
S Mollusca
8
9S
Ss ;
Echinodermata
Entecopneusta (Secondary Radial)
rm = pa
= ryozog
By Crrtaea N& NGS fi]. 1/4 irene ee
Nemertined ‘& SN
(Coelom?) S 8 Rotifera
Nemathelminthes = (Boal Cavity)
(Body Cavity) aS (___ Alimentary Canal
Platyhelminthes
eae | _____Silateral Symmetry
Ctenophora : : ; ;
eae en (Biraniol) —-S=2== __Trploblastic
Coelenterata
(Radial)
Peis «TAVERN eR Umer Os omits Wie,’ Diploblastic
Porifera
(Rahal)
\ Protozoa
ZO0q

First "Life
Fic. 394.—Diagram of a phylogenetic tree illustrating one conception of the relation-
ship of the animal phyla, and also suggesting the uncertainty attending the relationship
of most of the different phyla above the Coelenterata. Levels of organization are indicated
by horizontal lines of dashes. Retrogression is suggested by the branch turning downward.
Enteropneusta is a class of Hemichordata which includes Balanoglossus.

CLASSIFICATION OF ANIMALS 577

sented it as possessed of one main trunk with all of the branches springing
from it. The relationship between the various phyla above the coelen-
terates is so uncertain that in the present state of our knowledge it seems
impossible to present exact continuity here. In interpreting such a
diagram the fact should also be borne in- mind that no existing type is
ancestral to any other existing type but that if the two are related this
relationship is through a common ancestor, now extinct. With these
facts in mind the accompanying scheme (Fig. 394) is presented.

CHAPTER LXXV

HISTORY OF ZOOLOGY

The field of medicine, and in connection with it certain aspects of
zoology, was developed to a considerable extent by the early Egyptians,
a medical papyrus translated by Ebers dating from as early as the fifteenth

Fic. 395.—Aristotle, 384-322 s.c. (From Shull, ‘Principles of Animal Biology,”
after Hekler, ‘Greek and Roman Portraits,’ and by the courtesy of McGraw-Hill Book Com-
pany, Inc.)
century before Christ. The beginnings of modern science, however,
must be sought in the writings of the Greeks.

623. Greeks.—The history of zoology in Greece may be begun with
the name of Thales (624-548 B.c.), who had a theory that the ocean
mothered all life. EEmpedocles and others have been referred to in the
chapter on evolution. Hippocrates (460-370 B.c.), who placed medicine
on a scientific basis, has been called the father of medicine. The work
of the early Greeks was largely of a philosophical nature and was not
based upon exact observation. Aristotle (Fig. 395), however, to whom
we owe the scientific method, the basis of which is the gathering of facts
from direct observation, used that method in his work and thus reached
scientific conclusions. In addition to inherited wealth, social position,
and excellent education, he had the advantage of the first endowment for

578

HISTORY OF ZOOLOGY 579

zoological research of which we have any record. By his royal patron,
Alexander the Great, he was given a grant of 800 talents, which is equiva-
lent to about $1,150,000 of our money but with a purchasing value corre-
sponding to $4,000,000 today (Durant). With this assistance Aristotle
gathered extensive collections and made many very precise observations.
He knew that some sharks are viviparous and that an attachment is
formed between the young and the wall of the uterus of the female. He
also observed the development of the chick and viewed that of animals in
general as a process of the gradual building of complex structures from a

Fic. 396.—Andreas Vesalius, 1514-1564. (From Shull, ‘‘ Principles of Animal Biology,”
after Garrison, ‘‘ History of Medicine,’ and by the courtesy of McGraw-Hill Book Company,
Inc.)

simple beginning. Although many of his ideas were erroneous, Aristotle’s
work was the beginning of scientific zoology, and consequently he has been
known as the father of zoology. He also furnished the first classification
of animals. That his work was not the first is evident from his reference
to facts gleaned from ‘‘the ancients,’’ but those facts were inconsiderable
in value compared to those which he accumulated. The Greeks who
followed him added little. Galen (131-201 a.p.) was a physician and an
anatomist whose contributions were based largely on actual observations
of animals and who in his work brought together all the anatomical
knowledge of his time.

624. Dark Ages.—From the decay of Greek civilization to the revival
of learning in the sixteenth century little advance was made. Pliny

580 GENERAL CONSIDERATIONS

(23-79 a.p.), a Roman general, compiled a great work of 37 volumes in
which he undertook to bring together all of the knowledge of the time,
but in this work facts were so inextricably confused with legendary matter
and superstitions of all sorts that it was practically worthless. Through-
out this period authority reigned supreme and curious conceptions held
sway, including the belief that man had one less rib than a woman, since,
according to the biblical account, one rib had been taken from Adam to
create Eve. Another curious belief was in a resurrection bone which
was believed to be the foundation from which the new body was to be
developed after resurrection from the dead. Scientific observation had

Fig. 397.—William Harvey, 1578-1657. (From Shull, ‘‘Principles of Animal Biology,”’
after Garrison, ‘‘ History of Medicine,” and by the courtesy of McGraw-Hill Book Company,
Ine.)

given way to speculation. It is interesting to note, however, that during
this time the evolutionary conception was kept alive through the influ-
ence of members of the church, although it was, at the same time, the
influence of the church which led to this overemphasis on authority.
625. Vesalius.—The first name that stands out during the Renaissance
is that of Vesalius (1514-1564), a Belgian anatomist (Fig. 8396) who defied
authority and grounded his human anatomy on the dissection of the
human body. He thus effected a great reform in anatomical teaching,
which up to that time had been based entirely upon the works of Galen.
Vesalius gave a detailed description, with illustrations, of the veins and
arteries and knew that they came close together peripherally but con-
sidered each separate from the other. He believed that in each there

HISTORY OF ZOOLOGY 581

was an ebb and a flow of the blood. His classical work on the structure
of the human body was published in 1543.

626. Harvey.—Harvey (1578-1657), an English physician (Fig. 397),
revived the experimental method in physiology. His most noteworthy
contribution was the proof of the circulation of the blood. Aristotle
believed that the blood was elaborated from the food in the liver, was
conducted to the heart and then out from the heart through the veins.
He and others who followed him thought that the arteries contained no
blood but that they carried the vital spirits, or pneuwma. Galen dis-
covered that the arteries did contain blood; he distinguished nerves from
tendons, which look much like them, and believed the former carried the
pneuma. Vesalius proved that the two sides of the human heart were
completely separated, and Servetus (1511-1553) outlined the circulation
in the lungs. Jacobus Sylvius (1478-1555) described valves in the veins.
The complete circulation was first discovered and definitely described
by Harvey. He used no microscope, however, and neither saw the
capillaries nor demonstrated their existence; this work remained for
Malpighi and Leeuwenhoek.

627. Microscopists.—The actual discovery of the microscope is
unknown, though by many its discovery is attributed to Galileo. With
the improvement of this instrument an era of discoveries began which
was to affect the whole future of biology. The earliest of the microsco-
pists and perhaps the most eminent was Malpighi (1628-1694), an Italian,
whose greatest work was done on the anatomy of the silkworm but who
among other observations saw the flow of the blood in the lungs and
mesenteries of the frog. Swammerdam (1637-1680) and Leeuwenhoek
(1632-1723) were Hollanders. The former, who was a physician and
naturalist, made extensive researches in the structure and life histories
of animals, mostly insects. He championed the preformation theory
(Sec. 631). The latter, who was a microscope maker, discovered red
blood corpuscles and observed the connections of capillaries with veins
and arteries in the tail of a tadpole; he first observed rotifers and a great
many minute organisms, including protozoans and bacteria. Hooke
(1635-1703), an English physicist and microscopist, discovered the cell.
The work of these men was followed by that of many others who unfolded
a wealth of detail in regard to minute anatomy and the smaller living
organisms.

628. Comparative Anatomy.—To Cuvier (1769-1832) is ascribed the
founding of the science of comparative anatomy. He was the son of a
French army officer and early in his life showed a pronounced liking for
paleontology and zoology. Most of his life was spent in Paris, where he
occupied a commanding position in science. He was given a title, devoted
considerable time to public education, and was appointed chancellor of
the Imperial University by Napoleon. His great work was ‘“‘Le Régne

582 GENERAL CONSIDERATIONS

Animal,” published in 1817, which influenced classification for some
time. He did not accept the classification outlined by Linnaeus but
presented one dividing the animal kingdom into four branches, Verte-
brata, Mollusca, Articulata, and Radiata. Cuvier (Fig. 398) believed
in the theory of special creation and explained the existence of dissimilar
fossils in different rock strata as evidence of a series of creations, the life
arising from each past creation being destroyed completely by some great
catastrophe. This theory is known as catastrophism ; the opposite view—
that geological succession has been a continuous process—is known as

Fic. 398.—Georges Cuvier, 1769-1832. (From Shull, ‘‘ Principles of Animal Biology,”
after Locy, ‘‘ Biology and Its Makers,”’ and by the courtesy of McGraw-Hill Book Company,
Inc.)

uniformitarianism. Cuvier was followed by Owen (1804-1892), a great
English physician and anatomist, to whom we owe the ideas of homology
and analogy. The founder of the modern science of histology was Bichat
(1771-1802), a French anatomist and physiologist.

629. Physiology.—The Greeks believed in spirits and humors in the
body and in the existence of the pnewma which, taken into Latin became
spiritus, equivalent to vital force. Harvey contributed to physiology
by applying to it the experimental method. During the sixteenth and
seventeenth centuries two schools of physiologists arose, one seeking to
put the subject on a chemical foundation, the other to base it on physics.
Johannes Miller (1801-1858), a German, for the first time brought to
bear on physiology not only the facts of chemistry and physics but also

HISTORY OF ZOOLOGY 583

those of human and comparative anatomy. For that reason he is usually
credited with having founded modern physiology.

630. Cell Theory.—This has been reviewed in one of the early
chapters (Sec. 43) in which was described the work of Hooke, Dujardin
(1801-1860), Von Mohl (1805-1872), Schleiden (1804-1881), Schwann
(1810-1882), and Schultze (1825-1874).

631. Embryology.—It has already been stated that Aristotle studied
the embryology of the chick and its gradual development. Harvey
analyzed development in a critical manner, and Malpighi gave a very
complete description of the development of the hen’s egg. Just before
the time of Caspar Wolff (1733-1794), however, a theory of preformation
had become dominant; this theory was to the effect that existent in the
egg was a miniature adult which needed only the stimulation of the sperm
cell to develop. Others thought that this homunculus existed in the
sperm cell and that the egg cell was simply a bed of nourishment for it.
The proponents of this theory were driven logically to the assumption
that succeeding generations were also represented in some way one within
the other. Wolff successfully championed the antagonistic theory of
epigenesis, which was that the structures of the adult were gradually
developed from an egg cell and that each generation began anew with
another such cell. The modern science of embryology, however, dates
from the time of Von Baer (1792-1876), who established the facts in
regard to the germ layers and put the subject on a comparative basis.
He was the author of the biogenetic law, which has also been called Von
Baer’s law.

632. Taxonomy.—The names of those most prominently connected
with the development of taxonomy have been given in the preceding
chapter and will not be repeated here.

633. Evolution and Genetics.—Theories of spontaneous generation
have been treated earlier in this text, where the work of Redi (1626-1694),
Pasteur (1822-1895), and Tyndall (1820-1893) was discussed. The
steps in the development of the evolutionary conception have been taken
up in a separate chapter (Chap. LX XII), and statements made there
need not be repeated. The greatest name, and the one which marks the
beginning of precise knowledge in the field of genetics, is that of Mendel.

634. Pasteur.—Louis Pasteur (Fig. 399) was a chemist who made
noteworthy contributions in the fields of microbiology and preventive
and curative medicine. He pursued extensive investigations on fermen-
tation and discovered that the bacteria in milk could be killed by raising
it to a temperature much below boiling point and keeping it there for a
time, a process now known as pasteurization. He discovered the micro-
organism causing a disease in silkworms known as pébrine and thus
saved the silk industry in France when the silkworm was threatened
with extinction by this disease. He also discovered a method of inocula-

584 GENERAL CONSIDERATIONS

tion which prevents the development of hydrophobia if used promptly
after the bite of a rabid dog. This followed previous similar work on
anthrax and chicken cholera.

635. Recent Advances.—No attempt will be made to follow the
course of recent investigations in any of the fields of zoology, though

Fic. 399.—Louis Pasteur, 1822-1895. (From Shull, ‘‘Principles of Animal Biology,”
after Garrison, ‘‘ History of Medicine,” and by the courtesy of McGraw-Hill Book Company,
Inc.)

since the beginning of the present century tremendous advances have
been made and knowledge is now so extensive and progress so rapid
that only the specialist in any field can hope to keep fully abreast with the
discoveries in that field.

GLOSSARY

The words included are selected from those occurring in the text and are those of
more general significance or most likely to be mispronounced. Some of the more
frequent roots from Greek and Latin are inserted. The names of all persons referred
to in the text—and these only—are also included. Pronunciation and definitions,
generally, are based upon the last edition of Webster’s International Dictionary (1933).

The vowel sounds are indicated by the following symbols:

4 as in may 6 as in dot
4 as in mat 6 as in for

A as in care 6 as in obey
4 as in far 6=o+e
a as in sofa 00 as in boot
é as in be 00 as in foot
é as in met Ti as in mute
é as in fertile ti as in but
é as in event 0 as in fur

I as in line ti as in unite
i as in bin u=ucte
6 as in old

Certain consonant sounds are indicated in the following manner:

soittc =s soitg =j
harde =k hard th = +h
hard ch = k nm = asn in anger

The word Greek is abbreviated to G., and Latin to L., in indicating roots from those
languages. Roots used only as prefixes are followed by a hyphen and those only as
suffixes preceded by one.

A-, or an- (4, in). G.; not, without.

ab- (Ab). L.; away from.

abdomen (ib dd’ mén). The posterior region of the body proper with the axis hori-
zontal, the lower, with the axis vertical; adj., abdominal (ab dém’ 1 nal).

abiogenesis (ib 16 jén’ ésis). The conception that living organisms can arise from
nonliving matter at any time when favorable conditions exist: the same as
spontaneous generation.

aboral (4b 0’ rail). Opposite the mouth.

absorption (ib sérp’ shtin). The entrance of substances in solution into the body of
an organism.

acantho (a kan’ thd). G.; thorn.

accommodation (4 kém 6 da’ shiin). The adjustment of the eye to distinct vision at
different distances.

acetabulum (is étib’ iliim). The socket on each side of the pelvis which receives
the head of the femur.

achromatic figure (4k rd mit’ tk). All of the mitotic figure except the chromosomes.

acontium (i kn’ sh{tim). Threadlike defensive organ composed largely of nemato-
cysts, thrown out of mouth or special pores of a sea anemone.

acquired character. A character in an organism developed by the action of the
environment upon the somatic cells.

actin (Ak’ tin). G.; ray.

585

586 GLOSSARY

action pattern. The structural and functional relationships in an organism which
determine the character of its behavior.

ad- (fd). L.; to, toward.

adaptation (id Ap ta’ shitin). The fitness of an organism for a certain environment;
the process of adjustment involved; or a characteristic which so adjusts an
animal.

adductor (4 dtik’ tér). A muscle acting so as to draw a part toward a median line.

adipose (id’1pds). Pertaining to fat.

adjustor neuron. A neuron in a nerve center by which an impuise is passed from the
receptor neuron to the effector cell in a reflex act.

adolescence (Ad 6 lés’ éns). The period of life between childhood and maturity.

adrenal (id ré’ nal). A ductless gland near the kidney; in man, often called supra-
renal; the hormone produced is adrenalin (ad rén’ 4 lin).

aeri (4’ ér1). L.; air. aero (a’érd). G.; air.

afferent (Af’ ér ént). Incoming, toward a center.

ala (a’ 14). L.; a wing.

albinism (XI/ bi niz’m). Lack of pigment when normally it is present; in higher
vertebrates shows itself in the lack of color in skin, hair or feathers, and iris.

alimentary (Al{mén/ tari). Pertaining to food.

allantois (4 lin’ t6 1s). An embryonic membrane in land vertebrates, primarily for
respiration; adj., allantoic (& lan to’ tk).

allelomorphs (4 1é’16 mérfs). A pair of corresponding genes in homologous chromo-
somes; but each produces a different character.

allergy (Al’ ér ji). In a broad sense, a modification of the reaction of the body to a
substance after having been once subjected to the action of that substance,
including both immunity and anaphylaxis; in a narrow sense, natural acute
sensitiveness to a substance which does not act like a poison by causing the
development of an antitoxin.

alternation of generations. Metagenesis.

altricial (Al trish’ al). Pertaining to birds hatched in a naked, blind, and weak
condition.

alveolar (Al vé’ 6 lar). Pertaining to an alveolus, which is a small cavity, or pit.

ambergris (im’ bér grés). A substance produced in the stomach of a sperm whale
and used in the manufacture of perfumes.

ambulacral (Am bila’ kril). Pertaining to the tube feet of an echinoderm.

ameboid (a4 mé’ boid). Like an ameba; putting out pseudopodia.

amitosis (im1td’ sls). Direct nuclear division, neither spindle nor asters being
formed.

amnion (Am/ni{én). An embryonic membrane in land vertebrates serving for the
protection of the embryo; adj., amniotic (im nif dt’ fk).

amphi- (im/’ fi). G.; on both sides, of both kinds.

amphiaster (im’ ff 4s tér). The mitotic figure at its fullest development.

amphibious (im fib’ itis). Capable of living both in water and outside it.

amphimixis (Am fi mik’ sis). The union of a sperm cell and an egg cell.

ampulla (im pil’ 4). A flasklike dilatation.

amylopsin (im 1lép’sin). An enzyme produced by the pancreas, which changes
starches to sugars.

ana- (in’ a). G.; up, back, or again.

anabolism (An Ab’ 6liz’m). The building-up steps in metabolism, from ingestion to
assimilation, inclusive.

analogy (anal’ 6 j1). Resemblance involving function but not structural plan.

anaphase (An’ afaz). The phase in mitosis during which the chromosomes migrate
from the equator of the spindle to the poles.

GLOSSARY 587

anaphylaxis (in afi lk’ sis). The acute susceptibility of the body to an albuminous
substance which acts as a poison; or increased susceptibility to a toxin after
recovery from poisoning by that toxin.

Anaximander (in 4k si man’ dér). Greek philosopher; 611-547 B.c.

animal pole. That pole of an egg cell toward which the protoplasm is accumulated.

ante- (in’ té). L.; before.

antenna (in tén’ 4). A sensory appendage on the head of an arthropod, the functions
of which do not include sight; adj., antennal.

anterior (iin té’ riér). In front; in a bilaterally symmetrical animal with the axis
horizontal, toward the head.

anthropoid (4n’ thré poid). Manlike.

anti- (4n’ ti). G.; against.

antimere (in’ timér). One of the similar parts into’ which a radially symmetrical
animal may be divided.

antitoxin (An ti tok’ sin). A substance in the body which neutralizes a poison, or toxin.

aorta (4 or’ ta). A large artery arising from the heart of vertebrates, the blood in
which is sent over the body; branches which pass around the pharynx and reunite
to form a dorsal aorta are called aortic arches.

appendage (4 pén’ daj). A projecting part on a metazoan body that is movable and
has an active function.

Aquinas (4 kwi’ nas), Thomas. Italian scholastic; 1225?-1274?

arboreal (fr bo’ ré Al). Pertaining to life in trees.

arch- (irk). G.; first, beginning.

archenteron (ir kén’ tér6n). The gastrular cavity in metazoan embryos which
serves as a primitive digestive cavity; adj., archenteric (dr kén tér’ ik).

Aristotle (Ar’ is tdt’1). Greek philosopher; 384-322 B. c.

artery (iir’ téri). A vessel carrying blood away from the heart; adj., arterial
(ar te’ ri 41).

arthr (ar’ thr). G.; joint.

asexual (a4 sék’ sht 41). Not related to sex, or having no sex.

assimilation (4 sim ila’ shtin). The incorporation of food into the living protoplasm;
adj., assimilative (a sim’ f la tiv).

associative memory. Memory involving the relating of previous experiences.

aster (is’ tér). G.;star. A term applied to the rays which surround the centrosomes
in a mitotic figure.

asymmetry (asim’ étri). Lack of symmetry.

atoll (&t’ ol). A circular coral island inclosing a lagoon.

atriopore (a’ trid por). The external opening of an atrial cavity in one of the lower
chordates.

atrium (4’ trittim). A cavity on the outside of the body of lower chordates, also
called atrial cavity; an auricle in the heart; adj., atrial.

Augustine, Saint (sant 6 gis’ tin). Bishop of Hippo; 354—430.

auricle (6’ rf k’l). A chamber in the heart which receives blood, also called atrium;
an earlike projection on the side of the head; adj., auricular (6 rik’ t lar).

auto (6’ td). G.; self.

autonomic (6 td n6m’ ik). Having powers of its own; applied to the sympathetic
nervous system.

autosome (6’ td som). Any chromosome except a sex chromosome.

autotomy (6 tot’é mi). The automatic breaking off of a part by an animal.

axolotl (ik’ sé 16t’1). A salamander larva which has acquired sexual maturity, or a
sexually mature salamander which preserves the larval form.

axon (ik’sdn). A nerve fiber which conducts impulses away from the body of the
cell of which it is a part.

588 GLOSSARY

Baker, Frank C. American zoologist, at University of Illinois; 1867—.

basement membrane. A thin connective tissue membrane to which epithelial cells
are attached.

behavior. The sum total of an animal’s movements in response to changing environ-
mental conditions or to changes within the organism.

benthos (bén’ this). The life of the sea bottom, especially of the deep seas; it does
not properly apply to the littoral fauna of shallow water; adj., benthal.

bi- (bi). L.; two, or twice.

Bichat (bé shi’), Marie F. X. French anatomist and physiologist; 1771-1802.

bilateral symmetry. Symmetry which involves the possibility of dividing a body
only into two parts, which are mirror images of each other.

bile. The secretion of the liver in vertebrates.

binary (bi’ nari) fission. The division of an organism into two similar organisms.

binomial (bi nd’ mi 41). Consisting of two names; in the accepted nomenclature these
are the names of the genus and the species.

bio (bi’ 56). G.; life, related to life.

biogenesis (bid jén’ésis). The conception that since the beginning of life living
things have arisen only from preexisting living things.

biogenetic (bi 6 jé nét’ ik) law. The doctrine that in the embryogeny of any higher
animal there appear stages or conditions similar to those which were present in
the adults of lower types ancestral to it.

biota (bi 0’ ta). A collective term for the animals and plants of a given area; adj.,
biotic (bi dt’ ik).

biparental. Involving two parents.

biradial symmetry. A combination of radial and bilateral symmetry.

biramous (bira’ mtis). Having two branches.

bivalent (bi va’ lént). Having the value of two.

bladder. A membranous sac; the air bladder contains gases, the gall bladder serves
for the temporary storage of bile, and the urinary bladder serves for the accumu-
lation of urine.

blast (blist). G.; germ, relating to the early stages of the embryo.

blastocoel (blis’ to sel). The cavity of a blastula; the cleavage, or segmentation,
cavity.

blastoderm (blis’ t6 darm). The sheet of cells surrounding the cleavage cavity ina
blastula; adj., blastodermic (blis t6 dir’ mik) or blastodermal.

blastomere (blis’ t6 mér). A cell of the blastoderm.

blastopore. The opening into the archenteron.

blastula (blis’ tila). An embryo in the stage when it is composed of a blastoderm
inclosing a blastocoel, like a hollow ball of cells; adj., blastular.

blepharoplast (bléf’ 4rd plist). A granule in a cell from which a flagellum or cilium
arises.

brachio (brik’16). G.; arm.

brachy (brik’¥). G.; short.

branchi (brin’ ki). L.; gill. Branchial (brin’ ki al). Pertaining to gills.

breathing. The passage of air into and out of a cavity in an animal body leading to
the interchange of gases through the wall of the cavity, an interchange which is
respiration.

bronchus (brin’ kiis). An air passage within a lung.

Brown, Robert. Scottish botanist; 1773-1858.

Bruner (broo’ nér), Lawrence. American entomologist; 1856-1937.

buccal (bik’ 41). Pertaining to the mouth.

budding. The production of a young animal from the body of a parent and its
separation, while still small, from the parent, after which it is able to live inde-
pendently.

GLOSSARY 589

Buffon, de (dé bii f6n’), Georges L. L. French naturalist; 1707-1788.
byssus (bis’ tis). A tough thread or a group of threads attaching a mussel to a solid
object.

Caecum (sé’ ktm). A blind outpocketing of the intestine; adj., caecal.

calc (kAlk). L.; lime. Calcareous (kal ka’ ré tis). Limy.

capillary (kip’{la ri). A fine tube, such as a blood or bile capillary.

carapace (kir’a pas). A hard dorsal covering of a body, such as that of a turtle or a
crayfish.

carbohydrates (kir bé hi’ drats). Organic compounds, including starches, sugars,
and cellulose, composed of carbon, hydrogen, and oxygen, the number of atoms
of hydrogen and oxygen being usually as 2:1.

cardio (kir’ di6). G.; heart. Cardiac. Pertaining to the heart.

carnivorous (kirniv’ ériis). Flesh-eating; a flesh-eating animal is known as a
carnivore (kir’ ni vor).

carpus (kiir’ ptis). The wrist bones; adj., carpal.

Castle, William E. American zoologist, at Harvard University; 1867-.

catalysis (ka til’ isis). The acceleration of a chemical reaction by a substance—an
enzyme—known as a catalyst or catalyzer, which is not changed in the process;
adj., catalytic (kAt 4 lft’ ik).

caudal. Pertaining to the tail.

cell. A mass of protoplasm containing a nucleus; cells may exist as individual organ-
isms or may be parts of many-celled organisms.

cell membrane. A protoplasmic layer surrounding a cell; the plasma membrane.

cell theory. The theory that all plants and animals are composed of cells; now so
well-established as to have become the cell doctrine.

cellulose (sél’ ilos). A carbohydrate which usually makes up the walls of plant
cells; found in animals only in the tunicates.

cell wall. A nonliving wall about a cell, secreted by the cell itself.

central body. A clear mass in a cell, near the nucleus, containing one or two cen-
trioles; also called centrosome, centrosphere, or attraction sphere.

centralization. The development of a central nervous system.

central nervous system. That part of the nervous system which receives afferent
impulses and sends out efferent ones; it may consist of a pair of ganglia and two
ganglionic nerve cords, as in the planarian; a chain of connected ganglia, as in
the annelids and arthropods; or the brain and spinal cord of vertebrates.

centriole (sén’ tri dl). A minute granule contained in the central body or seen at
the center of the aster in mitosis; also called centrosome.

centrolecithal (stn tré lés’ 1 thal). Adjective applied to an egg cell with the yolk
massed toward the center.

centrosome (sén’ tr6sdm). A name which has been used in different senses; has been
applied to both the central body and centriole; adj., centrosomal (sén tré sd’ mal).

cephalo (séf’ als). G.; head. Cephalic. Pertaining to head.

cephalization (séf 4l{ za’ shin). The development of a head containing a brain.

cephalothorax (séf 416 thd’ riks). A part of the body composed of the fused head
and thorax.

cercaria (stir ka’ ria). A stage in the development of a fluke.

cerebellum (str é bél’ tim). The fourth part of the brain in vertebrates; contains the
centers for muscular coordination; adj., cerebellar.

cerebrum (sér’ ébriim). The anterior division of the brain in vertebrates; in higher
forms it is the center of intelligence and reason; adj., cerebral, which also may be
used in reference to any brain.

cervical (sfir’ vi kil). Pertaining to the neck.

590 GLOSSARY

chaet (két). G.; bristle. Chaeta (ké’ ta). A bristle set in the body wall and form-
ing the locomotor organ in many annelids.

character. A heritable and visible characteristic in an animal connected with the
presence or absence of a gene, or the interaction of two or more genes.

chelicera (ké lis’ ér 4). One of the anterior pair of appendages of an arachnid.

cheliped (ké’ li péd). The first walking leg of a crayfish.

chemotropism (ké mot’ ré piz’m). The response by an organism to chemical stimu-
lation; adj., chemotropic (kém 6 trép’ ik).

Child, Charles M. American zoologist, at University of Chieago; 1869—.

chitin (ki’ tin). A substance that gives hardness to the cuticula of many invertebrates.

chlorophyll (kld’ ré fil). The green coloring matter contained in the chloroplasts of
cells and carrying on photosynthesis.

chondrin (k6n’ drin). The substance which forms cartilage.

chord (kérd). G.; a string.

chordate (kér’ dit). An animal belonging to the phylum Chordata and possessing a
notochord at some time during its life.

chorion (k6’ ri dn). The outer embryonic membrane of land vertebrates; the same
name is also applied to the shells of insect eggs; adj., chorionic (k6 ri 6n’ ik).

choroid (k6’ roid). The vascular middle coat of the vertebrate eye; adj., choroidal
(k6 roi’ dal).

chromatin (kro’ matin). The deeply staining substance in the nucleus of cells; adj.,
chromatic (kré mAt’ ik).

chromatophore (kr6o’ ma t6 for). A cell containing pigment.

chromidia (kré mid’ a). Masses of chromatin scattered through the cytoplasm of a
protozoan cell which represent a ‘distributed’ nucleus.

chromomere (krd’ m6 mér). An individual chromatin granule in a chromosome.

chromosome (kro’modsdm). A characteristic mass of chromatin developed from
the chromatin network in a nucleus during mitosis; adj., chromosomal
(kr6 m6 sd’ mil).

chyle (kil). That part of the food, consisting largely of emulsified fats, which is
absorbed through the walls of the small intestine and collected by the lymphatic
vessels.

chyme (kim). Food reduced to liquid form.

cilium (sil’{ tim). One of many minute, hairlike, vibratile structures on the surface
of a cell; also the eyelashes; adj. ciliary, referring to either of the above or to many
structures in the vertebrate eye.

cirrus (sir’ rtis). A slender, usually flexible, and often branched appendage having
one of many functions, such as copulation in trematodes and mollusks, respiration
in some annelids, and touch in many animals; also one of the legs of barnacles.

cleavage. The division of the egg cell and of the blastomeres in the early stages of
embryogeny.

Cleveland, Lemuel R. American parasitologist, at Harvard Medical School; 1892-.

cloaca (klé 4’ ka). A common passageway to the outside of the body for the digestive,
excretory, and reproductive systems; adj., cloacal.

cnidoblast (ni’ dé blast). A cell in a coelenterate in which develops a nematocyst.

cocoon (ké kdon’). A protective covering for a larva, a pupa, a mass of eggs, or even
one formed about an adult animal.

coel (sél). G.; a hollow chamber.

coelom (sé’lém). A body cavity inclosed by mesoderm; in embryogeny the gonads
arise from its wall and into it the inner ends of the excretory ducts open; adj.,
coelomic (sé lém’ ik).

colloid (kél’ oid). A substance of gluelike or jelly-like consistency which in solution,
or suspension, will not pass through an organic membrane; adj., colloidal
(k6 loi’ dal).

GLOSSARY 591

colony. An association of individuals descended from a common ancestor and
remaining in organic connection.

com- (kom). L.; with, together.

commensalism (k6 mén’ sil iz’m). An association of two animals of different species
in which one is benefited and the other not affected.

commissure (kém’ishdor). A bundle of nerve fibers connecting two nerve centers
and crossing the median line.

Comstock (kiim’ stdk), John H. American entomologist; 1849-1931.

conditioned reflex. A reflex act which is modified by impulses arising in a “‘higher’”’
nerve center, one which dominates the center in which the adjustor neurons
involved in the reflex are are located.

conductivity. The property of protoplasm which enables it to transmit the effect of a
stimulus from one point to another.

congenital (k6én jén’{ tal). Existing at birth.

conjugation (k6n joo ga’ shitin). A temporary union of two cells resulting in an
exchange of material.

connective. A bundle of nerve fibers uniting two ganglia on the same side of the body.

contractile vacuole (kon trik’ til vik’ 161). A vacuole in a protozoan which alter-
nately fills and empties.

convergence (k6n vir’ jéns). Resemblance between two unrelated types because of
living under similar conditions.

copulation (kép i 14’ shiin). Temporary union of a male and female animal involving
the passage of sperm cells from the former to the latter.

corium (k6’ ritim). The dermis, or connective tissue layer, in the skin of vertebrates.

corn (kérn). L.; horn.

cornea (kér’ né a). A sheet of transparent epithelial tissue admitting light to an eye;
adj., corneal.

corp (kérp). .; body.

cortex (kér’ téks). An outer layer; the outer dorsal layer of the cerebrum and
cerebellum of higher vertebrates when it is organized into layers and areas; adj.,
cortical.

cranium (kra’nitim). The cartilaginous or bony box surrounding the brain of
vertebrates; adj., cranial.

crop. An enlargement of an anterior part of the alimentary canal serving for the
temporary storage of food.

cross-fertilization. The fertilization of the egg cell of one animal by the sperm cell
of another.

crossing over. The transfer of genes from one chromosome to another during
synapsis.

cutaneous (ki ta’ né tis). Pertaining to the skin.

cuticle (kii’ tik’l).. The epidermal layer of the skin of a vertebrate; also the firm
outer layer of a protozoan; sometimes used in the same sense as cuticula; adj.,
cuticular (ki tik’ t lar).

cuticula (ki tik’ ila). A hard covering on an animal body secreted by the epi-
thelium.

Cuvier (kii vya’), Georges L.C. F.D. French naturalist; 1769-1832.

cyclosis (si kl0’ sis). A continued regular movement of endoplasm within a cell.

cyst (sist). A hard covering secreted around a mass of living matter; adj., cystic.

cysticercus (sis tistr’ kts). The encysted stage in the development of a
tapeworm.

cyt (sit). G.; a hollow vessel; in biology, a cell.

cytoplasm (si’ td pliz’m). The protoplasm of a cell outside the nucleus; adj., cyto-
plasmic (si t6é pliz’ mik).

592 GLOSSARY

Dactyl (dik’ til). G.; finger.

Darwin, Charles R. English naturalist; 1809-1882.

Darwin, Erasmus. English physiologist; 1731-1802.

de- (dé). L.; down, from, off.

degeneration (dé jén ér 4’ shtin). The simplification or loss of structure or function
in an animal organism.

delamination (dé lim ina’ shtn). The splitting off of a layer of cells.

dendrite (dén’ drit). A branched fiber from a nerve cell which carries impulses
toward the cell body; same as dendron; adj., dendritic (dém drit’ ik).

dent (dént). L.; tooth.

derm (dirm). G.; skin.

dermis (dir’ mis). The inner connective tissue layer of the vertebrate skin; adj.,
dermal.

desiccation (dés1ka’ shitin). Drying up.

De Vries (dé vrés’), Hugo. Dutch botanist; 1848-1935.

di- (di). G.; two, double.

dia- (di’ 4). G.; through, apart.

dialysis (di Al’ sis). The separation of substances in solution or suspension into
colloids and erystalloids by means of a permeable membrane.

diaphragm (di’ afrim). The partition between the thoracic and abdominal cavities
of a mammal.

diastase (di’ 4 stas). An enzyme which changes starches to sugars.

diecious (di é’ shtis). A species of animal in which the male and female organs are
in different individuals.

diencephalon (di ¢n stf’ alin). The second region of the vertebrate brain, including
the optic thalami, optic tracts, and pineal and pituitary bodies; also called
thalamencephalon.

differentiation. The process by which parts differing in structure and function are
produced from a whole which originally was all alike.

digestion (di jés’ cht). The changing of food to a liquid and absorbable form;
adj., digestive.

dihybrid (di hi’ brid). The offspring of parents differing by two characters.

dimorphism (di mor’ fiz’m). The appearance of a species of animal in two forms;
adj., dimorphic.

diplo (dip’ 16). G.; double.

diploblastic (dip 16 blas’ tik). Composed of two germ layers.

diploid (dip’ loid). A term applied to the full number of chromosomes found in
somatic cells.

discoidal cleavage. Cleavage in which the blastomeres form a disc at the animal
pole of the egg cell.

dissimilation (di s{m 114’ shin). The breaking-down processes in a cell by means
of which chemical compounds are reduced to simpler form and energy is set free;
adj., dissimilative (di sim’ I la tiv).

divergence (di vir’ jéns). The acquirement of different characteristics by related
forms when placed under different environmental conditions.

division of labor. The distribution of specific functions to separate parts.

dominance (dém’{nins). The suppression of one character by a contrasted charac-
ter when the genes which correspond to the two are both present; also the exercise
of control by one part of the body over another.

Dujardin (dii zhar din’), Félix. French zoologist; 1801-1860.

duodenum (di 6 dé’ ntim). The first part of the small intestine in mammals; adj.,
duodenal (di 6 dé’ nil).

Durant (di rint’), William J. American author; 1885-.

dyad. A chromosomal mass formed by the division of a tetrad.

GLOSSARY 593

Ebers (4’ bérs), Georg M. German Egyptologist; 1837-1898.

ecdysis (&k di’ sis). The shedding of the cuticula of an arthropod or of the horny
scales of a snake or lizard.

economic (é k6 ndm’ ik). Pertaining to utilitarian values, either positive or negative.

ect- (ékt). G.; without, outside.

ectoderm (&k’ tédtirm). The outer germ layer in embryogeny; adj., ectodermal
(ék té dir’ mal).

ectoplasm (&k’ t6 pliz’m). The outer part of the cytoplasm of a protozoan; adj.,
ectoplasmic (&k t6 pliz’ mik).

effector (éf fk’ tér). A term applied to either a cell or an organ that completes a
reflex act.

efferent (éf’ fér ént). Outgoing, from a center.

egestion (é jés’ chtin). The passing out of feces.

egg. An egg cell with its protective coverings.

egg cell. The reproductive cell of a female.

electrotropism (élék trot’ ré piz’m). Response to an electrical stimulus; adj.,
electrotropic (é lék tro trop’ Ik).

elimination (é lim ina’ shiin). The passage out of the body of liquid waste; adj.,
eliminative (é lim’ I na tiv).

embryo (ém’ brid). An animal in the early stages of its development; adj., embry-
onic (ém bri dn’ tk).

embryogeny (ém bri 6j’énf). The stages in the embryonic development of an
animal.

Empedocles (ém péd’ 6 kléz). Greek philosopher; about 500-430 B.c.

en- (én). G.; in.

encephalon (én séf’aldn). The brain of a vertebrate.

encystment (én sist’mént). The formation of a hard covering, or cyst, about an
organism.

endo- (&én’ dé), or ento- (én’ td). G.; within.

endocrine (én’ dé crin). Pertaining to internal secretions or ductless glands.

endomixis (én dé mik’ sis). A process of reorganization of the nuclear material in a
protozoan cell.

endoplasm (én’ dé pliz’m). The inner portion of the cytoplasm of a protozoan cell;
adj., endoplasmic (én dé pliiz’ mik).

endoskeleton (én dé skél’ é tiin). An internal skeleton; adj., endoskeletal.

endothelium (én dé thé’ li tim). An epithelium of mesodermal origin lining coelomic
cavities, serous spaces, and blood and lymph vessels; adj., endothelial.

enteron (én’ tér én). A digestive cavity; adj., enteric (én tér’ ik).

entoderm (én’ t6dirm). The inner germ layer; also called endoderm; adj. ento-
dermal (én t6 dtr’ mAl).

entozoic (én t6 z0’ ik). Living within an animal body.

enzyme (én’ zim). A catalytic ferment.

epi- (ép’ 1). G.; upon.

epibole (ép fb’61é). The growth of a fold of the blastoderm over the surface of the
embryo, leaving an archenteron between the two.

epidermis (@p 1 dtr’ mis). The outer epithelial layer of the skin of vertebrates; adj.
epidermal (&p i dtr’ mAl).

epigenesis (&p I jén’ ésis). The conception that the parts of an organism arise from
an undifferentiated germ cell, in contrast to the idea of preformation.

epithelium (ép{thé’ litim). A tissue covering a free surface; adj., epithelial.

epizoic (ép 1 z0’ Ik). Living upon the surface of an animal.

equatorial (6 kwa td’ ri Al) plate. The group of chromosomes lying in the equatorial
plane of the spindle in mitosis.

594 GLOSSARY

esophagus (éséf’Agtis). That part of the alimentary canal lying between the
pharynx and stomach; adj., esophageal (é so faj’ é al).

estivation (és ti va’ shtin). The assumption of a dormant condition by an animal
during summer.

eu- (i). G.; true.

eustachian (ti sta’ ki in) tube. A tube connecting the pharynx and middle ear.

evagination (é viij 1na’ shin). The protrusion by eversion of an internal surface.

evolution (év 61t’ shtin). Progressive development; adj., evolutionary.

ex- (éks). G.; out of, off from, without.

excretion (tks kré’ shtin). The passing out from a cell of a substance produced in it,
this substance being waste to the cell and of no value to the body of which it is
a part; also the substance itself; excretory (éks’ kré t6 ri), pertaining to excretion.

exhalation (ks hala’ shtin). The passage of gases outward from the lungs.

exoskeleton (ék sé skél’ é tin). An outer skeleton; adj., exoskeletal.

expiration (&k spi ra’ shtin). The escape of carbon dioxide from the organism; or its
passage from the tissues into the blood.

exteroceptor (éks tir 6 stp’ tir). Sense organs for the reception of external stimuli.

extra- (éks’-tra). L.; beyond, outside.

eye. An organ for vision.

eyespot. A mass of pigment or a pigment-containing organ for the perception of
light rays but not giving vision.

F,, F2, F;, etc. Abbreviations used in genetics and referring to successive filial
generations.

family. Parents and their offspring; also a taxonomic group ranking between genus
and order.

fat. An organic compound made up of a fatty acid and glycerin and containing
carbon, hydrogen, and oxygen, with a relatively small number of oxygen atoms.

fathom (fith’ im). A nautical term of a measure of length containing 6 feet.

fauna (f6’ na). A collective term for the animals of a certain region; adj., faunal.

feces (f@’ séz). The indigestible portion of the food which is passed through the
alimentary canal and out of the body; adj., fecal.

-fer (fér). L.; bearer, carrier.

fertilization (ffir ti li za’ shin). The union of an egg cell and a sperm cell.

fetus (fé’ ttis). A young mammal after about one third of the time from the beginning
of development to birth has elapsed; in the case of man, after the end of the third
month; adj., fetal.

fibril (fi’ bril). A slender fiber; or a component part of a cross-striated muscle fiber.

fibrin (fi’ brin). The fibrous material in a blood clot; derived from the fibrinogen in
the blood.

fil (ffl). L.; a thread.

fission (fish’ tn). The division of an organism into parts approximately equal in
size, each part being a young organism.

flagellum (fla jél/itim). A long whiplike structure attached to a cell; adj., flagellar.

flame cell. A hollow cell containing a mass of vibratile cilia which carries on excretion
in planarians and rotifers.

fossorial (f6 sd’ ri Al). Fitted for digging.

fragmentation. An asexual method of reproduction in lower Metazoa in which the
body divides into a number of parts each of which becomes an individual.

fraternal twins. Twins produced from two egg cells.

Galen (ga’ lén), Claudius. Greek physician; 130-200?
Galilei (gi lé 14’ é), Galileo. Italian astronomer; 1564-1642.
gam (gim). G., marriage. Gamet (gim’ét). G., husband or wife.

GLOSSARY 595

gamete (gim’ ét). A germ cell; adj., gametic (ga mét’ ik).

gametogenesis (gim é td jén’ ésis). The stages in the development of a germ cell, or
reproductive cell.

ganglion (gin’ glién). A circumscribed group of nerve-cell bodies; adj., ganglionic
(gin gli on’ ik).

gastr (gis’t’r). G.; stomach. Gastral. Stomach-like. Gastric. Pertaining to
the stomach.

gastrovascular (gis tré vis’ ki lar) cavity. A cavity in the lower Metazoa which
serves for both digestion and distribution of food.

gastrula (gis’ troo la). A stage in embryogeny when the embryo has two layers and
contains a cavity called the archenteron; adj., gastrular.

gastrulation (gis troo 1a’ shtn). The development of a gastrula from a blastula.

gemmation (jém 4’ shitin). Reproduction by budding.

gemmule (jém’ il). An internal bud of a fresh-water sponge, composed of cells
separated internally and inclosed in a cyst, which lives over winter and develops
in the spring.

gen (jén). G.; producing, causing; gener (jén’ ér), born.

gene (jén). That in the chromatin of a germ cell which by interaction with other
genes and the cytoplasm determines the production of a character in the indi-
vidual developed from the cell; the unit of inheritance.

gener (jén’ ér). L.; race, kind.

genotype (jén’ dé tip). An organism considered with reference to its total genetic
constitution; adj., genotypic (jén 6 tip’ ik).

geotropism (jé ot’ ré piz’m). The response of an organism to the force of gravity;
adj., geotropic (jé 6 trdp’ ik).

germ cell. A cell concerned with reproduction; adj., germinal (j(ir’ mi nil).

germ layers. Cell layers formed early in embryogeny from which the definitive
tissues are developed.

germplasm (jirm’ pliz’m). The portion of the protoplasm of a germ cell which
transmits the hereditary characters.

gill. An organ for respiration under water.

gizzard. A portion of the alimentary canal in annelids and birds the function of
which is to grind the food.

gland. An organ which produces a secretion; adj., glandular (gliin’ di lar).

glochidium (glé kid’itim). The larva of a fresh-water mussel.

glomerulus (glé mér’ 00 lis). A mass of tissue containing a knot of blood capillaries
inclosed by the end of a kidney tubule of the mesonephroi and metanephroi of
vertebrates.

glottis (glét’ is). A slitlike opening from the pharynx into the larynx or trachea in
the higher vertebrates.

glycogen (gli’k6 jén). The form in which carbohydrate food is stored in the liver
and other tissues; also called animal starch.

gnath (nith). G.; jaw.

gon (gon). G.; seed, referring to reproduction.

gonad (gon’ id). An organ which produces egg cells or sperm cells.

Grassi (griis’ €), Giovanni B. Italian zoologist; 1854-1925.

gregariousness (gré ga’ ri tisnés). As used in this text, the tendency of animals of
different species to gather together.

gustatory (gtis’ ta t6 ri). Pertaining to the sense of taste.

Habit. A mode of action, or an act which when first performed has no relation to
an action pattern, but in connection with the continued repetition of which an
action pattern is developed, and which comes to control the animal in the same
manner as does an instinct.

596 GLOSSARY

habitat (hib’1t&t). The area in which a species of animal or a group of animals
lives.

Haeckel (hék’ él), Ernst H. German biologist; 1834-1919.

haplo (hip’ 16). G.; simple.

haploid (hip’ loid). The reduced number of chromosomes.

Harvey, William. English anatomist and physician; 1578-1657.

head. An anterior region of the body containing the dominant part of the nervous
system and the chief sense organs.

Helmholtz, von, Hermann L. F. German physicist; 1821-1894.

hem, haem (hém). G.; blood.

hemi- (hém’1). G.; half.

hemocoel (hé’ m6 sél). Coelom-like spaces devoted to the circulation of the blood.

hemoglobin (hé m6 glo’ bin). A protein found in the blood which by combining with
oxygen increases the amount of the gas which can be distributed over the body.

hemolysis (hé mol’ {sis). Solution of the red blood corpuscles.

hepatic (hé pit’ ik). Pertaining to the liver.

herbivorous (hér biv’ ériis). Plant-eating; a plant-eating animal is known as a
herbivore (hir’ bi vor).

hermaphrodite (hér mif’ ré dit). An animal containing both male and female
gonads; adj., hermaphroditic (hér mAaf ré dit’ ik).

Herodotus (hé réd’ 6 ttis). Greek historian; 484?-425 B. c.

hetero (hét’ ér 6). G.; other, different.

heteronomous (hét ér dn’ émtis) metamerism. A type of metamerism involving
unlike metameres.

heterosis (hét ér 6’ sis). Increased vigor due to hybridity.

heterozygote (hét ér 6 zi’ got). An organism in which two corresponding genes or
characters are unlike; adj., heterozygous.

hibernation (hi bir na’ shtn). Dormancy during winter.

Hippocrates (hi pok’ ri téz). Greek physician; 460-377 B. c.

holo (hdl’ 6). G.; whole.

holoblastic (hdl 6 bls’ tik). A term applied to egg cells possessing total cleavage,
involving the whole cell.

holophytic (hdl 6 fit’ fk). Plantlike as to type of nutrition, involving the utilization of
inorganic substances and the carrying on of photosynthesis.

holozoic (hdl 6 zd’ ik). Animal-like as to type of nutrition, involving the ingestion
of organic food.

hom (hém). G.; the same.

homolecithal (hd m6 lés’{ thal). A term applied to an egg cell with the yolk uni-
formly distributed.

homology (hé mil’ 6 ji). Structural similarity due to common origin, both evolu-
tionary and embryonic; adj., homologous (hé mol’ 6 gis).

homonomous (hé mon’ 6 mts) metamerism. A type of metamerism involving like
metameres.

homozygote (hd m6 zi’ got). An organism in which the corresponding genes or
characters are alike; adj., homozygous.

Hooke, Robert. English mathematician and microscopist; 1635-1703.

hormone (hor’ mon). An internal secretion which, carried by the blood from the
organ which produces it, influences other organs or growth processes.

host. An organism that harbors a parasite.

Huxley, Thomas H. English biologist; 1825-1895.

hybrid (hi’ brid). The offspring of parents differing in species or in genetic con-
stitution; the production of such is hybridization.

hydr (hi’d’r). G.; water.

GLOSSARY 597

hydrostatic (hi dré stat’ tk). A term applied to an organ which regulates the specific
gravity of an aquatic animal, like a fish, in relation to that of water.

hyp- (hip). G.; under, less.

hyper- (hi’ pér). G.; above, beyond, over.

hypermetamorphosis (hi pér mé&t 4 mor’ fo sis). A metamorphosis in insects involving
more than the stages of egg, larva, pupa, and adult.

hypodermis (hi pé dir’ mis). A layer of epithelial cells under a superficial cuticula.

hypostome (hi’ p6 stom). A conical projection in coelenterates at the tip of which is
the mouth.

Identical twins. Twins which are alike and which presumably come from a single

egg cell.

ileum (il/étim). The last and longest of three divisions of the small intestine of
mammals.

ilium (il’i tim). The dorsal bone of the pelvis in Amphibia and higher vertebrates;
adj., iliac.

imago ({ma’ go). The adult of insects; adj., imaginal ({ m4j’ 1 nal).

immunity (1 mii’ ni ti). The absence of susceptibility to disease.

inbreeding. The production of young by two closely related individuals.

individuality. That which belongs to one individual organism as distinct from all
others.

ingestion (in jést’ chtin). The taking of food into the digestive cavity and its prepa-
ration for digestion.

inhalation (In ha 1a’ shtin). The taking of air into the lungs.

inhibit (in hib’ it). To check or restrain.

insectivorous (in stk tiv’ értis). Insect-eating; an insect-eating animal is known as
an insectivore (in sék’ ti vor).

inspiration (in sp! ra’ shin). The taking of oxygen into an animal organism; or
its passage from the blood into the tissues.

instar (in’ stiir). A period between molts in an insect larva.

instinct (in’ stinkt). A mode of action determined by an inherited action pattern
which under appropriate conditions and when brought into play by the proper
stimulus causes a series of associated reflex acts leading to a definite end; adj.,
instinctive (in stink’ tiv).

integration (in té gra’ shtin). The development of unity in an organism.

intelligence. A mode of action freely modified by previous experience.

inter- (in’ tér). L.; between, among.

intercellular. Between cells.

internal secretion. A secretion passed into the blood instead of into the lumen of a
gland or out upon a surface.

interoceptor (in ter 6 sép’ tor). A sense organ stimulated by some agent from within
the body.

intestine (In tés’ tin). That part of the alimentary canal in which absorption takes
place; in mammals it is divided into the small and the large intestine; adj.,
intestinal.

intra- (in’ tra). L.; within.

intracellular. Within a cell.

intussusception (in tiis sti sép’ shitin). The introduction of new particles between
those already related in a mass.

invagination (in vij 1nd’ shtn). The infolding of a sheet of cells or membrane to
form a cavity.

ion (i/ {n). A free atom or radical in a solution, which bears an electrical charge.

irritability. The capacity to respond to a stimulus.

598 ' GLOSSARY

Jejunum (jé joo’ ntim). The second region of the small intestine in mammals.
Jennings, Herbert S. American zoologist, at Johns Hopkins University; 1868-.

Kary (kir’ i). G.; a nut; in biology, a nucleus.

karyokinesis (kir 16 kiné’ sis). Mitosis.

karyosome (kir’i6sdm). A nucleolus composed of chromatin.

katabolism (ka tib’6liz’m). The processes in metabolism concerned with the
breaking down of protoplasm and with getting rid of waste; adj., katabolic
(kat a bol’ tk).

Kelvin (kél’ vin), Baron; William Thomson. Scottish physicist; 1824-1907.

keratin (kér’ atin). A nitrogenous substance forming the chemical basis of horn,
hair, nails, feathers, and epidermal scales.

kidney. The organ of elimination in vertebrates.

King, Albert F. A. American physician; 1841-1914.

Labium (la’ bi tim). An exoskeletal mass, composed of several pieces, which forms
the posterior boundary of the mouth in insects; adj., labial.

labrum (la’ brim). An exoskeletal mass which forms the anterior boundary of the
mouth in insects.

lachrymal (lik’ rimil). Pertaining to the tears.

lact (Akt). L.; milk.

lacteal (lik’ té 41). As a noun, a lymph vessel in the wall of the small intestine into
which fats are absorbed; as an adjective, pertaining to milk.

Lamarck, de (dé la mark’), Jean B. P. A. de M. French zoologist; 1744-1829.

lamella (la mél’ 4). A thin layer or plate; adj., lamellar.

larva. The young of an animal which undergoes metamorphosis after it has hatched
from the egg; usually an active stage characterized by rapid growth; adj., larval.

larynx (lir’ inks). In vertebrates the enlarged anterior end of the trachea containing
the vocal cords; adj., laryngeal (lér in’ jé al).

Laveran (lav riin’), Charles L. A. French physician; 1845-1922.

Lavoisier (la vwa zya’), Antoine L. French chemist; 1743-1794.

Leeuwenhoek, van (viin la’ vén hook), Anton. Dutch naturalist; 1632-1723.

lens. A transparent body in the eyes of animals which focuses the rays of light; adj.,
lenticular (lén tic’ t lar).

leucocyte (la’ ké sit). A white blood corpuscle.

ligament (lig’ imént). A band of fibrous tissue connecting structures in the body
other than muscles; adj., ligamentous (lig 4 mén’ tiis).

Light, Sol F. American zoologist, at the University of Amoy; 1886-.

linin (li’ nin). The delicate reticulum in the nucleus upon which are the granules of
chromatin.

linkage (link’ aj). The constant association of certain genes in particular chro-
mosomes.

Linnaeus (li né’ tis), Carolus. Swedish naturalist; 1707-1778.

lipase (lip’ as). A fat-decomposing enzyme.

lipoid (lfp’ oid). Like a fat.

Loeb (lob), Jacques. German-American biologist; 1859-1924.

Lotsy (l6t’ si), Jan P. Dutch botanist; 1867-1931.

lumen (li’ mén). The cavity in a tubular gland, duct, canal, or vessel.

lung. A respiratory organ of the air-breathing vertebrates.

lymph (limf). The blood plasm, together with white corpuscles, in the lymph vessels
or in the tissues.

lymphatic (lim fat’ tk). A vessel conveying lymph; found in vertebrates. As an
adjective, pertaining to lymph.

GLOSSARY 599

MacBride, Ernest W. English biologist; 1866—.

Mach (mikh), Ernst. Austrian physicist; 1838-1916.

macro (mAik’ rd). G.; large.

macrogamete (mik r6é gi mét’). A female sex cell, or egg cell.

macronucleus (mik ro nti’ klé tis). The larger of two nuclei in the cells of infusoria.

Malpighi (mal pé’ gé), Marcello. Italian anatomist; 1628-1694.

mandible (min’ dib’l). In invertebrates, a mouth part for chewing; in vertebrates,
the lower jaw; adj., mandibular (min dib’ t lar).

mandibulate (min dib’ i lat). Having mandibles.

mantle. A fold of the body wall which more or less envelops the body; in most
mollusks it secretes a shell.

marsupium (mir sti’ pittm). An external pouch for carrying the young; in the fresh-
water mussels, a part of the gills.

Mast, Samuel O. American biologist, at Johns Hopkins University; 1871—.

maturation (mat ira’ shtin). As applied to germ cells, the final stages in gameto-
genesis, involving chromosome reduction.

maxilla (mak si’ la). In invertebrates, an accessory mouth part situated just back
of the mandible and used for handling food; in vertebrates, the upper jaw; adj.,
maxillary (mak’ si la ri).

mechanism (mék’ aniz’m). The view that life phenomena are to be interpreted in
terms of the same chemical and physical laws that govern the phenomena of
inorganic nature.

medulla (méditl’ a). The posterior region of the vertebrate brain, also called
medulla oblongata; also the soft central part of a gland or other organ; adj.,
medullary (méd’ t 1a ri).

medullary. In a particular sense pertaining to the embryonic structure from which
the nervous system of chordates develops, as the medullary groove or tube.

medullated (méd’ t lat éd). Term applied to a nerve fiber which possesses a fatty
sheath.

medusa (mé di’ sa). A jelly fish, which is a free-swimming individual coelenterate.

meiosis (mi 6’ sis). The reduction division in the maturation of germ cells; adj.,
meiotic (mi Oot’ ik).

membrane (mém’ bran). A thin sheet of tissue; also a thin sheet of matter secreted
by cells or in a cell; adj., membranous (mém’ bra nis).

Mendel, Gregor J. Austrian monk and botanist; 1822-1884.

mer (mér). G.; part.

meridional (mé rid’{6 nal). Applied to lines or planes running from pole to pole.

meroblastic (mér 6 blis’ tik). The term applied to an egg cell which in cleavage is
only partly divided into blastomeres.

Merriam, C. Hart. American biologist; 1855—.

mes (més). G.; middle.

mesencephalon (més én séf’alén). The third region, or midbrain, of vertebrates,
including the optic lobes.

mesenchyme (més’ én kim). A mesodermal mass of branched, irregular cells in the
embryo from which arise the connective tissues generally; also written mesen-
chyma (més én’ ki ma), a term which is also applied to the mass of connective
tissue that occupies the center of the body in some lower forms, such as the
planarians.

mesentery (més’ én téri). A double sheet of tissue attaching the alimentary canal
to the body wall in vertebrates; or thin sheets of tissue connecting the stomodeum
to the body wall in sea anemones; adj., mesenteric (més én tér’ ik).

mesoderm (més’ 6diirm). The middle germ layer in embryogeny; adj., mesodermal
(més 6 dir’ mal).

600 GLOSSARY

mesoglea (més 6 glé’ 4). A layer, mostly noncellular, between the ectoderm and
entoderm in coelenterates; if it contains scattered cells, these do not have the
character of a mesoderm.

mesonephros (més 6 néf’ rds). The kidney of the lower vertebrates, from the lamprey
to the amphibians; adj., mesonephric.

mesothelium (més 6 thé’ li tim). A mesodermal sheet of cells in the embryo from
which arise particularly the epithelia lining coelomic cavities and the striated
muscles; adj., mesothelial.

meta- (mét’ 4). G.; after, behind.

metabolism (mé tib’ 6 liz’m). The sum of the chemical changes in a living organism,
accompanied also by physical changes; adj., metabolic (mét 4 bol’ ik).

metagenesis (mét 4 jén’ ésis). The regular alternation of sexual and asexual types
of reproduction in a given species of animal.

metamere (mét’ Amér). One of a lineal series of sections into which the bodies of
the higher invertebrates and the vertebrates are divided; adj., metameric
(mét a mér’ ik).

metamerism (mé tim’ ériz’m). The existence of metameres in the body of an
animal.

metamorphosis (mét 4 mor’ fé sis). A pronounced change in appearance during the
development of an animal.

metanephros (mét Anéf’ rds). The kidney of reptiles, birds, and mammals; adj.,
metanephric.

metaphase (mét’ 4faz). That period in mitosis when the chromosomes, having
become lined up on the equator of the spindle, are divided into two, or, if splitting
has previously occurred, when they become so arranged.

metaplasm (mét’ 4 pliz’m). Nonliving matter in the cytoplasm of a cell.

metencephalon (mét én s#f’aloén). The fourth region of the vertebrate brain, includ-
ing the cerebellum and pons.

micro (mi’ cré). G.; small.

microgamete (mi kré ga mét’). A male sex cell, or sperm cell.

micronucleus (mi kré nu’ klé tis). The smaller of two nuclei in an infusorian.

micropyle (mi’ kré pil). The minute opening in the covering of an egg of any one of
certain types of animals through which a sperm cell enters.

migration (mi gra’ shtin), periodic. A periodic movement shared by all animals of
a species, or all of those occupying a certain area, from that area to another on
the earth’s surface.

milt. The spermatic fluid of a male fish.

mimicry (mim’ ik ri). A resemblance of one organism to another organism of a very
different character.

miracidium (mir asid’{tim). The first larval stage in flukes.

mit (mit). G.; thread.

mitochondria (mit 6kén’ dria). Structures in a cell which seem to be normally
present but the significance of which is unknown.

mitosis (mI td’ s!s). Normal cell division in which the chromatin material is equally
divided; adj., mitotic (m1 tot’ fk).

molt. To cast off an outer covering, such as a cuticula, scales, or feathers.

mon- (mon). G.; single.

monecious (m6 né’ shiis). A species of animal having both male and female gonads
in the same individual.

monoblastic (mon 6 blis’ tik). Having one germ layer—the blastoderm.

monohybrid (mon 6 hi’ brid). An offspring of parents differing by one character.

Morgan, Thomas H. American zoologist, at California Institute of Technology;
1866-.

morph (mérf). G.; form, structure.

GLOSSARY 601

morula (mor’ 6014). An embryo in an early stage when composed of a solid mass of

cells.

mucosa (mtikd’ sa). A membrane containing glands by which is formed a mucous
secretion.

mucus (mi’kiis). A slimy secretion containing mucin (mi’ sin); adj., mucous
(mi’ kis).

Miiller, Johannes P. German physiologist and anatomist; 1801-1858.

mutation (mi ta’ shtin). A sudden, heritable change in a character possessed by an
organism due to a change in one or more genes.

mutualism (mi’ ti Aliz’m). An association of two animals of different species
which is of advantage to both.

myelencephalon (mi élén séf’alén). The fifth, and most posterior, region of the
vertebrate brain, including the medulla oblongata.

myo (mi’6). G.; muscle.

myoneme (mi’6ném). A contractile strand in the cytoplasm of a protozoan.

myria (mir’ia). G.; myriad, literally ten thousand.

myx (miks). G.; slime.

Naiad (na’ yid). The immature stage of an aquatic insect with incomplete meta-
morphosis.

nares (na’ réz). The openings into the nasal chamber in vertebrates; the anterior
nares open externally and the posterior nares open into the mouth or pharynx.

nasal (na’ zal). Pertaining to the nose.

natural selection. The process in nature by which the types best fitted for their
particular environment survive while those least fitted are eliminated.

nauplius (nd’ plitis). The larva of certain of the Crustacea.

negative response. A response to a stimulus in which the organism turns or moves
away from the stimulus.

nekton (nék’ tén). That part of a pelagic aquatic fauna which is independent of the
action of winds and waves.

nemato (ném’ até). G.; thread.

nematocyst (ném’ 4 td sist). One of the stinging bodies found in coelenterates.

neo- (né’ 6). G.; new, recent.

neoteny (né ot’ éni). The indefinite prolongation of the immature condition of an
animal; in tailed amphibians it results in the production of axolotls, and in ter-
mites it is shown in the development of reserve females which take the place of
lost queens.

nephridium (néfrid’{tim). An excretory organ found in certain invertebrates.

nephros (néf’ rds). A vertebrate kidney.

nerve. A collection of nerve fibers inclosed in a sheath.

nervous. Pertaining to the nervous system or to any of its functions.

nervous impulse. The effect of stimulation transmitted along a nerve fiber.

nervure (ntr’ vir). A stiffening riblike structure in the wing of an insect.

neural. Pertaining to a nerve or to the nervous system.

neurilemma (ni rilém’ 4). The delicate membrane covering a nerve fiber.

neuron (nti’rén). A nerve cell, including the cell body and all the branches, or
fibers.

nid (nid). L.; nest.

noct (nokt). L.; night.

nomenclature (nd’ mén kla tytir). A system of naming objects or concepts.

noto (nd’ td). G.; the back.

notochord (no’ t6 kérd). A rod of cells, derived from the entoderm, in the chordates
lying in the median line below the spinal cord; it disappears as the vertebral
column is developed; also called chorda dorsalis, or simply chorda.

602 GLOSSARY

nuclear sap. The more fluid part of a nucleus; also called karyolymph.

nucleolus (ni klé’ 6 lis). In the nucleus a sharply defined body the nature and
function of which vary in different nuclei and in many cases are unknown.

nucleoplasm (nii’ klé6pliz’m). The protoplasm contained in the nucleus, as
distinguished from cytoplasm.

nucleus (ni’ kléiis). A portion of the protoplasm of a cell which is set off by a
membrane and which contains the chromatin and also produces enzymes that
stimulate the activities of the cytoplasm; adj., nuclear.

nurse cell. An egg cell which contributes its substance to another egg cell, which is
developing, and therefore does not itself produce an embryo.

nymph (nimf). The immature stage of an insect which undergoes incomplete meta-
morphosis; also the resting stage in the development of certain other inverte-
brates, coming in between the larva and the adult, as in certain mites; adj.,
nymphal.

Ocellus (6 s#l’ tis). A simple eye, as in an insect.

-oid. G.; like, resembling.

olfactory (61 fik’ to ri). Pertaining to the sense of smell.

ommatidium (Sm a tid’{tim). A rodlike unit in a compound eye.

ont (Ont). G.; a being.

ontogeny (in tdj’éni). The development of the individual from the egg cell to the
adult condition; adj., ontogenetic (dn t6 jé nét’ tk).

00 (6’6). G.; egg.

oocyte (0’ osit). An egg cell during the maturation period in oogenesis.

oogenesis (6 6 jén’ ésis). The development of a female germ cell from the primordial
sex cell to the mature egg cell.

oogonium (66 g6’nitim). The female sex cell during the multiplication and growth
periods in oogenesis.

operculum (6 pir’ kiltim). A fold of skin covering the gills of an amphibian larva;
a similar fold, containing scales, covering the gills of fishes; a horny or limy plate
closing the opening of a snail shell; and other structures which are lidlike.

optic (Sp’ tik). Pertaining to the eye.

optimum (ép’ timtim). The most favorable condition; adj., optimal.

oral. Pertaining to the mouth.

organ. An assemblage of tissues all contributing to the performance of some
function.

organic (6r gin’ ik). That which relates to living things, or has been so related.

organism (6r’ gin iz’m). A mass of living matter capable of maintaining independent
existence and all parts of which contribute more or less to the activities of the
whole.

organogeny (6r ga ndj’éni). The development of organs in embryogeny.

ortho- (6r’ thé). G.; straight.

orthogenesis (dr thé jén’ ésis). The theory that animals tend to develop along lines
leading constantly in the same direction and which are determined by internal
factors.

Osborn, Henry F. American paleontologist, at American Museum of Natural
History, New York; 1857-1935.

osmosis (3s md’ sis). The passage of miscible fluids through a semipermeable mem-
brane, usually from a region of higher concentration to one of lower concentration.

osmotic (és mot’ ik) pressure. The unbalanced pressure due to differences of con-
centration in solutions which are on opposite sides of a semipermeable membrane.

ossicle (6s’{k’l). A small bone.

oste (6s’ té). G.; bone.

oto (o’ td). G.; referring to the ear; adj., otic (6’ tik).

GLOSSARY 603

otocyst (6’tésist). An organ supposed to be one of hearing found in many
invertebrates.

ovary (6’ vari). An organ which produces egg cells; adj., ovarian (0 va’ ri in).

oviduct (3’ vidtkt). The duct leading from the ovary either to the outside of the
body or to a cavity opening externally.

oviparity (6 vi pir’it!). The condition which involves the laying of eggs; adj.,
oviparous (6 vip’ arts).

ovipositor (6 vi péz’itér). An organ which functions in the deposition of eggs,
especially in insects.

ovo (0’ vo). L.; egg.

ovoviviparity (6 vé viv 1 par’{ti). The condition which involves the laying of an
egg that contains a living embryo.

ovum. An egg cell; also an egg, including the egg cell and its protective coverings.

Owen, Richard. English zoologist; 1804-1892.

oxidase (6k’ si das). An enzyme which promotes oxidation.

oxidation (5k s! da’ shitin). A chemical reaction involving the addition of oxygen in
chemical combination.

Pale-, palae- (pa’lé). G.; old, ancient.

palpus (pil’ piis). An appendage, particularly in an arthropod, bearing organs for
touch and taste; adj., palpal.

pancreas (pin’ kré as). A gland in vertebrates producing a variety of digestive
secretions which are passed into the small intestine; adj., pancreatic (pin kré-
At’ ik).

papilla (pa pil’ 4). A small nipple-like projection, either from a surface or of one
tissue into another; a papilla may contain a sense organ.

para- (pir’ 4). G.; beside.

parasite (pir’ 4 sit). An animal which lives in or upon another animal, to the injury
of the latter but normally without causing its death. The phenomenon is
parasitism (pir’ 4 sit 1z’m), and the adj., parasitic (par a sit’ Ik).

parenchyma (parén’ kima). A loose mesodermal tissue contained in the bodies of
some lower invertebrates, or the essential tissue which forms the mass of an organ.

Parker, George H. American zoologist, at Harvard University; 1864-.

parthen (pir’ thén). G.; virgin.

parthenogenesis (pir thé no jén’ ésis). The production of an offspring from an
unfertilized egg cell; adj., parthenogenetic (par thé no jén ét’ ik).

Pasteur (pas ttir’), Louis. French chemist, 1822-1895.

pathogenic (pith 6 jén’ ik). Disease-producing.

pectoral (pék’ to ril). Pertaining to the region of the body opposite the fore limbs.

ped (péd). L.; foot. Pedal (pé’ dal). Pertaining to the foot.

pedipalp (péd’{ pAlp). One of the second pair of appendages of arachnids.

pedo-, paedo- (pé’ do). G.; child.

pedogenesis (pé dé jén’ ésis). The production of young by an immature animal.

pelagic (pé1ij’ ik). Pertaining to the open water in an aquatic environment.

pelvic (pél’ vik). Pertaining to the region of the vertebrate body opposite the hind
limbs; a bone or bones in this region articulating with the vertebral column and
giving attachment to the hind limbs forms a pelvis (pél’ vis).

penis (pé’ nis). A male copulatory organ.

pepsin (pép’ sin). An enzyme produced in the stomachs of vertebrates which
changes proteins to peptones.

peri- (pér’ i). G.; around.

pericardium (pér{ikir’ ditim). A coelomic or hemocoelic cavity in many inverte-
brates containing the heart; in vertebrates, a membranous sac inclosing a cavity
which contains the heart; adj., pericardial.

604 GLOSSARY

periosteum (péridés’ tétim). A fibrous membrane covering a bone; adj., periosteal.

peristalsis (péristal’ sis). A succession of rhythmical contractions of the wall of
the intestine, or other muscular tubes, which drives the contents onward; adj.,
peristaltic.

peritoneum (périté né’ tim). The membrane lining the coelomic cavity; in mammals
limited to the abdominal cavity; adj., peritoneal.

Pfliiger, Eduard. German physiologist; 1829-1910.

phag (faj). G.; eat.

phagocyte (fig’ 6sit). A leucocyte when engaged in the engulfing and destruction of
other cells or various foreign objects.

pharynx (fir’ inks). A region of the alimentary canal between the mouth cavity and
the esophagus; adj., pharyngeal (fa rin’ jé al).

phenotype (fé’ né tip). A type of organism considered as a complex of visible char-
acters; adj., phenotypic (fé né tip’ ik).

phore (for). G.; bearer.

photo (f6’ td). G.; light. Photic (f6’ tik). Pertaining to light.

photosynthesis (f6 to sin’ thé sis). The production of carbohydrates from carbon
dioxide and water by chlorophyll, using the energy of sunlight.

phototropism (f6 tot’ rd piz’m). The response of an organism to light; adj., photo-
tropic (f6 td trop’ Ik).

phyl (fil). G.; race, or tribe.

phylogeny (filoj’éni). The series of stages passed through in the evolution of a
group of animals; adj., phylogenic (fi 16 jén’ ik) or phylogenetic.

physiological state. A condition of an organism at any given time which is the
resultant of the metabolic processes that have preceded and which determines the
manner in which the organism will respond to stimulation.

phyt (fit). G.; plant.

pia mater (pi’ 4 ma’ tér). A delicate connective tissue membrane closely adherent
to the spinal cord and brain of vertebrates.

pineal (pin’ é 41) body. A dorsal outgrowth of the diencephalon in vertebrates; it
probably served as an eye in primitive vertebrates now extinct and remains as a
rudimentary structure in living forms, in some of which it may function as an
endocrine gland. Also called pineal eye and pineal gland.

pituitary (pi ti/{tdri) body. A ventral outgrowth of the diencephalon in verte-
brates, to which is added tissue from a dorsal outgrowth of the mouth cavity; it
functions in many forms as an endocrine gland.

placenta (pla stn’ ta). An organ by which the young of a mammal becomes attached
to the wall of the maternal uterus and through which it receives food and oxygen
and disposes of waste; it is derived in part from the chorion of the embryo, in
some cases including the allantois, and in part from the uterine wall; adj.,
placental.

plankton (plink’ ton). That part of a pelagic fauna made up of small and weak
organisms which are at the mercy of winds and waves.

plano (pli’ no). L.; flat.

plasm (pliz’m). G.; anything formed; in biology, living matter.

plasma (pliz’ ma). The liquid part of the blood.

plasma membrane. A living membrane on a cell, as distinguished from a nonliving
cell wall.

plasmosome (pliz’mésom). A nucleolus not composed of chromatin.

plast (plist). G.; an organized particle or granule, including a cell.

plastid (plis’ tid). A body in the cytoplasm of a cell carrying on some constructive
chemical process, as the chlorophyll bodies, or chloroplasts.

platy (pli’ ti). G.; flat, broad.

GLOSSARY 605

pleural cavity. That part of the coelom in mammals which contains the lungs; the
lining membrane is the pleura.

plexus. A network of nerves.

Pliny (plin’ 1); Caius Plinius Secundus, the Elder. Roman naturalist; 23-79.

pneumo (nt’ m6). G.; air, breath, consequently soul or spirit.

pod (pdd). G.; foot.

polar body. A nonfunctional cell produced in the maturation divisions of an egg
cell.

polarity (pé lar’ 1 ti). That condition in a body connected with the dominance of
one part over another, or in a conducting substance that which determines the
direction in which conduction will take place.

poly (pol’ i). G.; many.

polyandry (pol i An’ dri). The mating of-one female with several males.

polygyny (po lig ini). The mating of several females with one male.

polymorphism (pol { mor’ fiz’m). The existence of more than one form of the same
species; adj., polymorphic; if there are only two forms, the condition is spoken of
as dimorphism.

polyp (pdl’ ip). An attached coelenterate.

portal system. A capillary system interposed in the course of a vein; when used
without qualification it is that part of the venous system which passes blood
through the liver.

positive response. A response in which an organism, turns or moves toward the
stimulus.

post- (post). L.; after, behind.

posterior (pds té’ ri ér). Behind; in a bilaterally symmetrical animal with the axis
horizontal, away from the head.

precocial (pré kd’ shal). Term applied to a bird the young of which have down, leave
the nest, and are active as soon as they have hatched.

predaceous (pré da’ shiis). Preying on other animals; the condition is known as
predatism (préd’ 4 tiz’m).

preformation (pré fér ma’ shin). The conception formerly held that the parts of an
organism were preformed in the germ cell.

primates (pri’ mats). Animals belonging to the mammalian order Primates (pri-
ma’ téz).

primordial (pri mér’ di 4l). The first in order, original.

pro- (pra). G.; before.

proboscis (pré bos’ is). A forward extension of the head, especially of the snout in
mammals.

proctodeum (prok té dé’ tm). An invagination of the surface of an embryo which
meets the posterior end of the archenteron and forms that portion of the alimen-
tary canal just before the anus.

proglottid (pré gldét’ id). One of the segments of a tapeworm.

pronephros (pro néf’ rds). A primitive vertebrate kidney, functional only in the
hag; adj., pronephric.

pronucleus (pré ni’ klé tis). The nucleus of a sperm cell after it has entered an egg
cell and also that of the egg cell itself before the two have united in fertilization;
they are termed, respectively, male and female pronuclei.

prophase (pro’ faz). The first phase in mitosis, lasting until the chromosomes are
lined up in the equator of the spindle.

proprioceptor (pr6é prid stp’ tir). A receptor contained within the tissues of the
body and stimulated by conditions in the tissues themselves.

prostate (prés’ tat) gland. A gland connected with the male reproductive system
which produces a secretion that stimulates the sperm cells to activity.

606 GLOSSARY

prostomium (pré std’ miiim). A lobe overhanging the mouth of an earthworm which
functions as a lip; it is not considered a metamere; adj., prostomial.

protective resemblance. A resemblance of an animal to its environment which tends
to make it inconspicuous.

protein (pro’ téin). An organic compound, containing carbon, hydrogen, oxygen,
and nitrogen, and which is an essential constituent of protoplasm.

proto- (pro’ td). G.; first, primary.

protoplasm (pro’ té pliz’m). The living substance; of very complex composition;
adj., protoplasmic (pro té plaz’ mik).

protrusible (pré troo’ si b’l1). Capable of being put out or protruded.

proventriculus (pro vén trik’ i lis). The anterior glandular portion of the stomach,
or a dilation of the alimentary canal in front of the gizzard.

pseudo- (si’ dé). G.; false.

pseudopodium (sii dé pd’ ditim). A temporary locomotor protrusion from the sur-
face of a cell.

pter (tér). G.; wing.

ptyalin (ti alin). The enzyme in the saliva that changes starches to sugars.

pulmonary (pil’mé nari). Pertaining to the lungs.

pulmonate (piil’ m6 nat). Possessing lungs.

pupa (pi! pa). The usually quiescent stage between larva and adult of insects in
which complete metamorphosis occurs; adj., pupal.

Purkinje (poor kin’ yé), Johannes E. Czechish biologist; 1787-1869.

pylorus (pi1d’rtis). The opening from the stomach into the intestine; adj., pyloric
(pi lér’ tk).

Radial (ra’ di Al) symmetry. Applied to an organism in which the body can be divided
into a number of parts separated by radial planes.

Ray, John. English biologist; 1627-1705.

re- (ré). L.; again.

reaction (ré Ak’ shtin). The response which follows the application of a stimulus to a
living organism.

reason. Asa basis for behavior, the ability to analyze previous experiences, perceive
analogies, and by a logical process arrive at an abstract conception which may
determine subsequent action.

recapitulation (réka pit yi la’ shim) theory. The conception that stages passed
through in the evolution of the race to which an animal belongs are repeated in
the development of the individual.

receptor (ré stp’ tor). A sense organ serving for the reception of stimuli; also a
solitary cell acting in the same fashion.

recessive (ré sés’ lv). One of a pair of allelomorphic characters which does not appear
because of the dominance of the gene corresponding to the other of the pair.

rect (rékt). L.; straight.

rectum. The terminal portion of the large intestine in higher vertebrates and also
in some invertebrates; adj., rectal.

Redi (ra’ dé), Francesco. Italian naturalist; 1626-1694.

redia (ré’ dia). A stage in the development of a fluke; rediae are produced partheno-
genetically in the sporocyst.

reduction. The halving of the number of chromosomes in the maturation period of
gametogenesis.

Reese, Albert M. American zoologist, at University of West Virginia; 1872-.

reflex arc. A chain of cells which in the simplest form of the are are three in number—
a receptor neuron on the surface which receives a stimulus and passes the effect
as an impulse to an adjustor neuron; this in turn passes it on to an effector cell
that performs an appropriate act.

GLOSSARY 607

reflex action. An action involving one or more reflex arcs.

regeneration (ré jén ér 4’ shin). The replacement of lost parts.

renal (ré’ nil). Pertaining to the kidney.

rennin (rén’ In). An enzyme produced in the stomach of mammals which coagulates
the proteins in milk.

reproduction (ré pré dik’ shtin). The production of a new organism by an older one.

respiration (rés pi ra’ shtin). The exchange of oxygen and carbon dioxide between
an organism and its environment, or between the blood and different tissues.

response (réspdns’). An action on the part of an organism caused by a stimulus.

resting cell. A cell not undergoing division.

rete (ré’ te). A network, diminutive reticulum (ré tik’ i lim); adj., reticular.

retina (rét’{na). The receptor cells of the eye; adj., retinal.

retractile (ré traik’ til). Capable of being withdrawn.

retro- (rét’ rd). L.; backward.

retrogression (rét rd grésh’ tin). The going backward by an animal during its
development to a condition characterizing animals lower in the scale of life.

reversion (ré vir’ shtin). The reappearance of an ancestral character after a lapse
of several generations.

thabd (ribd). G.; rod.

rheotropism (ré dt’ r6 piz’m). A response by an organism to stimulation by a current
of air or water; adj., rheotropic (ré 6 trép’ tk).

rhiz (riz). G.; root.

thyncho (rin’ kd). G.; snout.

rhythmicity (rith mis’i ti). Variations repeated at regular intervals.

roe. The eggs of fishes.

Ross, Sir Ronald. English physician; 1857-1932.

rota (ro’ ta). L.; wheel.

Saliva (sa li’ va). The secretion of the salivary glands.

sapro (sip’ré). G.; decayed, rotten.

saprophytic (sip ro fit’ ik). Securing nourishment from the products of organic
decomposition.

sarco (sir’ kd). G.; flesh.

sarcode (sir’ kod). The term first applied to protoplasm by Dujardin.

sarcolemma (sir kélém’ 4). The delicate membrane about a cross-striated muscle
fiber.

sarcoplasm (sir’ ké pliz’m). The protoplasm of a striated muscle fiber outside the
sarcostyles (sir’ ké stils), or longitudinal fibrils.

scaph (skif). G.; something hollow, boat.

Schleiden (shli’ dén), Matthias J. German botanist; 1804-1881.

Schultze (shool’ tsé), Max J. S. German biologist; 1825-1874.

Schwann (shvin), Theodor. German zoologist; 1810-1882.

Sclater (skla’ tér), Philip L. English zoologist, 1829-1913.

scler (sklér). G.; hard.

sclerotic (sklér dt’ ik). The outer dense fibrous coat of the vertebrate eye.

scolex (sk6’ léks). The attached end or head of a tapeworm.

scut (skit). L.; shield.

scute (skit). A ventral scale of a snake which is placed transversely and extends from
one side of the body to the other.

scyph (skif). G.; cup.

scyphistoma (si fis’ t6 ma). An attached polyp-like stage in a scyphozoan.

sebaceous (sé ba’ shiis). A term applied to oil glands connected with the hairs of
mammals.

608 GLOSSARY

secondary sexual characters. Characters of an animal associated with the sex but
not connected with the reproductive system.

secretion (sé kré’ shtin). The passage out from a cell of a substance produced in it
which can be utilized by the body; also the substance itself secretory (sé’ kré to rl),
pertaining to secretion.

Sedgwick, Adam. English zoologist; 1854-1913.

segregation (stg ré ga’ shtin). The separation of paired genes during the maturation
of the sex cells, which thereby pass on only one gene and are “‘pure”’ for the
corresponding character.

self-fertilization. The fertilization of an egg cell by a sperm cell produced in the
same individual.

semi- (sém’i). L.; half.

seminal receptacle (stm’ {nil ré stp’ ta k’l). A sac in the body of a female animal
in which the sperm cells are stored until used.

seminal vesicle (vés’{k’l). A sac in the body of a male animal in which sperm cells
are stored until transferred to the female.

semipermeable membrane. A membrane permitting the passage of solvents but not
of substances in solution unless they can be dissolved in the membrane.

senescence (sé nts’ éns). The period during which the organism is growing old.

sensation. The effect of a stimulus when registered in a center of consciousness in
the brain.

sense organ. An organ for the reception of stimuli.

serosa (sé rd’ sa). A membrane which secretes a watery fluid; the secretion is termed
serous (sé’ rtis).

serum (sé’ riim). The plasma of the blood from which the clot has been separated.

Servetus (sér vé’ ttis), Michael. Spanish author; 1511-1553.

sessile (sts’ il). Attached and not capable of locomotion.

seta (sé’ ta). A fine bristle or spine; used in the annelids as a locomotor structure.

sex. The sum of the characters that distinguish male and female individuals; adj.,
sexual (stk’ shi al).

sex chromosome. A chromosome the presence or absence of which in a sex cell is an
important factor in determining whether the animal produced will be a male or
female.

sex-linked. A term applied to a character which is associated with sex, the gene
corresponding to it being in the sex chromosome.

Shelford, Victor E. American zoologist, at the University of Illinois; 1877—.

skeleton (skél’ é tiin). The firm supporting parts of an animal body; adj., skeletal.

skull. The bones of the head in a vertebrate.

society. An association of animals of the same species.

som (som). G.; body.

somatic (sé mit’ ik). Referring to the body cells as distinguished from the germ
cells; or to the wall of the body as distinguished from the viscera contained in it.

somatoplasm (so’ ma té pliz’m). The protoplasm of the body as distinguished from
the germ plasm.

somite (so’ mit). A metamere.

special creation. The conception that each species of animal is the result of a particu-
lar creative act.

specialization. The distribution of functions to certain organs or parts, which become
adapted to the performance of the respective functions.

species (spé’ shéz). A distinct kind of animal; adj., specific (spé sif’ ik).

sperm, sperm cell, or spermatozoon (sptir ma td z0’ 6n). The sex cell of the male.

spermary. A gonad producing sperm cells.

GLOSSARY 609

spermatid (spir’ ma tid). A male sex cell after the second maturation division in
spermatogenesis, before its metamorphosis into a sperm cell.

spermatocyte (sptir’ ma té sit). A male sex cell during the period of maturation in
spermatogenesis.

spermatogenesis (sptr ma td jén’ ésis). The production of sperm cells from pri-
mordial germ cells.

spermatogonium (sptrma td gd’ nitim). A male sex cell during the periods of
multiplication and growth in spermatogenesis.

spermatophore (sptr’ ma td for). A mass of sperm cells, usually inclosed in a mem-
brane or capsule.

sphincter (sfink’ tér). A muscle surrounding an opening which, by its contraction, it
closes.

spicule (spik’ il). A minute limy or siliceous crystal-like object embedded in the
tissues of an animal which serves to stiffen or support the body or part.

spinal cord. Of vertebrates the central nervous system exclusive of the brain.

spinal column. The bony column, or vertebral column, which incloses the spinal cord.

spindle, mitotic. The portion of a cell in mitosis lying between the centrosomes,
having the shape of a spindle and appearing in a microscopical preparation as if
containing a framework of fibers.

spiracle (spir’4k’l). In insects, one of the openings into the tracheal tubes; in
sharks, a modified and usually non-functional gill st opening internally into the
cavity of the pharynx and externally upon the surface of the head; in amphibians,
the external opening of a chamber on the inner wall of which are the external
openings of the gill slits.

spiral cleavage. Cleavage in which the blastomeres are spirally arranged.

spireme (spi’rém). A thread of chromatin appearing early in the prophase of mitosis
which later breaks transversely into chromosomes.

splanchno (splaink’ nd). G.; viscera; adj., splanchnic (splink’ nik), pertaining to the
viscera.

spongin (sptin’ jin). The horny substance which forms the fibers of fibrous sponges.

spontaneous generation. Same as abiogenesis.

spore (spor). A minute reproductive cell produced by a protozoan, either sexually or
asexually, and usually contained in a shell, though in some cases motile.

sporocyst (sp0’ ré sist). A stage in the development of the liver fluke following the
encystment of the miracidium.

sporulation (spor 00 1a’ shitin). The production of spores by a protozoan.

squam (skwim). L.; a scale; adj., squamous (skwa’ mis).

stato (stat’ 6). G.; standing, fixed.

statoblast (stit’ 6 blast). An asexual winter bud of a bryozoan, which is inclosed
in a chitinous shell.

statocyst (stit’ d sist). An organ of equilibrium and orientation in many inverte-
brates.

steapsin (sté ip’ sin). The enzyme in the pancreatic secretion which changes fats to
fatty acids and glycerin.

stigma (stig’ma). An eyespot in protozoans; also the same as a spiracle in an insect.

stimulus (stim’ ti lis). Any condition either external or internal to the body which
causes a response in a living organism.

stom (stom). G.; mouth.

stomach (sttim’ ik). An enlarged portion of the alimentary canal in which the food
is accumulated and in which it also may be reduced to fine particles and partly
digested.

stomodeum (sto mé dé’ tim). An invagination of the surface of an embryo which
meets the anterior end of the archenteron and forms the mouth cavity.

610 GLOSSARY

stratum (stra’ ttm). One of a series of layers; stratified (strit’1fid), arranged in
layers. In ecology, stratification refers to vertical distribution at a particular
location.

striated (stri’ 4t éd). As applied to muscle fibers, cross-banded.

strobila (stré bi’ la). A chain of individuals produced by budding from the sey-
phistoma of a scyphozoan.

sub- (stb). L.; under, below.

succession (stik sésh’ tn). The successive occupation of a given area by different
types of animals; adj., successional.

super- (su’ pér). L.; over, above.

superficial (sti pér fish’ 41) cleavage. Cleavage which results in a layer of blastomeres
surrounding a central mass of yolk.

supra- (sii’ pra). L.; over, above.

suture (sti’ tytir). A line of junction.

Swammerdam (swiim’ érdim), Jan. Dutch zoologist; 1637-1680.

Sylvius (sil’ vi tis), Jacobus. French anatomist; 1478-1555.

symbiosis (sim biG’ sis). An intimate association of two organisms of different
species, neither of which can flourish in the absence of the other; adj., symbiotic
(sim bi ot’ tk).

symbols. Selected symbols in common use in zoology are:

o signifies male.
@ signifies female.
9 signifies neuter.
w signifies micron (149909 millimeter).
up signifies millimicron (14 999 micron).
X signifies crossed with; also magnified by.
= signifies a synonym.
? signifies doubt.
! signifies affirmation.

symmetry (sim’ mé tri). Regularity of form, or balance between parts; adj., sym-
metrical (si mét’ ri kal).

syn- (sin). G.; together (syn- becomes sym- before b, m, and p and syl- before 1).

synapse (si nips’). The area of contact of two nerve fibers from different neurons.

synapsis (s! nip’ sis). The union of two similar chromosomes, one from each parent,
in the growth period of gametogenesis; adj., synaptic.

syncitium (sin sish’itim). A mass of protoplasm containing many nuclei but not
divided into separate cells; adj., syncytial.

syngamy (sin’ gami). The union of two sex cells to form a zygote.

system (sis’ tém), anatomical. An association of organs for the performance of some
general activity of the body.

systemic (sis tém’ ik). Applying to the body generally.

Tact (tikt). L.; touch; tactile (tak’ til), pertaining to touch.

tarsus (tir’ stis). The ankle bones in vertebrates; the distal segments of the legs in
insects and spiders; adj., tarsal.

taste bud. A group of gustatory cells with supporting cells forming an organ of
taste.

tax (tiks). G.; an arranging, or arrangement.

tegumentary (tég i mén’ tari). Pertaining to the skin, or integument.

tele (tél’ é). G.; far, far off.

telencephalon (tél én stf’alon). The most anterior region of the vertebrate brain,
including the cerebral hemispheres and olfactory lobes.

GLOSSARY 611

telo (tél’ 5). G.; end.

telolecithal (tél 6 lés’ {thal). A term applied to an egg cell with the yolk massed at
one pole.

telophase (tél’ 6 faz). The last stages in mitosis, in which the cytoplasm is divided
and the nucleus returns to the condition seen in a resting cell.

tendon (tén’ diin). A mass of white fibrous connective tissue fibers forming an attach-
ment for a muscle; adj., tendinous.

tentacle (tén’ tak’l). A soft elongated, nonarticulated appendage found in a great
variety of invertebrates and serving a great variety of functions; a tentacle
may be used as a grasping organ, or it may bear sense organs; adj., tentacular
(tén tak’ ti lar).

terrestrial (tér rés’ tri il). Living on or in the ground; used in distinction from
aquatic.

testis (tés’ tis). The male reproductive organ, in which sperm cells are produced.

tetanus (tét’ ants). A state of continued contraction in a muscle fiber due to con-
stantly repeated stimulation; adj., tetanic (té tin’ ik).

tetrad (tét’ rid). A chromatic body divided into four parts representing the halves
of each of two chromosomes which have united in synapsis and have been preco-
ciously divided.

Thales (tha’ léz). Greek philosopher; seventh and sixth centuries B.c.

therm (thirm). G.; heat.

thermocline (thir’ mé klin). A horizontal plane appearing in a deep lake in summer
below which the water is stagnant and above which frequent circulation is caused
by wind and other agencies.

thermotropism (thér mot’ ré piz’m). The response of an organism to a heat stimulus;
adj., thermotropic (thir m6 trdp’ ik).

thigmo (thig’ m6). G.; touch.

thigmotropism (thig mdt’ rd piz’m). The response of an organism to a contact
stimulus; adj., thigmotropic (thig m6 trop’ ik).

thorax (thd’ riks). A portion of the body of many animals lying between the head,
or head and neck, and the abdomen; adj., thoracic (thd ris’ ik).

tissue (tish’ i). A mass of similarly differentiated cells.

tome (tom). G.; cut.

tonsil (tén’ sil). A mass of lymphoid tissue located, in man, one on each side of the
passage from the mouth to the pharynx; adj., tonsillar.

tonus (td’niis). A state of continual moderate contraction in a nonstriated muscle
fiber; adj., tonic (tdn’ ik).

toxin (tdk’ sin). Any substance that acts as a poison in an animal body; adj., toxic.

trachea (tra’ ké 4). A tube conveying air into the body for the purpose of respiration;
found in many arthropods and in all lunged vertebrates; adj., tracheal.

traumatism (tr6’ ma tiz’m). A condition in the body due to a physical injury.

Trembley, Abraham. Swiss naturalist; 1700-1784.

trial and error. A method of finding the most favorable condition or environment;
in animals possessing intelligence it is done by conscious experimentation, in
lower animals by automatic responses to stimuli due to random sampling.

trich (trik). G.; hair.

trichocyst (trik’ 6 sist). One of many minute elongated sacs in the ectoplasm of a
paramecium and similar protozoans, set at a right angle to the surface, the con-
tents of which when thrown out form rodlike bodies used as weapons of defense
and perhaps of offense.

trihybrid (tri hi’ brid). An offspring of parents differing by three characters.

triploblastic (trip 16 blis’ tik). Having three germ layers.

trocho (trok’ 6). G.; a wheel.

612 GLOSSARY

-trope (trop). G.; a turning.

-trophy (tro’ fi). G.; nutrition.

tropism (trd’ piz’m). The automatic response of an organism to a stimulus; adj.,
tropic (trd’ pik).

trypsin (trip’ sin). An enzyme which changes proteins to amino acids and is pro-
duced by the pancreas.

tundra (tdon’ dra). A level, or gently undulating, treeless plain characteristic of the
arctic regions of both hemispheres.

tympanum (tim’ panim). In vertebrate anatomy, the cavity of the middle ear;
in zoology, the term is generally applied to any organ for the reception of sound
waves and frequently to a membrane having that function; adj., tympanic
(tim pin’ ik).

Tyndall (tin’ dal), John. English physicist; 1820-1893.

Umbilical (tim bil’{k&l) cord. The cord that unites a mammalian fetus to the
placenta.

umbilicus (tim bil’ { kts). The point of attachment of the umbilical cord to the young
animal.

uncinate (iin’ sin 4t). Hooked.

unguiculate (iin gwik’ i lat). Clawed.

uniparental (ii ni pa rén’ til). Involving only one parent.

unit character. A hereditary character that behaves as a unit in its transmission to
offspring.

urea (uré’ 4). The substance which contains most of the nitrogenous waste of the
animal body; adj., uric (i’ rik).

ureter (ii ré’ tér). The duct leading from the kidney and conveying the urine either
to a urinary bladder or to the outside.

urethra (ii ré’ thra). The duct from the urinary bladder to the external surface; adj.,
urethral.

uterus (ii’ tér tis). A dilated portion of the oviduct in which egg cells are retained
while undergoing more or less of their development; adj., uterine (t’ tér in).

Vaccine (vik’ sén). Any substance introduced into the animal body to protect it
from infection.

vacuole (vik’ i ol). A space in the cytoplasm of a cell usually filled with liquid and
containing food or collecting liquid wastes to be eliminated.

vagina (va ji’ na). The passage found in the female of many animals leading from
the uterus to the outside; adj., vaginal (vij’ J nal).

vas (vis). L.; vessel.

vascular (vis’ ki lar). Pertaining to vessels, especially blood vessels.

vas deferens (vis déf’ érénz). The duct leading from the testis to the outside;
plural, vasa deferentia (va’ sa déf Gr én’ shi 4).

vas efferens (vis éf’ ér nz). One of a number of small ducts interposed between the
testis and the vas deferens; plural, vasa efferentia (va’ sa éf ér én’ shi 4).

vein. A vessel conveying blood toward the heart; adj., venous (vé’ nts).

velum (vé’ lim). An annular membrane projecting inward from the margin of the
umbrella of certain medusae.

ventricle (vén’ trik’l). A chamber in a heart from which blood is sent out; also a
chamber in the vertebrate brain.

ventro (vén’ tro). L.; belly. Adj., ventral.

verm (virm). .; worm.

vermiform appendix. The contracted end of the caecum in primates.

vertebra (viir’ té bra). One of the bones of the vertebral column, or backbone, in
vertebrates; adj., vertebral (vir’ té bral).

GLOSSARY 613

Vesalius (vé sa’ lf tis), Andreas. Belgian anatomist; 1514-1564.

vesicle (vés’1k’l). A small sac, especially one filled with fluid.

vestigial (vés tij/1 Al). Existing only as a remnant of a former condition.

villus (vil’ ts). One of numerous, minute, vascular, finger-like projections on a
surface, as on the surface of the small intestine or of the chorion in mammals.

Virchow (fér’ kd), Rudolph. German pathologist; 1821-1902.

virus (vi’ ris). Consists of ultramicroscopic particles having the power of growth
and reproduction. Certain kinds are thought to cause diseases in man, such as
measles and smallpox, and many diseases in other animals and plants.

viscera (vis’ ér a). The soft organs contained in a coelom; adj., visceral; the appli-
cation is often extended to cover the whole length of the alimentary canal, as
in the case of pharyngeal arches and the visceral portion of the skull.

vision. The perception of an image derived from nervous impulses originating in an
eye; a mental picture of objects external to the body.

vitalism (vi’ tal 1z’m). The conception of a vital force, neither chemical nor physical,
back of all life phenomena; adj., vitalistic (vi til Is’ tik).

vitamin (vit’ 4 min). In certain foods a substance which plays a vital réle in assimila-
tion.

vitreous (vit’ ré tis). Glassy.

viviparity (viv 1 pir’ 1t!). The condition which makes possible the producing of
living young; adj., viviparous (vi vip’ 4 ris).

volant (v0’ lint). Capable of flying.

Von Baer (f6n bar’) Karl E. Russian naturalist; 1792-1876.

Von Moh! (fén mol’), Hugo. German botanist; 1805-1872.

Wallace, Alfred R. English naturalist; 1823-1913.

wandering cell. A leucocyte moving about in the tissues of the body.

Weismann (vis’ min), August. German biologist; 1834-1914.

Wilson, Henry V. American zoologist, at University of North Carolina; 1863-1939.
Wolff (vélf), Caspar F. German naturalist; 1733-1794.

Woodruff, Lorande L. American zoologist, at Yale University; 1879-.

X-chromosome. A chromosome occurring single or paired, the presence of which
apparently determines sex.

Y-chromosome. An unpaired chromosome present in the cells of certain animals and
also associated with sex.

Yerkes, Robert M. American psychologist, at Yale University, 1876—.

yolk (yok). Nutritive matter stored in an egg cell; included under the term meta-
plasm.

yolk gland. A gland producing yolk.

yolk stalk. The connection of the yolk sac to the embryo in amniotic vertebrates.

Zoo- (z0’ 6). G.; an animal.

zooid (z0’ oid). In a colony a small animal which has been produced by asexual
reproduction.

zygo (zig’ 5). G.; yoke, pair.

zygote (zi’ got). The united sperm cell and egg cell.

zymogen (zi’ mé jén). A substance which is developed in a gland cell and which may
be changed to an enzyme.

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INDEX

Note.—References to pages on which a figure appears are in bold face type.
covers two or more pages, no indication of figures is given.
If two or more references to the same page exist the page

when occurring among several, is in italics.
number is not duplicated.

Abalone, 224
Abdomen, crayfish, 255
insect, 277, 280
spider, 302
Abdominal cavity, 323
Abiogenesis, 270
Abnormalities, inheritance of, 564
Abomasum, 441
Abscess, 519
Absorption, 33, 35, 488
Abundance rhythms, 510
Abyssal fauna, 511
Acanthias, 344
Acanthocephala, 180, 185
Accommodation, 338
Accretion, 18
Acetabulum, 327
Acidity, 507
Acids, 8
cyanic, 27
fatty, 35
Acinous glands, 468
Acipenser rubicundus, 351
Acontia, 148
Acquired characters, inheritance of, 545, 549
Acquired immunity, 519
Acropara, 154
Actinosphaerium eichhorni, 81
Action pattern, 500
Adambulacral plate, 196
Adaptation and adaptations, 168, 172, 365-369,
382-384
Adelochorda, 314
Adhesive papillae, tunicate, 318
Adjustment (see Adaptation)
Adjustor, 496, 498
Adolescence, 45, 460
Adrenal bodies, 373
Aedes, 292
Aeolosoma, 249, 253
Aepyornis, 409
Aerial types, 512
Aeronautic spiders, 305
Afferent fiber, 244
Afferent impulse, 165, 333
Afferent path, 243
Afterbirth, 447
Aftershaft, 405
Age immunity, 519

However, if a reference
Any outstanding reference in the text,

Ages, of fishes, 362
geological, 540
Agnatha, 341
Air bladder, 354
Air sacs, 281, 407
Albino crow, 550
Albumen, egg, 383
Alder flycatcher, 509
Alexander the Great, 579
Alimentary canal, 179, 186, 198, 327, 466
of man, 328
Alkalinity, 507
Allantoic cavity, 384
Allantoic placenta, 430, 447
Allantoic stalk, 384
Allantoic vessels, 385
Allantois, 384, 447
Allelomorphs, 556
Allergy, 517, 520
Alligator gar, 351
Alligator mississippiensis, 399
Alligators, 387, 399
Alligator’s brain, 387
Alternation of generations, 108
Altitude, effect on distribution, 530
Altricial young, 418, 420
Alveolar connective tissue, 101
Alveolar structure of protoplasm, 23
Alveoli, lung, 370, 469
Amber, fossils, 538
Ambergris, 448
Ambulacral groove, 195
Ambulacral plates, 196
Ambulacral pores, 196
Ambulacral rows, 206
Ambystoma, 380
Ambystoma tigrinum, 371
Ameba, 33, 63-65, 66, 67, 68
Amebic dysentery, 88
Amebocytes, 200, 470
Ameboid cells, 133, 330, 356, 375, 468, 490, 518
Ameboid movement, 494
American hookworm, 182
Ametabola, 283, 284
Amino acids, 20, 35, 329
absorption, 35
Amitosis, 50
Ammocoetes, 342
Ammonoids, 541
Amnion, 383, 446
Amniotic cavity, 384

615

616 ANIMAL BIOLOGY

Amniotic fluid, 383 Annelids, 115, 234, 373, 468-470, 473 490,
Amniotic fold, 383 540, 541
Amniotic sac, 383 in general, 246-254
Amoeba, 63 Annual changes, 505, 510
Amoeba proteus, 63, 64 Annular cartilage, 342
Amphiaster, 48 Anodonta grandis, 212, 213
Amphiaster stage, 48 Anodontas, 219, 220
Amphibia, 340, 364, 370-381 . Anopheles, 90, 292
Amphibians, 314, 340, 365-381, 465-470, 491, Ant bears, 437
532, 533, 539, 543 Ant lions, 295
(See also Cecilians; Frog; Salamanders; Antarctica, 528
Toads) Anteaters, 534
Amphibolips confluens, gall, 291 spiny, 430
Amphicoelous vertebrae, 356 Antedon, 211
Amphineura, 221, 222 Antelopes, 439, 534
Amphioxus, 319, 320-321 Antennae, 189, 255-257, 266, 274, 276-278,
development, 124 282, 310, 311, 494
section, 313, 319, 320 Antennary glands, 494
Amphipods, 266 Antennata, 311
Ampullae, ear, 336, 337 Antennules, 255-257, 266
echinoderms, 197, 198, 208, 209 Anterior chamber, eye, 338, 339
Amylopsin, 329 Anterior ganglia, 244
Anableps, 360 Anthozoa, 145-148, 522
Anableps dovii, 360 Anthrax, 584
Anabolic processes, 461 Anthropoid apes, 449-454
Anabolism, 38, 43 Anthropology, 5
Anacondas, 396, 534 Antibodies (see Antitoxins)
Anaema, 570 Antimeres, 52, 574
Anal ciliated cells, 230 Antitoxins, 518, 519
Anal fin, 352, 354 Antrum, 335
Anal gland, 282 Ants, 286-288, 294, 296, 299, 300, 492, 509, 560,
Anal opening (see Anus) 572
Anal spot, 71 white, 457
Analogous resemblance, 572 (See also Termites)
Analogy, 54, 473 Anuraphis maidiradicis, 289
Anaphase, 48, 49 Anus, 71, 85, 140, 179, 195, 198, 203, 216, 234,
Anaphylaxis, 520 317, 329, 427, 466, 574, 575
Anatomy, 5, 579-581 Aorta, 319, 366, 386
Anaximander, 544 dorsal, 323, 342, 345, 346, 366
Andalusian fowl, inheritance, 556, 563 ventral, 345, 346, 366
Anger, 501 Aortic arch, 366, 367, 386, 388, 406, 427, 428
Angler fish, 364 Ape man, 452
Angleworm (see Earthworm) Apes, 437

anthropoid, 439, 449-454
Aphids, 289

corn root, 289
Aphis lions, 295
Apical plate, 230
Apiculture, 6

Animal biology, 6

Animal husbandry, 6
Animal organisms, 457-462
Animal pole, 120, 378
Animals, 424

in absence of oxygen, 491 Apis mellifica, 296-298
classification, 570-577 Apoda, 370, 374
compared and contrasted with plants, 41-43 Appendages, 54, 255, 256, 265, 303, 306, 322,
distribution, 4, 527-537 326, 354, 367, 369, 495
evolution, 544-554 jointed, 310
forms, 52, 54 (See also Fins; Legs; Wings; Tails; Antennae;
groups, 58 etc.)
limit of size, 4 Appendicular skeleton, 324
number, 3 Appendix vermiformis, 328, 427, 547
past distribution, 538-543 Apteryz, 533
relation of, to environment, 4 Aquatic environment, 488
to plants, 505 Aquatic habitat, 506
relations, 4 Aqueous humor, 227, 338
variety, 3 Aquiculture, 6

Annelida, 246, 249, 575, 576 Aquinas, Thomas, 545

INDEX

Arachnida, 302-309, 311
(See also Spiders)
Arbacia, 207
Arboreal life, 389, 437, 450, 453
Arch, foot, 451
Archaeopteryx, 409, 410, 543
Archaeopteryx macrura, 410
Archaeopteryx siemensi, 409
Archaeornithes, 409
Archean period, 540
Archenteron, 121, 122, 378, 446, 466
Archiannelida, 247
Archidiskodon, maibeni, 543
Arctic tern, 417
Argonauta argo, 228
Aristotle, 26, 364, 544, 570, 578
Aristotle’s lantern, 207, 208, 466
Armadillos, 424, 437, 534
nine-banded, 436, 568
Arms, 322, 452
Army worm, 290
Arrow worm, 188
Arteries, 330, 580, 581
Arthropoda, 265, 273, 276, 302, 310-311, 575, 576
Arthropods, 255-311, 465-467, 469, 490, 500, 541
diagrammatic structure, 310
Articular cartilage, 106
Artificial classification, 58
Artificial immunity, 519
Artificial parthenogenesis, 114-115
Artiodactyla, 439
foot bones, 435
Ascaris, 178-179, 491
Ascaris, 115, 116, 475, 526
Ascaris lumbricoides, 178, 178
Ascidian anatomy, 317
Ascidians, 316-317
Ascon, 131, 132
Asexual cycle, 89
Asexual reproduction, 46, 107, 136, 137, 142,
151-153, 171, 253, 254
Asexual spore, 89
Asexual zooid, 253, 254
Asphalt lake, fossils, 538
Asses, 447
wild, 535
Assimilation, 33, 34, 36, 43
Association fibers, 244
Associations of animals, 521-526
Associative memory, 505
Astacus, 258
Astacus fluviatilis, section, 258
Asterias feeding on clam, 199
Asterias vulgaris, 194, 195
bipinnaria larva, 200
Asteroidea, 204, 205
Asters, 47, 48, 114, 115
Astral rays, 47
Asymmetry, 52, 70
Atoll, 154
Atom, 8
Atrial cavity, 317, 319, 320
Atrial funnel, 316, 317
Atriopore, 317, 319, 320

Atrium, 320
genital, 162-164
of heart, 346
(See also Auricle)
Auditory meatus, 336
Auditory nerve, 336
Auditory organ, 277
(See also Ear)
Augustine, 544
Aurelia, life history, 151
Auricle, 214, 345, 346, 367, 386, 388, 406
Auricularia, 210
Austral region, 535
Australian lady beetle, 294
Australian region, 533, 534
Author’s names, 60, 573
Autonomic centers, 496
Autonomic nervous system, 333
Autosomes, 566
Autotomy, 187, 201, 264, 286, 517
Aves, 340, 403, 409
Avicularia, 191, 191
Avoiding reaction, 72
Axial gradient, 165, 242
Axial skeleton, 324
Axis cylinder fiber, 243
Axis cylinder process, 103
(See also Axon)
Axolotl, 371, 380
Axon, 102, 103, 243

Babbling thrushes, 535
Baboons, 439
Back swimmers, 289
Bacteria, 28, 86, 486, 540, 581
in chemical cycles, 487
in food cycle, 486
Bacterial action, 486, 487
Bacterial theory, 27
Bacteriology, 86
Badgers, 436, 445, 535
Bailer, 255
Baker, F. C., 223
Balance, animal life, 508
in body, 461
income and outgo, 458
Balancers, 280
Balanoglossida, 315, 316
Balanoglossus, 315, 576
Balantidial dysentery, 88
Balantidium, 88, 315
Balantidium coli, 84
Balanus hameri, 269
Balanus tintinnabulum, 269
Baleen, 443
Ballooning spiders, 305
Bandworms, 186, 187
Barbels, 336
Barbs, 404
Barbules, 404
Barn owl, 415
Barnacles, 268, 269, 522, 525
Barriers to dispersal, 528, 529

617

618

Basal disc, 136, 137, 141
Basal granules, 494
Basal metabolism, 493
Basal plate, 148
Basement membrane, 99, 260, 261, 465
Bases, 8
Basis of classification, 572
Basket stars, 206
Bats, 53, 432, 434, 435
wing, 53
Beach fleas, 267
Beaks, 386, 403, 412, 414
Bears, 436, 445, 535
foot, 435
Beasts, 424
Beavers, 437, 535
skull, 426
Béche-de-mer, 211
Bedbug, 275, 289
Bee milk, 297
Beebread, 297
Bees, 278, 296-300, 483, 491, 522, 572
proboscis, 278
Beetles, 276, 290, 294, 513
Behavior, 55-57, 497-504, 507, 521, 529-531
ameba, 66-67
amphibia, 376-377
amphioxus, 321
annelids, 252
arthropods, 273, 311
birds, 410-411, 419
chromosomes in maturation and fertilization,
556
coelenterates, 151
crayfish, 262
crustacea, 269
earthworm, 239
echinoderms, 210
fishes, 354, 360, 364
hydra, 141
insects, 295-298, 300
lampreys, 342
mammals, 443
modes of, 502
mollusks, 229
mussels, 217, 219-220
paramecium, 72—74
parasites, 177
planaria, 164-165
spiders, 305
sponges, 133
starfish, 200-201
Belly, muscle, 471
Benthos, 511
Bible, 26
Bichat, 582
Bighorn, 535
Bilateral symmetry, 52, 157, 166, 576
Bile, 329
Bile duct, 323
Binary division, 67
Binomial nomenclature, 59-60, 572, 573
Biogenesis, 270
Biogenetic law, 270, 333, 548, 583

ANIMAL BIOLOGY

Biogenetic series, 270-272, 325, 331-333, 366,
466, 547, 548
Biology, 6, 41
origin of word, 545
Biota, 505
Biotic factors, 507
Biotic succession, 508-510
Biparental reproduction, 107
Bivinnaria, 201, 210
Bipinnaria larva, 200
Biradial symmetry, 157, 576
Biramous appendages, 256, 265
Birds, 53, 113, 294, 314, 340, 382-385, 403-423,
460, 466, 470, 476, 478, 484, 503, 521, 528,
530, 633-5365, 537, 543, 567
beaks, 412
branchial arches, 366
colors, 416
feet, 413
lung, 469
wing, 53
Birds of paradise, 416
Birth, 447
Bisexuality, 107
(See also Diecious animals)
Bisons, 535
American, 440, 530, 535, 537
Biting lice, 288
Bittern, 509
beak, 412
Bivalve molluses, 221
Bivium, 203
Black widow spider, 306
Bladder, air, 350, 352, 354-355
gall, 329
swim, 354
urinary, 330
Bladder worm, 177
Blastocoel, 121-125, 142, 378, 444, 477
Blastoderm, 121, 122, 125, 142, 164, 263, 361,
362, 383, 477
Blastoids, 541
Blastomeres, 120-124, 164, 361, 378, 477
Blastopore, 121, 122, 378
Blastula, 121-125, 142, 151, 152, 201, 378, 444,
477
Blepharoplast, 79
Blind spot, 339
Blind worms, 374
Blister beetles, 287
Blood, 101, 216, 235, 236, 281, 330, 490, 520
corpuscles, 31, 236, 322, 330, 490, 581
plasma, 236, 322, 330, 490
platelets, 330
pressure, 490, 492
serum, 330, 519
vessels, 326
Blood-vascular system, 469, 490
nemertines, 186
(See also Circulatory system)
Blowing, of whales, 443
Boa constrictor, 393
Body, fighting disease, 518
self-regulation, 517

INDEX 619

Body cavity, 179, 180, 185, 188, 323, 576 Brush turkey, 420, 534
(See also Coelom; Hemocoel) Bryozoa, 190, 191, 574, 576
Body changes, 460 Bryozoans, 190-192, 465, 468-470, 541
Body coverings, 385, 484 fresh water, 192
Body divisions, 574-575 Bubonic plague, 294, 448
Body form, 353 Buccal cavity, 235, 237, 238, 327
Body heat, 483 (See also Mouth)
Body mass, 215 Buccal tentacles, 319, 321
Body plan, 322, 323 Budding, 86, 107, 134, 136, 137, 142, 151-153,
Body regions, 310 191, 253, 521
Body stalk, 444, 447 Buffalo, 440
Body surface in respiration, 468 Buffon, 545
Bog lake, 509 Bug, 289
Bone, 100, 101, 326, 491 Bugula avicularia, 191
cell, 31 Bulbus arteriosus, 347
corpuscles, 326 Bull snake, eggs, 394
Bones, 105, 106, 324-326 head, 390
dermal, 350 Bullhead, 336
(See also Skeleton) Bumble bees, 298
Bony fishes, 350 Burrowing insects, leg, 279
Bony ganoids, 350, 351 Burrowing owl, 537
Bony labyrinth, 336 Bursa, 182, 185
Book gills, 309, 468 Burying beetles, 295
Book lungs, 303, 304, 307, 468 Butterflies, 278, 286, 290, 572
Boreal region, 535 wing, 54
Borers, 290 Butterfly snails, 224
Botany, 41 Buttons, pearl shell, 230, 232
Bottom forms, 511 Byssus, 225
Bouton, 278
Bowfin, 351 Cc
Bowman’s capsule, 332
Brachiolaria, 210 Caducibranchs, 370, 371
Brachiopoda, 193, 574, 576 Caeca, 196, 198, 282, 303, 427, 467, 547
Brachiopods, 193, 465, 468-470, 541 Cake urchins, 207
section, 193 Calcarea, 130, 131
Brachystola magna, 277 Calciferous glands, 237
Brain, 229, 313, 318, 320-323, 334, 546, 575 Calcium, 19
alligator, 387, 388 Calcium metabolism, 492
bird, 408 Calf, embryo, 548
fish, 357, 358 Callinectes sapidus, 266
frog, 376, 377 Calosoma scrutator, leg, 279
honey bee, 281 Cambarus, 258
insect, 282 section, 257
lamprey, 342, 343 Cambarus diogenes, 256
mammals, 427 Cambarus obesus, 256
man, 451 Cambarus virilis, 25T
rabbit, 430 Cambrian period, 309, 539-543, 548
shark, 346, 347 Camels, 439, 447, 535, 554
Brain coral, 154 foot, 435
Branchial arches, 357, 366, 367 Campodea staphylinus, 284
Branchial arteries, 345, 346, 366 Canal systems, ctenophores, 156
Branchial pouch, 380 sponges, 131, 132
Branchiostegal membrane, 358, 359 Canaliculi, 101, 326
Branchiostegal rays, 352 Canary, 423
Breaking point, 264 Canine teeth, 425, 427
Breathing, of fishes, 358, 359, 427 Canis familiaris, 59
Breeding, 562, 563 Cankerworm, 290
Bridges, 527 Capillaries, 330
Brittle stars, 204, 205, 541 Capsule, joint, 106
Broadbills, 535 Carapace, 255, 388, 401
Bromine, 20 Carbohydrates, 20-21, 34, 35, 39, 482, 492
Bronchi, 469 absorption, 35
Brood pouch, 268, 269 Carbon, 19, 20, 486
Brown, Robert, 32 Carbon cycle, 486, 487

Bruner, L., 287 Carbon dioxide, 36, 37, 43, 330, 486-488, 490

620 ANIMAL BIOLOGY

Carboniferous period, 309, 369, 539, 542 Cells, protozoan, 516
Cardiac muscle, 327 receptor, 472
(See also Heart muscle) retinular, 260
Cardiac opening, 328 secretory epithelial, 100
Cardinal, 537 sex, 31, 109-113
Cardinal vein, 323, 345 sperm, 31
Carnivora, 429, 436 Cellulose, 42, 316
Carnivores, 447, 467 Cement gland, 188, 189, 574
feet, 435 Cementum, 324, 426
Carolina wren, 537 Cenogenetic adaptations, 479
Carotid artery, 345, 366, 367, 386, 428 types, 479
Carpals, 325 Cenozoic era, 543
Carpet beetle, 290 Centipedes, 274, 274, 275
Carpocapsa pomonella, 291 Central body, 30, 32, 47, 48, 114, 115
Carpus, 327, 368, 404 Central nervous system, 238, 259, 282, 333, 472,
Carrion beetles, 295 473, 496
Cartilage, 100, 101, 491 Central sulcus, 451
bone, 325 Centralization, 166, 282, 283, 459, 473, 496
of ear, 547 Centriole, 30, 32
Cartilaginous ganoids, 350, 351 Centrolecithal egg cells, 119-121, 269, 476, 477
Cassowaries, 534 Centrosome, 30
Castle, W. E., 562 Centrum, vertebra, 323
Catalysis, 14 Centrurus, 306
Catalyzers, 14 Cephalization, 282, 283, 310, 459, 473, 496
Cataract, congenital, 565 Cephalochordata, 314, 320, 576
Catastrophism, 582 Cephalopoda, 221, 225, 466, 541
Caterpillar, 284, 499 Cephalothorax, 255, 256, 302, 306, 309
Catfishes, 336, 354 Cercaria, 173, 174
Cats, 4386 Cere, 407
foot, 435 Cerebellum, 334, 342, 343, 346, 347, 357, 358,
Cattle, 439 375-377, 387, 388, 408, 427, 430, 451
(See also Ox) Cerebral cortex, 388
Cattle fever tick, 307 Cerebral hemispheres, 334
Caucasian type, 453 (See also Cerebrum)
Caudal artery, 345 Cerebral vesicle, 321
Caudal cirrus, 186 Cerebratulus, 186
Caudal fins, 188, 352, 354, 355 Cerebratulus lacteus, 186
Caudal gland, 181 Cerebropleural ganglia, 214, 217
Caudal vein, 345 Cerebrosides, 20
Caudal vertebrae, 326 Cerebrospinal system, 333, 375
Cave faunas, 512 Cerebrum, 343, 346, 357, 375-377, 387, 388,
Caviar, 364 408, 427, 430, 451
Cavities, table of, 125 Cervical groove, 255
Caymans, 399 Cervical vertebrae, 325, 326
Cecidiology, 293 Cestoda, 167-169
Cecilians, 370, 373 Cetacea, 429, 441
scales, 465 Chaetoderma nitidulum, 222
Cecropia silkworm, 498, 509 Chaetognatha, 188, 574, 576
Cell, 29, 30, 31, 32, 42 Chaetopoda, 247-249, 541
division, 47 Chalaza, 383
(See also Mitosis) Chalcid flies, 296
doctrine, 32 Chalina oculata, 131
of insect wing, 279 Chalk, 82
organs, 85 Chambered nautilus, 228
theory, 32, 462, 583 Chambers, eye, 338, 339
Cell wall, 29 Chameleo vulgaris, 389
Cells, 29-32 Chameleons, 386-389
blood, 31 Chamois, 535
Poneses 260 Characteristics, 555
ae i Characters, 555, 556
imotazoan. 516 acquired, 545, 549, 564
muscle, 31 breeding for, 562

nerve, 31, 103 dominant, 556
pigment, 356, 365 recessive, 556

INDEX

Characters, sex-linked, 568
tests, 562
Chat, yellow-breasted, 537
Checkerboard diagrams, 559-561
Cheeks, 424
Chela, 255, 256, 258, 262
Chelicera, 302, 303, 306
Cheliped, 255
Chemical changes in body, 480
Chemical control, 459-460
Chemical cycles, 486-488
Chemical energy, 15, 458
Chemical organization, 21
Chemistry, 8
Chemotropism, 56, 67, 73, 133, 141, 164, 239, 263
Chick, 118, 478, 548, 579
Chicken cholera, 584
Chigger, 307
Child, C. M., 165
Chimpanzee, 449, 534
Chinch bug, 289
Chiroptera, 429, 434
Chitin, 100, 465, 491
Chitons, 221, 222, 541
Chloragogue cells, 236
Chlorine, 19
Chlorophyll, 32
Cholera, 519
Cholesterol, 20
Chondrin, 100
Chondriosomes, 30
Chondrostei, 350, 351
Chorda dorsalis (see Notochord)
Chordata, 312-314, 575, 576
Chordates in general, 315-321, 465-470, 477, 490
Chorion, 384, 444, 446
Choroid layer, eye, 338, 339
Chromatin, 30, 47, 48, 49
Chromatophores, 79, 356, 375, 390
Chromidia, 84
Chromosomal fibers, 49
Chromosomes, 47—50, 109-111, 114-117, 476, 545,
549, 555, 566
homologous, 568
reduction, 116, 556
Chrysalis, 284, 499
Chrysaora hyoscella, 147
Chyle, 489
Chyme, 489
Cicadas, 283, 290, 506
Cilia, 69, 70, 85, 98, 99, 156, 161, 164, 187,
189, 230
Ciliary body, 339
Ciliary muscle, 339
Ciliata, 79
Ciliated bands, 200, 230
Ciliated epithelium, 98, 99, 465
Circulation, 33-35, 367, 483-484, 489-490, 493,
581
ameba, 33, 65
amphioxus. 320
annelids, 251
birds, 385, 406
earthworm, 234
hydra, 140

621

Circulation, insects, 281
mammals, 428, 446, 447
mussel, 216
paramecium, 71
planarian, 163
shark, 346
spider, 303-304
tunicate, 317
vertebrate, 330

Circulatory system, 36, 104,

468-469, 490, 547, 574, 575
amphioxus, 320
ascidian, 317
bird, 385, 388, 406, 407
brachiopod, 193
crayfish, 258, 259
earthworm, 235-236
fishes, 357
mammals, 427, 428
mussel, 216-217
nemertine, 186
sea cucumber, 209
shark, 345-346
spiders, 303
vertebrates, 330

Circumesophageal connectives, 259

Circumpharyngeal connectives, 238

Cirri, 83

Cirripedia, 268, 525

Cirrus, 162, 163, 170, 186

Cistudo lutaria, skeleton, 400

Cladoceran, 267, 268

Clams, 225, 230, 521

Claspers, 344

Classes, 59, 571

Classification, 58-60, 570-877
amphibians, 370
annelids, 247
arthropods, 311
birds, 409
chordates, 314
coelenterates, 145
of Cuvier, 582
cyclostomes, 341
echinoderms, 204
fishes, 350
flatworms, 166
mammals, 429
mollusks, 221
protozoans, 78
reptiles, 386
sponges, 130
threadworms, 178
vertebrates, 340

Clavicle, 325, 327

Claws, 324

Clear-winged moth, 513

Cleavage, 118-125, 476-477
amphibia, 378
amphioxus, 321
crayfish, 263
crustacea, 269
ctenophora, 157
earthworm, 241
fishes, 361, 362

105, 366, 367,

622 ANIMAL BIOLOGY

Cleavage, hydra, 142

insects, 283

mammals, 445

mollusks, 229

mussel, 218

planarians, 164

reptiles and birds, 383

starfish, 201
Cleavage cavity, 121

(See also Blastocoel)

Cleavage planes, 120
Cleveland, L. R., 524
Climbing birds, 414
Climbing perch, 364
Clitellum, 234, 235
Cloaca, 329

amphibia, 370, 372, 377, 379

birds, 406

hookworm, 182

mammal, 427, 429

mussel, 214, 216

reptiles, 386, 401

rotifer, 188, 189

vertebrate, 323, 329
Clonorchis sinensis, 167, 168
Closed circulatory system, 469
Clothes moth, 290, 483
Clotting, blood, 330, 490
Clypeus, 277
Cnidoblast, 138, 139, 497
Cnidocil, 138, 139
Coagulation, 490

(See also Clotting)

Cobras, 396, 433
Coccidia, 83
Cochineal insect, 287
Cochlea, 336, 337, 337, 427
Cockroaches, 275, 288, 294

Cocoon, 164, 241, 251, 292, 304, 491, 498, 499

Codling moth, 290, 291
Coelenterata, 145, 574, 576

Coelenterates, 136-155, 466, 469-472, 477, 489,

494, 496, 497
Coeliac artery, 345
Coelom, 125, 478, 494, 574-576
amphioxus, 313, 319
annelids, 257
brachiopods, 193
bryozoa, 190
crayfish, 258
earthworm, 234-236
mammals, 427
reptiles and birds, 383, 384
rotifers, 188
starfishes, 198
vertebrates, 322, 323, 326
(See also Extra-embryonic coelum)
Cold sensations, 473
Cold-blooded animals, 484
Cold-bloodedness, 484
Coleoptera, 290
Collar, 226
Collar cells, 129
Collateral, 102
Colloblasts, 157

Colloidal emulsions, 13, 14
Colloids, 12, 13
Colon, 328, 547

Colonial hydroids, 145, 146, 148, 149, 151, 152,

457, 522

Colonies, 134, 145, 146, 148-150, 190-192, 249,

253, 269, 318, 319, 522

Color, amphibia, 375

annelids, 249

birds, 416

chaetopods, 248

chameleon, 390

crustacea, 265

echinoderms, 210

fishes, 356

hydra, 136

hydroids, 148-149

jelly fishes, 157

mollusks, 225

onychophora, 274

turbellaria, 167
Color-blindedness, inheritance, 568-569
Coloration, concealing, 513

recognition, 514

warning, 514
Columbia livia, anatomy, 407
Columella, 337, 368
Columnar epithelium, 98, 99
Comb, 297, 491
Comb jellies, 156
Combustion, 480
Commissure, 161
Common names, 60
Communities, animal, 507-508
Comparative anatomy, 546, 581
Comparative embryology, 547
Competition, 528, 544
Complete metamorphosis, 284, 478-479
Compound eye, 260, 261, 276, 278
Compounds, 7, 8, 20
Comstock, J. H., 298
Concealing coloration, 513
Conchology, 5
Conditioned reflexes, 496
Conductivity, 56, 495
Condyle, 382
Conjugation, 75-77
Conjunctiva, 339
Connective fibers, 491
Connective tissues, 100-101
Connectives, 161
Consciousness, 501, 503
Conservation, of energy, 16

of matter, 9
Continental islands, 532
Continuity, cell life, 50

chromatin, 50

germ plasm, 96
Continuous fibers, 49
Continuous phase, 13
Continuous stimulus, 55
Contour feathers, 405
Contractile cells, 471
Contractile fibers, 138, 139, 471
Contractile fibril, 102

INDEX

Contractile vacuole, 64, 65, 70, 79, 85, 86, 468,
469, 494
Contractility, 495
Contrast, living and nonliving matter, 17-18
Conus arteriosus, 345, 347
Convergence, 473, 474, 572
Coot, foot, 413
Copepoda, 268
Copper, 20
Copperhead, 396, 397
Copulation, 164, 240, 241, 281, 304
Copulatory appendages, 255, 258
Coracoid, 327, 400
Coral, 145, 148, 153-155, 541
Coral reefs, 154, 155
Coral rock, 155
Coral snakes, 396, 397
Coralline bryozoa, 191
Corium, 324, 465
Coriza, 279
Cormorant, foot, 413
Corn bollworm, 290
Corn root aphis, 289
Cornea, 227, 252, 260, 261, 338, 339
Corpuscles, blood, 236
(See also Blood corpuscles)
Cortex, cerebral, 377, 388
paramecium, 70
Cosmopolitan animals, 527
Costal plate, 400
Cotton mouth, 396
Cotton worm, 290
Coughing, 496
Cowbird, 419
Coxal glands, 304
Coyote, skull, 426
Crab, blue, 266
fishing, 270
soft-shelled, 265
Crabs, 265-266, 269, 523-525
Cranefly, 280
Cranial nerves, 333
Cranium, 325, 326, 398, 453
Crawdads, 255
Crawfish, 255
Crayfish, 52, 116, 255-264, 534
sections, 257, 258
Creation of life, 26—28, 41
Cretaceous period, 539, 543
Crickets, 279, 288, 294
Crinoidea, 205, 209, 541
Crocodiles, 340, 387, 399-400, 523
heart, 386
Crocodilia, 387, 399
Cro-Magnon race, 452, 453
Crop, 235, 237, 251, 282, 328, 406, 407
Cross-fertilization, 108, 143, 164, 228, 251, 317
Crossbill, beak, 412
Crossbreeding, 564
Crossing over, 569
Crossopterygii, 350, 351, 368
Crotalus confluentes, skull, 398
Crow, albino, 550
Crustacea, 265, 311

623

Crustaceans, 265-272, 465, 490, 494, 508, 522,
523, 540, 541
Cryptobranchus, 371
Cryptopsaras couesti, 363
Crystalline cone, 260, 261
Crystalline style, 225
Crystalloids, 12
Ctenoid scales, 353, 354
Ctenophora, 1466-158, 573, 574
Ctenophores, 156, 464, 466, 469, 471, 472, 478,
489, 490, 495
Cuckoos, 419, 535
Cucumaria planci, 209
Culex, 90, 292
Cultivation, modifications, 546
Curassows, 534
Currents, 55, 507, 528
Cutaneous artery, 367
Cutaneous sense organs, 334-336
Cuticle, 69, 79, 85, 137, 138, 465
(See also Skin)
Cuticula, 100, 166, 167, 188, 234, 236, 310,
465
Cuttlefish, 226, 227, 466, 541
bone, 233
eye, 227
Cutworms, 294
Cuvier, 581, 582
Cuvierian organs, 209
Cyanogen theory, 27
Cycles, chemical, 486-488
food, 508
life, 44-45, 460-462
Cycloid scales, 353, 354
Cyclops, 268
Cyclostomata, 340-343
Cyclostomes, 341
Cynodonts, 429
Cypris, 268
Cypris, 525
Cypselurus, tail, 355
Cysticercus, 177, 178
Cystoids, 541
Cytoplasm, 29, 30, 84, 102, 111, 112, 119-121
463

Daddy longlegs, 308

Damsel flies, 294

Danaus archippus, 286
Daphnia pulex, 268

Dark ages, 579

Darwin, Charles, 32, 242, 452, 545, 546, 549, 557
Darwin, Erasmus, 545
Darwinism, 545

Dasypus novemcinctus, 436
Day and night rhythms, 510
Deafness, heredity, 564
Dealated ant, 299

Dealated termite, 288
Death, 24, 458

Decapods, 265

Deciduous wings, 286
Deep-sea fishes, 363

624

Deer, 439, 534, 535
foot bones, 435
Virginia, 537
Defective genes, 564
Degeneration, 189, 203, 318, 320, 512
Delamination, 477
Demospongiae, 130
Dendrites, 102, 103, 243
Dendron, 103
(See also Dendrites)
Dendrostoma alutaceum, 252
Dentalium pretiosum, 225
Dentine, 324, 425, 427
Dentition, 426
Dermal bones, 350
Dermal gills (see Dermobranchiae)
Dermal layer, 132, 464
Dermal scales, 374, 424
Dermis, 324, 365, 425, 465
Dermobranchiae, 196, 200
Desert faunas, 510, 512
Desmognathus, 371
Determination of sex, 566—568
Determiners, 555
Development, ascaris, 181-182
butterfly, 286
fish, 361
frog, 378, 379, 380
Gordius, 184-185
hookworm, 182-183
locust, 285
mammal, 444, 446, 447
mosquito, 292, 293
reptiles and birds, 382-385
shrimp, 270, 271
Trichinella, 183, 184
tunicate, 317, 318
(See also Life history)
Devil fish, 226, 227
Devonian period, 350, 369, 539, 542
De Vries, Hugo, 550
Dextrose, 21
Diabetes, 492
inheritance, 564
Dialysis, 12
Dialyzers, 12
Diaphragm, 328, 400, 427
Diastrophus nebulosus, gall, 291
Didelphia, 429
Diecious animals, 107, 181, 189, 192, 198, 223,
229, 248, 269, 275, 321, 339, 471
Diencephalon, 334, 343, 346, 376, 399
Differentiation, 85, 95, 458-460, 475
Difflugia urceolata, 81
Diffuse nervous system, 472
Diffusion, 11
Digestion, 33, 35, 488-489
Digestive enzymes, 13, 65, 86, 99, 133, 140, 235,
327, 329, 467, 489
Digestive system, 104, 105, 327-329, 466-467,
574-575
annelids, 247
ascidian, 317
beetle, 282
bird, 406, 407

ANIMAL BIOLOGY

Digestive system, crayfish, 258
earthworm, 237
fishes, 356-357
honeybee, 281
leech, 250
mussel, 214, 216
planaria, 160
sea cucumber, 209
sea urchin, 208
shark, 345
spider, 303
starfish, 198
vertebrate, 323
(See also Alimentary Canal)
Digitigrade foot, 435
Dihybrids, 560-561
Dimorphism, 471, 572
Dinophilus, 247
Dinosaurs, 403, 410, 411, 543
Diphtheria, 519
Diphycercal tail, 355
Diploblastie condition, 130, 144, 574-576
Diploblastic embryo, 123
Diplodinium ecaudatum, 85
Diploid number, 110, 111
Dipnoi, 350, 352
Diptera, 292, 293
Direct response, 55, 497-498
Disc, basal, 134-136
starfish, 194, 195
Discoidal cleavage, 119, 120, 361, 362, 386, 477
Discontinuous distribution, 527
Discontinuous stimulus, 55, 495
Disease, 294, 484, 515-520, 564
Dispersal, animals, 190, 220, 528-529
Disperse phase, 13, 22
Dispersion medium, 13
Dissimilation, 33, 34, 36, 38, 43, 65, 71, 170
Dissociation, 12
Distribution, characteristics in hybrids, 588
discontinuous, 527
geographic, 5, 527-537, 549
marine life, 511
past, 527, 538-543
Diurnal rhythms, 510
Divergence, 473-474
Diving beetles, 509
Diving birds, 414
Division of labor, 95, 297, 301, 458-459, 522
germ cells, 113
Dodo, 422
Dogfish sharks, 344, 345, 353
brain, 346
circulatory system, 345
Dogs, 58, 436, 502
Dolichoglossus kowalevskii, 316
Dolphins, 441, 474
Domesticated birds, 423
Dominance, 300, 503, 558
incomplete, 563
Dominant genes, 556, 558-560, 564
Dormancy, 24, 458
Dorsal aorta (see Aorta, dorsal)
Dorsal blood vessel, 236, 237
Dorsal fins, 352, 354

INDEX

Dorsal line, 179
Dorsal nerve cord, 179
Dorsal pores, 234
Dorsal root, 333, 334
Dorsal tubular nervous system, 313
Dorsal vessels, 236, 237, 258
Double circulation, 367
Down, 405
feathers, 405, 406
Dragon flies, 294, 542
Dreaming, 502
Drones, 296, 297
Drosophila, 549, 550, 569
Dryophantes tanata, gall, 291
Duckbill, 429, 431
Ducks, 414, 420, 423, 528
beak, 412
foot, 413
Duct, 467
Ductless glands, 373-374, 492-493
Dugong, 441
Dujardin, 19, 32, 583
Dura mater, 375
Durant, W. J., 579
Dwarfness, 565
Dyad, 476
Dysentery, 88
Dytiscus, leg, 279

E
Ear, 336-338, 347, 375, 386, 389, 390, 407, 408,
428, 473
fish, 360
human, 336

Ear muscles, man, 547
Ears, planarian, 159
Earthworm, 52, 234-242, 491
Earwigs, 294
Ebers, George, 578
Ecdysis, 264
(See also Molting)
Echidna, 430
Echinococcus granulosus, 171
Echinodermata, 203-211, 575, 576
Echinoderms, 115, 194-202, 465, 466, 469, 470,
472, 490, 491, 494, 511
Echinohynchus ranae, 185
Echinoidea, 204, 206
Echinopluteus, 210
Echiurus pallassii, 252
Ecology, 5, 505-514
Economic importance, amphibians, 381
annelids, 254
birds, 423
coelenterates, 155
crayfish, 264
crustacea, 270
ctenophores, 158
earthworms, 242
echinoderms, 210-211
elasmobranchs, 347
fishes, 364
flatworms, 171, 177
insects, 296-301

625

Economie importance, Jampreys and hagfishes
343
lower chordates, 321
mammals, 447
mollusks, 230
protozoa, 81—84, 88-91
reptiles, 401
sharks, 348
spiders, 306-309
sponges, 135
starfishes, 202
threadworms, 185
Ectoderm, 121-125, 129, 138, 143, 187, 196, 444,
446
Ectoparasites, 525
Ectoplasm, 63-65, 66, 70, 85
Ectopterygoid, 398
Edentata, 429, 437
Edible snail, 223
Eel, 341
electric, 534
migration, 530
Eelworm, 178
Effector cells, 165
Effectors, 165, 243, 496, 498
Efferent impulse, 165, 333
Efferent path, 244, 333
Egestion, 33, 34, 37, 38, 65, 71, 104, 140, 163,
181
Egg cells, 31, 45, 46, 118-120, 122, 476, 521,
556, 558, 560, 566-568, 583
cestodes, 169, 176
coelenterates, 151-153
development of, in birds, 384, 385
earthworm, 240, 241
hydra, 137, 142
mammals, 445
metazoa, 107-108
mussel, 218
nematodes, 181
origin of, 109-113
planaria, 163-164
sponges, 134
starfish, 198
types, 119
vertebrate, 340
Egg-laying mammals, 429
Eggs, 108, 113, 476
annelids, 251
birds, 383, 384, 385, 418, 419
bryozoa, 192
bull snake, 394
butterfly, 286
cecilian, 373
crayfish, 263
crustacea, 268, 269
fish, 361, 362
frog, 374, 378, 379
grasshopper, 285
hen, 383
honeybee, 297
humming bird, 418
insects, 283
mollusks, 229
monotremes, 429

626

Eggs, mosquito, 292
mourning dove, 420
mussel, 218
myriapods, 275
ostrich, 418
parasitic nematodes, 181
reptiles, 383
robin, 421
rotifer, 189, 190
sharks, 347
sheep liver fluke, 173
spider, 304
tapeworm, 176
Texas nighthawk, 419
turtles, 401
Egyptians, medicine, 578
Eimeria stiedae, 82
Elasmobranchii, 340
Elasmobranchs, 340, 344
extinct, 347
Elastic fibers, 101
Electric eels, 534
Electric organs, 347
Electric rays, 347
Electrical energy, 38
Electrolysis, 12
Electrolytes, 11
Electrons, 8
Electrotropism, 56
Elements, 8, 19
Elephantiasis, 184
Elephants, 441, 442, 534, 551-553
evolutionary series, 552
Elephas, 552
Elimination, 33, 34, 37, 43, 104, 329, 331, 469-
470, 493-494, 516
(See also Digestive system)
Elk, Irish, 551
Elk-horn coral, 154
Elytra, 280
Emaciation, 44
Embryo, 108, 176-177, 361, 362, 378, 384, 444,
476-478, 547, 548
(See also Eggs)
Embryogeny, 118-125, 201, 445, 475-478, 546
variations, 123
(See also Life history)
Embryology, 5, 118, 475, 583
Embryonic circulation, mammals, 446
Embryonic development, mammals, 444
Embryonic modifications, reptiles and birds,
382
Emotions, 492, 501
Empedocles, 26, 544, 578
Emu, 420, 534
Emulsion, 11
Enaema, 570
Enamel, 100, 324, 354, 426, 427
Encephalon, 334
Encystment, 68, 517
Endamoeba, histolytic, 88
Endocrine glands, 373, 374, 459, 461
Endoderm (see Entoderm)
Endolymphatie duct, 336, 337
Endomixis, 76, 77

ANIMAL BIOLOGY

Endoparasites, 525
Endoplasm, 63, 64, 70, 85
Endoskeleton, 322, 324, 465-466
Endostyle, 317—320
Endothelium, 469
Energy, 15-16, 38, 458, 492
changes in organisms, 480-485
conservation, 16
Engine, and body, 481
Engraver beetle, work, 290
Enteron, 137, 160
(See also Alimentary canal; Gastrovascular
cavity)
Enteropneusta, 576
Enterozoa, 130, 574, 575
Entoderm, 121, 122, 124, 125, 129, 130, 137, 144,
157, 312, 444
Entomology, 5
Entomostraca, 267, 268
Entozoic, 80
Enzymes, 14, 329
(See also Digestive enzymes; Ferments)
Eocene period, 551
Eohippus, 553
Ephelota gemmipara, 86
Ephydatia fluviatilis, 131
Ephyra, 152
Epibole, 123, 378, 477
Epidermis, 324, 365, 465
Epigenesis, 583
Epiglottis, 427
Epigynum, 303
Epimere, 478
Epipharynx, 278
Epithelia, 98-99, 104, 132, 324
(See also Skin)
Epithelial cells, 31
Epitheliomuscular cells, 23, 138
Epizoic associations, 522
Equal cleavage, 118, 120, 477
Equatorial plate, 47
Equilibrium, 347, 473
crayfish, 261
fish, 360
man, 337
Equilibrium, sense, 335, 347, 408
Erect posture, 451
Erector muscle of hair, 324
Esophagus, 198, 214, 216, 235, 237, 258, 259,
282, 328, 342, 407, 438
Esophageal pouch, 237
Esox lucius, scale, 353
Estivation, 381
Ethiopian region, 533, 534
Ethmoidal cells, 335
Eugenics, 569
Euglena, 79
Eunice viridis, 254
Euplectella, 131
Eupomotus gibbosus, mouth, 358
Eurosta solidaginis, gall, 291
Eurypterids, 542
Eustachian tube, 335-337, 368
Eutheria, 429
Evaporation, heat regulation, 483-484

Evening primroses, mutation, 550
Evolution, 6, 36, 544-554
evidence of, 546
fin to limb, 367
in man, 454
Evolutionary series, 551
elephant, 552
horse, 553
Excretion, 33, 37, 493-494
(See also Excretory system)
Excretions, 65, 71, 135
Excretory pore, 160, 168, 178

Excretory system, 104-105, 323, 469-470, 494

crayfish, 259

earthworm, 236

fishes, 357-358

mussel, 217

planaria, 160, 161

rotifer, 189

spiders, 304

tapeworm, 170

threadworm, 169-170

vertebrates, 330-333
Excretory tubes, 160, 168
Excretory tubule, 161
Exoskeleton, 259, 313, 324, 465-466
Expiration, 33, 37, 43, 71, 467-468

(See also Respiration)

External gills, 379, 468
External inspiration, 36, 37
External structure, arthropods, 310

birds, 403-404

crayfish, 255-258

ctenophores, 156-157

earthworm, 234

fishes, 353, 363

hydra, 136-137

insects, 276-281

mammals, 424

mussel, 212-213

planaria, 159

reptiles, 386

spiders, 302-303

sponges, 130

starfishes, 195-196
Exteroceptors, 473
Extra digits, inheritance, 565
Extracellular digestion, 140
Extra-embryonic coelom, 384, 444
Eyeball, 338
Eyelids, 368, 370, 375, 391, 424, 547
Eyes, 187, 222, 248, 338-339, 368

amphibia, 375

annelids, 252

birds, 408-409

compound, 276, 261

crayfish, 256, 260-261

crustacea, 266

fish, 359, 363

insects, 276-278, 282

mammals, 427

mollusks, 222, 223, 225, 227

pineal, 399

reptiles, 389, 395

simple, 302

INDEX

Eyes, spider, 303
squid, 226
stalked, 256
tunicates, 318
Eyespots, 187, 247, 252, 321

Fi, F:, etc., generations, 559-561
Factors, 555
Family association, 521
Family name, 59, 573
Fangs, snakes, 398, 399
Fasciola hepatica, 172, 173
Fat body, 373
Fat tissue, 100
Fatigue, 495
Fats, 20, 34, 39, 482
Fatty acids, 35
Fauces, 425
Faunal divisions, 534-537
Faunas, cave, 512
desert, 512
forest, 512
fresh-water, 512
island, 532-534
marine, 510-511, 529-530
stream, 512
terrestrial, 512
Fear, 501
Feather stars, 205, 209, 211
Feather tracts, 406
Feathers, 324, 385, 404, 405, 420
Feces, 37
(See also Excretions)
Feeble-mindedness, inheritance, 565

Feeding and sex determination, 566-568

Feet, bird, 413

carnivore, 435

even-toed ungulates, 435
Felidae, 573
Felis, 573

627

Female, 45, 106, 181, 182, 263, 266, 268, 288,

296-299, 331, 471, 566-568
Femur, 277, 325, 327, 400, 404
Fenestra ovalis, 336, 337
Fenestra rotunda, 336, 337
Fermentation, 14, 583
Ferments, 14, 28, 104, 492

(See also Enzymes)
Fertilization, 46, 114-117, 476

internal and external, 108
Fetal circulation, 446
Fetus, 478
Fibrillar structure, protoplasm, 23
Fibrin, 490
Fibrinogen, 490
Fibrous tissue, 100
Fibula, 325, 327, 400, 404
Filaria, 184
Filial generations (see F'1, F2, ete.)
Filoplumes, 405, 406
Fin rays, 319, 320, 323, 367
Final host, 172
Fingers, 409

628

Fins, 188, 226, 319, 323, 352, 354, 355, 367, 373
Fire and body compared, 480
Fireflies, 283
Fish lice, 267
Fishes, 341-364, 365, 367, 502, 5238, 530, 539,
542, 543, 548
Fission, 67, 74, 107, 142, 253
Fissipedia, 436
Flagella, 79, 133, 494
Flagellata, 79
Flagellated cells, 137
Flagellated chambers, 132
Flame cells, 161, 173, 470, 494
Flatworms, 159-177, 466, 470-472, 490, 498
Fleas, 294, 525
Flex gliding, 411
Flexor muscles, bird, 414
Flicker, foot, 413
Flies, 280, 283, 287, 293, 513, 525
foot, 279
Flight, 410-413, 435
Flight feathers, 405
Fluid, 9
Flukes, 167-168, 172-177
Fluorine, 20
Flying dragon, 391, 392
Flying fish, 356
squirrels, 433
tail, 355
Flying foxes, 534
Flying spiders, 305
Follicle mites, 308
Food, 33-40, 42, 43, 65, 230, 294, 297-298,
352, 374, 430, 482, 516
Food chains, 508
Food cycle, 508
Food vacuoles, 64, 70, 466, 488
Foot, 212, 213, 215, 221-225, 279, 325, 368, 389,
404, 414, 432, 435, 451
Foramen, 193
Foraminifera, 82, 541
Forebrain, 334
Foregut, 467
Forest faunas, 512
Fossil men, 456
Fossilization, 538
Fossils, 538
Four-o’clocks, inheritance, 562
Fovea centralis, 339
Fowls, domestic, 112, 420, 563
Foxes, 436, 534, 537
Fragmentation, 107, 171
Fraternal twins, 568
Fresh-water faunas, 512
Frog, 340, 370-381, 481
Frontal sinus, 335
Fruit fly (see Drosophila)
Fry, 362
Functions, animal organisms, 486-496
Fungia, 154
Funiculus, 101

Galen, 579, 581
Galileo, 581

ANIMAL BIOLOGY

Gall bladder, 328, 329

Gall flies, 292

Gall gnats, 108

Gall mites, 308

Gall wasps, 292

Galls, 291, 293

Gametes, 45, 86, 89, 142, 475, 559, 532

Gametocytes, 89

Gametogenesis, 109, 142, 475, 546
variations in, 113

Ganglia, 160, 161, 224, 244, 333, 472

Ganglionic synaptic nervous system, 473

Ganoid scales, 353, 354

Ganoids, 340, 350, 351, 542

Ganoin, 354

Gar pikes, 351

Gars, 351

Garter snakes, 392, 509

Gases, 9, 491

Gastraea, 272

Gastral cavity, 130, 132, 466

Gastral layer, 132

Gastric gland, 282

Gastric gland cells, 138

Gastropoda, 221, 222, 513

Gastrostomus bairdii, 363

Gastrovascular cavity, 137, 144, 160, 166, 466,

574, 575

Gastrula, 121-124, 151, 218

Gastrulation, 121-124, 477

Gavials, 399

Geckos, 390

Geese, 419-423

Gel, 13, 14

Gemmation, 86

Gemmules, 134

Gene mutations, 563

Genealogical tree (see Phylogenetic tree)

General physiology, 486-496

Generic names, 59, 573

Genes, 555-557, 559, 560, 562-565, 568

Genesis, 26

Genetics, 6, 545, 555-569, 583

Genital atrium, 162, 163

Genital duct, 214, 216

Genital opening, 223

Genital pore, 159, 162, 163, 168, 170

Genotype, 556

Genotypic ratio, 560-561

Genus, 59

Geographical distribution, 571

Geological ages, 540

Geological time scale, 539, 540

Geotropism, 56, 72, 74

Gephyrea, 247, 251, 252

Germ cells, 96, 109-113, 473-479
division of labor, 113
potential immortality, 479

Germ layers, 122, 123, 463, 574, 575

(See also Ectoderm; Entoderm; Mesoderm)

Germ plasm, 96, 479, 545

Germinal area, 383

Germinal disc, 362

Germinal epithelium, 98

Giant fibers, 244

INDEX

Giant tortoises, 401
Giardia, 80
Giardia lamblia, 80
Gibbons, 449, 450, 534
Gila monster, 392, 402
Gill arches, 319
Gill chambers, 257, 258
Gill filaments, 215, 357
Gill rakers, 357
Gill slits, 319, 322, 329-330, 358, 359, 379
Gills, 215, 280, 357, 378, 379, 468
Giraffe, 441
Girdles, 325
Gizzard, 235, 237, 282, 406
Glacial lakes, succession, 509
Gland cells, 23, 137, 138, 243, 467
Glands, 467, 468, 484
alveolar, 467
calciferous, 237
coxal, 304
mucous, 365
parathyroid, 492
tubular, 467, 468
Glandular epithelium, 98, 467
Glass snake, 390
Glenoid cavity, 327
Gliding, 411
Globigerina ooze, 82
Glochidium, 218
Glomerulus, 332
Glomus, 332
Glottis, 391, 427
Glowworms, 283
Glue cells, 157
Glycerin, 20, 35
Glycogen, 21, 329
Gnathostomata, 341
Goats, 439, 535
Gobies, 360, 364
Goblet cells, 99, 100
Golden plover, 418
Goldfish, 364
Golgi bodies, 30
Gonads, 108, 142, 150-152, 157, 214, 317, 319,
345, 493
Gonangia, 152
Gordiacea, 180, 185
Gordius, 184
Gorgonia, 149
Gorilla, 449, 450, 534
Grafting, 165, 242
Grain moth, 290
Grantia ciliata, 131
Granular structure, protoplasm, 23
Graptolites, 541
Grasshopper, 277
Grassi, 91
Gray matter, 333, 334
Grayfish, 349
Great Barrier Reef, 154, 155
Greeks, 578
Green glands, 258, 259, 470, 494
Green hydra, 457
Gregarina blattarum, 82
Gregarina polymorpha, 82

629

Gregarines, 83
Gregariousness, 522
Grosbeak, beak, 412
Ground beetles, 279, 294
Ground hog, 445
Ground squirrels, 443, 484, 537
Groups, animal, 58
Grouse, spruce, 537
Growth, 18, 44, 51, 65, 492
cycles, 44
in plants and animals, 42
Growth period, 109-112
Grub, white, 185, 284
Gryllotalpa hexadactyla, 279
Guanin, 356, 364, 375
Guano, 423
Guans, 534
Guinea fowls, 534
Guinea pigs, 559, 560, 562
Gullet, 69, 70, 79, 146
Gulls, 414
Gustatory cells, 335
Gypsy moth, 290

Habit, 201, 263, 305, 361, 377, 500-501, 503
Habitat, 506
Haeckel, 41, 571
Hag fishes, 340-343
Hair, 54, 324, 424, 425, 534, 547
Hair feathers, 405, 406
Hair worms, 184
Halteres, 280
Hand, 325
Haploid number, 110, 111
Hares, 437, 537
Harvest flies, 285
Harvestmen, 308
Harvey, 580, 581
Haversian canals, 101
Hawks, 414, 509

beak, 412

foot, 413
Hay fever, 520
Head, 159, 222, 248, 322

lizard, 390

snake, 390
Head kidney, 332
Health, 515, 516, 520
Hearing, 283, 334-338, 389, 408, 428, 473
Heart, 216, 236, 237, 257-259, 281, 317, 322, 330,

367, 386, 406, 407, 428, 469, 481
Heart muscle, 102
Heat, 38, 458, 473, 483-485
(See also Temperature)

Heat regulation, 483
Hedgehogs, 434

skull, 426
Heidelberg man, 453
Heliosphaera inermis, 81
Heliozoa, 83
Helix pomatia, 223
Hellbender, 371
Helmholtz, 27

630

Helminthology, 5
Hemichordata, 314, 315, 469, 576
Hemimetabola, 284, 285
Hemiptera, 289
Hemocoel, 259, 281, 478
Hemocoelic body cavity, 273
Hemocyanin, 490
Hemoglobin, 236, 330, 490
Hemogregarine, 82
Hemolymph, 490
Hemolysis, 490
Hemophilia, inheritance, 564
Hen, egg, 383
Hepatic artery, 345
Hepatic caeca, 198
Hepatic portal system, 345, 346, 370, 388, 428
Hepatic vein, 345
Herbivores, 467
Hereditary units, 97, 549, 555, 557
(See also Genes)
Hermaphroditism, 107
Hermit crab, 266, 524
and sea anemone, 266, 523, 524
Herodotus, 523
Herons, 414
Herpetology, 5
Heterocercal tail, 344, 355
Heterometabola, 284
Heteronomous metamerism, 52
Heterosis, 564
Heterozygous condition, 556, 558-560
Hexactinellida, 130
Hibernation, 381, 444, 484
Hind-brain, 334
Hind-gut, 281, 467
Hinge ligament, 212-214
Hinge teeth, 213
Hippocrates, 578
Hippopotamus, 439, 534
Hirudinea, 247, 250
Hirudo medicinalis, 250
Histology, 5, 582
History, of evolution, 544
of zoology, 570, 578, 584
Hoatzin, 534
Holoblastic cleavage, 120
Holoblastic egg cell, 120, 476, 477
Holometabola, 284, 286
Holophytic, 79
Holostei, 350, 351
Holothurioidea, 204, 207, 541
Holozoic, 79
Homing instinct, 529
Homo, 452
Homo sapiens, 453
Homocercal tail fin, 355, 356
Homoiothermous animals, 484
Homolecithal egg cell, 118, 118-121, 476
Homologous chromosomes, 568
resemblance, 572
Homology, 53, 54, 265, 326, 327, 404, 473, 546,
572
fore and hind limbs, 327
Homonomous metamerism, 52
Homoptera, 289, 290

ANIMAL BIOLOGY

Homozygous condition, 556, 558-560
Homunculus, 583
Honey, 287
Honey sac, 281
Honey suckers, 534
Honey tube, 289
Honeybee, 281, 296, 297, 298
head, 278
Honeydew, 290
Hoofs, 324
Hooke, 32, 581, 583
Hooks, tapeworm, 171
Hookworm, 182
Horizontal life zones, 530
Hormones, 459, 461, 480, 492, 501, 549
Horn, 100
Hornbills, 535
Horned toads, 391
nest of bald-faced, 300
Hornets, 300
Horns, 54
Horse, 53, 441, 447, 502, 535, 553
evolutionary series, 553
fore leg, 53
Horsehair snake, 184
Horseshoe crab, 309
Host, 172, 525
House centipede, 275
House fly, 293
foot, 279
Hovering, 411
Human cochlea, 337
Human ear, 336
Human parasites, 171, 172, 175, 178, 182-184,
251, 286-288, 307
Human skeleton, 325
Human taste buds, 335
Humerus, 325, 327, 367, 400, 404
Humidity, 529
Humming birds, 410, 535
Hundred-legged worms, 274
Hunger, 473, 529
Huronian period, 539, 540
Huxley, 545
Hyallela, 267
Hyallela dentata, 267, 528
Hybridization, 108, 551, 559
Hybrids, 550, 558, 559
Hydatid cysts, 171
Hydatids, 171
Hydra, 136-143, 153, 457, 472, 524
green, 457
nerve-net, 472
Hydra oligactis, 140
Hydra viridissima, 136, 143
Hydranths, 152
Hydration, 488
Hydrochloric acid, 65, 328, 489
Hydrogen, 19
Hydrogen-ion concentration, 507
Hydroids, 145, 146, 148, 149, 150, 266, 522, 523,
541
color, 148
Hydrophobia, 584
Hydrostatic organs, 350

Hydrozoa, 145

Hyenas, 434, 436
Hygiene, 6, 520

Hyla versicolor, 373
Hymenoptera, 292, 296
Hyoid arch, 382
Hypermetamorphosis, 286, 479
Hypertrophy, 293
Hypodermis, 236, 244, 465
Hypomere, 478
Hypopharynx, 278
Hypophysis, 317, 408
Hypostome, 136, 139
Hyracotherium, 553

I-beam principle, 406
Ibex, 535
Ice fish, 362
Icerya purchasi, 294
Ichneumon flies, 296
Ichthyology, 5
Ichthyophis, 373
Ichthyosaurus, 474
Identical twins, 568
Idiacanthus ferox, 363
Idiosyncrasy, 517
Iguanas, 392
Tleum, 328, 547
Iliac artery, 345, 428
Iliac vein, 345, 428
Ilium, 327, 400
Imago, 286
Immunity, 519-520
Impulse, nervous, 103, 243
Inbreeding, 563, 564
Incisors, 425
Income and outgo, organism, 458
Incomplete dominance, 563
Incomplete metamorphosis, 284, 285
Incubation, 420
Incurrent canals, 132
Incus, 336, 337, 428
Independent effectors, 496
Indirect response, 57
Individuality, 460, 481, 517
Infection, 517
Infective organisms, 517
Infertile females, 296
Infusoria, 7, 83, 84, 88
Ingestion, 33, 35, 65, 104, 358-359, 374-375
Inheritance, abnormalities, 564
acquired characters, 545, 564
diagram, 558
disease, 516, 564
in guinea pig, 559, 560
in organisms, 555-569
small eyes, 565
tapering fingers, 566
(See also Genetics)
Inherited immunity, 519
Ink sac, 226
Inner cell mass, 445
Inner chamber, eye, 338, 339

INDEX 631

Inner ear, 336-338, 347, 360
Inorganic matter, 17
Insanity, inheritance, 564-566
Insect powders, 294
Insecta, 276-301, 311
Insectivora, 426, 429
Insects, 276-301, 310, 501, 506, 509, 533, 538,
542
Insertion, muscle, 471
Inspiration, 33, 34, 36, 71, 491
(See also Respiration)
Instars, 284
Instinct, 263, 305, 311, 361, 375, 377, 498-500,
503
Integration, 459, 475
Integument, 324
(See also Skin)
Intelligence, 305, 311, 361, 377, 501-504
Interaction of genes, 562
Interambulacral plates, 207
Interambulacral rows, 206
Intercellular differentiation, 95
Interlamellar junctions, 215
Intermediate hosts, 172
Internal anatomy, birds, 406-409
crayfish, 258-259
ctenophores, 156-157
earthworm, 234-239
fishes, 356-358
flatworms, 172
hydra, 137
insects, 281-282
mammals, 424-429
mussel, 215
planaria, 159-163
reptiles, 388-389
roundworms, 178
shark, 345-347
spider, 303-304
sponges, 130
starfish, 196-198
vertebrates, 322-323
Internal gills, 380
Internal inspiration, 36, 37
Internal secretions, 373, 459, 492
Internal skeleton, 313, 322
Internal structure (see Internal anatomy)
International Commission on Zoological Nomen-
clature, 573
Interoceptors, 473
Interradial septum, 198
Interradii, 195
Interstitial cells, 138, 142, 470
Intertidal fauna, 511
Intestinal worms, 168, 170, 171, 178, 181-183,
525
Intestine, 179, 214, 236, 258, 282, 345, 489
(See also Alimentary canal)
Intracellular differentiation, 85, 95
Intussusception, 18, 27, 65
Invagination, 121, 237, 321, 477
Invertebrates, 119, 540
Involuntary muscle tissue, 102
Involuntary muscles, 327
Iodine, 20

632

Ionization, 11, 23, 507
Tons, 11

Iris, 227, 338, 339, 496
Trish elk, 551

Tron, 19

Irritability, 18, 495
Ischium, 327, 400
Ischnochiton, 222
Island faunas, 532
Islets of Langerhans, 492, 493
Isolation, 528

Isopods, 266, 267

Itch mite, 307, 308
Ivory, 448

Java ape man, 452

Jaws, 248, 252, 344, 393, 424, 426
Jelly fishes, 144-147, 149, 151, 460
Jennings, 201

Jewelers coral, 153

Jugular vein, 345, 428

Jumping mice, 445, 537

Jungle fowl, 423

Jurassic period, 229, 539, 541, 543

Kallima, 513

Kangaroo, 432, 433

Karyokinesis, 49

Karyosome, 30

Katabolic processes, 461

Katabolism, 38, 43

Katydids, 283, 288

Keel, sternum, 406

Kelvin, 27

Keratin, 100

Keweenawan period, 539, 540

Kidney, 214, 217, 329, 331, 332, 470, 494
(See also Excretory system; Mesonephros;

Metanephros; Pronephros)

Kidney types, 331

Killdeer, 417

Kinetic energy, 15, 38, 480
(See also Heat; Light; Movement; etc.)

King, A. F. A., 91

King crabs, 309, 542

King of herrings, 364

Kingfisher, foot, 413

Kiwi, 533, 534

Labial palps, 216

Labium, 277, 278

Labrum, 277, 278
Labyrinths, ear, 336
Lace-winged flies, 295
Lachrymal glands, 339, 368
Lacunae, bone, 101, 326
Ladybird beetles, 294

Lake communities, 507
Lamarck, 545, 571

ANIMAL BIOLOGY

Lamellae, branchial, 138, 215
Lamellibranchiata, 225
Lamp shells, 193
Lamprey, 340, 342, 342, 470
Lampsilis ligamentina, 218
Lampsilis luteola, 220
Lancelet, 320
Langerhans, islets of, 492, 493
Large intestine, 327, 329
Larvae, 118, 479, 513
ascaris, 181
blindworms, 374
crustacea, 268, 272
hookworm, 182
insects, 283, 286, 288, 291-297
jellyfish, 151, 152
lungfish, 352
mussel, 218
myriapods, 275
pilidium, 187
sponges, 133, 135
tapeworm, 176, 177
tornaria, 316
Larval organs, 479
Larval stage, 284
Larval thread, 218
Larynx, 328, 330, 427
Lasius niger americanus, 289
Lateral canals, 196, 197
Lateral fissure, 423
Lateral line, 179, 334, 352, 359
Lateral vein, 323, 345
Latrodectus mactans, 306
Laurer’s canal, 168, 169
Laveran, 91
Lavoisier, 480
Law, biogenetic, 270, 572, 583
of partial pressures, 491
of segregation, 558
Von Baer’s, 583
Leaf beetle, 290
Leaf butterfly, 513
Leaf insect, 513
Learning, 501
Lecithin, 20
Leeches, 250-251, 254, 491, 526
Leeuwenhoek, 581
Legs, 511—512
anthropoid apes, 450
arthropods, 310
crayfish, 255-258, 264
crow, 414
crustacea, 265-268
frogs and toads, 372
insects, 277-279
kangaroo, 433
myriapods, 273-275
spiders, 303
vertebrates, 322
Leg bones, 404, 451
Lemming, 530
Lemuria, 528
Lemurs, 437, 534
Lens, eye, 252, 338, 339, 368, 375
Leodice viridis, 254

INDEX

Lepidoptera, 290, 296
Lepisosteus, tail, 355
Lepisosteus osseus, scales, 353
Lepisosteus tristoechus, 351
Lepus cuniculus, brain, 430
Lethal genes, 563
Leucocytes, 101, 330, 518
Lice, 288, 294, 525, 526
fish, 267
Lids, eye, 363, 339, 375
Life, 18, 25-28, 480
Life cycle, 44, 460-462
(See also Life history)
Life history, Aurelia, 151
butterfly, 286
grasshopper, 285
hemimetabolous insect, 285
holometabolous insect, 286
liver fluke, 173
locust, 285
lung trematode, 175
malarial parasite, 88-91
mussel, 214
Obelia, 152
physiological, 506
tapeworm, 176
(See also Development)
Life zones, 511, 530, 535-537
Ligaments, 100, 101, 320
Light, 15, 38, 56, 67, 458, 507, 531
Light, C. F., 321
Limbs, 325, 327, 367-369, 410, 546, 553
(See also Legs)
Limpets, 224
Limulus polyphemus, 309
Linckia guildingii, 201
Lines of growth, 212, 213
Lingula, 193
Linin, 30, 48, 49
Linkage, 569
Linnaeus, 59, 186, 570-572
Lion, 434, 534
Lipoids, 20
Lips, 424
Liquid, 9
Littoral fauna, 511
Liver, 214, 216, 257, 304, 319, 323, 328, 329, 345,
428, 459
Living conditions and disease, 516
Lizards, 340, 386, 388, 389-392
head, 390
Lobate foot, 413
Lobe-finned ganoids, 350, 351
Lobsters, 29, 265, 270
Localization, 139
Localized stimulus, 139
Location, sense of, 418, 529
Lock and key relationship, 500
Locomotion, 54, 494-495
ameba, 66
birds, 410-414
crayfish, 262
fishes, 354
hydra, 141
leech, 250

633

Locomotion, mussel, 213
paramecium, 71—72
snake, 393-394
squid, 226
starfish, 197-198
Locusts, 276-277, 285, 529
diagram of body parts, 277
Loeb, J., 114
Loligo pealei, 226
Lophophore, 191, 193, 574
Lotsy, J. P., 550
Love, 501
Love birds, 423
Lumbar vertebrae, 325, 326
Lumbricus terrestris, 234, 235
Lumen, gland, 467
Luminescence, 38, 82, 149, 283, 458
Luminescent organs, 363, 364
Lung books, 306
Lung fishes, 340, 350 352, 353, 368, 534, 542
Lungs, 313, 323, 328, 366, 388, 407, 427, 428, 468
Lycosa carolinensis, 303
Lymnaea bulimoides, 173
Lymph, 330
Lymph glands, 330
Lymph nodes, 330
Lymphatic vessels, 330
Lymphatics, 329, 330
Lyre birds, 416, 534

Macaques, 439, 534
MacBride, E. W., 199
Mach, E., 458
Machine, and body, 481
Mackerel shark, 474
Macrogametes, 45, 86, 89, 90, 470
Macrogametocytes, 89, 90
Macromeres, 378
Macronucleus, 70, 71, 74-77, 85
Macropus rufus, 432
Madreporite, 194, 195, 197, 201, 207-209, 211
Magellania lenticularis, 193
Maggot, 284, 287
Magnesium, 19
Malacobdella, 187
Malacostraca, 265
Malagasy region, 534
Malaria, 88-91
life cycle, 89
Male, 45, 106, 178, 182, 189, 266, 268, 288, 296—
299, 331, 471, 566-568
Mallard, 423
Malleus, 336, 337, 428
Malpighi, 581, 583
Malpighian tubules, 274, 281, 282, 303, 470, 494
Mammalia, 340, 424, 429
Mammalogy, 5
Mammals, 113, 119, 120, 123, 314, 340, 366,
424-454, 460, 469, 470, 477, 485, 521, 532,
539, 543, 568
egg-laying, 429
hibernation, 484
migrations of, 530

634

Mammals, origin of, 429
viviparous, 429
Mammary glands, 324, 424, 459, 491, 547
Mammoth, 535, 538, 543
Man, 53, 116, 437, 448, 449-454, 485, 502-504,
545-547, 568-569
arm, 53
life cycle, 44
(See also Human parasites)
Manatees, 441
Mandibles, 255, 258, 277, 278, 325, 425
Mandibulate insects, 277, 282, 294
Mandrils, 439
Manganese, 20
Mantids, 279, 288
Mantle, 193, 215, 216, 317, 318, 575
Mantle cavity, 215, 223
Mantle fibers, 49
Manubrium, 144, 145
Margaropus annulatus, 307
Marginal canal, 144, 145
Marginal lappets, 146
Marginal spines, 195, 196
Marine distribution, 531
Marine faunas, 510, 511
Marmosets, 534
Marmots, 535
Marrow, 106, 326
red, 326
yellow, 326
Marsh associations, 522
Marsh wren, 509
Marsupialia, 429
Marsupials, 429-430, 445, 534, 549
Marsupium, 218, 430
Marten, pine, 537
Mass, 7, 15
conservation, 15
Mast, S. O., 65
Mast cell, 101
Mastax, 189
Mastication, 496
Mastigophora, 78, 79, 80, 88
Mating, 521
Matter, 7-14
living and nonliving, 17-18
Maturation, egg cell, 111, 112
germ cells, 476
Maturation period, 109-112
Maturity, 44, 460-461
Maxillae, 255, 277, 278, 398
Maxillipeds, 255, 257, 274
Meadow mice, 509
Meandrina meandrites, 154
Meandrina sinuosa, 154
Mechanical energy, 38, 39, 458
Mechanism, 25
Medicinal leech, 245, 254
Medicine, 6, 520, 578, 583
Medulla, 334, 343, 346, 375-377, 387, 408, 451
Medullary groove, 378, 378
Medullary sheath, 102
Medullary tube, 313, 379
Medullated nerve fibers, 103
Medusae, 144-148, 149-152

ANIMAL BIOLOGY

Megalops, 269
Megarhyssa lunator, 296
Meiosis, 110, 111
Melanoplus femur-rubrum, 285
Membrane of Reissner, 337
Membrane bones, 325
Membranelles, 85
Membranes, 11, 12

basement, 99, 260, 261, 465

cell, 29, 69

egg, 383

nuclear, 30, 47—50

plasma, 29, 30

semipermeable, 12, 22, 29

tympanic, 336, 337

undulating, 71

vestibular, 337
Membranous labyrinth, 336, 360
Memory, 492, 497
Mendel, 557, 561, 583
Mendelism, 557
Meridional canals, 156, 157
Meroblastic egg cells, 120, 382, 383, 445, 477
Merriam, C. H., 535
Mesencephalon, 334
Mesenchyme, 100, 122, 464, 469, 478
Mesenchyme cells, 187, 469
Mesenteric artery, 345, 428
Mesenteric vein, 428
Mesenteries, 146-148, 323
Mesoderm, 122, 124, 125, 129, 130, 157, 160,

166, 313, 444, 419, 477-478

formation, 477
Mesodermal pouches, 478
Mesoglea, 137, 138, 144, 466
Mesomere, 478
Mesonephric duct, 323, 331, 332
Mesonephric tubules, 331, 332
Mesonephros, 323, 331, 332, 347, 357, 370, 470
Mesopterygium, 367
Mesosoma, 306
Mesothelium, 122, 478
Mesothorax, 276, 277
Mesozoic era, 539, 541, 543
Metabolic gradient, 165, 242

water, 483
Metabolism, 17, 18, 22, 33-40, 42, 43, 488-494,

515, 516

ameba, 65

annelids, 251

coelenterates, 150

earthworm, 235

flatworms, 170

hydra, 140

mollusks, 229

mussel, 216

paramecium, 71

planaria, 163

planes of, 483

roundworms, 180

spiders, 304

sponges, 133

starfish, 199-200
Metacarpals, 325, 327, 404
Metagenesis, 108, 152-153, 177, 320

INDEX 635
Metals, 8 Moas, 533
Metameres, 52, 234, 235, 237, 238, 242, 250, 277, Modification of types, 529
331-332, 575 under cultivation, 546

thoracic, 255
Metamerically arranged ganglia, 246
Metamerism, 52, 246, 310, 314, 575
Metamorphism, 540
Metamorphosis, 478

ascidian, 318

butterfly, 286

erustacea, 269

frog, 378, 379

insects, 283-286

mosquito, 292

sheep-liver fluke, 173

shrimp, 270—272

starfish, 201

tunicates, 319
Metanephric tubules, 331, 332
Metanephridium, 470
Metanephros, 332, 382, 494
Metaphase, 48, 49
Metaphytes, 41
Metaplasm, 30
Metapleural folds, 319, 320
Metapterygium, 367
Metasoma, 306
Metatarsals, 325, 327, 404
Metathorax, 276, 277
Metazoa, 41, 95-97, 107-108, 463, 468, 470,

477, 494, 516
Metencephalon, 334
Meteoritic theory, 27
Methods of evolution, 549
Methods of reproduction, 107
Metridium dianthus, 147
Mice, 437
Microgametes, 45, 86, 89, 90, 470
Microgametocytes, 89, 90
Micromeres, 378
Micronucleus, 70, 71, 74-77, 85
Micropyle, 476
Microscope, 581
Microscopists, 581
Mid-brain, 334, 375
Middle ear, 337, 368, 370
Middle layer, 132
Midges, 108
Mid-gut, 467
Migration, 529-530

birds, 417-418

fishes, 360
Milk dentition, 425
Milk glands (see Mammary glands)
Millepora, 153
Millipedes, 275
Milt, 361
Mimicry, 513, 514
Mind, 503
Minks, 436
Miracidium, 173, 174, 175
Mites, 307, 308, 525
Mitochondria, 30
Mitosis, 47-51, 48, 546
Mixtures, 10

Modifications, birds, 413
insect legs, 279
Moeritherium, 551, 552
Molars, 425, 551, 552
Mole cricket, 279
Molecule, 8
Moles, 434, 509
Mollusea, 212, 221, 575, 576
Molluscoidea, 193
Mollusks, 115, 212-233, 465, 468, 477, 490, 494,
509, 522, 541
Molting, 182, 264, 284, 394, 416
Monarch butterfly, 286, 530
Monaxon spicules, 132, 133
Monecious animals, 470
balanoglossida, 316
bryozoa, 192
etenophores, 157
earthworm, 240
hagfishes, 339
leeches, 251
mollusks, 229
snails, 223
tunicates, 317
Mongolian type, 453
Mongoose, 433
Monhystera sentiens, 181
Monitors, 392
Monkeys, 437, 534
Monoblastic embryo, 122
Monodelphia, 429
Monohybrid, 559, 561
Monotremata, 429, 430
Monotremes, 427, 429, 430, 445, 485, 534
Moose, 437
Morgan, T. H., 569
Morphological differentiation, 95
Morphological unit, 29
Morphology, 5
Morula, 121-123, 444, 477
Mosaic image, 260
Mosaic theory of creation, 26
Mosquitoes, 90, 292, 293, 526
comparison, 292
malarial, 292
proboscis, 278
Moss animals, 190
Mother-of-pearl, 214
Moths, 276, 278, 290
Motion, 55
Motor end plate, 102, 243
Motor fibers, 333
Motor functions, 494-495
Motor neuron, 244
Mountain, faunal zones, 530-531
Mountain goat, 535
Mouse, northern jumping, 537
Mouth, 466
bryozoans, 192
crayfish, 258
deep-sea fishes, 363
earthworm, 237, 238

636

Mouth, hydra, 137
infusoria, 79
insects, 276, 277, 278
lamprey, 342
leech, 250
mussel, 214
nemathelminths, 178, 182
paramecium, 69, 70
planaria, 159, 160
rotifer, 189
sea anemone, 147
sea urchin, 207
shark, 334
snake, 393
spider, 303
squid, 226
starfish, 195, 198, 211
vertebrate, 327
Mouth cavity, 327
Movement, 42, 494
Mucous canals, 347
Mucous glands, 324, 327, 365
secretion, 327, 341, 467, 491
Mud chimney, 262
Mud flat faunas, 511
Mud puppy, 370, 371
Mulattoes, 563
Mules, 554
Miller, Johannes, 582
Multiple hybrid, 561
genes, 563
Multiplication period, 109-112
Mure», eye, 227
Musca domestica, 293
Muscle cells, 23, 31, 101, 102, 179, 180, 234, 463,
471
Muscle contraction, 495-496
Muscle, fibers, 196
Muscle origin, 471
Muscle tissue, 101, 102, 161, 327, 471
Muscles, 105, 106, 471, 495
adductor, 213, 217
circular, 236, 238
ear, 547
eye, 339
flexor, 414
heart, 102
invertor, 185
longitudinal, 236, 238
nonstriated, 102
retractor, 214, 217
sphincter, 329
striated, 102
voluntary, 326, 327
Muscular coordination, 408
Muscular energy, 482
Muscular movement, 495
Muscular system, 104, 326-327, 471
Mushroom coral, 154
Musk, 448
ox, 439, 537
Muskrats, 509
Mussels, 212-220, 225, 232
Mutant, 562
Mutation, 550, 562

ANIMAL BIOLOGY

Mutualism, 523

Muzzle, 424
Myelencephalon, 334
Myonemes, 85, 471
Myrianida, 253
Myriapoda, 274, 310, 311
Myriapods, 274, 275, 470, 542
Myrmeleon, 295

Mysis, 270, 271
Myxinoidea, 341
Myxinoids, 341-342, 470

Nacre, 214
Naiad, 284
Nails, 324
Naked snails, 224
Nares, 335
Nasal chamber, 328
Natural classification, 58
Natural immunity, 519
Natural selection, 544, 545, 549, 550
Nauplius, 270, 271, 525
Nausea, 473
Nautiloids, fossil, 541
Nautiluses, 221
types, 226-228
Neanderthal man, 452, 453
Nearetic region, 532, 535
Necator americanus, 182
Necessity, 545, 549
Neck, 222, 322, 386, 403
Nectar, 297
Necturus, 370
Necturus maculosus, 371
lung, 469
Negative response, 56
Negroid type, 453
Nekton, 511
Nemas, 180, 181
Nemathelminthes, 178, 180, 574, 576
Nemathelminths, 178-185, 470, 489
Nematocysts, 137-139, 148, 150, 574
Nematoda, 180
Nematodes, free-living, 180, 181, 495
Nemertinea, 186, 574, 576
Nemertines, 184-186, 465, 469, 470, 495
Neornithes, 409
Neoteny, 380
Neotropical region, 532, 534
Nephridia, 193, 236, 238, 273, 319, 470, 494
Nephridial system, 321
tubules, 331
Nephridiopore, 236, 237, 250
Nephros, 470
Nephrostome, 236, 237, 332
Nereis, 247
Nereis virens, 247
Nerve, 106, 160, 161
cells, 28, 31, 102, 103, 139, 243, 472
center, 168
cord, 124, 160, 196, 201, 259, 310, 321, 384
endings, 324
fibers, 102, 103, 138

INDEX 637

Nerve, net, 140, 464, 472, 496, 498 Nidifugae, 420
ring, 181, 182, 201 Night blindness, 565
Nerves, 161, 319, 326, 333 Nighthawk, beak, 412
cell body of, 103 nest, 419
peripheral, 238 Nitrogen, 19, 486
Nervous activities, 243, 495 Nitrogen cycle, 486, 487
Nervous impulse, 103, 243 Noctiluca, 81
Nervous stimulus, 243 Noctiluca scintillans, 80
Nervous system, 57, 104-106, 161, 217, 238, 283, Nomenclature, 59-60, 573, 576
333, 471-473, 495-496, 500, 574, 575 Nonmedullated nerve fibers, 103
amphibia, 375 Nonmetals, 8
arthropods, 310 Nostrils, 370, 386, 390
ascaris, 179 Notochord (chorda), 124, 312, 316, 318-320, 322,
bird, 408 323, 342, 378, 384, 466, 575
cephalopods, 229 Nuclear membrane, 30, 48
chordate, 313, 317 Nuclear sap, 30
crayfish, 259 Nucleolus, 30
dorsal tubular, 313 Nucleoplasm, 29, 30
earthworm, 238-239 Nucleus, 29, 30, 31, 32, 47, 48, 64, 79, 84, 86, 102,
fish, 358 114, 115, 118-121, 127
fly, 283 Nudibranchs, 225
frog, 375-376 Nurse bees, 298
Helix pomatia, 224 Nurse cells, 113, 164, 241
honeybee, 281 ‘ Nymphs, 284
insects, 282
mammals, 427 O
mussel, 217
nemertines, 187 Oarfish, 364
planaria, 160-162 Obelia, 152
reptiles, 388 Obstetrical frog, 372
rotifers, 189 Oceanic distribution, 531
shark, 347 Ocelli, 277, 278, 282
snail, 223 Octopus, 221, 226, 227, 229
spiders, 304 Oculina, 154
squid, 226 Odonata, 294
starfish, 200-201 Oecium, 191
termite, 283 Oil, 448
types of, 472 gland, 100, 324, 413, 467
vertebrate, 333-334 (See also Sebaceous glands)
water beetle, 283 Old age, 461
Nervous tissues, 103, 123 Olfactory bulb, 346, 430
Nervures, 279 Olfactory cells, 151
Nesting, 503 Olfactory lobes, 334, 343, 346, 347, 357, 374-377,
Nests, birds, 419 387, 408, 427
California bush-tit, 421 Olfactory. membrane, 335, 428
dove, 420 : Olfactory organ, 226, 277, 282, 335, 359
robin, 421 Olfactory sac, 342, 347, 359
Texas night hawk, 419 Olfactory tentacles, 222, 223
Neural arch, 323 Olfactory tract, 346
Neural groove, 361, 362, 418 Oligochaeta, 248
Neural tube, 323, 362, 378 Oligochaets, 248-249
Neurilemma, 102 Omasum, 441
Neurocoel, 313 Ommatidium, 260, 261
(See also Neural tube) Oniscus asellus, 267
Neuroglia, 463 Ontogeny, 118, 272
Neuromotor center, 85 Onychophora, 273-274, 310, 311
Neuromuscular cells, 133, 471 Oocyst, 89
Neuromuscular mechanism, 138-139 Oocytes, 109, 111, 112, 558, 567
Neurons, 243, 472 Oogenesis, 109, 111, 112, 475
motor, 244 Oogonium, 109, 111
receptor, 244 Ookinete, 89
Neuroptera, 295 Opalina, 83
Newts, 370-372 Opalina ranarum, 84
Nictitating membrane, 375, 409, 427, 547 Operculum, 224, 249, 350, 352, 357, 358

Nidicolae, 420 Ophiopluteus, 210

638 ANIMAL BIOLOGY

Ophiuroidea, 204-206 Ostracoderms, 542
(See also Brittle stars) Ostriches, 409, 419, 420, 534
Opossums, 431, 534, 537 egg, 418
Optic lobes, 334, 343, 346, 347, 357, 375, 376, 387, foot, 413
408, 427 Otocyst, 318
Optic nerves, 227, 260, 261, 339 Otter, 436
Optic tract, 408 Outer chamber, eye, 338
Optimum, 141 Outer ear, 336-338, 424
Oral funnel, 316, 317 Ovary, 137, 142, 162, 163, 168-170, 176, 179, 240,
Oral groove, 69, 70 303, 323, 331, 408, 460, 470, 493
Oral hood, 320, 321 Oviduct, 162, 163, 169, 240, 303, 323, 331
Oral papilla, 273 Oviparity, 108
Oral ring, 320 Oviposition, 285
Oral sucker, 168, 379 Ovipositor, 277, 292
Oral tentacles, 209 Ovum, 31, 109
Oral valves, 358, 359 Owen, 582
Orang-utan, 449, 534 Owls, 414, 415
Order, 59 barn, 415
Ordovician period, 5389-543 burrowing, 537
Organ-pipe coral, 154 Ox, musk, 439, 537
Organelles, 85, 463 Oxidases, 480
Organic matter, 17 Oxidation, 39
Organismal concept, 462 Oxidations in body, 39, 480
Organisms, 17 Oxygen, 19, 458, 480, 484, 488, 491
animal, 457-462 Oxygen cycle, 488
behavior, 497—504 Oxyhemoglobin, 236, 330, 491
compared to engine, 481 Oyster drill, 229
compared to fire, 480 Oysters, 202, 225, 230
definition, 457
development, 475-479 12
energy changes, 480-485
function, 486-496 P generation, 559, 560, 563
in health and disease, 515-520 Paddle plates, 156, 574
infective, 517 Pain, 473, 501, 502
inheritance, 555-569 receptors, 334
relations between, 521-526 spots, 473
Organization, 17, 21, 23, 24, 28, 166 Paired characters, 556
Organogeny, 122, 478 Paired units, 557
Organs, 104-106, 122, 123, 160—163, 166, 463-464 Palaeomastodon, 552
adrenal, 492 Palate, 335
of Corti, 337 hard, 426
hydrostatic, 350 soft, 328, 426
and systems, 104-106, 464 Palatine bone, 398
Origin, muscle, 471 Palearctic region, 535
Origin, of life, 26 Paleontology, 538
birds and flight, 410 Paleozoic era, 539, 541, 542
germ cells, 475 Paleozoology, 5, 538-543, 548
mammals, 429 Pallial line, 213, 214
sex cells, 109-113 Pallium, 214
species, 545 Palolo, worm, 253, 254
Orioles, 419 Palpi, insects, 248, 277, 278
Ornithology, 5 Palps, 214, 216, 248
Ornithomimus, 411 Pancreas, 99, 323, 329, 345, 459, 492
Ornithorhynchus anatinus, 431 Paper nautilus, 228
Orthogenesis, 551 Paralysis, 565
Orthoptera, 288 Paramecium, 69-77, 82
Osborn, H. F., 27 Paramecium aurelia, 77
Oscula, 130-132 Paramecium caudatum, 69, 70, 76, 77
Osmosis, 12, 18, 35 Parapodia, 248, 575
Osmotic pressure, 12 Parasites, 29, 172, 174, 506, 524
Osphradium, 217 (See also Human parasites, and individual
Ossicles, 196, 207, 338 types)
(See also Ear) Parasitism, 168, 169, 172-177, 363, 524-526
Ostia, 130-132, 146, 147 Parasitology, 5

Ostracoda, 267, 268 Parathyroids, 492, 493

INDEX 639

Parazoa, 130 Perch, 352

Parelephas, 522 climbing, 364

Parenchyma, 160 scale, 353

Parental generation (see P generation) Perching birds, 414

Parker, G. H., 335 Perching mechanism, 414

Parotoid gland, 373 Perennibranchs, 370

Parrakeets, 423 Peribranchial vesicle, 318

Parrots, 416, 423 Pericardial cavity, 214, 216, 303, 323, 427

Parthenogenesis, 107, 268, 290, 471, 568 Pericardial glands, 494
artificial, 114-115 Pericardial sinus, 259

Parthenogonidia, 87 Pericardium, 257

Partial cleavage, 119, 120 Perioral membrane, 195

Partial pressures, law, 491 Periosteum, 106, 326, 327

Pasteur, 27, 83, 583, 584 Periostracum, 213, 214

Pasteurization, 583 Peripatus, 273, 527

Patella, 325, 405 Peripheral ganglia, 333

Patella, eyespot, 227 Peripheral nervous system, 238, 333
trochophore, 230 Perisare, 464

Pathogenic protozoans, 88 Perissodactyla, 441

Pathology, 5 Peristalsis, 236, 489

Pavement epithelium, 98, 99 Peristomal tentacles, 248

Pea fowls, 416, 423 Peristome, 69, 195

Pear leaf blister mite, 308 Peristomium, 248

Pearl button industry, 230, 232, Peritoneum, 196, 198, 235, 323

Pearl essence, 364 Permian period, 539, 543

Pearl shell buttons, 230 Personality, 481

Pearls, 228, 232, 364 Perspiration, 467, 484, 491

Pearly nautilus, 228 Petrifaction, 538

Peas, Mendel’s work, 561 Petromyzon marinus, 341

Peat bog fossils, 538 Petromyzontia, 341

Pébrine, 83, 583 Pfliiger, 27

Pecten, 408, 427 Phagocytes, 518

Pectine, 306 Phalangers, 433

Pectoral arch, 367 Phalanges, 327, 404

Pectoral fins, 322, 344, 352, 354 Phalarope, 420

Pectoral girdle, 326, 451 Pharyngeal clefts, 468

Pedal ganglion, 214, 217 Pharyngeal slits, 312, 314, 317, 319, 320, 378,

Pedicellariae, 195, 196, 201, 207 468, 548, 575

Pedipalpus, 302, 303, 306 Pharynx, 160, 168, 235, 237, 238, 281, 282, 317-

Pedogenesis, 108, 380 320, 323, 328

Peduncle, 193, 302 Pheasants, 416, 535

Peking man, 452 foot, 413

Pelagic fauna, 511 ring-necked, beak, 412

Pelecypoda, DDle 225, 541 Pheidole instabilis, 299

Phenotype, 556
Phenotypic ratios, 560, 561
Philodina roseola, 189
Phosphorus, 19, 488
Phosphorus cycle, 488
Photostomias guernei, 363

Pelican, beak, 412

Pellicle, 69, 70, 464

Pelvic fins, 322, 344, 352, 354
Pelvic girdle, 326, 350

Pelvic region, 322

Pelvis, 325, 326, 327, 451 Photosynthesis, 42, 43, 486

Pen, squid, 226 Phototropism, 56, 67, 141, 239, 377
Penaeus, 270, 271 Phyla, 59, 571, 573, 576, 577
Penaeus semisulcatus, 271 Phyla table, 574-575

Penguins, 414 Phyllium, 513

Penis, 163 Phylogenetic series, 544

Pennaria, 146 Phylogenetic tree, 573, 576
Pennsylvanian period, 539, 542, 543 Phylogeny, 272

Physalia pelagica, 150

Physical basis of life, 24
Physical environment, 516
Physical factors, 507

Physics, 8

Physiological differentiation, 95
Physiological life histories, 506

Peper coral, 153

Pepsin, 328, 329, 489, 492
Pepsinogen, 492
Peptones, 329

Perca flavescens, 352
Perca scale, 353

640

Physiological state, 57, 74, 142, 165, 239, 499, 501
Physiology, 5, 486, 582-583
Pia mater, 375
Pig, embryo, 548
foot bones, 435
Pigeons, 418, 422
anatomy, 407
limb skeletons, 404
Pigment cells, 356, 365
(See also Chromatophores; Eyespots)
Pigment layer, eye, 338, 339
Pigment spots, 151, 201, 320
Pike, scale, 353
Pilidium, 187
Pill bugs, 266
Piltdown man, 453
Pincer, 255
Pine marten, 537
Pineal body, eye, or gland, 343, 346, 357, 376, 399
Pinna, 336, 424
Pinnipedia, 436
Pisces, 340, 350-364
(See also Fishes)
Pit vipers, 398
Pithecanthropus, 452
Pituitary body or gland, 408, 492, 493
Pituophis sayi, head, 390
Place of origin, 528
Placenta, 430, 444-447
Placoid scale, 344, 345, 353, 354
Planaria maculata, 159
Planarians, 159-165, 478, 491, 494
Planes of metabolism, 483
Plankton, 358, 506, 511
Plant galls, 291
Plant lice, 107, 289, 290, 491-492
Plantigrade foot, 435
Plants, compared with animals, 41
contrasted with animals, 42
relations to animals, 505
Planula, 151, 152
Plasma, 236, 330
membrane, 29, 30
Plasmagel, 65, 66
Plasmalemma, 63, 65, 66
Plasmasol, 65, 66
Plasmodium, 91
Plasmodium vivax, 91
Plasmosome, 30
Plastids, 30, 32
Plastron, 400, 401
Platyhelminthes, 166, 574
Platyhelminths, 166-177
(See also Flatworms)
Pleasure, 501, 502
Plestiodon septentrionalis, head, 390
Pleural cavity, 427
Pleurobrachia bachei, 156
Pliny, 579
Ploughshare bone, 403
Plover, and crocodile, 523
golden, 418
Plumage, 414
Plumatella, 192
Plumatella repens, 192

ANIMAL BIOLOGY

Pneuma, 581, 582

Pocket mice, 445

Podophrya, 84

Poikilothermous animals, 484

Poison claws, 214, 274, 306

Poison glands, 303, 387, 398

Polar bodies, 111, 112, 114, 115, 444

Polarity, 119, 243, 496

Poles, egg cell, 120, 378

Polian vesicles, 197, 209

Polistotreme stouti, 341

Pollen, 297

Polyandry, 419, 521

Polyaxon spicules, 132, 133

Polycelis, 167

Polychaeta, 247

Polygordius, 246, 247

Polygyny, 419, 521

Polymorphism, 149, 150, 152, 298, 299, 459, 572

Polyps, 144-150, 152, 153
compared to madusa, 144

Polypterus, 350

Polypterus senegalus, 351

Polypus bimaculatus, 227

Pond communities, 507

Porcupines, 436

Porifera, 129-135, 573, 575, 576

(See also Sponges)

Porospora, 84

Porpoises, 441

Portal systems, 346

Portal veins, 428

Portuguese man-of-war, 145, 150, 475

Position, organs of, 473

Positive response, 56

Postembryonie development, 478

Posterior chamber, eye, 338, 339

Potassium, 19

Potato beetle, 290

Potential energy, 15, 38, 458, 480, 481

Potential immortality, 97, 479

Prairie chicken, 537

Prairie dogs, 535

Praying mantis, 279

Precaval vein, 345

Precocial young, 417, 420

Predatism, 526

Preformation theory, 581, 583

Pressure sensations, 473

Priapulus caudatus, 252

Primates, 429, 437

Primordial germ cells, 109-112, 475

Priority, 60, 576

Prismatic layer, 214

Proboscidea, 441

Proboscis, 159, 185, 186, 278, 552, 574

Procoracoid, 400

Proctodeum, 379, 466

Proglottid, 169, 170

Pronephric duct, 323, 331, 332

Pronephric tubules, 331, 332

Pronephros, 332, 342, 494

Pronghorn, 537

Pronuclei, 114, 115

INDEX

Prophase, 48, 49
Proprioceptors, 473
Propterygium, 367
Prosoma, 306
Prostate gland, 162, 163, 185
Prostomium, 234, 135, 137, 247, 248
Protective epithelium, 98
Protective resemblance, 513, 514
Proteins, 20, 34, 35, 39, 482
Proterospongia, 80, 129
Proterospongia haeckeli, 80
Proterozoic era, 539-541
Prothorax, 276, 277
Protista, 41
Protocercal tail, 355
Protonephridium, 246, 470
Protons, 8
Protophytes, 41
Protoplasm, 17, 19-24, 28, 41, 85, 457, 488, 546
Protopterus, tail, 355
Protopterus annectens, 353
Prototheria, 429
Protozoa, 41, 78-87, 576
symbiotic intestinal, 288
Protozoaea, 270, 271
Protozoans (in general), 41, 63-91, 461-471, 488,
489, 494, 495, 497, 508, 516, 522, 524, 525,
541, 581
health in, 516
Protozoology, 5
Protylopus, 554
Proventriculus, 235, 282, 406
Psalterium, 438, 441
Pseudopodia, 63-66, 78
Pseudopodiospores, 68
Pseudoscorpions, 308
Psychology, 5
Ptarmigan, 415
Pterodactyls, 410, 543
Pterygium, 367
Pterygoid, 398
Ptilosarcus quadrangularis, 149
Ptyalin, 327
Pubis, 327, 400
Pulmonary artery, 366, 367, 386, 428
Pulmonary chamber, 223
Pulmonary vein, 428
Pulp cavity, 425, 426, 427
Pulsating vacuoles, 64
Pulvillus, 279
Pupa, 280, 284, 286, 291-294, 297
Pupal stage, 284
Puparium, 292
Pupil, eye, 338, 339
Purkinje, 19
Pus cells, 519
Pygostyle, 403
Pyloric caeca, 196, 198, 356
Pyloric opening, 328
Pylorie sac, 198
Pyloric valve, 329
Pylorus, 198, 489
Python, 396
Pyura aurantium, 316

641

Quadrate bone, 382, 398, 428
Quadrinomial, 572

Queen ant, 301

Queen bees, 296-298

Queen termite, 288

Quill, 404, 405

Rabbit, 437
brain, 430
embryo, 548
Rabies, 519, 584
Raccoon, 433, 436, 445
Races, 59
of men, 452
Racial immunity, 519
Radial canal, 131, 132, 144, 146, 196, 197, 198
Radial nerve cord, 196
Radial septum, 148
Radial symmetry, 52, 157, 574, 575, 576
Radiata, 203
Radiations, 549, 562
Radii, 195
Radiolaria, 83, 540, 541
Radiolarian ooze, 83
Radius, 325, 327, 367, 400, 404
Radula, 223, 226, 229
Raja erinacea, 348
Rana pipiens, 379
Range, 527
Rat, 437, 448, 533
pouched, 535
Rattlesnakes, 395
skull, 398
Ray, 571
Rays, 340, 345, 347
echinoderms, 194-199, 201, 204, 205, 206, 209
Reactions, 14, 21, 66—67, 72-73, 177, 317, 507
(See also Responses)
Reactiveness, 18
Reasoning, 502
Recapitulation, law, 271
Recent advances, 584
Receptor cells, 165, 243
Receptor-effector-adjustor mechanism, 496
Receptor-effector mechanism, 496
Receptor neuron, 243, 244
Receptors, 165, 243, 244, 333, 472, 496, 498
Recessiveness, 556, 559, 560, 564
Recognition colors, 514
Rectal caeca, 198
Rectal glands, 281
Rectum, 328, 329
Red blood corpuscles, 13, 31, 322, 330, 581
Red gland, 355
Red marrow, 326
Red spider, 308
Red-winged blackbird, 509
Redi, 26, 583
Rediae, 173, 174
Reduction, 110-112, 116, 476, 556
division, 110, 111

642

Reese, 270
Reflex act, 165, 244, 472
Reflex action, 165, 2438-245 472
Reflex arc, 244, 333, 496
Reflex centers, 496
Regeneration, 143, 165, 187, 201, 206, 208, 230
242, 264, 286, 381, 391
Regions, 532, 533, 535
Remora, 523
Remora remora, 523
Renal artery, 345
Renal-portal system, 345, 346, 370, 388, 408
Renal vein, 345
Rennin, 328
Reproduction, 18, 45-46, 105, 470-471, 475-479,
481, 521
ameba, 67—68
annelids, 251, 253
ascidians, 317-321
birds, 419-420
coelenterates, 151-153
crayfish, 263-264
crustacea, 268, 269
earthworm, 240-241
echinoderms, 209-210
fish, 361-362
frogs, 373, 374, 377-380
hydra, 142-143
insects, 283-286
lizards, 391
mammals, 445-447
metazoa, 107-125
mollusca, 229-230
monotremes, 429
mussels, 217—219
myriapods, 275
nemathelminths, 181
paramecium, 74
planaria, 162-163
platyhelminths, 171
rotifers, 189-190
sharks, 347
sheep liver fluke, 173-176
snails, 223
snakes, 395
spiders, 304
sponges, 134-135
starfishes, 201
tapeworm, 176-177
types of, 44-46, 107, 268
vertebrates, 339-340
(See also Asexual
reproduction)
Reproductive epithelium, 98
Reproductive system, 104, 162-163, 240, 339-
340, 408, 470-471
(See also Reproduction)
Reptiles, 314, 382-385, 539, 543
hearts, 386
Reptilia, 340, 385
Reservoir, 79
Respiration, 11, 37, 42, 48, 104, 447, 467-468,
490-491, 515
ameba, 65
birds, 385, 407

reproduction; Sexual

ANIMAL BIOLOGY

Respiration, earthworm, 236
fishes, 358, 359
hydra, 140
insects, 280, 281, 293
mussel, 215
paramecium, 71
sea cucumber, 208
sea urchins, 207
snails, 224
vertebrates, 329-330
Respiratory system, 104, 329, 467-468
(See also Respiration)
Respiratory tree, 208, 209
Responses, 54-56, 183, 141, 151, 162, 201, 210,
239, 263, 269, 495
(See also Reactions)
Resting cell, 48
Resurrection bone, 580
Reticular nervous system, 472
Reticular structure of protoplasm, 23
Reticulotermes flavipes, 288
Reticulum, 441
Retina, 223, 338, 339
(See also Eye)
Retinal layer, 338, 339
Retinula, 261
Retinular cells, 260
Retrogression, 203, 319, 572, 575
Reversibility, 14
Rhabdites, 166, 167
Rhabdom, 260, 261
Rhabdophaga strobiloides, gall, 291
Rhagon canal system, 132
Rhea, 534
Rheotropism, 56, 72
Rhinceros birds, 441
Rhinoceroses, 441, 534, 535
Rhizocrinus lofotensis, 210
Rhizopoda, 78, 494
(See also Sarcodina)
Rhizostoma pulmo, 147
Rhodites rosae, gall, 291
Rhynchocephalia, 387, 398
Rhythmicity, 74, 482
Rhythms, 510
Ribs, 323-326, 425, 580
Ring canal, 197, 209
Rodentia, 429, 437
Rodents, 448
Rodolia cardinalis, 294
Rods, 252
Roe, 361
Rose coral, 154
Ross, Ronald, 91
Rostellum, 169, 170
Rostrum, 255, 256, 258
Rotalia freyeri, 81
Rotifera, 188, 574, 576
Rotifers, 108, 187-188, 470, 471, 495, 508, 581
Roundmouths, 340
Roundworms, 178
(See also Nemathelminths)
Routine, 520
Royal jelly, 298
Rudimentary organs, 544

INDEX 643

Rumen, 441
Ruminants, 441, 448
stomach, 438
Running birds, 414
Running insects, 278

Sabellids, 248, 249
Saber-toothed tiger, 551
Sacculina, 525
Sacculus, 336, 337, 368
Sacral vertebrae, 326
Sacrum, 319, 547
Sagitta, 188, 475
Sagitta hexaptera, 188
Salamanders, 340, 370, 371, 375
embryo, 548
Salientia, 370, 372
Salinity, 531
Saliva, 327
Salivary glands, 99, 327
Salmon, 361, 362, 530
brain, 357
tail, 355
Salts, 8, 21, 34, 39, 482
Samia cecropia, 498
Sand dollars, 204
Sandpipers, 414
Sandworm, 247
Sanitation, 520
Saprophytie types, 80
Sarcode, 19
Sarcodina, 78, 81, 82, 88
(See also Rhizopoda)
Sarcoplasm, 102, 180
Sarcoptes scabiet, 307
Sauropsida, 382
Sawfish, 347
Scab mites, 308
Scabies, 308
Scala tympani, 336, 337
Scala vestibuli, 336, 337
Scale insect, 294
Seale lice, 294, 295
Seales, 54, 324, 350-354, 385, 386, 391, 399, 437
Scallops, 225, 230
Scaphognathite, 255, 258
Scaphopoda, 221, 225, 541
Scapula, 325, 327, 400
Scarlet fever, 520
Scent glands, 424
Scents, animal, 492
Schistocerca americana, 276
Schleiden, 32, 583
Schwann, 32, 583
Schultze, Max, 19, 32, 583
Sclater, P. L., 534
Scleroblasts, 132
Sclerotic layer, 338, 339
Scolex, 169, 171
Scolopendra, 274
Scolytid beetle, work, 290
Scorpions, 306-307, 538, 542
Scutes, 390, 391

Scutigera, 274, 275
Scyllium canicula, brain, 346
Scyphistoma, 161, 152
Scyphozoa, 145, 146, 147, 541
Scyphozoan jelly fish, 147
life history, 151
Sea anemones, 145-147, 266, 523, 524
Sea angels, 224
Sea cucumbers, 204, 209
Sea fans, 145, 148, 149
Sea lilies, 205, 209, 210
Sea lions, 437
Sea mats, 190
Sea moss, 190
Sea pens, 145, 148, 149
Sea slugs, 225
Sea snakes, 396
Sea squirts, 317
Sea urchins, 113, 204, 206, 207, 208, 210, 466, 541
Seals, 437
Seasonal changes, 505, 510
Sebaceous glands, 100, 324, 424
Secretary bird, 534
Secretin, 459
Secretion, 33-37, 65, 99, 104, 328, 329, 491-493
Secretions, 29, 36-37, 98, 99, 168, 241, 273, 292,
304, 317, 327, 329, 341, 365, 413, 467, 491-493
Secretory epithelial cells, 100
Sedgwick, Adam, 274
Segmentation, 122
cavity, 121, 122, 124
(See also Blastocoel)
Segments, 52
Segregation, 558-563
Self-fertilization, 108, 143, 223, 470
Self-regulatory tendency, 517
Semicircular canals, 336, 337, 342, 347, 376
Semilunar ganglion, 334
Seminal groove, 235
Seminal receptacles, 168, 169, 240, 263, 303
Seminal vesicles, 162, 163, 168, 240
Semipermeable membrane, 12
Senescence, 44, 45, 460—462
Senility, 461
Sensations, 337, 338, 496
Sense organs, 106, 151, 189, 201, 217, 223, 226,
248, 334-340, 347, 359-360, 375, 473, 496
auditory, 277-278, 336, 337, 407, 427-428
equilibrium, 261
lateral line, 352
olfactory, 335
sight, 261, 338, 339
tactile, 272
taste, 335
Sensory cells, 138, 139
Sensory epithelium, 98
Sensory fibers, 333
Sensory papillae, 250
Sepia, eye, 227
Serous secretion, 467
Serpent stars, 204, 205
Serpula vermicularis, 249
Sertularia, 146
Serum, 330
Servetus, 581

644 ANIMAL BIOLOGY

Setae, 234, 236, 260, 575 Skin, gills, 468
Sex, 45 (See also Dermobranchiae)
determination, 566, 567 insect, 465
Sex cells, 45, 86, 96, 109-113, 135, 136, 151, 253, Skull, 324, 325, 342, 356, 424-425, 546
470, 476, 559, 560, 567 snake, 390
Sex chromosomes, 566, 568 types, 425
Sex hormones, 492 Skunks, 436, 445
Sex-linked character, 568-569 Sleeping sickness, 88
Sex organs, 169, 240 Sloths, 437, 438, 534
Sexual cycle, 89 Slugs, 224
Sexual dimorphism, 471, 572 Small intestine, 327, 328, 329
Sexual reproduction, 46, 87, 107, 134, 142, 151, Smallpox, 519
164, 253, 317-318, 470-471 Smell, 334-336, 359, 473
(See also Reproduction) (See also Olfactory organs)
Sexual spores, 89 Snails, 10, 221, 223-225, 230, 509
Sexual zooids, 253, 254 Snakes, 340, 386, 389, 393-398, 445, 532
Shaft, 404, 405 boa, 393
Shank, 404 copperhead, 397
Sharks, 340, 344-349, 528, 542, 579 coral, 397
brain, 346 eggs, 394
Sheep, 439, 535 garter, 392
liver fluke, 172-174 head, 390
skull, 426 rattle, 395
Shelford, 506 skeleton, 393
Shell, 192, 193, 212, 213-215, 219-224, 230-232, skull, 398
400-401, 541, 574, 575 water moccasin, 396
glands, 168, 494 Snapping turtles, 401
Shipworm, 225, 231, 233 Sneezing, 496
Shore faunas, 511 Snipe, 412, 413, 414
Short digits, 566 Snout, 424
Shrews, 434 Soaring, 411
Shrimps, 270, 271 Social insects, 296
Sight, 192, 193, 212, 213-215, 219-224, 230-232, Societies, 296-300, 521, 522
400-401, 541, 574, 575 Sociology, 5
Silicon, 20 Sodium, 19
Silk, 287, 303-305, 491, 499 Sodium carbonate, 330, 491
Silkworms, 83, 278, 287, 491, 498-500 Soft-shelled crab, 263
cocoon, 498 Sol, 13, 14
Silurian period, 193, 229, 309, 539-541 Soldier ant, 299
Simple epithelium, 98 Soldier termite, 288
Simple reflexes, 498 Solid, 9
Sinanthropus pekinensis, 452 Solitary bees, 298
Sinus venosus, 345, 346 Solitary life, 521
Sinuses, 259, 304, 335 Solute, 11
Siphons, 208, 212-214, 226, 227 Solutions, 11
Siphuncle, 228 Solvent, 11
Sipunculid, 252 Soma, 479
Sirenia, 429, 441 Somatic cells, 95, 96, 479
Size limit, 45 Somatic mesoderm, 125, 313, 383
Skates, 340, 347, 348 Somatoplasm, 96, 479, 545
Skeletal lamina, 85 Somatopleure, 383, 384
Skeletal muscle, 102 Song birds, 414, 416, 417
Skeletal system, 104, 105, 465 Songs, bird, 417
(See also Skeleton) Sounds, insects, 277

Skeleton, 132, 312, 313, 322, 324-326, 335, 350, Sow bugs, 266

Special creation, 26, 580
24-425, 450-451 3
OI Specialization, 204, 247, 300, 310, 467

bird, 404 (See also Adaptation)
fish, 355, 367 Species, 3, 59, 550, 571
man, 325 origin of, 545
snake, 393, 398 Sperm, 45, 109
turtle, 400 whale, 4, 441, 448
Skin, 105, 324, 345, 365-366, 373, 464-465, 467, | Sperm cells, 31, 45, 107-117, 134, 142, 152, 163,
491 164, 181, 218, 240, 241, 251, 300, 310, 340,

frog, 365 361, 377, 445, 476, 521, 556, 558, 560, 583

INDEX

Spermaries, hydra, 137, 142, 143

Spermatids, 109, 110

Spermatocytes, 109, 110, 567

Spermatogenesis, 109, 110, 112, 475

Spermatogonia, 109, 110, 142

Spermatophores, 163, 251

Spermatozoon, 109

Sphenodon, 533, 534

Sphenodon punctatum, 398, 399

Sphenoidal sinus, 335

Spherical symmetry, 52

Sphincters, 133, 329, 489

Spicules, 130, 132, 133, 178, 222

Spider crabs, 266

Spiders, 302, 303-305, 502, 513, 538, 542
Latrodectus mactans, black widow, 306

Spinal cord, 313, 323, 333, 334, 342

Spinal curvature, 451

Spinal nerves, 333, 334

Spindle, 47, 48

Spines, 54, 194-196, 198, 206, 207

Spinnerets, 302, 303

Spinning, 304, 498-499

Spiny-headed worms, 185

Spiracle, 277, 280, 302, 303, 306, 344, 379

Spiral cleavage, 123

Spiral valve, 344, 345

Spireme, 47, 48

Spiritus, 582

Spirobolus, 275

Spirochaetes, 88

Splanchnic mesoderm, 125, 313, 383

Splanchnopleure, 384

Spleen, 323, 328, 330, 345, 373

Sponges, 129-135, 464-466, 468, 469, 523, 541

Spongilla lacustris, 129

Spongin, 132, 491

Spongoblasts, 132

Spontaneous generation, 26, 544

Sporoblast, 89

Sporocyst, 173, 174

Sporozoa, 78, 82, 83, 88

Sporozoites, 87

Sports, 550

Sporulation, 67, 68, 89

Spotted fever, 448

Squalus, 344

Squalus acanthias, 344, 345, 348, 353

Squamata, 386, 389

Squamous epithelium (see Pavement epithelium)

Squash bug, 289

Squids, 221, 226, 226, 466

Squirrels, 433, 509, 537

Staghorn coral, 153

Stagmomantis carolina, 279

Stalked eyes, 256

Stapes, 336, 337, 368, 428

Starches, 20

Starfishes, 114, 194-202, 204, 210, 211, 541

Statoblast, 192

Statocysts, 151, 156, 157, 217, 226, 261

Statoliths, 217, 261

Steapsin, 329

Stegocephala, 543

Stegodon, 652

645

Stegomastodon, 552
Stentor, 83
Stentor polymorphus, 84
Steps, in embryogeny, 122
in fertilization, 115
Stercoral pocket, 303
Sternal artery, 258, 259
Sternum, 325
Sterols, 20
Stigma, 79
Stimuli, 55, 57, 103, 133, 141, 243, 334, 472, 473,
495-499
Sting ray, 347
Stomach, 187, 189, 191, 198, 199, 200, 208, 214,
216, 226, 251, 253, 281, 282, 303, 317, 318,
323, 328-329, 345, 406, 438, 459, 467, 489
Stomodeum, 146, 156, 379, 380, 466
Stone canal, 197
Storage, food, 39
Strainer, 259
Stratification, 507
Stratified epithelium, 98, 99
Stream fauna, 512
Strobila, 151, 152, 169
Strobilation, 151
Strongylocentrotus drébachiensis, 206
Structural characteristics (see Internal anatomy)
Structure of organisms, 463-474
Struggle for existence, 545, 549
Sturgeons, 351, 364
tail, 355
Stylonychia, 83
Stylonychia mytilus, 84
Subclavian artery, 345, 366, 386, 428
Subclavian vein, 345, 428
Subdermal cavity, 132
Subesophageal ganglion, 259, 281, 282
Subimago, 286
Subneural gland, 317
Subperitoneal rib, 323
Subpharyngeal ganglia, 238
Subspecies, 59, 573
Subterranean fauna, 512
Succession, 508, 509
Suckers, flukes, 168, 173
leeches, 250
tapeworm, 170, 171
tube feet, 195, 196, 197
Sucking, 496
dises, 225
fish, 523
flies, 526
lice, 288
Suctoria, 79, 83
Suctorial insects, 278, 294
Suctorial mouth parts, 278
Sugar, 20, 35, 489
Sulphur, 19
Summation of stimuli, 74
Sunbirds, 535
Sunfish, mouth, 358
Sunflower star fish, 204
Superficial cleavage, 119, 120, 263, 269, 283, 477
Supporting lamella, 138
Supporting tissues, 100, 101

646

Suprabranchial chambers, 214-216
Supraesophageal ganglion, 259, 281, 282
Suprapharyngeal ganglia, 237, 238
Surface films, 10
Surinam toad, 372
Survival of fittest, 544
Suspensions, 11
Suspensory ligament, 339
Swallowing, 496
Swammerdam, 581
Swamp sparrow, 509
Swans, 420
Swarming, 253, 298
Sweat glands, 324, 424, 484
Swim bladder, 354
Swimmerets, 256-258, 263
Swimming birds, 414, 609
Swimming insects, 279
Swine, 439

(See also Pig)
Sycon, 131
Sycotypus, 229
Sylvius, 581
Symbiont, 524
Symbiosis, 143, 457-458, 518-524
Symbiotic intestinal protozoa, 288
Symmetry, 52, 574-576
Sympathetic nervous system, 333
Synapse, 243, 244, 472
Synapsis, 109-112, 117, 476, 556, 569
Synaptic mates, 556
Synaptic nervous systems, 472, 473
Syncytium, 464
Syndactyl foot, 413
Syngamy, 46
Syphilis, 88, 516
Syrinx, 407, 417
Systematic zoology, 570
Systems, 104-106, 160-163, 464-473

(See also individual systems)

Syzygy, 82

at)

Tactile cells, 151

Tactile corpuscle, 324

Tactile organs, 186, 248, 252, 261, 277, 324, 335,
359, 473

Tadpoles, 118, 318, 350-352, 379, 490, 581

Taenia saginata, 168, 176-177

Taenia solium, 168

Tail, 54, 322, 368, 389, 391, 404, 547

Tail fin, 355-356

Tapeworms, 169-171, 176-177, 457

Tapirs, 441, 506

Tarpon, 364

Tarsus, 277, 325, 327

Taste, 334, 336, 359, 408, 429, 473, 496

Taste buds, 335, 336, 429

Taste organs, 282, 359, 429

Taxis, 55

Taxonomy, 5, 570-577, 583

Tears, 37, 491

Teat, 430

ANIMAL

BIOLOGY

Teeth, 352, 344, 356, 359, 386, 393, 400, 425, 426
437, 443
Tegmina, 276, 280
Tegumentary system, 104. 105, 464-465
Telencephalon, 334
Teleostei, 350, 352
Teleostcmi, 350, 352
Teleosts, 340, 344, 352, 357, 358, 543
Telolecithal egg cell, 119, 120, 362, 477
Telophase, 48, 49
Telson, 256-258
Temperature, body, 407, 413, 445, 483-485
external, 67, 72, 174, 507, 529, 531
receptors, 334, 473
Temporal bone, 336
Tendons, 100, 101, 106, 326, 471
Tentacles, 54, 135, 136, 144, 147, 148, 156, 157,
195, 209, 226, 320, 468
Terebratella transversa, 192
Teredo navalis, 231, 233
Termites, 283, 287, 288, 296, 457, 524
Terrestrial vertebrates, 365-368, 512
Testes, 106, 137, 162, 163, 168, 170, 198, 240, 331,
339, 460, 470, 493
Testudinata, 388, 400
Tetanie contraction, 55, 495
Tetanus, 495
Tetrabelodon, 552
Tetrad, 476
Tetraxon spicules, 132, 133
Texas cattle fever, 307
Thales, 578
Theca, 148
Thermocline, 512
Thermotropism, 56, 67, 72, 141, 164, 377
Thigmotropism, 56, 67, 72, 141, 164, 239, 377
Thoracic basket, 326, 382
Thoracic cavity, 323
Thoracic ganglia, 281, 282
Thoracic region, 322
Thoracic vertebrae, 325, 326
Thorax, 276
Thousand-legged worms, 275
Threadworms, 178
(See also Nemathelminths)
Threshold, 56
Thrombin, 490
Thymus glands, 373, 492, 493
Thyroid glands, 373, 492, 493
Tibia, 277, 325, 327, 400
Tibiotarsus, 404
Ticks, 306-307, 526
spotted fever, 448
Texas cattle fever, 307
Tiedemann’s bodies, 200
Tiger beetles, 294
Tiger salamander, 371
Tigers, 534, 535, 551
Time and area, 528
Tissues, 98-103, 122-125, 463
definitive, 463
formation, 478
heart muscle, 102
skeletal, 102
visceral, 102

INDEX 647

Toads, 340, 365, 372-374, 381, 465 Tuberculosis, 288, 516, 519
horned, 391 Tubipora, 154
Toes, 189, 373, 389, 404, 438, 439, 441, 423 Tubular nervous system, 312, 314, 473
Toleration, 517 Tularemia, 448
Tongue, 328, 389, 390, 400 Tundra, 535-537
cartilage, 342 Tundra region, 535
Tonic contraction, 495 Tunic, 316
Tonsils, 425 Tunicates, 314-320
Tonus, 210, 490, 495 section of larva, 318
Tooth, 427 Turbellaria, 166, 167, 495
(See also Teeth) Turbinate bones, 335
Tornaria, 316 Turkeys, 416
Tortoises, 388, 400, 401, 534 Turtles, 340, 388, 400, 401, 443, 481, 548
(See also Turtles) heart, 386
Total cleavage, 118, 120, 142, 476, 477 skeleton, 400
Toucans, 534, 535 Tusk shells, 221, 225
Touch (see Tactile organs) Tusks, 448, 551
Toxins, 518 ’Tween-brain, 334
Trachea, 328, 330, 407 Twins, 568
Tracheae, 273, 274, 280, 281, 303, 304, 311, 468 Tympanic membrane, 283, 336, 337, 368
Tracheal gills, 281, 468 Tympanum, 277, 283, 336, 337
system, 281 Tyndall, 27, 583
tubes, 308 Types, 577
Traction fibers, 49 Typhoid fever, 293, 519
Traumatism, 517
Tree frogs, 373 U
Tree toads, 373
Trematoda, 166-177 Ulna, 325, 327, 367, 400, 404
Trembley, 143 Umbilical cord, 444, 446, 447
Tremex columba, 291 Umbo, 212, 213
Trepang, 211 Uncinate process, 406
Trial and error, 73-74 Undulating membrane, 71, 83
Triassic period, 539, 541-543 Unequal cleavage, 116, 120, 218, 378, 477
Triaxon spicules, 132, 133 Unguiculata, 429, 434
Trichinella, 183, 184 Ungulata, 429, 439
Trichinella spiralis, 183 Uniformitarianism, 582
Trichinosis, 183 Unio complanatus, 218
Trichocysts, 70, 74 Unios, 218-220
Trihybrids, 561 Uniparental reproduction, 107
Trilobites, 309, 541, 542 Units, hereditary, 555, 557
Trinomials, 572 Universal symmetry, 52
Triploblastie condition, 123, 574-576 Unpaired genes, 562
embryo, 123 Urea, 36, 329
Triradiate spicules, 132, 133 Ureters, 214, 217, 323, 331, 332
Trivium, 203 Urethra, 331
Trochelminthes, 188 Urinary bladder, 323, 331
Trochophore, 229, 230, 246, 254 Urinogenital sinus, 358
of patella, 230 Urochordata, 314, 316, 576
Trochozoon, 230 Urodela, 370, 371
Trombicula irritans, 307 Uroglena, 81
Trophoderm, 444, 445 Uroglenopsis americana, 80
Trophozoite, 87 Uropods, 256, 257
Tropical region, 535 Urosalpinz, 229
Tropisms, 55, 56° , nae Use and disuse, 545, 549-550
(See also individual tropisms)

Uses of foods, 39, 482-483
Uterus, 162, 163, 179, 331, 347, 377, 445
Utriculus, 336, 337

Trunk, 322

Trunk fishes, 354
Trypanosoma gambiense, 80
Trypanosomes, 88

Trypanosomiasis, 88 Vv

Trypsin, 329

Tsetse fly, 88 Vaccination, 519

Tuatara, 398, 399 Vaccines, 519-520

Tube-dwelling worms, 187, 248, 249 Vacuoles, 30, 64, 65, 67, 70, 71, 79, 85, 86, 129, 488

Tube feet, 195-200, 204-209 Vagina, 162, 163

648 ANIMAL BIOLOGY

Vagus nerves, 459
Valves, blood vessels, 236, 259, 581
oral, 358, 359
spiral, 344, 345
Vampire bats, 534, 545
Vane, 405
Variation, 546
Varieties, 59, 573
Vas deferens, 162, 163, 168, 170, 217, 240, 257,
258, 339
Vasa efferentia, 162, 163, 168, 240, 331
Vascular systems, air-vascular (see Tracheae)
blood-vascular (see Circulatory system)
gastrovascular (see Gastrovascular cavity)
water-vascular (see Water-vascular system)
Vegetal pole, 120, 378
Vegetation regions, 535-537
Veins, 279, 304, 330, 580, 581
Velum, 145, 152, 230, 317, 319, 320
Velvet mites, 308
Vena cava, 214, 217, 428
Venomous snakes, 396-398
Ventral aorta, 347
Ventral fins, 354
Ventral line, 179
Ventral nerve cord, 179, 236, 238, 257, 259, 281,
282
Ventral root, 333, 334
Ventral sucker, 378, 379
Ventral vessel, 236
Ventricle, 214, 345, 346, 367, 386, 388
Ventriculus, 282
Venus’ flower basket, 131
Venus’ girdle, 157
Vermes, 186
Vermiform appendix, 328, 427, 547
Vertebrae, 322, 326, 451
Vertebral column, 312, 322, 324, 325, 326, 451,
466, 548
Vertebral discs, 425
Vertebrata, 314, 322-340, 576
Vertebrates, 322-454, 468-470, 472, 491, 494,
546, 548
diagrammatic longitudinal section, 323
parallel stages in development, 548
skeleton, 324
Vertical distribution, 531
Vertical life zones, 530, 531
Vertical migration, 269, 531
Vesalius, 579, 580
Vestigial structures, 546, 547
Villi, 35, 329, 467
Villi, placental, 444
Vipers, 398
Virchow, 50
Virus, 517
Visceral ganglion, 214, 217
Visceral mass, 215, 223
Visceral muscle, 102
Visceral pain, 473
Visceral part of skull, 326
Visceral sensations, 473
Viscosity, 9
Vision (see Sight)

Vital activities, 22
Vital force, 25, 582
Vitalism, 25
Vitamins, 34, 38, 483
Vitelline membrane, 383
Vitrella, 260
Vitreous body, 338
Vitreous retina, 227
Viviparity, 108
Voluntary muscles, 326, 327
Volvozx, 81, 95
Volvox aureus, 80
Vomiting, 496
Von Baer, 270, 583

law, 583
Von Mobhl, 19, 583
Vorticella, 83, 84

WwW

Wading birds, 413, 414
Walking legs, 251, 253, 255, 413
Walking sticks, 279, 288, 513
Wallace, 545, 549
Walruses, 437, 448
Wandering cells, 133
(See also Leucocytes)
Warbler, beak, 412
Warm-blooded animals, 484
Warm-bloodedness, 427, 484
Warning coloration, 514
Warts, 372
Wasps, 296, 300, 513
Water, 21, 39, 482, 483, 488, 507, 528
beetle, 279, 283
birds, 420
boatmen, 279, 289
fleas, 108, 268
moccasin, 396
striders, 10, 289, 509
tubes, 215, 216
vacuoles, 64
Water-vascular system, 197-198, 204, 575
Wax, 287, 491
Weasels, 436
Webs, spiders, 305
Webworm, 290
Weevils, 290
Weismann, 96, 479, 545
Whale sharks, 441
Whalebone, 443, 448
Whales, 4, 424, 429, 441, 443, 448
White ants, 457
White body, 227
White corpuscles, 330
White fibers, 101
White matter, 333, 334
Wilson, H. V., 132
Wind, 507, 528
Wings, 53, 54, 278-280, 392, 403, 410-415, 432,
473
bones, 404
deciduous, 286
Winking, 496

INDEX
Wireworms, 290 Yellowlegs, beak, 412
Wolf spider, 305 foot, 413
Wolff, Caspar, 583 . Yellowthroat, 509
Wolves, 426, 436, 535 foot, 413
Woodchuck, 445 Yerkes, 377
Woodpeckers, 413, 418, 509, 537 Yolk, 111, 112, 118-121, 164, 173, 383
beak, 412 cells, 164, 173
young, 418 circulation, 385
Woodruff, L. L., 76 ducts, 168, 169
Worker ants, 299 glands, 162, 163, 168, 170, 173, 189
Worker bees, 296, 297 plug, 378
Worm lizards, 390 sac, 361, 384, 444, 446
Worms, 477, 494, 522, 525 stalk, 384, 446
(See also various types by name)
Wrigglers, 293 Z
Wuchereria bancrofti, 184
Zebra, 534
Xe Zinc, 20
Zoaea, 270, 271
X-body, 475 Zona pellucida, 444
X-chromosomes, 566, 568-569 Zooecium, 190, 191
X-rays, 549, 562 Zoogeographical regions, 532, 533
Zoogeography, 5, 510, 527-537
VE Zoology (general), 3-6, 41
Zoophytes, 145
Y-chromosomes, 566, 567-568 Zygote, 46, 87, 89, 96, 475, 556, 566-568

Yellow marrow, 326 Zymogen, 99, 100, 492

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