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Marine Biological Laboratory 


,M July 11, 1941 


Accession No. "^ 


Given By McGraiY-Hill Bock Co., Inc. 
Place i^er^ York City 



A. FRANKLIN SHULL, Consulting Editok 



Selected Titles From 



A. Franklin Shull, Consulting Editor 

Baitstil • Human Biology 

Burlingame ■ Heredity and Social Problems 

Chapman ■ Animal Ecology 

Goldschmidt ■ Physiological Genetics 

Graham • Forest Entomology 

Haupt ■ Fundamentals of Biology 

Hymun ■ The Invertebrates: Protozoa through Ctenophora 

Metcalf and Flint • Insect Life 

Mitchell ■ General Physiology 

Mitchell and Taylor ■ Laboratory Manual op General Physi- 

Pearse ■ Animal Ecology 

Reed and Young ■ Laboratory Studies in Zoology 

Riley and Johannsen ■ Medical Entomology 

Rogers ■ Textbook of Comparative Physiology 

Laboratory Outlines in Comparative Physiology 

Senning ■ Laboratory Studies in Comparative Anatomy 

Shull ■ Evolution 


Shull, LaRue, and Ruthven • Principles op 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. 

^"90 . 




Late Professor of Zoology, University of Nebraska 

Second Edition 
Second Impression 



Copyright, 1933, 1940, by the 
McGraw-Hill Book Company, Inc. 


All rights reserved. This book, or 

parts thereof, may not be reproduced 

in any form without permission of 

the publishers. 



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 accomphshed by the staff of the Department of 
Zoology at the University of Nebraska as a memorial to a former 
friend and colleague. 


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. 



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" 



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 XXXIX, 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 byF. 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. 


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. 

Lincoln, Nebraska 
August, 1933. 

(Go ^ -^#^*' A cp^ 


Preface to the Second Edition vii 

Preface to the First Edition ix 


Fundamental Principles 


The Field of Zoology 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 


Matter ' 

Definitions — Constitution of matter — Elements and compounds — Acids, 
bases, and salts — States of matter — Surface films — Mixtures — Ionization — 
Colloids — Colloidal emulsions — Reactions. 


Energy 15 

Forms of energy — Chemical energy — ^Laws of thermodynamics. 


Living and Nonliving Matter 17 

Contrast between living and nonliving matter — Tests of life. 


Protoplasm 19 

Historical facts — Chemical character of protoplasm — Physical characteristics 
of protoplasm — Microscopical structure of protoplasm — Appropriateness of 
protoplasm as living substance — Life is a consequence or concomitant of 


Life 25 

Definition — Vital force — Vitalism and mechanism — Origin of life — Possi- 
bility of creating life. 


Cells 29 

Definition — Sizes and shapes of cells — Numbers of cells — Structure of cells — 
General physiology of the cell — Development of knowledge of the cell — Cell 
theory and cell doctrine. 


55^3 t>i 


Metabolism 33 

Definition — Food — Steps in metabolism — Ingestion — Digestion — Absorp- 
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. 

Plants and Animals 41 

Comparison between plants and animals — Biology — Differences between 
plants and animals. 

Growth and Reproduction 44 

Growth cycles — ^Limit of size — Reproduction. 


Mitosis 47 

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


Forms of Animals 52 

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

Behavior 55 

Stimuli — Direct response — Conductivity — Part played by the nervous sys- 
tem — Physiological state. 


Classification and Nomenclature 58 

Definition — Arrangement of groups of animals — Nomenclature. 


Ameba 63 

Occurrence and appearance — Structure — ^Metabolism — Locomotion — Be- 
havior — Reproduction. 


Paramecium 69 

Occurrence — Structure — Metabolism — Locomotion — Behavior — Reproduc- 
tion — Con j ugation — Endomixis . 

Protozoa in General 78 

Classification — Mastigophora — Sarcodina — Sporozoa — Infusoria — General 
facts — Sexual reproduction in protozoa. 



Protozoa and Disease 88 

Pathogenic protozoa — Malarial parasite. 

Metazoa in General 


Metazoa ^^ 

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


Tissues ^° 

Definition — Epithelia — Supporting and connective tissues — Muscular tissues 
— Nervous tissues. 


Organs and Systems 104 

Definitions — Systems — Organs belonging to different systems — Other parts 
of the body. 


Reproduction in the Metazoa 107 

Methods of reproduction in metazoa — Sexual reproduction — Uniparental 
reproduction — Types of fertilization — Oviparity and viviparity — Metagene- 


Origin of the Sex Cells 109 

Gametogenesis — Spermatogenesis — Oogenesis — Comparison and contrast 
between spermatogenesis and oogenesis — Division of labor between the germ 
cells — Variations in gametogenesis. 


Fertilization H^ 

Steps in fertilization — Chromosome reduction — Significance of synapsis. 


Embryogeny 11^ 

Types of egg cells — Forms of cleavage — Steps in embryogeny — Variations in 
embryogeny — Germ layers — Coelom. 

Metazoan Phyla 


Sponges 1^9 

Relationship of sponges — Classification — Structure — Canal systems — Skele- 
ton—Histology — Metabolism — Behavior — Reproduction — Uses of sponges 
— Cultiration of sponges — Relations to other animals. 




Hydra 136 

External features — Internal structure — Nematocysts — Neuromuscular 
mechanism— Metabolism — Behavior — Reproduction — Symbiosis — Regen- 



Polyps and Medusae — Classification — Hydrozoa — Scyphozoa — Anthozoa — 
Color — Polymorphism — Metabolism — Behavior — Reproduction — Metagen- 
esis — Corals — Distribution and economic importance. 


Phylum Ctenophora 1^" 

Structure — Advances in body plan — Activities. 


Fresh-water Planarian 1^9 

S t ru c t u r e— I n t e r n al structure — Metabolism — Reproduction— Behavior- 


Phylum Platyhelminthes 166 

Classificatio n—Turbellaria—Trematoda—Cestoda— Metabolism— Repro- 
duction — Occurrence and economic importance. 


Parasitism 1'^ 

Structure of parasites— Sheep liver fluke— Life history of the sheep liver 
fluke — ^Life history of a tapeworm — Behavior of parasites — Practical aspects. 


Phylum Nemathelminthes 178 

Structure of an ascaris — Characteristics and advances — Classification — 
Free-living nematodes — Metabolism — Reproduction — Life history of the 
pig ascaris — American hookworm— Trichinella — Filaria — Hairworms — 
Spiny-headed worms — Economic importance. 


Other Unsegmented Worms 186 

Phylum Nemertinea — Phylum Chaetognatha — Phylum Rotifera — Phylum 
Bryozoa — Phylum Brachiopoda. 


Starfish 1" 

External appearance — Skeleton and musculature — Water-vascular system — 
Internal organs — Feeding and metabolism — Nervous system and behavior — 
Reproduction — Regeneration and autotomy — Economic importance. 


Phylum Echinodermata 203 

Retrogression — Specializations — Classification — Asteroidea — Ophiuroidea — 
Echinoidea — Holothurioidea — Crinoidea — Reproduction — Behavior — Color 
— Occurrence and economic importance. 



Fresh- WATER Mussel 212 

External appearance — Shell — Internal anatomy — Body mass and foot — 
Mantle — Gills — Digestive system and metabolism — Circulatory system — 
Excretory system — Musculature — Nervous system — Behavior — Reproduc- 
tion — Other fresh-water mussels. 


MoLLusKS IN General 221 

Classification — Amphineura — Gastropoda — Scaphopoda — Pelecypoda — 
Cephalopoda — Metabolism — Behavior — Reproduction and regeneration — 
Economic importance. 


Earthworm ^34 

External characteristics — Internal structure — Alimentary canal and metabo- 
lism — Circulatory system — Excretory system — Musculature and locomotion 
— Nervous system — Behavior — Reproduction system and reproduction — 
Regeneration — Economic importance. 


Reflex Action 243 

Nervous functions — Reflex acts — Anterior ganglia. 


Annelids in General 246 

Classification — Archiannelida — Chaetopoda — Hirudinea — Gephyrea — 
Metabolism — Behavior — Reproduction — Occurrence and economic impor- 


Crayfish 255 

External characteristics — Internal structure — Eyes and vision — Statocyst — 
Feeding habits — Behavior — Reproduction — Regeneration and autotomy — 
Economic importance. 

Crustacea 265 

Malacostraca — Entomostraca — Behavior — Reproduction — Economic impor- 
tance — Biogenesis. 


Onychophora and Myriapoda 273 

Onychophor a — M y r i a p o d a — Centipedes — Millipedes — Reproduction in 

Class Insecta 276 

External characteristics — Internal structures — Senses of insects — Reproduc- 
tion — Autotomy — Injuries due to insects — Benefits from insects — Injurious 
types — Combating injurious insects — Beneficial insects — Social insects. 




Class Arachnida "'"^ 

External structure of spiders— Internal structures— Metabolism— Reproduc- 
tion — Spinning activities — Behavior — Economic importance — Scorpions — 
Mites — Other arachnids. 


Arthropods in General "1" 

Characteristics and advances — Classification — Behavior. 


Phylum Chordata 312 

Characteristics — Advances shown by the chordates — Classification. 


Lower Chordates 315 

Hemichordata — Urochordata — Cephalochordata — Economic value. 


SuBPHTLUM Vertebrata 322 

Distinguishing characteristics— Body plan— Skin— Skeleton— Muscular sys- 
tem — Digestive system — Respiratory system — Circulatory system — Excre- 
tory system — Nervous system — Sense organs — Ear — Eye — Reproductive 
system — Advances shown by vertebrates — Classification. 


Class Cyclostomata 341 

Classification — Myxinoids — Lampreys — Relationship of the cyclostomes — 
Economic importance. 


Class Elasmobranchii 344 

Dogfish sharks — Other sharks — Skates and rays — Extinct elasmobranchs — 
Economic facts. 


Class Pisces 350 

Classification — Crossopterygii — Chondrostei — Holostei — Teleostei — Dipnoi 
^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. 


Terrestrial Vertebrates 365 

Changes incident to the acquirement of a terrestrial mode of life — Origin of 
terrestrial adaptations. 


Class Amphibia 370 

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. 




Reptiles and Birds 382 

Structural characteristics — Embryonic modifications — Egg — Amnion — 
Allantois — Body coverings. 


Class Reptilia 386 

Classification — Internal structure — Squamata — Chameleons — Lizards — 
Snakes — Venomous snakes — Rhynchocephalia — Crocodilia — Testudinata — 
Economic importance. 


Class Aves 403 

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. . 

Class Mammalia 424 

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

Anthropoid Apes and Man 449 

Manlike apes — Erect position — Evidences in man of former arboreal life — 
Intermediate forms — Fossil men — Present-day man. 

General Considerations 

Animal Organisms 457 

The organism — Definition — Income and outgo — Differentiation — Integra- 
tion — Centralization — Chemical control — Individuality — Life cycle in birds 
and mammals — Other life cycles — Practical considerations — Organismal 

Structure of Organism 463 

Grades of organization — Germ layers and tissues — Relationship of cells 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. 


Development of the Organism 475 

Germ cells — Origin of germ cells — Maturation of the germ cells — Egg — 
Fertilization — Cleavage — ^Blastula — Gastrulation — Mesoderm formation — - 


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


Energy Changes in Organisms 

Chemical changes in the body-Organism compared to a fire-Organism 
compared to an engine-Organism more than a machme-Individuahty- 
Rhythmicity— Uses of foods— Planes of metabolism— Body heat-Heat 
regulation— Warm-blooded and cold-blooded animals-Temperature of the 

human body. 



Functions of Animal Organisms 

Chemical cycles- Water— Digestion and absorption— Circulation— Respira- 
tion— Secretion— Internal secrttions- Excretion and ehmmation— Motor 
functions — Nervous activities. 


Behavior of Animal Organisms 

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 



Animal Organisms IN Relation TO Their Environment 505 

Facts of ecology— Relations of animals to plants-Physiological life histories 
—Habitat— Ecological factors— Reactions of the ammal— Communities— 
Succession— Rhythms-Marine faunas-Fresh-water animals— Terrestrial 
faunas— Mimicry and protective resemblance. 


Animal Organisms in Health and Disease 

Definitions-Health in a protozoan-Comparison of protozoan and metazoan 
cells-Conditions of health-Causes of disease-Effect of mdividiiality- 
Self-regulatory tendency in the body-Toxins and antitoxins-How the 
body fights disease-Immunity-Anaphylaxis and allergy-Maintenance of 
health in human beings. 


Relations between Animal Organisms 

SoUtary life— Associations of animals of the same species-Mating— 
Families— Colonies— Societies— Associations of animals of different species— 
Gregariousness-Epizoic associations— Commensalism— Mutualism— hym- 
biosis — Parasitism — Predatism . 



Distribution of Animals 

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. 



Past Distribution of Animals 538 

Fossils — Stages in fossilization — Geological ages — ^Geological time scale — 
Metamorphism — Animals of the past. 

Evolution of Animals 544 

History of evolution — Evidences of evolution — Causes of evolution — 
Methods of evolution — Evolutionary series. 

Inheritance in Organisms 555 

Organisms from the genetic viewpoint — Determiners or genes — Behavior of 
chromosomes in maturation and fertilization — Effect of chromosonae 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. 


Classification of Animals 570 

History — Species — Polymorphism — Basis of classification — Basis of nomen- 
clature — Rules of nomenclature — Phyla — Phylogenetic tree. 

History of Zoology 578 

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

Glossary 585 

Index 615 



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 ahve 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 Uved 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. 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 63=^-ton 

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. 

Fig. 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 Uving 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 sohtary existence, in 
which one animal fives 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 five 
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 stifi 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 fives in the soil, and others that enjoy the 
power of flight pass much of their active existence in the air. 


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 hfe. 

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 ^vith 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; taxonomy, 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 ammah; 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 wdth the worms; entomology, which is 
the study of insects; conchology, the study of mollusks; ichthyology, the 
study of fishes; herpetology, the study of reptiles and amphibians; or7ii- 
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. 


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 aquiculture, 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 

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. 


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 famiharity 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 hving 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 

A few common forms of matter consist of only one kind of matter, 
such as a mass of gold, silver, iron, the Kquid 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 compourid, 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 Hmit 
of visibiUty 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 ahke. This smallest particle of any com- 



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 /errwm), 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 (0), nitrogen (N), carbon (C), sulphur (S), 
silicon (Si), phosphorus (P), chlorine (CI), and iodine (I). 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 calc'um 
carbonate (CaCOs); and blue vitriol, or copper sulphate (CUSO4). In 
all chemical combinations the number of atoms of each element in a 


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

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 fluid. 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 sofids 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 hquid, or a solid but the same amount is 
present in each state. 


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. 
_____^ I^ i^ ^^^ presence of this film and 

{~}*^\J*^\~J the resistance it offers to penetration 

lj / \ which makes it possible, with suffi- 

r~^ (^ cient care, to lay a dry needle upon 

the surface of a liquid and for it to 

CN /^ remain there. The resistance which 

/ V-^ this film offers to the penetration of 

OV-^ /^ any object is also responsible for the 

>-^ ^^ dimpling of such a surface when a 

C!^ V-x dry object is pressed down upon it. 

/ Ky If the object is pressed with a con- 

FiG. 2.— Diagram illustrating the bal- stantly increasing force the dimple 

anced attraction exerted upon a molecule , i n i j. • i 

inamassof liquid (a), and the unbalanced becomes gradually deeper until 

attraction upon a molecule at the surface finally, wheU the force beCOmeS SUffi- 
(6). The two-pointed arrows («*) indicate • . . , +V. fil +V^ K' + 

mutual attraction between two molecules. Cient tO rupture tne mm, tne ODjeCt 

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 hquid 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 



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 hquid 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 hquid 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 

Fig. 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 or 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 hquids 
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. 


Those ions which are metanic in nature carry positive charges, and those 
which are nonmetalUc carry negative charges; they are termed, therefore, 
positive or negative ions. Table salt (NaCl) in solution separates mo 
sodium (Na) and chlorine (CI) ions; sodmm sulphate (Na^SO 4), into 
Na and SO4 ions, SO4 being a radical. Na is a positive ion; CI and bU4 
are negative. When, by evaporation of the solution or by precipitation, 
the substance which is in solution is made to reappear again m sohd form, 
the ions combine, and the charges neutralize one another and disappear. 
This separation of ions in solution is known as eledrolyszs, or dzssonahon; 
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, 

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 hqmds are m contact 
with the two sides of such a membrane, the Uquids tend to mmgle by the 
passage of molecules or atoms through the openings. When one or both 
of these two Uquids contain soUds, other liquids, or gases m 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 m solution are 
capable of passing through such a membrane are said to be crystaloidal 
and If thy normally exhibit this character are often o.lled cry siaUo^ds 
while those which will not pass through are termed colloidal and called 
colloids. But many crystalloids may be made to assume ^ ^^ ^^daj 
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 
eS in atoms o^ 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 hquids in the form of suspensions, and do not form true 
so utions. These colloidal suspensions are thicker or more like glue than 
are crystalloidal solutions. From crystalloidal solutions the subs ance 
are easily obtained in crystalline form, but this is not true of col oidal 
Tspensions. Oils and fats and proteins, such as the a bumen w^^^^^^^^^^ 
the white of eggs, are colloidal. This separation of ^^''^ff'^Z^^ 
loidal substances by membranes is known as dialysts and the membranes 
which effect the separation, as dialyzing membranes or ^^^^^^j;- 

Other membranes are found in the body which under similai conditions 
permit some hquids or gases to pass through them and P--nt other 
Lm so doing, but the passage takes place as a result of solution m the 
colidal memk-ane and not through openings in it. Such a -embrane is 
termed a semiper^eaUe membrane. This process is known as 0^08^2^ 
the force behind the movement is called osmotic pressure. In the human 



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 veiy important 
phenomenon in all living things. The intake into Hving 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 necessaiy for life activities come 
into living tissues, while the harmful chemical waste substances that are 
produced in living tissues are eUminated from them. 

.7 % so// /n 
'/as ma 


More wa/-&T~ 
Less sa// 

Less ivofer' 
More sa/f 

Fig. 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 Uquid 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 



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. 

A B C D 

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 emulsion is a sol. In B the droplets are shown taking up 
liquid and swelling. In C this is continued until they press upon one another. In D the 
droplets are so crowded as to become 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 


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 apphcation 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, 



inorganic compounds, as water, have relatively small amouiits of chemical 
energy, whereas organic compounds, as gasohne and other, coal 
and w^od, usually have large amounts. When a compound is resolved 
ink, its component atoms, the chemical energy reappears as kmetio 
energy This is the source of the charges carried by the ions m an ,on zed 
solution. Because ot their greater complexity, a greater arnount of 
Hnetic energy can be produced in an animal body by the disintegration 
of organic compounds than by that of inorganic compounds Th^ 
kinetic energy may appear in various forms, as movement, hght, heat, 

"'" ^rtlw if Thermodynamics.— Thermodynamics is a part of physics 
which deals with energy transformations. One law has long been con- 
Idered fundamental in this Held : It is the law of the canservaUon of energy 
which states that the sum total of energy iu this universe is the same at 
Tu times, being neither created nor destroyed but simply changed from 
one forn^ to another. Machines are mechanisms for transforming one 
form of energy into another. When coal is burned in an engine, its 
potential eneily is changed into the kinetic form ot heat and then changed 
agaL into tlf kinetic form of mechanical energy. This may dnve a 
dynamo and be transformed into electric energy, which in turn may be 
transformed into the radiant energy of light. 


Living matter or matter which has been arranged under the influence 
of Hfe 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 hving matter with the formation of wastes, 



such as urea, water, and carbon dioxide, and the Hberation of kinetic 
energy mainly evident as heat and movement. Living matter possesses 
the po4er 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 metaholism. If the material taken m 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 hving 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 m the former 
growth occurs by the introduction of new particles among those already 
present, which is growth by intussusception. ,^ ;. • 

4 Revroduction.-FavtmWy due to metabohsm and growth, hying 
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 froni the 
parent mass gradually assumes the size and form of the parent. I^ on- 
living things do not possess this power. 

5 Irritahility.-Uying things, generally speaking, have the power ot 
responding to changes in their environment, such changes acting as 
stimuli This quahty is termed irritability, 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 m the 
nature of adjustment, is destructive in its effect, and the thing cannot of 
itself regain its former character. 

26 Tests of Life.— Living matter, however, is not always to be recog- 
nized *by any characteristics which it possesses. A Uving seed may 
appear as inert as any bit of inorganic matter, and some animals may 
eZt 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 hfe phenomena. 



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 bodj^ 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 





















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 


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 coohng, 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 Hving 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 famihar 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. 


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 
ahke. 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 



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 ionization contributes to the speed of chemical reactions within the 

30. Microscopical Structure of Protoplasm. — The droplets of the 
disperse phase in protoplasm are mostly too small to be within the range 
of microscopic \dsion, 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 e\adence 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 

Fig. 6. — Semidiagrammatic sketches illustrating the different appearances exhibited 
by protoplasm. A, the granular type; represents cells from the liver of the mouse. B, 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 froTn Wilson, ''The 
Cdl," 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 
epithehal 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 Ufe activities, and are necessary in a 
substance in which Ufe 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 


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 supervene 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. 


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 
pecuHar 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 is 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 
fife 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 hving 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 nonUving matter and, as far as is known, throughout the 

35. Vitalism and Mechanism. — Those who have beheved 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 hving matter 
as distinguished from nonliving matter, but that does not mean that the 
chemical changes in protoplasm are precisely the same as those occurring 



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. As a 
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, Pfiiiger, 
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 hke 
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 


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. 


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 distinction 
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 

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. 




The nucleus, which is set off from the cytoplasm by a nuclear mem- 
brane, shows a fine network of fibers known as Unin 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 

Ce/J membrane 
or wall 



Plasma membrane 

Centra! body con- 
taining two 



Cfiro matin 










Fig. 1 .- — 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 

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 soUd 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. Vacuoles are transparent droplets seen regu- 
larly in certain cells or at certain times in other cells. 



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 

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. X 700. F, human blood corpuscles. X 1000. G, nonstriated 
muscle cell from mammalian intestine. X 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 


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 hving 
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. 



Reference has been made previously to the fact that one of the char- 
acteristics of hving matter is its abihty to carry on metabohsm — that is, 
its abihty to take material into the body and work it over in such a way 
as to make it a part of the hving 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 


9. Excretion 


W. Elimination 

10. Expiration 
Fig. 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 pro\ide 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. 




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. 

\. Ingestion 

\O.Eypi ration 
5. Inspiration 
4. Circulation 

9. Excretion of 
nitrogenous waste 

4. Circulation 
3. Absorption 

11. Elimination 

In every cell 
of the body ^ 
6. AssimiJcrtion 
1. Dissimilation 
9. Excretion 

-2. Digestion 

8. Secretion of 
gastric Juice 


Fig. 10. — Diagram to suggest the steps in metabolism as they occur in the human body. 

For comparison with Fig. 9. 

47. Steps in Metabolism. — Metabohsm 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 



6/ycer/n J<^'^^s) 

/no adds 

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 metaboUsm 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 
the head of ingestion also occurs all 
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 (clrbSi ydrak), 
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 
digestion. In the higher animals diges- 
tion may begin in the mouth and be 
continued in the stomach and intes- 
tine. Only organic foods need to be 
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, 

Fig. 11. — Diagram of a villus, one of 
the finger-like projections in the small 
intestine of a mammal, to show how the 
absorption of organic foods takes place. 
The lymph vessels are in solid black, the 
blood vessels stippled. 



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

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). 

Fig. 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 


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 ceU 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 external expiration, 
which is its passage from the body (Fig. 12). Expiration relieves the 
organism of its gaseous waste. 

58. Elimination. — ^Liquid waste may be eUminated 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 ehmina- 
tion should not be confused with egestion. As an example of the differ- 
ence between excretion and ehmination 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. — Egestion 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 soHd 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, 


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 
Bi (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 (antiscorbutic vitamin) is found in fresh vegetables and citrus 
and other fruits and prevents scurvy. Vitamin D (antirachitic 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- 


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 

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 Hver 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. 


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 


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 fine 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 intelhgible 
the difficulty in distinguishing between the simpler forms of the two. 
Metabohsm 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 hving 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 



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 



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 utiUze all the oxygen they can get and give off 
carbon dioxide as waste. 

Eli rninafion 
Water and Urea 

Expi ration 

Eotestion ^ 

a. Animal 

Excess of 

Fig. 13. 


and CO 2 

b. Plo(n+ 
■Diagrams contrasting the metabolic and food-manufacturing processes in plants 
with the metabolic processes in animals. 

Because of the difference in metabohsm of plants and animals plants 
have often been referred to as predominantly anabolic in their activities, 
while animals are characteristically katabohc. Also, since in the presence 
of light the processes concerned with photosynthesis outweigh those 
concerned in the metabohsm 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 
metabohc standpoint, on the same plane as animals. 



Whenever during the hfetime 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 Hfetimes, 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 


Age 21 

Age 45 

Age 60 Death 

Fig. 14. — Diagram illustrating the growth cycle in 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 hmited a Hmit 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 hving 
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, 



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 Hmit 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 hmit 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 Hfe by 
the multiplication of hving 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 generaUzed or unspe- 
ciahzed cells are set aside for the purpose of reproduction and are reheved 
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 sex 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. 


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. 


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 
Unin network, begin to collect together into a slender and very much 
tangled thread, or spireme, and the Unin 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 Unin 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 




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 aniphiaster 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 


Central body 
and cen- 

Lin in 



(Linin disappears) 

Spireme shor- 
tens and tiiickens 







Nuclear Asters 

membranes and Spjndle 

appear disappear 

Fig. 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 mesh work, from which finally are produced 


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. 


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 vai'ious 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 hap 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 vitahty, 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 hfe 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 


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 possibihties which 
have been realized in all living things that have since come from that fife. 
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 accomphshed by the sHpping 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. 



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 




^^u-' ^^•^^"t^i'ies of the fore limbs and their skeletons, of man (A), horse (B), bat (C) and 
bird (D), to illustrate homology. The dotted lines pass through corresponding joints. 



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 
defined as a projecting part, capable 
of movement and performing some 
active function. Immovable horns, 
spines, hairs, and scales are not 
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- 
ally or morphologically different in 
plan 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. 

Fig. 17. — Outline of the wings of a 
butterfly and the veins in them, for compar- 
ison with D, Fig. 16, to illustrate analogy. 



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. — All 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 troyism. 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 




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

1. Thigmotropism, or response to contact. 

2. Thermotropism, or response to temperature. 

3. Phototropism, or response to light. 

4. Chemotropism, or response to chemical stimulation. 

5. Rheotropism, or response to mechanical currents. 

6. Electrotropism, or response to currents of electricity. 

7. 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 

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- 


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

88. Part Played by the Nervous System. — In 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 metabohc 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 abihty 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 inabihty 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. 


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 
to them. If this grouping is based only upon the place where animals Uve 
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 

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. 



Kingdom: Animalia (all animals) 

Suhkingdom: Metazoa (many-celled animals). 
Phylum: Chordata (chordate animals). 
Subphijlum: 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 in a species. In a 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 famihes. 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 itahcized. Generic names are capitalized and usually itali- 
cized, while specific and subspecific names are italicized but not capitahzed. 

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 apphcation, 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 


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 appUcation 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. 




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 5 mm. (3<5 inch) in length. 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 




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 



Pse udopodium 


Fig. 19. — Amoe6a proteus Leidy. X 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 

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. 


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 

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 plasrnalemma, which 
is very thin and elastic; the clearer ectoplasm; the plasrnagel, which is the 


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. 


b a 

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 outhne. There 
is a curious resemblance in the manner of functioning of the plasmalemma 
to the continuous metalhc belt which forms the tread in a caterpillar 

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



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 Hves it is indifferent; 


Fig. 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 

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 


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 E). 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. 




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 di\dsion 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 hve. 

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 obhquely 
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 cilia, 
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 

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 




into a great number of very small hexagonal areas. From the center of 
each of these areas arises a cihum. Just below the pelhcle 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 ciUa are described as having an 
axial thread continuous with the cortex and a covering continuous with 

Ectoplasm and 



Food vacuole 



Contractile vacuole 

Oral groove 


\ — Micronucleus 


g — Mouth 

Contractile vacuole 

Fig. 22. — Paramecnnn caudatum Ehrenberg. X 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, he 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 




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 
through a narrow passage, the body contracting as it does 
so, but on its exit from the passage it immediately 
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 

Fig. 23. — ^The spiral path followed by a swimming Paramecium. Fiq, 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. 



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 

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

Fig. 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 
is an 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 



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 

Fig. 26. 

Fig. 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 

Fig. 26. — Diagram illustrating the method of trial and error in Paramecium. (Modified 
from Borradaile and Potts, " The Invertebrata," after Kuhn, 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 suffi- 
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 


is the same as that seen in the "trial-and-error" mode of behavior of 
higher animals, and so this term is often appHed 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 

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 fission, 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 



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. Micronuoleus divides. Macronucleus begins to dis- 

3. Micronuclei divide and three of four disappear 

T/7/S animal 
not followed 
further; same 
as other 

4. Remaining micronucleus divides unequally 

5. Smaller micronucleus crosses into other animal 

C. 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 

9. Fusion micronucleus divides 

10. Two micronuclei divide 

11. Four micronuclei divide 

12. Four micronuclei become macronuclei, three dis- 
o#o*«#o# P appear, one remains 

1.3. Micronucleus divides and animal divides 

14. In each of two micro- 
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 



to fertilization in higher animals which possess sex, the smaller mieronu- 
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 

Micronuclei divide. Macronucleus disintegrates 

3. Micronuclei again divide 

Six out of eight micronuclei disappear. Animal divides 

Each animal with one micronucleus 

Micronucleus of each divides 

7. Micronuclei divide again. 

S. Two micronuclei in each animal become 

Micronuclei divide and animals divide 

Four ordinary individuals 

Fig. 28. — Diagram illustrating endomixis in Paramecium aurelia Miiller. (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 
vitahty. Woodruff, however, has succeeded in maintaining a culture 
since May 1, 1907, without conjugation. In the thirty-second year, 


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 Mliller (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. 


The phylum Protozoa (pro to zo' 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 (mas ti gof o ra; G., mastix, whip, and phoros, 
bearing), or Flagellata (fla gel la' ta).— Have a limited number of long 
whiphke locomotor appendages known as fiagella. 

2. Sarcodina (sar ko di' na; G., sarkodes, fleshy), or Rhizopoda 
(ri zop' o 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 zo' 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 pons, nominative, 
for the Greek word for foot. If a word comes from the Greek through the Latin, 
the Greek is given. 









■ Contractile 


4. Infusoria (in fu so' ri a; L., infusus, poured into, crowded), or 
Ciliata (sil i a' ta). — Have a very large number of permanent small hair- 
like 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 

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 hleph- 
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 
vacuoles pour their contents, opens into the 
gullet. Close to the reservoir is a mass of 
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 

Euglena is a type which possesses some 
of the characteristics of plants. Other 
members of this class show these to such a 
degree that they are by botanists considered 
plants and classified by them as such. 
A plantlike characteristic is the abihty, 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 Hghted 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. 


Fig. 29. — Euglena viridia 
Ehrenberg. {Slightly modified, 
from Doflein, "Lehrbuch der 
Protozoenkunde," by the courtesy 
of Gustav Fischer.) X 800. 



The Mastigophora (Fig. 30) are divided into two groups: (1) Those 
which are animal-hke and which may be holozoic, saprophytic, or ento- 
zoic. Saprophytic imphes the absorption of nonUving organic matter in 
solution directly through the surface of the body. Entozoic 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 

Fig. 30. — Several different species of Mastigophora. A, Proterospongia haeckeli Kent. 
(From Kent, "A Manual of the Infusoria.") X 530. B , Giardia lambha Stiles. {After 
Wenyon, in Archiv fur Protistenkunde, Suppl. I.) X 2200. C, Trypanosoma gambiense 
Button. (From Wenyon, "Protozoology," by the courtesy of William Wood & Company.) 
X 1330. D, Noctiluca scintillans (Macartney). {From Kent.) X 40. E, Vohox 
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 am,ericana (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, 
hves in the small intestine of man. These are both animal-like. 



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 Volvox, 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 

\A.. ,\U,'/'/// 


Fig. 31. — Different types of Sarcodina. A, Rotalia freyeri. {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.") X 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.) X 40. One of the Heliozoa. D, Heliosphaera inermis 
Haeckel. {From Bronn, " Klassen und Ordnungen des Tierreicha," 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 Nocfiluca. This animal frequently collects 
on the surface of the sea in enormous numbers, the jelly-hke 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 



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 




Nucleus of 



Fig. 32. — Examples of Sporozoa. A, a hemogregarine in the red blood corpuscle of a 
frog. {From Hegner and Taliaferro," Human Protozoology.") X 550. B, section through 
the intestinal epithelium of a rabbit, showing infection with one of the Coccidia, Eimeria 
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.) X 60. 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 Macm,illan 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. 


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 Sarcodina 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 

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 
pehrine. 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) occur in 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. In addition to cilia, infusoria frequently possess undulating mem- 
branes or cirri, formed by the fusion of numerous ciha. 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 
Slentor 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 



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 % inch, in length. Most of them are not 





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. X 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, Steritor polymorphus Mliller. {From Kent.) 

X 60. Attached individual. E, Balantidium coli Malmsten. {From Thomson and 
Robertson, "Protozoology") X 400. F, Styloiiychia mytilus Ehrenberg. {From Kent.) 

X 100. (C by the courtesy of The Macmillan Company; E by that of William Wood & 

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

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- 



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 membranelle 

yOral cilia 










Oral cilia 


'Esophagus " 


Skeletal lamina 


Rectum ' 


Fig. 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- 


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, Hving and dead. Their metaboUsm 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 httle 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). 

A S 

Fig. 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 planthke 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-Uving, 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, hke egg cells, are usually not active; and the smaller microgam- 
etes, which, hke the sperm cells of higher animals, are active. When 


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 ^\^thout fertihzation. When this occurs, they remain 
^^dthin 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 E). 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. 



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 E), which cause a type of dysentery known as halantidial dysen- 
tery. This is most common in the tropics. 

Among the Sarcodina, one ameba, Endamoeha 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 hfe cycle 
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. 




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 fife history. The setting 
free of spores from many infected corpuscles corresponding to the starting 


,rit'^ rec^ b/ooafcoA-^ 


in man 

/ \ \ GamefO' 

Asexual spore'l^ {m'j 7 

Enter mosQu/fo 

Sex-ucil cycle 

in mosquito 

Macro- {v^|8 



13 Oocyst 

Fig. 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 1.3, 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 


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

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 cuHcid 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, 


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. 
Plasmodiuvi 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 Itahan, 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. 




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 intercellular 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 metabohc 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 cell and in which certain 
reproductive cells are differentiated. It thus appears that 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 



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 speciahzation in the work of the individual and the exchange of the 
results of that work develop in proportion as civihzation 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. 

of soma . 


First 9eneration Second ojenerati on Third generation 

Fig. 37. — Diagram to illustrate Weismanii'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 immortahty. 

The genetic continuity of the germ plasm was emphasized in the work 
of Weismann, whose conception is illustrated in the accompanying 
diagrain (Fig. 37). 


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 Hkened 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 
reahzed and manifested such of these possibilities as, taken together 
equip the individual with its characteristic features. 



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. 




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 basernent membrane (Fig. 38 C to E) to which 
the epithehal 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 roothke projections which penetrate the tissue below them. Neither 
nerves nor blood vessels are ordinarily found in epitheha, though this does 
not apply to nerve terminals in sensory epithelia. 

.- ^. si. ■^ ^■ Y "^ 

' — y — ■' Y~\- — >t-~>- — ^ — 
oTo oToToToTo o 




Basement membrane 

Fig. 38. — Semidiagrammatic sketches illustrating different types of epithelia. A, 
simple pavement epithelium, seen in surface view and in section. B, section of simple short 
columnar, or cubical, epithelium, also seen in surface view and in section. C, sectional view 
of a simple columnar epithelium. D, section of simple ciliated epithelium. E, section of a 
stratified pavement epithelium. All highly magnified. Figure SA 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 



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 

A prominent function of these tissues is support, 
either of the body as a whole or of some particular 
part. Among supporting tissues having this func- 
tion are fibrous tissues, which are characterized by 
JheUa^ceTii?" A^ cdis bundles of fibers or single fibers between the cells, 
of such a gland as the White, nonelastic fibers are usually collected in 
ifeZLt^'lZZn bundles, while yellow, elastic fibers are, in most cases, 
granules accumulated single and, sincc they branch and run together, tend 
idiltnfto ttmt to form a network. The fibrous tissues also serve 
or cavity, of the gland, to bind parts together and to hold them in place. 
tt'TntStilTe ;f;'r- Another type of supporting tissue is cartUage, in 
tebrate showing the which the space between the cells is occupied by a 
accumulation of mucus g^bg^ance known as chondrin, or "gristle." Still 

and its extrusion into _ _ _ ' ^ 

the lumen of the intes- 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 

Fig. 39. — Figures 
to illustrate the secre- 



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 


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


Havers/an cana! 

'Fiber bundle 

Fiber bundle 

Tendon cell 



Chondrin E Cartilage 

tissue cor- 

Fig. 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. S E). 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. E, 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 




D end rites 

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 
organs under the control of the will 
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 WMscZe, 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, 
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. 


Confracfile fibril 





Fig. 41. 



Nucleus of 




^y^i Muscle 

Fig. 42. 

Fig. 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. B, three nonstriated muscle fibers, or 
cells. C, several cardiac muscle cells. All highly magnified. 

Fig. 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 


the walls of the aHmentary canal, the blood vessels, and the gland 

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 an impulse. 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 axon, or axis cylinder process. Some nerve 
fibers have a fatty sheath and are said to be medullated; others, which lack 
this, are nonmedullated. 



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.— Trotection, 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.— Vroteciion and support. 



7. Muscular System. — Motion and locomotion. 

8. Nervous Syste^n. — Reception of stimuli, sensation, coordination, 
and causation of muscular and secretory activity. 

9. Re-productive 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 



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 




Articular Ml, 

of Joint 







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 hmb contains organs of 
the tegumentary, circulatory, skeletal, muscular, and nervous systems. 


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. Fission 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 hiparental. 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 



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 

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 vivi- 
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 external 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 jelly fishes, in connection A\ith the study of which it will 
be more fully described. 



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 epitheUum, which in turn is developed from the cells Uning 
the coelom, or body ca\ity. 

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 beheved that these cells were themselves animals 
li\'ing 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 multipUcation 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 




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 

Primorolial germ cell 


- Spermatogonia 

\ r-\ r^\ /^^ / ^, ' r> n L- •^- ' 

and so on for a very famenumber of cell divisions 

At^M any one spermatogonium 



,, Synapsis 

Primary spermatocyte 

Meiotic division 

'/ \\ Secondary 
— J Spermatocyte 

Sperm ce/fs 

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 



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 

^^^^^Xm Primordial germ celt 


and so on for a larye f^^J^^^er of cell d/i^/s/ons 

^FyT^ any one oocjonium 



Primary oocyte 

Moif urcation 

Secondary oocyte 

egg cell 

C/ \ Me /otic division 
1 (i^ First polar body 

® ® 

Polar bodies 

^Second polar 

°\ ,''Yolk accumulated in the 
egg cell 

Fig. 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 
hquid medium at a relatively high rate of speed. 


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 haK 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 Httle 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 ahke; the more important simi- 
larities may be enumerated as follows: 

1. Both start with a primordial germ ceU. 

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 

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 


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 multipUcation 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 fertihzed 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 fertihzation 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. 



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 


D E p 

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 pronuclei, 
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. E, 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. F, the first cleavage division, which follows the pause, at the beginning of the 

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 




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 
egg 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 httle cytoplasm. The 

Fig. 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. B, 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. E, a pause corresponding to that in 
Fig. 46£. 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 


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 (vl scans) 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 


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


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. 



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 

D E 

Fig. 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. E, 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 




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 

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 lower 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 

Fig. 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. E, 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 

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 


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 hlastomeres. 

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 holohlastic 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 E). In some cases the two 
cells first formed differ m 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 

In telolecithal egg cells the division of the cytoplasm results in an 
embryonic disc at the animal pole, and accordingly the cleavage is termed 
discoidal (Fig. 49 J5). As development proceeds and the cells continue 


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 httle 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 E). 

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 Hke 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 hlastocoel. 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 archenteron, 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 



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 7). 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 sohd 
masses of cells, which later become hollow. In both of the latter cases 
the cells surrounding these cavities form mesothelium (Fig. 51 7 and J). 


Blastocoel Blastoderm 

G Blastopore H Archenteron I 

Fig. 50. — Diagrams illustrating the steps in an ideal embryogeny. A, the egg. B, 
the two-cell embryo. C, the four-cell stage. Z), the eight-cell stage. E, the sixteen-cell 
stage. F, the morula, a solid mass. G, section of the blastula, with the blastocoel. H, 
section of the gastrula. /, 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 fertihzed). 

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. 


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 

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 epithehum 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. 



From the entoderm is derived the epitheUum hning 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 

D Nerve cord 



F hferve cord 

, chord- 

Nerve cord 

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. D, cross section of the blastula. E, 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. 7, 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 
hning 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 



and the blood which they contain, and the epithehal Hning 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 hning the greater part of the digestive cavity 
is entodermal, and the epithelium lining the cavities within the mesoderm, 
including the Hning 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, splanchriic. 

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 

of appearance 

Lining of wall 


Segmentation or cleav 
age cavity 









Disappears as the next is 

Becomes the digestive 
cavity of the adult 

Becomes in the adult the 
body cavity and cavities 
derived from it 




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 vacuofe 
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, Nucleus 

-^ ^ . ,^. orv J \ r\ iU- Fig. 52. — A number of collar cells, or 

Proterospongia (i^lg. 30.4). Un tJllS choanocytes, from one of the flagellated 

account the sponges were for a time chambers of a fresh-water sponge, Spon- 

, .,-1 1 ■ 1 n n X T-> gilla lacustris (Linnaeus). (From Borra- 

claSSlfied as colonial flagellate Pro- ^^-^^ ^^^^ p„^;^_ .. j.^^ Invertebrata:' after 

tOZOa. They differ from them, how- Vosmaer, by the courtesy of The Macmillan 

ever, in the fact that the body is °'^P'^'^y-> 

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 cefls, 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 



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 (p6 rif er a; L., porus, pore, and ferre, 
to bear) is divided into three classes : 

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

2. Hexactinellida (hex ak ti nel' li da; G., hex, six, actinos, ray, ella, 
Latin diminutive, and eidos, form). — Sponges with siliceous spicules 
having three axes; confined to the deep sea. 

3. Demospongiae (de mo spun' gi e; 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 fiat 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. 



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. 

Fig. 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 
Eupledella sp., a hexactinellid sponge known as Venus' flower-basket, showing the form and 
general structure; the spicules are white and like spun glass. X }4. 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 H- D, Ephydatia fluviatilis (Linnaeus), 
a fresh-water sponge belonging to Demospongiae. {From Zacharias, ''Die Tier- undPflanz- 
enwelt des Silsswassers.") X %. 

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. 



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- 
oscu/i/fT? Oscu/utn 

/ffae://ipr/ ccrncf/ 

'^ Oscu/um 
"^ ^ Gcfsfra/ cav/^y 


A B C 

Fig. 54. — Diagrams of canal systems of sponges. A, ascon type. 5, 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 scleroUasts. The spicules may be straight rods with 
one axis, the monaxon type; or they may have three rays in one plane 
and be triradiate; or four rays lying in four planes, in which case they 
are known as tetraxon. They may have six rays, the ends of three axes, 
in which case they are triaxon; or they may have numerous rays and be 
poly axon (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 



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 

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- 
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 
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 




Fig. 55. — Types of spicules. {From 

-r-, 1 1, , 1 " • f Sollas, "Cambridge Natural History," 

Each cell excretes and respires for 'f , ; 

by the courtesy of The Macmillan Com- 
The cells which are not collar pany.) a and 6, monaxon; c, triradiate; 

d, tetraxon; e, triaxon;/, polyaxon. 


cells receive their food more or 

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 cihated and swim about, but the adults are attached 
and never move from their position. Some sponges possess fiber-hke 
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 



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 


jr:0 ci 


Fig. 56. — Gemmules of a fresh-water sponge, Carterius tubisperma Mills. A, fragments 
of old sponge and gemmules on a piece of wood. X about 3. B, 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. X 78. {Drawn by E. F . 

existence. If budding continues and the individuals remani 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 


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 fertiUzation takes place. An embryo is formed which escapes 
through the wall of the body and becomes a free-swimming ciUated 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. 




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 soHd 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 
is contracted assumes an approximately spherical 
shape (Fig. 57). The attached end of this 
Fig. 57. — Hydra viri- body IS known as the hasal disc. The power of 

attachment is due to an adhesive substance pro- 
duced by gland cells in this disc. The free end 
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 huds 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 


dissima Pallas. A speci- 
men possessing a bud, 
shown in a partially ex- 
tended, and in b fully- 
contracted. X about 12. 



they are spermaries, or testes ; if they are more knobUke 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 



flagellated cell 

Fig. 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 gasir avascular 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 


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 

164. Nematocysts. — Interstitial cells in the ectoderm of the body 
in which are formed nematocysts are known as cnidohlasts. A nematocyst 
(Fig. 59) consists of a sac of fluid within which is coiled a thread. When 
fully developed, the cnidoblast 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 E). By thus coiling around the spines and 
hairs of the prey these nematocysts impede its movements. No cnido- 
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- 



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. 



A Nuc/euSy 


Fig. 59. — Sketches illustrating nematocysts and their action. A, a cnidoblast contain- 
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 

The nerve cells seem to be most numerous around the basal disc 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. 



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 







Fig. 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 disc 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 locahzed 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 strugghng 


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 

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 been 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. 




The same individual may produce both spermaries and ovaries at 
the same time, in which case self-fertihzation 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 
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 
the algal cells. 

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 gree^'^hyd^TSX ''Ssi'^a! 
develop into complete animals. While regen- showing the minute algal 
eration may result in an increase in numbers, plants (represented by the 

■^ ^ ' larger black spots) withm the 

it is not a normal method of multiplication entoderm cells. (From WhU- 
and cannot, therefore, be considered as repro- "^^' ^i^i^^sicai 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 

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. 



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 


Gasfro- vcrscu/ar 

Fig. 62. — Diagrams illustrating the comparison of the structure of a polyp (A) with that of 

a medusa (5). 

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, 



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 (sel en ter a' ta; G., 
koilos, hollow, and eyiteron, intestine) is divided into three classes: 

1. Hydrozoa (hi dro zo' 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 (si fo zo' a; G., skyphos, cup, and zoon, animal). — 
Includes the larger jellyfishes. 

3. Anthozoa (an tho zo' a; G., anthos, flower, and zoo7i, 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 



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 7J4 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 jellyfishes in not hav- 
ing a velum; in having a com- 
plexly branched system of radial 
canals; in the fact that the mar- 
gin of the bell is divided into sec- 


colonial marine 
sp. X about 3. 


Fig. od. Colonies of 
hydroids. A, Pennaria 

Medusa buds are shown attached to the sides 
of the polyps from which they have been tions by notcheS, in each of which 
developed B Sertularia sp X 2. 5a ^g ^ j^. ^f ^^ -j^^^ j^^ ^^ . ^^^ 
portion of a branch showing three pairs of . . ft- j > 

polyps retracted into the sessile hydro thecae. in many cases, in the abundance 

^ ^^' 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. i65). The stomodeum, in turn, is fastened to the body wall 

by radially arranged membranes called mesefiteries. 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. 



A B 

Fig. 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. 

Sfotrtocfeum feao/incr 

wfo en-/-eron i/p about moufh 




Pn'tnary mesentery 
Reproo/ucfi've or(yan 

A Edl^e of /nesentery 

Fig. 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. B, diagram of transverse section, reduced in size, showing the general plan of 
the mesenteries. 




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 (Sec. 185). The 
upper surface of the polyp is covered with many hollow tentacles the 

cavities of which communicate 
1 /''v^ _..</ with the enteron. Nematocysts 

are found on the tentacles and 
also on the acontia, which are 
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. 
'^Basof/ p/afe gg^ anemones usually exist as 

Fig. 66.-Diagram to illustrate the forma- • ^ polyps, though grOUpS 
tion of coral by a coral polyp. {From Thomson, '-^ & i J i^ ; & & i 

''Outlines of Zoology," after Pfurtscheller, by the may be formed by buddlUg. 

courtesy of D. Appieton & Company.) This Individuals may attain a diam- 

shows the formation of a basal plate and radial •' 

septa; it does not show the external wall or theca etcr of a foot Or more. 

which rises gradually with the basal plate and ^ ^^^^j animal, which is 

radial septa as the coral is deposited. 

usually an anthozoan polyp, 
secretes lime under the basal disc and around the side of the body, form- 
ing a cup. 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 hme 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). Sohtary coral polyps exist which may be 
several inches in diameter, but very frequently coral animals five in 
large colonies. The colonial polyps average smaller than the sohtary 
ones, the smallest not exceeding Ke 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 shghtly brown or yellow tint. Anthozoan polyps 
are often very brightly colored. Many jelly fishes are perfectly transparent 



and the mesoglea reflects rays of light with crystal-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. 

Fig. 67. — A, sea pen from Puget Sound, Ptilosarcus quadrangularis Moroff. X J^. 
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, Gorgonia sp. X 'e- This 
colony 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- 



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- 
sae. There remain to be added 
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- 
FiG. 68. — A Portuguese man-of-war, Phy- nished as they are with batteries 
:;!ri'ZSr X «^'Th?f»tt;f t"?.: of nematocysts which soon para- 

pable of extension to a length of over 40 feet, lyzC the struggling victim, they 
and bear thousands of minute nematocysts. ^^^^^ ^^^^^ ^^^-^^ 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. 



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, 



£gg cell - 

4-ceU sf-age 


2- eel I stage 

sperm cell from 
another animal 


B C "* E 

Fig. 69. — Diagram illustrating the life history of a scyphozoan jellyfish {Aurdia). 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 (5), then a gastrula (C), which is 
sno-rni in section, and finally develops into a ciliated planula larva (D). After a time this 
becomes attached and changes to a scyphistoma (£) , from which is developed a strobila {¥) . 
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 fertihzation, 
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 scyphozoan jellyfishes occurs an interesting type of 
budding called sfrohilation (Fig. 69). The planula, after becoming 



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 strohila. 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. 


cell from 



Fig. 70. — Diagram illustrating metagenesis, and also polymorphism, in the life history 
of a species of Ohelia. 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 
(E), 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 Ohelia may be 
taken as an example (Fig. 70). A colony of Ohelia 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 



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 jelly fishes, the medusae 
reproducing sexually and the strobila asexually. The hydra does not 
show metagenesis, because the same individual exhibits both types 
of reproduction. 

Fig. 71.— a, portion of a hydrozoan coral, the pepper coral {MiUepora 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.— Corai is a deposit of Ume 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 hving 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 ooral and the red, or precious, coral fall in a third 
group, produced by polyps related to sea fans and sea pens. Coral 



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 

Fig. 72. — Several types of anthozoan corals. A, brain coral, Meandrina sinuosa 
Lesueur. X }{. B, rose coral, Meandrina meandrites (Linnaeus). X %. C, portion of 

an elk-horn coral (Acropora sp.). X ^3. D, portion of another branching coral (Oculina 

sp-). X H- E< mushroom coral {Fungia sp.). X%. F, part of an organ-pipe coral 
{Tuhipora sp.). X %. The last is made by an animal related to the sea fans and sea 
pens; the rest fall into 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 



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

■piG, 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 

Fig. 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 

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. 






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

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 (te nof o ra; 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 
-Paddle plafe 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 jelhes. Because 
^ _- . ** , D7 these rows of paddle plates form 

Fig. 75. — A ctenophore, Fleuro- ^ ' 

brachia hachei A. Agassiz, from Puget roughened ridgcs running from one 

Sound. A, the organism seen from j^ ^^ ^j^^ ^^^iev SOmewhat resemb- 

the side. Somewhat diagrammatic. '^ 

X iM- B, diagram of a cross section ling the ridgcs on a walnut, cteno- 
of the animal to show the relations of phores are often Called sea walnuts. 

the canals. ^ 

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 






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 hiradial. 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 
ctenophores 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 


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. 



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, fiat, 
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 Eye spot 
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 is elongated, flattened dorsoventrally, 
blunt at the anterior end, and tapering to a J^^^ Mouih^ 
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 
the base of the head which are termed ears, but sal view 
these neither hear nor have any other function ^'"'f ' ^""^ ^,f ^/ Mus Comv. 

'' ZooL, vol. 31.) B, the ani- 

of their own. mal turned to show features 

On the upper side of the head and near each "^ ^^^ ^^ntrai surface, x 6. 
other in the median hne 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 



Oenifal ■ 


7 6 . — P lanar ia 

Leidy. A, dor- 

{From. Wood- 



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 



Opening oF 








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 enter on^ 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 gastr avascular cavity. 



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 ciHa 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 
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 
layers of external and internal 
longitudinal fibers which are 


of excretion 

^, Flame 
f cells 

of the cell 



Fig. 78. — Semidiagrammatic sketches to 
illustrate the flame cells of a planarian. {From 
Benham, in Lankester's, " A Treatise on Zoology," 
,1 , f -I y by the courtesy of A. and C. Black.) A, part 

separated by a set OI oblique ^j ^^le system of excretory tubes, showing 
Bundles of muscle fibers the relation of the flame cells to them. Nuclei 

are seen in the walls and tufts of cilia project- 
ing into the tubes, adjacent to the nuclei. 


also pass through the body dorso- 

ventrally, between the branches B, flame cell. 

of the intestine. 

The nervous system (Fig. 77 C) is very simple. Two masses of 
nervous tissue containing nerve cells and fibers lie 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 



distributed to the internal structures of the body and to the surface, 
particularly to the head, which is the most sensitive region. Since the 


Yolk glands 



Geniivjl atrium 


Yasa efferentia 

Vas deferens 

Prostate gland 
Seminal vesicle 
Genital pore 

Fig. 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 


each of these testes is connected a tube of thin epitheUal 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 th'ese 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 pouchUke 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 cihated 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 fission. 


Although both male and female germ cells are produced in the same . 
individual, cross-fertilization rather than self-fertihzation occurs. The 
egg cells received by the atrium from the vagina are passed into the 
uterus, where they are fertiUzed. When cross-fertiUzation 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 

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 shde over the 
surface on which it moves. The motion is rhythmic, waves of move- 
ment passing backward from the head. The ciha beat in a mass of 
shmy 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 stimuh which are effective in 
other lower organisms— that is, to contact, to temperature, to fight, 
to chemicals, and to water currents. They find their food by both 
chemical and contact stimuh. 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 fight-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. 


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 gangUon 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 ijjf'g';'^^^""^ 
margin of the piece which was nearest the head in the uiustrate the 
animal from which it came, while a new tail is developed '^^}^}'?}^l 

- gradient in a 

on the opposite margin. The explanation of this was tor planarian. 

a long time obscure but has been furnished by recent experi- J^^J/^j^^^^^jf 

ments. These show that in an animal possessing a head the black line 

and a tail, as do planarians, there is a gradient in metabolic shows the 

' • J J. +1, varying de- 

activity extending from near the anterior end to tne grees of meta- 
posterior end. The rate of metaboHsm is greatest at the b o Hc activity 

J ,, J. at different 

anterior end of this gradient and decreases gradually trom ig^eis, such as 

this end to the other (Fig. 80). Thus it is that any «,^^f'. c-^ The 

fragment will differ in the metaboHc activity of its different c o u r s e , ' n o t 

portions corresponding to their position with respect to the confined to the 

^ 1- o , ,. ,1 -1 median line 

axial gradient. Consequently, from the margin where ^ut extends 
metabolic activity is greatest a head will develop, and from from one side 

.1 rr^i • J • J! . 1 , to the other. 

the other margin a tail. This conception of an axial meta- 
bohc 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 



The phylum Platyhelminthes (plat i hel min' thez; 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 symmetry 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 centralizatio7i. 

193. Classification. — This phylum is divided into three classes 
as follows: 

1. Turhellaria (ter bel la' ri a; L., turhella, 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 (tre ma to' 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 



may also be other suckers on other parts of the body. The trematodes 
are all parasites. 

3. Cestoda (ses 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 presen(!e 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 



(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 

-Ora/ sucker 

'Merve center 







Ven traf sucken 

Yolk gr/an(^S 

_ Seminal 


Yolk c/uct 

Vas efferens 


Fig. 81. — Clonorchis sinensis Cobbold. {Redrawn from Hegner, Root and Augustine, 
"Animal Parasitology," after Faust.) Dorsal view, showing internal structure. X 8. 

the body. Added to the female organs is a sheM 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. 






196. Cestoda. — This class includes the tapeworms, which are common 
in the alimentaiy canals of vertebrates. A typical tapeworm (Fig. 83^) 
consists of a more or 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 an embryo. 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- , ^'«- 82 —Diagrammatic sketch 

' . ... showing relationships of female organs 

ence to the relation of the individuals in Clonorchis sinensis. (From a sketch 

in a coelenterate strobila (Fig. 69F). ^^ h w Manter.) The egg cells are 

^ " formed in the ovary and passed into the 

The scolex and neck correspond to the oviduct, where they are fertQized by 

scyphistoma and the proglottids to the ^p^/™ """^^^ ^i"°™i^'' T^'^'f receptacle 

•' ^ . and provided with yolk and shell-form- 

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 ^Z introduced through Laurer's canal, 
' which opens to the outside. In some 

unity m the organization of some SyS- other flukes, however, there is no ex- 

tems common to the entire tapeworm, t^^^i opening to this canal and it seems 

. . . to be a vestigial organ. 

The effects of parasitism are carried 
much further in tapeworms than in the flukes. 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 

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 



possess hundreds of proglottids and which may reach a length of many 


197. Metabolism. — The steps in metabohsm in a free-hving 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 






She// Va//f 

Vasc^efereris -^^^^^ ^^""^^ ^''^^-^ 


Fig. 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.") X }4- B, scolex and 
neck in an extended condition. The rostellum bears no hooks and the tapeworm is spoken 
of as unarmed. {From Leuckart.) X 5. C, proglottid, showing the sex organs. {Also 
from Leuckart.) X 7. D, ripe proglottid, showing the uteius 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, 




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 gastrovasciilar ca\'ity 
to the outside in turbellarians, but the extent to which these can 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- 
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 indiiddual. 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 
vertebrates is infected by the parasitic forms. 
From an economic standpoint the free-living flat- 
worms are of no importance, but both trema- 
todes and cestodes produce a great deal of injury 
to domestic animals and to man. Among trema- 
todes several flukes are parasitic in man. The . , , 
most serious of these are the blood flukes (genus the two sides are not 
Schistosoma). Two species of blood flukes occur ±°^''^ i^ true perspective. 

. „ . . , _-. T T They should be shown in 

m man m Africa, one m 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. 

Fig. 84. — Scolex of the 
pork tapeworm, Taenia 
solium Linnaeus. {From 
Leuckart, " Parasiten des 
Menschen.") X30. 
Shows a rostellum with 
hooks, this being an armed 
tapeworm. The illustra- 
tion is not artistically cor- 




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. 




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. 



Yo/k „ 
<re// B 

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. E, the sporocyst containing developing rediae, in liver of the snail. (C and 
E from Leuckart, " Parasiten des Menschen.' ) F, 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.) H, encysted 
cercaria on vegetation. 7, 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 


u1;ems 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 ciha and becomes transformed 
into a sacHke form known as the sporocyst. The sporocyst is the second 
larval form in the hfe cycle. Within it are groups of cells, usually con- 
sidered as germ cells, which develop, without fertiUzation, into larvae 
known as rediae (singular, redia). The sporocyst, therefore, carries on 
reproduction. The redia is also sachke but, unhke the sporocyst, pos- 
sesses a mouth and a simple enteron. Rediae escape from the sporocyst 
and develop in the hver 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 unfertihzed germ cells. 
The cercaria possesses a body and a tail, the body having an oral and a 
ventral sucker and a forked enteron hke the adult. It is the fourth type of 
larva in the life cycle. 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 hver, 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. 



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. 


1 Jn?; fr!:^T^^ ''J^ ^'l*u''^ °^ ^ ^T^ trematode, Haematoloechus medioplexus, of the frog. 
1. adult trematode from the lung of the frog, Rana pipiens. {From W. W Cort 1915 ) I 
egg contaming miracidium. Passes from frog in feces; does not hatch until ;aten by 3 
Planorhula armigera, a snail. The eggs hatch in this snail and sporocysts develop in the 
liver. 4, sporocyst containing developing cercaria. 5 mature cercaha. It leaves the 
snail, swims about in the water, enters the branchial basket of dragonfly nymphs. 6, 
ZTJ'^A ^^^ ""^J' '^'''^"f encysted in nymph. 8, mature dragonfly containing 
encysted cercana When eaten by a frog, the trematode larvae emerge and pass up thS 

ri^?^-'/?"'" '^p'r^.- ^""'n'"^ "■ ^- ^''^"'^'•' ''^Laboratory Manual ir. Animal Para- 
sitology, Burgess Publishing Company. Figure 7 from Krull, 1930.) 



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. D, later stage in the 
development of the cysticercus. E, cysticerci (about natural si2e) 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 


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 ahmentary 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 cercaria. 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 

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. 


-^ ^5p/cu/es 


The representatives of the phylum Nemathelminthes (n?m a thel 
min' thez; 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- 
£xcre-/vr ^copic, but a small number reach a larger 
pore size and a few may even be several inches in 

206. Structure of an Ascaris. — An ascaris 
may be taken as a type of the phylum. 
Logically a free-Uving 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 lumhricoides hinnsieus, 
is found in the intestines of pigs and human 
beings. The ascaris found in man and in 
pigs are morphologically identical but physi- 
,. . 1,. V 1/ '^R^ ologically different. Because of certain phys- 

Linnaeus. A, male. X ,^t. o> => '^ . . ~ 

the anterior end of the body, c, iological differences, some authorities prefer 
Jhowi^Hhrl hpl "a p^steno; the name Ascans suum for the pig ascaris and 
end. (S to D from Leuckart, AscaHs Iwmhrico ides for the ascaris of man. 

"Para^itendesMenschen.") Mag- ^^^ ^^^^j^ .^ ^^ ^^^ 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 


Fig. 88. — Ascaris lumhricoides 

X H- 



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. 

Dorsof/ /;he 

nerve cord 







Ver7¥^rcr/ ^ Ventral 

nerve cord //he 

Fig. 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 

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 



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 aUmentary 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 eariier 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 
food some other means must exist, 
and this is furnished by the body 

208. Classification.— The 
phylum Nemathelminthes may be 
considered as including three classes : 

1. Nematoda (nem a to' da; G., 
nematos, thread, and eidos, form). — 
Either parasitic or free-Hving. 

2. Gordiacea (gor di a' she a; L., 
gordius, referring to a complicated 
knot) . — Parasitic in the larval stages 
and free-living and aquatic as adults. 

3. Acanthocephala (a kan tho sef '- 
a la; G., akantha, thorn, and kephale, 
head). — All parasitic. 

209. Free-living Nematodes. — 

contractile part of the cell is indicated by ^y^ inCOnCcivably large number of 
lines, the nou-contractile part by stippling. . , „ t • j i i 

mmute 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 beUeved 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 

Process /ectcfi'n^ /<? nerve 



'Con-f-rcrctj'/e pa/^f 

of ce/l 

Fig. 90. — A muscle cell from an ascaris. 
These cells run longitudinally and are 
shown in cross section in Fig. 89. At the 
right are shown two sections of the same 
cell to show how the appearances seen 
in Fig. 89 are produced. {The cell from 
Leuckart, " Parasiten des Menschen.") The 




A/erve ce//s 




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 Trichinella, 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 inj ure the organ- 
ism. The larva, which at first may 
be free and may remain so in free- 
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 S^^^"^ 
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 
passed out with the feces and 

mixed with the soil in the hog ^j^ ^i.—UonhysUra sentiens Cobb, a 

lot, are taken up by the young pigs free-living nematode. Side view of female. 

, , 1 • , 4. u i. Probably the most widely spread nematode 

as soon as they begm to root about; g^^^^^^ ^^^^^ -^ ^^.^^^^ ^ater, in the sea, and in 

or if infested soil is caked upon soil. (From Cobb, in Ward and Whipple's 
,, , 1 ~ ., ,, ,, "Fresh-water Biology," by the courtesy of John 

the body of the mother, the eggs j^^^^y ^ sons. inc.) x 94. 
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. 


^Egr^ C£'//S 





This journey through the body is injurious to the young pig, retarding 
its growth. The passage of the worm through the kings 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 milhons, 
known as "poor whites," who were 
noteworthy for their shiftlessness, 

"Tropical Diseases," after Paclencia, by although they were the descendants of 
the courtesy of Cassell & Comimny.) , . . , , i rm 

very good immigrant stock. These 
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 



Fig. 92. — N ecator americanus (Stiles). 
A, male. B, female. {From Manson, 



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. 

214. Trichinella. — A third parasitic nema- 
tode is that known as Trichinella spiralis 
(Owen) , which is the cause of a disease known 
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 hve 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 vilh (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, Hme 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 Hfe 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 

Fig. 93. — Section of pork 
containing encysted larvae of 



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 

branches of water milfoil Wuchereria bancTofti (Cobbold), which is a 

{Myriophyiium) in which is threadworm living in the lymphatic system of 

man. 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 

Fig. 94. — Portions of two 

coiled a specimen of Gordius 
Natural size. 


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). 

^^^ Proboscis „ / ^ , 

^j^\y Bra//^ Testes 

W,t*\ ^__/^_-_ V\^ Pr-<?sfcfte gf/a/7c/s 

/fiver for musc/es Refractor musc/es ^-~-^=^ 

Fig. 95. — Echinorhynchus ranae (Schrank), showing general organization of a young 
male. (Fro7n 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 Nemathclminthes 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 produc- 
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, feechng upon the injurious forms or destroying 
various injurious microorganisms. 



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 (nem er tin' e a; G., nemertes, 




Catfi^cf/ C/rrus 


Fig. 96. — Cerebratulus, a nemertine. A, Cerebratulus lacteus (Verrill), a common 
Bpecies 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 fiatworms, 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 




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 








Fig. t)7. — 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, Malacohdella, is parasitic in a European marine bivalve 

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 abihty of an animal itself to break its body into 

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). 


220. Phylum Chaetognatha.— The group Chaetognatha (ke t6g'- 
nath a; 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 annehds. 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 gangha, 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. 

221. Phylum Rotifera.— The phylum Rotifera (ro tif er a; L., rota, 
wheel, and ferre, to bear) includes small, aquatic animals, many of them 

^an^//or> Ovary Anus vesicle 

Fig. 98. — Sagitta hexaptera D'Orbigny. Ventral \'iew. {From Lang, "Text-book of Com- 
parative Anatomy," after 0. 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 ciha which in some cases are grouped 
in circles. Such circular groups, owing to waves of motion which pass 
rhythmically around the circle of ciha, 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 bilateraUy 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 (trok hel min' thez; 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 



the posterior end of the body. The term cloaca is apphed to any cavity 
into which open both the aUmentary 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. 


-Dors^f/ fee/er 
Eye spot 

Flcume ce/l 



Cewe/r/' ^/a/^c/s 


Fig. 100. 

Fig. 99. 

Fig. 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 and swims. A common American species. {From Jennings, 
in Ward and Whipple, ''Fresh-water Biology," after Weber, by courtesy of John Wiley & 
Sons, Inc.) X 300. 

Fig. 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 simphfication 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 o^'iduct 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 



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 

A marked peculiarity of certain rotifers is their power to undergo 

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 

222. Phylum Bryozoa. — The animals 

included in the phylum Bryozoa (briozo'a; 

G., bryon, moss, and zoon, animal) are known 

-c^ -.f^, rr.. ..^ rr , in a general way as moss animals and sea mats. 

Fig. 101.— The rotifer Hyda- „, *= , • i i- wi • 

Hnasenta Ehrenberg. A, mature i hey are colouial lorms and their manner of 
female. B small degenerate male growth reminds ouc of the coloiiial hydroids, 

showing the large sperm sac but , 

c, but their structure distinguishes them very 

Like the hydroids they have plant- 

lacking a digestive system. 

parthenogenetic egg covered with dearlv 

a thin membrane and developing . . 

within a few hours into a female, hkc characteristics which often cause them to 
D, smaller parthenogenetic egg be included in Collections of dried seaweeds 

covered with a thin membrane 

and developing within a few brought as souvcnirs from a Seaside trip, 
hours into a degenerate male The biyozoaiis are mostly marine, though a 

which IS sexually mature at time . 

of hatching. E, fertilized egg few livc 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- 

mentai Zoology.) 

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, ner of growth and suggest the name sea moss 
{From Whitney, Journal of Experi- (Fig_ ^02). Other colonies form mathke 

masses and are quite appropriately termed 
sea mats. 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 zooecium, 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 



support for the colony. Sometimes lime is added. 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 cihated 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 is "somewhat similar. 
develop in a modified portion of the body cavity ^^^l^^^ j^^l^^lHistir^" 
called an ooecium which serves as a brood pouch, by the courtesy of The 
The larva is in many respects like a trochophore. Macmiiian Company.) 

•^ ^ Natural size. 

Certain species produce nidividuals of a peculiar 
type called avicularia. These are highly modified forms possessing a pair 
of strong jaws, which probably are used in defense. 

Fig. 102.— a colony 
of marine bryozoans, 
Bugula turbinata Alder. 
A British species; an 
American species, 
Bugula turrita (Desor), 




Musc/e -^o 
boofy wa/J 






Jaws oper 

/vfasc/e to 
boc/y wa// 


Fig. 103. — Bugula avicularia (Pallas), a European species. Two zooids are shown, the 
one at the left entire, that at the right turned 90 degrees to the first and sectioned. Two 
avicularia are shown, one with the jaws closed and the other with them open. An ooecium 
is also included in the figure. {Redrawn from Parker and Haswell, "Text-book of Zoology," 
somewhat modified.) Much magnified. 



The fresh-water bryozoans (Fig. 104) form moss-Hke 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 


; Lophophor-e 


Fig. 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 crawhng along on its base. These fresh-water forms have developed 
a type of winter egg known as a statohlast (Fig. 105), which is produced in 

Fig. 105. Fig. 106. 

Fig. 105. — Statoblasts of two genera of fresh-water bryozoans. A, Plumatella. B, 
Cristatella. Much magnified. 

Fig. 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. 



223. Phylum Brachiopoda. — The animals included in Brachiopoda 
(brak i op' o 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 

/ op/70pho/'e' 











Fig. 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, Ldngula, 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). 



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 Httle 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 

Fig. 108. — Aboral surface of a starfish, Asterias vulgaris Verrill. 

specimen. X }'2- 

From a preserved 

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. 




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- 


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 efTort to 
turn over. X J^. 

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 interradii. 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 amhulacral groove (Fig. 110 .4) 
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 



spines, known as marginal spines. These project more or less over the 
groove, which is nearly filled \\'ith 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 

Per/'brar/^ch/'a/ space 


Tube /bof 



Py/or/c cctecufTT 

Paolia/ ca/^a/ 
Lafera/ carrcif 

'Ambu/acral p/a-^e 
^' ^ ^A/fcrrgi/'/^a/ sp/ne 


Ambcz/acra/^ A(pfa/77bu/cfcra/ 

p/a-^e g p/ai^e 

Fig. 110. — Ray of a starfish. A, diagrammatic cross section to show especially the 
arrangement of the ambulacral plates and the tube feet. 5, ambulacral and adambu- 
lacral plates of a dried and cleaned specimen, showing the pores for the tube feet. X 3. 

face to face on either side of the mid-ambulacral line and nearly at right 
angles to it (Fig. 110 5). 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. 



226. Water-vascular System. — The water-vascular system, which is 
pecuhar 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 ca\ity to a ring canal 
which lies at the outer margin of 

■5 to fie caA7a/ 




■ff/n^ rar7a/ 




the perioral membrane. In the 
stone canal just below the madre- 
porite, as well as on the inner sur- 
face of the madreporite itself, are 
ciha, the movement of which forces 
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 
the ampullae and the tube feet. 
The ampullae are sacs lying inside 
the wall of the ray; the tube feet 
lie outside it and in the ambulacral 
groove. The tube which connects 
each ampulla with its tube foot 
runs through an ambulacral pore 
(Fig. 110 A), formed by the separa- 
tion of adjacent ossicles. These 
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 

Fig. 111. — Diagram to illustrate the 
arrangement of parts in the water-vascular 
system, a, tube foot and ampulla, enlarged. 




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 


Py/onc ccfeca^rr 


£ye Spot 
Tube 7€»^/ 

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. Each 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. 



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-dwelhng 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 


Fig. 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. Wlien 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. 



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 
caeca but also in another and very curious manner. Ameboid cells, 
known as amehocytes, lying in the coelomic fluid, pick up particles of 
waste and make their way out through the walls of the dermohranchiae — 
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 


Preorcr/ c//i'cr^ec^ 











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 ^. 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 wdthin 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. 
Ill) 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 



central ganglion. The system consists of a nerve ring encircling the 
perioral membrane, radial nerve cords lying at the bottoms of the ambula- 
cra] 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 restrauiing 
these rays, Jennings succeeded in 
developing in individuals the use of 
others. One animal was trained in of a group of starfishes noted for their 

180 lessons ten on each of eiffhteen '"^generative ability. A to C, three speci- 
iOU lessons, len on eacn OI eigmeen ^^^^ showing regeneration of the whole 

days, so to use certain rays; after animal from one ray. in such a case five 

an intprvq] of spvpn davs and whpn arms are regenerated making six altogether, 

an mier\ ai oi seven aays ana wnen g^^ ^^^^^^ ^-^^ normal number. When the 

left to its own initiative it was still disk is uninjured only the rays lost are 

using them. This type of action '^s^''^'^^^^' ^« i" ^- 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 embryo. A larva known as a hipinnaria (Fig. 114) 
is produced which is bilaterally symmetrical but which gradually meta- 
morphoses into the radially symmetrical adult. 

Fig. 115. — Linckia guildhigii Gray, one 


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 disc will 
regenerate the whole animal (Fig. 115). 

The starfish also possesses the power of autotomy. Ordinarily the 
part dropped off is regenerated. This abiUty 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). Ownere of oyster 
beds fonnerly were in the habit of using drags made of ftayed 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. 



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 hivium and the three nonadjacent 
rays the trivium. 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 radial symmetry. 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. 




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 

Fig. 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 six rays. 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 0. Wade.) 

235. Classification. — The phylum Echinodermata (e kl no der' ma ta; 
G., echinos, hedgehog, and dermatos, pertaining to skin) is divided into 
five classes: 

1. Asteroidea (as ter oi' de a; G., aster, star, and eidos, form). — The 

2. Ophiuroidea (6 fi u roi' de a; G., ophis, serpent, ou7-a, tail, and 
eidos, form). — The brittle stars and serpent stars. 

3. Echinoidea (ek i noi 'de a; G., echinos, hedgehog, and eidos, form). — 
The sea urchins and sand dollars. 

4. Holothurioidea (hoi o thu ri oi' de a; G., holothourion, water polyp, 
and eidos, form). — The sea cucumbers. 



5. Crinoidea (kri noi' de 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 

Fig. 117. — The long-armed brittle or serpent star, Amphiodia occidental is (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 disc is small and the rays are long and slender; in 
others the disc 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 


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 discs 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 Hke the tails of so many snakes 

5p/'ne i\H[Jl| 

rate ^ooi>^M^m 

Fig. 118. — A sea urchin, Strongylocentrotus drobachiensis (Miiller). From a pre- 
served specimen. The spines have been stripped from the right half; they and the tube 
feet show on the left. X ^. 

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, cHnging 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 ani- 
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 interamhulacral 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 interamhulacral 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 interamhulacral bands run up around the side of the body and ter- 




Ocular- plof-e 


Jf7-f-efambu/acrc,/ . Ambu/ccrc^/ 

p/afes ^ p/afes 


rainate 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 
and known as an Aristotle's lantern 
(Fig. 120). This is made up of nu- 
merous ossicles, lies \vithin 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 a circle around the mouth. 
The tube feet are also said to be 
respiratory. The latter may be 
exceedingly long if the spines on the 
surface of the animal are long, since 
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 
urchins and sand dollars are exceed- 
ingly flat forms ^^^.th numerous very 

small and short spines ; they are found B, the oral surface, with the perioral 
^■^ ««„J u u u • iU .„ membrane torn loose for about two-thirds 

on sandy beaches, burying them- ^^ ^,3 attachment. Dried pedicellariae 

selves just under the surface of the are still attached to this membrane. 

sand as the tide goes out but mov- 
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 


Fig. 119.— Dried 
of the genus Arbacia. 
ment of plates. A, 


shell of a sea urchin 

Shows the arrange- 

the aboral surface. 



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 





Fig. 120. — Internal structure of a sea urchin. {From Ddage and Herouard, " Traite de 
Zoologie Concrete," 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 



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 

O/'a/ feni-ac/es 

P/n^ cai/7a/ 



Gen/-^/ o, 


musc/e banc/ 



rl ~'\^i\/^^^^^por/'-^e 


;7~ ~\\^/n-/-esf/ne 

— b/ooof vessel 

b/ooo/ vessef 

^^^^ - Pesp/'ra-Z-ory 

Paef/'a/ musc/es 

Fig. 121. Fig. 122. 

Fig. 121. — A sea cucumber, Cucumaria planci Brandt, from the Mediterranean. 
From a preserved specimen. X /s- 

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 



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 
formation has been referred to in the preceding 

243. Color. — The echinoderms when living are 
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. 
There are also varying tints from cream to almost 
white, and innumerable shades of buff, brown, 
green, blue, and purple, some being almost black. 
When a group of sea cucumbers are seen on the 
bottom with their tentacles spread and possessing 
varied and brilliant hues, they appear as striking 
as a bed of variously colored flowers. 

244. Occurrence and Economic Importance. 

Fig. 123. — Rhizocri- 
nus lofotensis Sars. {From 
Bather, in Lankester, 
"A Treatise on Zoology," . riij. u u- 

after Wyviiie Thomson, Becausc of the possessiou of skeletons by echi- 
by the courtesy of A. and C. noderms their parts have been preserved in 
^' 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. 



Of this phylum only starfishes and sea cucumbers are of much eco- 
nomic importance. As has been previously indicated, the starfishes 
are enemies of the oyster. Sea cucumbers are used as food among the 

'■A bora/ 
Macfreponte fenfades 

Fig. 124. — A feather star, Antedon sp. From a preserved specimen. Natural size. The 
animal is in a breeding condition as shown by the dilatation of the base of the pinnules. 

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 heche-de-mer and trepang. 




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 

Un7bo^ ^H/n^e ligament 


' siphon 

Line of 

Fig. 125 — Anodonta grandis Say. From a preserved specimen from Nebraska. 
X M. 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 outhne of a conventional heart (Fig. 127). The animal is 




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 siphons, 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 

Hin^e li'^cfmerr-f- 


Aifac/imenf area 
of an^er/or 

Aitachment area 

o-f posfen'or refracfor 


area of 



Showing inner surface of a right-hand valve. 

Fig. 126. — Anodonta grandis Say. Showing inner surface of a right-hand valve. X J^. 

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 umbo, 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 toothhke 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 



together. A little way from the ventral margin and parallel to it is seen a 
small groove, known as the palUal line, which marks the attachment of 
the muscle layer of the mantle, or pallium. 












Fig. 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) 


o-f ure-her 





Pcsfenor refmcfor muscle 

ac/ductor muscle 

Visceral gang //on 

Anal opening 




Labial palp 


Foof Peolal 


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, 


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, w^ere 
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 
Hme 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 slithke 
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 



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 

Openiha of 







Mant/e cayj'fy Oufer ^//l 

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- 


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 cerehro-pleural ganglia near the mouth, the yedal 
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 httle 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 enabhng 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 



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 ciha 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. 




Fig. 130. — Diagram to illustrate the life history of a fresh-water mussel. A to E are 
of TJnio 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, Bxdl. Bureau 
of Fisheries, vol. 30.) A, the adult. X about }i. B, the egg. X 55. C, Two-cell 
stage, showing unequal cleavage. X 55. D, section of the gastrula. X 122. E, 
glochidium. X 92. 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 



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 


Fig. 131. — Shell of a unio, Quadrula 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. 


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 unios, 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 


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 Lmnysilis 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. 


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 lus' 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 (amfinu'ra; G., amphi, on both sides, and neuron, 
nerve). — The chitons. 

2. Gastropoda (gas trop' o da; G., gastros, stomach, and podos, foot).^ — 
The snails. 

3. Scaphopoda (ska fop' 6 da; G., skaphe, boat, and podos, foot). — 
The tusk shells. 

4. Pelecypoda (pel e sip' o da; G., pelekys, hatchet, and podos, foot). — 
The bivalve mollusks. 

5. Cephalopoda (sef al op' o 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 




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 distinctly 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 










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 a chiton. {From 
Cooke, "Cambridge Natural History," after Hubrecht.) D, a primitive, wormlike species, 
Chaetoderma nitidulum Loven. {Also from Cooke, "Cambridge Natural History.") X 2%. 
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. 


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- 

Mar;f/e co'v/fy or 
pa/fT70/^ary chan7ber 


^ ferfac/e 


Intest/he \ ope/o/ngr 

Open/'ng' /hto 
pulmonary c/7t;*^^£>er 

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 

The number of ganglia in the gastropods is much greater than that in 
the mussel, and they lie closer together in the body (Fig. 134). 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 sHme 
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. 



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 


Fig. 134. — Somewhat diagrammatic representation of the central portion of the nervous 
system of Hdix -pomatia. {From Lang, " Text-book of Comparative Anatomy.") 

be extended into a spire. On the other hand, the mouth of the shell may 
be greatly expanded and the coihng be reduced to one turn or even a part 
of a turn, as in 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 Hd 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 disc 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, 






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 moUusks 
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 hyssus, 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 sac or caecum connected 
with the intestine secretes a rodlike body, or crystalline 
style, the function of which is unknown. 

264. Cephalopoda. — The cephalopods make up a 
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. 

Fig. 135.— 
A tusk shell, 
Dentalium preti- 
osum Sowerby, 
from Puget 
Sound. Natural 










Ink sac 



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 siphon, out of which water 
can be forced by the contraction of the mantle, driving the animal back- 
ward through the water. The shell in the 
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- 
sory organs include two very highly 
developed eyes, two organs of equilib- 
rium, or statocysts, and one which is 
probably an olfactory organ. The eyes 
(Fig. 137) are large and superficially 
similar to those of the vertebrates but 
when critically compared with them are 
found to be fundamentally different in 
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 

Flo. 136. — Common squid of the 
Atlantic coast, Loligo pealei Lesueur. 
From a preserved specimen. X M. 
A, lateral view. B, diagram to 
show position of certain internal 
structures. The body is placed 
so as to bring out the position of the 
morphological axes; its normal posi- 
tion in the water is at an angle of 
90 degrees to this, with the anterior 
surface above (Sec. 264). 



cavity and siphon resemble those of the squid and are used in a similar 
way. Large octopuses are dangerous antagonists and are feared by 


Cornea (-f-rcrnsp,) 





(Tar-f'/ /ct^t 

Op'f/'c r?erve 

Optic /lerve 

Fig. 137. — Eyes of mollusks. A, eye spot of Patella, a gastropod, possessing only an area 
of pigmented epithelium, or retina, and optic nerve. B, 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 Physiologic 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. 



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 


Dor sat/ /obe 



She// masc/e 


befweer? chambers 

Fig. 139. — A female chambered nautilus. {From Hertwig and Kingdey, "Manual oj 
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 siphuncle. 

Another type of cephalopod is 
the paper nautilus (Fig. 140). By 
means of glands in certain tentacles 
the female secretes a small and deli- 
cately coiled shell which is really a 
basket for the eggs, and which, because 

"Text-book of Zoology:' by the courtesy of ^f j^g method of formation, is not 
The Macmillan Company.) X about }-'i. , , .,i ,i i n r j.i. 

homologous with the shells ot 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, 

Fig. 140. — Paper nautilus, Argo- 
nauta argo Linnaeus. Female, swim- 
ming. {From Claus and Sedgwick, 


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, Urosalpinx, 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 fiatworms. 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 

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 MoUusca, 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 ciHa about the margin of the 
expanded upper portion and with an eyespot at the apex of the body. 



The mouth opens near the ring of ciUa 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. 

Fig. 141. — Trochophore of Patella, one of the limpets. 
side. B, median section of a slightly older trochophore. 
"Text-book of Embryology.") 

A, viewed from the ventral 
(From Korschelt and Heider, 

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. S. 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 




//7 -f-i/te 



Fig. 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 Mobius, by the courtesy of The Macmillan Company.) 
Natural size. B, the shell, mounted in position at the end of a tunnel, lateral view. {From 
Miller, Univ. of California Pub. in Zool., vol. 26, by the courtesy of the University of Cali- 
fornia Press.) X 53^. 



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 

Fig. 143. — Mollusca 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. C, 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." 

success 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 


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 fives. 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 pifing, 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. 




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, Lumhricus 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 XXXI or XXXII to XXXVII 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 clitellum. 

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 (/) 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 








Openm0 of 
Is/as c/eferens 


one end of the animal to the other the aUmentary canal extends, a 

tube within a tube. It is held in place by the partitions, the coelomic 

spaces thus becoming ringhke. 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 fined by a delicate epi- 

thefium known as the yeritoneum. 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; yxKVE- 
a thin- walled ciliated crop, or proventriculus; 
a thick-walled muscular gizzard; and a thin- 
walled intestine, 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 wdth 
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. 
Feces are egested at the anal opening. They appear in the form of cast- 
ings outside the burrows. Enzymes are produced in the intestine which 
act on proteins, fats, and carbohydrates. 

272. Circulatory System.— The circulatory system of the earthworm 
is a compficated system of blood vessels through which the blood is 
forced by a sort of peristaltic action of the muscle fibers in the walls of 




Fig. 144. — An earth- 
worm, Lumhrictis terrestris 
Linnaeus. Ventral side of 
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. 



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 oxyhemo- 


■ muscles 


First toop^ o-f 

Thirc^ hop o-f 

Second /oop ot- 


Position ot^ 
''n terseg/n^en ta/ 


'rcu/ar musc/e 

^ /ntest/na/ 
6/ooc^ vessel 

Ch/or-a - 





f/ep t) rostome 

Vent fa/ 
r?efVe cordis 


\ So&intest'>-'a/ 
biooai vesse/ 

(j/arnt f/'/berS 
Subneur-a/ h/ooaf vesse/ 

Fig. 145. — Diagrammatic cross section of an earthworm at about the middle of the 
body. {From Marshall and Hurst, "Practical Zoologij," by the courtesy of G. P. Putnam's 
Sons.) The nephrostome of the segment in front is shown for the sake of completeness. 

glohin. 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 hberating 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 cihated 
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 



Buccal cav/^y 








Dorsa/ b/ood 

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. 


-Nephrostome ^^Nephn'p/i'opore 
Intersex werjfc*/ 


Fig. 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. 



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). 








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 

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 suprapharyngeal 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 circunipharyngeal 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 


backward to the posterior end of the body showing a gangHon in each 
metamere, each ganghon 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. 

Earthworms 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 httle 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 hght 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. 

Earthworms 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 hmit 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 



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. 

A/erve cor^ 






Vas c/eferens 

Fig. 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 



and through the length of the chtellum 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 
seminal receptacles 

Opening of 
vas deferens 

Seminal groo ve 




Fig. 150. — Reproduction in the earthworm. {From Curtis and Guthrie, " Text-hook of 
General Zoology,^' by the courtesy of John Wiley & Sons, Inc.) A, two worms enclosed in 
bands of mucus. B, 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 

Asexual reproduction does not occur in the earthworm. 



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. 




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- 
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 

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. 


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 impulse, 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 abihty 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 

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 axon, 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 




are carried from the center to the effector cells is known as the efferent 


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 

Motor neuron 

Efferent nerve 

6/anf fibers 

venfrcrf ganglion 

nerve fiber 



Venfrai longitudinal 


Fig. 152. — Diagram illustrating reflex action in an earthworm. {From Parker, in Popular 

Sci. 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 gf an/ fibers— though they are really bundles of fibers 
—which he 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- 


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. Either 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. 



The most striking advance shown by annehds 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 annehds 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 , 

Bye. spots 


Pro ton ephriolium 

Mouth' x.^r.^^x .^--^ ^^^/ 

Protonephrt- >5^ ^^^ -^ / 

^Anus -^^^^ 

A B ^ 

Fig. \b'6.— 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 gangha or 
nerve tracts, shows the development of metamerically arranged ganglia 
connected by a ventral nerve cord. An earthworm is really composed of 




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 
involving the body as a whole. Still another advance is 
seen in the greater degree of specialization exhibited by 
the digestive system, which is here divided into a larger 
number of regions than heretofore, each region having 
a special function to perform. 

283. Classification.— The phylum Annehda (a nel' i 
da; L., anellus, a little ring, and G., eidos, form) is 
divided into four classes: 

1. Archiannelida (ar kl a nel' i da; G., archi-, first, + 
annelida). — Primitive annelids which possess neither 
setae nor parapodia. 

2. Chaetopoda (ke t5p' o da; G., chaite, horse's mane, 
and podos, foot). — Annelids with setae, and in one of 
the two subclasses with parapodia, which are fleshy 
lateral outgrowths of the body wall. 

3. Hirudinea (hi roo din' e a; L., hirudo, leech). — 
AnneUds possessing neither setae nor parapodia but 
having suckers, which are an adaptation to parasitic 

4. Gephyrea (je fi re' a; G., gephyra, bridge). — A 
small and heterogeneous group, apparently more 
appropriately placed in Annelida than in any other 

284. Archiannelida. — A type of this class is Poly- 
gordius, a small worm about an inch and a half long, 
Hving 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 
and respiratory. The other members of this class are 
also small in size, simple in structure, and are marine. 
One type, Dinophilus, has eyespots, moves by means of 
cilia, and has other characters reminiscent of the 

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 Nereis, the sandworm (Fig. 154). 

Fig. 154.— a 
sandworm, Nereis 
virens Sars. From 
a specimen. The 
parapodia bear foli- 
aceous lobes which 
are conspicuous in 
the figure. X %. 



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 feeUng 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 
sexes are separate in Nereis, which 
is generally the case in all the poly- 

Certain polychaets show a tend- 
ency to the division of the body into 
portions which have been likened 
to the thorax and abdomen of arthro- 
pods. Some of the polychaets reach 
a large size, attaining a length of 
three feet. These frequently con- 
struct tubes, various in nature, in 
which they live. The tube may be 
limy, cylindrical, and attached to 
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-hke, 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 


with four eyes 

Setae in 

Fig. 155. — Anterior end of a sandworm 
{Nereis) with prostomium and peristomium 
protruded and extended. {From Wieman, 
"General Zoology," after Ehlers. by the 
courtesy of McGraw-Hill Book ComiMny, 



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 



Fig. 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. 



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). 



Opening of 
vois deferens 

Opening of 




fat diverticulum 
of crop 

lOth diverticulum 
of crop 



■ Rectum 


sticker ' 

Fig. 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. D, 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 


teeth make incisions in the skin thus permitting the blood and lymph 
to flow freely. At the same time sahvary 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 

Leeches are hermaphroditic, but, as in the case of the earthworm, 
cross-fertilization takes place. The sperm cells are collected in bundles 
called sperfnatophores 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 
ovo viviparous. 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. 



289. Behavior. — The behavior of the earthworm has been described. 
That of other types seems to be similar, subject to the possibihties and 
Umitations depending upon different modes of Hving and in some cases 
to hfe in a fixed tube. Many annehds 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 Herouard, 
" Traite de Zoologie Concrete.") X ^2- -S, 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.) X 4. C, a priapulid, Priapuhis 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 
hght. 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 



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 
Pacific 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 
continues for two or three days. 
The worms which lie in burrows at 
the sea bottom have their whole ^2/ ^^^^ coY^esy o/ i/mr^, //o/< * Companj/.) 

Ine budded individuals develop in order, 
posterior portions modified for the the one at the end of the chain being the 

production and retention of the sex °|"^e!*: compare with formation of pro- 

glottids m the tapeworm. 

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. 

Fig. 159. — A chain of individuals 

formed in an annelid, Myrianida sp., by 

budding. {From Hertwig and Kingsley, 

'Manual of Zoology," after Milne-Edxvards, 


The larva of most of the marine anneUds 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 anneUds, 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 


200 ic^ 

Fig. 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. 



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 
cai'apace. 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 maxillae. 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 maxillipeds, 
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 




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 hne, they 



?Si ■ '. -'■'jViiJI 

— Anfennule 


•5^-' -'i^ 


\-5i-alked Eye 


(Wblking Legs) 




Fig. 161 — Camharus 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. obeaua Hagen).] X % 

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. 






Opening of 
green gland 







Telson Anus 

Fig. 162. — Under side of Cambarus virilis Hagen, a common species in the central states. 
From a male specimen from Wisconsin. X %. 


Vas deferens 

Sterna f artery 



Ventre/ / I ^„f ^^J^ 
thoracic artery \ °'^ '^3 

Ventral nerve 

Fig. 163. — Diagrammatic cross section through a crayfish (Cambarus) in the posterior part 
of the cephalothorax, to show the gill chambers and gills. 



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 



\/as deferens "^^^^'^ 
\ Heart 

"Dorsal ab- ~~ 
\ dominal drferv 

gang! I on 



Telson ,, J . - - ,.,„ , 

Ventral / Wernal 

nerve cord / artery 

Opening of 
vas deterens 

Walking' . 

Fig. 164. — Partly diagrammatic longitudinal section of the European crayfish, Astacui 
fluviatilis Fabricius. {From Borradaile and Potts, "The Invertehrata," after Shipley and 
MacBride, by the courtesy of The Macmillan Comjyany.) The indi\-idual is a male and the 
first two swimmerets are modified to form copulatory appendages. 

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 



-a^ecpil gang- 

ageal con- 

~ Esophagus 

-ageaf gang- 

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 
setae which permits the food to pass only when 
it has been ground into very fine particles. 

The circulatory system includes the heart, 
seven arteries leading to various parts of the 
body, and spaces in the tissues called sinuses 
which communicate with a large space around the 
heart known as the pericardial sinus. Valves are 
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 
esophagus, the ducts from which open through 
papillae on the basal segments of the antennae. 

The crayfish possesses a well-developed mus- 
cular system, the muscles being attached to the 
various portions of the exoskeleton. 

The nervous system (Fig. 165) is in many 
respects similar to that of the earthworm, includ- 
ing a supraesophageal ganglion; circumesophageal 
connectives; a subesophageal ganglion, representing 
a fusion of the ganglia of the metameres from III 
to VII; and a ventral gangliated nerve cord with 
ganglia in each segment posterior to the seventh. 
This condensation of metameric ganglia in the 
subesophageal ganglion promotes coordination of 
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- Fig. 165.— The central 
oped in different parts of the body, particularly ^Th" uTacT«)°' Vr7m 
upon such parts as the chelae of the walking legs, Lang, "Text-book of Com- 
the mouth parts, the ventral surface of the abdo- "v^^^'Za YungT'''' "^^"' 
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. 


- Verrtrat 





294. Eyes and Vision. — The crayfish possesses com-pound 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, mtrellae, 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 gangha. The rhabdom, 
a refractive body secreted by the proxi- 
mal retinular cells, lies in the axis of the 
ommatidium at its inner end. The cor- 
neal facet, vitrellae, crystalline cone, and 
rhabdom are all transparent. The re- 
tinular cells cover the surface of the 
ommatidium and contain pigment. In 
bright hght this pigment is distributed 
throughout the length of the omma- 
tidium; in dim light, however, it con- 
tracts toward the respective ends leaving 
much of the surface without this dark covering. 

A compound eye sees just as many Httle 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 fine 
with the axis of the ommatidium. In dim hght 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 hght falhng 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 hmiting the distance to which vision is possible. 
It has, however, the great advantage that it more readily perceives move- 


Opf/c nerve 

166. — Diagrammatic representa- 
tion of the eye of a crayfish. 



ment in the visual field, since any motion almost inevitably results in a 
stimulus being withdrawn from one retinula and appHed to another. 

295. Statocyst. — At the distal end of the basal segment of each 
antennule is a sac, hned 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 

Cornea/ facet 

- Cornea 
'~Corneogen cell- 
-Cap of cone 

-Disfal refinu/ar 
pigment ce// 

■ CrystaJIins cone 

-Proximal portion 
of cone ceJf 

-Proximo) ret I nu Ian 
pigment- cell 



'Optic nerve fibers 

A B 

Fig. 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 fines the statocyst is lost with the rest, 


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. 


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 ^^^th 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. A^atur., 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 mnter; 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 w^alking 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, w'hich correspond to 


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. 


Crustacea (krus ta' she a; L., crusta, a hard shell), to which the 
crayfish belongs, is a class included in the phylum Arthropoda (ar throp' 
o 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 
k6s' 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 

The decapods, which agree in having five pairs of walking legs, include 
crayfishes, lobsters, crabs, and shrimps. The more famihar 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 




of molting after it has shed its old shell and before the new one has 
hardened. Among the various types of crabs is the 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 




Fig. 169. — The blue or edible crab of the Atlantic coast, Callinectes sapidus Rathbun. 
From preserved specimens. A, upper surface. X 3^. B, under surface of female to 
show breadth of abdominal metameres between which and the thorax the eggs are carried, 
attached to the swimmerets. X M- C, under surface of body of male to show the 
narrowness of abdominal metameres. X M- 

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 isopods 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. 



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 
the gills; the last three pairs are directed posteriorly and are the jumping 

Fig. 170. Fig. 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.) X 3. 

Fig. 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,.) X 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 (en to mos' 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 dos' er a; G., klados, sprout, and keras, horn), also known as water 
fleas; and Copepoda (ko pep' o da; G., kope, oar, and podos, 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. 



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 

Fig. 172. — Three types of entomostracans. A, a copepod, Cyclops sp. (From Bronn, 
" Klassen und Ordnungen des Tierreichs," after Claus.) Female with eggs. 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- 

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 pe' 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 



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 

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- 

A B 

Fig. 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 -wTth the shell closed. (From Bronn, "Klassenund 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 i'n 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 

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. 


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 109,000,000 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. S. 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 hfe 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 appHcation 
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- 



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. 

Fig. 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-hook of Comparative Anatomy," after Fritz Milller.) 

Fig. 175. 

-An adult shrimp, Penaeus semisulcatus. X M- 
Zoology," after de Haan.) 

{From. Huxley, "The Study of 

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 


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. 



Onychophora (onikSf ora; 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 & ComjMny.) 

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 a marked 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 

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 




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 


308. Myriapoda. — Myriapoda (mir i ap' oda; 
G., myrios, ten thousand, and podos, foot) is a 
third class distinguished particularly by four 
characteristics: (1) The metameres are many, 
all metameres back of the head are alike in 
appearance, and there are numerous pairs of 
similar running legs; (2) the head has a pair of 
antennae, a pair of mandibles, and one or two 
pairs of maxillae; (3) the animal breathes by 
tracheae opening to the outside by metamerically 
arranged pores; and (4) the excretory organs are 
malpighian tubules, like those present in insects, 
opening into the posterior end of the intestine. 
The body of all myriapods is elongated, some- 
times nearly cylindrical and at other times 
dorsoventrally flattened, rarely being com- 
pressed from side to side. Myriapods are widely 
distributed and flourish under a variety of condi- 
tions. The class is divided into several orders, 
two of which are larger and better known than 
the rest, one including the centipedes and the 
other the millipedes. 

309. Centipedes. — The centipedes, or hun- 
dred-legged worms (Fig. 177), have a body 
which is, generally speaking, flattened dorsoven- 
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 

Fig. 177. — A Tropical 
American centipede, Scolo- 
pendra sp. From a dried 
specimen. X %. 



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. 

Fig. 178. — The long-legged house centipede, 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 

Fig. 179. — A millipede, Spirobolus sp., from South Carolina. 

Natural size. 

From a preserved specimen. 

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. 



The fourth class of Arthropoda is Insecta (in sek' 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 


Prof h or ax 

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 




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 



La brum 



nr e: Y m 

Femur Spiracles 



■ Tarsus 

Fig. 181. — Diagram of a locust with parts of the body separated to show metameres and 
other structures. Abdominal metameres numbered in reman numerals. {Modified from 
several previous authors.) 

are tactile, olfactory, or auditory in function. 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 
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 labium. 
Between these and meeting in the 
median line are two strong jaws, 
or mandibles, and behind the 
mandibles is a pair of maxillae. 
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 
the escape of the food from 


Compound eyS 






Labial palpus 

Fig. 182. — Front view of the head of 
a lubber grasshopper, Brachystola magna 
between them. The maxillae help Girard, from Nebraska. X 3>s- Illustrates 
, r J xu r J • i- j-u biting mouth parts. 

to feed the food into the man- 
dibles, while the palpi are sensory, sending impulses into the nerv- 
ous system which determine the activity Oif the other mouth 



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 


eyes - 


palpus ' 




Labi urn- 

p^ 3? Souton 

Fig. 183. — Suctorial mouth parts, ^.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 hke the locusts and 



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 

Claw - ^PoM^'os 

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, 
Corixa 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. X M. D, part of under surface of a 
ground beetle, Calosoma scrutator Fabricius, to show hind leg fitted for running. X l\i. 
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 \Aang and 
divide its surface into areas called cells. While wings are growing and, in 


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 
(Fig. 185). 
Fig. 185. — A crane fly, showing the Insects generally possess tracheae, 

modified hind wings or halteres. Male, u xi,- ± -u u- i. j. 

adult. (From Sanderson, -Insect Pests," ^^ brcathmg tubcS, whlch OpCU to 
after Weed, by permission.) About natu- the OUtside by laterally placed 

"^^' 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 threadUke 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 



of apparatus used in copulation, egg laying, or stinging. This results in a 
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 

Base of 


Supra esophageal 
ganglia or brain 

Compound eye 



Honey sac 



nerve cord 

'Rectaf g/ancts 
Hind gut 

Fig. 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 j&ner branches reach all parts of the body. In insects of active 
flight the tracheae are dilated in certain places and form air 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 






Pro venfri cuius 


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 tuhules. 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). 

The nervous system of insects is 
similar in general plan to that of the 
earthworm, but there are two ventral 
nerve cords (Fig. 186). The two gan- 
glia of each pair are usually fused and 
communicate by commissures. In 
the lower insects there is a pair of 
ganglia to each segment, but in 
the higher forms the number is 
reduced. In the latter forms the 
thoracic ganglia are increased in size 
and the supraesophageal and sub- 
esophageal ganglia are not only 
increased in size but tend to be 
brought together by the shortening of 
the circumesophageal connectives. 
This results in such a degree of 
centrahzation and cephalization that 
these anterior ganglia may properly 
be called a hrain (Figs. 186 and 188). 
Their removal interferes with the 
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 ocelh 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 


Fig. 187. — Digestive system of a 
beetle, Carahus aurahis Linnaeus, as an 
example of a mandibulate insect. 
{From Lang, "Text-book of Comparative 
Anatomy," after Dufour.) 



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 wdngs. The auditory organ of a locust is a pit, or 
tymyanum, on the first abdominal metamere, closed by a thin kidney- 
shaped tympanic membrane (Fig. 181). Organs believed to be auditory 

•<. II 

A B c 

Fig. 188. — Nervous systems of three insects to illustrate condensation and cephaliza- 
tion. A, a termite, one of the lowest insects. B, a water beetle. C, a fly, the highest type. 
{From VanCleave, "Invertebrate Zoology," A after Lespes, B and C after Blanchard, by the 
courtesy of McGraw-Hill Book Company, hic.) 

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 fight, fike 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 
fertiUzed 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 (am e tab' o 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. 



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 
(hem i me tab' o la; G., hemi, half, and 
metabole, change) or Heterometabola 
(het er o me tab' o 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, Jiaiads. 
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. 

A third group of insects, according to 
development, is Holometabola (ho lo me- 
tab' o la; G., holos, whole, and metabole, 
change). These pass through a complete 
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 

Fig. 189. — Young and adult of 
Campodea staphylinus Westwood. 
The simplest living insect, and an 
example of Ametabola. (From 
Kellopg, "American Insects," by the 
courtesy of Henry Holt & Company.) 
X 11. 



Fig. 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; E. 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 & 



Some insects exhibit in their Hfe 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. 

Fig. 191. — Life history of a holometabolous insect, a butterfly, Danaus archippus 
Fabricius. a, egg. b, larva, c, pupa, d, adult. Natural size, (a to c 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- 


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 

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 heahng 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 



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 

Being vegetable feeders and likely to attack growing crops, locusts 
have affected man since the very beginning of agriculture. Migrations 

Fro. 192. — A termite, Reticulotermes flavipes Kollar. A, winged male. B, larva. 
C, dealated queen. D, soldier. E, 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 (or thop' ter 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 Hce. 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 
Hce 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. 



The term bug in its proper sense is applied only to the Hemiptera 
(he mip' ter a; G., herni, 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 

Honey tube 


Fig. 193.— T-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. B, 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 

The order Homoptera (ho mop' ter a; G., homopteros, 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- 



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 

Fig. 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 (kol e op' ter a; G., koleopteros, sheath- winged), 
which includes the beetles, contains many destructive forms. Among 
these are borers in the trunks and hmbs of trees (Fig. 194) and stems of 
other plants; and the leaf beetles, which destroy fohage. The potato 
beetle is a leaf beetle. Weevils attack seeds and fruits, carpet beetles 
injure rugs and carpets, and wire worms damage the roots of plants. 
Both the larvae and imagos of beetles cause damage. 

The order Lepidoptera (lep i dop' ter a; 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 web worm, 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 



Fig. 195. — Codling moth, Carpocapsa pomonella Linnaeus. {From Farmers' Bull. 171, 
by Simpson, and 283, hy 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 l}4; C, X H- 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 cre\-ice, 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. 

Fig. 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, Eurosta 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.) 



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 M«lc<rial or Anopheles Yellow Fever or Aedes 


Fig. 197. — Comparison of three important kinds of mosquitoes, Culex, Anopheles 
(Sec. 115), and Aedes, showing eggs, larvae, pupae, and adults. X 2 or 3. (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 men op' ter 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 


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' ter 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 ^^^ igg.— House fly, Musca domestica 

them for a time, but its exhaustion Linnaeus. A, larva. B, pupa. C, adult. 

1 .u 4. • 4. +U^ +^v. X 4. {From Howard, Farmers' Bull. 459, 

makes another trip to the top Q ^ ^^^^ ^^^^ 
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 ahghting 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. 



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 

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, a twig of range, showing the scale lice, and larvae and adults 
of the beetle. A to 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 (o 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 



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 (nu rop' ter a; G., neuron, nerve, and pteron, 
wing) includes the lace-winged flies, sometimes called aphis hons because 
the larvae, as well as the adults, feed on plant lice. Their eggs are laid 


Fig. 200.— r>Ant lion, Myrmeleon sp. 

A, larva, X 2. B, pit, 


with concealed larva. 


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-hon 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. 



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 1}^ 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 
which has been most intimately 
associated with mankind and has 
reached the highest degree of domes- 
tication. For ages before man learned 
how to manufacture sugar he depended 
for his sv^^eets very largely upon the 
bees, and honey was an important 
element in human diet. 
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 fives 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 aU the labor of the society. They five 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 

Fig. 201. — An ichneumon fly, 
Megarhyssa lunator (Fabricius), de- 
positing its eggs in the tunnels of 
Tremex columba (Linnaeus), on the 
larva of which its larva feeds. Natural 
size. (From Graham, "Principles of 
Forest Entomology,'' by the courtesy of 
McGraw-Hill Book Com,pany, Inc.) 
The figure shows the characteristic 
attitude of the insect when drilling. 



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. 

Fig. 202. — The honeybee. Apis mellifica Linnaeus. A, worker. B, queen. C, drone 
D, portion of comb showing queen cells. E,egg. F, young larva. G, old larva. //, pupa. 
A to C, somewhat enlarged; D, natural size; E to H, much enlarged. {From Phillips, 
Farmers' Bull. 447, U. S. Dept. Agr.) In Z) 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 


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 


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 



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 

Fig. 203. — Polymorphism as illustrated in an ant, Pheidole instahilis Emery, a, 

soldier, h to e, intermediate types between soldier and worker. /, typical worker, g, 

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 



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 
haiiy, 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, 

Fig. 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 


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, inchiding 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 



The spiders and allied forms make up the fifth class of Arthropoda, 
known as Arachnida (a rak' ni da; G., arachne, spider, and eidos, 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 fv reproc^uctive organs 


Fourth leg 


if Spinnerets 


Fig. 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 chehcerae, 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 pedipalpi 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). 




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 spinnerets (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 m.y ^ „jw/j,, t."'Vl^c/?e//'cera 
front of them may be a single spiracle. 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 ^ „„^ ^ 

•^ ^ ,11 Fig- 206. — Front view of 

each side, a slit placed transversely, which is head of Lycosa caroiinensis 
the opening into an air sac. The spiracle Waickenaer. A large female 

from Lincoln, Nebraska. X 4. 

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 M/y/r,ir,h;^,n 

complexly branched fu^'S'^'s''" 

diverticula /Heart '^'^'^'"^^ 

Poison gland 


c'ZTA^ ''°''rA rr^cJ/sfn^s,/ Anus 

Peapclpas stumps, f legs , . / Opehingof Sp,n„erefs 

'^ -^ Seminal oviduct 


Fig. 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, 


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 hook 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 

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 

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 



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 beadUke 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. 

Fig. 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 



have many activities which are as justly considered inteUigent as are 
those of many insects. 

329. Economic Importance. — Spiders are of httle 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 countiy 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 




Fig. 209. 



-A Tropical American scorpion, Centntrus 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 Uke 



the chelipeds of the decapod Crustacea. There are four pairs of hing 
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 


Fig. 210. — Mites. A, an itch mite, Sarcoptes scabiei DeGeer. Male from above. 
Greatly enlarged. B, Texas cattle iever tic]i., Margaropus annulatits (Say). Male. X 16. 
C, chigger, Trombicula irritans (Riley). Larva. X 75. 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 


\\athout 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 is said at one time to have 

Fig. 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 folhcles 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 hke 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 



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, 


Fig. 212. — King crab, Limulus polyphemus (Linnaeus). 


A, dorsal view. B, 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. 



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 

Antenna ^Supraesophageal 

Bloodvessel Alimenfviry canal 



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: 



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 subcyUndrical (millipedes) ; possess tracheae, Malpighian 
tubules, and one pair of antennae. 

4. Insecia. 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 cephalo thorax 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 inteUigence. 
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 intelhgence is attributed to the two groups men- 
tioned, it should also be recognized in them. 


The last of the phyla, and from the standpoint of efficiency the 
highest, is Chordata (kor da' ta; G., chords, 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 discs 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 







vessels making it possible for respiration to take place. In the higher 
vertebrates, however, these sUts, 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 sHts. 
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, 
called the medullary tube (Fig. 214). 
This development of a central nerv- 
ous system by invagination is pecul- 
iar to the chordates. Typically 
this tube, when first formed in the splanch. 
embryo, opens at the anterior end mesoderm 
to the outside and at the posterior 
end turns ventrally around the end 
of the notochord to become con- 
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. 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 

from which 
will develop 
the body 



Fig. 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. 


pharyngeal slits offer a mode of respiration more effective than that 
allowed by any type of respiratory organ possessed by lower forms 
because the shts 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 (hem i kor da' ta; G., hemi, half, and chorde, cord), 
or Adelochorda (ad e lo kor' da; G., adelos, concealed, and chorde, cord). 
Includes tw^o or three types of marine wormlike animals, which are 
all small, some very minute. 

2. Urochordata (u ro 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 (sef a lo kor da' ta; G., kephale, head, and chorde, 
cord). — Includes a marine type known as the amphioxus which has a 
somewhat fishlike form. 

4. Vertehrata (ver te bra' ta; L., vertebra, a joint). — Includes fish, 
amphibia, reptiles, birds, and mammals. 



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 




Fig. 215. — Dolichoglossus kowalevskii (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 




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 

^Apical sense organ 
Ciliated band 



Pore entering 
proboscis cavity 

^Anal ciliated 

Oral funnel 



Fig. 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 
possess a large number of pharyngeal 
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 
composed of animal cellulose the com- 
position of which is very similar to that 
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- 

FiQ. 217. — Pyura aurantium 
(Pallas), a sessile ascidian from 
Labrador. X %. {From VanName, 
Proc. Boston Soc. Nat. Hist., vol. 34.) 



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 sHts, respiration occurs and food is strained out. On 
the side of the pharynx which corresponds to the ventral surface is a 
cihated groove called the endostyle. A sticky mucous secretion produced 

/ — Ora/ /uy!ne/ 

Dc/Cf o-f hypophys/'s 



kJ)'^^^^\\ z'-^^'^''^/ cav/fy 

Afr/cf/ fuiotiel — - 



WiM"^ IWV^ — Mcrnf/e 

n .j%i t^~ 1 

■ m: 


MnUS f 




?z -• '^»f h\i\ Itm liwJ— --i 

( ! -x 1 -A 


^^^^ JIl'JIll ^/^~ 'r'i 


A'f'y^/ty/ /'^uv/i/ \k V )^ 

X,; ' -'Y V -" ■ ^S'^^ ^ \ a ' '" "^'^ -- .Xi • *-• vo 

r^/r/i^/ ^ *^ y * ' / w 


£sopha^us ■ 


Fig. 218. — Anatomy of a typical ascidian. {From Newman, " Vertebrate Zoology," after 


in this endostyle is continually being passed onward to the intestine and 
serves to convey food particles to that portion of the ahmentary 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 ganghon 
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-fertiUzation is the 
rule, fertilization taking place outside the body. From the egg is pro- 


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 

/Verv^ core/ 


Stomach „^^„^ . 

nearr per/ branchial 


Fig. 219. — Section through the anterior part of the body of a tunicate larva, after 

fixation. From the right side. (From Delage and Herouard, " Traite de Zoologie Concrke," 

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. 



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 

F/n and fin-rays 
Velum X =-/» 

^L^^43UJ'LVM^a^Ul^t^^^;x.;^;r^jJOl^^^V^^ . --_/ 

Pharnyngeal / Gonad 

Slifs / A 


Metapleural folds 


Anal _ 


Fig. 220. — ^An amphioxus. ^, seen from the side; B, from below. Some details of internal 
anatomy indicated by dotted lines. From a specimen. X 2^^. 

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 

■Dorsal fin 








Dorsal nerve 
Nerve cord 





Fig. 221. — Cross section through the gill region of amphioxus. {From Hertwig and Kings- 
ley, "Manual of Zoology,'' after Lankester and Boveri.) 

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 



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. 

of r;er\/e cor-c^ 

Dorscr/ f/^ ra/s 

A/e^^e cord 



Oral hooc/ 
Oral r/r^^ 

Oral fen-f-ac/es 

^ I Afn'um 

Fig. 222. — Median longitudinal section of an amphioxua. (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 


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 systern resembling that of some of the annelid 

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 w^ater, 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. 


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 e7ido skeleton. 

(3) As a rule two pairs of ap-pendages, 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 brazji 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 tail. 

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. 




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 

Brain \ Neurali-ube 








Heart Stomach 



Fig. 223. — Diagrammatic longitudinal section of the female of a lower vertebrate. 

drawn, with modifications, from Hegner, "College Zoology," after Wiedersheim.) 


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 

arch , 




Centrum y' 
of vertebra//^ 


^ Spinal cord 

orsal aortcf -^Ji ((O- 
Ca retinal — / 1 f/~j^ 


A V\ winter muscular 
<le^A — 'Mesonephros 

V;: ^ 


Iv 7 

Mesonephric \^ 
duct /V^ 


\Mj Pronephric duct 

Lateral vein /^ 







Fig. 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. 



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 epitheUum; 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 

//a/r Hair 



' rierye I Sweat duct 


Dermis ■' 

'^•1 ' corpuscle 

muscle ofrne 




Blood vessel Nsrve 

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 
glands, 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 exo- 
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 axial and appendicular portions (Fig. 226). The former includes 
the skull, the vertebral column, the ribs, and in the higher forms a 



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. 


















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. 


Bone tissue consists of an intercellular matrix of salts of lime which 
masks the cellular elements. Under the microscope, however, the cells, 
called hone 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 sUts 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 lumbar 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- 



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 




cavity Pelvis 




Coracoid /7y 

Radius — 7///^ 

'4^ / 








Metatarsals' , 
Metacarpals \ foS\ 

^ ^'Phalanges -^^==^^^0 


^■—^^ Phalanges 

Pig. 227. — Diagram to illustrate the homology between the skeletons of the fore and hind 


divided into an ahmentary 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 huccal 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. 



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. 




— Pharynx 











Ileum I ^^Recfum 

Fig. 228. — Alimentary canal of man. {From Sobotta, " Deskriptive Anatomie,' 

courtesy of J. F. Lehmann' s Verlag.) 

by the 

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 


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 Hquid. Thus as the food is gradu- 
ally reduced to Uquid 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 Hning 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 

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- 


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 lymphatics. 
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. 



The kidneys serve for the eUmination of Uquid waste which contains the 
end products of metaboUsm. The ducts connected directly to the 
kidneys are known as ureters; they meet to form the urethra. Where 














o « 




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 



(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 


of myo- 













Fig. 230. — Diagrams to show relations of kidney tubules. A, pronephros; 5, 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 



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 ceyitral 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 


Spina/ Sensory 

cord nerve Dorsal root 


(sense organ) 


Ventral root 

Gray matter 

Motor neri^e 

Fig. 231. — Diagrammatic cross section of spinal cord and diagram of a spinal nerve, 
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 


the body, where they control the involuntary muscles in the walls of 
these vessels. The largest ganglion in this system, the semilunar 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 tw^o. 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 
dienceyhalon. From the mid-brain, or mesence-phalon, 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 he 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 



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 

nafe bone 






Opening of 



Palate Nasal 


Supporting cell 


and lower 
turbinates A 

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. B, 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 epitheUal 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 
some aquatic vertebrates, have for a long time 
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 
in the water which are important in the securing 
of food, the avoidance of enemies, and adjustment 
to currents. 

The most important organs of smell (Fig. 
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 
OD its surface. The cells are stimulated chemically, and the result is the 

Surface of 



Fig. 233. — -Human taste 
bud, somewhat diagram- 
matic. Highly magnified. 



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 huds (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. 



Stapes closing 
fenestra o^al/s 

Incus \ Semicircular 

''>^l \ Utriculusyfcarna/s 

'■ "t— ^-._ ^/. Auditory 

Ampullae \nerve 




«' Tympanic 

closing fensi ^ 
f\a rotunda 


Eustachian tube 

Fig. 234. — Diagrammatic section of the human ear. 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 memhranous 
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 


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 equihbrium. 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 


Vestibular J^^^^^ "'^m ..,^^^^^^^°"^ 

Oangilor, ^^^l^~-^cociilear nerve 

Fig. 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 
Bcala 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 eusta- 
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- 


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 iris, 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 



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 ciliary 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 



■Sclerotic layer 

Choroid layer 
Retinal layer 





Somewhat diagrammatic section of the human eye. 

lens becomes less 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 


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 klo sto' ma ta; G., kyklos, circle, and stomatos, 
mouth). — Roundmouths, including hagfishes and lampreys. 

2. Elasmohranchii (e las mo bran' ki i; G., elasmos, metal plate, 
and L., hranchia, gill). — Cartilaginous fishes, including sharks, skates, 
and rays. 

3. Pisces (pis'ses; L., pisces, fishes). — Bony fishes, including all 
ganoids, teleosts, and lungfishes. 

4. Amphibia (am fib' i a; G., amphibios, leading a double life). — 
Salamanders, frogs, and toads. 

5. Reptilia (rep til' i a; L., reptilis, creeping). — Lizards, snakes, 
turtles, and crocodiles. 

6. Aves (a' vez; L., aves, birds).- — Birds. 

7. Mammalia (ma ma' h a; L., mammalis, of the breast). — Mammals. 



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 o sto' ma ta; G., gnathos, jaw, and 
stomatos, mouth), which includes all the other classes. The lack of jaws 


Fig. 237. — Cyclostomes. A, California hagfish, Polistotrema stouti (Lockington) , with 
12 gill slits. X 3'3- {Redrawn from Chidester, "Zoology," after Dean.) B, sea lamprey, 
Petromyzon marinus Linnaeus, with seven gill slits. X 3^. 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' de a; G., myxinos, 
sHme fish, and eidos, form), includes the hagfishes; the second, Petro- 
myzontia (pet ro mi zon' ti a; 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. 237^), 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 fill a bucket. Hagfishes do not lead a strictly parasitic 




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. 2375) 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 aorfa 

Nasal opening 
Olfactory sac 

Dorsal cartilages 

■Annular cartilage 

cavity with 
chitinous teeth 


Internal \ 

openings of Esophagus 
gill slits 

Fig. 238. — Median longitudinal section of the anterior part of the body of an adult sea 

lamprey. From a specimen. X /'4. 



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 



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 


A B 

Fig. 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. 



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 pecuhar type of scale known 

Fig. 240. — Dogfish shark, Squalus acanthias Linnaeus. X Ho- {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 sHts 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 Elasmobranchii. 

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. 




The body of the dogfish is covered with placoid scales (Fig. 250), 
each of which is composed of a sharp toothUke 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 

Fig. 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 CoTnpany, 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 


Precai^al vein ■ 
Sinus venosus^ 




, Dorsal 
V aorta 

- Cardinal i/ein 
Coeli'ac artery 


vein ~ 





First gill slit 

branchial artery 

Venfrcfl aorta 



la'f'eral vein 
Hepatic portal vein ' 



Subclavian artery 

Fig. 242. — ^Diagrammatic lateral view of the circulatory system of a dogfish shark. 
Vessels carrjdng oxygenated, or arterial, blood in black, those carrj-ing 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. 



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 


Fig. 243. — Brain of a European dogfish shark, Scyllium 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-yortal 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- 


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 sac. 

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. AVhale 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 M'ith 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 



and also more primitive than any other known fishes, either Uving 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 


' • . KlSyi'*, 


Fig. 244. — Common skate, Raja erinacea Mitchill. X 

{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 shts 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 


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. 


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 

369. Classification. — The class Pisces may be divided into two 
subclasses— Teleostomi (tel e 6s' to ml; G., teleos, complete, and stoma, 
mouth), or bony fishes proper, and Dipnoi (dip' no i; G., dipnoos, with 
two breathing apertures), or lungfishes. There are four divisions of 
Teleostomi: (1) Crossopterygii (kro s6p ter ij' i i; G., krossoi, fringe, 
and pterygion, fin), or lobe-finned ganoids; (2) Chondrostei (kon dros' te i; 
G., chondros, cartilage, and osteon, bone), or cartilaginous ganoids; 
(3) Holostei (holos'tei; G., holos, whole, and osteon, bone), or bony 
ganoids; and (4) Teleostei (tel e os' te 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 Polijpterus (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. 




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- 

FiG. 245. — A crossopterygian. Polypterus senegalus Cuvier. o, the adult. X H- 
h, the larva. X 2%. {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 rMWcMW^MSiLeSueur. X Hs- {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 

Fig. 247. — Alligator gar, Lepisosteus tristoechus (Block and Schneider). X J^eO- (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 



ganoid or an amphibian than Hke 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 



^Anal Caudal 
fin fin 

\\\©s"^V ^i-'eivic 


Fig. 248. — Common perch, Ferca flavesceris (Mitchill). X P3. 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. 



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 

Fig. 249. — African lungfish, Protopterus annectens Owen. X M- {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. 

C D 

Fig. 250. — Scales of fishes. A, cycloid scale of northern pike, Esox Indus 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, and d, X 60. D, ganoid scales of Lepisoateus 
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, 


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 ganoid 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. In some fishes the scales 
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 



coelom and which is a hydrostatic organ. It arises as an outgrowth 
from the anterior portion of the aUmentary 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 

Fig. 251. — Diagrams to illustrate various types of caudal fins in fishes. A, diphycercal 
type (dipnoan, Protopterus). B, heteroeercal 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 
heteroeercal 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 heteroeercal 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 


bottom, and it is, therefore, possovssed 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 

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 chromatoTphores. 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, 



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 , poirf 
between removed 

Pineal body 

Optic lobe 


Spinal cord 

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 



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 



Oral va/ve 

Mouth opening 

Upper valve 

Lower valve 

Fig. 253. — Diagrams to illustrate the mode oi 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 

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 hving 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. 


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 hranchiostegal 
meynhrane. 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 hranchiostegal 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 hranchiostegal 
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 hranchiostegal 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- 


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 equihbrium. 

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 

J'' Sii; 

Fig. 254. — Anableps dovii Gill, a Tropical American fresh-water surface fish, the eyea 
of which are divided by a partition into two parts, one for seeing outside of the water, the 
other in it. X M- (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 



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 milt, which 
contains the sperm cells, over the eggs, called roe, at the time of laying; 

anc/ nac/eus 




f^eural groove 



Head ^ ^'^''^ ^'^^ 

Fig. 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. E, 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. H, 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. 


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 fertihzation 
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 developed 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, five only 
a year. Such short-lived fish have a definite size which they may attain. 
Others, Uke 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 



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. 

Fig. 256. — Deep-sea fishes. A, Photostomias guernei Collett; length Ij^^ 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; 
2j^ 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. 


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 }'2 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. — Fish 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. 



The fourth class of the vertebrates, Amphibia, exhibits a transition 
from hfe 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 

Ducf Pigment cells 


gland (b) 

Dermis - 

gland (a) 

\^ gland (c) 

Pigment cell 

Fig. 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 h 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. 




A moist skin may, under certain conditions and in certain types of 
amphibians, serve also for respiration. Generally speaking, how- 




to fiead 




Dorsal aorta Carotid artery artery 

Capillary q 
System " 


Left aortic 



lan ^ 

to fore 



Pulmonary arte, 
Righ f aortic arch 



ian , \ 
to "^i 
^ I fore leg \ 



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, E being again 
a lateral view; F, a reptile (lizard) ; G, a bird; H, a mammal. 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- 

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 



change. Gills are still possessed by the larvae but these are lost during 

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 


Bony rays 

Horny rays 

Me sop f^rygiunfy-^^ 

Me ftrp ferygium 


^ Three parts 

of pterygium 


Pectoral arch 


Radius Metapferygium 

Fig. 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 Wirheltiere" ; 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 w^hich 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 


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 


the same Devonian period have been discovered in Greenland. Previous 
to these no remains were known earUer 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. 



The name Amphibia indicates that these animals Uve at different 
times in their Ufe 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 

394. Classification. — The class Amphibia is usually divided into 
three orders: 

1. Urodela (u ro de' la; G., oura, tail, and delos, visible). — Amphibians 
with tails, including salamanders and newts. 

2. Salientia (sa II en' shI a ; L., salientis, leaping). — Tailless amphib- 
ians, including frogs and toads. 

3. Apoda (ap' o 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 perennihranchs; those which lose their gills 
upon becoming adult and assume a terrestrial mode of life are known 
as caducibranchs. 

Among the perennihranchs 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. 260.4). As in all of the perennihranchs 




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, Cryptohranchus, 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 }-i- B, axolotl larva of Ambystoma tigrinum (Green). X %■ C, tiger salamander, 
Ambystoma tigrinum (Green). The axolotl and the salamander from Nebraska. X % 
A and B from preserved specimens, C from a li\ang 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 



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 o\aduct is protruded through the cloaca and passed forward 
between the back of the female and the abdomen of the male. As the 

Fig. 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 
4 to 5 years to grow to mature size. Warts on man cannot be contracted by handling toads. 
(Photographed and contributed by George E. Hudsoyi.) 

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. 



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, \\dth 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 

Fig. 262. — Common eastern tree 
toad, Hyla versicolor heConte. Male, 
from Staten Island, New York, 
X %. (Redrawn from Dicker son. 
"Frog Book:') 

Fig. 263. — .\siatic cecilian, Ich- 
thyophis sp., with eggs. (Modified 
from Thomson, " OutliTies of Zoology," 
after P. and F. Sarasin.) 

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 



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 
greatly enlarged feet, 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 Hve in a 
moist locality. 

397, Apoda, — The Apoda, or ceciHans, some- 
times called blmdworms, 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-hke organ 
lying in a groove between the eyes and nose by 
and is inverted in the means of which it feels its Way about. In one 
action; the insect adheres type found in southern and southeastern Asia the 
wMr'^^LtTeat^Tci female lays her eggs in masses in a shallow hole 
is drawn back into the near the water and coils herself about them 

(Fig. 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 hfe. Some types of Apoda are viviparous. The 
whole group is to be looked upon as the result of a very pronounced 

398. Food. — The food of frogs and toads is composed of any Hving 
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 

Fig. 264. — Showing the 
manner in which a toad 
takes an insect. The 
tongue is extruded in an ex- 
ceedingly rapid movement 

mouth. (Modified from 
Dickerson, "Frog Book.") 



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 moUusks 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 chromatopliores, 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. 


Fig. 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 

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 vascvilar pia mater. The cerebrospinal 
system includes 10 pairs of cranial nerves and also 10 pairs of spinal 

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 



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 hues 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 

^t. ;■■ '•■ ' %<^ Cerebrum 






Fig. 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 


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 
intelHgence 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 stimuli. 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 

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 intelhgence 
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 

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 



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 




G F 

Pharyngeal s/ifs 


, Neural lube 


in vagi not/ on 
I J Yolk K 

Fig. 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 E, section of it. F, beginning gas- 
trulation by epibole. and G, the process more advanced. E, stage showing the yolk plug, 
and /, somewhat later. /, 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 obhterated 
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 swelHng 
appears on each side near the anterior end of the body. Below each 
swelhng 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- 



tion called the stomodeum 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 external gills are formed which project 
outward from the branchial arches. At the same time muscle segments 

Oral sucker 

Fig. 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. E, 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. 

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 ciUa 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 


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 aUmentary 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 axolotls (Fig. 2Q0B). 
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. 


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 hfe. 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 sUghtly 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 estivation. 

406. Economic Importance. — Amphibians are almost mthout 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 


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 
Hne. (3) Respiration is carried on throughout life by lungs, and though 
branchial arches appear early in embryonic hfe, their development ceases 
before gill shts 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 



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. 

^memb%ne - Germinal area 



'Egg shell 

^Egg membrane 

White Yello w yolk 


Fig. 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 E). 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 



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 corcf 

Nerve cord 

No to chord 


o . o o y ; 

.0.0 o o 
'o ■ O o o . o 

o ■ 0.0.0 


A mniofic 

A mniofic 



coelom Amniotic 



O o' *> 

^' o. o o o 

■ 00" 


Fig. 270. — ^Diagrams of the development of a bird's egg. A, cross section of an 
amphibian embryo for comparison with 5, 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 £ 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 allantois, 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 E). The somatopleure and allan- 
tois together form what has often been termed a chorion,, which is spread 



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. 

/ anc^ blood 

Yolk circulcff-ion 

Fig. 271. — Diagram of a stage in the development of a bird's egg, later than showTi 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. 



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 








subcla vian 



Left aortic 











Fig. 272. — Reptilian hearts. A, heart and associated blood vessels of snapping turtle, 
viewed from in front. From a specimen, but somewhat diagrammatic. B, 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 m 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 (skwa ma' ta; L., squamatus, scaly). — Chameleons, 

lizards, and snakes. 




2. Rhynchocephalia (rin ko se fa' li a; G., rhynchos, snout, and 
kephale, head). — One living type, a lizard-like animal found only in New 

0/fac/vry lobes 

Optic lobe 



Fig. 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 (krok o dil' i a ; G., krokodeilos, crocodile). — Crocodiles 
and alligators. 


4. Testudinata (tes tu 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. 272 A), 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), w^here 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 

Fig. 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. 



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 Hzards 
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 

Fig. 275. 

-Common chameleon of southern Europe, Chamaeleo vulgaris Daudin. 

upon figure in Brehm, " Thierleben.") 

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 



independently and it is stated that the pecuUar structure of the lids 
makes it possible for the animal to locate objects very exactly. The 


Ear opening 

Fig. 276. — Heads of a bull snake, Pituophis sayi (Schlegel), and of a lizard, Plestiodon 
septentrionalis Baird. A, head of the snake, from the side. B, from below. C, head of the 
lizard, from the side. D, from below. A and B, X % ; 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 

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. 
This is true of the so-called glass 
snakes of Europe and America, which 
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 

Fig. 277. — Wall gecko, Tarentola 
mauritanicus (Linnaeus), of southern 
Europe. {From Brehm, " Thierleben.") 




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 Uzards 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 

Fig. 278. — Common horned lizard, Phrynosoma douglassii 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 



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 

Fig. 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- 

FiG. 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 Pickwdl.) 

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. 



418. Snakes. — Snakes differ from lizards and chameleons in the 
structure of the lower jaws; in the absence of both free Hmbs and 



Fig. 281. — Skeleton of a bull snake, Pituophis 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 
objects actually much greater in diameter 
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 
and that side of the mouth is carried forward 
over the object being swallowed, after which 
those teeth are again set in. Then the 
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 

Fig. 282. — A constricting snake, 
Boa sp., clinging to a post and 
maintaining its position for more 
than an hour with about a third of 
its body extended. 



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 

Fig. 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 



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 eyehds 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 bhnd. 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 

Fig. 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 vi\aparous. The 
idea is prevalent that snakes can swallow their young when danger 
threatens but this is not supported by scientific observation. 



Snakes are more abundant in the tropics than elsewhere and are 
frequently absent from islands, though found on the adjacent continents. 
Some snakes Hve 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 

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 



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. 280. — A copperhead, Agkistrodon 77iokasc>i Beauvois, ranging from Massachusetts 
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. 

fc . - ■ ■ - ■-. - - -'• 

Fig. 287. — Coral snake; harlequin snake, Micrurus fuhius 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. 



The pit vipers (Fig. 288) have long, curved fangs near the anterior 
end of the upper jaw. When the mouth is closed they he 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 



Fig. 288. — Skull of rattlesnake, Crotalus confluentes Say. From a young specimen from 
Montana. A, entire skull, seen from the side and below. B, 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 

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 


small neighboring islands and is threatened with extinction. It is 
lizard-like (Fig. 289), about two feet in length, hves 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 Hzards, and is often called the pineal gland or pineal body. It is 

Fig. 289. — Tuatara, Sphenodon punctatum (Gray), of New Zealand, an example of the 
Rhynchocephalia. X about Je- 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 
Hzards have sprung but rather a highly modified type which has persisted 
from earlier times. 

Fig. 290. — North American alligator, Alligator mississippiensis (Daudin), an example of 
the Crocodilia. X about He- From several sources. 

421. Crocodilia. — This order contains some of the largest hving 
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- 
codiHa are hzard-hke 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 



project from the top of the head, the animal can He 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 






Fig. 291. — Skeleton of a European turtle, Cistudo lutaria (Marsili). From a mounted 
preparation. A, the carapace with the internal skeleton. B, the plastron, removed. 
The limb skeletons are inside the ribs. X /^. 

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 


of a dorsal carapace and a ventral plastron. Into this shell may be 
drawn the head and neck, Hmbs, 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. 




Fig. 292. — Common snapping turtle, Chchjdra 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. {Photograj)hed 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 


have been noted as being sources of food; and on the other hand one 
type, the Gila monster, is poisonous. Crocodiles and alhgators are 
feared in the countries in which they hve 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. 


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 forehmbs. 

The forehmb is relatively slender, possesses muscles nearly to the 
tip, and is covered by feathers. Along the posterior margin are attached 
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, chmbing, 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 




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). 

^^<^ hrPJiaJanges 

A Phalanges g 

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 

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 
harhs (Fig. 294). The barbs in turn bear still smaller and slenderer 
lateral projections known as harhules. When the barbs lie parallel 
and the barbules are hooked together so as to make of the whole a 



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. 

A B D 

Fig. 294. — Types of feathers. A, flight feather. B, contour feather. C, down feather. 

D, filoplume. From specimens. X %■ 

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 forehmbs 


and along the lateral margins of the tail. (3) 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) Filoplumes, 
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 

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 uncinate 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 
apphcation in their internal structure of the engineering principles 
involved in the use of thin plates at right angles to one another as in an 

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 hmbs 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 fining which protects its 
wall during this process. The afimentary 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. 



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 air 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 


Ear opening 





Fig. 295. 

-Dissection of a pigeon, Columba livia Linnaeus. (Based upon a Pichler chart, by 
the courtesy of Mariinus 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, 



among which, however, they are not developed to the degree seen in 

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 









■•■•■■ •'%^l 


' ' "'ff-"^-'' 

sa^U, - , ">xii\i 






\ -^^NTX^ 


^^^ VIII, 



V\ ' \ 

ST Medulla 

)\is \^ 


Optic lobe 


Fig. 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- 



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 (ar ke or' ni thez ; G., 

Fig. 297. — Fossil remains of Archaeopteryx siemensi Dames, representing a specimen in the 
Berlin Museum. {From Steinmann-Doderlein, " Elemente der Palaeontologie.") X /^. 

archaios, ancient, and ornithes, birds) and represented by a single genus, 
Archaeopteryx, and a second subclass, the Neornithes (ne or' ni thez; 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 



hummingbird found in Central America is only 1}^^ 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. 

Fig. 298. — One conception of the appearance of Archaeopteryx macrura Owen, based 
upon a specimen in the British Museum. {From Wieman, "General Zoology," after Romanes, 
by the courtesy of McGraw-Hill Book Company, Inc.) X J^. 

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. 



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. 

"Reptiles and Amphibians.") 

{Redrawn from Barbour, 

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. — A 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. 



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 

I K 

Fig. 300. — Beaks of birds. From mounted specimens. A, generalized beak of ring- 
necked pheasant. X }i- B, straining beak of canvasback duck. X H- C, spearing 
beak of bittern. X }4. D, probing beak of greater yellowlegs. X 3^. E, beak of 

brown pelican. X J^. F, chisel-like beak of hairy woodpecker. 


beak of Swainson hawk. X j'i- H, 
insectivorous beak of myrtle warbler. X 
grosbeak. XJi- K, beak of red crossbill. 

insectivorous beak of nighthawk. X 'ji- I, 

G, carnivorous 


l^. J, graminivorous beak of black-headed 


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- 



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- 

Feet of birds. From mounted specimens. Showing the inner side of the 

foot, except J. A, wading foot of greater yellowlegs. X 3^. B, totipalmate foot of 
cormorant. X /i- C, swimming foot of blue-winged teal duck. X x'S- D, generalized 
foot of ring-necked pheasant. X %. E, perching foot of yellowthroat. About natural 
size. F, raptorial foot of Swainson hawk 

I foot o 
ostrich. X Hs- 

H, lobate foot of coot. X %. /, clinging foot of flicker. X /"a- 

X ^i- G, syndactyl foot of kingfisher. X % 

J, running foot of 

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 




number of modifications. 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 
• Fig. 302.-^ Mechanism of perch- ^^^^ ^^ ^j^^ posterior end of the body 

ing in birds. Preparation from a ^ 

crow. The flexor muscles are shown where they serve effectively as propellers 
ending in tendons which pass behind -^^ underwater Swimming. 

the joint between the tibiotarsus ^ 

and the tarsometatarsus and under 432. Plumage. — ^The plumage of birds 

a bony arch near the upper end of the y^rics Considerably, fitting them for vari- 

tarsometatarsus. Running under a * i n- i i 

ligament at the bases of the toes ous modes of living. Among the flightless 

they are distributed to the individual ^^^.^g ^^^ ^ ^\{\Qh live in dcSCrt regions 

digits. Because of this structure, ^ <- i i j 

when a bird flexes its legs and sits and the feathers of which are slender and 
upon the perch, the toes grasp the ^^ ^^^^ overlap. The wings of penguins 

perch with a very powerful grip. i r i i • i i 

are covered with feathers which are scale- 
like. There are also numerous curious modifications of the feathers 
on the head, wings, and tail. Especially conspicuous among birds 



Fig. 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.) 

Fig. 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.) 



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 hght, caused in part by ridges and furrows 


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. 



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 syrinx, 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.) 

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 



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 

Fig. 307. — Eggs of the largest bird, an ostrich, Struthio camclus, 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 

Fig. 308. — Ahricial young birds. A, a blind, naked, weak and helpless 2U-hour old 
woodpecker. (Photo, by Ralph S. Palmer, U. S. Nat. Mtis. Bull. 174, Arthur C. Bent.) 
B, a scantily feathered, blind but stronger 3-day old domestic pigeon. (Photo, by Edson H. 

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. 



435. Reproduction. — Birds may mate either for the rearing of a 
single brood or for life. The nesting locaUty 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 

Fig. 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 


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. 



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. 

Fig. 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. Some are 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. 



Fig. 311. — A more complex nest of the robin, Planesticus m. migraforius 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.) 


Fig. 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.) 



Fig. 313. — The passenger pigeon, Edopistes 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.) 


Fig. 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.) 


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 fertiUzers. 

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 w^hich 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. 




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 mth the term mammals, some 
using the term animals in the same sense and others 
the word beasts. 

437. External Characteristics. — Mammals are dis- 
tinguished by the possession of hair, in connection 
with wdiich they have developed sebaceous, or oil, 
glands. They also possess sweat glands and mam- 
mary glands and in some cases scent glands. Lips 
and cheeks are found in all except the whales, and 
there is a fleshy and cartilaginous lobe about the 
external opening of the ear known as a pinna (Fig. 
234). The eyes are protected by lids, the upper of 
which is the movable one, in contrast with the birds, 
in which the lower lid is movable. The facial portion 
of the skull usually projects to form a muzzle, or snout. 
Typically, mammals possess four feet with five toes 
on each foot. Both the feet and toes are modified in a 
variety of ways. 

438. Hair. — Hairs are lifeless epidermal struc- 
tures arising from a Hving 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 





Fig. 315.— Dia- 
gram of a longitudinal 
section of a typical 
hair, which shows the 
main parts. {From 
Hausman, American 



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. 

Fig. 316. — Hair, magnified, showing the outer covering of the thin, horny, transparent, 
plates of various forms. A, golden mole. B, red bat. C, beaver. D, woodchuck. E, 
mink. F, skunk. G, cat. H, horse (Percheron). {Redrawn by Paul T. Gilbert from 
Hausman, "Natural History," and the Sci. Man.) 

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), 



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 epitheUal 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. 

Fig. 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 %• B, coyote, or prairie wolf, a carnivore. X }i- C, beaver, a rodent. 
X %. D, sheep, an ungulate. X M. 

The aUmentary 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 




Pulp cavify 


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 epiglottis. 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 
spaces called pleural cavities, which are the 
lateral portions of the thoracic cavity, the 
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 
mechanism. The heart is four-chambered 
and the systemic and pulmonary circula- 
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 

^^'-''^O^-B/ooc/ vessel 

Fig. 318. — Canine tooth of a mam- 
mal, diagrammatic. 



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 

Carotid artery 
Vugular vein 

artery and 

Aortic arch 

artery and 

Vena caycj 

and vein 

vessels to 

Fig. 319.— Diagram of mammalian circulation. 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 


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: 


Subclass I. Prototheria (pro 15 the'rl a; G., protos, first, and therion, mammal). — 
Egg-laying mammals. 
Order 1. Monotremata. 
Subclass IL Eutheria (u the' ri a; G., eu, true, and therion, mammal). — Viviparous 
forms, including all the rest of the mammals. 
Division I. Didelphia (dl d6l' fl 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. Monodelphia (mon 5 d&V fl a; 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 (im gwik u la' ta; L., unguiculatus, provided with 
claws). — Clawed mammals. 
Orders 3 to 10. Include Insectivora, Chiroptera, Carnivora, Rodentia, and 
Section II. Primates (as a zoological group name, pri ma' tez; L., primatis, 
one of the first in rank). — Contains mammals with nails on the fingers 
and toes. 
Order 11. Primates. 
Section III. Ungulata (tin gii la' ta; L., ungulatus, provided with hoofs). — • 
Hoofed mammals. 
Orders 12 to 16. Include, besides the typical hoofed forms, the Sirenia. 
SecUon IV. Cetacea (se ta' she 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 (mon 6 trem' a ta; G., monos, 
single, and trenia, 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 



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. Echidna, possesses a temporary 
pouch in which the eggs are incubated. 


^ /Cerebrum 


Fig. 320. — Brain of European rabbit, Lepus cunicuhis 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 Marsupiaha (mar su 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 



until it has become sufficiently developed to move about and resume 
its attachment when it wishes to feed. 

Fig. 321. — The Australian duckbill, Ornithorhynchus anatinus (Shaw). (Drawn from 
Lydekker, " Wild Life of the World," vol. II.) X H- 

The marsupials were distributed ages ago over both Europe and 
North America, but now they are confined to Australia and neighboring 

Fig. 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 from a specimen in the 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 




7'"< I'lih \ 

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. 

Fig. 324. — A mother red bat, Nyderis horcalis (Muller), 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 Pickwell.) 



have adapted themselves to various modes of hving, some of them having 
become carnivorous, and others molehke, shrewHke, or rodent-Hke. 

Fig. 325. — A mongoose, Hcrpcstcs mungo, ready to attack the deadly cobra, Naja 
tripudians, 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 

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. — Raccoon, 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 from a 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). 



444. Unguiculata. — Insectivora (in sek tiv' 5 ra; L., insectum, insect, 
and vorare, to devour) includes tlie liedgehogs, moles, and shrews. They 

^> ^ '♦.' ...i 

«-•* ' ,.- /'. 

""^ '^'•lA 

Fig. 327. — Hyenas. ('n'i',,t,i. niiicuIhIii, uf Sovitli -MVirn. ilicy an- ii^.i mnal, 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. 

Fig. 328. — Lion and lioness, Felis leo, of Africa are among the largest and most powerful 
of the cat family. (Photographed from specimens in the University of Nebraska State 

They are all insect eaters and are, generally speaking, beneficial. 

The order Chiroptera (kl r6p' ter a; G., cheiros, hand, and pteron, 
wing) includes the bats. These are the only true flying mammals, the 



wings being formed by membranes stretched between the greatly elon- 
gated digits of the forelimbs and between those and the hind limbs 


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 

A B C 

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 

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). 



Carnivora (kar niv' o 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, Erethizon dorsatum (Linnaeus). An example of the 
arboreal porcupines which are limited to the New World. They are large clumsy animala 
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 pe' 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 

Fig. 332. — Nine-banded armadillo, Dasypus novemcinctus Linnaeus, found from Texas 
and southern New Mexico to Argentina. From a mounted specimen. X J^. 

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 (pm 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 



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 

The order Rodentia (ro den' shi a; 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, 
porcupines, and beavers; it is 


very rich in species, these numbering 

about one-third of all of the species of 


Edentata (e den 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 
are toothless; the others possess teeth, 
but these lack enamel and are absent 
in the front part of the jaws. The 
sloths are interesting because they are '^^^ tetradactyla. A partly arboreal 

. . edentate, with prehensile tail, long 

distinctly arboreal animals, having a snout, small mouth, long and wormlike 
habit of hanging from the underside of t^^^s^®' ''''^ '?\*f.*.^- ^* eats mainly 

^ . . . termites, and inhabits forests oi tropical 

branches and following them m this America. {Photographed from a speci- 
position in locomotion (Fig. 334). The J/^J^^^/ University of Nebraska state 

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 

Fig. 333. — ^Lesser anteater, Taman- 



Fig. 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.) 





Fig. 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. 



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. 

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 Hve 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 (ar ti o dak' ti la; G., artios, even, 
and dadylos, 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 Ufe. 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; 



Fig. 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.) 

Fig. 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 



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 
ahomasum (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 W 
other chambers, being digested in the 

The odd-toed ungulates, or Peris- 
sodactyla (pe ris o 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 ^ , ^^| '• 


Proboscidea (pro b5 sid' e a; G., 

pro, in front, and hoskein, 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 re' ni a; G., seiren, a sea 

nymph) includes the manatees and 

dugongs, which are aquatic and herbiv- 
orous animals with several character- in nature elsewhere. They have an 

, . , . , extremely long neck which enables them 

IstlCS betraymg a relationship to the ^o browse upon trees. Even though so 

elephants. The manatees are found in long, there are only seven vertebrae in 
, „ . 1 ii J. c the neck, the normal mammalian num- 

the fresh waters along the coasts Ot ^er. (Photographed from a specimen in 

southern North America, northern the University of Nebraska State 

South America, and Africa; the du- 
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 

Fig. 339. — A male giraffe, Giraffa 
reticulata. These ruminants live only 
in Africa and apparently never existed 



Fig. 340. — African tiepiiants, Loxoduula afncaniL, have heads which slope back more 
and have much larger ears than the Asiatic species. They are also more savage and difl&- 
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 tuska. (Photo- 
graphed from grouped specimens in the University of Nebraska State Museum.) 

"* ;"sr*«'^ 


Fig. oH. Thf 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.) 



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 haleen. 
Whalebone is a horny material developed from the epidermal lining 

Fig. 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 






^^^ Blastocoel 
P (yolk sac) 




Amniotic cavity 




Entoderm G 

Extra -embry- u 
onic coelom " 


Last trace 
of extra- em- 
bryonic coelom 



Fig. 343. — Diagrammatic representation of the stages in mammalian development. 
A, egg cell, in section. B, two-celled stage. C, morula. D, section of morula. E, 
blastula, in section. F, development of entoderm. G, formation of amniotic cavity, and 
separation of embryonic disc into entoderm and ectoderm, i/, development of mesoderm 
and extra-embryonic coelom. 7, formation of body stalk and beginning of placenta. 
/, cross section of same stage as I. 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 noiesoderm stippled. 


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 tme 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 hke 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 

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. 343). 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 in7ier 
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 



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 




Fig. 344. — Diagram of the embryonic membranes and circulation of a mammal. Stage 
between / and K in Fig. 343. For comparison with Fig. 271. Arteries erosslined, veins 
black. Arrows show direction of blood flow. {From Wilder, "History of the Human 
Body," by the courtesy of Henry Holt & Com.pany.) 

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 


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 
\^^th 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 umbilical 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 


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 


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. 



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 civihzed 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 

Fig. 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, chimpanzees (Fig. 345), and gorillas (Fig. 346). All these 




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 

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 



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 

CenfrcfJ sulcus 

Fig. 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 



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 opposabihty 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. 


Fig. 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 apehke. 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 a part. 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 hmestone cave near Peiping, China, include 
one almost perfect brain case. 


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 apeUke 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. 3485). 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 dra^\^ngs 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. 


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. 




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 

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, 



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: 


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. 


1. Material outgo. 

a. Solid wastes, egested. 

b. Liquid wastes, eUminated. 

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 small 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 

460. Differentiation. — It has been seen that differentiation, accom- 
panied by division of labor, presents itself in organisms in several ways 


(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 pz'ocess 
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 ce-phalization, 
which is the setting apart of a head region containing the brain and many 
of the organs of special sense (Chap. XXXI 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 


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 distmguish 
individuals, the result of their combined activities determinmg 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 orgamsms are ever 
precisely alike, though they may resemble each other very closely. 1 his 
individuahty 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 a stream. 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. A living 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 mamtams its 
individual character. It has been said that the body changes once m 
every seven years. This is a statement which is not exactly true, though 
it suggests a truth. Some materials in the body, such as bone, remam 
the same throughout all or a large part of the individual's he, while m 
other structures of the body, as in the cells of the skin, replacement is 
continually taking place. i,- u^„4. 

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 matunty 


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. For a 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 hfe 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 (Sees. 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 life cycle. 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. 


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 as many. 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 



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- 




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 syncytium 
(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 



Fig. 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 

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 


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. 

Connective tissue 

Fig. 350. — Diagramniatic section of an insect's skin. 

The tegument ary 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, 


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, 

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 stoniodeum, 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 proctodeum. 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 


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 



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 moUusks gills 
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 

B C 

Fig. 351.— Diagrams showing types of glands. A, simple acinous gland. B, 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 



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- 


Fig. 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, 
wath simple alveoli. C, lung of a lizard, showdng 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. E, lung of a mammal showing the branching of the 
bronchi. F, 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 wdthin a closed system of vessels; and 
the other the opeyi 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 ehminative struc- 
ture is the contractile vacuole. Sponges, coelenterates, and ctenophores 


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 amehocytes, 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 ahke 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 

In coelenterates the sex cells are derived from interstitial cells, and the 
mass of cells in which they are produced is either the testis 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- 
fertihzation is avoided by structural conditions which prevent it or by 


differences in the time of maturation of the two kinds of sex cells. In the 
higher metazoans the diecious 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 ctenophores 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 origin; the attachment pulled upon, the iyiser- 
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 



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 disc (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. 77 C). This also leads to a type of reflex action. 
A typical reflex act has been described in connec- 
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 fiat- 
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. 

Fig. 353. — A 
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 dcr Zoolog.- 
Institut der Univer- 
aitdt Wien, vol. 17.) 


2. Ganglionic Synaptic Nervous System.— 'Nervous systems of this 
type possess neurons the cell bodies of which are in ganglia. In the 
mollusks these gangha 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 cephahzation. 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, hgaments, 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, 


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 

Fig. 3.54. — 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 upon 
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. 




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, emhryogeny 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 a--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 x-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 fining 
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. 



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 

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 fertihzation. 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 
fertihzation 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 (Se